Measurements of impure reactor-grade plutonium using the increase in totals neutrons from plutonium-241 decay

We have developed a new technique to obtain accurate measurements of impure plutonium mass using neutron counting. The technique is based on making two or more totals neutron measurements with a time separation that is long enough to measure the decay of 241Pu (T1/2=14.7 y) to 241Am. Because of the excellent precision for totals neutron counting, short measurement intervals are adequate and time separations as short as a few months can give useful results. The known change of the plutonium isotopics with time can be used to calculate the alpha value [(α, n) to spontaneous fission ratio]. The alpha value can be accurately calculated for pure oxide samples; however, for impure samples, the alpha value must be determined from a measurement such as multiplicity counting or the totals neutron counting in the present technique. For bulk samples with significant neutron multiplication, the neutron coincidence (doubles) count is needed to calculate the 240Pu-effective mass. However, for waste samples such as 200-l drums or other very dilute plutonium loadings, the multiplication is unity and the plutonium mass can be calculated directly from the two totals counts without the requirement for coincidence counting. For many present and future applications of reactor-grade plutonium verification, impure plutonium is in a storage condition as opposed to active chemical processing. In these storage conditions, the plutonium samples routinely receive repeat measurements over time for the purposes of accountability and international inspection. This storage of plutonium facilitates the present dual measurement approach. This paper gives the concept of the new technique as well as an evaluation of the accuracy for different types of plutonium.