Reactor neutron energy spectrum

The LSL-M2 program package determines the neutron energy spectrum based on information obtained from a combination of neutron flux calculations and threshold foil activation measurements. The results of LSL-M2 are used primarily for the determination of radiation damage to reactor components and... 

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. 

Irradiation in test reactors with mixed neutron energy spectrum produces basically the same damage morphology as in commercial reactors. Available size and volume of specimens are similar to or larger than those in surveillance of commercial reactors. These are great advantages of test reactor irradiation, although influences of differences in flux and neutron... 

In principle, in a reactor, three components of the neutron energy spectrum (O Fig. 57.3) can be distinguished (Erdtmann 1976 Erdtmann and Petri 1986) ... 

In PWR fuel rods, the axial plutonium distribution is closely correlated to the local burnup, as can be derived from the y scans of the fuel rods. By contrast, in BWR fuel rod operation the axial shift in the neutron energy spectrum, due to the decrease in moderator density in the upper region of the reactor core, results in axial transuranium element distributions which do not completely coincide with the axial burnup distribution. Besides these inhomogenieties of plutonium distribution in the fuel, which are caused by nuclear effects, a thermal migration of the transuranium elements in the fuel is not to be expected because of the relatively low temperatures prevailing in LWR fuel. 

The classification of the neutron-energy spectrum of a given reactor is determined principally by the neutron-moderating materials which it contains. If the nuclear masses of the nonfuel components of the reactor are relatively low, then the neutron spectrum will correspond to that of a thermal reactor (cf. Fig. 1.4c) if they are large, a fast spectrum will result (cf. Fig. 1.4o). The spectrums of intermediate reactors may be due to a number of nuclear characteristics, the presence of nuclear masses of moderate magnitudes being one cause. 

Fig. 1.4 Neutron-energy spectrum in (a) fast, (b) intermediate, and (c) thermal reactors.

These results are entirely consistent with those of our previous analysis of the bare reactor using the Fermi age model (refer to Sec. 6.3). In this formulation, Eq. (8.281a) describes the neutron-energy spectrum and is easily recognized as the integral equation for the collision density in an infinite homogeneous system. If we select, for example [cf. Eq. (4.36)],... 

The SSR operates with a neutron energy spectrum quite unlike any gas-cooled reactor concept studied during the past 20 years. Nuclear data evaluations for uranium, carbon, oxygen, and cadmium isotopes for the epithermal and fast energy ranges must be reviewed, probably reevaluations performed, and likely additional cross section measurements made. Because the concept relies on low excess reactivity, the predicted critical mass should be confirmed. The value of the temperature coefficient must be confirmed. Subcriticality under accident conditions should be assured through applicable benchmark experiments. 

Reactor physical characteristics have also drawn much attention. The control rod worth, including the differential worth and integral worth, were calculated by the Monte Carlo code for neutron and photon transport (MCNP) for the 2 MW TMSR-SE (Zhou and Liu, 2013). The measurement of the neutron energy spectrum was also theoretically and experimentally studied (Zhou, 2013). Parametric study of the thorium-uranium conversion rate was conducted to optimize the core structure for the improvement of the economics of the TMSR using the standardized computer analyses for licensing evaluation (SCALE) code (Wang and Cai, 2013). 

Zhou, X., 2013. A Study on Measurement of Neutron Energy Spectrum for Thorium Molten Salt Reactor. Shanghai Institute of Applied Physics. The University of Chinese Academy of Sciences. 

Preliminary reactor core concepts have been proposed at various institutions. Among them, the concept with mixed neutron spectrum proposed by SJTU [95,96] has achieved the special attention of Chinese researchers. The mixed spectrum SCWR core combines the merits of both thermal and fast spectra as far as possible. The basic idea is to divide the reactor core into two zones with different neutron spectra. In the outer zone, the neutron energy spectrum is similar to that of PWRs. To assess the performance of the reactor core, a coupled neutron-physics and thermal-hydraulics analysis was conducted [96]. 

The neutron dose to graphite due to irradiation is commonly reported as a time integrated flux of neutrons per unit area (or fluence) referenced to a particular neutron energy. Neutron energies greater that 50 keV, 0.1 MeV, 0.18 MeV, and 1 MeV were adopted in the past and can be readily foimd in the literature. In the U.K., irradiation data are frequently reported in fluences referenced to a standard flux spectrum at a particular point in the DIDO reactor, for which the displacement rate was measured by the nickel activation [ Ni(np) t o] reaction [equivalent DIDO nickel (EDN)]. Early on, neutron irradiation doses to the graphite moderator were reported in terms of the bum-up (energy extracted) from imit mass of the adjacent nuclear fuel, i.e., MW days per adjacent tonne of fuel, or MWd/Ate. 

Some of the alternative TOF instrument designs involve replacing the beryllium filter with either a crystal or a mechanical chopper to monochromate the incident beam. With this change, the spectrometer can be used with a higher incident neutron energy (typically E 50 meV) so that a smaller momentum transfer Q is possible for 5 the same energy transfer (21,22). With a monochromatic incident beam, a beryllium filter is sometimes substituted for the chopper after the sample in order to increase the scattered intensity but with a sacrifice in the,minimum Q attainable. Energy transfers up to 100 meV (800 cm" ) can be achieved with TOF spectrometers at steady state reactors before the incident neutron flux is limited by the thermal spectrum of the reactor. (With hot moderators such as at the Institut Laue-... 

The spectrum of neutron energies in nuclear reactors is, in general, relatively broad. Furthermore, it varies with the type of reactor, and in the same reactor with... 

Another approach to the determination of Li in RPV steel was by neutron activation analysis (NAA) followed by radiochemical analysis for H. This was far more complex than the ICP-MS analysis as it involved the irradiation of the inactive steel specimens in the CONSORT reactor at Ascot and transport of the activated steel to the Springfields Laboratory for radiochemical analysis. Six inactive specimens of RPV steel from TRA were irradiated for 63 hours in a neutron flux comprising flux values of 960 x 10 n cm s 44 X 10 n cm s and 300 x 10 n cm s for thermal, epithermal and fast regions of the energy spectrum respectively. 

The international group has identified six Generation rV reactor systems for development. All of these reactors should be ready for deployment by 2030. The fast neutron spectrum reactors can use the fuel values of all of the fissile and fertile transuranic isotopes in reprocessed fuel. This does not occur in the current thermal spectrum reactors. Producing energy... 

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