When one wants to simply estimate tritium breeding ratio (TBR), the TBR may be reduced from a "local" TBR for the breeding zones of a blanket module by multiplying the breeder coverage (= the surface area of effective breeding region / the surface area of the first wall around plasma). When blanket modules are arranged, the gap between neighboring modules and the frames of the modules are regarded as non-breeding zones. On the other hand, neutrons scattered in the non-breeding zones can enter breeding zones, contributing to tritium production. This means that the estimation method mentioned above tends to underestimate TBR. In order to assess the scattering effect quantitatively, we carried out a three-dimensional Monte Carlo N-particle transport MCNP-5 calculation. It was found from the calculation that there is little decrease in TBR for gaps less than 4 cm when the blanket thickness is 70 cm. The result indicates that such a wide allowance of the gap will facilitate access of remote handling equipment for the replacement of blanket modules and improve access of diagnostics.
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