To improve the nuclear data testing and validation of advanced reactors, such as pebble-bed high-temperature gas-cooled reactors, molten salt reactors, and heat pipe microreactors, a conceptual design for a novel critical assembly using a horizontal split table (HST) was developed jointly by Oak Ridge National Laboratory and Lawrence Livermore National Laboratory. The mechanical design is led by Lawrence Livermore National Laboratory, whereas the neutronics considerations are led by Oak Ridge National Laboratory. The characteristics of the designed HST and the benefits of performing such an experiment to the community are included. As a proof of concept, a proposed critical experiment using tristructural isotropic fuel particles and a graphite moderator/reflector is described, mimicking a pebble-bed-type advanced reactor based on the HTR-10. A critical configuration corresponding to a footprint of about 4.5 m2 was determined with SCALE/KENO-VI to fit the planned dimensions of the HST. The similarity of the pebble-bed design and the HTR-10 reactor application was assessed using SCALE/TSUNAMI, and a similarity coefficient, ck, of 0.9982 was obtained, proving that the concept will be useful for cold-critical validation and for nuclear data validation and assimilation of pebble-bed-type advanced reactors.

In the proposed design, the materials with the highest keff sensitivity are graphite and uranium, which demonstrates that particular care must be given to carbon-related cross-section data. A cross-section library study was performed to test the influence of the different recent releases of the ENDF/B cross-section library on the concept’s keff. The effect of mechanical uncertainties between the fixed and moving tables was also assessed by calculating the reactivity change caused by vertical and horizontal gaps, as well as angular and torsion offsets between the two sides of the HST concept. As a last analysis step, the performed nuclear data assimilation of the hypothetical experiment showed that uncertainties can be reduced by several hundred pcm. The same analysis process is currently being used to create a molten salt advanced reactor–type HST concept based on the Molten Salt Reactor Experiment.