Gamma Ray Signatures For Identifying Plutonium Content Changes In MSRs

Liquid-fueled Molten Salt Reactors (MSRs) have gained significant interest globally due to the potential improvements they offer compared to current light water reactors in terms of safety margins, fuel utilization, and economics. While MSRs are a promising reactor concept, many technical challenges remain, including those facing nuclear material accounting for nuclear safeguards. Online processing of the nuclear fuel-moderator mixture introduces pathways for fissile material diversions and the liquid, semi-homogenous nature of the molten salt fuel precludes any ability to count discrete items or fuel elements, such as rods or assemblies. Thus, safeguarding of MSRs will require the development of MSR-specific fissile material diversion detection strategies. To address the need for identification of diversion signatures and methods to reliably and quantitatively measure them, a 2-D Serpent model of the Molten Salt Demonstration Reactor (MSDR) was developed to estimate isotopic changes based on different operational or diversion scenarios. Serpent has recently added the capability to model continuous feed and removal of nuclides, as is required for the analysis of most MSR designs. The model allows the examination of the bulk fuel salt, materials in the reprocessing streams, and in the waste streams under various conditions. For this work, the model was used to simulate the effects of various Pu concentrations in the salt and their effect on isotopic inventories. As a preliminary examination of possible diversion signatures, GADRAS was used to quantify variations in the observed gamma ray spectral lines in response to the varying isotopic inventories. These results have revealed potential isotopes of interest that correlate with changes in Plutonium content within the fuel, such as 106Tc and 156Pm. Further exploration of this method may identify signatures relevant to material accounting and safeguards for MSRs. This work will present simulated results for potential gamma ray signatures associated with changes in plutonium content and for different fuel streams.