Reactor neutron spectrum

Figure 2, The general features of a reactor neutron spectrum. Key A, thermal region B, epithermal region and C, fast region.

In Eq. (30.38), the term Qo(a)//in the factor (1 + Qo( )//) corrects for activation by epithermal neutrons. This factor is the heart of the ko method and was the key innovation that made the method possible. It was necessary to predict, with high accuracy, the relative reaction rates for two different (n,y) reactions in any reactor neutron spectrum. For each reaction, the Qo value, the ratio of the resonance integral to the thermal neutron activation cross section, and / the ratio of thermal flux to epithermal flux for the irradiation channel used, is needed. ... 

The control of the reactor neutron spectrum (i.e., energy dependence), which has a strong effect on the reactor operating characteristics... 

The assessment of the uncertainties in the prediction of reactor properties is an important requirement, both for economic and safety reasons. Allowances must be made in the design and operation of reactors to cover uncertainties by introducing suitable margins. A very high level of confidence in the safety aspects of reactors must be achieved. Uncertainties in nuclear data contribute to the overall uncertainty. Consequently the uncertainties in the data and the sensitivity of calculated reactor parameters to these uncertainties must be estimated. The estimation and representation of uncertainties in evaluated differential cross-sections is a complex problem. However, since the required information is the imcertainty in reactor neutron spectrum averaged values of cross-sections, or in ratios of such averages, the required... 

Reactor Neutron spectrum Core coolant Maximum achievable temperature (°C) Envisioned fnel cycle Power rating (MWe,) Applications... 

Reactor Neutron spectrum Coolant Temperature °C Pressure Fuel Fuel cycle Uses... 

The determination of Pb by reactor activation normally requires chemical separation of the 3.3 h pure emitter ° Pb. The use of the short-lived isomeric state 207mp j activated by the Pb(n,y) ° Pb and PbfnjnO Pb reactions is a useful instrumental alternative. The method has been investigated by Lukens using the triga reactor, and by Wiernik and AmieF and Henkelmann et Again the sensitivity of the method will depend upon the reactor neutron spectrum used. In the latter case, a reactor fast-neutron flux of 1.8 X10 cm s" allowed analysis with a limit of detection of 200 MS, somewhat poorer than the other methods. However, in such a flux spectrum the interference from other (n,y) activations will be considerably reduced. 

Estimates of the effective cross section in typical homogeneous-reactor neutron spectrums (except for 2200 m/sec value) these values include contributions due to resonance absorptions. (Although these values were not used in the calculations presented, they are believed to be more accurate than the ones employed. Values used for Pa were 133, 118, and 97 bams at 20, 100, and 280°C, respectively.)... 

Figures Comparison of nuciear reactor and pulsed spaliation sources. For reactor sources (steady-state method), a narrow band of wavelengths is seiected with a monochromator crystal and the scattering angle (26,) Is varied to scan dspacings. Pulsed sources (time-of-flight method) use almost the entire avail-abie neutron spectrum, fix the scattering angie (26,), and simultaneousiy detect a neutron while determining its time of flight.

Compared to the simplicity of the relative method, with its simple measurement equation, there is a hidden complexity in the k0 method complex algorithms, dedicated software for reactor neutron fluxes and gamma ray measurement efficiency and many problems associated with spectrum deconvolution. The method relies on a complex set of written standards which are not always fully understood by the average user. It uses non-transparent instrumentation and measurement processes. In short the method becomes, forgive the terminology, non-traceable to the user and this is, I believe, worse than non-traceable to SI units. 

The second best choice on the basis of traceability concepts, the k0 method, contains a major flaw (in essence, there is no neutron spectrum for every irradiation position of every reactor available at BIPM in Sevres ), neither is it transparent in its concepts and its error budget. 

Future nuclear reactors are expected to be further progressed in terms of safety and reliability, proliferation resistance and physical protection, economics, sustainability (GIF, 2002). One of the most promising nuclear reactor concepts of the next generation (Gen-IV) is the VHTR. Characteristic features are a helium-cooled, graphite-moderated thermal neutron spectrum reactor core with a reference thermal power production of 400-600 MW. Coolant outlet temperatures of 900-1 000°C or higher are ideally suited for a wide spectrum of high temperature process heat applications. 

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... 

This procedure, as described, requires that a known quantity of each element to be measured be irradiated under the same conditions as the unknown sample. If the relative production rates of the radionuclides used are measured accurately in a particular irradiation facility, subsequent measurements can be done with less labor by irradiating only a few monitor elements to characterize the neutron spectrum. Published values of cross sections have been used as a measure of these relative reaction rates, but the inaccuracy of tabulated data and the poorly known neutron spectrum of most reactors makes this approach unsatisfactory when good accuracy is needed. 

Calculated fox the neutron spectrum of a typical pressurized-water reactor. Bennett [B3]. 

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. 

The composition of irradiated fuel to be fed to a reprocessing plant varies widely. It depends on the composition of the fresh fuel charged to the reactor, the neutron spectrum in which the fuel is irradiated, the specific power or rate of heat generation in the fuel, the duration of irradiation, and the length of time the fuel is cooled -the interval between end of irradiation and start of reprocessing. 

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. 

The low cross-section of the reaction of I(n,p) I with fast neutrons (cf Table 2.3) and a low abundance of neutrons with energies higher than 9MeV, which are needed for this reaction, in the neutron spectrum of a nuclear reactor result in a detection limit which is not sufficient for iodine determination in most types of foodstuffs, even if an RNAA procedure is applied. However, this reaction, which is completely independent in relation to the reaction of I(n, ) I with thermal and epithermal neutrons may be useful for cross-checking results in analysis of foodstuff samples with higher iodine contents, using the so-called self-verification principle in NAA (Byrne and Kucera, 1997). Detection limits of various NAA modes, which were achieved in the authors laboratory are compared in Table 2.4. 

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. 

X 10 n/sec. The neutron spectrum of this source is similar to that of reactor neutrons, and therefore, for practical applications, the source is placed in a moderator or thermalizer. The useful thermal-neutron flux density available in a typical facility is about 3 x 10 n/(cm -sec). 

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