Water Reactor Fuel Extended Burnup Study PDF Download

To utilize energy resources as efficiently as possible has become a necessity today. The purpose of this study is to see how this can be done by extending burnup in a light water reactor. Specifically, an in-out refueling scheme might extract the maximum energy from nuclear fuel during its redidence in the reactor. In principle, a reactor loading with minimum neutron leakage and minimum parasitic absorption will best utilize the neutrons. For this, having more fissions near the center of the reactor will reduce leakage, and parasitic absorption by control elements could be reduced by having smaller reactivity changes between reloads. An in-out refueling scheme, i.e., loading the fresh fuel into the center of the reactor, moving the older fuel outward and removing the oldest fuel from the edge, could best utilize the reactivity of fresh fuel. This is because neutron importance is highest in the center. Placing control preferentially near the center then requires relatively less absorption in control elements to achieve a given degree of reactor control. The non-uniform flux distribution, with peaking at the center, will save neutrons because the fuel assemblies of the lowest neutron production capability are located at the edge of the reactor, and leakage from these fuels is neither very large nor very important. To simulate the in-out operation of the reactor in this study, two computer codes have been established. One is a one-dimensional code utilizing two group diffusion equations in an iteration scheme, and the other is a two-dimensional code using spatial flux synthesis. Studies have been made of the effects of the in-out refueling scheme on reactor cycle length, fuel burnup level, power peaking factor and other reactor characteristics. Results show that an in-out refueling scheme could have a fuel burnup benefit over the conventional (out-in) refueling scheme. The benefit can be up to 13 percent or more, depending on the frequency of refueling, the fuel design and the reactor size, compared with out-in refueling under the same circumstances. The in-out refueling scheme with short cycle length gets part of its benefit from frequent refueling. However, frequent refueling tends to expose the fuels at the center of the core to very high power peak ing. Peaking is a function of batch size (the smaller the batch size the higher the peak), and is closely related to the initial reactivity of the fuel, as well as to the method of fuel management. High power peaking can be alleviated by confining the reactivity-controlling absorber to the center of the reactor. Moreover, with this type of control the cycle length and discharge burnup become larger, for a given replacement batch size, than for a case in which control is applied over the whole core. Enrichment certainly could elongate the cycle length and so the discharge burnup, but the gain of burnup per enrichment increment decreases with enrichment level. Lattice design could also have some effects on the discharge burnup. More moderation of neutrons in a looser lattice increases the initial reactivity of the fuel, although it speeds up the rate of reactivity loss per flux-time. The low conversion of the fertile material in the fuel of a loose lattice does not apparently have as much influence on the discharge burnup as the initial reactivity does.