Physical Description of LWR Fuel

Per F. Peterson

Department of Nuclear Engineering, University of California, Berkeley, CA 94720-1730

NE-161 Sample Report

Key words: LWR, BWR, fuel

Abstract

This document provides a physical description and photographs of Boiling Water Reactor (BWR) fuel.

Contents

Introduction

In light water reactors, spent fuel consists of bundles fuel rods. Each fuel rod is a zircaloy tube, welded closed at both ends and containing ceramic fuel pellets. Fuel rods are then assembled into bundles, held together by upper and lower tie plates and fuel rod spacers.

Photographs of BWR Fuel

Most light water reactors (LWR) use ceramic pellets of uranium dioxide (UO2) as fuel. In some cases plutonium dioxide (PO2) may also be mixed with the UO2 to make mixed oxide fuel (MOX) pellets. Neutron absorbing species may also be mixed in at small concentrations to provide a burnable poison (gadolinium). Figure 1 shows typical fuel pellets for a boiling water reactor. Pressurized water reactor fuel pellets are quite similar, though smaller diameter.

LWR fuel pellets are fabricated by compacting and then sintering the powdered oxides and then grinding the pellet to size. The powder is usually compacted and sintered to around 95% of its theoretical maximum density to provide void space for fission gases. The resulting ceramic pellet is similar to other ceramics, like those used for dishware and mugs.

SAMPLE EQUATION

Fig. 1 Typical BWR ceramic fuel pellets, 1cm in diameter (0.416 in). (Click image for larger photo, 11K.) Courtesy of GE Nuclear Energy.

Fuel rods are made by stacking fuel pellets inside a Zircaloy cladding tube. The tube is then evacuated, backfilled with helium to around 3 atmospheres pressure and sealed at each end by weld plugs as seen in Fig. 2. The Zircaloy cladding tube contains the gaseous fission products released from the fuel, and provides the second barrier to fission product release.

Large amounts of heat are generated in LWR fuel during reactor operation, primarily from kinetic energy deposited in a short distance by the high-energy fission products (83%). A typical BWR fuel rod generates around 44 kW/m (13.4kW/ft) of heat. Because the thermal conductivity of UO2 is relatively low, a large temperature gradient exists from the center of pellets to their surface, giving pellet surface temperatures around 650°ree;C (1200°ree;F) and centerline temperatures around 1260°ree;C (2300°ree;F). These temperatures are well below the melting temperature of UO2 2800°ree;C (5072°ree;F), but because the nonuniform temperture causes the center material to expand more that the surface material, the pellets normally crack under power. Thus in spent fuel, the pellets are usually cracked and broken.

Fig. 2 BWR fuel rods showing welded tube ends, and separate end plugs. (Click image for larger photo, 11K.) Courtesy of GE Nuclear Energy.

LWR fuel rods are assembled into square bundles (Fig. 2) using lower and upper tie plates (Fig. 4 and Fig. 5) and grid spacers (Fig. 6)

Fig. 3 BWR fuel bundle, consisting of rods, upper tie plate, lower tie plate, and spacers. (Click image for larger photo, 28K.) Courtesy of GE Nuclear Energy.

Fig. 4 BWR lower tie plate (nosepiece). (Click image for larger photo, 32K.) Courtesy of GE Nuclear Energy.

Fig. 5 BWR upper tie plate. (Click image for larger photo, 35K.) Courtesy of GE Nuclear Energy.

Fig. 6 BWR fuel rod spacers. (Click image for larger photo, 25K.) Courtesy of GE Nuclear Energy.

Conclusions

References