Fission spectrum average

A. L. Pope and J. S. Story, Minigal Output from UK Nuclear Data Library—NDLI Thermal Cross-sections, Resonance Integrals and Fission Spectrum Averages, AEEW-M1191 (1973). 

The isotope is the only spontaneous fission (SF) source of neutrons easily available. It provides fission spectrum neutrons with an average of 2.3 MeV. The characteristics of isotopic neutron sources are given in Table 15.4. 

Before fire testing, the Fire Science Group tested lined containers for their neutron-absorption properties. After exposing each container to flames for 10 to 20 min, they reevaluated its neutron attenuation. For a Cf fission spectrum moderated by 25 cm of DjO, the liner reduced the low-energy neutron flux by an average factor of about SO before and after fire testing. These tests also showed the neutron-absorption properties of the liner were not greatly affected despite the partial collapse and loss (as much as 8%) of absorber in the form of steam. 

This result may be written in a more compact manner by introducing an average resonance escape probability for neutrons released according to the fission spectrum thus,... 

Fig. 8.35 Fission spectrum and average fast-flux distribution in lethargy.

Cf spontaneous fissions have a fast neutron energy spectrum, shown in Figure 3, with an average energy of 2.2MeV. On average, 3.76 neutrons are emitted per spontaneous fission. The neutron emission rate is 2.34 X 10 n/(s-g)... 

Transmutation. Recycling actinides to the LWRs will decrease the average material neutron multiplication factor by only 0.8 percent, provided that they are of high purity [C2], Recycling to LMFBRs, however, will be preferred. There will be less neutron capture in impurities, such as lanthanides, and the average fission-to-capture ratio of the actinides should be higher in a fast spectrum than in a thermal one. 

Some spontaneously fissioning radionuclides, such as produce 3-4 neutrons per fission with an energy spectrum of average energy of about 2-3 MeV. The neutron generation rate depends on the amount of which emits (2-3) X 10 neutrons s g ... 

The runs show that between 6 and 7 of the 20 runs done fall outside the one sigma deviation, exactly what would be expected. This give an average multiplication factor of 1.037 0.001. The second set of runs was focused on what the spectrum of the neutrons causing fissions in the reactor during accident cases. The results are shown in Figure 5-17... 

For a fission neutron spectrum, it is possible to define an average cross section ... 

Consider, therefore, a normalized function j(u) which describes the distribution of neutrons appearing from fission sources. If r(u w ) is the age of neutrons of lethargy u which first appeared from j at lethargy u then we define the average age to u for the spectrum from 0 to w by the relation... 

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