Modelling small reactivity effects at the VENUS-F fast reactor

Introduction

SCK CEN is actively involved in the development of advanced nuclear systems, with a strong focus on lead-cooled fast reactor concepts. A key asset for this development is VENUS-F, one of the few operational fast-spectrum research reactors worldwide. Designed as a neutronic mock-up for heavy-metal-cooled systems, VENUS-F is a flexible, zero-power facility which flexibility enables a wide range of experiments to support reactor design and validate neutronic models.

Among the experiments conducted at VENUS-F, reactivity measurements play a central role. Reactivity quantifies the deviation from reactor criticality. It is a fundamental parameter for reactor monitoring, operation, and safety assessments. In VENUS-F, reactivity changes following system perturbations are measured. Monte Carlo neutronic codes are extensively used to model such effects, typically by computing the difference between perturbed and unperturbed reactor states. This can be computationally demanding. Perturbation theory (Bell G. & Glasstone S., 1970) offers a more efficient approach to estimate the impact of small physical changes on reactor parameters. Despite its theoretical maturity, only a few Monte Carlo codes natively implement exact perturbation theory capabilities.

Objectives

An exact perturbation route has been recently developed and implemented within a well-established Monte Carlo code (Serpent2) used for VENUS-F neutronic simulations. The first objective of this thesis is to test, validate, optimize, and ultimately achieve stable use of this new implementation. This feature is crucial for accurate evaluation of reactivity effects measured at the VENUS-F reactor.

A second objective is to apply the validated methodology to the computation of reactivity effects of various materials in the VENUS-F reactor. These results will support the design of future experimental campaigns, guide measurement strategies, and aid the interpretation of collected data. Student participation in VENUS-F experiments is considered an opportunity to gain hands-on experience that complements the computational work.

Workflow

The project begins with a thorough literature review covering reactor physics, adjoint transport theory, Monte Carlo methods for reactor analysis, and the interpretation of neutronic experiments. VENUS-F related literature is also essential to establish the necessary experimental context. (1 month)

With this theoretical basis, the computational work starts with the verification and validation of the perturbation theory implementation using selected test cases. These configurations are to be simulated with the new methodology, including proper uncertainty quantification. The convergence of the results is assessed, as well as the agreement with reference calculation results and experimental data. (4 months)

After confidence in the methodology has been achieved, the workflow moves to the application phase. Simulations are performed to quantify the reactivity impact of various samples inserted into the VENUS-F core, with each configuration evaluated using the exact perturbation method previously validated. (1 month)

Finally, the results of the reactivity effect of each sample are gathered into a comprehensive dataset. This dataset will support ongoing and future experiments at VENUS-F, guiding experimental design and aiding the measurement interpretations.

References

Bell G., & Glasstone S. (1970). Nuclear reactor theory. Van Nostrand Reinhold Company.