Hand Calculation of Fuel-Cycle Performance

Hie preceding example shows that accurate representation of fuel-cyde performance requires such detailed examination of the time-varying dianges in power distribution and fuel composition at many points in a nuclear reactor as to necessitate use of a digital computer. The purpose of this section is to develop an approximate procedure for calculating fuel-cycle performance that, although complicated, can be carried out by hand calculation. The FWR described in Sec. 4.1 will be used as example, except that its rated electric output is taken as 10S4 MWe instead of 1060 MWe. 

The neutron energy cycle of the one-group reactor-physics model to be used is shovm in Fig. 3.28. 

Consider a unit volume containing N atoms of fissile material ( U, Pu, or 

Those fast neutrons that have energies greater than about 1 MeV may cause a limited amount of fission of fertile material. To account for this, the reactor designer usually specifies a quantity e, called the fast-fission factor, which is defined as the ratio of the net rate of production of fast neutrons to the rate of production of fast neutrons by thermal fission. The fraction e — 1 of the fast neutrons comes from fission of fertile material with fast neutrons e — 1 may be of the order of a few hundredths in a thermal power reactor. The net production rate of fast neutrons from fission is er N a 4 - 

The fraction of the fast neutrons that do not escape from the reactor as they degrade from fission to resonance energy depends on the size and moderating properties of the reactor. This fraction is denoted as Pi, the fission-to-resdrmnce nonleakage probability. Hence, the rate at which fast neutrons degrade into the resonance region is er) N a l)Pi. 

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