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

Okawa, Tsuyoshi; Gi Hong, Ser*; Greenspan, E.*

The feasibility of designing a 125 MWth 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 0.5%. Six molten-salts are considered: NaF(57)-BeF2(43), 7LiF(66)-BeF2(34), LiF(46.5)-NaF(11.5)-KF(42), NaF(50)-ZrF4 (50), LiF(42)-NaF(29)-ZrF4(29) and LiF(43)-RbF(57). Only the first three are considered for the neutronic analysis. Six structural materials are considered: SS304, Hastelloy-N, HT-9, Mn-316SS, PCA, and SiC. 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. Its drawbacks include a relatively large positive coolant temperature coefficient of reactivity and a relatively small negative Doppler coefficient. Of the structural materials considered, SiC gives the largest p/d ratio, lowest plutonium loading, flattest power distribution and largest reactivity worth of the control elements. Hastelloy-N is the worst structural material; it requires the smallest p/d and gives the largest peak-to-average power density and smallest reactivity worth of the control elements.