The Fukushima disaster (part 2)

I ended part one of this story by describing the destruction caused by the hydrogen explosions in the reactor buildings, and how this must complicate efforts to get the cooling systems operational. I just found this status report on the BBC News site:

  • Reactor 1: Was first to be rocked by an explosion on Saturday; fuel rods reportedly 70% damaged
  • Reactor 2: There are fears a blast on Tuesday breached a containment system; fuel rods reportedly 33% damaged
  • Reactor 3: Explosion on Monday; smoke or steam seen rising on Wednesday; damage to roof and possibly also to a containment system
  • Reactor 4: Hit by a major blaze (possible blast) on Tuesday and another fire on Wednesday

According to the BBC, all four reactor buildings have experienced explosions or fires or both. There are two more reactor buildings on site (total of six), but the other two were offline at the time of the earthquake. These two are also reported to have elevated fuel rod temperatures, but like many of the reports, this is difficult to evaluate — who is measuring these temperatures, and with what instruments?

Desperate measures

Most of us have seen the video of the helicopters attempting to dump water into the spent fuel rod storage pool, located on the upper level of a reactor building. This brings up another dangerous situation, which I haven’t addressed yet. The Fukushima plant stores ”spent” fuel rods in water tanks on the upper floors of the reactor buildings, as well as some in a common pool. The number of these rods dwarfs the number of rods in the actual reactors — over 600,000 rods are stored on site. Each rod consists of around 5kg of uranium oxide pellets, sealed in a zirconium tube — that’s about 3 million kilograms of hot radioactive material outside of the reactor vessels, that must also be submerged in continuously circulating water. Needless to say, the pumps responsible for this circulation are not functioning, either. One of the serious flaws in this reactor design, that has been heavily criticized in the past, is the fact that these storage pools and their support systems are not nearly as robust as the main reactor systems. They are not “hardened” against natural disasters or protected by multiple redundancies. In the case of  Fukushima 1 through 4, it is entirely possible, perhaps even likely, that some or all of these systems were irreparably damaged by the hydrogen explosions. The fact that you have a hole in the roof of the containment building large enough to attempt a helicopter water drop is a pretty good indication that the interior is seriously messed up. And when the official spokesman is reduced to expressing hope that there might possibly be some water left in the storage pool, I think we can assume that if it is not dry now, it is going to be very soon.

What prompted the managers to attempt such low-probability stunts as helicopter drops and water cannons (which would seem to have the same chance of success as trying to fill your neighbor’s backyard pool by blindly tossing water balloons over the fence)? Well, they probably felt they had to do something, considering the alternative. Left to themselves, the spent rods would eventually boil off the remaining water, becoming exposed to air, at which time the temperature would increase enormously. The zirconium casings might even catch on fire (it was speculated that this may have been the source of the fire in building 4); eventually the fuel would melt through the tank and drop through to the lower levels of the building (where it would continue to do damage), all the while releasing high levels of radioactivity, preventing workers from approaching the area and essentially forestalling any mitigation measures. All things considered, squirting water from a distance may have been the only viable option.

What happens next?

The latest reports state that TEPCO workers have succeeded in bringing grid power to Building 2. This is great news, but it is not in itself a solution. The suppression chamber at reactor 2 is reported to have been breached; if true, this will need to be repaired before water flow can be restored to the reactor. Then there is the question of the building utilities — considering all the various types of damage that have been inflicted, starting the pumps is not going to be a simple matter of flipping a switch or resetting breakers. Circuits and pipelines will need to be tested for integrity, computers will need to be rebooted, safety systems reset or even overridden. In the other buildings, which have gross visible structural damage, this task will be even more difficult. One encouraging sign is that the situation has not become obviously worse over the last two days — there have been no new explosions or columns of smoke, or sudden spikes in radiation (although the effective capacity for monitoring radiation at the site has been questioned). My guess is that right now the outcome is still in the balance — they may be starting to get a grip on events, but there is still potential for matters to spiral out of control on several fronts.

Meanwhile, I would like to send a deep bow in the direction of the heroic workers.

Related stories:

Japan steps up moves to cool stricken nuclear reactors

Fukushima Coverup: 40 Years of Spent Nuclear Rods Blown Sky High

Nuclear fuel (Wikipedia)

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