Thermal utilization factor

The cross sections for neutron capture increase for all atoms for thermal energy neutrons. As a result, even though low cross section materials are used some neutrons are captured by the structural and moderator materials. The probability for the non-capture of thermal neutrons in this fashion is signified by/, the thermal utilization factor, which in our case can be assumed to be 0.9. Thus of the original N neutrons 112 thermal neutrons remain in the second generation to cause fission in the nuclear fuel. 

A cubic unreflected graphite moderated natural uranium reactor contains 3 % enriched uranium as UC homogeneously dispersed in the graphite matrix the weight ratio CIV = 10. The resoiunce passage and thermal utilization factors ate both assumed to be 0.9 c = 1.00. Make an estimate of the critical size of the cube. 

In order to better estimate the effect of the water, a comparison calculation for a 2 region system consisting of a 1.25 cm radius metal cylinder concentric with an 11.1 cm radius graphite cylinder has been carried out. The distributions at thermal and resonance energies are shown in the figures. The thermal utilization factor, for such an arrangement is 1.185, the resonance absorption factor, is 1.100 and so n = x - = 1.304. 

Table HI lists the neutron absoiption cross sections for many of the metals described above, as well as their cross section relative to the typical reactor material, zirconium. Materials with a very large cross section relative to zirconium would result in a reduction in the thermal utilization factor f and hence a reduction in Nff. Consequently, Ta, W, V, Mo and Ni based alloys would be impractical choices for a reactor core. From this literature survey, it appears that Fecralloy would provide the greatest promise as a containment material for liquid lead. In addition Tantiron may be an alternate choice. More extensive studies on the applicability of inhibitors such as Ti should be undertaken to determine their affect on the corrosion resistance of these materials. 

In other words, the thermal utilization factor changes... 

Poison addition to the reactor is the final general factor affecting neutron multiplication. Poisons can be in the form of boron in control rods xenon and samarium fission products or any absorbing nucleus that is introduced into the reactor. Poisons increase the denominator in the thermal utilization factor making the overall value of f decrease. In the case of resonance absorbers, resonance escape probability also decreases. 

Reproduction factor increases slightly but remains effectively constant, Thermal utilization factor decreases as fuel is depleted. Rods are withdrawn (BWR) or chemical shim diluted (PVR) to restore the thermal utilization factor and maintain criticality. 

Thermal utilization factor decreases, For resonance absorbers, resonance escape probability also decreases. 

For the following neutron life cycle, calculate the fast fission factor, fast non-leakage probability, resonance escape probability, thermal non-leakage probability, thermal utilization factor, reproduction factor, Keff and core reactivity. 

The amount of water affects Kgff through thermal utilization factor, resonance escape probability, fast fission factor, and the non-leakage probabilities as shown in Figure 6.3(a), Of these factors, thermal utilization factor and resonance escape probability are affected more strongly. 

The thermal utilization factor will be highest if there is no water in the core, The thermal utilization factor is basically considering the percentage of neutrons in the core that are absorbed in the fuel. If there were no water and no the hydrogen and oxygen atoms, then a higher neutrons would be absorbed by the fuel atoms, simplified reactor of only fuel and moderator,... 

As xenon-135 and samarium-149 are formed in a reactor, they reduce the multiplication factor by decreasing the thermal utilization factor, f, Since the formation of fission product poisons is a direct function of the fission rate, as power level changes the amount of poison present in the reactor also changes. Control system reactivity insertions such as rod motion and chemical shim must be made to compensate for fission product reactivity. 

If the reactor is in the subcritical condition only because of the negative reactivity of a control rod, the reactor will approach critical as the rod is removed and at some control rod position will reach criticality. In order to duplicate the fuel approach the aim here is to predict the critical control rod position - starting with the rod in the down position. Each rod removal step will introduce positive reactivity just as if a few grams of were added, because both act primarily through the thermal utilization factor f to increase kgff. The relationship between f and kg is shown by the fundamental reactor equation... 

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