Answer to Question #984 Submitted to "Ask the Experts"

Q

What is the radiation dose rate in rems h^-1 at 1 meter for 5 kg of Pu-SWG (surplus weapons-grade plutonium) (97.2% ²³⁹Pu 2.8% ²⁴⁰Pu)?

A

This is a very complex and difficult question to answer, for hidden within this apparently simple and straightforward question are a number of complexities, some subtle and some not so subtle. Not the least of these is that a 5 kg lump of ²³⁹Pu is likely greater than a minimum critical mass! The geometry is also important from the standpoint of self attenuation and from determination of the specific uncollided neutron and photon flux. Another important consideration is air attenuation or any other shielding material, since it is the low-energy L x rays that are the primary photons emitted by ²³⁹Pu and ²⁴⁰Pu, and these are readily absorbed within the mass of the plutonium as well as by the air. A similar problem exists with respect to the neutron dose calculation; the neutron dose will devolve not only from spontaneous fission neutrons but from alpha, neutron reactions within the mass of the plutonium. So in summary, more specifics need to be known before a reasoned calculation (and a very difficult one at that) can be attempted. What can be said is that under most conditions, the dose rate would likely not exceed a few tens or hundreds of mrem h^-1. Ron Kathren, CHP