Publication
Efforts are underway worldwide to develop advanced simulation methods of nuclear power plants, with an improved resolution in space, angle and energy. These tools aim to enhance operational efficiency through the predictions of local phenomena critical for optimizing the plant power output without degrading safety. Verification and validation procedures are essential to demonstrate the accuracy of the high-resolution information produced by these novel simulation methods; yet access to experimental data remains limited, especially regarding localized quantities such as the neutron spatial distribution within a fuel rod. The NECTAR experiments, conducted on the CROCUS zero-power reactor operated at EPFL, aim to bridge this gap by providing precise intra-pin reaction rate experimental data. The CROCUS reactor serves as an ideal testbed for validating high-fidelity simulation codes due to its relative ease of access and available space; as well as challenging core geometry. Therefore, an instrumented fuel rod for intra-pin reaction rate measurements has been designed, licensed, built and installed in CROCUS. The rod can accommodate dosimeters which can be divided post-irradiation into either 6 radial or 8 azimuthal sections, allowing to precisely measure the reaction rate spatial variations within the dosimeter. Relative radial and azimuthal intra-pin 197Au(n,g) reaction rate distributions, were measured in 3 positions within the CROCUS core lattice. Each measured reaction rate is provided with a maximum experimental uncertainty of 0.17%, which is one order of magnitude lower than existing datasets available publicly. Repeatability within the quoted experimental uncertainty was achieved for both radial and azimuthal profiles. With respect to the modeling aspects of this work, we began by developing a low-fidelity model of CROCUS using the GeN-Foam SP3 solver and an unstructured mesh description. This model's predictions were compared to existing reaction rate measurements in the core moderator, reducing discrepancies in the outer core lattice compared to an existing structured model using the nodal code PARCS. To obtain pin-resolved predictions, we developed 2 high-fidelity models: one using GeN-Foam with the discrete ordinates method and the other using MPACT with the Method Of Characteristics. Both models were verified against Serpent Monte Carlo predictions in terms of keff values, pin power maps, and inter- and intra-pin thermal neutron flux profiles. Despite discrepancies in pin power predictions for both models in the range of ±6%, attributed respectively to the definition of the cross-section homogenization regions and library issues, predictions for thermal neutron flux in the fuel were within 1% of Serpent's. Finally, predictions of the high-fidelity models were compared to the NECTAR's reaction rate measurements. GeN-Foam showed minor yet statistically significant discrepancies in terms of intra-pin radial distributions with a maximum diff