Thermal aspects of uranium nitride, mixed oxide and thoria fuels as applied to supercritical water-cooled nuclear reactors

Generation IV International Forum (GIF) design options consist of 6 reactor concepts. One concept is a SuperCritical Water-cooled nuclear Reactor (SCWR). The coolant in this option is light water heated and pressurized to supercritical pressures and temperatures, i.e., 25 MPa and 350 -- 625oC, respectively. SuperCritical Water (SCW) Nuclear Power Plants (NPPs) are beneficial, because they will have increased thermal efficiencies by 10 -- 15% compared to that of existing subcritical-water-cooled NPPs. Additionally, SCW NPPs will utilize a simplified steam circuit as they can operate with a direct cycle, eliminating the need for steam generators, steam dryers, etc. Furthermore, SCW is a single-phase fluid, which has no dryout phenomena. The objective of this paper is to demonstrate the feasibility of alternative nuclear-fuel options such as uranium nitride (UN), Mixed OXide (MOX) and Thoria (ThO2) in application to SCWRs. The latter two fuels are currently considered as alternatives to uranium dioxide (UO2) accounting for fast depleting uranium resources. Moreover, the UN fuel may be a suitable fuel choice to UO2, MOX and ThO2 due to its higher thermal conductivity, which will have significantly lower fuel centerline temperature. A generic pressure-tube-type SCWR fuel channel is analyzed with a 43-element Inconel-600 bundle filled with either UN, MOX or ThO2 fuel. A uniform Axial Heat Flux Profile (AHFP) is applied. The design constraints are: 1) fuel centerline temperature must not exceed the industry accepted limit of 1850oC and 2) sheath temperature must not exceed the design limit of 850oC. The bulk-fluid, sheath and fuel centreline temperatures and Heat Transfer Coefficient (HTC) profiles for each nuclear fuel with a uniform AHFP were calculated along the heated length of a fuel channel. (author)