The molecular diffusion model [1, 2] was developed within RELAP5-3D [3, 4] in 2005. Its objective was to implement the capability of calculating the ingress of air into a particular Very Hight Temperature gas-cooled Reactor (VHTR) by molecular diffusion following an accident, such as a large break in the primary coolant system, resulting in a complete loss of flow and depressurization. The existing coding was verified and validated [4]. However, the implementation was confined to only two different gases for the primary fluid, water vapor and helium. It has become of interest to implement other gases as the primary coolant, primarily for modeling designs for new Small Modular Reactors (SMR) and microreactors. This project adds the capability to specify, through use input, a variety of noncondensable (NC) gases as the primary coolant.

The approach for molecular diffusion modeling has two parts. The first part solves the diffusion equation at each time step to calculate the various components of gas added or subtracted from each RELAP5-3D control volume due to molecular diffusion during the time step. The second part modifies the RELAP5-3D field equations with the molecular diffusion calculations so that the overall fluid behavior modeling accounts for molecular diffusion. The theory, algorithm, coding implementation and verification are given, and some numerical validation and calculational results are given.

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