Neutron Measurements On Boron Carbide Added To A Plutonium Waste Form

A recent Nuclear Criticality Safety Assessment concluded that a nominal amount of neutron absorber (boron carbide) should be mixed with plutonium prior to disposition at the Waste Isolation Pilot Plant. Boron, like other light elements, interacts with alpha particles to create neutrons. Thus, adding boron carbide to plutonium as a neutron poison actually increases the neutron exposure for workers handling the materials. Multiple batches of diluted plutonium material were created with approximately one tenth the total material of an actual dilute and dispose container. These batches were measured by neutron multiplicity counting prior to dilution, after addition of the diluent without boron carbide, and with the diluent containing boron carbide. Two forms of boron carbide were identified: One form was plain boron carbide particles, and the other form was boron carbide particles coated with metallic nickel to block the alpha particles from reaching the boron. Both uncoated and nickel-coated boron carbide particles were added to separate batches of adulterated material for neutron measurements. The addition of the uncoated boron carbide increased the neutron rate by almost 33% over the material with diluent only. In comparison, the nickel-coated boron carbide increased the neutron rate by only 4%. While the addition of boron carbide to the plutonium used in these experiments was observed to increase the neutron rate, this increase is not large enough to substantially impact the precision of neutron-based non-destructive assays. Similarly, the neutron absorption of the boron was not significant enough to bias the neutron assays.