If, as seems increasingly likely, the cooling pumps can’t be restarted in each and every block of the Fukushima Daiichi nuclear power plant, we will lose all the blocks due to release of excessive radiation from whatever block melts down first. The only option left will be to seal the power station under a huge amount of concrete, a sarcophagus like in Chernobyl.
Two weeks after the quake and tsunami hit the plant, the situation is no better than it was, if anything it has deteriorated since then, despite how it has been spun by Tepco and the government in the Japanese mass media.
On the upside, external power has been restored to inside the control rooms and limited cooling has been established via splashing water from outside and injecting water through the fire extinguisher system. Freshwater is being used for that now, after nothing but seawater had been available for two weeks. The use of will relieve worries about problems from salt buildup inside the plants (over 100 tons of sea salt are supposed to have accumulated already).
On the downside, the basements of the turbine halls under units 1 through 4 were flooded with highly radioactive water (#1: 0.4m, #2: 1m, #3: 1.5m, #4: 0.8m). On Thursday three workers were injured while trying to replace a cable in the turbine hall basement for unit 3 when they walked in the water which had not been observed the day before. It contained a staggering 3.9 million becquerels of radioactivity per cubic centimeter which is 10,000 times the usual amount inside the reactor (or 13 million times the Japanese safety level for drinking water for adults). That water is now being pumped out, but Tepco is not sure how to dispose of it.
An analysis of radioactivity in water from the basement of unit #1 showed that most of the radioactivity (1.8 million becquerel) was from cesium-137, which has a half life of 30.2 years. This is much more long lived than iodine-131 (half life: 8 days), which so far dominated tests around Japan before. Besides cesium-137 the water in unit #1 contained cesium 134 (160,000 becquerel), cesium-136 (17,000 becquerel) and iodine-131 (210 000 becquerel). The dominance of cesium-137 is a major worry: While radioactivity from iodine-131 drops off rapidly within weeks and months, pollution from cesium-137 will be dangerous for decades and centuries.
Both cesium and iodine are fission products normally contained within the uranium oxide (or uranium/plutonium oxide in the case of unit #3) of the fuel rods inside zirconium alloy tubes held inside a thick steel pressure vessel (reactor core) inside a reinforced concrete containment vessel. In the case of spent fuel rod assemblies in the storage pools there is no pressure vessel or containment.
When the fuel rods overheat the zirconium alloy will melt at temperatures over around 1800C, allowing volatile fission products to diffuse out of he oxide tablets into the pressure vessel or the storage pool.
It is not clear if the highly radioactive water in unit 1, 2, 3 and 4 came from the reactor core or from a storage pool. In the latter case, there could either be a leak in the pool (it consists of a stainless steel liner inside a reinforced concrete structure) or the pool could have overflowed during attempts to refill it so it doesn’t boil until dry.
If the water came from the reactor core it could be due to a damaged containment and reactor core or it could be due to problems with the pipes or valves connecting the reactor to the adjacent turbine hall.
Either way the leaks make the turbine hall a hostile environment for technicians trying to restore the cooling system for the damaged reactors. The reactor cores in unit 1, 2 and 3 would have to be cooled for about the next 2 years to prevent the fuel rods from melting through the reactor core. This will be next to impossible to achieve without reactivating the cooling pumps and restoring their control system.
With significant damage to the fuel rods as presumed by Tepco, any primary cycle cooling water will be loaded with dangerous fission products. Dealing with leaks of coolant or bleeding pockets of air and gas from coolant pumps as needed before resuming pumping could expose workers to life threatening doses of radiation. Under these circumstances, if any of the pumps turn out to be damaged there is little prospect of being able to replace them, even if spare parts could be manufactured and brought in.
The bottom line is that getting proper cooling working again for all cores at Fukushima Daiichi is a long shot, especially considering how long the cooling will still be required. The power station units have been damaged so badly by overheating, hydrogen explosions and sea water flooding that time is running out. The more radioactive fission products are leaked, the more difficult it becomes for humans to work and survive inside the plant. There will come a point when all of Fukushima Daiichi is a death zone that no one can enter and get out again alive.
At that point the only option is to find a way to completely and permanently seal the plant off from air and water by entombing it inside concrete like the stricken block in Chernobyl. The reason Tepco has not started doing that yet is not that they’re still trying to salvage their property: Since seawater and boron was pumped into the reactor cores on the first weekend the reactors have already been beyond rescue, as boron is a “neutron poison” and seawater is highly corrosive. At best the reactors would have to be mothballed indefinitely after that. They could never have been restarted to provide power again.
Why then is Tepco not pouring concrete over the reactors yet? Partly the answer may be that it’s like trying to dismount a tiger one is riding. While the reactor is still exposed to the outside one can still try to do things like cooling it down with fire hoses or replenishing water in the spent fuel storage pool. Once it is partly buried under concrete that becomes more and more difficult. As a result radioactivity might spike before the concrete could securely enclose the mortally wounded reactors.
A durable sarcophagus in this earthquake zone not only requires ample quantities of concrete but also steel reinforcements, which is hard to do if humans aren’t safe near the reactors. This will be no ordinary construction job by any means. The construction effort may have to involve remote controlled vehicles and other novel engineering approaches.
A primitive approach could simply bury the entire plant area under a mountain of concrete, perhaps piled up via remote-controlled trucks and bulldozers, later sealed with a skin of reinforced concrete to deal with earthquakes.
One would hope that Tepco started work on various alternative plans for increasingly severe scenarios as soon as they realized the cooling systems failed on March 11. Unless a miracle happens and all reactor cooling systems can be restored quickly, a concrete “Mt. Fukushima” probably is the only way to save Eastern Japan or all of Japan from massive radioactive pollution.
UPDATE (2011-03-28):
The lack of discussion of the concrete sarcophagus solution by Tepco suggests they are still committed to a “Three Mile Island” solution: Try to reestablish adequate cooling, wait about 5 years and then open the containment and pressure vessel to remove the radioactive mess in the core, clean up the contamination in the building. In other words, they still see the accident as a TMI-like level 5 accident and hope to keep it there, even though both the French nuclear safety authority ASN and its counterpart in Finland have rated the accident as level 6 on the INES scale.
The gradual melt-down of cores 1, 2 and 3, the uncertainty about whether the containment of block 2 is cracked or not plus the problems with the spent fuel storage pools 1, 2, 3 and 4 raise serious questions about how realistic a TMI-type recovery and cleanup is.
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