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Japan Nuclear Watch, May 19: Critical Safety Vent Failures Could Happen Here Too

7:19 am in Uncategorized by Scarecrow

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

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The New York Times’ coverage of the ongoing nuclear plant catastrophe in Japan has been generally excellent, thanks largely to an experienced reporting team familiar with plant safety issues. In the second part of an expanded series this week, they reported on the utility owner TEPCO’s findings concerning the sequence of failures.

As we’ve explained in previous Japan Watch posts, that sequence led inexorably to near total meltdown of the fuel core inside the reactor vessels at Units 1-3. (All of Unit 4′s fuel was in the storage pool, which presents problems.) TEPCO now concedes these meltdowns were likely total. Though the details varied by unit, the common sequence started with the loss of both grid power from the earthquake and backup power when the tsunami flooded the backup diesel generators. See video/photos. Extended TEPCO videos here via the Times.

That was soon followed within hours or a day or two by failures of the last resort cooling system. That system circulates pressurized steam from the reactor into the suppression pool (“torus”). But it also requires battery powered controls/pumps to help circulate cooled water back into the reactor. When the batteries ran out or the pumps failed (one did), all designed cooling systems were helpless, making melting of the fuel rods inside reactor Units 1-3 virtually inevitable.

Although earlier expert analyses and TEPCO announcements suggested only partial meltdowns and varying levels of water covering parts of each reactor’s core, it now appears the cores were completely uncovered for some period early on –within the first hours or days — resulting in near total meltdown at each unit. That means the melted fuel is now puddled at the bottom of each reactor vessel, though it is thought (because of low temperatures) to be covered with whatever cooling water can be injected into each reactor vessel.

Earlier this week, they realized Unit 1′s vessel has one or more holes, leaks, allowing water and possibly melted fuel to leak out of the vessel onto the containment structure floor. The reports vary, but they suggest that tons of water, which they’ve been injecting into the core in desperate efforts to keep the core covered, have now found its way into the reactor building’s basement. Water from other units has leaked from other places, including adjacent turbine buildings and some has reached the ocean.

The meltdowns would have been bad enough. But the extreme conditions were also separating hydrogen from water and thus causing a buildup of hydrogen gas inside the reactors, which then accumulated first in the containment structure and possibly other parts of the buildings. As the pressure builds inside the reactor, it diminishes the ability to inject water into the reactor, especially using weaker ad hoc injection measures. There is a last ditch safety valve to release this pressure directly from the reactor vessel, which must be used at the right moment, both to relieve the pressure and prevent an explosion should the gas ignite.
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Japan Nuclear Watch, April 23: Can You Rebuild a Cooling System Inside a No-Go Zone?

8:53 am in Uncategorized by Scarecrow

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

Japan Nuke Watch, Sat Nite (JST): Power to Site, Radiation in Food

5:38 am in Uncategorized by Scarecrow

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

It is Saturday morning here; Saturday night in Japan.

Some important developments that provide rays of hope and areas of concern. Concerns first.

First, some local produce has become contaminated with radiation. The New York Times reports authorities finding unacceptable levels of radiation in food in Fukushima:

The government said on Saturday that they had found levels of radioactive materials above safe limits in spinach and milk in Fukushima and Ibaraki prefectures, the first confirmation by officials that the nuclear catastrophe unfolding at power plants nearby has affected the nation’s food supply.

Yukio Edano, the chief cabinet secretary, said that the radioactivity contained in the average amount of spinach and milk consumed during an entire year would be equal to the amount received in a single CAT scan. Mr. Edano said that abnormal amounts of radioactivity were found only in these two products, though other foods were tested.

It was not known how much affected milk and spinach had already been shipped.

Subsequent reports indicate the government has banned the sale of produce grown in the Fukushima prefecture. They are still maintaining their 20 kilometer evacuation zone with warnings to stay indoors out to 30 km from the site.

[Update: Kyodo News reports findings of slightly higher levels of radioactive iodine in water samples in Tokyo and above legal limits in tap water at Fukushima prefecture.]

Second, they’re about to test the new electrical connection from the grid. TEPCO reports they have completed installing the new power connection from the grid to the site and then on to Units 1 and 2. They’re also working to extend to Units 3 and 4, though it’s unclear whether that implicates radiation levels near those two units.

As I suspected, not only did they need a new line from the grid; they needed to bypass much of the original interconnection points on the site that link to each of the Units and other buildings. These interconnection points were vulnerable to the tsunami and may be unusable. Recall that the back-up diesel generators used whenever grid power is lost were located in a low elevation building closer to the sea and were inundated by the tsunami.

Authorities say they now have power connections to the station, but have not tested the electrical components. They’ll need to make sure the interconnections to the individual Units are sound first, then carefully test connected electrical equipment, control panels and switching. Only then will we know whether the pumps and other critical cooling equipment that require electric power are still operable. Those tests are supposed to occur on Sunday.

Third, they’re bringing in the robots! If you watch the periodic reports on the NHK tv feed, they show an unmanned fire truck with water cannon shooting water into Unit 3. The truck is connected by a hose 800 meters long so they can constantly feed water into the unmanned truck-cannon, rather than expose workers with manned hoses. (Authories also report they are bringing in emergency fire crews from other regions to relieve those who have already reached their exposure limits.)

The remotely supplied water cannon allowed several hours (seven?) of continuous spraying on Saturday, and officials claim they sent 1260 tons of water aimed at Unit 3′s spent fuel storage pool. They also claim to have slightly lowered the temperature in the pool to below boiling, but it’s not clear whether they’ve confirmed this.

In addition, American and other western nations are sending in robots that can perform certain tasks in nuclear emergencies, including dragging fire hoses. From the NYT (h/t 4cdave):

At the request of the Japanese military, a Massachusetts company, iRobot, said it put four robots on a plane for Japan on Friday. Colin Angle, the chief executive, said it had sent two small robots that could measure radiation levels close to the reactors and two larger ones that could pull hoses to spray water on the fuel rods. He said Japanese soldiers could operate the robots from a protected vehicle.

Fourth, they’ve restarted another generator at Unit 5. This appears to be a generator on site, not power from the new power line extensions from the grid. The added generation has allowed them to resume more water pumping into Units 5 and 6 storage pools.

Both reactors were down form maintenance when the quake/tsunami struck, but they both have fuel in their reactor cores (per NYT Unit summaries) and more in their spent fuel storage pools. The generators can help keep the units cooled, so those units are now at significantly lower risk.

More: Could Unit 4 Storage Pool be leaking? US authorities have argued that the loss of cooling water in Unit 4′s spent fuel storage pool could be at least partially explained by a leak or damage to the storage pool walls or the “gate” that opens to allow transfer of fuel rod assemblies between the pool and the reactor vessel.

Union of Concerned Scientists has an explanation of one plausible reason for a leak in the “gate.” The gist is, the gate is normally sealed by an air pressurized seal driven by an electric air pump. The pump gets its power from the grid. Lose that and the air pump can’t seal the gate to ensure against leakage of water from the pool. As UCS notes, this scenario actually occurred at a US reactor, the Hatch facility in Georgia in 1986. It’s worth reading for the clear explanation, diagrams and actual pictures from other BWR plants.

More updates as we get them, and I want to thank our commentors who continue to bring in reports and updates. Thanks especially to 4cdave and others who maintained the late night vigils.