NEUTRON LIFE CYCLEDOE-HDBK-1019/2-93Reactor Theory (Nuclear Parameters)The value of the resonance escape probability is determined largely by the fuel-moderatorarrangement and the amount of enrichment of uranium-235 (if any is used). To undergoresonance absorption, a neutron must pass close enough to a uranium-238 nucleus to be absorbedwhile slowing down. In a homogeneous reactor the neutron does its slowing down in the regionof the fuel nuclei, and this condition is easily met. This means that a neutron has a highprobability of being absorbed by uranium-238 while slowing down; therefore, its escapeprobability is lower. In a heterogeneous reactor, however, the neutron slows down in themoderator where there are no atoms of uranium-238 present. Therefore, it has a low probabilityof undergoing resonance absorption, and its escape probability is higher.The value of the resonance escape probability is not significantly affected by pressure or poisonconcentration. In water moderated, low uranium-235 enrichment reactors, raising thetemperature of the fuel will raise the resonance absorption in uranium-238 due to the dopplereffect (an apparent broadening of the normally narrow resonance peaks due to thermal motionof nuclei). The increase in resonance absorption lowers the resonance escape probability, andthe fuel temperature coefficient for resonance escape is negative (explained in detail later). Thetemperature coefficient of resonance escape probability for the moderator temperature is alsonegative. As water temperature increases, water density decreases. The decrease in water densityallows more resonance energy neutrons to enter the fuel and be absorbed. The value of theresonance escape probability is always slightly less than one (normally 0.95 to 0.99).The product of the fast fission factor and the resonance escape probability ( p) is the ratio ofthe number of fast neutrons that survive slowing down (thermalization) compared to the numberof fast neutrons originally starting the generation.ThermalUtilizationFactor,(f)Once thermalized, the neutrons continue to diffuse throughout the reactor and are subject toabsorption by other materials in the reactor as well as the fuel. The thermal utilization factordescribes how effectively thermal neutrons are absorbed by the fuel, or how well they areutilized within the reactor. The thermal utilization factor (f) is defined as the ratio of thenumber of thermal neutrons absorbed in the fuel to the number of thermal neutrons absorbed inany reactor material. This ratio is shown below.f number of thermal neutrons absorbed in the fuelnumber of thermal neutrons absorbed in all reactor materialsThe thermal utilization factor will always be less than one because some of the thermal neutronsabsorbed within the reactor will be absorbed by atoms of non-fuel materials.NP-03Rev. 0Page 4
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