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The values listed for helium buildup in HasteHoy N are based on an initial ‘OB concentration of 2 atom ppm (about 2 .wt ppm of natural boron). Since the boron reaction is essentially a thermal-neutron reaction, the helium from this process was unaffected by the assumed shape of the delayed-neutron energy spectrum. On the other hand, the ”Ni(n, a) reaction has a high energy threshold, so two values are listed. The first or lower value ‘corresponds to the neutron emission spectrum that went to zero at about 5 MeV, while the higher value was obtained with the spectrum that extended to 10 MeV. Since all of the helium values from ”Ni are quite low, the only potentially significant difference produced by the shape of the delayedneutron spectrum is in the reactor outlet line. The results in Table 2.2 consider only the internal neutron sources - delayed fission neutrons plus those produced by fssions within the components. One calculation was performed to estimate the effect of core-leakage neutrons on the rate of helium production. The maximum amount of helium produced in the outer shell of the heat exchanger in 30 equivalent full-power years (EFPY) was about 500 ppb, 80% from ”Ni(n, a) reactions. Thus helium production from core-leakage neutrons may completely overshadow, at least locally, the production from delayed neutrons, unless appropriate shielding is provided to reduce the fast-neutron flux from this source. The activation processes in the coolant salt were found to be essentially independent of both the high-energy end of the delayed-neutron source spectrum and the incidence of core-leakage neutrons. The 24Na calculation is applicable only to the NaBF4 coolant, and the value listed is the total steady-state inventory for the 100@MW(e) MSBR reference design. The production rates of tritium in the various coolant salts are all much smaller than the rate of production in the fuel salt (2400 Ci/day). In the case of coolant containing natural lithium, however, the production in the coolant is not negligible. The damage rate in Hastelloy N by atom displacement was calculated for the highest flux regions in both the reactor outlet line and the heat exchanger with NaBF4 coolant. Since the neutron flux spectrum outside the core is quite different from any fission-reactor spectrum, the rate of accumulation of such damage was expected to be different also. Consequently, atom displacement rates were computed using effective “damage cross sections” evaluated by Jenkins.’ These cross sections were obtained by explicit evaluation of the energy transferred to primary recoil atoms and the Kinchin-Pease model for secondary atom displacements.

In the calculations, the effect of the assumed delayedneutron specthm was examined for the heat exchanger; however, none of the calculations included the effects of core-leakage neutrons. The effect of the delayedneutron source spectrum was appreciable in terms of the Ni(n, a) reaction rate, but not significant in terms of overall helium production. However, all of the flux spectra in the out-of-core components were substantially “softer” than a typical test-reactor flux spectrum (e.g., that of HFIR). Consequently, the rates of atom displacement per unit flux (E> 100 keV) were lower by factors of 1.2 to 1.3 than those that would be encountered in a reactor spectrum with the same total flux above 100 keV. Thus, the total number of metal-atom displacements produced in these components by operation of an MSBR for 30 EFPY could be produced by irradiations to 2 to 4 X 10’ nvt (E > 100 keV) in a test reactor like HFIR. Although the total number of atom displacements is not an entirely accurate measure of metal damage (since it completely ignores annealing effects), the low equivalent HFIR fluences suggest that damage by this process is probably not of major concern for MSR components. 2.2.2


MSBE Nuclear Characteristics J. R. Engel L

Perturbation calculations were added to a twodimensional, nine-group diffusion calculation for the core of the reference concept MSBE12 to provide estimates of some of the reactivity coefficients of this reactor. Although these calculations are subject to refinement when a detailed core design is evolved, they serve to illustrate the kinds of values that may be expected. The results are summarized in Table 2.3, along with previously published results for the singlefluid MSBR.13 A prominent difference between the MSBE values and the comparable quantities for the larger MSBR is in the reactivity effect of fuel-salt density. In the MSBE the density coefficient is positive, so that voids decrease reactivity (-0.1% 6k/k for 1% voids in the salt). This effect appears to be a consequence of the higher neutron leakage from the physically smaller MSBE core. Also significant is the 11. J. D. Jenkins, RICE: A Program to Calculate Primary Recoil Atom Spectra from ENDFIB Data, 0RNL.-TM-2706 (Feb. 14,1970), 12. J. R. McWherter, Molten-Salt Breeder Experiment Design Bases, ORNGTM-3177 (November 1970). 1 3 MSR Program Semiannu. Rogr. Kep. Feb. 28, 1970, ORNL-4548, pp. 63-64.