Influence of LOTUS Concrete Structure, Boron-Loaded Sheets, and B₄C Filter on the Integral Tritium Production of a Natural Lithium Graphite-Reflected Blanket and Comparison with Experiment

Integral tritium production rate (TPR) measurements are important in comparisons of calculations to ascertain the suitability of computer codes and cross-section sets used in calculation. At the LOTUS facility, one of the objectives is to make measurements with different types of pure fusion and hybrid blankets and compare the results with calculations. Since the concrete cavity housing the blankets is small, it is of direct relevance to determine the influence of room-reflected neutrons on the integral TPR and, if possible, to reduce this effect by special absorbers. The effects on the TPR of a stainless steel—natural lithium—graphite-reflected blanket due to the concrete structure, B₄C filter, and boron-loaded sheets covering the assembly are studied. Calculations are performed by the MCNP Monte Carlo code. Since the room-returned component depends strongly on the composition of the concrete and, moreover, does not correspond to a real blanket situation, it is advisable to compare measurements with calculations for the region where such interference is minimal. A central region measuring 30.15 × 26.25 × 60 cm³ is identified for the purpose of comparison. In addition to calculations for a fully homogenized blanket, the important central blanket region is considered in the form of rods, and the remaining blanket as a homogeneous region, to assess the effect of neutron streaming on the TPR of the assembly. An experiment is done by irradiating several Li₂CO₃ probes positioned in each tube so that the central region of interest is fully covered. The activity of the probes is measured by the standard liquid scintillation method, and the TPR for the entire region can be derived from the experimental reaction rate data. The complete details of the calculational model and the experimental procedure are provided. Good agreement is found between the calculated and experimental TPRs after accounting for various sources of errors. This suggests that the three-dimensional description of the source and the blanket arrangement employed for the calculations are quite satisfactory.