High Fidelity Ion Beam Simulation of High Dose Neutron Irradiation

The promise for developing new, advanced nuclear reactor concepts and the extension of life of existing light water nuclear reactors rests heavily on understanding how radiation degrades materials that serve as the structural components in reactor cores. In high dose fission reactor concepts (GEN-4) structural materials must survive up to 200 dpa of damage at temperatures in excess of 400C. At such high damage levels, the major degradation modes are likely to be driven by void swelling and phase stability. As materials degradation due to irradiation is both a life-limiting and a concept-validating phenomenon, it is truly the grand challenge for the growth and vitality of nuclear energy world-wide.
Traditionally, research to understand radiation-induced changes in materials is conducted via radiation effects experiments in test reactors, followed by a comprehensive post-irradiation characterization plan. However, test reactors cannot create radiation damage significantly faster than that in commercial reactors, meaning that radiation damage research often cannot "get ahead" of problems discovered during operation.
A promising solution to the problem is to use ion irradiation that can produce high damage rates with little or no residual radioactivity. The advantages of ion irradiation are many. Dose rates (typically 10-3 to 10-4 dpa/s) are much higher than under neutron irradiation (10-7 to 10-8 dpa/s) which means that 200 dpa can be reached in days or weeks instead of decades. Because there is little activation the samples are not radioactive. Control of ion irradiation experiments is much better than experiments in reactor. Measurement of temperature, damage rate and damage level is difficult in reactor, resulting in reliance on calculations to determine the total dose, and estimate irradiation temperature. By contrast, ion irradiations have been developed to the point where temperature is extremely well controlled and monitored, and damage rate and total damage are also measured continuously throughout the irradiation and with great accuracy.
However, ion irradiation has several potential drawbacks; the volume of irradiated material, the effect of high damage rate on the resulting microstructure, and the need to account for important transmutation reactions that occur in reactor, such as the production of He and H. Understanding and modeling the microstructure-property relationship, and the development of micro-sample fabrication and testing, while not a replacement for bulk property determination, hold the promise for minimizing the drawback of limited irradiated volume. The extent to which high damage rates can produce microstructures relevant to reactor conditions is a major challenge, but significant progress is being made to address this issue.
The strategy to account for transmutation reactions is to simultaneously irradiate a target with heavy ions while also bombarding it with He and/or H. Such a process requires multiple accelerators coupled in a double or triple beam facility. Recently, the UK has begun development of a plan to create such a double beam facility at its Dalton Cumbrian Facility and the proposed project will underpin this activity.
To qualify ion irradiation it is necessary to reproduce as best as possible both the neutron irradiated microstructure and the neutron-induced macroscopic property changes using ion irradiation. This task is best addressed using a combination of state of the art experimental techniques closely coupled to modeling, which can yield mechanistic understanding of the defect development process, while taking into account in the experimental design and theoretical modeling the possible confounding factors mentioned above.

The industrial and technological impact of this work lies in the role it will play in making best use of all the different types of data, existing and future, on the effects of irradiation damage in materials. Neutron-irradiation experiments are in most senses definitive, but are slow and expensive to perform. Ion irradiation experiments provide accelerated testing environment, are widely used, produce non-active materials that can be readily tested in "ordinary" laboratories, but there are many problems with relating results from these experiments to those from neutron irradiation.

This project aims to use experiment and modelling to firmly link results from these two complementary experimental methods. As such there will be a very high degree of potential and actual impact in the use of structural materials in the nuclear power industry. This will be for both current nuclear installations - lifetime extension, and effects of material embrittlement in potential accident situations, and for nuclear new build, where the rapid assessment of existing and potential new materials in the higher radiation flux and higher temperature environments projected for the next generation of nuclear materials is highly important.