Serious concerns remain for safety in Fukushima (Greepeace blog)

[jpak]

http://www.greenpeace.org/international/en/news/Blogs/makingwaves/serious-concerns-remain-for-safety-in-fukushi/blog/33715

Risk differences between unit 1 (which was our main concern yesterday) and unit 3 (which is our main concern as of this update):
- Unit 3 is about two thirds larger, which means that the operators have correspondingly more residual heat to deal with.
- Unit 3 contains mixed plutonium-uranium fuel (MOX), which is hotter than “conventional” uranium fuel.

The MOX fuel in unit 3 can fail more easily when heated up because the melting point is lower and there is more gas buildup with the fuel elements.

There is much less experimental data and practical experience of the behavior of MOX fuel under accident conditions compared with conventional uranium fuel.

If the fuel melts, the risk of “recriticality” or a spontaneous nuclear reaction, is higher with MOX fuel. This is the absolute worst case scenario. In accident conditions, borated cooling water is used to prevent criticality, but the presence of larger amounts of fissible plutonium reduces the effectiveness of boron. (Further reading)

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