UC Berkeley

Molten salt reactors (MSRs) are a class of nuclear reactor which uses a molten ionic liquid

as either the coolant or also as the fuel. While a 8 MWth MSR was successfully operated

in the 1960s it was not until the early 2000s that MSRs gained widespread attention. Since

then MSRs have enjoyed plentiful research support. Despite such support only one general

MSR fuel cycle analysis tool is available for use by the research community and even this

this tool lacks features necessary for modelling a MSR, and so possibly providing answers of

a lower quality.

In this work a method is proposed and implemented within the SERPENT 2 reactor

physics Monte-Carlo code. This method, named ADER - the Advanced Depletion Extension

for Reprocessing - is a seamlessly incorporated source code modification to the SERPENT

2 base code which allows the user to define arbitrary collections of elements, isotopes, and

chemicals as well as relationships among them.

Furthermore ADER allows the user to specify a variety of mass flows subject to the

constraints as defined through the collections of elements, isotopes, and chemicals the user

has structured. Along with support for constraints involving both corrosion and nuclear

control concerns, ADER allows the user to optimize the solution against a quantity of interest

- e.g, total uranium fed into the system.

Through these structures much of the complex chemistry, corrosion modelling, and nuclear concerns of operating a MSR can be linearized and solved against an optimization

target, which is necessary given the large number of constraints and variables in such a

problem space. Linearization reduces the problem complexity and eliminates concerns over

local versus global optimization targets. Within the typically narrow operating parameters

of MSRs linearization is an appropriate approximation to the higher dimensional equations

representing the phenomenon involved.

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This linear system and optimization target may then be passed to a linear optimization

solver, 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

uses this solution to create a brand new depletion matrix which SERPENT 2 then solves

using the CRAM approximation method.

From this algorithm a more accurate modelling of MSR fuel cycles and physics may

be arrived at through the consideration of chemistry driven limitations, corrosion driven

limitations, nuclear driven limitations, and operator driven limitations. Results from this

implemented method indicate that ADER drives the MSR fuel cycle simulations towards

a more physically representative result. Unfortunately, as detailed later in this work, an

underlying 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

adoption of a quadruple-precision floating-point linear optimization library over the current

implementation of CLP as used in ADER.

In the following chapters an introduction to MSRs and their fuel cycle modelling is

given. Following this the theory behind ADER and its implementation within SERPENT 2

is discussed after which the results from one of the less numerically unstable simulations is

presented after which concluding remarks are given.