For most, but not all fission-derived spectra, stainless steels are relatively immune to transmutation, espe­cially when compared to other elements such as aluminum, copper, silver, gold, vanadium, tungsten, and rhenium,5,21,24-27 each of which can rapidly become two or three component alloys via transmu­tation arising from thermal or epithermal neutrons. Whereas the properties of these metals are particu­larly sensitive to formation of solid transmutation products, stainless steels in general do not change their composition by significant amounts compared to preexisting levels of impurities, but significant amounts of helium and hydrogen can be produced in fission-derived spectra, however.

In stainless steels the primary transmutant changes that arise in various fission and fusion reactor spectra involve the loss of manganese to form iron, loss of chromium to form vanadium, conversion of boron to lithium and helium, and formation of helium and hydrogen gas.4,28 While each of these changes in solid or gaseous elements are produced at relatively small concentrations, they can impact the evolution of alloy properties and behavior.

For instance, vanadium is not a starting compo­nent of most 300 series stainless steels, but when included it participates in the formation of carbide
precipitates that change the distribution and chemi­cal activity of carbon in the alloy matrix. Carbon plays a number of important roles in the evolution of microstructure1 and especially in grain boundary composition. The latter consideration is very impor­tant in determining the grain boundary cracking behavior, designated irradiation-assisted stress corro­sion cracking (IASCC), especially with respect to the

29

sensitization process.

The strong loss of manganese in highly therma — lized neutron spectra has been suggested to degrade the stability of insoluble MnS precipitates that tie up S, Cl, and F, all of which are elements implicated in grain boundary cracking.30 Late-term radiation — induced release of these impurities to grain bound­aries may participate in cracking, but this possibility has not yet been conclusively demonstrated.

In some high-manganese alloys such as XM-19 manganese serves to enhance the solubility of nitrogen which serves as a very efficient matrix strengthener. In highly thermalized spectra the loss ofmanganese via transmutation has been proposed to possibly lead to a decrease in the strength of the alloy and perhaps to induce a release of nitrogen from solution to form bubbles.31

The overwhelming majority of published trans­mutation studies for stainless steels and high-nickel alloys steels have addressed the effects of He/dpa ratio on mechanical properties and dimensional instabilities. Much less attention has been paid to the effect of H/dpa ratio based on the long-standing perception that hydrogen is very mobile in metals and therefore is not easily retained in steels at reactor-relevant temperatures. As presented later, this perception is now known to be incorrect, espe­cially for water-cooled reactors.

The focus of most published studies concerned the much higher helium generation rates anticipated in fusion spectra (~3—10 appm He/dpa) compared to the lower rates found in fast reactors (~0.1—0.3 appm He/dpa).32 It was later realized that in some highly thermalized test reactors, such as HFIR, very large generation rates could be reached (~100 appm He/dpa), and even in pressurized water reactors the rate could be very high (^15 appm He/dpa).3 In heavy water reactors the rate can be much larger, especially in out-of-core regions.34,35

While some helium arises from (n, a) reactions with thermal and epithermal neutrons interacting with the small amounts of boron found in most stainless steels, the major contribution comes initially from high-energy threshold-type (n, a) reactions with the major alloy components. This type of reac­tion occurs only above high neutron threshold ener­gies (>6MeV). Figure 8 shows that nickel is the major contributor to helium production by (n, a) reactions,36 and thus the helium generation rate scales almost directly with nickel content for a large number of commercial steels.

A similar behavior occurs for production of hydrogen by transmutation via high-energy neutrons, where nickel is also the major source of hydrogen compared to other elements in the steel.4,7 In this case, the threshold energy is around 1 MeV with 58Ni being the major contributor.

This generality concerning nickel as the major source of He and H is preserved in more energetic fusion-derived spectra, although the He/dpa and H/dpa generation rates in fusion spectra are much larger than those of fast reactor spectra. When moving to very energetic spallation-derived neutron and proton spectra, however, the observation that nickel accounts for most of the helium and hydrogen is no longer correct. Iron, nickel, chromium, cobalt, and copper produce essentially the same amounts of helium and hydrogen for energies above ^100 MeV as shown in Figure 9.6

Another very important helium-generation pro­cess also involves nickel. Helium is produced via the two-step 58Ni(n, g)59Ni(n, a)56Fe reaction sequence.37,38 This sequence operates very strongly in mixed-spectrum reactors. 59Ni is not a naturally occurring isotope and is produced from 58Ni. Thus, this helium contribution involves a delay relative to

Figure 8 Cross-sections for (n, a) reactions as a function of neutron energy for common elements used in stainless steels. Reproduced from Mansur, L. K.; Grossbeck, M. L. J. Nucl. Mater. 1988, 155-157, 130-147. Nickel dominates the production of helium at higher neutron energies.

250 both steps of the sequence involve cross-sections that increase with decreasing energy and the second step exhibits a resonance at 203 eV, the generation rate per dpa in fast reactors increases near the core boundaries and out-of-core areas.

It is in thermalized neutron spectra characteristic of light and heavy water-cooled reactors, however, where the 59Ni(n, a) reaction can produce He/dpa generation rates that are significantly larger than those characteristic of fusion-derived spectra.

Nickel has five naturally occurring stable isotopes with 58Ni comprising 67.8% natural abundance, 60Ni comprising 26.2%, and ~6.1% total of61Ni, 62Ni, and 64Ni. There is no natural 59Ni or 63Ni at the beginning of radiation. During irradiation in a highly therma — lized neutron spectrum, all nickel isotopes are strongly transmuted, primarily to the next higher isotopic number of nickel. 59Ni has a half-life of 76 000 years and is progressively transmuted to 60Ni while 58Ni is continuously reduced in concentration. Therefore, the 59Ni concentration rises to a peak at a thermal neutron fluence of 4 x 1022 n cm-2 where the 59/58 ratio peaks at ^0.04 and then declines, as shown in Figure 10.

This transmutation sequence in nickel is an exam­ple of the isotopic shift category of transmutation defined earlier. For other elements used to make stain­less steels, there are no consequences to such a shift since the total amount of the element is unchanged

6.1%

total

and isotope shifts induce no significant consequences. However, in the case of nickel there is an intimate linkage between the displacement and transmutation processes that arises from the isotope shift.

The recoil of the 59Ni upon emission of the gamma ray produces only about five displacements per event, and usually is not a significant addition to the displacement dose. However, the isotope 59Ni undergoes three strong reactions with thermal and resonance (~0.2 keV) neutrons, two of which are exceptionally exothermic and can significantly add to the dpa level. These reactions, in order of highest-to-lowest thermal cross-section, are (n, g) to produce 60Ni, followed by (n, a) and (n, p) to produce helium and hydrogen, respectively.

Even at relatively low thermal-to-fast neutron ratios, the reaction sequence can produce significant amounts of helium. For example, He/dpa ratios in the order of ^3-8 appm dpa-1 can be experienced along the length of a 316 stainless baffle bolt in the baffle-former assembly of a pressurized water

reactor4’33’39 while comparable rates in fast reactors are in the order of 0.1—0.2 appm dpa-1. In therma — lized spectra the latter two reactions can quickly overwhelm the gas production produced by nickel at high neutron energies.

As mentioned previously, the thermal neutron reactions of 59Ni are quite exothermic in nature and release large amounts of energy’ thereby causing increases in the rate of atomic displacements’ and con­comitant increases in nuclear heating rates. Nuclear heating by elastic collisions with high-energy neu­trons is usually too small to be of much significance.

The 59Ni(n, a) reaction releases 5.1 MeV, produc­ing a 4.8 MeV alpha particle which loses most of its energy by electronic losses, depositing significant thermal energy but producing only ^62 atomic dis­placements per each event. However, the recoiling 56Fe carries 340 keV, which is very large compared to most primary knock-on energies, and produces an astounding 1701 displacements per event.

The thermal (n, p) reaction of 59Ni produces about one proton per six helium atoms, reflecting the difference in thermal neutron cross-sections of 2.0 and 12.3 barns, and is somewhat less energetic (1.85 MeV), producing a total of ^222 displacements per event.7,40 In addition, approximately five dis­placed atoms are created by each emission-induced recoil of60Ni. This reaction occurs at six times higher
rate compared to the 59Ni(n, a) reaction, resulting from a thermal neutron cross-section of 77.7 barns. In effect, the dpa rate increases during irradiation due to the three 59Ni reactions even though the neutron flux-spectrum may not change.

The major point here is that use of standardized computer codes to calculate dpa does not track shifts in isotopic distribution and therefore will underpredict the dpa level when 59Ni production is an important consideration.

A strong example of this time-dependent increase in dpa rate in highly thermalized light water spectra is shown for pure nickel in Figure 11 for a thermal — to-fast ratio of 2.0. Note that the calculated increase in this figure addresses only the 59Ni(n, a) reaction. Additional increases occur as a result of the 59Ni(n, p) and 59Ni(n, g) reactions, resulting in almost doubling of dpa by the three 59Ni reactions before a calculated dose of ^40 dpa is attained.

Recently, however, an even stronger example of the linkage of the 59Ni transmutation effect and the displacement process has been observed.34,35 In-core thermal-to-fast ratios in heavy water-moderated reactors such as CANDUs are in the order of ~10, but far from the core the ratio can be near ^1000. Compression-loaded springs constructed of high — nickel alloy X-750 were examined after 18.5 years of operation far from the core and were found to be
completely relaxed. Calculating the 59Ni contribu­tion, it was deduced that full relaxation occurred in ^3-4 years rather than the 650-700 years one would predict based on dpa calculated without taking into account the 59Ni contribution.

Therefore, in this case 59Ni contributed ^95% of the dpa. Additionally, 1100 appm of helium was cal­culated to have been produced at the mid-section of the spring in ^3 years, with ^20 000 appm helium having been produced when the spring was examined after 18.5 years of exposure.

There is another consequence of the 59Ni sequence that causes the temperature to increase during irradia­tion. At the peak 59Ni level reached at 4 x 1022 n cm-2, the nuclear heating rates from the energetic (n, a) and (n, p) reactions are 0.377 and 0.023 Wg-1 of nickel, significantly larger than the neutron heating level of -0.03 Wg-1 of natural nickel. Thus, an increase in nuclear heating of —0.4 W g — of nickel must be added to the gamma heating rate at the peak 59Ni level. Fractions of the peak heating rates that are pro­portional to the current 59Ni level should be added at nonpeak conditions. Depending on the nickel level of the steel and the level of gamma heating, which is the primary cause of temperature increases in the interior of thick plates, this additional heating contribution may or may not be significant.

Gamma heating is also a strong function of the thermal-to-fast (T/F) neutron ratio and the neutron flux, being —54 W g-1 in the center of the HFIR test reactor where the T/F ratio is —2.0. In pressurized water reactors at the austenitic near-core internals, however, the T/F ratios are lower by a factor of 2-10, depending on location, and the gamma heating rates in the baffle-former assembly are — 1-3 W g-1. In this case, an additional 0.4 Wg-1 of nuclear heating can be a significant but time-dependent addition to total heating, especially for high-nickel alloys.

It should be noted that thermal neutron populations can vary during an irradiation campaign with conse­quences not only on 59Ni production but also on gamma heating levels. In PWRs boric acid is added to the water as a burnable poison at the beginning of each cycle. As the 10B burns out the thermal neutron population increases, leading to an increase in gamma heating and transmutation.3,4 Over successive cycles there is a saw­tooth variation of gamma heating rate in the baffle — former assembly and therefore in A T, with the latter reaching values as large as ±20 °C in the worst case.

Additionally, another concern may arise in that small radiation-induced nickel-rich phases such as g0, Ni-phosphides, and G-phase may become less stable. This concern arises due to cascade-induced dissolution as the 56Fe from the 59Ni(n, a) reaction recoils within the precipitates, thereby altering the phase evolution in thermalized neutron spectra com­pared to nonthermalized spectra typical of fast reac­tors. These precipitates are known to form as a direct result of irradiation and contribute to hardening, swelling, and irradiation creep processes.1 The size of these precipitates at PWR-relevant temperatures (290-400 °C) is often comparable to or smaller than the —80nm range of the recoiling 56Fe atom.

Finally, another significant source of helium can arise from the implantation of energetic helium resulting from collisions with neutrons into the sur­face layers of helium gas-pressurized or gas-cooled components, often involving hundreds and often thousands of appm of injected helium. In gas-cooled reactors helium injection has been investigated as a possible degradation mechanism of alloy surfaces.41

In fast reactor fuel cladding helium was found to be injected into the inner surface, coming from two major sources, ternary fission events (two heavy fis­sion fragments plus an alpha particle) in the fuel and from helium recoiling from the pins’ helium cover gas as a result of collisions with neutrons.42

The injection rates from these two sources of injected helium are slowly reduced during irradia­tion, however, as heavy fission gases build up in the space between the fuel pellet and the cladding. These gases slow down the energetic helium atoms, reducing their energy sufficiently to prevent most of them from reaching the cladding. Helium injection at high levels was also found on the inner surface of helium-pressurized creep tubes.42 Although helium injection tends to saturate in fuel pin cladding with increasing dose, it does not saturate in pressurized tubes due to the lack of increasing fission gases to reduce the range of helium knock-ons in the gas phase.

Some studies have cited this early source of helium as contributing to the embrittlement of fuel pin clad­ding and its poor performance during transient heating tests,43 although more recent studies have linked the major mechanism to delayed grain boundary attack by the fission products cesium and tellurium.44,45