Japan's Onagawa Nuclear Power Station Unit 2 Plans to Replace Hydrogen Concentration Monitoring Equipment

Tohoku Electric Power has consulted with Miyagi Prefecture, Ishinomaki City, and Onagawa Town in advance regarding a change to the method of monitoring hydrogen concentration inside the containment vessel under severe accident conditions at Unit 2 of the Onagawa Nuclear Power Station. The company plans to submit an application to amend the reactor installation license to the Nuclear Regulation Authority (NRA) of Japan on the 2nd.

Unit 2 of the Onagawa Nuclear Power Station is located in Onagawa Town and Ishinomaki City, Miyagi Prefecture. Tohoku Electric Power previously installed a total of four hydrogen concentration detectors inside and outside the reactor containment vessel of this unit, intended to monitor changes in hydrogen concentration within the containment vessel in the event of a severe accident. In 2025, some of these detectors experienced malfunctions. In August of the same year, Tohoku Electric Power replaced all four detectors, including the two faulty units.

Under the new plan, Tohoku Electric Power intends to replace the existing equipment with thermal conductivity-type hydrogen concentration analyzers. These devices measure hydrogen concentration based on changes in detector temperature and have already been applied at other nuclear power plants. The company states that thermal conductivity-type devices offer advantages in operability and maintainability, and have a longer service life than conventional equipment, eliminating the need for replacement during every periodic inspection.

The replacement is scheduled to be implemented during the periodic inspection following the conclusion of the current commercial operation cycle of Unit 2 at the Onagawa Nuclear Power Station. Since the new equipment will be installed outside the containment vessel, subsequent replacement and maintenance work can be carried out without shutting down the reactor.