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Preliminary neutronic studies have been performed in order to provide guidelines to the design of a liquid-salt cooled Very High Temperature Reactor (LS-VHTR) using Li{sub 2}BeF{sub 4} (FLiBe) as coolant and a solid cylindrical core. The studies were done using the lattice codes (WIMS8 and DRAGON) and the linear reactivity model to estimate the core reactivity balance, fuel composition, discharge burnup, and reactivity coefficients. An evaluation of the lattice codes revealed that they give very similar accuracy as the Monte Carlo MCNP4C code for the prediction of the fuel element multiplication factor (kinf) and the double heterogeneity effect of the coated fuel particles in the graphite matrix. The loss of coolant from the LS-VHTR core following coolant voiding was found to result in a positive reactivity addition, due primarily to the removal of the strong neutron absorber Li-6. To mitigate this positive reactivity addition and its impact on reactor design (positive void reactivity coefficient), the lithium in the coolant must be enriched to greater than 99.995% in its Li-7 content. For the reference LS-VHTR considered in this work, it was found that the magnitude of the coolant void reactivity coefficient (CVRC) is quite small (less than $1 for 100% voiding). The coefficient was found to become more negative or less positive with increase in the lithium enrichment (Li-7 content). It was also observed that the coefficient is positive at the beginning of cycle and becomes more negative with increasing burnup, indicating that by using more than one fuel batch, the coefficient could be made negative at the beginning of cycle. It might, however, still be necessary at the beginning of life to design for a negative CVRC value. The study shows that this can be done by using burnable poisons (erbium is a leading candidate) or by changing the reference assembly design (channel dimensions) in order to modify the neutron spectrum. Parametric studies have been performed to attain targeted cycle length of 18 months and discharge burnup greater than 100 GWd/t with a constraint on the uranium enrichment (less than 20% to support non-proliferation goals). The results show that the required uranium enrichment and discharge burnup increase with the number of batches. The three-batch scheme is, however, impractical because the required uranium enrichment is greater than 20%. The required enrichment is smallest for the one-batch case, but its discharge burnup is smaller than the target value. Therefore, the two-batch scheme is desirable to satisfy simultaneously the target cycle length and discharge burnup. It was additionally shown that to increase the core power density to 150% of the reference core value, the required uranium enrichment is less than 20% in the single-batch scheme. This higher power density might not be achievable in the two- or three-batch schemes because the fuel enrichment would exceed 20%

Preliminary neutronic studies for the liquid-salt-cooled very hightemperature reactor (LS-VHTR).