Fission neutron spectrum

It is obvious that the neutron energy spectrum of a reactor plays an essential role. Figure 19.4 shows the prompt (unmoderated) fission neutron spectrum with 2 MeV. In a nuclear explosive device almost all fission is caused by fast neutrons. Nuclear reactors can be designed so that fission mainly occurs with fast neutrons or with slow neutrons (by moderating the neutrons to thermal energies before they encounter fuel). This leads to two different reactor concepts - the fast reactor and the thermal reactor. The approximate neutron spectra for both reactor types are shown in Figure 19.4. Because thermal reactors are more important at present, we discuss this type of reactors first. 

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

X 10 n/cm s (>0.1 Mev) with fission neutron spectrum, which was measured by means of metal foil threshold detectors. The thermal neutron flux was of the order of 10 n/cm s. The y-dose rate was estimated to be roughly 1.1 x lO R/h. 

The oblective here is-to provide an assessment of both the stuq>e and mean oiergy of the fission neutron spectrum for slow-neutron fission in the Labora-... 

N. M. STEEN, Analysis of the Fission Neutron Spectrum of Uranium-233 and Criticality Computations for Homogeneous Uranium-233—H2O < heres and Cylinders (LWBR Development Program), WAPD-TM-997 Bettis Atomic Power Lab. (1972). 

Percent of neutrons absorbed by a specific fission product element compared to the neutrons absorbed by all the fission products in a PWR neutron spectrum. 

Table 2.15 gives direct fission yields y [B3], effective thermal-neutron absorption cross sections a and half-lives (cf. App. C) for radioactive decay that are used below to evaluate the poisoning ratio for this chain. Effective cross sections were calculated from cross sections for 2200 m/s neutrons and for neutrons of higher energy from cross-section data given by Bennett [B3], applied to the neutron spectrum of a typical pressurized-water reactor. 

Also shown in Table 8.2 are the effective thermal cross sections for the individual nuclides, calculated for the neutron spectrum of a typical PWR and including the contributions from resonance absorption. The cross sections are multiplied by the atoms per fission-product pair to obtain the effective cross sections per fission-product pair listed in Table 8.2. Although the total effective cross section of 89.2 b/fission-product pair is calculated for the mixture of radionuclides existing 150 days after fuel discharge, it is a good approximation for the effective... 

The smallest critical sizes are obtained for homogeneous systems of pure fissile nuclides with maximum neutron reflection. For neutrons with the fission energy spectrum, the critical mass of a metallic sphere of pure is 22.8 kg, that of is 7.5 kg, and that of Pu is 5.6 kg, assuming a 20 cm uranium metal neutron reflector. For fission by thermal neutrons the smallest critical size of a spherical homogeneous aqueous solution of 1102804 without reflector requires 0.82 kg of in 6.3 1 of solution. The corresponding figures for are 0.59 kg in 3.3 1, and of Pu, 0.51 kg in 4.5 1. 

The fission neutrons produced in nuclear reactors have a continuous kinetic-energy spectrum, mostly in the range of 1-10 MeV. Since (n, y) reactions are of more widespread analytical use, fission neutrons must be slowed to thermal energies by passing them through HjO, D2O, or graphite, which act as moderators. Depending on the type of nuclear reactor and the irradiation position in the reactor, the neutron spectrum may vary widely. Therefore, both (n, y) and threshold reactions can occur in samples placed in nuclear reactors. Threshold reactions may produce interferences, of which the experimenter should be aware. 

Increasing the thickness of both the rhenium liner and the uranium nitride pin were required in the reactor design to meet the safety conditions. During normal operations the rhenium had a negative effect on the k-effective but was countered by the extra fuel in the core. In two of the three accident scenarios, the neutron spectrum is more thermal than during normal operation due to the addition of water to the core. For the dry sand accident case, the spectrum is faster than the normal operation case. For the accident scenarios, the extra thermal absorption of neutrons from the additional rhenium dominated the effects from the additional fissionable fuel. 

These effects are illustrated in the sketch on p. 4.46 which is based on some (XINL experiments. Fission neutrons enter the iron from the left. The observed relaxation length of about 9 cm in the iron is probably due to slowing down and absorption of neutrons in the low-energy part of the spectrum. The neutrons which emerge from the iron are predominantly grouped near the "window in the iron cross-section which occurs around 1 Mev. 

A neutron zero power facility with only SOkg U-235 has been built up in 1970, then moved to the South-West center of Reactor Engineering in Sichuan P rovince. Basic zero power physics experiments have been done at diis facility including critical parameter measurements, fission rates, neutron flux distribution, neutron spectrum, material reactivity etc. in 1988, it was removed to ClAE again, and now it has been rebuilt and will be used for proving of the neutronics experiment medtods which will be served to CEFR first start-up and to primary test for die neutronic and other radiation detectors. It is considered also it will be valuable to the evaluation of some specimen nuclear cross section using its hard spectrum. 

Some of the MA nuclides (Np, Am, Cm) contained in residual waste from reprocessing have extremely long-term radio toxicity. Means of reducing the radio toxicity of the MA nuclides are presently under investigation. The MA nuclides could produce useful energy if converted into short-lived fission products by neutron bombardment. From this standpoint, a nuclear reactor provides the obvious means for transmutation of MA nuclides. Among the various nuclear reactors, a fast reactor is considered to have the greatest potential to transmute MA effectively, because of its hard neutron spectrum. 

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