The Role of Geometry on Alpha-radiation Modelling of Spent Nuclear Fuel/water Interfaces - 22153

In both interim wet storage and geological disposal facilities (GDF), fractured spent fuel has the potential to be exposed to water. Loss of containment of nuclear waste in interim wet storage can cause sludge build-up within the fuel pond. Examples of this can be found in the K-basin and Sellafield legacy site sludges. Over much larger time scales groundwater could also pose a risk to spent fuel inside GDFs. When building a safety case for these scenarios, radiolysis rates and localized concentration of oxidizing species is key in understanding the dissolution of fuel and hydrogen build-up in the surrounding water. To do this, predictive tools are needed to produce dose rates and dose rate distributions in multiple geometries, as a function of fuel type and age. A predictive model to determine the dose rate of both planar and spherical alpha-emitting surfaces has now been developed. The approach presented is a computationally efficient mathematical model using stopping range data from the Stopping Ranges of Ions in Matter (SRIM) software. Investigating the influence of geometry on alpha dosimetry has shown significant effects for smaller radii, showing clear differences in dose rate curves below 50 mm. These considerations are essential for any accurate prediction of the dissolution rate and hydrogen gas release, driven by the radiolytic yields of particulate spent nuclear fuel. The alpha dose rates as a function of distance from irradiated UO₂ spent fuel surfaces were produced for benchmarking with previous modelling attempts. This method can replicate Monte Carlo (MCNPX) studies of an irradiated UO₂ fuel surface within 0.7 % of the resulting total dose rate and displays an equivalent dose profile while dramatically reducing the number of calculations required. This has led to the development of the open-source Alpha Dose Rate Calculator (ADRC) modelling tool and web application, Rad-dose. (authors)