Neutrons, fast, effect

From reaction yields and cross-sections, measured under varying irradiation conditions and neutron energies, it was found that fast neutrons were effective in processes 1 and 2, an indication that the target nucleus was 238U, but that the yield was greater when thermal neutrons were used, which is typical of neutron capture. 

Szilard, also working at Columbia, became interested around this time in what is now called the fast effect. The fast effect, is the increase in the multiplication constant obtained by the emission of neutrons by which is induced to fission by the fission neutrons before they are slowed down. Szilard measured both the cross section of such fission neutrons to induce fast fission and also their inelastic cross section, i.e., the probability for their being slowed down below the fast fission threshold by an inelastic collision with uranium. He concluded on the basis of these measurements that one may obtain an increase of as much as 6-8% in the multiplication constant by using large and metallic lumps of uranium. Szilard was also somewhat discouraged by the low multiplication constant which Fermi s experiment gave but was far from giving up hope. 

Papers 29 and 30 served as the basis for the earhest calculations of the multiplication constant in an infinite heterogeneous reactor. By early 1942, the significance of fast fission was recognized, and all later calculations included estimates of e, the fast effect. This gave the four-factor formula, k = rjepf rj being the number of neutrons released per neutron absorbed in U. The quantity actually calculated in the following reports was rj = (1/ep/), k = r /rj ... 

There is no relevant difference between the fast effect in a finite and infinite lattice. However, the probability p that a neutron with an energy... 

In these and the above equations, the a are cross sections per imit volume, the a in (8) is scattering cross section, the average loss in r per collision. The are used because the material may contain different types of atoms. The (Ta is the thermal absorption cross section r(r) the resonance absorption cross section per unit volume. The = qef is the multiplication constant divided by the resonance escape probability. The product of thermal utilization / and (Ta is the effective cross section of uranium per unit volume, i.e., its cross section per unit volume multiplied by the thermal neutron density in it and divided by the average thermal neutron density. One can write, therefore, (Tu for f(Ta- If one multiplies this with rj the result is the same as crfU where fission cross section for thermal neutrons per unit volume, p the number of fast neutrons per fission. As a result, the third term in (7) can be written also as e is the multiplication by fast effect)... 

INP day 94i u creasing number of fast neutrons. The effective repro-... 

One of the merits of test reactor irradiation is the higher dose irradiation at higher flux compared to irradiation at vessels or surveillance capsules in commercial power reactors. As of the end of 2013, the maximum fast neutron fluence of surveillance data was around 9 x lO n/cm E> MeV) while the maximum fluence at the inner surface of a RPV may exceed 1 x 10 °n/cm E > IMeV) for a 60-year operation in a pressurized water reactor (PWR). Test reactor irradiation is the only way to obtain experimental data for embrittlement behaviour at fluences relevant to 60 years or longer operation, together with an understanding of neutron flux effects on embrittlement behaviour. 

On the other hand, the value of the second integral is very sensitive to the manner in which the fuel is disposed. If the fuel is in the form of discrete lumps fission neutrons will be much more likely to encounter and cause fission than if the fuel is uniformly divided. Thus the fast effect—i.e., essentially the second term in (13)—is considerably greater in lumped (heterogeneous) systems than in dispersed (homogeneous) systems. 

Criticality analyses were carried out using KENO-IV (Ref. 7) Monte Carlo calculations, and few-group PDQ (Ref. 8) diffusion calculations were benchmarked against these. The KENO-IV calculations employed a 123-group cross-section set that has sufficient fine groups to model thermal and fast neutron disadvantage effects, but in the... 

Seguchi, T., Hayakawa, N., Yoshida, K., and Tamura, N., Fast neutron irradiation effect. II. Crosslinking of polyethylene, ethylene-propylene copolymer, and tetra-fluoroethylene-propylene copolymer, Radiat. Phys. Chem., 26, 221-225 (1985). Keller, A., and Ungar, G., Radiation effects and crystallinity in polyethylene, Radiat. Phys. Chem., 22, 155-181 (1983). 

Xhe reproduction factor is defined as the average number of therma fission neutg released per neutron absorbed in the fuel excluding resonance capture in and absorptions accounted for in the fast effect. Xhe reproduction factor may be written as... 

Ehe thermal utilisation is defined as the ratio of the total number of neutrons absorbed in the fuel to the total number of neutrons absorb ed in all materials In both eases the resonance captures in U 3o the absorptions accounted for in the fast effect are excluded. An expression for calculating the thermal utilization is... 

The calculation of is made by homogenising the fuel, cladding and coolant internal to the fuel assembly, and calculating the collision probabilities for the three fast neutron groups described in Section 2 5on the fast effect calculation. 

The unit-cell method offers a relatively simple computational procedure for determining the various factors in /c . As already demonstrated, the calculation of these factors in each case reduces ultimately to the determination of the neutron spatial distributions for the entire energy range. For the thermal utilization we require the thermal-flux distribution and in particular the thermal-flux depression (i.e., the thermal disadvantage factor). The resonance-escape probability, on the other hand, requires a knowledge of the spatial distribution of resonance neutrons, and finally, of course, the calculation of the fast effect involves, essentially, the determination of the spatial distribution of successive (cascading) generations of fission neutrons. 

Note that if the relation (10.217) is used to obtain 2i. then Eq. (10.218), along with a computed 2t can be used to determine 2,. The approach outlined here for estimating the fast-neutron cross sections is found to give reasonably good agreement with experiment in the case of U. Table 10.6 lists the various cross sections which have been used in the past to relate the theory to actual measurements on the fast effect. 

Placing a thin zirconium hydride layer between the seed and blanket fuel assemblies was found effective in changing the reactivity with steam density in the study of a steam cooled fast reactor [103, 104]. The typical geometry and calculation result are shown in Figs. 1.58 and 1.59, respectively. The effectiveness was explained in the subsequent studies [105,106]. The mechanism is described in Sect. 7.3. The fast neutrons are generated in the seed assemblies. They are moderated by the thin zirconium hydride layer between the seed and blanket. The layer is installed in the blanket assemblies in the present Super FR design. The moderated neutrons are effectively absorbed in the blanket fuel by the capture of U-238. The... 

The First Reactor. When word about the discovery of fission in Germany reached the United States, researchers thereafter found that (/) the principal uranium isotope involved was uranium-235 (2) slow neutrons were very effective in causing fission (J) several fast neutrons were released and (4) a large energy release occurred. The possibiUty of an atom bomb of enormous destmctive power was visualized. 

Properties. Most of the alloys developed to date were intended for service as fuel cladding and other stmctural components in hquid-metal-cooled fast-breeder reactors. AHoy selection was based primarily on the following criteria corrosion resistance in Hquid metals, including lithium, sodium, and NaK, and a mixture of sodium and potassium strength ductihty, including fabricabihty and neutron considerations, including low absorption of fast neutrons as well as irradiation embrittlement and dimensional-variation effects. Alloys of greatest interest include V 80, Cr 15, Ti 5... 

Radiation Effects. Alpha sihcon carbide exhibits a small degree of anisotropy in radiation-induced expansions along the optical axis and perpendicular to it (58). When diodes of sihcon carbide were compared with sihcon diodes in exposure to kradiation with fast neutrons (59), an increase in forward resistance was noted only at a flux about 10 times that at which the increase occurs in a sihcon diode. In general, it appears that sihcon carbide, having the more tightly bound lattice, is less damaged by radiation than sihcon. 

Birch, M., Schofield, P., Brocklehurst, J.E., Kelly, B.T., Harper, A. and Prior, H., The combined effects of fast neutron damage and radiolytic oxidation on the physical... 

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