Reactor Containment & Fuel Storage from UCS; (h/t commenter lobster)

The good news is that over the last two weeks or so at the Fukushima Daiichi Nuclear plant, there have been no further spectacular explosions, no new massive breaches of containment or as far as we know, massive releases of radiation, though there continue to be dangerous levels inside the reactors, in nearby water and in surrounding areas.

The bad news is the Japanese authorities have been unable to make substantial progress against the massive quantities of contaminated water still leaking from the damaged units. In the last three days, for example, they attempted to pump contaminated water out of the flooded trench outside Unit 2′s turbine building, but managed to lower the water level by only a few centimeters. In previous weeks, they would pump some out one day, but then find the water rising back the next with varying degrees of radiation, because water injected into the reactors leaked and found its way out and downhill.

Also discouraging, as they slowly and haltingly begin to take a closer look inside the reactor buildings — see TEPCO photos here — they’re discovering such massive structural damage that each day’s plans and assumptions get scrapped. They’re now on ad hoc plan #xqb and tomorrow it may be #yrz.

As lobster reported a week ago, the utility owner, TEPCO, released what it called a “roadmap” and some news media called a “blueprint” but which was neither. To me, it seems a package of optimistic goals laying out in logical sequence the problems that must be overcome, with some vague timeframes — three months, six to nine months — designed to reassure the Japanese people and themselves that there is hope things will get better over time.

I thought the most important revelation then was TEPCO’s acknowledgement that their best case recovery plan, which assumed the normal cooling systems could be restarted soon after external electrical power and controls were restored, has been abandoned. This was not suprising; it always seemed likely that critical pumps, valves, seals, meters and controls that operate those cooling systems would have been at least partially damaged by the 9.0 quake that greatly exceeded design capacity.

The only question was: how much damage had these systems sustained? And indeed, in the weeks after external power was restored to the control rooms at each unit, TEPCO admitted that various pumps and valves were too damaged to repair and would need to be replaced. It sounded like they just needed to order a few parts. That was then.

The “roadmap” documents let us know that TEPCO now realizes that if it wants something better than its current ad hoc water injections, it must completely rebuild the cooling systems, or even create an entirely new cooling system. The new system would replace the current ad hoc approach of just dumping water from above (Unit 4′s spent fuel pool) or injecting it from the outside (Units 1-3 reactors) through external hoses and pumps.

The current system is not a closed loop, and it leaks badly; they inject water one day, and it boils off or leaks out the next, requiring continuous reinjections with more and more water. And that’s just to stabilize the assumed level of fuel exposure and melting that’s already occurred in the Units 1-3 reactor vessels and Unit 4′s spent pool. The new system would presumably be a closed loop, just like the original system, so that if water boiled to steam, the steam would be captured, condensed back to water, cooled and returned to the reactor vessel for more cooling, while keeping the core covered.

That immediately raised the question, which I’ve not seen even discussed, is how do you build such a system? It would be one thing to take a never used reactor vessel outside a contaiment structure and refit it with new injection/release pipes and seals and attach those to valves, pumps, pipes, controls, etc. But how do you do that to the still hot reactor vessels inside the contaimment structure at Units 1-3 that have full radioactive cores and that have to be continuously cooled with the existing ad hoc water injection? If there’s even a conceptual design for that, I haven’t seen it mentioned.

Let’s take the problem one step further. Let’s assume that eventually they can do better than the tiny robots that have only managed to crawl a few meters into the reactor building to take photos and measure radiation. They’ll need much larger robots and heavy equipment to clear away the explosion debris and allow radioactive cleanup crews to make it possible to work inside for more than a few minutes. What next?

In the April 2 post where I summarized the AREVA presentation on the accident sequence, we explained a sequence in which the space between the outer containment structure and the reactor vessel inside that containment could gradually fill with radioactive steam and hydrogen. Some of that steam could condense and leave water inside the containment structure. As we explained elsewhere, water is also probably leaking into the containment structure from, for example, damaged seals for the piping that carries water/steam to/from the reactor vessel, making it difficult to keep the core covered, because the leak points are below the top of the fuel rods.

About the same time, the New York Times had a report describing other concerns by US experts. One of those concerns, buried at the end of the story, was that the ad hoc measures to pump outside water into the system would gradually fill up the containment structure, which was not designed to hold lots of water. Water is heavy. The weight/force of that water on the structure, coupled with continuing afterquakes, might eventually cause the containment structure to fail.

Now it seems that warning is being taken seriously. According to this report, TEPCO is now worried about how much water the containment structure at Unit 1 can safely hold if there is another significant afterquake. And if that’s a problem at Unit 1, it could become a problem at Units 2 and 3.

Is excessive water injection also a concern at Unit 4? Apparently so. This NHK World report says TEPCO is now worried that the pace of water dumping into Unit 4′s spent fuel storage pool — and remember, that exposed pool has a full core load of non-spent fuel — could make the structure supporting the storage pool, and thence its walls and steel lining, vulnerable to a serious afterquake. It’s a concern similar to that at Unit 1, except the pool is totally outside any containment structure. If the integrity of that pool, which may already be leaking, is suspect, their “roadmap” boils down to hoping Mother Nature gives them a break.

Sources:

NHK World
Kyodo News
Hi-res photos
IAEA Updates
Union of Concerned Scientists