Experimental Determination of the Ratio of ²³⁸U Capture to ²³⁵U Fission in LEU-HTR Pebble-Bed Configurations

The shift toward low-enrichment uranium (LEU) fuel for gas-cooled high-temperature reactors (HTRs) has revealed a lack of experimental data for validating neutronics codes that are used for the design and licensing of such systems. In the framework of the LEU-HTR experimental program at the PROTEUS critical facility, the safety-related effects of accidental moderation increase (ingress of water or other hydrogeneous compounds) in pebble-bed HTR core configurations employing low-enriched (16.7%) fuel were investigated. An important neutron balance component in this context is the integral reaction rate ratio of ²³⁸U capture (C₈) relative to ²³⁵U fission (F₅).

It was necessary to develop new experimental techniques for the accurate measurement of C₈/F₅ in the doubly heterogeneous fuel pebbles. These have involved the utilization of specially prepared particle foils on the one hand and the counting of whole fuel pebbles on the other. Core-center measurements employing both experimental methods have been carried out in two different HTR-PROTEUS configurations (with and without accidental moderation increase simulation, respectively). In each case, satisfactory agreement was obtained between the experimental results based on the two techniques. By carrying out a comparison of particle-foil C₈/F₅ measurements in the PROTEUS reactor's thermal column with the results of standard foil-activation measurement techniques, the systematic uncertainty (1) of the core-center measurements could be reduced by ~0.6%, yielding a net experimental error of ±1% with either of the new methods. A comparison of the experimental results with calculations based on the MICROX-2/TWODANT codes in conjunction with JEF-1 cross sections has indicated that this calculational route overpredicts the core-center C₈/F₅ value by ~2.5% in both the investigated configurations.