One of the concerns associated with groundwater interacting with spent nuclear fuel waste packages in a geologic repository is the potential for criticality. A nuclear criticality event is the occurrence of a self-sustaining nuclear chain reaction in fissionable material. Groundwater that contacts a spent nuclear fuel waste package could eventually infiltrate the package and (1) act as a moderator and a reflector; (2) initiate/accelerate material degradation resulting in changes to the internal geometry; and (3) result in separation of neutron absorbers from fissionable material within the package as well as transport fissile material out of the waste package. The time frame till breach will be dependent upon several factors including repository geologic host media, seepage rate, and engineered design features. Understanding how the components in the waste package degrade is an important aspect of the criticality evaluation since the degradation products may remain in many forms, such as insoluble neutron absorbers, insoluble corrosion products that displace water (moderator), hydrated clayey materials, or solutes affecting either the solubility or the degradation rate of the internal structures and waste form. In addition, understanding the waste package effluent chemistry changes as it moves through the geologic environment is important for predicting the materials that may precipitate out of solution and accumulate in regions external to the waste package. The reactivity level and potential for criticality of spent nuclear fuel can be sensitive to the groundwater composition. For example, chlorine ions are expected to be present in a variety of groundwater compositions in geologic media under consideration for disposal of spent nuclear fuel. Sensitivity analyses are performed to evaluate the impact ionic species such as Cl- (35Cl has a moderate thermal neutron capture cross-section of ~44 barns) present in ground water or other constituents (e.g., material degradation products) could have on system reactivity.

Impacts of groundwater ionic species on spent fuel reactivity

247th National Meeting (2014)

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