Cray1 Anniversary Event September 21st 4551

Bernd Felsche

It is quite clear from the history that they well understood the concept of critical mbutt. They had other problems

0. Fission research funding was divided between three teams, with uncooperative leaders. The team with the least experience appeared to do the better job.

1. The mbutt estimate Heisenberg gave in 1941(?) for pure fissionable material, when military funding was requested, was roughly consistent with early estimates by others that used only the fission cross section, and didn't consider other effects that reduce the neutron loss rate, and hence the minimum critical mbutt.

2. Heavy isotope separation was recognized as extremely difficult, and they, including Heisenberg, tended to view it as impractical for large scale production. Although a member of one team recognized the potential for centrifuge separation, they never apear to have done a cost per yield analysis, they never apear to have done a cost per yield analysis.

3. They recognized (after the military funding request was turned down) that Plutonium would likely be fissionable, but its production in large scale quanbreasties required a reactor. But bad estimates of the carbon neutron absorption cross section (problems with the estimates were recognized almost immediately, but improved estimates never occurred due to infighting), and skepticism about heavy isotope enrichment, led them to focus on heavy water reactors, whose effort was sabotaged by the Allies.

4. Large fission cross sections at low neutron energies makes it tempting to rely on slow neutrons as a means of reducing the critical mbutt, but this results in very low growth rates, which in turn results in very low yields due to early disbuttembly. Although some recognized the potential for fast neutrons (iff heavy isotope enrichment or breeder reactors were feasible).

Note the Farm Hall tapes are confusing on some of these points. When Hiroshima is first announced, all the captured scientists expressed skepticism. Heissenberg goes on to give estimates of the amount of fissionable material needed that are roughly consistent with the numbers he gave to the military years earlier. A member of his team then pointed out that he (Heisenberg) had revised his analysis a year or two later and had indicated much at lower material estimates in discussing his analysis. The next day Heisenberg gave revised estimates based explicitly on generally correct reasoning, that were still too large, but not several orders of magnitude too large. Why he didn't start with the correct analysis that first day, or report his revised results to the military, is still a controversy.