Where There’s Steam, There’s … a Steam Generator

Kenneth Karwoski
Senior Advisor for Steam Generators
 

News articles recently brought the phrase “steam generators” into the national conversation, but we’re not talking about teakettles. pwr[1]Steam generators provide vital technical and safety functions at many U.S. nuclear power plants.

In the United States, steam generators are only found in 65 pressurized-water reactors, one of the two types of U.S. reactors. There can be two to four steam generators for each reactor unit. The generators mark the spot where two closed loops of piping meet. The first loop sends water past the reactor core to carry away heat, and this loop is at such high pressure that the water never boils. The second loop is at a lower pressure, so the water in this loop turns to steam and runs the plant’s turbine to generate electricity.

The steam generator’s main technical job is to let the first loop pass its heat to the second loop as easily as possible. To do this, a steam generator packs thousands of small tubes closely together, allowing the maximum area for heat to pass through the tubes and into the second loop’s water.

At the same time, the steam generators provide an important safety barrier – the first loop can contain radioactive material, so the tubes must keep the two loops of water separate. NRC rules require plants to closely monitor the second loop and immediately shut the reactor down if a tube leak exceeds very strict limits.

The NRC’s rules for inspections, maintenance and repair of steam generator tubes help ensure the tubes continue providing the safety barrier. If an inspection shows a tube is starting to get too thin, the plant will repair or even plug a tube to maintain safety.

Steam generator tube material has improved over time. The first steam generators had tubes made from a type of stainless steel that experience showed could be corroded by the chemicals, temperatures and pressures in the first and second loop. Over time, plants have replaced those steam generators with ones using more advanced alloys that are less likely to corrode. Steam generator replacement only happens when the reactor is shut down for refueling, and plant owners bring in hundreds of specialized workers to safely remove the old generators and install the new ones. The old generators have to be safely disposed of as low-level radioactive waste.

Two Important Reports about Steam Generators at SONGS Go Public

Victor Dricks
Senior Public Affairs Officer
 

The NRC made public today redacted versions of two reports prepared by Mitsubishi Heavy Industries concerning the steam generator replacement at the San Onofre Nuclear Generating Station (SONGS).

The Steam Generator Root Cause Analysis Report and a Supplemental Technical Evaluation Report  were prepared by Mitsubishi Heavy Industries as part of its effort to determine what contributed to the unusual wear in the steam generators after they were installed in 2010 and 2011 at Units 2 and 3, respectively.

The NRC is using a variety of regulatory actions, such as inspections and investigations, to ensure that it is comprehensively addressing the issues that have arisen at the SONGS nuclear power plant.

On Sept. 28, 2012, the NRC began an expansive investigation on the completeness and accuracy of information that Edison provided to the NRC regarding the steam generator degradation under the NRC’s regulatory requirements.

These reports are included in an array of documents being reviewed by the NRC as we investigate whether Edison demonstrated sufficient due diligence in its oversight of the redesign of the steam generators; how design changes that were made or rejected may have affected the safety of the steam generators; and the truthfulness and accuracy of all the information Edison has provided to the NRC regarding the redesign and replacement of the steam generators.

Separately from the ongoing investigation, the NRC is evaluating Edison’s responses to questions the NRC has raised about their request to restart Unit 2 at the plant.

Additionally, the Atomic Safety and Licensing Board is reviewing issues related to the Confirmatory Action Letter issued by the NRC staff to Southern California Edison.