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Effect of Aggregate Specification in Heavy Concrete for Fusion Reactor Shield on Neutron Dose Evaluation

Heavy concrete is one of the most powerful, reliable and inexpensive radiation shield materials. It can be applied to a fusion reactor as an effective and strong shield. However, heavy concrete inherently has heterogeneous structure due to the absorber aggregates which can cause heterogeneity effect on the shielding performance. In the present study, the aggregate-specification dependent heterogeneity effect in neutron dose evaluation has been studied for D-T neutron field using measured leakage neutron spectra from heavy concrete samples with D-T neutrons and the analysis results conducted with the Monte Carlo code MCNP-CFP. From the results, the heterogeneity effect of heavy concrete could be observed in the evaluation of neutron dose equivalent. The effect was reproduced accurately by the MCNP-CFP calculation. The ratio of dose for heterogeneous heavy concrete to that of homogeneous one was mostly less than unity, and the effect became large with increase of the packing fraction, shield thickness and aggregate diameter. The reason was that the spatial distribution of the aggregates usually tends to be biased towards the center of the shield. In such a case, MCNP-CFP becomes a powerful tool to give the exact shielding performance.