PhD Defense | Federico Grimaldi | Neutron Data Benchmarking at the VENUS-F Zero Power Reactor for MYRRHA

Neutron Data Benchmarking at the VENUS-F Zero Power Reactor for MYRRHA 

Research reactors are vital assets for providing experimental data needed to validate reactor physics codes and nuclear data evaluations. The VENUS-F zero-power fast reactor, operated at the Belgian Nuclear Research Centre (SCK CEN), contributes to the development of lead-cooled systems such as MYRRHA. A systematic review of past VENUS-F critical cores (2011–2022) revealed consistent calculation-to-experiment discrepancies in criticality (k_eff) and threshold-to-non-threshold fission rate ratios (threshold spectral indices).

To investigate these discrepancies, three experimental campaigns were conducted between 2024 and 2025 using a tailored fast core configuration (CC14). The CoRREx campaign analyzed the influence of the epithermal spectrum, finding that epithermal flux adjustments have a limited impact on the under-prediction of threshold spectral indices. The PERLA campaign confirmed high experimental repeatability and reproducibility within propagated uncertainties. The VALUE campaign introduced new observables, including the first ²³⁷Np/²³⁵U spectral index measurements at VENUS-F. The results showed significant sensitivity to different ²³⁷Np evaluations, leaving open whether underestimations stem specifically from ²³⁷Np measurements or broader fast-spectrum modeling. In the radial reflector, under-prediction of experimental values occurred across all evaluated libraries for ²³⁸U(n,f)/²³⁵U(n,f) and ²³⁷Np(n,f)/²³⁵U(n,f). The calculation of ²³⁷Np(n,f)/²³⁵U(n,f) far from the active core was in better agreement with the experiments than for ²³⁸U(n,f)/²³⁵U(n,f), highlighting ²³⁷Np(n,f)/²³⁵U(n,f) as an observable of great potential.

Nuclear data uncertainty propagation highlighted ²³⁸U and ²³⁵U as the primary uncertainty contributors for k_eff and spectral indices. Strong spatial and threshold-dependent correlation patterns were identified; core-to-core k_eff correlations exceeding 99% are reported. Transposition analysis demonstrated the impact of benchmark bias and uncertainty when highly correlated experiments are considered for assimilation. Overall, these findings offer valuable insights into the performance of current nuclear data libraries and codes, directly influencing the JEFF-4.0 evaluation (feedback on ²³⁸U). Beyond criticality, this work demonstrates the role of integral spectral index measurements in refining nuclear data pipelines and guiding experimental programs for next-generation fast reactors.

Promoter(s):

Pierre-Etienne Labeau, Université libre de Bruxelles

Patrick Blaise, FRAMATOME

SCK CEN mentor(s):

Antonin Krása