Japan: TEPCO aims to achieve 'cold shutdown' for reactors in 6-9 months

Kyodo News ^ | 04/17/11

[chimera]

I’m sure the nuclear industry here will be examining the Japanese situation for lessons learned.

You're right, this is an industry that learns from experience. I am sure we'll see things like added redundancy for emergency power, such as hardened generator buildings, fuel lines, and storage tanks. What you won't hear in the press is that everything at Fukushima survived both the earthquake and the tsunami with some manner of functionality except the diesel fuel tanks and fuel lines. Something simple, like buried tanks and lines with waterproof seals will solve that issue.

But the press will take something like that and say "see, see, that company was stupid, the people are dumb, it's a poor design, nyah nyah nyah". That's the only "lesson" the media and anti-nuke kooks will take. It's easy to second-guess and armchair-manage a company. The truth is, those plants were designed for an earthquake twice as powerful as any ever recorded there, an a tsunami almost three times bigger than any for that region. Then they had an earthquake five times the design basis magnitude, and a tsunami twice as big as the design basis. No society can reasonably plan for such outlier events, unless you want to go back to living in caves and scratching in the dirt for survival.

[chimera]

I’m guessing there will probably be procedural changes such as limiting the amount of fuel held in the elevated pool. Unit 4 had a full core offload plus some other older fuel in the elevated pool. They will likely require more of the older fuel to be moved to the common pool when doing a full-core offload, at least where common pools, like at Fukushima Daiichi, are available. Lowering the heat load in the event of loss of SFP cooling is probably one outcome of the events we have seen at Unit 4.

[SteamShovel]

The Mark 6 I’m familar with housed the spent fuel pool within a concrete building.

Aren't these spent fuel pools primarily used for short term storage during fuel unloading and shuffling?

I had thought there were other pool facilities built on-site for longer term cooling in the US when the government started playing games with fuel reprocessing and then disposal storage. If not, it is an area of concern that will probably require investment based on what has been learned here at Fukushima.

[meatloaf]

The nuclear sites here already have redundant diesel generators in hardened buildings. Fuel tanks are also protected.

[meatloaf]

The spent fuel pools at the Japanese plants are located outside the primary containment but within the secondary containment.

[chimera]

Those that have them will likely be asked to take another look and offer some form of analysis to show that they can handle the anticipated events. There was a big controversy in the 1970s about plants sited around the Great Lakes and the question of seiches. Quite a lot of work was done to show that this kind of event could be handled. Same with tornadoes for plants located inland. It is almost assured that offsite power is going to be lost. Primary containment had been shown to handle debris impact from tornadic winds, but some effort had to be made to assure that secondary structures likewise could manage it. In particular, switchgear and connections from the diesels to the safety buses.

[meatloaf]

Utilities have had to build dry storage facilities here due to the government reneging on providing long term storage such as Yucca Mountain.

[gandalftb]

Please apply your Rensselaer Nuclear Engineering degree to these thoughts:

Cold shut down means core temps under 212F. Unit 1 core is still at 392F, with the steam picking up an additional 84 degF from the exposed core above the water level. A water-zirconium reaction producing free hydrogen generally occurs above 500F so we should not see anymore hydrogen detonations, paticularly with the nitrogen injection.

But, after weeks of water injection the top of the rods in 1,2 and 3 are still exposed by 4-5 feet. 1 and 2 are still cooking at over 300F. Getting down to under 212F may take much more than 6-9 months.

[gandalftb]

Our fuel tanks are in concrete bunkers and truck mounted backup generators are offsite. At Cluster Fukushima, the diesel generator fuel tanks were mounted on open concrete pads and the truck mounted backup generators parked next to them.

Here comes the big wash and adios power to the pumps. Amazing incompetence by their safety engineers whom were also warned years ago to raise the sea wall 3 meters and never did.

[meatloaf]

The NRC would not allow that in the United States.

[SteamShovel]

I knew about the dry storage. I’m talking about wet storage. For years I thought I had heard about expanding wet storage thinking it was in a building other than the reactor building (I’m a turbine engineer, not a Rx engineer, so I’ve never been near containment).

So I must have been mistaken. I’m now thinking it is near impossble to move a bundle that still requires wet storage from the Rx building pool to another pool outside the Rx building. You’d have to build some sort of underwater transport channel or water cooled and lead shielded transport vehicle.

[gandalftb]

It was done at Comanche Peak not only for the main generators, but also for the diesel fire pumps I helped design.

[sefarkas]

Please apply your Rensselaer Nuclear Engineering degree to these thoughts:
Cold shut down means ...

You ought to look it up before you type of the top of your head. Cold shutdown at a BWR is MODE-4: less than 200DEGF and reactor vessel head bolts fully tentioned (see Table 1.1-1 in NUREG-1434 V1 (“In MODE 4, decay heat is low enough for the RHR System to provide adequate cooling, and reactor pressure is low enough that the overpressure limit is unlikely to be approached by assumed operational transients or accidents. In MODE 5, the reactor vessel head is unbolted or removed and the reactor is at atmospheric pressure. The S/RV function is not needed during these conditions. ”) See NUREG-1434 V2.

Try your hand in a Thermodynamics Class someday, then you will start to understand that the system has reached an equilibrium given the decay heat rate and the ability of the various systems and methods to accept the heat being produced. The decay heat rate exponentially decreases naturally. The engineers can add heat removal capacity (e.g., run both trains of RHR if electric power supply permits) to achieve cold shutdown sooner.

[gandalftb]

Read my post, I said "under 212F", last I checked, 200F is under 212F. Quibbling about a cold shutdown at either 212F or 200F is irrelevant when the temps of 1 & 2 are still 300F+ weeks later and 5+ feet of the top of the fuel rods are still exposed. Obviously cold shutdown means less than boiling temps.

In my Thermodynamics Class you don't have cooling equilibrium if the core is still venting steam which 1 & 2 are. Obviously they know they are a long way from stasis if they still feel the need to inject nitrogen to interfere with zirconium oxidation that produces dangerous hydrogen.

If they could add more "heat removal capacity" they'd be doing it now.

[bvw]

I always win against arrogant degree-boasting a**holes and I just did again. Thanks for playing.

[bvw]

It all comes down to decay heat management, which as we have seen with Fukushima is exactly the cause of the problems they face.

Decay heat management, containing the residual radioactivity, and since 9/11 it should have been clear -- but evidently was not -- preventing the breach of the pit by a single missile or explosive. I would not call the pit a "secondary containment" area, given how easy it is to breach. Did all the air in that outer shell of a steel building at the Tepco plant go through filters, go was it open to atmosphere?

True, a pit at the top of vessel is needed during fuel cycling, but leaving much fuel for any length of time there is bad, and the reason -- the ENGINEERING reason, for good engineers learn from mistakes -- because when that pit is breached the whole of the plant -- the primary containment and the major plumbing, controls and power supplies are made very difficult to impossible to access.

In general on-site spent fuel has to be better secured.

[bvw]

That kind of man is a danger to all around him in a real crisis. Maybe a good designer, but an absolute ruination WTSHF.

[chimera]

My guess is that there will probably be some procedural changes as to how much fuel can be in the SPF at any one time. Unit 4 had a full core offload, which adds significantly to the heat load, as well as some older fuel. There will probably be some rule making to the effect that older fuel should be removed to the common storage pool, for those facilities that have such features.

It is not clear to me at this point if the Unit 4 SPF has been significantly damaged. There was some concern that there was fuel damage resulting from overheating when they lost the SPF cooling capabilities, or perhaps from debris falling onto the fuel from the damage to the roof and overhead crane when they had their hydrogen detonation. The pictures I've seen taken from above show a lot of debris and it is difficult to get a clear look. Until there is a close-up inspection it is all guesswork.

[bvw]

It’s clear that the pits and the medium term storage of the spent fuel could use some major re-engineering to reduce event risks (1) during re-fueling and (2) during regular operations.

[chimera]

What do you suggest? From everything I have seen so far, the root cause of this event is the loss of emergency power. Lose that, and you lose heat removal capacity. That leads to cladding damage, hydrogen evolution and detonation, which everything else flows from.

Until we know the damage to the unit 4 SPF, we likely cannot offer anything specific at this point. We know there was likely boil-off of the coolant inventory, but again that relates to loss of onsite power. If there was damage to the SFP from the earthquake itself, then some strengthening is indicated. Some have suggested further redundancy in the cooling systems but in this case, without onsite AC, those won't do you any more good than the existing systems.

I don't think there are any generic issues related to the elevated SFP design used in these BWRs. In fact, it has some significant advantages in terms of ALARA implementation during refueling. Procedurally, yes, you could address some of the concerns by limiting SPF inventory during a full-core offload.