Criticality effect of neutron channeling between boron carbide granules in boral for a spent-fuel shipping cask

Commercial fixed-neutron poison materials typically consist of a mixture of a neutron absorber such as boron in an aluminum or steel matrix. The neutron absorber is not evenly distributed (homogenized) within the matrix if the boron is in the form of boron carbide particles as is the case for Boral, which can allow neutrons to pass through the Boral between the boron carbide granules without being attenuated. This integranular channeling effect increases the transmission of neutrons through a Boral sheet and reduces the effectiveness for criticality control or neutron shielding. The channeling of neutrons between neutron absorber particles in fixed neutron poison materials can reduce the criticality control effectiveness of the material. This effect must be included in shipping cask licensing calculations and typically could result in a change in k/sub eff/ of 1 or 2%. The intergranular channeling effect is caused by a particulate form of the neutron absorber and would be less significant for poison materials consisting of alloys of steel or aluminum with a boron or gadolinium neutron absorber. Such alloys could be used in future cask designs because of their contribution to the structural strength of a fuel basket and their relative insensitivity to intergranular channelingmore » effects.« less