Thermal Aspects of Uranium Nitride, Mixed Oxide and Thoria Fuels as Applied to SuperCritical Water-Cooled Nuclear Reactors

Abstract

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 ― 625°C, 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 1850°C and 2) sheath temperature must not exceed the design limit of 850°C. 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.