The Mudcat Café TM
Thread #59418 Message #3984961
Posted By: Rapparee
28-Mar-19 - 08:23 PM
Thread Name: BS: The Mother of all BS threads
Subject: RE: BS: The Mother of all BS threads
As I noted earlier, the process of assembling the supercritical mass must occur in significantly less time than the average interval between spontaneous fissions to have a reasonable chance of succeeding. This problem is difficult to accomplish due to the very large change in reactivity required in going from a subcritical state to a supercritical one. The time required to raise the value of k from 1 to the maximum value of 2 or so is called the reactivity insertion time, or simply insertion time.
It is further complicated by the problem of subcritical neutron multiplication. If a subcritical mass has a k value of 0.9, then a neutron present in the mass will (on average) create a chain reaction that dies out in an average of 10 generations. If the mass is very close to critical, say k=0.99, then each spontaneous fission neutron will create a chain that lasts 100 generations. This persistence of neutrons in subcritical masses further reduces the time window for assembly, and requires that the reactivity of the mass be increased from a value of less than 0.9 to a value of 2 or so within that window.
Do not try this at home. Leave it to the professionals.