Physics analysis of the LS-VHTR: Salt coolant and fuel block design

The Liquid-Salt-Cooled Very High Temperature Reactor (LS-VHTR), also known as the Advanced High Temperature Reactor (AHTR), is a new, large [>2400 MW(t)], passively safe, high-temperature reactor concept. It uses a graphite-matrix coated-particle fuel and a graphite moderator similar to the fuel used in modular high-temperature gas-cooled reactors, but with a clean liquid-fluoride salt coolant. The neutronics properties of various salt coolant options are considered with respect to their coefficients of reactivity for various reactor configurations. In addition, several variations on the basic graphite block design of the AHTR are considered that would simplify refueling. The results show that the coolant coefficients of reactivity are negative or very small relative to other reactivity feedbacks, such as the fuel Doppler feedback. This allows several salt-coolants, even some with a positive coolant density coefficient, to be considered for use in the AHTR. In addition, parametric studies of assembly-type clustered rod configurations show that there is minimal impact on the reactivity coefficients and multiplication factors with appropriate cluster design choices. (authors)