Thermal Hydraulics: Heat, Water, Nuclear Power and Safety

Scott Krepel
Reactor System Engineer

One of the most important safety questions in a nuclear power plant is: Can you cool the very hot nuclear fuel in an accident when normal cooling is disrupted? The scientific field best equipped to answer this question is called “thermal hydraulics.”

bwrThe first part of the term, “thermal,” relates to heat transfer, such as the movement of heat from the burner on a stove to the water in a pot via the metal of the pot. The second part, “hydraulic,” relates to the flow of a fluid such as water. The combination, “thermal hydraulics,” can be applied to systems where both the flow of fluid and the transfer of heat are important – such as a nuclear power plant.

I work in the NRC’s Office of Nuclear Regulatory Research as part of a team dedicated to expanding our understanding of thermal hydraulics and applying that understanding in nuclear power plant safety. Over time, we’ve put much effort into incorporating existing knowledge into the NRC’s thermal hydraulics computer simulation program, TRACE. This program allows NRC staff to construct computer models of the cooling systems of a nuclear power plant and then simulate accidents such as pipe breaks (but not wildly improbable events such as the considerable destruction caused near the end of a typical superhero action movie).

TRACE is constantly being pushed to become more accurate, reliable and versatile. Universities and test facilities around the world are conducting experiments and accident simulations to collect real-world data that can be used to determine TRACE’s ability to accurately predict specific phenomena. We use the outcomes to update the program as needed to make it more accurate and to better capture certain phenomena.

Sometimes, new safety issues may result in further investigation of certain scenarios and further evolution of TRACE. Ultimately, the goal of this work within the research arm of the NRC is to continuously expand our understanding of situations which may impact the cooling of the nuclear fuel. This knowledge can then be used to ensure that the public and the environment are protected in the unlikely event of an accident at an U.S. nuclear power plant.

How the NRC is Responding to the Cooling Water Leak At the Palisades Nuclear Plant

Prema Chandrathil
Region III Public Affairs Officer

On Friday, the Palisades plant in Covert, Mich., shut down so plant personnel could find and repair a leak somewhere in the reactor’s cooling water system. Soon after, the NRC dispatched an additional inspector from the Regional III office, located in Lisle, Ill., with a background in mechanical testing and repairs. He supplements the two NRC resident inspectors as they evaluate the plant’s repair activities.

palisades_smallFor more than a week now, the NRC resident inspectors on site have been following the actions taken by workers at Palisades to find the leak. The resident inspectors reviewed the data. They also watched plant workers as they isolated different parts of the system to conduct tests to try and identify exactly where the leak was coming from.

Plant workers caught the problem because the water level in the component cooling water system was going down slowly. This system uses non-reactor water to cool certain safety equipment. Per NRC regulations the system is required to be monitored. When the plant shut down the system was leaking about 35 gallons per hour. This water was captured and released to Lake Michigan through an established monitored release path. The leak did not place the plant or the public in danger.

It’s now believed a heat exchanger in the system is the source of the leak. A heat exchanger is basically a box that contains around 2,000 tubes. The tubes have water running through them to remove heat from equipment, such as seals or pumps. This heat exchanger plays an important role to cool necessary equipment during normal operation, but also during potential accident scenarios.

Palisades has two safety-related heat exchangers in this particular system; both are required by NRC regulations to be in working condition and ready to respond at a moment’s notice. With one of the two exchangers potentially not working right the plant decided to shut down before the regulations required it. NRC regulations state if there is a problem with the heat exchanger it would need to be fixed within 72 hours. If that’s not possible the regulations require the plant to shut down to find the leak and make the appropriate repairs. The plant will only be able to restart when the heat exchanger is working correctly.

Over the weekend all three NRC inspectors continued to monitor and assess the repair work to find and fix the leak. The NRC will continue to closely follow this event and observe how the plant goes about these activities with safety in mind from start to finish. We know the community is interested and concerned about these types of issues and continue to work to keep our commitment to ensure they are informed.

One of our initiatives is to provide summaries of conversations between the NRC and plant staff to the public. A summary of such a conversation about this leak, which took place on Thursday, Feb.14, will be available to the public in the near future. Our assessment of this issue will also be documented in a publically available inspection report.