Cold shutdown that doesn't require coolant circulation?

[HowlerMonkey]

"cold shutdown" that doesn't require coolant circulation?

I was wondering what is preventing a plant being built that can be truly shut down and not require coolant circulation.

Is it that efficiency would be reduced to unacceptable levels?

 

[Simon Bridge]

You mean design a reactor so the fuel can be completely removed?
What makes you think this does not already happen?

 

[Astronuc]

I was wondering what is preventing a plant being built that can be truly shut down and not require coolant circulation.

Is it that efficiency would be reduced to unacceptable levels?

Coolant circulation is simply a mechanism of energy transport. A consequence of the fission process is the accumulation of fission products and transuranics in the fuel. Fission products undergo decay (beta and gamma), while transuranics and their decay products undergo (alpha, beta, gamma) decay - well after the reactor is shutdown. Initially, the decay heat rate is a few percent of steady-state operation, but it rapidly decreases as short-lived isotopes decay quickly in seconds, minutes, hours, days to stable isotopes.

It is the decay (or residual) heat that must be removed from the core. Normally there is a residual heat removal (RHR) system, which is tied into the primary system, but is only used after shutdown. There is also an emergency core cooling system (ECCS), which is also tied into the primary system, but only used if the primary cooling system fails.

The RHR and ECCS are active systems in that they have valves and pumps, which usually require power. Some systems can be driven from steam, otherwise electrical motors are required. Modern plant design have far fewer valves, piping and pumps/motors, and usually some large volume of cooling water for passive cooling - which usually means gravity and natural convection.

In general, natural convection uses much lower flow velocities, and hence lower heat transfer coefficient, than force convection. Lower heat transfer coefficient means higher temperatures. It is necessary to keep the fuel temperature sufficiently low to ensure that internal gas pressure in conjunction with cladding temperature and corrosion do not lead to failure.

 

[nikkkom]

I was wondering what is preventing a plant being built that can be truly shut down and not require coolant circulation.

Is it that efficiency would be reduced to unacceptable levels?

Fukushima Unit 1 had such a system - Isolation Condenser. It is nearly passive - no pumps required. The only active measurements needed to operate IC for days is to pour more water into IC tanks as it evaporates.

Nothing prevents IC designs with larger tanks and/or with more trains (Fukushima had 2). So yes, fully passive cooling is possible.

As far as I understand, other units don't have IC for a quite appalling reason: for units with bigger design power, IC also needs to be bigger. Active systems can be made more compact. So IC was replaced by them.

There goes "safety first" PR of nuclear industry... in practice, they did sacrifice safer systems for somewhat more compact active ones.

 

[Hiddencamper]

The AP1000 westinghouse design still requires circulation, but it uses natural forces, and can keep the reactor cooled for 72 hours with no human interaction or electrical power once the initiation signal fires off. Gen 4 designs are primarily passively cooled and require no active systems at all for extended periods of time (as low as a week or indefiniately)

Regarding nikkom's comment about the IC being removed, the IC is a great system in concept, but for the evolution of the BWR it was a bottleneck. The IC in BWR series plants only contains enough water for about 20-30 minutes per train (at 2 trains is about an hour at most). IC was replaced by RCIC. RCIC is a pump which is driven using steam from the reactor. It initially draws water from an outside tank and can function for up to 8 hours using external water before switching over to the suppression pool for cooling. It requires no AC power, only DC power to initiate and control. It can also be black started without DC power, but it requires manual control.

RCIC has a much longer injection and cooling time than IC does. Additionally IC ONLY removes decay heat, it does not provide any injection. RCIC is capable of removing a fair amount of decay heat AND injecting.

The RCIC system at Fukushima unit 2 ran for 70 hours. Unit 3 ran on RCIC for about 32 hours (not sure why it failed, probably loss of DC power coupled with no injection water to use and overheated suppression pool water flashing in the pump causing pump damage...but we won't know for a while. Without active cooling or an external water source, RCIC with no DC power will eventually overheat and fail, but it still lasts for drastically longer than the IC would have). If unit 1 had a RCIC system, the whole accident may have been less of an issue. The operators wouldn't have wasted so much time on trying to figure out if the IC was functioning or not and would have been able to prioritize real issues. They spent a disproportionate amount of time and resources trying to get the IC to work in unit 1. Personally I think IC plants should be shut down. RCIC all in all is the better system. IC requires an active feed pump to maintain it filled, while RCIC self powers and pumps using reactor steam. The plant I'm at was notorious for running for several days on RCIC after BOP scrams to maintain the reactor in hot standby and save a few hours off of their next restart. IC also contaminates the area outside the plant. It's low level, but the IC uses reactor grade water on the shell side, which contains small amounts of tritium and some other radioisotopes. Dresden plant in illinois (before security fences prevented this), they would have to decon the first 2 or 3 rows of cars outside the reactor building after running the IC.

 

[nikkkom]

Regarding nikkom's comment about the IC being removed, the IC is a great system in concept, but for the evolution of the BWR it was a bottleneck. The IC in BWR series plants only contains enough water for about 20-30 minutes per train (at 2 trains is about an hour at most).

In Fukushima Unit 1, IC capacity was 8 hours of cooling. It's trivial to design an IC with bigger water tank.

IC was replaced by RCIC. RCIC is a pump which is driven using steam from the reactor.

Pump is an active system. It can break, it can't function if steam pressure is lost (say, a fissure in RPV's top).

It requires no AC power, only DC power to initiate and control. It can also be black started without DC power, but it requires manual control.

Unit 3 ran on RCIC for about 32 hours (not sure why it failed, probably loss of DC power

That's exactly why we want to see passive systems, which don't need DC to work.

Without active cooling or an external water source, RCIC with no DC power will eventually overheat and fail, but it still lasts for drastically longer than the IC would have).

IC tank is at atmospheric pressure and can be refilled by a very ordinary equipment. A fire truck will do. Try using it to cool overheating suppression chamber. Good luck.

If unit 1 had a RCIC system, the whole accident may have been less of an issue. The operators wouldn't have wasted so much time on trying to figure out if the IC was functioning or not and would have been able to prioritize real issues.

And if operators would have even minimal training for SBO, and in particular, how to activate the IC during SBO, the disaster may be averted altogether.

IC also contaminates the area outside the plant.

?!

It's low level, but the IC uses reactor grade water on the shell side, which contains small amounts of tritium and some other radioisotopes.

WHAT? Why a clear water can't be used in IC?

 

[Hiddencamper]

I just pulled the FSAR for a plant which contains an IC [Dresden http://pbadupws.nrc.gov/docs/ML0719/ML071910096.pdf Same model as unit 1. Go to section 5.4.6]. The IC is made up of 304 SS tubes, and contains enough water inventory for about 20 minutes prior to boil-off. The IC's accredited makeup source is the condensate storage tank (CST), via diesel driven transfer pumps. The condensate transfer pumps, fire protection system, and probably some other stuff, can be used to get water into the IC. The valves for the IC still require AC power for the MOV and control power to be actuated (so if they were potentially closed, like when the tsunami hit unit 1, it's useless). I'm not sure if they fail close on loss of power.

When you look at the water in the CST, there is more than enough for 8+ hours of IC for decay heat removal. But the shell of the heat exchanger only has 20 minutes of water. (If you want more evidence of this, go to the NRC teleconferences the day of Fukushima. They are available through the FOIA links. They state it in there, each IC has 20-30 minutes of water in it. I think its in the first of like the 7 teleconferences posted.)

>Pump is an active system. It can break, it can't function if steam pressure is lost (say, a fissure in RPV's top).

RCIC and IC are not ECCS systems, and are not required to function for a primary system line break. The RCIC pump is powered passively (decay steam) which gives it some inherent advantages to something which fails after 20 minutes. If you had a line break, HPCI(high pressure coolant injection)/HPCS(high pressure core spray) are accredited for high pressure injection. Assuming single failure of HPCI or HPCS, then on level 1 water level (low-low-low alarm water level), the ADS (automatic depressurization system) activates to blowdown the vessel and inject with all three LPCI (low pressure coolant injection) systems and the LPCS (low pressure corespray) system. Neither RCIC nor IC are accredited for loss of primary loop integrity accidents.

>That's exactly why we want to see passive systems, which don't need DC to work.

Additionally, RCIC's main limit is suppression pool temperature, as it uses that water to cool itself when it is in recirculation mode. During SBO, you lose RHR's ability to remove containment heat, and containment venting is the only heat removal you have to attempt to reduce suppression pool temperatures.

>IC tank is at atmospheric pressure and can be refilled by a very ordinary equipment. A fire truck will do. Try using it to cool overheating suppression chamber. Good luck.

IC doesn't inject water into the vessel or containment. It also does not cool containment. RCIC tank is at atmospheric pressure and can be refilled just as easily. Additionally RCIC injects, removes decay heat, and can help manage suppression pool and containment temperature/pressure (adding colder water into the vessel from the outside rather than recirculating suppression pool water).

>And if operators would have even minimal training for SBO, and in particular, how to activate the IC during SBO, the disaster may be averted altogether.

One of the identified issues (as can be seen in INPO IER 11-05 August 2012 addendum found here: http://www.nei.org/resourcesandstat...t-the-fukushima-daiichi-nuclear-power-station), is that Japan deviated from many decisions and lessons learned the US had after Three Mile Island. One of those lessons learned was that operators should train in a simulator that matches the plant they are working in. At Fukushima, operators trained in the Unit 2-4 simulator, as they did not have a unit 1 simulator, and as a result never had "hands on" experience with the IC. That lack of knowledge combined with no AC/DC instrument power or control power made figuring out if unit 1 had cooling near impossible. If unit 1 had a RCIC system, this wouldn't have been an issue, as RCIC does not need to be cycled to prevent violation of reactor vessel cooldown rate technical specifications like the IC does, and would have remained in service as it did in unit 2 and 3 when the tsunami hit.

>WHAT? Why a clear water can't be used in IC?

Editing this section: Clean water or reactor grade condensate can be used for the IC. Clean water is much more expensive to make, and may not have been the preferred source 20+ years ago (which is where the stories I know of regarding contamination come from). Condensate contains low level contamination. The IC system does have radiation monitors which will isolate the atmospheric vents at high radiation levels, but this is intended to protect against a leak path from damaged fuel, out through a damaged heat exchanger tube, to the atmosphere. I do not believe there is filtering on the atmospheric vents.Personally I think the IC is a big danger. It requires active diesel pumps for more than 20 minutes, and you still need control power to line it up. It can't inject (so you are still reliant on HPCI if you are in hot-standby and need to inject), and it cools the vessel down faster than the 100 degree F per hour limit on BWR reactor vessels and requires operators to cycle it constantly. With RCIC, it will auto stop and start between reactor water level 8 (high high alarm) and level 2 (low low alarm) [respectively]. It has a flow controller and can be dialed to inject to match decay heat directly to prevent excessive cooldown, while injecting. It can inject outside water. It functions with no AC, and its pump does not require an active energy source (as decay heat is the energy source). Maybe I'm slightly biased, but I restate my point, at Fukushima, unit 1 was a huge concern, and it had no RCIC, while units 2 and 3 lasted 70 and 36 hours respectively.

Side note: Great discussion/comments.

 

[rmattila]

As nikkkom said, IC capacity of the order of 8 hours has been built e.g. at Fukushima Dai-ichi unit 1, as well as several other plants of the same generation. 20 minutes is obviously too short, but it doesn't mean the IC wouldn't - if designed properly wrt. capacity and valve fail safe modes - be a great device for passive shutdown cooling.

 

[Hiddencamper]

As nikkkom said, IC capacity of the order of 8 hours has been built e.g. at Fukushima Dai-ichi unit 1, as well as several other plants of the same generation. 20 minutes is obviously too short, but it doesn't mean the IC wouldn't - if designed properly wrt. capacity and valve fail safe modes - be a great device for passive shutdown cooling.

I'm not sure if I understand, but again, the IC does not have 8 hours of capacity built in, it has 20 minutes, and requires active systems to function for longer than that.

The IC as it exists in BWR/2 and /3 plants is pretty awful as it is limited capacity and requires active pumping systems, where RCIC can run for DAYS.

Now in comparison, the IC as designed in the ESBWR, and the PXS heat exchanger in the AP1000 (basically an IC), are excellent, but that requires a full plant/containment design to support it. The IC as exists in older BWRs is a vulnerability in my professional opinion, and I that's what we saw at Fukushima.

 

[nikkkom]

The IC as exists in older BWRs is a vulnerability in my professional opinion

You might be right about that particular design. I don't know the details.

and I that's what we saw at Fukushima.

If operators would engage IC on Unit 1 and it didn't save the day, I'd agree with you.

But that's not what happened.

In Fukushima after SBO, operators failed to keep valves from RPV to IC open and thus IC couldn't do its job.

 

[rmattila]

I'm not sure if I understand, but again, the IC does not have 8 hours of capacity built in, it has 20 minutes, and requires active systems to function for longer than that.

There appears to be different designs of the IC regarding the shell size capacity. that post by Tsutsuji-san cites the Japanese NSC saying

In BWR-3 plants, the IC can provide cooling for 6 hours with the isolation condenser as water source, but as it can be replenished via the fire extinguishing line from the filtrate water tank, its cooling capacity can be prolonged for 10 more hours.

The IC was run for an hour between the quake and tsunami, and its failure was due to spurious isolation signal due to loss of DC, as translated by Tsutsuji-san in the "Japan Earthquake: nuclear plants" thread around page 750. Last October, when the IC:s were checked, they were still more than half full, so running out of shell side water was not the cause of loss of the IC capacity - it was the spurious isolation signal.

 

[Hiddencamper]

You might be right about that particular design. I don't know the details.
If operators would engage IC on Unit 1 and it didn't save the day, I'd agree with you.

But that's not what happened.

In Fukushima after SBO, operators failed to keep valves from RPV to IC open and thus IC couldn't do its job.

They manually closed the valves prior to the tsunami hitting (actually this happened several times, they cycled the IC in and out of service because the IC cools the reactor down too quickly). But yea, once they lost control power and instrument air it was pretty much over for unit 1. BWR vessels have a 100 degree F/hr tech. spec. cooldown/heatup rate, and each reactor vessel is only rated to exceed this once in its lifetime. To date, I do not believe any plant has actually done so (at least not in the US). Based on differing reports, they were trying to re-start the IC, and its not entirely clear what happened. I agree with the above poster than an isolation signal may have come in on loss of DC, but at the same time, the AC system was out of service around the same time, so it is possible the valves were half shut or in some unknown intermediate state. Regardless they spend a LOT of time trying to figure out the state of the system which could have been better spent on other issues.

If the valves were lined up correctly and the diesel driven pumps were functioning and the makeup tank was still intact then yes, it would have helped a LOT at Fukushima. They still would have been limited to 8 hours, but that would have let them prioritize getting DC power and instrumentation restored.

 

[Hiddencamper]

Just posting another follow up, I likely made an error. While Dresden is nearly identical in design to Fukushima Daiichi, Dresden operates 2 reactors at over 800 MWe (over 2400 MWth), while Fukushima Daiichi unit 1 is a single unit which operates around 460 MWe (just over 1380 MWth most likely).

This difference in thermal output also directly correlates to a difference in decay heat, and would explain why Dresden only accredits their IC for 20 minutes while Fukushima Daiichi news posts claim up to 6 hours on shell side water inventory.

My opinion is still the same regarding RCIC vs. IC, but I apologize if I made this confusing.

 

[Astronuc]

IIRC, Dresden and Quad Cities are the largest of the BWR/3 class with 724 assemblies in the core. The units have been uprated.

Dresden and Quad Cities Uprates - http://pbadupws.nrc.gov/docs/ML0100/ML010080047.pdf

The proposed changes will allow the DNPS and QCNPS units to operate at an uprated power level of 2957 megawatts thermal (MWt). This represents an increase of approximately 17 percent rated core thermal power over the current 100 percent power level of 2527 MWt for DNPS and an increase of approximately 17.8 percent rated core thermal power over the current 100 percent power level of 2511 MWt for QCNPS.

FK1-1 is a small core like Garona with 400 assemblies. Units 2, 3 and 4 were moderate-size cores of 548 assemblies.

The IC, RCIC and ECCS are sized according to the core. Some systems can be challenged, i.e., are less effective, for larger cores.

 

[clancy688]

IC also contaminates the area outside the plant.

?!

I think he may have misphrased this one. An IC doesn't necessarily contaminate the area outside the plant, but in case of leaks there is the possibility of it doing serious contamination.

As far as I understand both systems, RCIC is fully integrated into the PCV, therefore any pipe leak anywhere in the piping would vent coolant into the primary containment, but not into the environment.

Contrary to that, the IC-piping naturally has to be at least partially outside the primary containment - since the coolant pool is there as well. Therefore in a reactor with an operating Isolation Condenser, there's a direct and unfiltered connection between the RPV and the environment, with the primary containment vessel being circumvented. If the IC piping outside the PCV would be damaged, the fuel would be directly connected to the atmosphere.
And that's the reason why it had been constructed fail-safe. You absolutely don't want a backdoor in your containment you may not be able to close in case of accidents. Therefore make sure that it's closed on default.

To go back to Fukushima:
I'm not so sure anymore if constructing it fail-safe had been the wrong decision. Let's say it had been constructed fail-operational.
Then Unit 1 would have survived for 8 hours with adequate cooling, after that the reactor would've boiled dry anyway, high pressure steam would have ruptured the IC-pipes outside the reactor and fission products released from the failing fuel rods would've found their way directly into the atmosphere.

 

[Hiddencamper]

I think he may have misphrased this one. An IC doesn't necessarily contaminate the area outside the plant, but in case of leaks there is the possibility of it doing serious contamination.

As far as I understand both systems, RCIC is fully integrated into the PCV, therefore any pipe leak anywhere in the piping would vent coolant into the primary containment, but not into the environment.

Contrary to that, the IC-piping naturally has to be at least partially outside the primary containment - since the coolant pool is there as well. Therefore in a reactor with an operating Isolation Condenser, there's a direct and unfiltered connection between the RPV and the environment, with the primary containment vessel being circumvented. If the IC piping outside the PCV would be damaged, the fuel would be directly connected to the atmosphere.
And that's the reason why it had been constructed fail-safe. You absolutely don't want a backdoor in your containment you may not be able to close in case of accidents. Therefore make sure that it's closed on default.

To go back to Fukushima:
I'm not so sure anymore if constructing it fail-safe had been the wrong decision. Let's say it had been constructed fail-operational.
Then Unit 1 would have survived for 8 hours with adequate cooling, after that the reactor would've boiled dry anyway, high pressure steam would have ruptured the IC-pipes outside the reactor and fission products released from the failing fuel rods would've found their way directly into the atmosphere.

The IC shell side can be filled using condensate water (primary loop), which IS contaminated. Dresden had to regularly decontaminate cars in the parking lot after IC use in the early 90s before they switched to using demineralized water instead.

The IC is not a direct unfiltered path, as the heat exchanger tubes are the RCS boundary in this case. The IC does have rad monitors which will isolate the IC should high radiation levels which could indicate an unfiltered release are detected, but that shouldn't be happening as the heat exchanger tubes shouldn't have leakage. The isolation system valves fail closed to ensure that if the IC would become part of the RCS, that there would not be a release path.

Also the IC pipes should not rupture. They are ASME code, so they should be tested to withstand 3 times reactor pressure. Additionally, the safety relief valves prevent the RCS from exceeding 1300 PSI, so the IC would never have to deal w/pressures that high (in fact, the IC, even if it was open at unit 1, would never have had pressures above 1200 due to SRV lifts)

 

[clancy688]

Also the IC pipes should not rupture. They are ASME code, so they should be tested to withstand 3 times reactor pressure. Additionally, the safety relief valves prevent the RCS from exceeding 1300 PSI, so the IC would never have to deal w/pressures that high (in fact, the IC, even if it was open at unit 1, would never have had pressures above 1200 due to SRV lifts)

Well, some time ago I found a patent of a (newer) isolation condenser where they specifically addressed the problem of bursting pipes:

https://www.physicsforums.com/showpost.php?p=3881754&postcount=12998

 

[Hiddencamper]

Well, some time ago I found a patent of a (newer) isolation condenser where they specifically addressed the problem of bursting pipes:

https://www.physicsforums.com/showpost.php?p=3881754&postcount=12998

For nuclear reactor safety analysis you HAVE to assume you are going to have pipes burst. It is a requirement. That's precisely why isolation valves exist. The pipes are ASME code and shouldn't burst or rupture, but you need to have defense in depth to get licensed to operate a nuclear power plant. It is for that reason that the IC valves are energize to open, and its also why there are radiation monitors which will close those valves if high radiation levels are detected leaving the plant.

This is consistent with GE designed isolation valves in other systems, including the main steam lines, the reactor building normal ventilation system, drywell purge systems, etc. When high radiation is detected in a system, that system is isolated.

The patent doesn't add anything new about pipe burst. A pipe burst can always happen. But the key thing here is it shouldn't.

It's just like LOCA. All the reactor piping is designed, inspected, and maintained in accordance with ASME codes and legal requirements, and as such, the piping should NEVER break in the lifetime of the plant. However you still have to assume they do, in order to overdesign your facility and ensure that you can prevent a release of radioactive material.

 

[rmattila]

In later ASEA BWRs, the containment isolation valves in the emergency cooling lines do not fail close, and their isolation signal is generated by energizing the circuit, unlike in other penetrations. I don't know if that (preferring core cooling to containment isolation) would be possible under the US regulations.

 

[Hiddencamper]

In later ASEA BWRs, the containment isolation valves in the emergency cooling lines do not fail close, and their isolation signal is generated by energizing the circuit, unlike in other penetrations. I don't know if that (preferring core cooling to containment isolation) would be possible under the US regulations.

It really depends. In the US you have to assume certain types of failures then calculate the maximum leakage from the containment under those situations to come up with a dose delivered to the public. Any valves which don't fail closed would probably need to be considered open, and it would just count as a penalty against your accident analysis.

There are some valves that require energy to close, but typically they aren't main steam valves.

Thanks for the interesting info though. I wouldn't have thought ASEA would have gone in that direction. I know GE BWRs over seas do some stuff that the US BWRs would never dream of doing (island startup, REVABS, RHR steam condensing, etc).

 

[nikkkom]

If the valves were lined up correctly and the diesel driven pumps were functioning and the makeup tank was still intact then yes, it would have helped a LOT at Fukushima.

Exactly. Unit 1 explosion wouldn't happen in this case, and that explosion added a lot of complications to the situation and to personnel morale.

They still would have been limited to 8 hours

No. If they would be able to replenish IC with a relatively clean water, IC can operate for days or even weeks. The limiting factor is scale formation from evaporation.

 

[nikkkom]

As far as I understand both systems, RCIC is fully integrated into the PCV, therefore any pipe leak anywhere in the piping would vent coolant into the primary containment, but not into the environment.

"fully integrated into containment" and "efficiently removes heat" are mutually exclusive design goals.

An IC doesn't necessarily contaminate the area outside the plant, but in case of leaks there is the possibility of it doing serious contamination.

True, BWR water is radioactive, and leaking it is not a good thing, but we are talking about a major emergency here. Even *if* (far from a given!) IC would leak some BWR reactor water while being used in Fukushima-like scenario, it would be *many orders of magnitude* better than core meltdown and resulting massive releases.

the IC-piping naturally has to be at least partially outside the primary containment - since the coolant pool is there as well. Therefore in a reactor with an operating Isolation Condenser, there's a direct and unfiltered connection between the RPV and the environment

No, there is no such connection. Reactor steam is fully contained by IC's piping. Unless piping leaks, the steam has no path to the outside.

If the IC piping outside the PCV would be damaged, the fuel would be directly connected to the atmosphere.

As I said:
(1) That's a big "if".
(2) Even if it would happen, it nowhere nearly as bad as what happened in Fuku when IC was not engaged.

I'm not so sure anymore if constructing it fail-safe had been the wrong decision. Let's say it had been constructed fail-operational. Then Unit 1 would have survived for 8 hours with adequate cooling, after that the reactor would've boiled dry anyway,

You are assuming operators would sit and look at IC steam plumes for 8 hours and do nothing?

high pressure steam would have ruptured the IC-pipes outside the reactor and fission products released from the failing fuel rods would've found their way directly into the atmosphere.

Which they did anyway. How is this worse than what in fact happened?

 

[clancy688]

"fully integrated into containment" and "efficiently removes heat" are mutually exclusive design goals.

Well, doesn't the RCIC "drop" heat in form of steam into the wetwell which's part of the primary containment? Down there steam condenses and the resulting water is pumped into the RPV again (hot steam going down into the wetwell powers a pump which's transporting the water back up into the RPV). Wetwell is acting as a heat sink. So either you have to cool the wetwell from outside or you're going to lose your heat sink after a certain time and coolant circulation stops (which afaik happened in Unit 2 and 3).

As I said:
(1) That's a big "if".
(2) Even if it would happen, it nowhere nearly as bad as what happened in Fuku when IC was not engaged.

Why? If cladding fails and fission products escape, and if IC-pipes burst, the radioactive steam would be released directly to the atmosphere. That's not the same what happened in Unit 1 and 3. There the radioactive steam was first transported into the wetwell which "scrubbed" it of many radioactive particles. Therefore there was an additional filter between the reactor and the atmosphere, which wouldn't have been in case of IC-pipe bursts.

You are assuming operators would sit and look at IC steam plumes for 8 hours and do nothing?

Nope. But I don't assume them to have superhuman powers either. The whole plant was devasted, tsunami debris everywhere. Afaik it took them considerably more than 8 hours to only drive the single available fire engine to Unit 1.

Which they did anyway. How is this worse than what in fact happened?

As I said, most (if not all) of what was released in Unit 1 and 3 went through the wetwell first, where most of the dangerous particles stayed. What happened at Unit 1 and 3 was a filtered release. Not a 100% filtered-one, but a filtered-one nonetheless. Whereas the IC release path would be close to 0% filtered...

 

[nikkkom]

Well, doesn't the RCIC "drop" heat in form of steam into the wetwell which's part of the primary containment? Down there steam condenses and the resulting water is pumped into the RPV again (hot steam going down into the wetwell powers a pump which's transporting the water back up into the RPV). Wetwell is acting as a heat sink. So either you have to cool the wetwell from outside or you're going to lose your heat sink after a certain time and coolant circulation stops (which afaik happened in Unit 2 and 3).

EXACTLY my point! RCIC does not remove heat from containment - it more like smears heat inside it.

Why? If cladding fails and fission products escape, and if IC-pipes burst, the radioactive steam would be released directly to the atmosphere.

While IC operates properly, cladding won't fail. Correctly operating IC gradually cools down RPV's water to sub-100C temperatures. Ergo, fuel would be intact, the water would contain a typical, relatively low amount of radionuclides (compared to meltdown scenario).

Meltdown can happen only after IC fails, or boils dry and not replenished, or if RPV leaks (in IC or elsewhere). IC is not meant to fight RPV leaks - this should be mitigated by other systems. IC's task is to cool down an intact RPV.

That's not the same what happened in Unit 1 and 3. There the radioactive steam was first transported into the wetwell which "scrubbed" it of many radioactive particles. Therefore there was an additional filter between the reactor and the atmosphere, which wouldn't have been in case of IC-pipe bursts.

Again, IC pipe burst would expose relatively _uncontaminated_ steam, nowhere near the levels of stem coming out of a core which melts down.

Nope. But I don't assume them to have superhuman powers either. The whole plant was devasted, tsunami debris everywhere. Afaik it took them considerably more than 8 hours to only drive the single available fire engine to Unit 1.

We need some facts here:

"12 March 05:50: Fresh water injection into reactor 1 is started."
That is 14 hours after the tsunami.

So, you are right. IC should have bigger tanks :) and there should be more fire engines prepared and sheltered around the plant.

(Sometime ago I wondered whether NRC ordered more fire engines to be made available in US NPPs? I still have no idea. Judging by Lochbaum's stories about NRC, I imagine if it would ever be done, it'd take a decade or two to implement </sarcasm>).

 

[HowlerMonkey]

The premise of this thread was whether there are designs that can survive an extended time period without power or generators whether recently scrambled or simply shut down and whether it is even possible or feasible.

Let's call that period of time 3 months without any factors other than not having electricity to run pumps or fuel supply to run generators.

 

[rmattila]

The SWR1000 (later renamed Kerena) has (had?) an IC inside the containment and without any valves, activated by reactor level drop.

 

[Hiddencamper]

The premise of this thread was whether there are designs that can survive an extended time period without power or generators whether recently scrambled or simply shut down and whether it is even possible or feasible.

Let's call that period of time 3 months without any factors other than not having electricity to run pumps or fuel supply to run generators.

Generation 3+ plants (AP1000, ESBWR) get 72 hours with no operator action or AC power. AP1000 in particular gets 1 week with a diesel engine pump and some battery power (simply to fire squib valve charges for depressurization) in the first 72 hours.

Generator 4 plants should be weeks or months or indefinite w/out electrical power.

No gen 2 or early gen 3 design can go more than a 1/2 a day without active cooling. Conservative analysis show that uncovery happens in 3-6 hours, fuel damage in another 2, vessel in another 2-4, containment in another 4-8, in generally near worst case conditions.

 

[nikkkom]

The premise of this thread was whether there are designs that can survive an extended time period without power or generators whether recently scrambled or simply shut down and whether it is even possible or feasible.

Re feasibility: it is certainly feasible. Isolation condenser design can be augmented to provide much longer, potentially unlimited, passive cooling.

Remember that decay heat power does drop off. In one day after scram, it drops to ~0.5% of full power. That will be 15 MWt for a 3 GWt plant.

The passive cooling system can be designed to sacrificially (e.g. boiling water in IC) absorb the initial high decay heat power, and to be able to dump the prolonged, but lower-power heat output _without_ the need to consume water: 15 megawatts thermal can be dissipated by passive air cooling.

A passive air cooler of this size would be a rather large device, though. I'm afraid the obstacle is that NPP industry doesn't want to spend money on it, since it will stand idle.

 

[nikkkom]

Another note: it should be possible to rig a quite simple "poor man's passive cooling system" in a BWR reactor. Before fuel rods start to crack and melt, depressurize RPV to 1 atm and keep it that way, then gravity-feed it with fresh water, letting water boil in the RPV itself.

This requires some designing: gravity-fed water must be available. It, and steam dump must be possible to activate w/o AC or DC. Otherwise, the system won't be fully passive.

This method of cooling would release some radioactive steam (unless initial steam dump is quenched), but again, compared to what actually happened at Fukushima, it'd be a *much* milder accident.

 

[Hiddencamper]

Another note: it should be possible to rig a quite simple "poor man's passive cooling system" in a BWR reactor. Before fuel rods start to crack and melt, depressurize RPV to 1 atm and keep it that way, then gravity-feed it with fresh water, letting water boil in the RPV itself.

This requires some designing: gravity-fed water must be available. It, and steam dump must be possible to activate w/o AC or DC. Otherwise, the system won't be fully passive.

This method of cooling would release some radioactive steam (unless initial steam dump is quenched), but again, compared to what actually happened at Fukushima, it'd be a *much* milder accident.

This model would also require that the containment be opened/vented, as you need a way to remove that pressure from the containment.

The issue is there really isn't a way to gravity feed in a BWR. (not counting the ESBWR, but none of those have been built or even purchased yet). Just about all lines into the vessel go up from a lower elevation (the lines are all going down to lower elevations to help with NPSH on the pump suctions).

 

[Hiddencamper]

Re feasibility: it is certainly feasible. Isolation condenser design can be augmented to provide much longer, potentially unlimited, passive cooling.

Remember that decay heat power does drop off. In one day after scram, it drops to ~0.5% of full power. That will be 15 MWt for a 3 GWt plant.

The passive cooling system can be designed to sacrificially (e.g. boiling water in IC) absorb the initial high decay heat power, and to be able to dump the prolonged, but lower-power heat output _without_ the need to consume water: 15 megawatts thermal can be dissipated by passive air cooling.

A passive air cooler of this size would be a rather large device, though. I'm afraid the obstacle is that NPP industry doesn't want to spend money on it, since it will stand idle.

Well remember, if I build a passive air cooler, I have the following engineering challenges:

I need to to not be in service when the plant is normally operating, but I need it to come in service when called upon under all conditions (requires DC power for squib valves or MOVs).

If the reactor is not boiling (natural circulation), I need the water level above the steam dryer (at least at the separator skirt), and enough decay heat to drive natural convection.

I need to have penetrations and piping which allow reactor coolant to go outside the containment to be cooled (in a situation where I potentially don't have power to close those valves upon a pipe rupture).

I need the structure (as it is safety related), to be protected from all external hazards, so it needs to have a shield structure around it. But that same shield structure is going to impinge upon air flow. Because of this I need natural chimney effect and other means to promote cooling.

It needs to be very large, but it also needs to be seismically qualified.

I need at least 2 of them for redundancy.

Ideally, they should be able to handle a wide range of events as not to be cost excessive (which was your reason why the industry didnt want to do it).

When you look at all of the above, the AP1000 design makes a lot of sense, but it also sheds some insights on why you can't apply this to a BWR design easily, and why you still need evaporation flow for an extended period of time for the AP1000 design. The biggest thing is the fact that you would need something outside of containment (again AP1000 being the exception as the heat transfer surface IS the containment), but the consequences of a line break would be very severe outside of containment.

I'm just throwing some thoughts out there. I agree it's physically possible and feasible, and it is an even better idea for molten salt or high temperature reactors where you can get much more bang for your buck in terms of heat transfer, but it is a very difficult thing to build and justify when you have things which already provide more than the necessary amount of safety which can also perform other functions. AP1000's PCS is a containment and a decay heat removal structure. PXS can be used for all DBAs. Combined, you find out that the AP1000 does not have safety related diesel generators, and that all AC powered ECCS systems are considered non-safety in the AP1000 (they are now "asset protection" systems and are given augmented quality, even though they are not safety related).

 

[nikkkom]

Well remember, if I build a passive air cooler, I have the following engineering challenges:

...

If the reactor is not boiling (natural circulation), I need the water level above the steam dryer (at least at the separator skirt), and enough decay heat to drive natural convection.

I don't understand why that should be difficult. If RPV is cooled by an IC or a theorized passive air cooler, and RPV not damaged, you aren't losing water from it.

I need to have penetrations and piping which allow reactor coolant to go outside the containment to be cooled (in a situation where I potentially don't have power to close those valves upon a pipe rupture).

These penetrations already exist for routing steam from RPV to IC.

I need the structure (as it is safety related), to be protected from all external hazards, so it needs to have a shield structure around it. But that same shield structure is going to impinge upon air flow. Because of this I need natural chimney effect and other means to promote cooling.

I don't see why you absolutely must encase the air cooler in some sort of massive shielding. This will drive costs up and make it even more likely that utilities wouldn't want to build it at all. Ergo, making it seemingly "better" in fact drives us to a situation where we don't have a passive, indefinitely-operating cooling.

I agree it's physically possible and feasible, and it is an even better idea for molten salt or high temperature reactors where you can get much more bang for your buck in terms of heat transfer, but it is a very difficult thing to build and justify when you have things which already provide more than the necessary amount of safety which can also perform other functions. AP1000's PCS is a containment and a decay heat removal structure. PXS can be used for all DBAs. Combined, you find out that the AP1000 does not have safety related diesel generators, and that all AC powered ECCS systems are considered non-safety in the AP1000 (they are now "asset protection" systems and are given augmented quality, even though they are not safety related).

Good to hear that :)Okay, here's my design which is very likely isn't going to work because I'm just a software engineer, but I'd like to hear where I'm wrong :)

Let's take an IC design similar to Fuku Unit 1. Its tanks are somewhere up in the reactor building. Modify it by adding many (a dozen or two) independent heat pipes whose lower ends are submerged into the IC tanks and then they are routed up to the roof and have a significant length of heat pipes all over it. (And, of course, I assume that operators know how to activate IC in the event of SBO. Post-Fukushima, it's kind of an obvious fix, right?)

Heat pipes are efficient at heat transfer starting from a few degrees above melting point of the internal liquid they use. In this case, water will do nicely. In this design they should be working to keep IC tank water cooled down to the outside ambient temperature, unless it is below ~+2 C in which case they will stop conducting heat (IOW: they won't try to freeze IC tank during winter).

During an accident, when IC tank water goes to 100 C and starts boiling, heat pipes will transfer and dissipate some of this heat to the outside air on the roof.

I wonder is it practical to size IC tanks so that, as decay heat rate goes down, they will stop boiling vigorously and start transferring most of the heat via heat pipes before they boil dry?

This design has redundancy (heat pipes are independent, leak and resulting loss of heat transfer ability in one of them doesn't impair others) and does not increase radiological hazards (since heat pipes' water is physically separated from IC water, not to mention reactor steam).

 

[Astronuc]

This might help the discussion: Boiling Water Reactor, GE BWR/4 Technology Advanced Manual, BWR Differences
http://pbadupws.nrc.gov/docs/ML0230/ML023010606.pdf

Standard Technical Specifications — General Electric Plants (BWR/4): Bases (NUREG-1433, Revision 3, Volume 2)
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1433/r3/v2/
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1433/r3/v2/sr1433r3v2.pdf


http://www.oecd-nea.org/press/2011/BWR-basics_Fukushima.pdf

 

[Hiddencamper]

I don't understand why that should be difficult. If RPV is cooled by an IC or a theorized passive air cooler, and RPV not damaged, you aren't losing water from it.

The reactor does lose water, mostly through wet seals and small known leaks (valve packings and the like) and does need makeup. What I was specifically referring to was the fact that you cannot bring a reactor to cold shutdown using IC alone. Once water stops boiling, you need to have the water level raised above the steam separator to have ANY natural cooling circulation in the reactor, and optimally you would have it flooded through the steam separator for natural convection. Once boiling stops I need a way to get water, not steam, to the external cooling source and I'll need a large water injection to do that.

These penetrations already exist for routing steam from RPV to IC.

Not true. the IC is inside the secondary containment boundary. If the IC heat exchange tubes were to break, monitors will detect it and isolate the leak, and any released radiation will be captured by the secondary containment standby gas treatment system. The IC does not actually put reactor coolant outside the containment boundary.

I don't see why you absolutely must encase the air cooler in some sort of massive shielding. This will drive costs up and make it even more likely that utilities wouldn't want to build it at all. Ergo, making it seemingly "better" in fact drives us to a situation where we don't have a passive, indefinitely-operating cooling.

You can't take any credit for a passive air cooler if it cannot withstand all of the environmental effects it could be faced with. Since you have to run reactor coolant through it, and it will likely need to be outside or in some type of chimney structure, if it is hit by a missile or an airplane (which US nuclear plants are required to be designed for now) it no longer will be a functional system, and in fact, can create a credible leak path for reactor coolant and fission product release. This is why a shield building is required, and is part of the reason that the AP1000 heat transfer structure is its primary containment.

Another thing to remember, if I can't take credit for my passive air cooler in all cases, that means I now need active systems which are safety grade to back it up, and that's expensive.

Okay, here's my design which is very likely isn't going to work because I'm just a software engineer, but I'd like to hear where I'm wrong :)

Let's take an IC design similar to Fuku Unit 1. Its tanks are somewhere up in the reactor building. Modify it by adding many (a dozen or two) independent heat pipes whose lower ends are submerged into the IC tanks and then they are routed up to the roof and have a significant length of heat pipes all over it. (And, of course, I assume that operators know how to activate IC in the event of SBO. Post-Fukushima, it's kind of an obvious fix, right?)

Heat pipes are efficient at heat transfer starting from a few degrees above melting point of the internal liquid they use. In this case, water will do nicely. In this design they should be working to keep IC tank water cooled down to the outside ambient temperature, unless it is below ~+2 C in which case they will stop conducting heat (IOW: they won't try to freeze IC tank during winter).

During an accident, when IC tank water goes to 100 C and starts boiling, heat pipes will transfer and dissipate some of this heat to the outside air on the roof.

I wonder is it practical to size IC tanks so that, as decay heat rate goes down, they will stop boiling vigorously and start transferring most of the heat via heat pipes before they boil dry?

This design has redundancy (heat pipes are independent, leak and resulting loss of heat transfer ability in one of them doesn't impair others) and does not increase radiological hazards (since heat pipes' water is physically separated from IC water, not to mention reactor steam).

The decay heat rate will still be on the order of 10s of MW for days and on the order of MWs for years. I don't think the heat pipes will provide any real benefit in this case. When water boils in the IC, the majority of the heat being removed is due to the latent heat to bring liquid water to boiling. Sticking some heat pipes in the IC will still require a large heat transfer surface, and some way to force air to pass over them (As simple radiative heat transfer is not sufficient for large reactor sizes). It's really a matter of scales here. You wouldn't be able to remove large enough amounts of heat prior to the IC tanks going dry because the decay heat loads remain very large for quite a while.

Sorry I'm being negative here. In concept/theory if you make it large enough you can have a system that works (again see the AP1000 containment), but in terms of a realistic integrated design there's a lot more to it then that. To make it worth using it needs to be credited for just about all accident scenarios and environmental hazards it could be subject to, and to make it cost effective it needs to be able to replace active safety systems.

 

[Hiddencamper]

This might help the discussion: Boiling Water Reactor, GE BWR/4 Technology Advanced Manual, BWR Differences
http://pbadupws.nrc.gov/docs/ML0230/ML023010606.pdf

Standard Technical Specifications — General Electric Plants (BWR/4): Bases (NUREG-1433, Revision 3, Volume 2)
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1433/r3/v2/
http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1433/r3/v2/sr1433r3v2.pdf


http://www.oecd-nea.org/press/2011/BWR-basics_Fukushima.pdf

I really appreciate the first link. I've worked in 5s and 6s but don't have the same kind of experience with 3s and 4s. my knowledge of them is limited to reading their procedures, design documents, and SARs

 

[rmattila]

I've never been quite able to understand the difference between the "differences in BWR:s" document and the information about the Oyster Creek & Dai-ichi #1 found on the net: the document talks about one 29000 gallon IC tank and 90 min capacity, whereas Fuku 1 and Oyster Creek apparently have two IC tanks and several hours of capacity.

 

[Hiddencamper]

I've never been quite able to understand the difference between the "differences in BWR:s" document and the information about the Oyster Creek & Dai-ichi #1 found on the net: the document talks about one 29000 gallon IC tank and 90 min capacity, whereas Fuku 1 and Oyster Creek apparently have two IC tanks and several hours of capacity.

I think I'll be able to find out oyster's actual IC capacity.

That differences document states a 20 minute IC capacity (which is in line with Dresden's IC). I do know that IC capacity of the shell itself in some plants (like dresden) is only about 20-30 minutes per IC (i found this chapter of their FSAR online), but it can be extended to several hours with water pumped in via a diesel driven or electric driven motor from a water tank sitting outside the plant which refills the IC.

part of the reason for the smaller time is the greater decay heat load. Dresden is a very high power plant for the core size, so it has much more decay heat to deal with.

I've read claims that Fuku #1 has a 6 hour IC capacity in the heat exchanger shell...I'm a little skeptical (because I know some BWRs have much smaller ICs)...and there could be a translation issue here, but I have no verified evidence that they don't have a 6 hour IC capacity. I doubt we will ever get Fukushima's specifics for its IC size (in the form of a design document), but Oyster's should be in chapter 5 or 6 of their FSAR (they have a different FSAR layout so I'm not positive which chapter), and that's semi-publicly available. I know someone at oyster so I'll see if I can get a rough number.

 

[rmattila]

I have very limited knowledge of the GE BWRs. I know that the only ASEA BWR with IC, Oskarshamn 1, has 6 hour shell side capacity and capability to gravity-fill from the SFP (see http://www.ensreg.eu/sites/default/files/Swedish national report EU stress tests 111230.pdf , page 160), but that the capacity in SBO is limited to 2 hours due to the battery capacity needed to keep the valves open.

EDIT: The Spanish 466 MWe Santa Maria de Garona NPP, which AFAIK is close to FK1/1, apparently only has 1 hour worth of water on the shell side: http://www.ensreg.eu/sites/default/files/Spain_Stress-Tests.pdf , p. 157.

 

[Astronuc]

This might also be of interest in understanding BWRs.

https://netfiles.uiuc.edu/mragheb/www/NPRE%20402%20ME%20405%20Nuclear%20Power%20Engineering/Boiling%20Water%20Reactors.pdf

Oyster Creek and Nine Mile Point 1 are BWR/2 units without jetpumps, but with direct cycle.


FYI - Passive Safety Systems and Natural Circulation in Water Cooled Nuclear Power Plants
IAEA TECDOC 1624 - http://www-pub.iaea.org/books/IAEAB...culation-in-Water-Cooled-Nuclear-Power-Plants

 

[Hiddencamper]

I've never been quite able to understand the difference between the "differences in BWR:s" document and the information about the Oyster Creek & Dai-ichi #1 found on the net: the document talks about one 29000 gallon IC tank and 90 min capacity, whereas Fuku 1 and Oyster Creek apparently have two IC tanks and several hours of capacity.

Thanks to a colleague at Oyster, I've seen copies of Oyster's safety analysis report and their operations training manuals. The IC at Oyster can operate for 45 minutes each without a makeup water supply. so 2 ICs in service is 90 minutes. It also takes about 100k gallons to actually do a full cool down on the oyster creek reactor. This "several hours capacity" is not correct. The IC is only sized for a little over 90 minutes w/out makeup.

 

[nikkkom]

You can't take any credit for a passive air cooler if it cannot withstand all of the environmental effects it could be faced with.

Interesting. Your position boils down to "we can't build an air-cooled heat exchanger which can withstand 9M earthquake, therefore let's not have it at all".

Since you have to run reactor coolant through it

Wrong.

if it is hit by a missile or an airplane (which US nuclear plants are required to be designed for now) it no longer will be a functional system, and in fact, can create a credible leak path for reactor coolant and fission product release.

You are criticizing something different from my proposal, because in my proposal reactor water does NOT go directly thru air cooler; but anyway:
I am 100.00% sure Fukushima refugees would take a small, TMI-like transient radiation leak instead of a massive Cs-134/137 plume and ensuing permanent evacuation any day, thank you very much!

 

[nikkkom]

What I was specifically referring to was the fact that you cannot bring a reactor to cold shutdown using IC alone.

In a SBO, your primary concern is not to bring reactor to cold shutdown. In a SBO, your goal is to not let it melt down. If IC would be able to stabilize RPV at 120C for days without any power, I am a happy camper.

 

[Hiddencamper]

Interesting. Your position boils down to "we can't build an air-cooled heat exchanger which can withstand 9M earthquake, therefore let's not have it at all".



Wrong.



You are criticizing something different from my proposal, because in my proposal reactor water does NOT go directly thru air cooler; but anyway:
I am 100.00% sure Fukushima refugees would take a small, TMI-like transient radiation leak instead of a massive Cs-134/137 plume and ensuing permanent evacuation any day, thank you very much!

Ok so are you talking about SBO or are you talking about design basis accidents?

In either case, why are you going to install something that isn't capable of functioning in ALL environmental and accident conditions. You can't even accredit it as safety. There is absolutely no purpose in nuclear to install a piece of equipment with a safety function if it cannot handle the design basis earthquake, floods, weather events (wind snow tornado), plus any effects from design basis accidents including jet impingment/pipe whip due to High energy line breaks, LOCA, LOOP, etc. So yes, if you cannot build a structure that can withstand all of that, then it is not worth it to built it at all in nuclear.

As for not having to run reactor coolant through it, I'm curious... how are you going to transfer heat from one loop to another? So you are going to use reactor natural circulation combined with gravity for a primary loop heat removal...but how are you going to get the secondary loop to do the same. It is a large challenge, but not an insurmountable one, but I have a feeling (based off of experience) that adding in another loop to a natural air cooled heat exchanger for LWRs is not going to be effective without AC electrical power or some other motive force.

 

[Hiddencamper]

In a SBO, your primary concern is not to bring reactor to cold shutdown. In a SBO, your goal is to not let it melt down. If IC would be able to stabilize RPV at 120C for days without any power, I am a happy camper.

There is still operational leakage even during SBO, so makeup is an issue as well (albiet a long term one). If you cannot bring the system to cold shutdown, it is very difficult to makeup to the vessel using external pumps, and is an issue we saw at Fukushima, when they couldn't get RPV and PCS pressures low enough to allow injection.

The IC on its own extracts too much heat from the reactor, a detailed analysis would need to be performed, but its possible it would cool the system down so rapidly that if would lose natural circulation for a period of time. You need motive force to somehow turn the IC "on" and "off". Additionally you still need motive force plus a makeup source to have the IC maintain the vessel in a hot standby condition, although that force does not necessarily need to be AC power.

 

[nikkkom]

Ok so are you talking about SBO or are you talking about design basis accidents?

SBO must be a design basis accident. Otherwise we will have more Fukushimas.

 

[nikkkom]

If you cannot bring the system to cold shutdown, it is very difficult to makeup to the vessel using external pumps, and is an issue we saw at Fukushima, when they couldn't get RPV and PCS pressures low enough to allow injection.

They couldn't bring RPVs to atmospheric pressure because some idiot decided that prolonged SBO "can't happen" and therefore accident planning and personnel training regime does not need to include instructions and drills for venting RPVs in SBO conditions.

This corporate/regulatory blunder has nothing to do with technical merits of ICs.

If anything, if IC maintains RPV internals at near 100 C and pressure just a tad above 1 atm, that makes injection easier, not harder.

 

[Hiddencamper]

SBO must be a design basis accident. Otherwise we will have more Fukushimas.

SBO can't be a design basis accident. Under plant design you can never have that many failures happen to put you in prolonged SBO. It wasn't a decision, its the fact that's how your design is of your facility. SBO is like any other extensive damage or severe accident, the fact that you got there in the first place meant that circumstances occurred which could not be prevented by design, and as such, you cannot plan for it like you would plan for any normal accident scenario.

Also, there are accident scenarios and procedures for venting the RPV, even in SBO conditions. The issues involved were due to the Japanese position to not vent until double the maximum containment design pressure. There were a lot of things that occurred as a result of this decision, such as not enough SRV accumulator pressure to actuate the SRVs in ADS or relief mode, failure of penetrations and seals in the PCV (and a potential breach in unit 2 PCV), etc.

 

[rmattila]

The term "design basis" is internationally a bit vague, since in some countries it refers to the original US definition, whereas in some countries additional more extensive conditions ("design extension conditions") and even severe accidents are in fact within the design basis. For example, the Finnish event classification is described in that post: https://www.physicsforums.com/showpost.php?p=3671859&postcount=566 and a SBO falls under the DEC B category, where systems are only required to withstand external conditions with frequency once per 1000 years, not once per 100 000 years as the DBC4 systems.

 

[rmattila]

As for not having to run reactor coolant through it, I'm curious... how are you going to transfer heat from one loop to another? So you are going to use reactor natural circulation combined with gravity for a primary loop heat removal...but how are you going to get the secondary loop to do the same. It is a large challenge, but not an insurmountable one, but I have a feeling (based off of experience) that adding in another loop to a natural air cooled heat exchanger for LWRs is not going to be effective without AC electrical power or some other motive force.

They're doing something like that at the Kudankulam VVER being built by Russia in India: http://www.frontlineonnet.com/fl2824/stories/20111202282403300.htm

http://www.frontlineonnet.com/fl2824/images/20111202282403305.jpg

 

[nikkkom]

SBO can't be a design basis accident. Under plant design you can never have that many failures happen to put you in prolonged SBO. It wasn't a decision, its the fact that's how your design is of your facility. SBO is like any other extensive damage or severe accident, the fact that you got there in the first place meant that circumstances occurred which could not be prevented by design, and as such, you cannot plan for it like you would plan for any normal accident scenario.

Also, there are accident scenarios and procedures for venting the RPV, even in SBO conditions.

Glad that you are sure there are.

From where I sit, empirical evidence (Fuku) says that those procedures are not known to people operating NPPs, and when SBO occurred, they had no idea what to do.