Neutronics study on optimizing the US helium-cooled ceramic breeder (HCCB) test blanket module for deployment in ITER

The U.S. Helium-cooled Ceramic Breeder (HCCB) Test blanket module (TBM) planned to be placed in ITER test port is undergoing an optimization process to improve its neutronics performance. A 2 mm thick beryllium layer is used as a plasma facing material on the FW area subjected to 0.78 MW/m2 neutron wall load. It is desirable to maximize tritium production rate, TPR with a minimum use of Be, while tailoring nuclear heating rates in the toroidal, poloidal and radial direction to ensure that the breeder is operating within its temperature window and with adequate design margins. Since heating rate is largely dependent on the rate of tritium production through the 6Li(n,α) reactions which produce ∝4 MeV per reaction, we started to explore how to vary 6Li enrichment such that TPR are maximized in regions where the coolant temperature is low and minimize the enrichment where the coolant temperature is high. With this strategy, one can have more or less even heat production distribution which maximizes the heat extracted from the TBM while ensuring adequate TPR to satisfy tritium self sufficiency criterion. To achieve this goal, our first parametric round of analysis was to launch several 1-D calculations in which the 6Li enrichment was varied across the TBM in the radial direction with various possible combinations to determine the optimal 6Li enrichment variation that ensure both high TPR and at the same time smooth transition in heating rate profiles between regions. This initial calculation showed that we can reduce heat load in zones whose boundary coolant temperature is high by letting the 6Li enrichment in the radial direction to be 20%-30%-50%-90% respectively (i.e. low enrichment at the front and high enrichment at the back of the breeder zone). On the other hand, heat generation in zones characterized to be ''cold regions'', we let 6Li enrichment to be varied as 90%-70%-50% and 30% (i.e. high enrichment at the front and low enrichment at the back of the breeder zone). It is therefore clear that varying Li-6 enrichment should not only be mapped in the radial direction, but also should be varied in the toroidal as well as in the poloidal direction for maximum optimization. To achieve this goal, 3-D calculations with MCNP Monte Carlo code have been started to find out the optimal 6Li distribution throughout the TBM that achieve our design goal. The results of this lengthy design process are discussed in this manuscript. (orig.)