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.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.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?
In Fukushima Unit 1, IC capacity was 8 hours of cooling. It's trivial to design an IC with bigger water tank.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).
Pump is an active system. It can break, it can't function if steam pressure is lost (say, a fissure in RPV's top).IC was replaced by RCIC. RCIC is a pump which is driven using steam from the reactor.
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.
That's exactly why we want to see passive systems, which don't need DC to work.Unit 3 ran on RCIC for about 32 hours (not sure why it failed, probably loss of DC power
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.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).
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.If unit 1 had a RCIC system, the whole accident may have been less of an issue. The operators wouldnt 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.
???!!!IC also contaminates the area outside the plant.
WHAT??? Why a clear water can't be used in IC?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.
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.
You might be right about that particular design. I don't know the details.The IC as exists in older BWRs is a vulnerability in my professional opinion
If operators would engage IC on Unit 1 and it didn't save the day, I'd agree with you.and I thats what we saw at Fukushima.
There appears to be different designs of the IC regarding the shell size capacity. that post by Tsutsuji-san cites the Japanese NSC sayingI'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 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.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.
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.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.
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.???!!!IC also contaminates the area outside the plant.
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.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.
Well, some time ago I found a patent of a (newer) isolation condenser where they specifically addressed the problem of bursting pipes: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)
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.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
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.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.
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.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.
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.They still would have been limited to 8 hours
"fully integrated into containment" and "efficiently removes heat" are mutually exclusive design goals.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.
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.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.
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.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
As I said:If the IC piping outside the PCV would be damaged, the fuel would be directly connected to the atmosphere.
You are assuming operators would sit and look at IC steam plumes for 8 hours and do nothing?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,
Which they did anyway. How is this worse than what in fact happened?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.
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)."fully integrated into containment" and "efficiently removes heat" are mutually exclusive design goals.
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.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.
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.You are assuming operators would sit and look at IC steam plumes for 8 hours and do nothing?
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...Which they did anyway. How is this worse than what in fact happened?
EXACTLY my point! RCIC does not remove heat from containment - it more like smears heat inside it.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).
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).Why? If cladding fails and fission products escape, and if IC-pipes burst, the radioactive steam would be released directly to the atmosphere.
Again, IC pipe burst would expose relatively _uncontaminated_ steam, nowhere near the levels of stem coming out of a core which melts down.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.
We need some facts here: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.