Published on Feb 27, 2018: The Denki Shimbun (The Electric Daily News)
Shojiro Matsuura
President & CEO

Recently, an open symposium entitled “Use of Risk Information for Improving the Safety of Nuclear Power Generation” was held under the sponsorship of the Central Research Institute of Electric Power Industry’s Nuclear Risk Research Center (previously reported in this newspaper on February 9). RIDM, which has noted in headlines, is code for Risk-Informed Decision-Making, the fundamental concept for realizing the symposium’s main theme.

The intense interest demonstrated by people associated with the nuclear power business in this symposium arises out of the Nuclear Regulation Authority’s new regulatory inspection system, which will be introduced on a trial basis in FY2019 and then fully enforced beginning in FY2020. That is to say, the situation today is one where regulators and operators are both aware that effective utilization of RIDM is very important for complying with new regulatory systems, and continuing to operate nuclear power plants in a safe and stable manner.

With most technology systems, the construction of actual objects has been based upon trustworthy methodology, which is grounded in technical experience and empirical testing and research, technical standards, and a scope of technical conformity determined by these two elements, and has been further improved incrementally so that the technology reaches a stage of maturity. This evolution has been regarded as an outcome achieved through “deterministic judgments.”

Deterministic judgment has also been the method adopted in the challenge of developing and utilizing nuclear power generation technology systems for civilian use, which began in the 1950s. With nuclear power technology systems in particular, there is the presumption and mission “to reduce to the extent practically feasible any risk that hindrances posed by radiation or radioactive materials produced through such use have on people and the environment.” For this reason, significant paradigm shifts have taken place in the fundamental concept, technology systems, and their operation in pursing safety assurance every time one of several major accidents has occurred at a nuclear reactor around the world.

Among these, the accident at Three Mile Island indicated the importance of decision-making that applies not only deterministic judgments but also probabilistic assessments. The Three Mile Island accident created the likelihood and necessity for a major paradigm shift in our approach to improving safety.

Prior to this accident, it was believed that “a large rupture of a main steam pipe would be a major accident for a pressurized water reactor, but that such an event was unrealistic in engineering terms and there was no possibility such an accident would occur.” However, the Three Mile Island accident, in fact, did entail an event on par with a small-to-medium diameter pipe failure causing a major accident that led to core damage. Because probabilistic risk assessment (PRA) had already predicted that such an event could occur many years before, the potential of PRA then came to be firmly recognized.

Since the Three Mile Island accident, PRA research and its use have been vigorously advanced in the United States. The U.S. Nuclear Regulatory Commission (NRC) started using PRA in practice to improve regulatory objectivity and effectiveness. The amazing progress made in computational science, which drove this development, has been a tremendous contribution.

The NRC comprehensively fused the previous deterministic risk assessment and the new probabilistic risk assessment to create the strategic RIDM method. Moreover, at the beginning of the 21st century, the NRC put into effect a new regulatory method, the Reactor Oversight Process (ROP), with the intention of strengthening the scientific rationale for regulations, selectively focusing its regulatory resources on important issues, and encouraging power operators’ voluntarily endeavors to improve safety. RIDM is central to this process.

The results achieved with ROP have been outstanding. Several NRC leaders reminisced that, at the time ROP was introduced, “Tremendous preparation and effort as well as unwavering resolve were essential to both regulators and operators for its implementation.” It is my earnest hope that the time will soon come when results achieved through the sincere effort and resolve of Japan’s regulators and operators to take up this challenge will be properly recognized by society.

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