The proposed designs of GenIV reactors call for high operating temperatures and long exposure times, aimed at increasing thermal efficiency, enabling hydrogen production, decreasing waste and increasing safety, among other goals. These requirements will require imply the materials will be subjected to higher radiation damage doses than seen in light water reactors, requiring an even higher degree of alloys stability. In addition the higher temperatures and exposure times in corrosive environments will require much higher corrosion resistance than current materials. One of the main challenges of designing these materials is that our ability to predict material response in the face of the synergistic effects of temperature, radiation damage and a corrosive environment is limited. However, efforts are underway to understand mechanistically the degradation processes so that they can be extrapolated to conditions beyond the experimental database.
In this context, it is interesting to consider the effort made by the nuclear power industry to qualify nuclear fuel for operation at higher burnup in existing light water reactors. In the last two decades, the average discharge burnup of nuclear fuel in light water reactors has increased almost two fold, thereby increasing the reactor exposure time and the amount of radiation damage withstood by the cladding. Experience has shown that the material degradation rates can increase at high burnup, sometimes through the synergistic operation of various processes. Two examples will be given to illustrate the complexity of the problem:
(i) The rate of irradiation growth of zirconium alloys increases significantly at high burnup. The mechanism has been shown to depend on the amorphization of intermetallic ZrCrFe precipitates with consequent release of Fe into the Zr matrix. This has been shown to help nucleate component dislocation loops which preferentially absorb vacancies, thereby increasing the net rate of growth. This causes the interstitial and vacancy fluxes both to contribute towards irradiation growth, thus increasing the rate.
(ii) One of the most significant obstacles to approval of cladding operation at high burnup is difficulty in the evaluation of the behavior of the fuel during a reactivity initiated accident (RIA). The degradation of the mechanical properties of the zirconium cladding with increasing corrosion and consequent hydrogen ingress, the radiation damage and the microstructure evolution in the fuel all have to be taken into account in evaluating the likelihood of severe fuel failure at high burnup. This problem is still being addressed in various research programs.
Such mechanisms and the efforts to qualify nuclear fuel for higher burnup in light water reactors will be reviewed as a means of illustrating the challenges (known and unknown) faced by GenIV reactor materials.