The Oil Drum's editor summarizes crisis to date and the potential outcomes.

[snagglepuss]

Euan Mearns, editor of TOD (The Oil Drum) has PhDs in Geology. This lengthy piece is very informative. As TOD is listed as Creative Commons.

A 'interesting' observation is that The reactor cores are more likely being destroyed by a super-fast corrosion process of steam acting on zircaloy and chloride acting on stainless steel.




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I am not a nuclear engineer and all should be wary of information reported by non-experts on the internet. Here I try to assemble information (not facts) gleaned from TheOilDrum comments, our email list, as well as mainstream media and news reports, in an attempt to cast light on status and potential outcomes. This involves considerable speculation.


On Friday 11th March, the day the earthquake and tsunami struck the west coast of Japan, reactors 1, 2, and 3 at the Fukushima Dai-ichi site were successfully shutdown by inserting control rods to shut down the nuclear fission chain reaction. Reactor 4 was non-operational at the time of the event, its spent fuel rods being stored in a cooling pond outside of the reactor containment system. The chart shows that heat flow in a reactor drops extremely quickly upon shutdown but that after the passage of days to weeks (where we are now) the rate of heat decay slows. Things are still very hot and need to be cooled. The source of the heat is radioactive decay of the fission products, many of which have very short half lives and decay away in the first minutes to hours, but the longer lived isotopes go on decaying and emitting heat and radiation for days, weeks and years.


There is therefore a world of difference in terms of energy that has to be contained between an operational reactor experiencing difficulties, as was the case with Three Mile Island, and reactors that have been shutdown, as is the case in Fukushima.


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Oil Drum commenter donshan left this comment on my Safety of nuclear power and death of the nuclear renaissance thread which I believe may provide an accurate picture of what is actually going on in the reactor cores:


I think I can answer this if I am correct that the Japanese reactors use conventional zirconium ( Zircaloy) fuel cladding with ceramic uranium oxide fuel pellets inside. I understand that Unit 3 has a mixed oxide pellet including plutonium oxide.

In 1956, my first job as a materials scientist was at the AEC's Hanford Laboratory in Washington State, operated by General Electric. Over 8 years I conducted many laboratory scale high-pressure autoclave experiments on the properties of zirconium alloys in high temperature and pressure water and steam. These tests were classified "secret" back then to prevent our technology from being obtained by the Soviets. Sometimes I fear that even though all this science is now declassified, this early science has not made into the education of today's engineers. I retired in 1995 and have followed TOD for 3 years now, having also worked on natural gas pipeline and geothermal system corrosion, but now feel I have expertise to share on this topic.

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But these fuel rods must NEVER be overheated. That is why multiple redundant cooling systems are required. All these backup-cooling systems failed in Japan. Even after reactor shutdown, if the fuel rods are uncovered cladding temperatures can rapidly rise to 800C or higher, due to fission product decay heat.

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This loss of the last battery-powered cooling, led to the fuel rods becoming uncovered in a manner similar to that in the Three Mile Island accident (although due to different reasons). When overheated in steam, the oxidation reaction above accelerates exponentially. As the zirconium oxidizes, the coating thickens, cracks, and turns white from internal fractures that increase the diffusion rate of steam to the metal. It then has the look and mechanical properties of eggshells. Hydrogen from this process is released, but is also absorbed by the underlying metal cladding, which causes embrittlement and metal fracture. Soon cracks form in the cladding, releasing the trapped fission products inside. This is not "melting', but rather catastrophic disintegration of the cladding structural integrity and containment of fission products. If the process continues, the cladding can fracture away, exposing the fuel pellets, which in the worst-case scenario can drop out and collect on the bottom of the reactor vessel. It is the worse case scenario that I believe is causing the Japanese to inject boric acid. Boron is a neutron absorber and will prevent any possibility of a pile of fuel pellets on the bottom of the vessel from going critical and restarting the chain reaction.

These reactors are now a total loss, but I am still disturbed by their inability to bring in portable diesel generators and restart the back-up cooling. I guess the chaos of the catastrophe is the cause.

I do question the use of seawater cooling. I hope the Japanese have considered the danger they have created by introducing oxygenated seawater into this stainless steel piping and pressure vessel at boiling temperatures. These stainless steels are extremely susceptible to chloride stress corrosion cracking:
http://www.tpub.com/content/doe/h1015v1/css/h1015v1_134.htm

Since residual weld stresses and tensile stress in piping, valves, control tubing, etc. are always present, Standard Operating Reactor water quality standards require keeping chlorides at parts per billion levels. Seawater has about 3.5% or 35 grams per liter of salinity!!!

I have no way of knowing how many days they have before a stainless steel component suddenly cracks, but if it were me, I would be advocating an emergency program to get pure deionzied cooling water back into this stainless steel system ASAP. In laboratory tests in boiling chlorides, cracking of stainless in tensile stress can occur within days- they have at most a few months if they keep boiling sea water in this system and yet another disaster occurs. I am sure there are competent scientists in Japan's nuclear industry and government regulators. I hope they are on top of this threat!


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What next?


At this point, it is necessary to lurch towards pure conjecture. Day by day, the status at Fukushima has worsened and until the situation is stabilized, it is impossible to predict the final outcome.

Much will depend on the status and location of the fuel rods and pellets in the reactor cores. If these remain largely intact and in place then they will be easier to cool and to moderate, i.e. to have the neutrons being released absorbed by boron or the control rods already in place.

If one of the cores is disintegrating and gathering as debris on the vessel floor then it becomes much more difficult to circulate cooling water and to absorb neutrons being produced. We have had much debate about whether or not it is possible for the fission chain reaction to re-start in a pile of reactor rubble. The consensus is that this is unlikely though possible. Should this happen then the energy to be contained escalates and the situation becomes more critical. Colleagues Joules Burn and Engineer Poet suggest that restarting the fission chain reaction would be self destroying since the energy produced would blow apart the pile of rubble, shutting down the fission process immediately.

The possibility remains that an explosion (hydrogen gas?) or fire (burning what?) destroys one of the containment vessels, rendering the site uninhabitable, in which case the fate of the other reactors would be left to nature. Fire in particular could spread high levels of radiation over a substantial area. Stuart Staniford at earlywarn.blogspot has produced this picture of what a Chernobyl scale disaster could mean for Japan.







http://www.theoildrum.com/node/7675

[Hawkowl]

1. Highly recommend

The link to the entire article is highly informative.