4.02.2.1 Atomic Displacements

What are the nature and origins of neutron-induced phenomena in metals? The major underlying driving force arises primarily from neutron collisions with atoms in a crystalline metal matrix. When exposed to displacive irradiation by energetic neutrons, the atoms in a metal experience a transfer of energy, which if larger than several tens of eV, can lead to displacement of the atom from its crystalline position. The displace­ments can be in the form of single displacements resulting from a low-energy neutron collision with a single atom or a glancing collision with a higher energy neutron. More frequently, however, the ‘primary knock-on’ collision involves a larger energy transfer and there occurs a localized ‘cascade’ of defects that result from subsequent atom-to-atom collisions.

There are several other contributions to displace­ment of atoms from their lattice site, but these are usually of second-order importance. The first of these processes involve production of energetic elec­trons produced by high-energy photons via the photoelectric effect, Compton Effect, or pair produc­tion.18 These electrons can then cause atomic displa­cements, but at a much lower efficiency than that associated with neutron-scattering events. The sec­ond type of process involves neutron absorption by an atom, its subsequent transmutation or excitation, followed by gamma emission. The emission-induced recoil of the resulting isotope often is sufficient to displace one or several atoms. In general, however, such recoils add a maximum of only several percent to the displacement process and only then in highly thermalized neutron spectra.4 One very significant exception to this generalization involving nickel will be presented later.

For structural components of various types of nuclear reactors, it is the convention to express the accumulated damage exposure in terms of the calcu­lated number of times, on the average, that each atom has been displaced from its lattice site. Thus, 10 dpa (displacements per atom) means that each atom has been displaced an average of 10 times. Doses in the order of 100-200 dpa can be accumulated over the lifetimes of some reactor components in various high-flux reactor types. The dpa concept is very useful in that it divorces the damage process from the details of the neutron spectrum, allowing comparison of data generated in various spectra, providing that the damage mechanism arises primar­ily from displacements and not from transmutation.

The use of the dpa concept also relieves research­ers from the use of relatively artificial and sometimes confusing threshold energies frequently used to describe the damage-causing portion of the neutron spectrum. Neutrons with ‘energies greater than

X MeV,’ where X is most frequently 0.0, 0.1, 0.5, or 1.0 MeV, have been used for different reactor con­cepts at different times in history. The threshold energy of 0.1 MeV is currently the most widely used value and is most applicable to fast reactors where large fractions of the spectra lay below 0.5 and 1.0 MeV. Many older studies employed the total neutron flux (E > 0.0) but this is the least useful threshold for most correlation efforts. Caution should be exercised when compiling data from many older studies where the neutron flux was not adequately identified in terms of the threshold energy employed.

There are rough conversion factors for ‘displace­ment effectiveness’ for 300 series austenitic steels that are useful for estimating dpa from >0.1 MeV fluences for both in-core or near-core spectra in most fission spectra. Examples are ^7 dpa per 1022 n cm~ (E > 0.1) for most in-core light water spectra with lower in-core values of ^5 dpa per 1022 n cm~2 (E > 0.1) for metal fueled fast reactors and ^4 dpa per 1022 n cm~2 (E > 0.1) for oxide-fueled fast reactors.4 Such con­version factors should not be trusted within more than (10-15%), primarily due to spatial variations across the core resulting from neutron leakage. For fast reactor spectra, E > 1.0 conversion factors are completely unreliable.

When E > 1.0 fluxes are employed in light water reactor studies, the conversion factor increases from ^7 dpa per 1022 n cm~2 (E > 0.1) to ~14 dpa per 1022 n cm~2 (E > 1.0). In Russia, a threshold energy of >0.5 MeV is popular for light water
reactors with ^9 dpa per 1022 n cm~2 (E > 0.5). All of these conversion factors assume that within several percent pure iron is a good surrogate for 300 series alloys. Note that other metals such as Cu, Al, W, etc. will have different conversion values arising from different displacement threshold energies and some­times different displacement contributions.

A standard procedure for calculating dpa has been published,19 although other definitions of dpa were used prior to international acceptance of the ‘NRT model’ where the letters represent the first letter of the three author’s last name (see Garner1 for details on earlier models). Caution must be exercised when compiling doses from older studies where displacement doses were calculated using other mod­els (Kinchin-Pease, Half-Nelson, French dpa, etc.) sometimes without clearly identifying the model employed. Conversion factors between the NRT model and various older models of dpa are provided in Garner,1 but all models agree within ^23%.

While sometimes controversial with respect to how far the dpa concept can be stretched to cover the full range of spectral differences for neutron and especially for charged particle environments, it appears that the dpa concept is very efficient to stretch over light water, heavy water, fusion, and spallation spectra, providing that all energy deposition and displacement processes are included. Note in Figure 1 how well the dpa concept collapses the data on neutron-induced strengthening of stainless steel into one response function for three very different spectra (light water fission, pure D-T fusion and ‘beam-stop’ spallation).20