Chapter 16 - Environmental considerations in the qualification of monitoring instruments for severe accidents

Abstract

A severe accident in a nuclear power plant may have significant consequences due to potential degradation of the sheath on nuclear fuel rods, which may subsequently lead to release of radioactive substances into the atmosphere. A severe accident management (SAM) program deals with mitigating the effects of such accidents and reducing related consequences through emergency actions. The availability of instrumentation systems to monitor relevant parameters of the plant during and after a severe accident is extremely important to support effective execution of a SAM program. The environment within the vicinity of the reactor can become extremely harsh and hazardous during the progression of a severe accident. Similarly, harsh conditions can also occur in the areas adjacent to high pressure boundaries, high temperature regions, and near the facilities/systems housing high-level radioactive materials, such as spent fuel bays. The accident could cause the levels of some physical parameters such as temperature, pressure, humidity, and radiation level to rise exceedingly higher than the typical levels during normal operation. As such, an accident can also lead to rapid degradation of performance of equipment and integrity of safety protection systems, and even render some systems fail completely. Hence, it is important that instruments used for monitoring plant conditions in SAM should be designed and constructed in such a way that their essential functions are protected under the worst-case scenarios. In this regard, it is imperative to understand, characterize, and assess the impacts of a severe accident on the environment within the plant where these instruments may be installed. Only then, one can effectively protect them from damage. Even though the exact condition after a severe accident may depend on specific plant design and the nature of the accident itself, there are many commonalities among them. This Chapter has summarized various actual and estimated/simulated severe accident environmental conditions reported in the literature. To put the instrument design requirements for severe accident conditions into perspective, several key industrial standards and regulatory guidelines have also been reviewed.