Advancing Ruggedness of Nuclear Stations By Expanding Defence In Depth in Critical Areas

The nuclear industry continues to rise above the challenges it has faced over the years from external events and internal events. Fukushima event has shed light on a few vulnerabilities that could be overcome by utilizing the current state of technology. Common cause from sea water ingression was not conceived to have the entire electrical power system including AC and DC disabled beyond reasonable recovery. Rather than focusing on the solutions for lessons from Fukushima, it is better to address 'Fukushima type' events and advance the resilience of the NPPs. The effort needs to be on exploring different approaches to overcome such vulnerabilities so that a variety of solutions are available to make appropriate choices on improving NPP ruggedness based on anticipated challenges in the regions. In a technology neutral approach for light water reactors (LWR) there are 4 critical areas that are significant for ensuring nuclear safety. (1) Reactor trip, (2) Depressurization, (3) Emergency Core Cooling, and (4) Containment integrity. The reactor trip had not suffered any significant setbacks in the immediate past but provisions to address Anticipated Transients without Scram (ATWS) were generally included in most designs. While the technology has advanced, software driven/assisted trips are becoming popular and desirable. However, a diverse approach with least probability of potential interference needs to be provided in the control room and remote shutdown area to advance the ruggedness of rector trip. Depressurization is essential for passive as well as active cooling systems and therefore the approaches to de-pressurize should have more than one approach to ensure its success. In the absence of diverse approaches to de-pressurize, it is more important to consider RCS cooling capability during accidents or transients while the reactor is at a higher pressure. In the area of Emergency Core Cooling, the events history demonstrates greater success on diversity than increasing redundancy. There are several events both external and internal that could cause the failure of AC motor driven cooling systems. DC operated steam driven systems and diesel driven cooling systems have avoided several near core melt conditions. Containment Integrity is the last defence for protecting the people and the environment. Diversity in containment cooling is essential for keeping the pressure transients under control. Design provisions to connect potable cooling systems for heat removal and capability to flood the reactor cavity are essential. Recognizing the remote possibility of a severe accident, reliable containment venting (capability to operate with potable energy sources) and filtering could be explored as an option for ensuring an additional layer of protection. These four critical areas need to be viewed as layers in the defence of depth and consequently would require a design that fully removes and common cause failures. Ruggedness of these layers can be achieved only when the process signal sources, power supply and processing of the logic is executed independently. The electrical power system should be re-evaluated for bringing flexibility and adaptability for achieving greater level of safety. (author)