Thermal Utilization Factor
It is obvious, the neutrons that escape the resonance absorption and remain in the core will be thermalized. In thermal reactors these neutrons continue to diffuse throughout the reactor, until they are absorbed. But there is many materials in the reactor core, in which these neutrons may be absorbed. The thermal utilization factor, f, is the fraction of the thermal neutrons that are absorbed in the nuclear fuel, in all isotopes of the nuclear fuel. It describes how effectively (how well are utilized) are thermal neutrons absorbed in the fuel.
The value of the thermal utilization factor is given by the ratio of the number of thermal neutrons absorbed in the fuel (all nuclides) to the number of thermal neutrons absorbed in all the material that makes up the core.
f ~ 0.70
From definition, this factor will always be less than one, because there will be always some of the thermal neutrons absorbed in non-fuel materials (usually referred to as a parasitic absorption). For homogenous reactor cores, this factor can be calculated in terms of macroscopic cross-sections by the following equation:
where Σa is the macroscopic absorption cross section, which is the sum of the capture cross section and the fission cross section, Σa = Σc + Σf. The superscripts U, M, P, CR, B, BA and O, refer to uranium fuel, moderator, poisons, control rods, boric acid, burnable absorbers and others, respectively. It is obvious, that the presence control rods, boric acid or poisons causes a decrease in the neutron utilization, which, in turn, causes a decrease of multiplication factor.
In heterogeneous reactors there is usually some difference between the neutron flux in the moderator and the neutron flux in the fuel. This difference is caused by the high absorption in the fuel and the ratio of thermal neutron flux in the moderator to that in the fuel is known as the thermal disadvantage factor defined as:
The thermal utilization factor is for heterogeneous reactor cores about 0.5 – 0.8, but it depends strongly on the reactor type.
Main operational changes, that affect this factor:
This factor is the key factor, that the reactor operator has the greatest control over. Control rods insertion/withdrawal and to borate/boron dilution from the moderator are operations, which influences precisely the thermal utilization factor. For example, control rods insertion causes an addition of new absorbing material into the core and this causes a decrease in thermal utilization factor.
The thermal utilization factor slightly changes with the fuel burnup. The fresh fuel at the beginning of the cycle comprises only the absorption by the 235U. As the amount of 239Pu and other higher transuranic elements increases because of the radiative capture of neutron by the 238U in the core, it is necessary to consider the change of fuel composition in determining the value of f at different times of the fuel cycle.
In general, the thermal utilization factor decreases in time as the total content of fissile isotopes decreases and the total content of neutron poisons (fission products with high absorption cross-sections) increases. But in the power reactors, in which the criticality must be maintained for long period (e.g. 12-month or up to 24-month) without refueling, the thermal utilization factor may not decrease. Such the decrease would imply inevitable reactor shutdown. The continuous decrease in ΣaU must be offset by the continuous decrease in ΣaB, which means the concentration of boric acid (in case of PWRs) must be continuously decreased as the fuel loses its reactivity (kinf). For reactors, in which the chemical shim can not be used, the excess of reactivity is compensated by compensating rods.
On the other hand, there is the fuel breeding, which plays very important role in the compensation of decreasing 235U content.
Much more important, however, is the reactor design, which determines the moderator-to-fuel ratio. Most of light water reactors are designed as so called undermoderated with the slightly harder neutron flux spectrum (the moderation is slightly insufficient) than in an optimum case.