There are significant differences in neutron spectra for water-cooled, sodium-cooled, and other types of fission-based reactors. It should be noted that there is a conventional but slightly misleading practice to differentiate between ‘fast’ and ‘thermal’ reactors. Thermal reactors have a significant portion of their spectra composed of thermal neutrons. Thermalized neutrons have suffered enough collisions with the moderator material that they are in thermal equilib­rium with the vibrations of the surrounding atoms. Efficient thermalization requires low-Z materials such as H, D, and C in the form of water, graphite, or hydrocarbons. At room temperature the mean energy of thermalized neutrons is 0.023 eV.

The designation ‘fast’ reactor, as compared to ‘thermal’ reactor, refers to the portion of the neutron spectrum used to control the kinetics of ascent to full power for each type of reactor. As shown later, this practice incorrectly implies to many that fast reactors have ‘harder’ neutron spectra than do ‘softer’ thermal reactors. Actually, the opposite is true.

Examples of typical flux-spectral differences in fission-based reactors are shown in Figures 2-5. The local spectrum at any position is determined primarily by the fuel (U, Pu) and fuel type (metal, oxide, carbide, etc.), the coolant identity and density, the local balance of fuel/coolant/metal as well as the proximity to control rods, water traps, or core bound­aries. Additionally, it is possible to modify the neutron spectra in a given irradiation capsule by including in it

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or enclosing it with a moderator or absorber. Metal hydrides are used in fast reactors to soften the spec­trum, while in mixed-spectrum reactors the thermal — to-fast ratio can be strongly reduced by incorporating elements such as B, Hf, Gd, and Eu.

The most pronounced influence on neutron spectra in fission reactors arises from the choices of coolant and moderator, which are often the same material (e. g., water). Moving from heavy liquid metals such as lead or lead-bismuth to lighter metals such as sodium leads to less energetic or ‘softer’ spectra.

Use of light water for cooling serves as a much more effective moderator. Counterintuitively,
however, this leads to both more energetic and less energetic spectra at the same time, producing a two — peaked ‘fast’ and ‘thermal’ distribution separated by a wide energy gulf at lower fluxes.

Such two-peaked spectra are frequently called ‘mixed spectra.’ The ratio of the thermal and fast neutron fluxes in and near such reactors can vary significantly with position and also with time.4 Using heavy water, we obtain a somewhat less efficient moderator that does not absorb neutrons as easily as light water, but one that produces an even more pro­nounced two-peak spectral distribution where the thermal-to-fast neutron ratio can be very large.

These spectral differences lead to strong varia­tions between various reactors in the neutron’s ability to displace atoms and to cause transmutation. Depend­ing on the reactor size and its construction details there can also be significant variations in neutron spectra and ‘displacement effectiveness’ within a given reactor and its environs, especially where more energetic neutrons can leak out of the core. Examples of these variations of displacement effec­tiveness for fast reactors are shown in Figures 6 and 7. Compared to fission-derived spectra, there are even larger spectral differences in various fusion or spallation neutron devices.

The reader should note the emphasis placed here on flux-spectra rather than simply spectra. If we focus only on light water-cooled reactors for example, there are in general three regimes of neutron flux of relevance to this review. First, there are the relatively low fluxes typical of many experimental reactors that

can produce doses of 10 dpa or less over a decade. Second, there are moderate flux reactors that are used to produce power that can introduce doses as high as 60-100 dpa maximum over a 30-40 year life­time and finally, some high-flux thermal reactors that can produce 10-15 dpa year in stainless steels.

Most importantly, fast reactors also operate in the high-flux regime, producing 10-40 dpa year-1. Therefore, the largest amount of published high — dpa data on stainless steels has been generated in fast reactors. Some phenomena observed at high exposure, such as void swelling, have been found to be exceptionally sensitive to the dpa rate, while others are less sensitive (change in yield strength) or essentially insensitive (irradiation creep). These sensitivities will be covered in later sections.

For light water-cooled reactors, the various flux regimes need not necessarily involve large differ­ences in neutron spectra, but only in flux. However, the very large dpa rates characteristic of fast reactors are associated with a significant difference in spec­trum. This difference is a direct consequence of the fact that fast reactors were originally designed to breed the fissionable isotope 239Pu from the relatively nonfissile isotope 238U, which comprises 99.3% of natural uranium.

In order to maximize the breeding of 239Pu, it is necessary to minimize the unproductive capture of neutrons by elements other than uranium. One

strategy used to accomplish this goal is to avoid thermalization of the reactor neutrons, which requires that no low atomic weight materials such as H2O, D2O, Be, or graphite be used as coolants or as moderators. For this purpose, sodium is an excellent coolant with a moderate atomic weight. The use of sodium results

in a neutron spectrum that is nominally single-peaked rather than the typical double-peaked (thermal and fast) neutron spectrum found in light water or heavy water reactors. The single-peaked fast reactor spec­trum is significantly less energetic or softer, however, than that found in the fast peak of light water reactors. Depending on the fuel type (metal vs. oxide) the mean energy of fast reactor spectra varies from ^0.8 to ^0.5-0.4MeV while light water-cooled reactors have a fast neutron peak near ~1.2 MeV.

One consequence of attaining successful breeding conditions is that the spectrum-averaged cross­section for fission is reduced by a factor of 300-400 relative to that found in light water spectra. To reach a power density comparable to that of a light water power-producing reactor, the fast reactor utilizes two concurrent strategies: increases in fissile enrichment to levels in the order of 20% or more, and most importantly, an increase in neutron flux by one or two orders of magnitude.

Thus, for a given power density, the fast reactor will subject its structural materials to the punishing effects of neutron bombardment at a rate that is several orders of magnitude greater than that in light water reactors. At the same time, however, the softer ‘fast’ spectrum without thermalized neutrons leads to a significant reduction in transmutation compared to typical light water spectra, at least for stainless steels and nickel-base steels.