Design of an economically optimum PWR reload core for a 36-month cycle

Abstract

The goal of making nuclear plants more economically competitive with other sources of electricity has motivated research on extended operating cycles. By increasing cycle length in currently operating PWRs, the outage frequency is reduced, and the capacity factor is improved, providing an economic benefit. On the other hand, fuel with higher enrichment is required, and the fuel fabrication costs go up. A single-batch strategy is required if the current burnup licensing limit (60 GWD/MTU) is to be maintained. Previous work has shown the technical feasibility of single-batch cycles up to 44 calendar months in PWRs. Parametric studies indicated that the economically optimum length for a PWR, single-batch core is about 36 calendar months. The goal of this thesis is to design a PWR reload core for a 36-month cycle ready for implementation in current reactors and capable of appealing to utility managers. The core design includes physics, fuel performance and economics analysis. For the neutronics study, the core is modeled in 3 dimensions and in the steady-state using the codes CASMO-3/SIMULATE-3. Several steps are considered in the design process. First, the fuel enrichment required for the cycle and the most suitable burnable absorber are selected. Then, an optimum design is obtained for the peripheral assemblies that minimizes fuel costs. Finally, axial blankets that reduce neutron leakage are analyzed, as well as the benefits of axially grading the poison loading. The fuel performance --key to the technical feasibility of the core-- is analyzed with the code FROSSTEY-2, and simple models are developed for cladding corrosion and fission gas release. Core costs are calculated and the influence of operational and economics parameters is studied. A PWR reload core is presented that meets current physics and fuel performance design limits for a cycle of 33.9 EFPM or 36 calendar months when operating at a capacity factor of 94.1%. Fuel is enriched to 6.5% U-235 and selected pins use gadolinia as burnable absorber mixed with U0 2. By including pins with two different concentrations of gadolinia in the asssemblies, very good reactivity control is obtained, and the power is evenly distributed over a broad region of the core. Fuel costs are optimized by loading the core periphery with reused assemblies. The rest of the assemblies are discharged after one cycle in the core. The design criteria for peak pin exposure, axial enthalpy rise hot channel factor, and total peaking factors are met. The fuel performance analysis indicates that fuel centerline temperature, rod internal pressure, cladding oxide thickness, clad surface temperature and fission gas release are within acceptable limits, although in general slightly larger than for a contemporary reference 18- month cycle multibatch loading strategy. The 36-month core is economically competitive with an 18-month reference core under certain operational conditions. Considering a refueling outage of 30 days and 3% forced outage rate, the 36-month core is about $5M/yr more expensive than an 18-month reference core. However, if the outage length increases to 42 days, costs are similar for both cores. Furthermore, the reduction in enrichment costs expected with the development of AVLIS technology will make the 36-month cycle more economically attractive and potentially cost competitive with the 18-month reference cycle.