Neutronics experiment on a helium cooled pebble bed (HCPB) breeder blanket mock-up

A neutronics experiment has been performed in the frame of European Fusion Technology Program on a mock-up of the EU Test Blanket Module (TBM), HCPB concept, with the objective to validate the capability of nuclear data to predict nuclear responses, such as the tritium production rate (TPR), with qualified uncertainties. In the experiment, the TPR has been measured using Li2CO3 pellets located at various depths at two symmetrical positions at each depth, one in the upper and one in the lower breeder cassette. Three independent measurements were performed by ENEA, TUD/VKTA and JAEA. The neutron flux in the beryllium layer was measured as well using activation foils.The measured tritium production in the TBM (E) was compared with the same quantity (C) calculated by the MCNP-4C code using a very detailed model of the experimental set up. A sensitivity and uncertainty analysis has also been performed to evaluate the calculation uncertainty due to the uncertainty on neutron cross sections. The results of such analysis, carried out using both Monte Carlo and deterministic codes, shows that the uncertainty due to cross sections amounts to 4–5% (2σ). C/E ratios were obtained with a total uncertainty on the C/E comparison of 9–10% (2σ). The C/E comparison shows that the calculation of the tritium production is underestimated by about 10% in average and is within the combined 2σ uncertainty margin of the measurements and the calculations. The obtained results indicate that design calculations of the tritium breeding ratio provide conservative results for the fusion reactor employing a HCPB type breeder blanket.