Helping NRC Advance Reactor Safety
Since its creation in 1974, the U.S. Nuclear Regulatory Commission (NRC) has been a principal sponsor of research to advance nuclear technologies. Because ORNL has been a leading participant in the performance of NRC research, we have been better able to maintain our role as a nuclear laboratory, especially over the past 15 years. "We are still doing important nuclear safety work for the NRC," says Claud Pugh, head of NRC programs at ORNL.
For example, in 1998 ORNL investigated whether electric utility industry restructuring would increase the risk of loss of off-site power (LOOP) to nuclear power plants. Risk analyses have shown that LOOP events lead to increased risk of reactor fuel core damage in the event of an accident. An ORNL team—B. J. Kirby of the Engineering Division and A. Bruce Poole and John D. Kueck, both of the Engineering Technology Division—found significant differences in the way restructuring is being carried out during their visits to 17 nuclear plants in 10 regions.
Historically, the electric power industry in the United States has been dominated by utilities that control the generation, transmission, and distribution of electricity in their service areas. With restructuring, power suppliers will compete for customers located anywhere on the national transmission grid, and regional grid control will be the responsibility of independent system operators (ISOs).
"It is uncertain how the ISOs will maintain reliability of the grid, and specifically, the reliability of the circuits that provide power to nuclear power plants," Kueck says. "The nuclear industry and NRC need to better understand the synergism among power plants within a given region." They also must understand how distribution of responsibilities and increased competition for the most economical power (often obtained from outside an ISO's geographic area) can affect the potential for LOOP events. To get better answers, probabilistic risk assessments may be performed.
In the Heavy-Section Steel Irradiation Program, Randy Nanstad, Shafik Iskander, Don McCabe, and Mikhail Sokolov, all of ORNL's Metals and Ceramics Division, have been developing data to account conservatively for irradiation embrittlement of reactor pressure vessel steels. Neutrons from reactor fuel cores cause the ferritic steels in reactor pressure vessels to become a little brittle, but they normally don't develop cracks from the high temperatures and water pressures typical of reactor operation. To determine how resistant these embrittled vessel steels are to fracture, the researchers, including ETD's John Merkle, have developed a new experimental fracture mechanics method that requires fewer experiments than were needed before for each steel examined.
"The method helps the reactor operator and the NRC determine whether the embrittled steel is sufficiently resistant to fracture to ensure that the continued operation of the vessel is safe," Nanstad says.
This "master curve" concept has been accepted as an American Society for Testing and Materials standard. Other countries are considering using the concept in their commercial nuclear power programs. This accomplishment is one of many reasons why ORNL has long been the NRC's lead laboratory for nuclear pressure vessel technology.
