Critical and Subcritical Source-Driven Transients Analysis of the VENUS-F reactor

Introduction

Predicting the dynamic behavior of nuclear reactors during operational changes or transients is crucial for reactor safety and operation optimization. While transients in power reactors are highly coupled and involve multiphysics phenomena, the problem significantly simplifies in zero-power reactors where temperature feedback is negligible and the system behavior is governed by reactor kinetics.

Reactor kinetics models predict how neutron flux responds to changes in core reactivity (Hetrick David L, 1971). This thesis aims to perform transient analysis of the VENUS-D reactor using a reactor kinetics model capable of simulating the reactor response to arbitrary reactivity insertion. As a first step, available point kinetics solvers are tested, compared, improved and validated on benchmark cases. The updated model will provide a robust tool to investigate transient phenomena and help optimize operational procedures.

The VENUS-F zero-power fast reactor serves both for model validation through the analysis of existing operational data and to analyze operational postulated transients to optimize them. The analysis extends not only for critical transients, but also for subcritical operation. This will allow studying VENUS-F transients with the start-up source inserted and potentially predict future coupled operation with an accelerator. Accurate modeling of source-driven subcritical operation requires the integration of external source terms into the kinetics model in a physically meaningful way, which constitutes substantial part of the thesis.

Objectives

The primary objective of this thesis is to analyze VENUS-F reactor transients using a reliable kinetics model capable of predicting the reactor neutron flux response to arbitrary reactivity perturbations: first, by testing the model on benchmark cases, and second, by verifying its applicability directly against VENUS-F operational and experimental data. This analysis of VENUS-F transients forms the core of the thesis.

 In order to extend the model to subcritical transients with the external source inserted, the actual VENUS-F source must be modeled. The effective source intensity will be deduced from dedicated experiments. Finally, the reactor kinetics model will be used to optimize control rod operational procedures and to assess the impact of key system features during subcritical transients. Throughout this process, the applicability and limitations of point kinetics for the analyzed cases must be assessed to identify situations where more advanced spatial kinetics models would be required.

Workflow

The project will follow a sequence of consecutive steps:

• Develop a flexible framework for reactor transient analysis using available numerical and analytical point kinetics solvers. These solvers will be tested and compared against benchmarked cases to verify their mathematical reliability and select the most suitable options for the thesis.
• Apply the model to VENUS-F critical transients to predict neutron flux evolution during operational or experimental procedures. This serves as the validation stage against direct VENUS-F data.
• Extend the analysis to subcritical transients by modeling the actual VENUS-F external source. This step involves analyzing data from dedicated experiments to deduce the effective source intensity, which will then be integrated as an external neutron source term into the kinetics model.
• Apply the model to subcritical operations using recent VENUS-F subcritical data and reactor startup procedures to verify whether a point kinetics model can accurately represent subcritical phenomena.
• Simulate potential critical and subcritical operational scenarios. For critical operations, control rod movements during power stabilization will be analyzed as operational transients, while period measurements will be simulated for reactivity evaluations. For subcritical operations, the model will be used to investigate the stabilization time as a function of the reactivity insertion shape, and to quantify the impact of the source at different power levels to determine when it becomes negligible.

Reference

Hetrick David L. (1971). Dynamics of nuclear reactors . University of Chicago press.