2.15.3.1 Fuel Rod Design

2.15.3.1.1 Basic structural design

In LWRs and FBRs, a number of fuel rods are formed into a fuel assembly. The fuel rod is a barrier (con­tainment) for fission products; it has a circular cross­section that is suited for withstanding the primary pressure stress due to the external pressure of the coolant and the increase in internal pressure by fission gas release. An axial stack of cylindrical fuel pellets is encased in a cladding tube, both ends of which are welded shut with plugs. A gas plenum is located at the top part of the rod, in most cases, to form a free space volume that can accommodate internal gas. Helium gas fills the free space at atmospheric pressure or at a given pressure. A hold-down spring, located in the gas plenum, maintains the fuel stack in place during shipment and handling. UO2 insulator pellets are inserted at both ends of the fuel stack, in some fuel designs, to thermally isolate metallic parts such as the end plug and the hold-down spring.

2.15.3.1.2 Fuel rods for LWRs

Table 2 summarizes LWR fuel rod design speci­fications.30 LWR UO2 fuel rods contain dense low-enrichment UO2 pellets in a zirconium alloy cladding; they are operated at a low linear heat rate with centerline temperatures normally below 1400 °C. The fuel pellets of the VVER have a small central hole (1.2-1.4mm in diameter).

Fission gas release is low under these conditions and no large gas plenum is needed. Burnable absorber fuel rods containing UO2-Gd2O3 pellets are located in some part of the fuel assemblies of LWRs to flatten reactivity change throughout the reactor oper­ation cycle.

Great efforts have been made in LWR fuel rod design in order to achieve the following good perfor­mance features: high burn-up, long operation cycle, good economy, and high reliability. Toward achieving these ends, many modifications have been made, such as the development of high-density UO2 pellets, axial blankets for reducing neutron leakage, ZrB2 integral burnable absorber, high Gd content UO2- Gd2O3 pellets, corrosion-resistant cladding materials, and optimization of helium pressure and plenum length in the rod designs.

LWR MOX fuel rods contain MOX pellets that have a low plutonium content. As the plutonium concentration is low, their irradiation behavior is similar to that of LWR UO2 fuel rods. No additional

aPartial length rod.

Mitsubishi developed alloy.

Source: Tarlton, S., Ed. Nucl. Eng. Int. 2008, 53, 26-36.

problems are apparent, with the possible exception of higher gas release and therefore an increase in rod internal pressure at high burn-up. Power degra­dation with burn-up is less in the MOX fuel than in UO2 fuel because of the neutronic properties of the plutonium isotopes and thus MOX fuel is irra­diated at higher power later in its life, releasing more fission gases. In addition, the slightly lower thermal conductivity of MOX may give rise to higher fuel temperatures, resulting in higher fission gas release. Design changes, such as lowering the helium filling pressure, increasing the plenum volume, and/or decreasing the fuel stack length in the rod, are applied to accommodate higher gas release in MOX fuel rods.