Predicting Fuel Salt Composition via Linear Optimization in Molten Salt Reactors

Molten salt reactors (MSRs) are a class of nuclear reactor which uses a molten ionic liquid/nas either the coolant or also as the fuel. While a 8 MWth MSR was successfully operated/nin the 1960s it was not until the early 2000s that MSRs gained widespread attention. Since/nthen MSRs have enjoyed plentiful research support. Despite such support only one general/nMSR fuel cycle analysis tool is available for use by the research community and even this/nthis tool lacks features necessary for modelling a MSR, and so possibly providing answers of/na lower quality./nIn this work a method is proposed and implemented within the SERPENT 2 reactor/nphysics Monte-Carlo code. This method, named ADER - the Advanced Depletion Extension/nfor Reprocessing - is a seamlessly incorporated source code modification to the SERPENT/n2 base code which allows the user to define arbitrary collections of elements, isotopes, and/nchemicals as well as relationships among them./nFurthermore ADER allows the user to specify a variety of mass flows subject to the/nconstraints as defined through the collections of elements, isotopes, and chemicals the user/nhas structured. Along with support for constraints involving both corrosion and nuclear/ncontrol concerns, ADER allows the user to optimize the solution against a quantity of interest/n- e.g, total uranium fed into the system./nThrough these structures much of the complex chemistry, corrosion modelling, and nuclear concerns of operating a MSR can be linearized and solved against an optimization/ntarget, which is necessary given the large number of constraints and variables in such a/nproblem space. Linearization reduces the problem complexity and eliminates concerns over/nlocal versus global optimization targets. Within the typically narrow operating parameters/nof MSRs linearization is an appropriate approximation to the higher dimensional equations/nrepresenting the phenomenon involved./n2/nThis linear system and optimization target may then be passed to a linear optimization/nsolver, in this work the CLP library as part of the COIN-OR package, from which an optimized system material composition and material flows solution may be found. ADER then/nuses this solution to create a brand new depletion matrix which SERPENT 2 then solves/nusing the CRAM approximation method./nFrom this algorithm a more accurate modelling of MSR fuel cycles and physics may/nbe arrived at through the consideration of chemistry driven limitations, corrosion driven/nlimitations, nuclear driven limitations, and operator driven limitations. Results from this/nimplemented method indicate that ADER drives the MSR fuel cycle simulations towards/na more physically representative result. Unfortunately, as detailed later in this work, an/nunderlying and pernicious numerical instability issue was uncovered within the linear optimization library selected for this work. Any future work on this method must begin with the/nadoption of a quadruple-precision floating-point linear optimization library over the current/nimplementation of CLP as used in ADER./nIn the following chapters an introduction to MSRs and their fuel cycle modelling is/ngiven. Following this the theory behind ADER and its implementation within SERPENT 2/nis discussed after which the results from one of the less numerically unstable simulations is/npresented after which concluding remarks are given.

Advisor: Fratoni, Massimiliano