Three 0.95-mm$^2$ miniature fiber-coupled scintillators have been used to perform cm-wise resolution measurements of the thermal neutron flux within experimental channels of the SUR-100 facility, a zero power thermal reactor operated by the Institute of Nuclear Technology and Energy Systems at the University of Stuttgart. The detection system is developed at the École polytechnique fédérale de Lausanne in collaboration with the Paul Scherrer Institut.
Reaction rate distributions are measured along the experimental channels I and II of SUR-100, which cross the reactor at the center and tangentially to the core, respectively. The results for the experimental channel I are compared to neutron activation measurements. In addition, reaction rate gradients across the 2.6 cm and 5.4 cm diameters of the channels are measured. The reactor was modelled with the neutron transport code Serpent-2.1.31, and the collected experimental data are compared with Monte Carlo simulations.
The comparison of experimental and computed reaction rate distributions showed a good agreement within the core region, with discrepancies within 2$\sigma$. An unexpected discrepancy, probably caused by a geometric inconsistency in the computational model of the reactor, was observed in the reflector region of the experimental channel I, where a 20$\%$ difference (i.e. 8$\sigma$) was found between experimental and simulated results. Significant discrepancies, respectively worth 10$\sigma$ and 15$\sigma$, were noticed at distance, in the lead shielding region, for both experimental channels I and II.
An horizontal reaction rate gradient of (9.09 $\pm$ 0.20)$\%$ was measured within 2.4 cm across the diameter of the experimental channel II, with a difference from computed results of 2$\%$. The absence of a vertical reaction rate gradient inside the experimental channel I was confirmed by measurements.
The performed measurements highlighted the suitability of the miniature fiber-coupled scintillators for high resolution reaction rate distribution measurements and for the characterization of highly localized gradients in reaction rate. The negligible flux perturbation induced by the miniature fiber-coupled detectors has been found to be a great advantage compared to the previously used manganese activation techniques. In the case of the activation technique, the presence of the samples holder caused a maximum 35$\%$ increase of thermal neutron flux in the core region of experimental channel I.