Cross-section sensitivity of tritium breeding in a fusion reactor blanket: effects of uncertainties in cross-sections of 6Li, 7Li, and 93Nb

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Abstract
The authors have examined the cross-section sensitivity of the tritium-breeding ratio in a fusion reactor blanket model which is representative of a large class of blanket configurations under current investigation. This blanket model exhibits the following tritium-breeding and neutronic characteristics: (1) liquid lithium of natural isotopic abundance is used for breeding; (2) the tritium-breeding ratio is in the "high" range, ~ 1. 5; (3) graphite is employed for neutron moderation; and (4) the neutron leakage from the blanket is relatively low, ~4% of the source neutrons. The sensitivity of the breeding ratio was evaluated by altering reference cross-section sets to reflect estimates of the uncertainties in the cross-section of the 6Li(n, α)t, the 7Li(n, n'α)t, and the 93Nb(n,2n) reactions. Our analysis leads to the following observations: The status of the 6Li(n,α)t cross-section data appears adequate for accurate (of the order of ~1%) tritium-breeding calculations. We emphasize that this observation applies to the class of blanket configurations considered here and that other classes must be examined to determine the generality of the observation. Uncertainties in the cross-section and secondary-neutron energy distribution of the 7Li(n, n'α)t reaction attach uncertainties in excess of 5% to the tritium-breeding ratio. A re-evaluation of the relevant data in the energy range above ~ 10 MeV might provide a smaller uncertainty, and we recommend that such a re-evaluation be undertaken. Additional measurements may be required to reduce the uncertainty in the tritium-breeding ratio to the order of 1 to 2%. Uncertainties in the (n, 2n) cross-section of 93Nb attach uncertainties of ~ 2.4% to the tritium-breeding ratio. New experimental data on the 93Nb(n, 2n) cross-section are available, and a recent evaluation of this data suggests that the variations adopted here may reflect too pessimistic an estimate of the uncertainty. This point will be pursued in future studies When the cross-sections of the 6Li(n, α)t, the 7Li(n, n'α) t, and the 93Nb(n, 2n) reactions are varied simultaneously, the calculated uncertainty in the tritium-breeding ratio is ~ 7 to 8%. This uncertainty approaches 10% when uncertainties in the secondary-neutron energy distribution of the 7Li(n, n'α)t reaction are included.

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