There's only on the order of 1 mg of Cf-252 in the source (my estimate) -- enough for the order of 10^9 neutrons/second. It's not a meaningful amount of fissile

There's only on the order of 1 mg of Cf-252 in the source (my estimate) -- enough for the order of 10^9 neutrons/second. It's not a meaningful amount of fissile material. By weapons-grade they mean the uranium in the neutron multiplier is >90% U-235. Cf-252: http://www.nndc.bnl.gov/chart/reCenter.jsp?z=98&n=154 http://www.nndc.bnl.gov/chart/reCenter.jsp?z=98&n=154