Have they located the melted fuel at Fukushima?

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zapperzero said:
Large tanks of what, please?

Water.
 
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nikkkom said:
Safety versus meltdown can be achieved only by designing in very robust emergency cooling systems. In my "armchair engineer" view, something like "reactor sitting in a stainless steel lined pit with no drains, and with a set of large tanks beside it which can be manually drained into the pit, no electricity needed" should work.

You just described PWR "Acumulators".
 
There should be robust "last resort" emergency cooling systems which can be manually operated by hand without electricity to supply the reactor with water. If such systems existed at Fukushima, the safety and stability of the reactors would have been ensured.

More water can be brought in via truck or helicopter if needed.

Speaking of, is it possible to bring in more diesel fuel to nuclear power plants by the truckload in case the emergency diesel generators run dry?

The roads leading to the Fukushima Daiichi NPP were blocked by debris from the Tsunami, making this impossible.
 
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jim hardy said:
You just described PWR "Acumulators".

No. Accumulators are high pressure tanks (nitrogen pressurized) with water which are meant to inject this water into primary coolant loop.

They require RPV to be depressurized. As you know, in Fukushima depressurizing RPV and PCV proved difficult. Fail.

What I described are tanks which can, if all else fails, flood reactor pit and submerge the reactor, i.e. cool reactor from the outside.
 
Kutt said:
Speaking of, is it possible to bring in more diesel fuel to nuclear power plants by the truckload in case the emergency diesel generators run dry?

There should be enough fuel for weeks. And fuel can be delivered, by air if needed. But it's not of much use if your diesels or electrical switchboards are flooded, right?...
 
nikkom said:
What I described are tanks which can, if all else fails, flood reactor pit and submerge the reactor, i.e. cool reactor from the outside.

Fair enough. You're suggesting a passive containment flood system.
Ours was active not passive and used pumps. It was intended to reduce containment pressure, it fed spray nozzles in upper containment near ceiling ..

Our accumulators were pressurized to about 1/3 reactor operating pressure so they'd flood vessel while pressure is on the way down following a break.

Fukushima operators might have been able to depressurize had they done it very early. It is suggested in the ORNL station blackout study to do that.
Hindsight is always 20/20........

we kept 30 days diesel fuel onsite. Yes, it was trucked in.
 
Kutt said:
There should be robust "last resort" emergency cooling systems which can be manually operated by hand without electricity to supply the reactor with water. If such systems existed at Fukushima, the safety and stability of the reactors would have been ensured.

More water can be brought in via truck or helicopter if needed.

Speaking of, is it possible to bring in more diesel fuel to nuclear power plants by the truckload in case the emergency diesel generators run dry?

The roads leading to the Fukushima Daiichi NPP were blocked by debris from the Tsunami, making this impossible.

Unit 2 and 3 have a RCIC system which uses reactor steam to pump water into the core. Unit 2 was cooled for 70 hours and unit 3 for 32 hours.

Unit 1 has a passive isolation condenser system, but operators were not properly trained on it and did not know if it was functioning properly. It likely wasn't
 
jim hardy said:
Fair enough. You're suggesting a passive containment flood system.
Ours was active not passive and used pumps. It was intended to reduce containment pressure, it fed spray nozzles in upper containment near ceiling ..

Our accumulators were pressurized to about 1/3 reactor operating pressure so they'd flood vessel while pressure is on the way down following a break.

Fukushima operators might have been able to depressurize had they done it very early. It is suggested in the ORNL station blackout study to do that.
Hindsight is always 20/20...


.....

we kept 30 days diesel fuel onsite. Yes, it was trucked in.


Per the ANS standard, 7 days of fuel PER GENERATOR OR the minimum amount of time required to get a resupply, whichever is greater, is the requirement. (I forget the US reg guide that endorses this).

With regards to depressurizing, you actually DONT want to depressurize the RPV in a Fukushima type situation. BWRs can utilize high pressure reactor steam to operate their RCIC, IC, or HPCI systems for extended core injection. While these systems do not provide for decay heat removal, you can achieve that function through early containment venting and portable pump injection. If you have no low pressure coolant injection pumps available, once you depressurize, you lower the water level remaining in the core (generally uncovering the fuel), and you lose ALL injection. Depressurization early in the event is good during short term station blackout, as depressurization gives you about 15-20 minutes of steam cooling after uncovery of the core and buys you some extra time to get portable pumps going, but ultimately containment venting and portable pump injection to the RPV and/or containment are REQUIRED to assure critical safety functions can be restored.

I've been involved with Fukushima responses for BWRs if anyone is interested on how the plants actually respond, and what we are doing and changing to respond to Fukushima-like situations.
 
As I've said i never spent any time around BWR's , just walked through one once.

I went by this report, which is an analysis and re-thinking of station blackout scenarios done around 1981. They used Brown's Ferry's design as their case study. I stumbled across it while following events at Fukushima on another forum. It's been posted here at PF back in 2011.
http://www.ornl.gov/info/reports/1981/3445600211884.pdf

The analysis shows, because of the loss of the drywell coolers, that it is necessary
for the operator to begin to reduce the reactor vessel pressure to about
0.791 MPa (100 psig) within one hour of the inception of the Station
Blackout. This depressurization reduces the temperature of the saturated
fluid within the reactor vessel and thereby decreases the driving potential
for heat transfer into the drywell, yet keeps the vessel pressure
high enough for continued operation of the RCIC system steam turbine.
With this action, the drywell average ambient temperature can be kept be
low 149°C (300°F) throughout the initial phase of a Station Blackout;
tests have shown that both the drywell structure and the equipment located
therein can be expected to survive temperatures of this magnitude.
The analysis also reveals an important second reason for operator action to
depressurize the reactor vesel early in the initial phase of a
Station Blackout. This depressurization removes a great deal of steam and
the associated stored energy from the reactor vessel at a time when the
RCIC system is available to inject replacement water from the condensate
storage tank and thereby maintain the reactor vessel level. Subsequently,
when water injection capability is lost for any reason, remote-manual re
lief valve operation would be terminated and there would be no further
water loss from the reactor vessel until the pressure has been restored to
the setpoint [7.72 MPa (1105 psig)] for automatic relief valve actuation.
Because of the large amount of water to be reheated and the reduced level
of decay heat, this repressurization would require a significant period of
time. In addition, the subsequent boiloff* would begin from a very high

vessel level because of the increase in the specific volume of the water
as it is heated and repressurized. Thus, an early depressurization will
provide a significant period of valuable additional time for preparative
and possible corrective action before core uncovery after injection capability is lost.

**The term "boiloff" is used to signify a monotonic decrease in reactor vessel water level due to intermittent loss of fluid through the
primary relief valves without replacement.
vxx

I'm not a BWR guy.
If you are one, you are certainly more versed than i in their station blackout approach .


old jim
 
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jim hardy said:
As I've said i never spent any time around BWR's , just walked through one once.

I went by this report, which is an analysis and re-thinking of station blackout scenarios done around 1981. They used Brown's Ferry's design as their case study. I stumbled across it while following events at Fukushima on another forum. It's been posted here at PF back in 2011.
http://www.ornl.gov/info/reports/1981/3445600211884.pdf
I'm not a BWR guy.
If you are one, you are certainly more versed than i in their station blackout approach . old jim

First off, amazing reference. I'm always looking for gems like that to add to my little collection.

The problem is the station blackout scenario is only a 4~8 hour scenario. I fully agree with their approach of keeping RCIC online, as you can bypass RCIC interlocks and run it down to about 45 psig, and by keeping the reactor close to depressurized you have the ability to immediately transition to shutdown cooling the moment it becomes available again (typical SDC interlocks are around 145 psig).

Another quick note about the normal 4-8 hour station blackout scenario. The acceptance criteria is power being restored in that 4 or 8 hour timeframe, the reactor being brought to cold shutdown, and the suppression pool temperature not exceeding some arbitrary limit (in the range of 175F +/- 15). The other main reason you want to keep vessel pressure low, is that as the suppression pool heats up, you have less capability for the pool to absorb the energy during a reactor blowdown, which can result in containment failure if a LOCA occurred during the event. By keeping pressure low, you can have much higher suppression pool temperature limits, and also prevent containment failure in the event a blowdown or LOCA occurs towards the end of the event. (note: Mark 3 and possibly mark 2 containments don't have an issue here, as it is my understanding that they can survive the entire SBO coping time without violating their suppression pool temperature limits)

The issue though, is the relief valves for BWRs utilize air accumulators and DC solenoids. The total air accumulator supply for all SRVs is usually enough for about 30 lifts against design containment pressure. There are backup air bottles, however you can always have a situation where these are unable to restore pressure to the instrument air system due to damage (it is non-safety grade for some obscure reason), or worse, you don't have DC/AC power in the right places to open the valves and bypass the interlocks on the instrument air system.

If you are in an extended situation where you do not have DC power (Fukushima had no DC power at the start of the event in units 1 and 2, and limited in 3), or if you have to do more than a few dozen SRV lifts and deplete your accumulators, you will lose the ability to use SRVs to control vessel pressure. This is where the extended scenario departs from the normal station blackout scenario. The SRVs are manual staged valves, and are either open or shut. To maintain vessel pressure at 100 PSI requires operators to be opening and closing these valves manually, depleting the air supply in them and exhausting your DC power supply.

Further complicating the event is the fact that RCIC's operation is dependent on the suppression pool as a heat sink. If you are in an extended situation, and you blowdown early, you introduce a large volume of heat to the pool, and limit how long RCIC can operate. RCIC is self cooled, and once suppression pool water passes 200F you start to wear the bearings and can fail the pump. Furthermore, as vessel pressure decreases, and drywell/containment pressure increases, you get less dP over the RCIC turbine which results in less injection to the vessel. I should note that heatup of the suppression pool was likely one of the direct causes of the RCIC system failing at Fukushima unit 2 (ran for 70 hours).

The strategy the plant I'm at is taking, is recognizing the extended blackout event, stripping all non-essential loads and shutting down a full safety division of DC power, cross tieing the various DC power busses to the RCIC system to ensure continued injection and automatic flow control, keeping the vessel pressurized. We will actually declare 10CFR50.54(x) to violate our HCTL (heat capacity temperature limit) on the suppression pool in order to keep utilizing RCIC and keep injecting to the core. Once we either restore power, or get portable pumps going, we will cool the suppression pool via "feed and bleed". Once the pool is cooled below the HCTL again, we will do a reactor blowdown by taking portable batteries and directly wiring them up to the SRV terminal blocks and transition to cooling the reactor using portable pumps using the suppression pool -> reactor -> relief valves -> suppression pool loop (alternate shutdown cooling), and either feed and bleed or portable pump setups to cool the suppression pool.

The scenario changes if you are at an isolation condenser plant like dresden or oyster (it changes greatly at oyster as they don't even have a HPCI...ONLY IC and LP corespray). ICs buy you more time, and allow use of engine driven pumps, provided you can get it started properly and identify its function (one of the Fukushima unit 1 flaws, operators were not well trained on the system). Oyster, provided their ICs are in service and portable pumps or the on-site diesel driven pumps are available, is probably in the best shape for extended SBO, as you are essentially injecting against atmosphere and can keep cooling as long as there is enough decay heat for natural circulation (likely days).
 
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Dang, I forgot that the Fukushima reactors had to be depressurized before any water could to be injected. The safety relief valves located inside the PCV were stuck shut and could not be operated because of the crushing pressure (thousands of PSI) inside the primary containment vessel pushing against the exterior of the safety relief valves.

art_1f_07.jpg


https://controls.engin.umich.edu/wiki/images/8/8e/Spring_loaded_safety_relief.jpg
 
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Kutt said:
Dang, I forgot that the Fukushima reactors had to be depressurized before any water could to be injected. The safety relief valves located inside the PCV were stuck shut and could not be operated because of the crushing pressure (thousands of PSI) inside the primary containment vessel pushing against the exterior of the safety relief valves.

art_1f_07.jpg

Containment pressure in the Mark I containment system only goes up to about 125-140 PSI. Around that point you either have your rupture disks blow (for the Japanese containment systems), or your penetrations and seals start to fail, causing pressure leakoff (likely one mode through which hydrogen migrated into the secondary containment building).

And not all of the relief valves issues were due to containment differential pressure. Between loss of DC power allowing solenoids to go back to the closed position during the event, some unique behaviors of the SRVs in general, and difficulties establishing portable air supplies to the SRVs, there were a number of reasons the SRVs had trouble.

The SRVs are designed to lift against the design containment pressure several times without being recharged, and should be capable of doing at least 1-2 lifts against double containment design pressure (which is what the Fukushima containments were getting up to). Leakage of the system, constant use of the SRVs, and inability to restore the Instrument Air Supply to recharge the SRVs was really what prevented the valves from opening/staying open. A higher containment pressure does require higher accumulator pressure to lift the SRVs, but it doesn't prevent the SRVs from lifting.

And a final note, this isn't just the "Fukushima reactors", of which you are referring to the BWR series of reactors, but this is an attribute of virtually all light water commercial power reactors, including PWRs.
 
Hiddencamper said:
Containment pressure in the Mark I containment system only goes up to about 125-140 PSI. Around that point you either have your rupture disks blow (for the Japanese containment systems), or your penetrations and seals start to fail, causing pressure leakoff (likely one mode through which hydrogen migrated into the secondary containment building).

And not all of the relief valves issues were due to containment differential pressure. Between loss of DC power allowing solenoids to go back to the closed position during the event, some unique behaviors of the SRVs in general, and difficulties establishing portable air supplies to the SRVs, there were a number of reasons the SRVs had trouble.

The SRVs are designed to lift against the design containment pressure several times without being recharged, and should be capable of doing at least 1-2 lifts against double containment design pressure (which is what the Fukushima containments were getting up to). Leakage of the system, constant use of the SRVs, and inability to restore the Instrument Air Supply to recharge the SRVs was really what prevented the valves from opening/staying open. A higher containment pressure does require higher accumulator pressure to lift the SRVs, but it doesn't prevent the SRVs from lifting.

And a final note, this isn't just the "Fukushima reactors", of which you are referring to the BWR series of reactors, but this is an attribute of virtually all light water commercial power reactors, including PWRs.

The pipework which supplied emergency pressurized air to the SRV's were severely damaged in the earthquake and rendered useless.
 
Kutt said:
There should be robust "last resort" emergency cooling systems which can be manually operated by hand without electricity to supply the reactor with water. If such systems existed at Fukushima, the safety and stability of the reactors would have been ensured.
Perhaps you should try calculating how many thousands of people cranking by hand would be required to pump enough water to cool a nuclear reactor...
 
russ_watters said:
Perhaps you should try calculating how many thousands of people cranking by hand would be required to pump enough water to cool a nuclear reactor...

What about a massive water tank that feeds water into the reactor (using gravity) which can activated by turning some valves?
 
Kutt said:
The pipework which supplied emergency pressurized air to the SRV's were severely damaged in the earthquake and rendered useless.

I assume we are talking about the instrument air system. Do you have a link to this, I do not recall seeing any absolute data suggesting this was the case, especially because SRVs were used in units 2 and 3 to support blowdown and transition to portable pumping equipment, which implies the IA system was not damaged.

Some info about IA, it is a non-safety system. It automatically isolates (shuts/locks out) on loss of power, and requires specific manual interlock overrides in order to restore it. I'm not sure if the IA system actually failed or if there were more complicated events and I would appreciate a source. Thanks.
 
Kutt said:
What about a massive water tank that feeds water into the reactor (using gravity) which can activated by turning some valves?

Basically the AP1000 or ESBWR designs. Both of those plants will automatically depressurize and utilize gravity type feeds for reactor cooling.
 
 
Hiddencamper said:
I assume we are talking about the instrument air system. Do you have a link to this, I do not recall seeing any absolute data suggesting this was the case, especially because SRVs were used in units 2 and 3 to support blowdown and transition to portable pumping equipment, which implies the IA system was not damaged.

Some info about IA, it is a non-safety system. It automatically isolates (shuts/locks out) on loss of power, and requires specific manual interlock overrides in order to restore it. I'm not sure if the IA system actually failed or if there were more complicated events and I would appreciate a source. Thanks.

I remember seeing a two-hour long docu-drama about the Fukushima incident with actors re-creating the events. It also went into technical detail to how the pipework which supplied the SRV's with enough pressurized air to open, were damaged and rendered inoperable by the earthquake.

I watched this documentary on youtube, but so far, I haven't been able to find it again.

I'll keep looking.
 
Kutt said:
I remember seeing a two-hour long docu-drama about the Fukushima incident with actors re-creating the events. It also went into technical detail to how the pipework which supplied the SRV's with enough pressurized air to open, were damaged and rendered inoperable by the earthquake.

I watched this documentary on youtube, but so far, I haven't been able to find it again.

I'll keep looking.

I wouldn't trust a youtube documentary.

Looking at the official report

http://www.cas.go.jp/jp/seisaku/icanps/eng/02Attachment1.pdf

it looks like loss of power caused an isolation of the instrument air system, which was not recoverable due to AC power being lost, and that the accumulators were depleted.

And there were two types of accumulators for the SRV, one for the relief valve (85L) and the other for the ADS (250L). Both of them were installed inside the PCV. Normally, nitrogen for the relief valve accumulator was supplied from the liquid nitrogen system (AC) line, and nitrogen for the ADS accumulator was supplied from the nitrogen cylinder. The pressure gauge set at each supply line was scheduled to be checked daily by the shift team who were on duty between 15:00 and 21:00. However, because the isolation valve of the supply line to the accumulator had closed due to external power loss after the tsunami, the supply to the accumulator had not been carried out as they were unable to do so. For this reason, the residual pressure that allowed the pressure to be applied to the SRV was the relief valve accumulator, ADS accumulator, and pressure remaining in the piping between the isolation valve (which had closed) and the accumulator side

In other words, the earthquake did NOT cause the recharging system to fail. Instead, the inability to open the isolation valves caused the issue. Even if the system did fail, it is not safety-grade and is not expected or required to operate post accident (although it probably should be).
 
Hiddencamper said:
I wouldn't trust a youtube documentary.

Looking at the official report

http://www.cas.go.jp/jp/seisaku/icanps/eng/02Attachment1.pdf

it looks like loss of power caused an isolation of the instrument air system, which was not recoverable due to AC power being lost, and that the accumulators were depleted.
In other words, the earthquake did NOT cause the recharging system to fail. Instead, the inability to open the isolation valves caused the issue. Even if the system did fail, it is not safety-grade and is not expected or required to operate post accident (although it probably should be).

Actually, this was a BBC documentary that was almost two hours long.
 
jim hardy said:
Fukushima operators might have been able to depressurize had they done it very early. It is suggested in the ORNL station blackout study to do that.
Hindsight is always 20/20...

Fukushima operators can be hardly blamed for it. Someone else should be held responsible.

Someone wrote accident manual which said that loss of all power is impossible (!). Someone decided that battery-backed lights are an unnecessary luxury in a nuclear power plant (what can possibly go wrong?). Someone didn't think about installing filters on emergency vent lines. Someone didn't think about training operators how to open said vents.

Operators were left in the dark, figuratively as well as literally. And wet.

The scary thought is that F1 would have melted down on any given day during last 40 years, if tsunami would happen on that day.
 
Hiddencamper said:
With regards to depressurizing, you actually DONT want to depressurize the RPV in a Fukushima type situation.

It depends. If in an emergency you have only low-pressure pumps available for injection (like fire trucks), then depressurizing RPV makes sense.

BWRs can utilize high pressure reactor steam to operate their RCIC, IC, or HPCI systems for extended core injection.

ICs do not require high pressure steam.

RCIC only circulates water from torus to RPV and back. This does not remove heat, it all remains inside PCV. This can't go on indefinitely.

While these systems do not provide for decay heat removal, you can achieve that function through early containment venting and portable pump injection.

Or you can depressurize and use the same portable pump to inject to *RPV* instead of the more complex setup you describe.

If you have no low pressure coolant injection pumps available, once you depressurize, you lower the water level remaining in the core (generally uncovering the fuel), and you lose ALL injection.

Surely, if you have no way to cool your reactor, then it will melt. That's obvious. RCIC is useful for delaying the meltdown. Useful, but not enough.

I want to have a system which pretty much *guarantees* to prevent meltdowns. Which means passive, power-independent system. Which means it should be gravity-fed. Which means very low pressure. Which means it can't inject water into pressurized PCV or RPV. Which means they need to be depressurized in order to allow this water to go in.

I've been involved with Fukushima responses for BWRs if anyone is interested on how the plants actually respond, and what we are doing and changing to respond to Fukushima-like situations.

Please do.
Are filters being installed on vent lines?
Do you have battery-backed lighting? Individual batteries in each light, or what?
Do your operators now know how to open the vent? Did they practice it? Without electricity?
Do you have gravity-feed water sources?
Etc...
 
The more I learn here, the scarier it looks. Safety relief valves need something (compressed air) to work? *Safety* valves? Really? Why? It's not possible to have valves which are actuated solely by the pressure they are intended to relieve??
 
Or you can depressurize and use the same portable pump to inject to *RPV* instead of the more complex setup you describe.

The issue here, is that you have to assume the plant has already gone 8+ hours without electricity. In this case, a blowdown of the reactor has the potential to result in damage to the containment as the HCTL (heat capacity temperature limit) of the suppression pool will be exceeded. You CANNOT blow down a BWR type reactor when you are above the HCTL, as the potential to damage the containment is not acceptable. In these cases, the best thing to do is to keep running RCIC, and work on cooling your suppression pool through venting and feed and bleed, and ultimately portable heat exchangers. Once you are below the HCTL and can blow down again, or you have manually and slowly blown down the reactor to a level where low pressure pumps can handle injection at a sufficient rate, then you can transition to using your portable pumps for core cooling. Damaging containment is NOT an acceptable response.

RCIC is useful for delaying the meltdown. Useful, but not enough.

RCIC will run as long as you can maintain the suppression pool relatively cold. If you vent containment and feed/bleed replacement suppression pool water, you can run RCIC indefinitely. There are several BWRs who would use their RCIC system for days in the 80s in order to shorten the time between restarts if there was an equipment failure, something which is not acceptable anymore.

I want to have a system which pretty much *guarantees* to prevent meltdowns. Which means passive, power-independent system. Which means it should be gravity-fed. Which means very low pressure. Which means it can't inject water into pressurized PCV or RPV. Which means they need to be depressurized in order to allow this water to go in.

Pretty much the ESBWR design. Take a look at it. The problem is you are limited based on the size of your initial pools (which is limited based on the design and cost of the structural loading on your containment, as the pools are all seismic class 1).

Are filters being installed on vent lines?

Mark I containment plants are upgrading their existing hardened vents, and Mark II containment plants are installing them. Mark III plants already have MANY containment venting systems. With regards to filters, this is a regulatory issue in the US right now. The industry wants to be required to have decontamination factor goals, which is consistent with NRC policy and direction right now, and allows for the greatest flexibility using equipment already available while avoiding unintended consequences. This is the same position that ACRS (Advisory committee for reactor safeguards) has, and is a position the NRC staff also agrees is a feasible approach. The staff however suggests installing mandatory filters as filters meet the commission's intent of a prompt action which could have a net benefit. There's a lot going on in this area and I think there's more information out there than I can give justice to right now. The short answer though is that the industry has a lot of concerns, many of which are NOT cost related, that have to do with the fact that filters are only applicable in roughly 3-4 out of 8 scenarios where containment venting or release is required, and that the installation of filters for beyond design basis events can REDUCE the reliability of the containment system during required design basis events. There are very large cost increases associated as well which were not accounted for in the backfit analysis, and there is no generic/common approach for plants to do this. Anyways.....the industry is putting together plants which will achieve decontamination factors on the order of 1000 to 10000 using FLEX equipment. This is a political issue at the moment and I really don't want to get into more than that.

Do you have battery-backed lighting? Individual batteries in each light, or what?
Emergency lighting is a fire-code thing. I can't speak for all plants, but at the plant I'm at now, and the few I've been at, emergency lights have individual lighting packs. The lights are separated into "normal" emergency lighting, and b.5.b emergency lighting, which is accredited for beyond design basis events where there are explosions and other significant site damage. Only b.5.b lighting would be assumed to function during a Fukushima-like event. There are b.5.b storage lockers with credited lights and equipment for operators to use in the event that all normal and emergency lighting and systems are lost. The emergency lights have their own backup batteries, and there are reserve batteries on site.

Do your operators now know how to open the vent? Did they practice it? Without electricity

YES! I even know the procedure number off the top of my head (and will not state it here because it may point back to my plant). Our extensive damage mitigating procedures, part of our b.5.b plan and 9/11 terrorist attack response plan includes things such as black starting the diesel generators with no control power, cooling the plant using a variety of crazy methods, running RCIC without electricity, AND, using credited b.5.b portable battery packs which are above ground in seismically qualified lockers and going to the SRV penetrations and hooking the batteries up to the appropriate connections to cause the SRV to actuate. These tasks have been "simulated", and are very clearly laid out in the procedure. The procedure includes a list of all SRVs, which contacts you have to wire to in order to read suppression pool temperature. Which contacts you have to wire up in order to energize each individual SRV, preferred energization order based on plant conditions, how to identify if the lift was successful, and how long the battery is expected to maintain the valve open. In a Fukushima like event, we would disable the relief mode of the SRVs (the air/power operated mode), and activate the ADS backup air supply, which will give us more lifts without having to install portable air bottles, but the options for installing portable air bottles or compressors do exist and are detailed in the extensive damage mitigation guidelines. We also have a remote shutdown panel with a select number of SRVs, and wiring up any power source to the RSP can allow lifting those valves without having to open up a penetration.

With regards to emergency procedures, the EOPs (emergency operating procedures), are high level guidelines/procedures for how to achieve critical safety objectives. The SAGs (severe accident guidelines) are even higher level goals to achieve critical safety objectives if core damage is a possibility. The EDMGs (extensive damage mitigation guidelines), are unique ways to meet those goals without using normal equipment or operating procedures, and assuming significant site damage. The new Fukushima/FLEX procedures further expand upon that and include very extended duration events. In other words EOPs/SAGs = goals, EDMGs and FLEX = extra means to fulfill those goals, should normal means become unavailable. All procedures are trained on by operators in the simulator and are part of their initial licensing and requalifications.

Do you have gravity-feed water sources?

BWR series plants do not have this. The AP1000 and ESBWR are the only two plants I know of that utilize some form of gravity fed source.

The more I learn here, the scarier it looks. Safety relief valves need something (compressed air) to work? *Safety* valves? Really? Why? It's not possible to have valves which are actuated solely by the pressure they are intended to relieve??

The valves are SAFETY-RELIEF valves. The SAFETY mode is spring operated, and requires no electricity or pressure to function. These valves are sequenced to lift against spring pressure as reactor pressure exceeds the relief mode setpoints. This is what prevents the reactor from exceeding its 1375 PSI ASME code limit post event with no power or ADS air.

The RELIEF mode is power actuated utilizing a pneumatic air supply. The RELIEF mode is the mode in which the plant's safety systems will automatically lift to limit pressure. The relief mode setpoints are below the safety mode setpoints. The operators can manually lift the valve in relief mode, and can also disable the relief mode, for each valves. Typically, if the instrument air supply to the containment is isolated or disabled for any reason, the operators will turn off the relief mode on all SRVs and allow the safety mode to actuate in order to preserve accumulator air for a blowdown, if required. Once the operators get a handle on the situation and are ready to blow down, they will either actuate ADS (which automatically lifts several valves in relief mode) or they will manually lift valves. The SRVs have enough air for a specific number of rated lifts against containment design pressure. Once lifted, if DC power is continuously applied, the valves will stay open for quite a while, as the leakage from the accumulators is rather low. For extended events, a means to assure recharging the accumulators may be vital to success. There are other ways to blowdown the reactor should the SRVs be all failed, but those means require AC power to be restored to some plant systems, while the SRVs only require DC power and pre-charged instrument air.
 
nikkkom said:
Fukushima operators can be hardly blamed for it. Someone else should be held responsible.
That wasn't my intent at all... sometimes what i don't say gets me in more trouble than what i do say...
I've consistently lauded the operators' heroism throughout the 2011-2012 thread.
Responsibility lies with some individual(or committee) who decided to ignore the warnings in 1990's from earth-scientists that tidal waves in excess of plant design for them were more probable than had been known at time of construction.
They should have put a submarine hull around the electrics.

Someone wrote accident manual which said that loss of all power is impossible (!).
That the plants had not considered and addressed station blackout in their procedures and training falls on some level of management. Those ORNL blackout analyses were around for nearly two decades. One doesn't wait for regulators to make you do something that important. We drilled on station blackout in our simulator.


Someone decided that battery-backed lights are an unnecessary luxury in a nuclear power plant (what can possibly go wrong?).
They are standard emergebcy equipment. Our batteries were 125 volt so they were plain rough service incandescent lightbulbs. (Now there's an interesting thought - EPA wants to outlaw incandescents.)
Fukushima's battery powered lights and instruments worked fine until the batteries ran down.
A small engine driven DC generator would have been a prudent accessory.
Probably all their welding machines got flooded, or had electric motors. We kept some gasoline driven ones around...


Someone didn't think about installing filters on emergency vent lines. Someone didn't think about training operators how to open said vents.
Had your first two issues been addressed, things wouldn't have got so far as to need such filters.


Operators were left in the dark, figuratively as well as literally. And wet.

The scary thought is that F1 would have melted down on any given day during last 40 years, if tsunami would happen on that day.
Yep. Placing diesels in basement made them safer from earthquakes but left them vulnerable to flooding.
Every old cowboy knows to pitch his tent above high water.

old jim
 
Thanks for such a detailed response! :)

Hiddencamper said:
>> Or you can depressurize and use the same portable pump to inject to *RPV* instead of the more complex setup you describe.

The issue here, is that you have to assume the plant has already gone 8+ hours without electricity.

Fukushima demonstrated a different scenario. When tsunami receded, even though operators were without power for only a few minutes so far, they *knew* that power is likely to be unavailable for days.

If at that moment they would have a way and training how to depressurize RPV and if they would have low-pressure injection source (like fire truck), they could just do that: keep RPV filled, at ~100 C and at ~1 atm. KISS. No RCIC. No need to think about temperature/pressure in the torus. (They wouldn't even absolutely need vent to be filtered, because the steam at that point was relatively uncontaminated).

Pretty much the ESBWR design. Take a look at it. The problem is you are limited based on the size of your initial pools (which is limited based on the design and cost of the structural loading on your containment, as the pools are all seismic class 1).

Yes, I looked at it and at AP1000. Looks better.
The pools may (should) have means to be refilled by a "fire truck".

The RELIEF mode is power actuated utilizing a pneumatic air supply.

Stupid question: is it impossible to have relief valves which can be operated manually?
 
I'm going to start with the easy ones first.

Stupid question: is it impossible to have relief valves which can be operated manually?
The SRVs are in the containment and are connected to the main steam system. Because they are part of the ASME class 1 piping, they must be within containment. This pretty much eliminates any possibility of a person going in and manually lifting the valve, even if such a design existed.

Fukushima demonstrated a different scenario. When tsunami receded, even though operators were without power for only a few minutes so far, they *knew* that power is likely to be unavailable for days.

The various units were already being cooled by IC/RCIC prior to the tsunami. When the tsunami hit, the operators did not *know* they would be out for days, and they were not trained on this type of scenario. It probably took over an hour to truly realize what the extent of the damage was to the AC and DC auxiliary power distribution systems in the plant. At the time the tsunami hit, they were likely trying to understand what indications they lost and trying to restore them, as indications are the most vital component of reactor operations. They likely radioed equipment operators to go out to the field to figure out what was going on.

If at that moment they would have a way and training how to depressurize RPV and if they would have low-pressure injection source (like fire truck), they could just do that: keep RPV filled, at ~100 C and at ~1 atm. KISS. No RCIC. No need to think about temperature/pressure in the torus. (They wouldn't even absolutely need vent to be filtered, because the steam at that point was relatively uncontaminated).

This really isn't KISS though. In a Fukushima like situation, the reactor is going to be boiling 600 gpm after shutdown, which only gives you about 10-15 minutes before your HPCI/RCIC systems start. RCIC and HPCI will add heat to the suppression pool before you could ever hook up your portable equipment. Additionally, a portable pump only provides injection, it does not support decay heat removal. You also cannot just "sit at 100 lbs" or 1atm of pressure. The SRVs are not control systems. They are either "OPEN" or "CLOSED". If you leave 1 SRV open, with a shut down core, your pressure will continue dropping to 0. This means, to maintain 100 lbs of pressure, the operators have to manually open and close them, which not only distracts them, but also introduces a human performance issue where if they leave it open, they can trip the RCIC system (requires an operator to manually reset it AT the turbine), or they leave it closed too long and the vessel heats up above the allowable HCTL. Cycling SRVs also rapidly depletes each SRV's reserve air supply and further complicates the event if your portable equipment does not come available. On top of it, you might not have available power to actuate your SRVs (like at Fukushima).

The KISS you are talking about also assumes the operators immediately recognize the situation, immediately perform an emergency blowdown, and can, within 15 minutes, connect and start injecting with portable pumps. This is not feasible given the number of actions which need to occur on top of correctly identifying the beyond-design-basis event. Operators are trained to not do anything for the first several minutes of an event but watch, and we are not allowed to credit ANY operator actions for safety for a minimum of 10 minutes post event, and most operator actions do not start until 30 minutes post event. (Our 10 minute post event action is to start suppression pool cooling. That's the only proceduralized action we have during accidents within 30 minutes). Another note is a reactor can only have 1 blowdown at a time. Blowdowns put a LOT of stress on the vessel, and it may not be possible to restart the plant after a blowdown. For this reason, rapid blowdowns are last resort options. It is much preferred to do a slow blowdown.

The bottom line, is your "KISS" approach is not KISS, requires constant operator attention taking them away from other potential issues, requires a lot of knowledge the operators will not have at that point in time regarding the extent of damage and feasibility of repair, requires many actions to be taken in a very short amount of time (faster than we ever credit for nuclear safety), and does not address all conditions.

If it was that easy, Fukushima would not have happened.
 
Thanks again!

Hiddencamper said:
The SRVs are in the containment and are connected to the main steam system. Because they are part of the ASME class 1 piping, they must be within containment. This pretty much eliminates any possibility of a person going in and manually lifting the valve, even if such a design existed.

But... existence of ICs in Unit 1!
ICs sit outside of PCV. They are easily accessible. And they have reactor steam piping which comes to them all the way from RPV, right?

So it's not merely possible to have such a line, it _exists_ in some BWRs. Why can't it have a manually operated valve and vent line?

Additionally, a portable pump only provides injection, it does not support decay heat removal.

Correct. They only provide water. The boiling of water and venting of the vapor removes heat.

You also cannot just "sit at 100 lbs" or 1atm of pressure. The SRVs are not control systems. They are either "OPEN" or "CLOSED". If you leave 1 SRV open, with a shut down core, your pressure will continue dropping to 0.

...and the problem with RPV pressure dropping to ~1 atm is?...

I see benefits: easy injection; low temperature.

the operators have to manually open and close them

I meant that in this hypothetical scenario steam vent from RPV should be opened and *remain open*.
 

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