- 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.