Fukushima Japan Earthquake: nuclear plants Fukushima part 2

[Hiddencamper]

These are quite interesting studies. The Japanese Fukushima reports also mention two papers on hydrogen explosions outside of primary containment, which they consider obscure (one modelling Olkiluoto NPP in Finland and the other Browns Ferry NPP). It appears a lot of theoretical work on severe accident mitigation was simply overlooked or at least not integrated to EOPs. Some of that was even Japanese experiences, such as using plant fire department fire engines for core injection, provisions which had been prepared after earthquake damage to other plants, but formal procedures apparently had not been updated to Fukushima EOP.

A completely another question is that even if there had been much more extensive formal severe accident mitigation guidance available, would they have really implemented it? One of the main human factors issues identified by the Japanese reports, especially the Cabinet ones, is the comparison of how F-2 managed the crisis by always being one step ahead of things. They always had a Plan A in action, while preparing for Plan B to be implemented immediately should there be indication of Plan A failure. And when they were switching Plan A, they tested the viability of implementation of the entire new plan several times before actually carrying the switch over.

In contrast in F-1 this was never achieved when it became obvious that RHR and other sea water reliant systems were going to be out of operation for days. After that, there was over reliance on Plan A continuing to work despite lack of monitoring data and Plan B formulation only started when information came in putting Plan A viability in doubt, sometimes only after several misunderstandings and delays in information flow.

If we accept for Unit 1 that IC in the heavily degraded condition with the internal isolation valves partially closed would not have delayed core uncovery sufficiently for work to fully restore it, even if all PCV external valves had been opened for both trains, and that there was insufficient 125VDC power to start HPCI, then it appears the logical course of action would have been to implement the fire cistern->fire engine->FP system->core spray and car batteries to the MRC for SRV remote manual depressurisation plan ASAP. The question if enough time was available for this would have to look at how long implementing the individual parts of this work took at later stages of the crisis, but with the same resources available.

It appears the biggest problems and longest delays in the accident response all came after the hydrogen explosions and when radiological conditions had degraded both inside key buildings and outside in close vicinity. Another system that took very long time to get to work was SC venting arrangements, which at the end still was only partially successful for Units 1 and 3, being unsuccessful for Unit 2 despite almost a day of trying. In F-2 it was undestood early that any work inside the RBs, including manual valve actuations should be done proactively with anticipated not forced need. They also lined up venting paths, without the need to ever use them. The same was also understood in F-1, but apparently only after observing how things had already gone sour in Unit 1.

Venting however should not have been needed for Unit 1 until many hours or couple days, had core cooling being restored before severe damage, considering how long the other units went with RCIC.

Japan's BWR EOPs were not well updated. My understanding is they were still using rev 1 or 2 (all other plants are on 3 or 4). They had to get dresden's EOPs and SAMGs to use.

They did violate EOPs in that they did not perform a blowdown at unit 1 when required. This resulted in a hot debris ejection which may have contributed to containment leakage. The only way to minimize the damage in this event was exactly as you said, which is also what EOPs say, to blowdown when level was below the fuel and flood vessel or dry well with fire pumps through the core spray header.

With no functioning level indication, and elevated containment temperature causing reference leg boiling, the operators had no indications to go off of. They didn't have enough to demonstrate that reference leg boiling was occurring, could not transition to the flooding EOP, and suffered core damage.

I probably should make another post about BWR EOPs in detail. In all cases they should have blown down the reactor if they didn't know where level was and transitioned to flooding. But they didn't have enough to know if they didn't know where level was. Pretty screwed up.

 

[jim hardy]

It appears a lot of theoretical work on severe accident mitigation was simply overlooked or at least not integrated to EOPs.

yes i recall thinking that at the time.

, then it appears the logical course of action would have been to implement the fire cistern->fire engine->FP system->core spray and car batteries to the MRC

being a PWR guy not BWR i don't know offhand what is a MRC

but i recall thinking "Why don't they hook a gasoline driven welding machine up to the battery bus and get some instruments back ?" I knew exactly where in my plant to hook them, some unused breakers in the DC panels.. Could be their welders were all flooded by the tidal wave i suppose.

The plant is at its simplest a big heat source with several heat sinks and the approach is to assure heat moves from source to sink. That heat transport requires water, and in a PWR pressure above saturation for whatever is temperature.
so yes the need is to get water in there by hook or crook .
If you're using a fire engine you need to get pressure (hence temperature) low enough for your fire engine to overcome it.
I think Mr Hidden' says same, hope I'm not mis-interpreting

With no functioning level indication, and elevated containment temperature causing reference leg boiling, the operators had no indications to go off of. They didn't have enough to demonstrate that reference leg boiling was occurring, could not transition to the flooding EOP, and suffered core damage.

In all cases they should have blown down the reactor if they didn't know where level was and transitioned to flooding.

Loss of DC is the nightmare that wakes you up shaking because even your pumps and diesels need DC to start. The more natural circulation in the heat transport system the better, imho.

I was a maintenance man not an operations guy so my knowledge of EOP's is not very deep. And it's nil for BWR's
But i do remember the dramatic changes to our PWR EOP's post TMI .
In the early days they were failure oriented
"If you have failure X do Y"
the trouble with that is the plant doesn't tell you it has "falure X" it only shows you symptoms, ie strange instrument readings.*
So the procedures were re-written to be symptom oriented :
"If you see indication X do Y "
What a good idea - act on what you see instead of what you think is happening.
I don't know if Tepco's EOP's were similar in that regard to US.
But when batteries failed they no longer had anything to see because the instrument power comes from the batteries..

Sorry for the ramble. A plant was my life for thirty+ years so it's difficult _{er, make that} not possible for me to feel unaffected.

* (well, except for a steamline break outside containment . That one you hear for miles.)

 

[mheslep]

They did violate EOPs in that they did not perform a blowdown at unit 1 when required.

Did they have sufficient control to do so at that point, post tsunami but pre H2 explosion?

 

[Red_Blue]

Did they have sufficient control to do so at that point, post tsunami but pre H2 explosion?

Looking at the timelines, we get this partial information:
15:30 IC trains A and B manually secured, loss of all cooling to Unit 1
18:18 to 18:25 partial operation of train A
18:45 earliest start of Unit 1 core damage per TEPCO November 2011 analysis

March 11 Unit 1 and 2 MCR instrumentation
“late afternoon” battery collection starts
20:00 2 x 12V and 4 x 6V batteries delivered
20:49 temporary AC lighting available
21:19 24V connection to reactor water level gage of unit 1 connected

March 13 Unit 3
06:00 battery collection starts
07:44 ten 12V batteries delivered to MCR, series connection starts
09:08 120V connection to SRV established

It should be noted that there was no initial rush to provide instrumentation power, because after initial loss of DC power from unit batteries, that power returned for a while and only then faded away for good. Also, time was lost in looking over paper wiring diagrams with flashlights in the MCR, when this could have been done in the ERC that had AC backup power and two working phone lines to each MCR to relay instructions with.

In the unit Unit 3 case they collected and connected batteries for the actual case of using the SRV remotely from the MCR, but here the conditions in Unit 3 & 4 MCR were much worse than earlier in Unit 1 & 2 MCR, because this was after the H2 explosion in Unit 3 and everybody had to wear full suits and masks, including rubber gloves for radiation protection. Also at that point there were only flashlights available. Also the battery collection efforts were significantly hampered by radiation and additional debris outside.

I think it would not be unreasonable that the battery collection time for Unit 1 SRV operation could have been reduced to less than an hour in March 11 late afternoon with the conditions then prevailing and also if decision had been made to utilise employee's personal vehicles instead of TEPCO and contractor vehicles, access to which apparently was much delayed.

If this battery collection time had been OTOH used by another team to prepare the connection supplies, tools and wiring instructions from the ERC where PCs and electronic records with better search capabilities were available, I believe it should have been possible to bring the connection time down to less than one hour as well. That would have still left about an hour to come up and decide to implement this plan, which should have been enough even with time to evaluate IC effectiveness before committing.

Obviously, to be really effective it would also have required lining up the FP system injection path and positioning a fire engine for it. This was actually only attempted starting on March 12th 02:00 and the first attempt failed to locate the injection port, because the plan was to just drive around the building and search for it with the directable searchlights of the truck. They were able to locate the correct water connection only after going back to the ERC and getting a person on board who actually knew where it was. Because of this little snafu, it took until 04:00 to do the connection. There was also no other preparatory work for this until after midnight of March 12th, except breaking one electrically locked gate and some road repair work that was being carried out for other purposes. When the water injection to Unit 1 finally started, radiation levels were already high around Unit 1 buildings and required periodic evacuations of contractor personnel.

The actual mission time from when correct personnel was onboard, was from 03:00 to 04:00, so one hour to position the first fire engine and connect the hoses.

It should also be noted that no priority was given to the fire engine plan until the DDFP and plant fire water system plan was tried for many hours and failed. Its failure could have been expected for at least two reasons by the Japanese reports. First is that the DDFP was at a lower level than the external water connection and had less exhaust pressure than the fire engines, so even if it did get water from the system, it would require very low reactor pressure to work. Apparently none of the units achieved low enough RPV pressure for it to work for any of them. Another problem was that the fire water system was damaged plant wide due to the earthquake and tsunami and there was never any assurance that more water than what was in some length of upstream pipes would ever reach the DDFP. The plant fire department had closed valves from the main filtered water tank due to extensive leaks in many fire water lines.

However, the valve line up work for the FP injection from either the DDFP or the fire engine connection via MUWC and CS took from 18:30 to 20:50. Work was hampered by the same team having several tasks, poor instructions and wrong keys, having to return to the MRC several times and then back to the RB to continue the work. With even a little better planning or execution this task might have been condensed to two hours as well.

I have not seen a clear accounting of personnel in any of the reports, but it appears to me like additional manpower resources were only sent to the main control rooms (in addition to about 12 per unit in the regular shift) for particular recovery work tasks from the ERC and everything else had to be done with the regular shift that also had to have people manually record unit data and communicate with the ERC. That could not have left many 2 man teams to simultaneously do several control or recon missions from the MRC to the RB or TB. I would have expected much faster actions with more people available, essentially standing by at the MRC and waiting for new tasks that might arise either locally or from instructions from the ERC, without the ERC having to gather and then send the necessary extra personnel from the ERC to the unit needing it.

 

[etudiant]

NHK reports that a sarcophagus structure is under consideration, to seal off the buildings with the fuel inside.
Given the groundwater issues, is this a plausible option for even the relatively short term?

http://www3.nhk.or.jp/nhkworld/en/news/20160713_25/

 

[jim hardy]

I'll not criticize those guys
i still remember the shock at what Hurricane Andrew did to my plant . We sat on our diesels for a week while system folks put the grid back together. Meantime we fixed the water treatment plant and put security fences back up.

Fukushima Lessons Learned are here, i only noticed these pages a few minutes ago
http://www.nrc.gov/reactors/operating/ops-experience/japan-dashboard.html
http://www.nrc.gov/reactors/operating/ops-experience/japan-dashboard/priorities.html
It will be interesting to explore them.

 

[mheslep]

I'll not criticize those guys...

Which guys? If you mean the operators on the job at the time, maybe they did the best they could with what they knew. Yet three reactors are a total loss, most of the reactors across Japan were shut down for some years, people are excluded from the area for some years, and all of this was avoidable with either better designs or better preparation. Criticism is appropriate for those who could have taken action before the fact. Criticism is necessary if clean nuclear power is to flourish, else expect more of the same.

 

[jim hardy]

Which guys? If you mean the operators on the job at the time,

that's indeed to whom i refer.

Criticism is appropriate for those who could have taken action before the fact.

I've said consistently that blame lies with " responsible design organization " who dismissed historical reports of huge tidal waves that surfaced i think in the 1990's.

Recall my allusion a few days ago to "bureaucratic potato toss" .old jim

 

[etudiant]

When considering the available options, was there no way to use the power from Daiichi 5 and 6 to serve the remainder of the site?
Afaik, they were fully operable even after the earthquake and had escaped the tsunami.

 

[Hiddencamper]

When considering the available options, was there no way to use the power from Daiichi 5 and 6 to serve the remainder of the site?
Afaik, they were fully operable even after the earthquake and had escaped the tsunami.

5/6 were physically separated from 1-4.

Additionally the switchgear, breakers, motor controllers at 5/6 weren't submerged like at 1-4 As 5/6, were built at higher elevation.

So even if you could get power to 1-4, there wasn't any way to power up pumps.

Plus there is the issue of Diesel engine loading. You can expect a LPCI pump to be between 0.7 and 1.2 MW. Meaning a large twin 20 cylinder engine could power 4 LPCI pumps, but more standard engines could only power 2.

 

[etudiant]

I understand there would be huge obstacles, it is just surprising to me that this considerable resource could not be made available in any way.
Even just to charge the batteries might have helped some.
To lose three reactors in good part because there is no power while there are gigawatts standing idle just up the street is truly 'stranger than fiction'.
Presumably there is no provision for site self support power at other nuclear complexes either. Would such an internal link not be feasible and possibly helpful?

 

[Hiddencamper]

I understand there would be huge obstacles, it is just surprising to me that this considerable resource could not be made available in any way.
Even just to charge the batteries might have helped some.
To lose three reactors in good part because there is no power while there are gigawatts standing idle just up the street is truly 'stranger than fiction'.
Presumably there is no provision for site self support power at other nuclear complexes either. Would such an internal link not be feasible and possibly helpful?

The "self power" thing is complex. I'm assuming you are talking about keeping the reactor online on house loads only after a grid disturbance.

First: the vast majority of nuclear units do not have 100% load reject capability. That means at full power, a turbine or generator trip WILL result in a reactor trip, as the steam side isn't rated or designed to handle the pressure/temperature excursions.

Some plants have or had complex logic and systems actuations to rapidly runback the reactor, temporarily relive steam pressure (even at the cost of condenser vacuum or lifting relief valves), and hopefully steadying out at a low power level with the generator supplying house loads only. Problems: if anything goes wrong or the initiating event knocked out one of the systems required for the runback, you usually end up with a much more severe transient on the plant and reactor than if you just allowed the trip to happen. For plants with full generator load reject capability, they typically have to take a thermal limit penalty on the core due to this.

Talking to the BWR/6 in Germany, I've been told their version of this works half of the time at best. Also in the case of Fukushima this would require no seismic damage to the secondary side of the plant, which is not rated for seismic protection and had known damage. Some other things to consider: modern high efficiency mono block turbines do not like low load or temperature swings, and would likely vibrate and damage/rub if left in this mode for too long (house loads only isn't enough to keep mono locks stable).

Furthermore, at least in the Fukushima case, this would not have been able to work as the tsunami still flooded all the electrical distribution. The secondary side of the plant was completely out of commission.

 

[jim hardy]

the tsunami still flooded all the electrical distribution.

@etudiant
salt water in switchgear renders it unuseable .

I used to live by saltwater
if an ordinary extension cord falls in,
the end smokes, starts sparking and burns itself up
and that's at just 115 volts. Imagine what 4.2 or 6.9 kv would do.
Design constraints:
One places his diesels low in the building.
They're massive locomotive engines, and F=MA, and an earthquake is all A.
The heavy diesels go in the basement so earthquakes don't amplify the ground acceleration and whip them around even more as the building flexes like a bullwhip.
One places the electrical switchgear near the diesels so as to keep those runs of huge cable not very long.
So, diesels and switchgear in the basement is a good for earthquakes but not so good for flooding.
They needed a submarine hull around them.
I keep coming back to somebody dismissed the possibility of huge tidal waves .

I have a saying -
"If you want to guarantee that something will happen-
just stand up, slam the table, and publicly stake your reputation that it won't."
old jim

 

[Rive]

I understand there would be huge obstacles, it is just surprising to me that this considerable resource could not be made available in any way.

I might be wrong, but as I recall they already had to crosswire U5 and U6 diesels to maintain cold shutdown there, because one of those diesels were down too.

 

[Hiddencamper]

I might be wrong, but as I recall they already had to crosswire U5 and U6 diesels to maintain cold shutdown there, because one of those diesels were down too.

They did.

 

[etudiant]

Thank you, hiddencamper, jim hardy and rive, for your expert inputs.
If I understand your input correctly, the reactors can't run at much less than full power, because they are set up to feed the grid and if the grid goes down, the reactors trip. That does help explain why there was no help possible from 5 and 6 as well as the regulatory concern about station blackout, which was clearly well founded.
Seems the only contribution that 5 and 6 could have made was if there could have been a separate site wide power link for the emergency diesels. Battery chargers were probably not available either.

That salt water does not play nice with electrical is understood. What is murky is why the instruments and valves were still operable, even if only on battery power.
Is there a separate set of control circuits that bypassed the flooded electrical switchgear?

Overall, it seems, based on the earlier discussions, that even with hindsight the reactors were doomed once the tsunami hit. Was there a course available that might have minimized the resulting damage?

 

[Hiddencamper]

Thank you, hiddencamper, jim hardy and rive, for your expert inputs.
If I understand your input correctly, the reactors can't run at much less than full power, because they are set up to feed the grid and if the grid goes down, the reactors trip. That does help explain why there was no help possible from 5 and 6 as well as the regulatory concern about station blackout, which was clearly well founded.
Seems the only contribution that 5 and 6 could have made was if there could have been a separate site wide power link for the emergency diesels. Battery chargers were probably not available either.

That salt water does not play nice with electrical is understood. What is murky is why the instruments and valves were still operable, even if only on battery power.
Is there a separate set of control circuits that bypassed the flooded electrical switchgear?

Overall, it seems, based on the earlier discussions, that even with hindsight the reactors were doomed once the tsunami hit. Was there a course available that might have minimized the resulting damage?

At very low power levels, you just dump excess steam to the condenser. You can run at any power level, but in general you won't have the turbine online below 25% for any extended amount of time.

Units 1/2 had no AC or DC, so they had no ability to control any valves or instruments.

Unit 3 did have DC power for a while, so they were able to operate RCIC/SRVs/HPCI.

Not sure why you think instruments or valves at units 1/2 were still working.

The only real way to minimize damage for unit 1 would be to get drywall temperature and vessel pressure measurements. They would have identified reference leg boiling, indicating their level instruments were frozen upscale high. Then they would enter the flooding EOP and could attempt to blow down and start flooding containment earlier. If unit 1 containment flooding occurred earlier, it would have minimized the release rates drastically and wouldn't have complicated saving units 2/3 which did have injection for some time.

 

[etudiant]

I misread some of the earlier discussion to indicate that 1 and 2 still had some instrumentation even after the tsunami, enough to allow the operators to run the RCIC. Is that a misperception so that they were basically dead electrically from the time the tsunami hit? Would depressurizing the reactors immediately before the flooding have been the least damaging choice, even though it would have probably also left the reactors scrap?

 

[jim hardy]

Seems the only contribution that 5 and 6 could have made was if there could have been a separate site wide power link for the emergency diesels.

that sounds right
The conductors for an extension cord sized for a diesel are big, like like fire hose size , not something you just uncoil and plug in .

What is murky is why the instruments and valves were still operable, even if only on battery power.
Is there a separate set of control circuits that bypassed the flooded electrical switchgear?

Switchgear powers big equipment directly, and little equipment indirectly through step down transformers and smaller power panels distributed throughout the plant.

One of the small loads is the station battery chargers. In my plant they and the batteries are located upstairs . Batteries power 130VDC to 120VAC inverters for instrumentation.
So instruments remain available until the batteries run down , a matter of hours.
So do some valves it they're powered by DC and didn't get flooded .

old jim

 

[Hiddencamper]

I misread some of the earlier discussion to indicate that 1 and 2 still had some instrumentation even after the tsunami, enough to allow the operators to run the RCIC. Is that a misperception so that they were basically dead electrically from the time the tsunami hit? Would depressurizing the reactors immediately before the flooding have been the least damaging choice, even though it would have probably also left the reactors scrap?

Unit 1 didn't have RCIC. It's an HPCI/IC plant. I still haven't seen a reason as to WHY they couldn't black start HPCI at unit 1, but I'm guessing HPCI inboard steam isolations went closed the same time the IC inboards went closed (likely use a similar 'fail safe' leak detection system).

Unit 2's RCIC was in service when the tsunami hit. RCIC uses DC power for most of its valves, and as long as the inboard steam isolation valve does not go shut (AC motor operated), you can black start RCIC by manually opening the trip/throttle valve and the injection valve.

When unit 2 lost DC power, the RCIC governor valve failed to the open position. With no servo current applied to the governor, it is spring loaded to fail open (maximum injection). The pump filled the reactor to the steam lines, then two phase flow went down the steam line into the RCIC turbine, causing it to slow down to around 1/3rd flow or stall out, until level dropped low enough to get clean steam through it and the RCIC turbine would spin back up. It was 'passively' controlling level at the steamlines until it overheated and stalled out. If operators were able to access the room, they should have manually controlled the trip/throttle valve to control injection rate/level, but to my knowledge they didn't have access due to the flooding.

As for depressurization. From a practical perspective, depressurization would have helped minimize the potential for containment damage when the core melted through the vessel, however there are a lot of limits/issues with this. For one, you need DC power to operate the Safety-Relief valve solenoids to perform the blowdown, so you couldn't do this easily at units 1/2 without battery packs. The other issue is that you cannot intentionally violate cooldown rate. The 100 degF/55degC per hour cooldown rate is a strict cooldown limit. EOPs do not allow exceeding this limit unless you hit an Emergency Depressurization Required contingency, and you are not allowed to anticipate the requirement to blow down early unless the steam dumps to condenser are available (they weren't). So you cannot do an "early" blowdown, only a normal cooldown.

Emergency depressurization would not "scrap" the reactors, GE reactors are designed for an emergency blowdown and reflood, and require a vessel analysis after that is complete to verify the integrity of the vessel. Some plants have blown down rapidly before, at Laguna Verde in Mexico, an SRV stuck open and depressurized the core in under an hour from NOP/NOT, and they are still operating the unit today.

For reference, the only times you can perform an emergency blowdown or exceed the cooldown limit for a BWR:

Level below top of fuel and steam or spray cooling cannot be established. Primary containment parameter being exceded and cannot be recovered (temp/pressure/torus level and temp, etc), secondary containment safe temp/rad limits exceeded due to a primary coolant leak, offsite rad release in excess of legal limits due to a primary coolant leak, and finally, if all level indication is lost in order to provide temporary steam cooling and flood the reactor to the steamlines.

 

[mheslep]

@etudiant
salt water in switchgear renders it unuseable ...

Marine high power electrical gear is somewhat robust to salt spray, and is not permanently unuseable even if flooded for a time after a clean up. Dropping the standard marine power cable in the water for instance, the kind found around every marina, to follow your example, won't cause it to burn up. For a coastal plant to not have electrical backup equipment without some resilience to salt water, if that is actually the case, is curious.

 

[Hiddencamper]

Marine high power electrical gear is somewhat robust to salt spray, and is not permanently unuseable even if flooded for a time after a clean up. Dropping the standard marine power cable in the water for instance, the kind found around every marina, to follow your example, won't cause it to burn up. For a coastal plant to not have electrical backup equipment with some resilience to salt water, if that is actually the case, is curious.

The plant was considered a "dry" site, which is why the switchgear and diesels were allowed to be at lower elevations and not in watertight cubicles.

 

[gmax137]

... The plant was considered a "dry" site ...

Criterion 2—Design bases for protection against natural phenomena. Structures, systems, and components important to safety shall be designed to withstand the effects of natural phenomena such as earthquakes, tornadoes, hurricanes, floods, tsunami, and seiches without loss of capability to perform their safety functions. The design bases for these structures, systems, and components shall reflect:
(1) Appropriate consideration of the most severe of the natural phenomena that have been historically reported for the site and surrounding area, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated,
(2) appropriate combinations of the effects of normal and accident conditions with the effects of the natural phenomena and
(3) the importance of the safety functions to be performed.

There are about 60 of these General Design Criteria. This one is Number 2 for a reason.

 

[Hiddencamper]

There are about 60 of these General Design Criteria. This one is Number 2 for a reason.

In order to meet that GDC, you need to demonstrate that you are protected from hazards. At the time the plant was build, it WAS in compliance with it's tsunami analysis. In 2009 when TEPCO identified that a tsunami much larger than anticipated could have struck the site, they were no longer in compliance with their tsunami analysis. The regulator allowed continued operation under the pretext that a loss of all seawater pumps could be coped with using compensatory actions and other installed equipment (like the air cooled diesel generators - similar to station blackout generators in the US), as well as TEPCO getting an independent analysis performed by another organization.

This is where the real flaw is, because the comp actions were not sufficient for the type and level of damage which could have occurred. It's clear that TEPCO did not properly evaluate the effect a massive tsunami could have on the site, and the regulator did not challenge them.

But from a pure design standpoint, the site was designed over 40+ years ago, to standards and analysis from 40+ years ago, which is WHY the original design is the way it is.

 

[mheslep]

The plant was considered a "dry" site, which is why the switchgear and diesels were allowed to be at lower elevations and not in watertight cubicles.

Was considered? Clearly. But by whom and via what rationale?

 

[Hiddencamper]

Was considered? Clearly. But by whom and via what rationale?

Some excerpts from the following:

http://www.tepco.co.jp/en/press/corp-com/release/betu11_e/images/111202e14.pdf

The major buildings of the Fukushima Daiichi NPS are located at an elevation of O.P. +10 m for Units 1 to 4, which suffered major damage, and at an elevation of O.P.+13 m for Unit 5 and 6. When obtaining the establishing permit, the Chilean tsunami had been envisioned as the greatest tsunami in history, and the tsunami height at that time was O.P. +3.1 m. At present, the tsunami height of O.P.+6.1 m, that was evaluated based on the “Tsunami Assessment Method” of the JSCE, is used for the design purpose. It was recognized that there would not be any tsunami that could run up to the level of the buildings.

Investigation on the plants revealed that EDGs are not located inside the reactor buildings that require air-tightness. U.S. plants that were under construction when Fukushima Daiichi Unit 1 was designed were designed to plant-specific seismic criteria as early as 1969, - 16 - using the existing subsurface conditions for the individual plants. U.S. designs are unique to the site soil conditions, supported by rock or a unique subsurface formation, or on spread-footer foundations. Hence, most of the buildings in which EDGs are installed did not require foundations built on base rock. In comparison, many buildings in Japanese NPSs have basement floors due to the necessity of being built on the base rock layer for seismic reasons. Due to such differences, EDGs were installed on the foundation (the lowest floor) in Japan in consideration of the large components’ seismic adequacy and vibrations.

Since Dr. Satake’s paper proposed wave source models, although they were not verified, TEPCO conducted a trial calculation using the two models proposed in the paper in December 2008. The result of the trial calculation showed a tsunami height of O.P. +7.8 m to 8.9 m (O.P. +7.8 m to 9.2 m, if a different accounting method for high tide is used) in front of the Fukushima Daiichi and Fukushima Daini NPS intake points.

 

[nikkkom]

NHK reports that a sarcophagus structure is under consideration, to seal off the buildings with the fuel inside.
Given the groundwater issues, is this a plausible option for even the relatively short term?

http://www3.nhk.or.jp/nhkworld/en/news/20160713_25/

I like this idea a lot.

 

[etudiant]

As for depressurization. From a practical perspective, depressurization would have helped minimize the potential for containment damage when the core melted through the vessel, however there are a lot of limits/issues with this. For one, you need DC power to operate the Safety-Relief valve solenoids to perform the blowdown, so you couldn't do this easily at units 1/2 without battery packs. The other issue is that you cannot intentionally violate cooldown rate. The 100 degF/55degC per hour cooldown rate is a strict cooldown limit. EOPs do not allow exceeding this limit unless you hit an Emergency Depressurization Required contingency, and you are not allowed to anticipate the requirement to blow down early unless the steam dumps to condenser are available (they weren't). So you cannot do an "early" blowdown, only a normal cooldown.

Emergency depressurization would not "scrap" the reactors, GE reactors are designed for an emergency blowdown and reflood, and require a vessel analysis after that is complete to verify the integrity of the vessel. Some plants have blown down rapidly before, at Laguna Verde in Mexico, an SRV stuck open and depressurized the core in under an hour from NOP/NOT, and they are still operating the unit today.

QUOTE]

Again, thank you hiddencamper for another very informative post.
It suggests that there needs to be a more effective emergency stop provision for current reactors. The Fukushima operators did their best to execute the shutdown procedures, yet their reactors would still have poisoned the Japanese heartland for centuries if the winds had not swept the debris out to sea.
Clearly reactors need a switch to allow them to fail in a manageable way if continued operator control is lost. Is that even possible?

 

[Astronuc]

Was considered? Clearly. But by whom and via what rationale?

TEPCO submitted an analysis and the regulatory authority accepted/approved it.

In the report cited by Hiddencamper, see section
3.4 Tsunami evaluation
(1) Evaluation of tsunami height
The establishing permits for the units of the Fukushima Daiichi NPS were obtained
between 1966 and 1972. At that time, there was no guideline for tsunami and the units were
designed based on the known tsunami traces. Specifically, the maximum tide level that was
observed at the Onahama Port (O.P. +3.122m), which was caused by the Chilean earthquake
and tsunami of 1960, was established as a design basis.

In 1970, the “Regulatory Guide for Reviewing Safety Design of Light Water Nuclear
Power Reactor Facilities” (hereinafter referred to as the “safety design review guidelines”)
was established. In the guideline, tsunamis were referred to as one of the natural conditions
that should be considered and the facility was required to be able to withstand the harshest
natural force that was foreseen based on past records. . . . .

Somehow, the utility and government regulator convinced themselves that the tsunami level was not going to be more than 5.4 to 5.7 m. Much of the eastern coastline in the Tohoku region was not adequately protected as is evidenced by the substantial flooding in areas like Sendai.

However, there were historical records of such tsunamis. One simply had to go looking for them.

 

[etudiant]

Fukushima can clearly be classified as a regulatory failure, as Astronuc's above post again makes clear.
Unfortunately, regulatory failure is not uncommon, as illustrated by experience elsewhere, in the financial industry for example.
That puts a heavier burden on the nuclear engineers, they have to allow for inadequate regulation in the design and operation of the plant.
The Fukushima disaster proves that that is beyond the capability of the current reactor installations. That threatens the industry's survival imho.
At this point, there are about 500 power reactors world wide, so maybe 15,000 reactor years of operation cumulatively. With at least 3 loss of reactor accidents in that time there is about one per 5000 reactor years. That suggests a going forward rate of one disaster per decade. It does not seem a sustainable path, so what are the possible remedies?