Hi Saul, Another thing I realized I wished I had added as a caveat in last night's post. There is also the issue of fast versus slow neutrons as triggers to fission. The role of the moderator is to slow neutrons so that they can be captured by materials that need slow neutrons to fission, of which U238 is one. If you have fast-fissionable material in the reactor, then the moderator isn't needed, and losing it doesn't help as a passive safety measure for those reactions. I believe that P239, created in breeder reactors, is of the latter type. I don't know details of fuel mixes in modern reactors, and how the roles of slow and fast neutrons are balanced. I expect that is in the public domain.
I also don't know what fraction (if any) of reactors in Japan are breeders. The power/waste ratio of breeders is much higher than pure-U238 reactors, but because P239 is bomb material, the security problems of breeders cause them to be received very differently in different countries. I believe France runs a high fraction of breeders (something like 70% many years ago when I checked?), while the U.S. currently uses none. If the reactors in discussion in Japan have a high fraction of fast-neutron fissionable material, some of what I said last night may be irrelevant. Certainly, the lack of Carbon as a moderator/explosion hazard will still be a help, but Nick has done some research, and finds that there can be other complicated reactions involving the non-fissionable materials on or in fuel rods, which can create other burn or explosion hazards. It would indeed be interesting to know the full range of technologies in use, and what design elements to ensure "passive" reaction quenching, versus needs for active interventions that require the installation to be functioning well, are in play. Thanks, Eric ============================================================ FRIAM Applied Complexity Group listserv Meets Fridays 9a-11:30 at cafe at St. John's College lectures, archives, unsubscribe, maps at http://www.friam.org
