Japan Earthquake: Nuclear Plants at Fukushima Daiichi

[Pakman]

We do know why they did not try (at least did not try hard enough). They were not prepared to SBO. Basically, they found themselves in a situation they were never trained for.

It's quite impolite to think about Fukushima operators as about circus monkeys who do only what they are trained for.

 

[Pakman]

Lack of lighting, failed instrumentation, high radiation, worrying about loved ones that may have been in the tsunami zone, I am thankful that I wasn't in that position.

All of us are. But the story's not abut us.

 

[steve olsen]

It's quite impolite to think about Fukushima operators as about circus monkeys who do only what they are trained for.

True training in disaster preparedness is unlikely to happen. Why spend money on a "black swan" which is so unlikely? As much as I like and respect the Japanese culture, they do far better at operating inside the box, than thinking outside the box.

 

[Pakman]

In our BWRs, those pressure relief lines that are equipped with DC operated control valves have their quick-opening valves failing open, which is a precaution to prevent high-pressure melt-through of the RPV, which might compromise containment integrity in a severe accident. Furthermore, these valves are equipped with pneumatic connections to enable keeping them open with pressurized nitrogen to make sure the RPV pressure can be lowered equal to the containment even in total blackout.

It looks like your NPPs are well prepared for total SBO.

With your SBO fail-safe logic, do you realize that in case of depressurization from nominal pressure to about atmospheric the reactor loses at least 38% of its Inventory immediately due to evaporation? With this in mind, TAF will be reached in a very short time, if any time is needed at all. You believe you can implement the firewater to cool the core before the melting starts?

 

[rmattila]

With your SBO fail-safe logic, do you realize that in case of depressurization from nominal pressure to about atmospheric the reactor loses at least 38% of its Inventory immediately due to evaporation? With this in mind, TAF will be reached in a very short time, if any time is needed at all. You believe you can implement the firewater to cool the core before the melting starts?

Most probably not, and due to the passive filtered venting in the containment, there would probably be too much backpressure anyhow to get firewater in. The purpose of the fail-open of these two small-capacity blowdown lines is rather to prevent high-pressure melt-through, not to enable injection of firewater in case of total loss of DC.

Normally (=if DC is available for the measurement circuits), the depressurization signal is triggered by TAF +0,7 m with some delay, and its purpose is to first of all switch from the high-pressure core injection mode to attempting low pressure core injection, and secondly to reduce the pressure in case the low-pressure core injection would fail and there was a risk of melt-through. This forced blowdown will use a larger number of blowdown lines, not just those two small ones with the fail-open valves.

The weakness of the later ASEA BWRs is the total reliance on AC driven emergency cooldown systems in contrast to the GE line of reactors. But on the other hand, this means that there's no need to keep the reactor pressure high to enable the operation of the cooling systems, which makes it easier to depressurize the reactor in case of emergency.

It's not obvious which approach is the best - it's all a matter of optimization. IC and RCIC are good since they don't need electrical power to operate, but if they fail, it seems that pressure relief was too difficult to manage to be succesful.

 

[Pakman]

I believe subsequent IC restart failures could have been caused by loss of natural circulation in the IC system due to non-condensible gases (hydrogen) air-binding the system. If that is the case the valve problems they encountered were of little impact.

I believe if the IC restart would be taken earlier, before melting, there would be no problem of natural circulation.
The melting started at about 17:40. SBO occurred at abut 15:37.
Two hours seems to be enough to stop fearing, to realize things and to start acting.

I just want to know, what does the phrase in report: "This was because it was wrongly assumed that the IC was operating normally" exactly mean. That Japanese NPP operators are trained to assume instead of to know?

 

[Pakman]

but we're trained to not go where it's unknown
and we're trained that to go into a very high area one takes special equipment and precautions and plans the entry carefully...

Here's what came to mind: to follow these rules in a severe accident is like for pilots of passenger plane to be prepared to bail out with parachute.

 

[jim hardy]

"Here's what came to mind: to follow these rules in a severe accident is like for pilots of passenger plane to be prepared to bail out with parachute.""

do you then suggest they descend into mountain clouds with no altimeter?

 

[Rive]

It's not obvious which approach is the best - it's all a matter of optimization. IC and RCIC are good since they don't need electrical power to operate, but if they fail, it seems that pressure relief was too difficult to manage to be succesful.

- are they really exclude each other?
- the cold leg of RCIC can be kept cold with low pressure input, it's just matter of heat exchangers in the wetwell.

 

[Yamanote]

Let's get back to presence, they are now dealing with a leak in the skimmer surge tank and/or the spent fuel pool of unit 4:

http://www.tepco.co.jp/en/press/corp-com/release/12010201-e.html

I think this could become a quite dangerous incident.

 

[steve olsen]

Let's get back to presence, they are now dealing with a leak in the skimmer surge tank and/or the spent fuel pool of unit 4:

http://www.tepco.co.jp/en/press/corp-com/release/12010201-e.html

I think this could become a quite dangerous incident.

Indeed, here is relevant part


P
At approx. 5:30 pm on January 1, 2012, we observed approx. 240 mm
decrease in the water level of the skimmer surge tank*1 of Unit 4 spent
fuel pool in the three hours between 2:00 pm to 5:00 pm (According to the
operation record so far, there had been an approx. 50 mm decrease.). As a
result of the site investigation later, we did not observe any leakage
around the Unit 4 reactor building, connecting points of primary system
pipes of Unit 4 spent fuel pool alternative cooling system, or its
installation space. The water temperature of Unit 4 spent fuel pool as of
5:00 pm on January 1 is 23°C (22°C as of 5:00 am on January 2). The spent
fuel pool alternative cooling system is still in operation and there is
no problem in cooling the reactor. Though the water level of the spent
fuel pool is kept stable without any problem, the water level in the
skimmer surge tank continues decreasing. Therefore, from 10:27 pm to
11:13 pm on January 1, we filled water in the skimmer surge tank. At
present, the water level in the skimmer surge tank keeps decreasing
approx. 90mm/hour. We will strengthen surveillance on observation of the
water level in the skimmer surge tank, increasing the frequency to once
in an hour from once in three hours. At the same time, we will continue
examination of the causes of the decrease in the water level, such as
site investigations based on such possibilities as the water may flow
into the reactor well*2 from the spent fuel pool.
At present, no leakage is observed outside the building. No significant
change is observed in the water level of the accumulated water in the
building.

*1 The tank installed in order to receive the water overflows from the
spent fuel pool. The water in the spent fuel pool is overflowed into
the skimmer surge tank in order to cool the fuel assembly and remove
impurities in the water. The water returns to the spent fuel pool after
passing through the heat exchanger and the filter.

*2 The reactor well is the space which contains the reactor pressure
vessel and the lid of the reactor containment vessel. During the
periodical inspections, the space filled with water and the fuel is
changed.

-At this moment, we don't think there is any reactor coolant leakage
inside the primary containment vessel.

 

[Yamanote]

It seems that the water level is only dropping in the skimmer surge tank:

Though the water level of the spent fuel pool is kept stable without any problem, the water level in the skimmer surge tank continues decreasing.

But I am not sure, many things are lost in translation, as their (Tepcos) English is as bad as mine.

Perhaps there is now more danger coming from the SFPs (which might leak or even collapse during an aftershock) than from the reactors itself (which are already molten and leaking anyway). So shouldn't they focus on getting the damaged fuel pools somehow empty asap?

 

[steve olsen]

It seems that the water level is only dropping in the skimmer surge tank:



But I am not sure, many things are lost in translation, as their (Tepcos) English is as bad as mine.

Perhaps there is now more danger coming from the SFPs (which might leak or even collapse during an aftershock) than from the reactors itself (which are already molten and leaking anyway). So shouldn't they focus on getting the damaged fuel pools somehow empty asap?

I think I was saying that around March 20th...empty all the fuel pools, including at 5 and 6. Plenty of Plutonium in the used rods.

 

[Yamanote]

I am not an expert, so I can't say how and within which timeframe it would be possible to empty the SFPs. But for sure they would get rid of one huge problem, if they could do so. The sooner, the better!

 

[Gary7]

Anomaly in the water level of the skimmer surge tank for SFP #4 was apparently due to the earthquake on the 1st of January.

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_120102_01-j.pdf

 

[tsutsuji]

http://www.47news.jp/CN/201201/CN2012010101000398.html The water level in the skimmer surge tank decreased at an 8 cm/hour speed between 02:00 PM and 05:00 PM on 1 January, instead of the normal speed of 1.7 cm/hour which is mainly due to evaporation.

http://sankei.jp.msn.com/affairs/news/120102/dst12010220560011-n1.htm Tepco presumes that as a consequence of the earthquake that occurred on 1 January at around 02:00, it became easier for the water in the spent fuel pool to flow into the adjacent pool whose water level is lower. When water was poured into the adjacent pool, the water level in the tank stopped to decrease.

http://mainichi.jp/select/weathernews/news/20120101k0000e040118000c.html A magnitude 7 earthquake occurred at 02:28 PM on 1 January in the sea near Torishima Island in the Izu archipelago at a 370 km depth. Intensity 4 was measured in a large area over Kanto and Tohoku.

http://www.tepco.co.jp/nu/fukushima-np/images/handouts_120102_01-j.pdf In a later survey, it was confirmed that the water level in the reactor well had risen by an amount corresponding to the water level decrease in the skimmer surge tank, and that the reactor well's water level was lower than that of the spent fuel pool. It is inferred from the above that as a consequence of the earthquake that occurred at around 2:30 PM on 1 January, the conditions of the interstices of the gate between the spent fuel pool and the reactor well were changed, and due to the increase of the water flow from the spent fuel pool into the reator well, the quantity of the overflow from the spent fuel pool into the skimmer surge tank decreased, causing a bigger than usual water level decrease in the skimmer surge tank. In order to reduce the water level difference between the spent fuel pool and the reactor well, water was injected into the reactor well from 11:50 AM to 11:59 AM on 2 January, and as of 04:00 PM no skimmer surge tank water level decrease was observed. The skimmer surge tank's water level will be continuously monitored in the future.

 

[CaptD]

It looks like your NPPs are well prepared for total SBO.

With your SBO fail-safe logic, do you realize that in case of depressurization from nominal pressure to about atmospheric the reactor loses at least 38% of its Inventory immediately due to evaporation? With this in mind, TAF will be reached in a very short time, if any time is needed at all. You believe you can implement the firewater to cool the core before the melting starts?

I'm not sure about the firewater cooling but I think it bears consideration!

 

[kaworu1986]

I have been following the discussions about Fukushima on this website for quite a while (they seem the most level headed and fact based ones around), with the exception of the last two months or so.
The latest video on http://www.fairewinds.com/content/tepco-believes-mission-accomplished-regulators-allow-radioactive-dumping-tokyo-bay , about the situation of the plant going into 2012, managed to surprise me, however, when it claimed that an aftershock in Fukushima could knock out the piping installed since the accident (which is easily believable) and would cause another meltdown within 40 hours.
Now, I understand there isn't much of a plant left to put into "cold shutdown", but I would have thought that by now the fuel's thermal output would have decayed enough to make further meltdown impossible.
Has the situation really improved so little that an aftershock could set it back to square one or worse?

 

[zapperzero]

I would have thought that by now the fuel's thermal output would have decayed enough to make further meltdown impossible.
Has the situation really improved so little that an aftershock could set it back to square one or worse?

Further meltdown is impossible for the simple reason that the meltdown is complete. So is the melt-out, for that matter. We can't talk about aftershocks because too much time has passed, it would be a whole new quake. There is no way to go back to square one, really, because square one had intact reactors in it.

If the water in the reactors somehow starts boiling again, radioactive emissions could resume. Hydrogen is being produced, so if the nitrogen injection stops there could conceivably be more explosions. Massive spillage from the waste water tanks (perhaps from a tsunami wave?) could mean the site becomes inaccessible. And so on, and so forth.

Generally, speaking, there used to be an NPP at Fukushima, which was pretty unsafe, as it turned out. Now it's an industrial accident site, which obviously is even more unsafe. Many things could go wrong. Does this mean that they will? What are you asking, really?

 

[gmax137]

At 9+ months after the reactor trip, the decay heat fraction is around 0.0005 (maybe one of you guys can check my number). That means the Unit 1 core is making around 0.7 MW and Units 2 & 3 are each making 1.2 MW. These are still non-negligible heat generation rates. Keeping the fuel (whatever its condition) covered with liquid is necessary to prevent it from heating back up.

 

[rmattila]

Once the fuel reaches melting point, iodine, Cs and noble gases have already been boiled out and what remains, does not migrate as easily. So emission-wise, a re-melt would not constitute a comparable risk to what already occurred, even if it would theoretically be possible (which is not necessarily the case, if the molten material has mixed with sufficient amount of structural materials).

 

[clancy688]

@It is too bad that TEPCO is not removing every rod they can to yet another location so as to minimize the number of "rods" that remain in the pool itself; the fewer rods the slower the remaining rods would tend to boil the volume of water they are in...

Um, we are all talking about removing the fuel from the SFPs, but is this even possible? It's my understanding that most of the fuel there is still "young", generating much radiation and therefore heat. Which's exactly the point why it's somewhere near the reactor - easy transfer.
So if you'd take a "young" fuel element out, you probably have to provide massive shielding and considerable cooling. And then there's the point of safe storage. Where to put those things? They just lost 10 of 17 SFPs (reactors) and 2 of 3 NPPs and as far as I'm informed there is no other storage site for used fuel elements in Japan than on top of or next to the reactors. Putting them into an unused swimming pool wouldn't make us any happier... ;)

 

[tsutsuji]

The latest video on http://www.fairewinds.com/content/tepco-believes-mission-accomplished-regulators-allow-radioactive-dumping-tokyo-bay , about the situation of the plant going into 2012, managed to surprise me, however, when it claimed that an aftershock in Fukushima could knock out the piping installed since the accident (which is easily believable) and would cause another meltdown within 40 hours.

http://www3.nhk.or.jp/news/genpatsu-fukushima/20111013/0500_kunren.html The drill assumed that tanks and pumps had been broken by an earthquake. 40 people installed fire trucks and 300 m of hoses, so that cooling was restored to one reactor in 1 hour 10 minutes. In the future Tepco will perform other drills assuming a tsunami with debris spread on roads, and occurrences at times when gathering people is more difficult, such as on holidays and during the night.

So if they can be as quick as in the drill, the water injection is restored before the 40 hours elapse.

There was another drill during the night on 7 December : http://www.tepco.co.jp/en/nu/fukushima-np/images/handouts_111208_02-e.pdf

 

[Rive]

@ kaworu1986
The Spent Fuel Rod (SFR) pool MUST be cooled constantly ...

On the early days (as the site was hardly reachable) it was a real problem, but for now they would be able to refill them and maintain a stable water level even if the cooling is lost. The fuel pools would run dry only after a week or so.

The PCVs/RPVs are more of a concern, but as they are at low pressure even pumps of fire trucks would be able to provide enough cooling to prevent really serious problems.

@Clancy688 (Say Hi to smilin' Tom)
Nine months is a long time, with a crash program TEPCO could easily build another "hardened" SPR POOL and start transferring PFR's ASAP into it; even if the transferring was done one at a time with the MOX rods having priority...

To start transferring the fuel rods they would need
- a fully functional and reliable crane system which is able to access the pools with high accuracy and able to lift even full transfer casks: the original crane systems are gone, not reliable or not accessible now
- free access to the pools (U1 pool has the whole roof on its top: U2 pool level is not accessible due the high radiation, U3 pool is a mess, U4 pool is not accessible due the debris of the roof. )

And there are no MOX rods in any of the pools AFAIK. U3 had some MOX rods loaded.

 

[Joffan]

And there are no MOX rods in any of the pools AFAIK. U3 had some MOX rods loaded.

... and even if there were, they wouldn't be significantly different from any other spent fuel rods. There is no magic excessive hazard to MOX rods.

 

[Yamanote]

Many things could go wrong. Does this mean that they will?

When it comes to nuclear engineering, it is quite useful to expect also the unexpected and to be prepared as good as possible, when something goes wrong. At F1 they were not and we can see now the outcome with all the consequences.

Even today - nine months later - there are still many questions unanswered how to cope with an accident, that was obviously beyond imagination. And as there is no book with answers available, they have to go the painful way finding answers on a trial and error basis.

So talking about answers and plans - what could be the plan for a massive water leak in one of the SFP? How to recover, when the water is dropping rapidly? Or is the only solution to pray, that this will never happen?

 

[zapperzero]

So talking about answers and plans - what could be the plan for a massive water leak in one of the SFP? How to recover, when the water is dropping rapidly?

"thinking aloud" here, for lack of time, sorry.

I think first you would put some more water in, using a fire engine or maybe one of those big concrete pumps. Even if you do not succeed in re-filling immediately, spraying the fuel may be enough to keep it cool. Of course, you would have a big problem with radioactivity if the pool actually empties.

Then, you could drop some filler material into the gap, or maybe patch it up from outside. It would be a very bad situation, for sure, but not, I think, unrecoverable.

How this could happen, I am not so sure. Perhaps the seal around the gate failing with age and lack of maintenance? The pools seem to have endured at least one major quake just fine and then explosions above three of them. I don't think they will suddenly crack like eggs. Maybe the steel lining could be corroded away? But that by itself is not enough to cause a major leak. The concrete would have to be eaten away too but this would take months, I believe.

 

[al2207]

In reactor 3 there was quite large explosion some people think of weak nuclear explosion originated from SFP after water was evaporated and rod were melting creating all necessary conditions to dissociate water, have uranium plutonium concentrated at bottom and first hydrogen and OX exploding with shock wave concentrating curium and starting nuclear explosion process .
just curious to have your comments

 

[Pakman]

The Chernobyl operators knew they were receiving large doses and they died without making any difference in the outcome. In any case according to repports I've read they stuck around for only two reasons - to make sure no one was trapped, and to try to see what the could toppass on to the responders. I don't think any mitigation actions were attempted by the operators. That was heroic but futile.

Well, let me tell you about a couple of such an action.
The first instruction was given immediately after the sound of explosion is to cool down the reactor with emergency rate. The next instruction was to start up the emergency cooling system and to open its valves, which was set to local mode before testing, so operator had to go to the valves location. Next instruction was for operators to go to reactor hall to manually insert the control rods.
Since there was no reactor any more, this actions could be called futile. But they didn't know for sure what's happened to reactor and they acted
The staff of the turbine hall also didn't sit without work. The debris of the destroyed #4 reactor building fell through the roof directly to the turbine and to the feedwater pumps. Fires broke out. The main objective for the operators was to save from fire each flammable material in turbine hall.
Operators removed hydrogen from damaged generator to prevent the explosion inside the turbine hall which is also the turbine hall for unit 3.
They drained oil from turbine oil tanks (100 m3 of extremely flammable oil) into the underground storage. They operated manually, since the power cable to drain valve was broken. There two operators had got their lethal doses of radiation, because there was a nuclear fuel fragment near this place outside the building.
They did many other usefull things saving #3 unit from fire. Here’s the difference in outcome.

 

[zapperzero]

Superhot "corium" will find the rebars and eat away through those little molten steel highways. Fuel Pool 4 did apparently crack/leak

It was the skimmer surge tank that leaked for some reason, not the pool. Also, I meant water would "eat" the concrete, not corium, as I don't believe even TEPCO is incompetent enough that they can't keep a damned pool filled with water, given the needed pumps (which are on-site anyway).