A methodology for mixing different waste types in an RH TRU waste shipment

Each container in the payload of transuranic-(TRU) waste shipping cask must be shown to comply with the 5% (by volume) limit on hydrogen that is converted into an allowable hydrogen gas generation rate limit. The primary mechanism for gas generation during TRU waste transportation is by radiolysis of the waste materials. The G-value defines the hydrogen gas generating potential of a material by radiolysis. Based on a bounding hydrogen G-value the decay heat of a payload container can be converted into a hydrogen gas generation rate for comparison to the limit. Payload containers of different contents, packaging, and different bounding hydrogen G values may be assembled together as a payload, provided that all containers comply with the worst-case (i.e., lowest) calculated hydrogen gas generation rate limit. This is currently the only approved mixing option available for transport of remote-handled (RH) TRU wastes. For contact-handled (CH-) TRU waste, however, a methodology has been developed and approved by the U.S. Nuclear Regulatory Commission to allow mixing of containers of different contents within a single payload. For CH TRU payload mixing, container specific limits are derived based on the characteristics of each container in the payload. An added complexity in RH TRU wastes is the presence of significant gamma (γ) activity. The γ-radiation emitted from one drum may interact with the waste materials in other drums and cause radiolytic gas generation in adjacent drums of the payload. This paper presents a methodology for mixing RH TRU containers of different contents in a single payload and accounts for potential gamma ray interactions between containers. The container hydrogen gas generation rate may be calculated through a bounding analysis for comparison to the limit. Alternatively, the container rate may be derived through detailed analyses that account for shielding materials, payload geometry, particle transport, radiation absorption, and interaction effects, and actual percentages of radiolytic gas generating materials present. The paper presents example calculations based on both bounding analyses and more detailed and realistic analyses. (authors)