Neutron spectrum effects

Coupling of irradiation tests in Joyo and JMTR for evaluation of neutron spectrum effects were started. 

In some cases, thermal neutrons can also be used to measure the absolute abundances of other elements. Transforming the neutron spectrum into elemental abundances can be quite involved. For example, to determine the titanium abundances in lunar spectra, Elphic et at. (2002) first had to obtain FeO estimates from Clementine spectral reflectances and Th abundances from gamma-ray data, and then estimate the abundances of the rare earth elements gadolinium and samarium from their correlations with thorium. They then estimated the absorption of neutrons by major elements using the FeO data and further absorption effects by gadolinium and samarium, which have particularly large neutron cross-sections. After making these corrections, the residual neutron absorptions were inferred to be due to titanium alone. 

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... 

Effective thermal cross sections for a typical neutron spectrum of a PWR, including contributions from nonthermal resonance absorption. 

Because of epithermal resonance absorption of neutrons in Pa, its effective cross section in a thermal-neutron spectrum is much greater than the 2200 m/s cross section listed in Table... 

Sands DG, De Laeter JR, Rosman KJR (2001) Measurements of neutron capture effects on Cd, Sm and Gd in lunar samples with implications for the neutron energy spectrum. Earth Planet Sci Lett 186 335-346 Schaeffer OA (1975) Constancy of galactic cosmic rays in time and space. 14th Inti Cosmic Ray Conf 3508-3520... 

Experience has shown that the proper values of w depend on both the shape of the neutron spectrum and the statistical errors of M, and that small changes in the Wj cause large changes in the errors of the result.This effect has been mitigated in the code FORIST, " which is a modification of FERDOR. In FORIST, the value of is obtained by an iterative process in terms of the desired statistical error of the result 5. Choosing the widths w by this method improves the resolution of the unfolded spectrum, for a fixed desired statistical error. 

The relationship between the neutron spectrum and the measured pulse-height distribution is given by Eq. 14.28. The response function k(E, E ) (Eq. 14.29) may be measured or calculated. In either case, the following effects have to be taken into account in obtaining k(E, ) i - 7 27.28... 

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. 

The formation of harder neutron spectrum has been influenced by LBC using, thus the higher value of CBR has been provided, which has had a key part in reaching the EUU biggest value. The calculations have demonstrated that for sodium coolant, which can moderate neutrons more effectively, it is hard to provide the reactor criticality if it has been made up by depleted uranium. 

Therefore, the problems which faced the would-be designers of chain reactors early in 1941 were (1) the choice of the proper moderator to uranium ratio, and (2) the size and shape of the uranium lumps which would most likely lead to a self-sustaining chain reaction, i.e., give the highest multiplication factor. In order to solve these problems, one had to understand the behavior of the fast, of the resonance, and of the thermal neutrons. We were concerned with the second problem which itself consisted of two parts. The first was the measurement of the characteristics of the resonance lines of isolated uranium atoms, the second, the composite effect of this absorption on the neutron spectrum and total resulting absorption. One can liken the first task to the measurement of atomic constants, such as molecular diameter, the second one, to the task of kinetic gas theory which obtains the viscosity and other properties of the gas from the properties of the molecules. The first task was largely accomplished by Anderson and was fully available to us when we did our work. Anderson s and Fermi s work on the absorption of uranium, and on neutron absorption in general, also acquainted us with a number of technics which will be mentioned in the third and fourth of the reports of this series. Finally, Fermi, Anderson, and Zinn carried out, in collaboration with us in Princeton, one measurement of the resonance absorption. This will be discussed in the third article of this series. 

The reaction rate of a moderated uranium pile depends on temperature. For example, in a reactor in which the neutron spectrum is approximately Maxwellian, the average velocity will increase with temperature, thus decreasing the absorption cross sections for the low-energy neutrons which vary as 1/v. Resonance levels will be broadened by the Doppler effect, and if lumping of the uranium has been made use of to decrease the total resonance absorption as proposed by Szilard, and later found experimentally to be effective, increased temperature will decrease the advantage thus gained. 

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. 

The Code specifies that surveillance capsules shall be located within the reactor vessel so that the specimen history duplicates as closely as possible the neutron spectrum, the temperature history and the maximum neutron flux experienced by the reactor vessel. A sufficient number of surveillance capsules shall be provided to monitor the effect of neutron radiation on the reactor vessel materials, that is, the transition temperature shift, ARTndt and the decrease in USE throughout its operating period. A minimum number of capsules is specified depending on the predicted ARTndt value of each testing material at the inside surface of the beltUne of the reactor vessel. In this section, the Japanese surveillance tests program is reviewed. The details of the JEAC 4201 can be found elsewhere (Tomimatsu et al., 2006). Table 4.8 summarizes major revisions of the JEAC 4201. 

---