Fast fission factor

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 subcadmium activation distributions were used in conjunction with cross sections computed by Westcott to calculate values of the thermal utilization f and the thermal migration area L in the usual way. A base value of V was calculated from Westcott values, assuming the neutron flux spectrum in the moderator to be Maxwellian at 2(PC. This value was then modified for flux hardening effects >y comparing the ratios of the 1/v activations (Cu and Mn) and the U-235 activations at various locations. Values of the fast fission factor < were obtained by comparing the fission product activities of natural and depleted uranium foils according to the technique described by Futch . The neutron age r was measured to indium resonance from isolated fuel assemblies in DjO. Corrections were calculated for the age to thermal energy and for lattice effects. 

A. Z. kranz, Measurements of Thermal Utilization, Resonance Escape Probability, and Fast Fission Factor of Water Moderated Slightly Enriched Uranium Lattices, WAPD-134 (Sept. 1955). 

The fast fission factor e, the material buckling B, the quantity Ap/AB were measured in critical assemblies of each fuel mixture. The thermal-neutron diffusion area L wa.8 calculated from thermal-neutron cross sections. The delayed-neutron fraction was determined from the delayed-neutron fractions for U and measured by Keepin and the "U-to- U fission ratio measured in the critical assemblies. 

Xhe constant ri In 2.2 1 Is the reproduction factor and is defined as the number of neutrons produced by Jbgrmal fission for each neutron absorbed In the fuel (exclusive of those captured in resonances) The constant is the fast fission factor and It accounts for fast fission In fast fission... 

A compilation of fast neutron cross sections from Reference 3 which are useful in calculating the fast fission factor is given in Table 2,3 2.2.1. 

Xhe fast fission factor following Spinrad s model is defined as the number of neutrons mskixig their first collision w 1th the moderators external to the fuel element per neutron arising from thermal fission. The... 

The calculation of the fast-fission factor e proceeds directly from results given in Table 10.5. In order to obtain we need simply sum the number of neutrons supplied to the slowing-down process from each fast-fission generation. These are the terms indicated in the last column. Thus we obtain... 

Answer Before the velocity of the newly-created fast neutrons is decreased to any great extent, they will cause a few fast fissions in the more abundant 1)238 nuclei, and also a negligible number in the relatively scarce 1) 35 nuclei. This effect is called the "fast fission" factor and it multiplies the number of neutrons created by thermal fissions each generation by a value slightly in excess of unity (about 1.03). 

Althou U-238 is not fissionable to slow neutrons, it will fission with a fast neutron, such as we get from fission. This is much like hitting the nucleus harder - so hard, in fact, that it splits. This is a fairly small effect, adding only about 3 to our neutrons, so now we have 1035 of 139> or about lii-35t of what we started with. This number, I.03 or 1035 is called the fast fission factor, and is denoted by the Greek letter epsilion, . So now we have fission neutrons from all fission. 

Which one of the following is the definition of the FAST FISSION FACTOR ... 

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 fast fission factor tends to be greater as the moderator to fuel ratio is reduced in a reactor. However, it does not exhibit a significant variation with the %20/ fuel 2is is shown in Figure 6.3(a). 

It is readily apparent from equation (4.7) why the best conversion ratio is obtained for a fast reactor system. Apart from the more favorable value of rj at high energies (see Fig. 4.1), the fast fission factor can rise to a value as high as 1.3 in a fast reactor spectrum. In addition, the third factor on the right-hand side is reduced owing to the absence of moderator. Consequently, values of C greater than unity can be readily achieved for a fast reactor. 

In fundamental reactor science, experimental studies display several essential concepts from neutron and reactor physics. Total neutron cross section is measured by a method involving basic neutron-beam techniques in other exercises, measurements of isotopic neutron cross sections and absolute neutron fluxes are made by use of activation methods in the thermal, resonance, and fast regions of the neutron spectrum. The resonance escape probability and fast fission factor are evaluated in the lattice of the Argonaut reactor. 

T) = neutrons born per thermal neutron captured in the urajiium, e = fast fission factor, p = resonance escape probability, f = thermal utilization,... 

Discuss the effect of the void on resonance escape, thermal utilization, fast fission factor, r), neutron age, and diffusion length. 

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