In the 1960s and 1970s, heavy ion irradiation was developed for the study of radiation damage pro­cesses in materials. As ion irradiation can be conducted at a well-defined energy, dose rate, and temperature, it results in very well-controlled experi­ments that are difficult to match in reactors. As such, interest grew in the use of ion irradiation for the purpose of simulating neutron damage in support of the breeder reactor program.1-3 Ion irradiation and simultaneous He injection were also used to simulate the effects of 14MeV neutron damage in conjunction with the fusion reactor engineering program. The application of ion irradiation (defined here as irradi­ation by any charged particle, including electrons) to the study of neutron irradiation damage caught the interest of the light water reactor community to address issues such as swelling, creep, and irradiation assisted stress corrosion cracking of core structural materials.4-6 Ion irradiation was also being used to understand the irradiated microstructure of reactor pressure vessel steels, Zircaloy fuel cladding, and materials for advanced reactor concepts.

There is significant incentive to use ion irradiation to study neutron damage as this technique has the potential for yielding answers on basic processes in addition to the potential for enormous savings in time and money. Neutron irradiation experiments are not amenable to studies involving a wide range of conditions, which is precisely what is required for investigations of the basic damage processes. Simula­tion by ions allows easy variation of the irradiation parameters such as dose, dose rate, and temperature over a wide range of values.

One of the prime attractions of ion irradiation is the rapid accumulation of end of life doses in short periods of time. Typical neutron irradiation experi­ments in thermal test reactors may accumulate dam­age at a rate of 3-5 dpayear-1. In fast reactors, the rates can be higher, on the order of 20 dpa year. For low dose components such as structural components in boiling water reactor (BWR) cores that typically have an end-of-life damage of 10 dpa, these rates are acceptable. However, even the higher dose rate of a fast reactor would require 4-5 years to reach the peak dose of ^80 dpa in the core baffle in a pressurized water reactor (PWR). For advanced, fast reactor con­cepts in which core components are expected to receive 200 dpa, the time for irradiation in a test reactor becomes impractical.

In addition to the time spent ‘in-core,’ there is an investment in capsule design and preparation as well as disassembly and allowing for radioactive decay, add­ing additional years to an irradiation program. Analysis of microchemical and microstructural changes by atom probe tomography (APT), Auger electron spec­troscopy (AES) or microstructural changes by energy dispersive spectroscopy via scanning transmission electron microscopy (STEM-EDS) and mechanical property or stress corrosion cracking (SCC) evalua­tion can take several additional years because of the precautions, special facilities, and instrumentation required for handling radioactive samples. The result is that a single cycle from irradiation through micro­analysis and mechanical property/SCC testing may require over a decade. Such a long cycle length does not permit for iteration of irradiation or material conditions that is critical in any experimental research program. The long cycle time required for design and irradiation also reduces flexibility in altering irradia­tion programs as new data become available. The requirement of special facilities, special sample handling, and long irradiation time make the cost for neutron irradiation experiments very high.

In contrast to neutron irradiation, ion (heavy, light, or electrons) irradiation enjoys considerable advantages in both cycle length and cost. Ion irradia­tions of any type rarely require more than several tens of hours to reach damage levels in the 1-100 dpa range. Ion irradiation produces little or no residual radioactivity, allowing handling of samples without

the need for special precautions. These features translate into significantly reduced cycle length and cost. The challenge then is to verify the equivalency between neutron and ion irradiation in terms of the changes to the microstructure and properties of the material. The key question that needs to be answered is how do results from neutron and charged particle irradiation experiments compare? How, for example, is one to compare the results of a component irradiated in-core at 288 °C to a fluence of 1 x 1021n cm~ (E > 1 MeV) over a period of one year, with an ion irradiation experiment using 3 MeV protons at 400 °C to 1 dpa (displacements per atom) at a dose rate of 10~5dpas_1 (~1day), or 5MeV Ni2+ at 500°C to 10dpa at a dose rate of 5 x 10~3 dpas-1 (~1 h)?

The first question to resolve is the measure of radia­tion effect. In the Irradiation assisted stress corrosion cracking (IASCC) problem in LWRs, concern has cen­tered on two effects of irradiation: radiation-induced segregation of major alloying elements or impurities to grain boundaries, which may cause embrittlement or enhance the intergranular stress corrosion cracking (IGSCC) process, and hardening of the matrix that results in localized deformation and embrittlement. The appropriate measure of the radiation effect in the former case would then be the alloy concentration at the grain boundary or the amount of impurity segregated to the grain boundary. This quantity is measurable by analytical techniques such as AES, APT, or STEM-EDS. For the latter case, the measure of the radiation effect would be the nature, size, density, and distribution of dislocation loops, black dots, and the total dislocation network, and how they impact the deformation of the alloy. Hence, specific and mea­surable effects of irradiation can be determined for both neutron and ion irradiation experiments.

The next concern is determining how ion irradia­tion translates into the environment describing neu­tron irradiation. That is, what are the irradiation conditions required for ion irradiation to yield the same measure of radiation effect as that for neutron irradiation? This is the key question, for in a postirra­diation test program, it is only the final state of the material that determines equivalence, not the path taken. Therefore, if ion irradiation experiments could be devised that yielded the same measures of irradiation effects as observed in neutron irradiation experiments, the data obtained in postirradiation experiments will be equivalent. In such a case, ion irradiation experiments can provide a direct substitute for neutron irradiation. While neutron irradiation will always be required to qualify materials for reactor
application, ion irradiation provides a low-cost and rapid means of elucidating mechanisms and screening materials for the most important variables.

A final challenge is the volume of material that can be irradiated with each type of radiation. Neutrons have mean free paths on the order of centimeters in structural materials. One MeV electrons penetrate about 500 pm, 1 MeV protons penetrate about 10 pm, and 1 MeV Ni ions have a range of less than 1 pm. Thus, the volume of material that can be irradiated with ions from standard laboratory-sized sources (TEMs, accelerators), is limited.