Assessment of a novel fuel block and core arrangement for use in a nitrogen-cooled direct cycle high temperature gas-cooled reactor

Abstract

This paper investigates the thermal performance of a new fuel block design utilising annular compacts supported in counterbored holes through a coupled thermohydraulic and neutron transport study of a prospective High Temperature Gas cooled Reactor (HTGR) core. The paper highlights the design freedom afforded by a fuel block design which permits irregular spacings of fuel channels without impacting the heat transport to the coolant channel. The approach of optimising the moderating ratio through variable fuel spacings can flatten the radial thermal flux profile, achieving thermal peaking flux factors of less than 1.15 for cores studied in this paper. Flattening the radial thermal flux is shown to minimise variations in coolant outlet temperatures and therefore significantly reduce peak fuel temperatures in the core.

Burn-up studies of the core demonstrate the benefits of a radially optimised thermal flux profile by demonstrating insensitivity to the fuel burnup of the power profile within the core. This insensitivity results in consistent peak coolant outlet temperatures and small variations in peak fuel temperature over the course of the core life.

The paper also demonstrates the design flexibility offered by using variable diameter coolant channel displacer rods within the centre of each fuel channel to enhance heat transfer, whilst also balancing the flow distribution within the core. Specifically, the approach of utilising variable displacer rod diameters to match local coolant mass flow with fuel column power is shown to reduce peak fuel temperatures where significant power peaking factors exist.