The overall objective of this Special Topic Report (STR) is to provide the knowledge of how the reactor environment (fast neutron flux, temperature, water chemistry, etc.) and the Zr-alloy microstructure, which is a function of material chemistry and manufacturing process, impacts fuel performance during normal operations, transients, design basis accidents and interim dry storage.
Although the performance of water reactor fuel has improved greatly relative to early experience, social, regulatory and operational incentives exist to both maintain the gains that have been achieved and to make further improvements in fuel reliability. The primary objectives in these areas are to increase fuel reliability by
- Reducing the primary fuel failure frequency and minimize the consequences of fuel failures when they occur and
- Minimizing operational effects due to factors such as fuel assembly and channel bowing, that can affect thermal margins (LOCA, DNB, Dryout) and core control capabilities (control rod insertion).
Maintaining and improving fuel reliability requires an understanding of the behaviour of fuel and materials as related to in-reactor conditions and the mechanisms that have been observed to cause fuel failures. A key factor in improving fuel reliability is the identification of the cause or causes of failure. Such information, in turn, requires the examination and analysis of irradiated fuel at reactor sites (poolside examinations), in hot cells and in related laboratories. Thus, to make progress toward ultra-high reliability fuel and to reduce the potential for post-failure degradation, it is imperative to examine both failed and non-failed (reference) fuel. The most cost efficient way to carry out these examinations is to begin with a good understanding of the known mechanisms of failure and degradation and of the principal methods for examining irradiated fuel. With such an understanding, fuel investigation and development programs can be focused on the likely causes of failure or degradation, while unnecessary costly and time consuming work can be minimized. One of the objectives of this Report is to provide such an understanding.