Molten salt cooled Encapsulated Nuclear Heat Source (ENHS)-like reactors

The feasibility of designing molten-salt cooled ENHS (Encapsulated Nuclear Heat Source)-like reactor cores with Pu15N-U15N nitride fuel for high temperature applications is assessed. The cores considered have uniform fuel composition and no blanket elements and solid reflectors. They are to operate for at least 20 effective full power years without refueling, without fuel shuffling and with burnup reactivity swing less than 0.52%. Three molten-fluoride-salts: NaF(57)-BeF2(43), 7LiF(66)-BeF2(34), and LiF(46.5)-NaF(11.5)-KF(42) are considered as the coolant and six materials: SS304, Hastelloy-N, HT-9, Mn-316SS, PCA, and SiC, are considered for the structures. It is found that, neutronically, ENHS-like cores can be designed for all combinations of molten-salt coolants and structural materials considered. Relative to the reference ENHS core, the molten-salt cooled cores require significantly tighter lattice, have softer neutron spectra, significantly more negative Doppler reactivity effect, much more positive coolant temperature and void reactivity effect and smaller reactivity worth of the control elements. Of the molten salts considered, LiF-NaF-KF offers the largest p/d ratio and is most suitable for natural circulation cooling.