Future US nuclear power plants might be safer, more efficient, and less expensive to build thanks to the efforts of several Sandians who are working with the US Nuclear Regulatory Commission (NRC) to develop new approaches for regulating reactors. This work is an extension of efforts to modify existing regulations for currently operating reactors.
Sandia, which has been doing probabilistic risk assessments (PRAs) for nuclear reactors for the NRC since the mid-1970s, is using that experience to help the agency revise decades-old regulations following a risk-informed regulatory approach.
Risk-informed regulation combines the results from PRAs with sound engineering practices to develop regulations that ensure a cost-effective approach to safety.
"The risk-informed approach allows the NRC to relax requirements that aren’t important and focus on those that are," says Allen Camp, Manager of Risk, Reliability, and Modeling Group 6410. "After 20 years of research, we’ve gained a clearer understanding of how accidents begin and progress. We know better which requirements make a plant safe — those that prevent or mitigate accidents — and which ones don’t."
The NRC has established goals stating that nuclear power plant operation should not expose the public to significant additional risk. PRAs have given the NRC considerable evidence to show that these goals are generally being met, indicating that nuclear energy continues to be a very safe method of generating electricity.
Over the years, PRAs have shown areas where the plants needed to be improved, and also areas where NRC regulations may require unwarranted conservatism.
Risk-informed alternatives
A risk-informed alternative to an existing regulation may eliminate or modify some requirements while imposing others. An important goal of risk-informed regulation is to use risk information to provide flexibility in plant operation and design, which can reduce construction and operating costs while enhancing safety.
In the early days of nuclear power plants — and then even more so after the Three Mile Island accident — the NRC regulations became extremely prescriptive, building many safety layers into the plant design. For example, the Three Mile Island accident raised concerns about the potential for ignition of hydrogen gas generated during an accident.
To address this concern, the NRC imposed new requirements to help prevent hydrogen combustion from becoming a problem in an operating plant. Later analyses showed that some power plant designs did not need all of these requirements.
"It wasn’t unusual for plant operators to spend training time preparing for a variety of extremely unlikely accidents instead of events that pose a much more real threat to the public," Allen says. "This is completely counter to safety."
Tom Sanders, Manager of Nuclear Initiatives Dept. 6406, says the NRC turned to Sandia to assist with risk-informed alternatives to regulations because of "its experience in risk assessment and severe accident analysis for nuclear power plants."
Unrealistic conservatism?
Over the past two decades, Labs personnel have visited many plants and built an understanding of their operations as part of nuclear power risk assessment activities. Sandia provides a unique perspective to the NRC on how the regulations affect nuclear power safety.
"The Labs’ role has been helping the NRC to figure out which parts of the regulations must stay, and which ones can be modified, and how to modify them,’ Tom says.
For example, Sandia is currently involved in identifying risk-informed alternatives to the regulations for emergency core cooling.
Emergency core cooling systems provide cooling water in a light water reactor in the event of a pipe break of other loss of coolant accident. Current regulations, which were developed in the early 1970s, require these systems to begin delivering water within seconds of a large pipe break, and to prevent core damage despite major failures within the system.
More recent risk information has shown several ways in which the methods used to evaluate these systems were unrealistically conservative. Reducing these conservatisms allows for more realistic training and testing of equipment, thus enhancing safety. According to Jeff LaChance (6410), changing these regulations could save the nuclear industry, and thus consumers of electricity, $1 billion or more because plants could operate at increased power levels even while reducing operating costs.
Current NRC regulations
The current NRC regulations were developed for light water reactors, such as those now in operation in the US. However, Sandia is working to bring the same risk-informed approaches to regulating advanced reactor designs being developed by the nuclear industry and DOE.
Risk-informed approaches have an even greater potential for savings in advanced reactors, because significant improvements can be made in the initial designs.
Sandia is supporting DOE’s Nuclear Energy Research Initiative as part of an industry/university/national laboratory team to develop risk-informed approaches for advanced designs.
The NRC is now preparing to regulate advanced reactor designs. Several companies have approached the NRC about the certification and licensing of new reactor designs.
Exelon Generation Co. is considering an application for a site permit during 2002, and several additional site permit applications are expected to follow. The type of reactor Exelon plans is a pebble bed modular reactor (PBMR) — as opposed to the light water reactor. Licensing for the PBMR will involve the use of risk information to adapt the current regulatory process to their design.
"Because no pebble bed reactors have ever been built in the US, the NRC does not have regulations to govern their design, construction, and operation," says Greg Wyss (6410). "While the first PBMR license will probably use a modified version of the current regulations, the NRC will likely develop completely new regulations for future reactors. And Sandia will help guide them in a risk-informed approach."
Given the initial successes of risk-informed regulation with the NRC, Sandia sees the potential for many other applications of these approaches, Tom says. Essentially any regulated industry or operation can make use of these ideas. For example, the new risk-informed regulations may include guidelines for recycling waste from the nuclear fuel cycle. Assessment of proliferation risk and development of related regulatory guidelines may also be possible.
Allen says the changes to the NRC regulatory process are proceeding and major changes for future reactors will happen over the next few years.
"The technical basis for this approach has been developed, and the NRC continues to be a leader among federal agencies in implementing risk-informed processes," Allen says.