Tohoku decides to decommission oldest Onagawa unit

25 October 2018

Unit 1 of the Onagawa nuclear power plant in Japan's Miyagi Prefecture is to be decommissioned after owner Tohoku Electric Power Company decided that safety upgrades required to restart it would be too expensive and time-consuming.

Onagawa unit 1 (Image: Tohoku)

Unit 1 of the Onagawa plant - a 524 MWe boiling water reactor (BWR) which began operating in 1984 - is of a different design to the other two larger (825 MWe) BWR units there, which began operating in 1995 and 2002, respectively. Tohoku also operates a single 1100 MWe BWR at its Higashidori plant in Aomori Prefecture, which started operation in late 2005.

Tohoku said today a problem unique to Onagawa 1 is the restricted space within its containment vessel in which to install additional safety equipment, such as fire extinguishing equipment, power supply equipment and alternative water injection pumps. It said it had taken the decision to decommission the unit after taking into account its generating capacity and the number of years it would be able to operate if it were restarted.

Tohoku will now seek approval from the Ministry of Economy, Trade and Industry to decommission Onagawa 1, as well as to submit a decommissioning plan for the unit to the Nuclear Regulation Authority (NRA).

"We will advance the various procedures in accordance with the decommissioning of Onagawa unit 1, and we will work on the decommissioning measures with the highest priority for ensuring safety," the company said.

Tohoku applied to the NRA for safety assessments of Onagawa 2 and Higashidori 1 in December 2013 and June 2014, respectively. These assessments - which will verify whether measures taken at the plant meet new safety standards - can only take place once construction of additional safety measures has been completed. The company expects to complete this construction work at Onagawa 2 in the second half of fiscal year 2018 (between this month and March 2019), with work at Higashidori 1 expected to be completed in fiscal 2019 (April 2019 to March 2020).

The utility plans to restart unit 2 at the Onagawa plant in fiscal 2020 at the earliest, as well as its Higashidori plant in fiscal 2021 or later, the Asahi Shimbun reported. Tohoku is also said to be preparing documents for NRA inspections for Onagawa 3.

The Onagawa plant is on Japan's northeastern coast and was the closest plant to the epicentre of the massive earthquake and tsunami of 11 March 2011. Although the earthquake knocked out four of the five external power lines, the remaining line provided sufficient power for the plant's three reactors to be brought to cold shutdown. Onagawa 1 briefly suffered a fire in the non-nuclear turbine building. A mission from the International Atomic Energy Agency in August 2012 concluded the plant was largely unaffected by the tsunami as it sits on an elevated embankment more than 14 metres above sea level.

Onagawa 1 becomes the tenth operable Japanese reactor to be declared for decommissioning since the Fukushima Daiichi accident.

Researched and written by World Nuclear News