Fission-fusion Correlation Research Articles

Defect interaction processes controlling the accumulation of defects produced by high energy recoils

Defect interaction processes involved in the microstructural evolution during irradiation with high energy recoils are discussed on the basis of experimental observations. First of all, an experimental procedure is outlined for identifying the nature of small defect clusters produced in cascades and subcascades. The procedure was the growth and shrinkage behavior of clusters produced in the cascades during subsequent irradiations with 1 MeV electrons in a high voltage electron microscope. It is shown that the analysis of the clusters produced during irradiation with high energy recoils can be used to identify the formation of sub-cascades. Various aspects of the one-dimensional glide of small self-interstitial atom (SIA) clusters and their role in defect accumulation are considered. The experimentally observed dose dependence of the cluster density is used to discuss the problem of fission-fusion correlation. Finally, some comments are made on the role of stochastic fluctuation of point defect reaction in the stability and lifetime of SIA clusters under cascade damage conditions.

Hydrogen generation arising from the 59Ni(n, p) reaction and its impact on fission—fusion correlations

While the influence of transmutant helium on radiation-induced microstructural evolution has often been studied, there is a tendency to overlook the influence of concurrently-generated hydrogen. Recent studies suggest that the influence of hydrogen may be enhanced in the presence of large amounts of helium, especially at lower irradiation temperatures typical of projected ITER operation. This interaction may have an impact on potentially hydrogen-sensitive processes such as irradiation-assisted stress corrosion cracking (IASCC) of stainless steel. In nickel-bearing alloys, one of the major sources of helium in neutron spectra with a significant thermal neutron component is the two-step 58Ni(n, γ) 59Ni(n, α) 56Fe reaction. It has generally been overlooked, however, that the competing (n, p) reaction on 59Ni can dominate the hydrogen production process since about one hydrogen atom is generated for every six helium atoms. Inclusion of the damage energy deposited by the (n, p) reaction in the alloy matrix also requires a slight modification in the 59Ni(n, α) damage enhancement formula published earlier. The impact of the (n, p) reaction on both hydrogen generation and displacement rates is evaluated in this paper for a variety of neutron spectra often employed in fission—fusion correlations.

The impact of spectral effects in fast reactors on data analysis and development of fission-fusion correlations

Models describing the production of freely migrating defects (FMD) during neutron irradiations were applied to materials studies in fast reactors, where neutron spectral effects have been largely ignored. Calculations for the Experimental Breeder Reactor II (EBR-II) and the Fast Flux Test Facility (FFTF) show that effects of spectral differences between in-core and out-of-core positions are significantly larger on the basis of freely migrating defects than on the basis of dpa. As an example, data on swelling behavior measured both in-core and out-of-core in EBR-II show nearly a linear dependence on FMD production but not on dpa.

New developments in fusion materials research using surrogate neutron spectra

The use of surrogate irradiation facilities to conduct fusion-relevant materials research requires that fission-fusion correlations be developed to account for the differences between the surrogate and fusion spectra. It is shown that with the exception of 59Ni isotopic doping of nickel-containing alloys and irradiation in a temperature-controlled environment it is almost impossible to study the effects of helium without introducing other important variables such as displacement rate, temperature history and solid transmutants. When helium effects are studied in the absence of differences in these variables, helium's impact on macroscopic properties is shown to sometimes be second order in magnitude. Differences in displacement rate and temperature history can completely dominate experiments directed toward the study of PKA recoil spectra and helium/dpa ratio.

New developments in fusion materials research using surrogate neutron spectra

The use of surrogate irradiation facilities to conduct fusion-relevant materials research requires that fission-fusion correlations be developed to account for the differences between the surrogate and fusion spectra. It is shown that with the exception of 59Ni isotopic doping of nickel-containing alloys and irradiation in a temperature-controlled environment it is almost impossible to study the effects of helium without introducing other important variables such as displacement rate, temperature history and solid transmutants. When helium effects are studied in the absence of differences in these variables, helium's impact on macroscopic properties is shown to sometimes be second order in magnitude. Differences in displacement rate and temperature history can completely dominate experiments directed toward the study of PKA recoil spectra and helium/dpa ratio.

Fission-fusion correlation by fission reactor irradiation with improved control

The necessity for the elimination of exposure to neutrons at lower temperatures during start-up and shut-down of the reactor is confirmed by experiments which compare the result of irradiation of metals with conventional and improved temperature control in JMTR. Only several percent of exposure to neutrons at lower temperatures is found to result in a one hundred percent difference of radiation induced microstructures in some cases. All differences can be understood from the microstructural development mechanisms, i.e. from the temperature dependence of the stability of point defect clusters and from the relationship of the transient temperature to the temperature for nucleation and growth. Fission neutron irradiation data with improved control are compared with fusion neutron irradiation data from RTNS-II. The differences of vacancy and interstitial clusters formed directly from cascades, observed in samples irradiated as thin foils, are understood when the difference in the primary recoil energy spectrum and the thermal stability of the clusters are taken into consideration. The defect structures which are developed and/or modified by the reactions of freely migrating point defects, such as vacancy clusters interstitial type dislocation structures and voids, observed in samples irradiated as bulk, are remarkably different in the two cases. Factors to be applied to the fission neutron irradiation dose to introduce microstructures equivalent to those by fusion neutrons are found to range widely, depending on the kind of microstructure, materials and irradiation temperature, from a value less than one up to 30 in the scaling of damage energy per atom.

Influence of transmutation and high neutron fluence on materials used in fission-fusion correlation experiments

This paper explores the response of three different materials to high fluence irradiation as observed in recent fusion-related experiments. While helium at fusion-relevant levels influences the details of the microstructure of Fe-Cr-Ni alloys somewhat, the resultant changes in swelling and tensile behavior are relatively small. Under conditions where substantially greater-than-fusion levels of helium are generated, however, an extensive refinement of microstructure can occur, leading to depression of swelling at lower temperatures and increased strengthening at all temperatures studied. The behavior of these alloys is dominated by their tendency to converge to saturation microstructures which encourage swelling. Irradiations of nickel are dominated by its tendency to develop a different type of saturation microstructure that discourages further void growth. Swelling approaches saturation levels that are remarkably insensitive to starting microstructure and irradiation temperature. The rate of approach to saturation is very sensitive to variables such as helium, impurities, dislocation density and displacement rate, however. Copper exhibits a rather divergent response depending on the property measured. Transmutation of copper to nickel and zinc plays a large role in determining electrical conductivity but almost no role in void swelling. Each of these three materials offers different challenges in the interpretation of fission-fusion correlation experiments.

Role of charged particle irradiations in the study of radiation damage correlation

Charged particle irradiations were originally expected to provide means to simulate the effect of neutron irradiations. However, it has been recognized that quantitative and sometimes even qualitative simulation of neutron radiation damage is difficult and the role of the charged particle irradiations has shifted to establishing fission-fusion correlation based on fundamental understanding of the radiation damage phenomena. We have been studying radiation effects in fusion materials using energetic ions from the latter standpoint. In this paper, we will review our recent results using a heavy-ion/electron microscope link facility together with sets of small heavy ion and light ion accelerators on cascade damage produced by energetic primary recoils and on the effect of helium on microstructural and microchemical evolution. Some of the other applications of the ion accelerators will also be mentioned.

Implications of neutron spectrum and flux differences on fission-fusion correlations at high neutron fluence

Abstract The application to fusion environments of materials data derived from fission reactors involves considerations related not only to neutron spectra but also the often dominant effect of displacement rate. It is shown in this paper that fission-fusion correlation experiments directed toward helium effects and PKA recoil spectra are frequently difficult to interpret due to the strong influences of displacement rate, low energy recoils from thermal neutron absorption and in some cases a large influence of solid transmutants. It is also shown that materials data published in earlier decades must be reevaluated in light of recent advances in defining irradiation parameters.

Fission-fusion correlation of void swelling in pure nickel

Abstract Microstructures have been compared in annealed and cold worked pure nickel irradiated with fission (JOYO) and fusion (RTNS-II) neutrons. The swelling is found to be considerably higher for the fusion neutrons than for the fission neutrons on the dpa basis. This difference is due mainly to the void size. Since the void size is found to have a close relation with the damage production rate, it is expected that the present fission-fusion difference is caused mainly by the difference in damage production rate. The effects of cold work on swelling are found to be also a function of the damage production rate. The present study has shown the importance of the damage production rate in the correlation studies. Quantitative estimation of the effects of PKA spectral or transmutative differences for fission and fusion neutrons seems to be possible only after fission and fusion neutron irradiations with highly controlled damage production rate as a parameter are achieved.

Fission-fusion correlations for swelling and microstructure in stainless steels: Effect of the helium to displacement per atom ratio

The irradiated fusion materials data base will be initially developed in fisson reactors. Hence, predictions of the in-service performance of materials in fusion reactors must account for the effect of a number of variables which differ between fission and fusion environments. Ideally such fission-fusion correlations should reflect sound mechanistic understanding. In this paper we review the effects of the helium to displacement per atom ratio on microstructural evolution, with emphasis on austenitic stainless steels. Charged particle data is analyzed to determine mechanistic trends. These results appear to be consistent with a more detailed comparison made between data from neutron irradiations of type 316 stainless steel in EBR-II (low helium) and HFIR (high helium). Finally, a model calibrated to the fission reactor data is used to extrapolate to fusion conditions.

A model based fission-fusion correlation of cavity swelling in stainless steel

The helium/dpa (He/dpa) ratio in stainless steel under fusion conditions will be about 30 times greater than in EBR-II and about 7 times less than in HFIR. To permit the meaningful use of fission reactor swelling data, the effects of various He/dpa ratios must be understood. A rate theory model with several novel features and which explicitly considers some potential He/dpa ratio effects has been calibrated using data from both EBR-II and HFIR. The calibrated model has been used to predict swelling under fusion conditions. The results indicate that simple extrapolations of swelling data from either EBR-II or HFIR may not be valid.