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OCR for page 109
Appendix D Use of a Nuclear Poison to Inhibit Nuclear Criticality Careful evaluation of the behavior of Molten Salt Reactor Experiment (MSRE) salts during remelting operations suggests that the risk of a critical excursion is very low. However, it is desirable to perform neutron criticality calculations to confirm this conclusion by modeling the salt system in real terms. Without introduction of a moderator such as water, there is no criticality hazard associated with the solidified salt system at present. A small possibility exists for aggregation of uranium compounds such as uranium trifluoride (UF3) during melting or fluorination. The addition of an excellent nuclear poison on top of the salt should provide additional insurance that criticality cannot be achieved during melting. If this is deemed necessary, the pane! advocates consideration of gadolinium trifluoride (G4F3) for this purpose. The advantages of G4F3 addition to the drain tanks would be the following: . It has a 49,000-barn thermal neutron capture cross section, with an additional 400-barn resonance integral. The chemical solubility of G6F3 in the LiF-BeF2-ZrF4 (lithium fluoride-beryIlium fluoride- zirconium fluoride) salt is expected to be high; additionally, the density is about 5.0, which implies that the gadolinium will descend rapidly through the molten fluid, dissolving as it falls through the liquid salt. The chemical properties of G6F3 are very similar to the properties of UF3, which would prevent segregation of poison and fissile materials. Only ~ kg of G4F3 would be needed to provide adequate protection; this material can be obtained as a fine powder that can be dispersed easily over the entire solidified cake of existing salt. D.l

OCR for page 109
D.2 AN EVALUATION OF DOE ALTERNATIVES FOR MSRE The compound G4F3 is not a listed chemical in any of the Environmental Protection Agency or supplementary state regulations (it is a natural fission product already present in the MSRE salt). Because of these desirable features, G6F3 appears to be a very useful poison for MSRE remediation purposes and does not have any known negative characteristics for this application. For the above reasons, the pane} believes that the addition of G]F3 should be considered if detailed reactivity calculations suggest the possibility of a criticality excursion due to precipitation and aggregation of a Missile species.