Post-Fukushima enhancements at CANDU NPPs - part 1

The events which took place at Fukushima Daiichi Nuclear facility in Japan prompted NPP utilities to evaluate and enhance their ability to mitigate challenges caused by extreme external events and manage challenges caused by those events. One such challenge is an extended loss of onsite and offsite electrical power which results in the unavailability of reactor heat removal systems. As a preparatory measure for such events, NPPs in Canada have adopted the use of portable/mobile equipment to provide emergency power to critical monitoring instrumentation and key components and to provide cooling water to critical loads. This concept is similar to the US FLEX strategy developed by the Nuclear Energy Institute (NEI). In Canada the concept is referred to as the Emergency Mitigation Equipment (EME) strategy and is designed to prevent a Beyond Design Basis Accident (BDBA) from progressing to a severe accident (SA). This paper will discuss the EME strategies and their application at Canadian utilities. If left unmitigated, the subsequent core damage from a loss of core cooling event will result in the pressurization of the containment envelope leading to an uncontrolled release of fission products to the surrounding environment. Therefore protection of the containment integrity is a key SA management objective and Canadian NPPs have developed implemented a number of diverse strategies in the plant designs (e.g., engineered safety features, EME, design changes specific for SA) to mitigate the potential containment challenges. This paper will discuss a number of the engineered safety features and design changes developed at Canadian utilities. The buildup of hydrogen in the containment system following a SA has the potential to cause a very large explosion, if not property mitigated. As seen in Japan at the Fukushima Daiichi NPP, detrimental effects from the explosion caused significant damage to the containment system, allowing a pathway to the environment for radionuclides. To control hydrogen accumulation in the containment, Canadian utilities have implemented measures to control hydrogen concentration inside the containment. This paper will discuss the engineered safety features and design changes specific to hydrogen control in containment.