Tohoku Electric Power Company has applied to Japan's Nuclear Regulation Authority (NRA) for approval of its decommissioning plan for unit 1 of the Onagawa nuclear power plant in Miyagi Prefecture. The company announced in October 2018 its decision to scrap the unit as it said required safety upgrades would be too expensive and time-consuming.Onagawa unit 1 (Image: Tohoku)
Unit 1 of the 524 MWe boiling water reactor (BWR) that began operations 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 plans to restart units 2 and 3 at the Onagawa plant, as well as its Higashidori plant.
Last October, Tohoku said 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 decided 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. Onagawa 1 became the tenth operable Japanese reactor to be declared for decommissioning since the March 2011 Fukushima Daiichi accident.
Its decommissioning plan for the unit, which it submitted to the NRA on 29 July, outlines the facilities and equipment to be dismantled and a timetable for completing the work. Decommissioning will take about 34 years and will be carried out in four stages. The first stage, lasting about eight years, will involve preparing the reactor for dismantling (including the removal of all fuel and surveying radioactive contamination), while the second, lasting seven years, will be to dismantle peripheral equipment from the reactor and other major equipment. The third stage, taking about nine years, will involve the demolition of the reactor itself, while the fourth stage, taking about ten years, will see the demolition of all remaining buildings and the release of land for other uses.
During the first stage, all fuel is to be removed from the operation of Onagawa 1. This includes 821 used fuel assemblies stored in unit 1's storage pool, which will be transferred to unit 3's storage pool. These assemblies will later be transported for reprocessing, together with 95 used fuel assemblies from unit 1 currently stored at unit 2 and 66 stored at unit 3. There are also 41 unused fuel assemblies stored at unit 1.
A total 60 tonnes of high-level radioactive waste is expected to be generated through the decommissioning of Onagawa 1, together with 740 tonnes of low-level waste and 5340 tonnes of very low-level waste. A further 12,400 tonnes of non-radioactive waste will also be generated through the clearance of the site.
Tohoku said it expects the decommissioning of the unit to cost a total of JPY41.9 billion (USD392 million), with dismantling activities costing JPY30.0 billion and waste disposal accounting for the remainder.
In March 2015, the Ministry of Economy, Trade and Industry's Agency for Natural Resources and Energy revised the accounting provisions in the Electricity Business Act, whereby electric power companies can now calculate decommissioning costs in instalments of up to 10 years, instead of one-time as previously. This enhanced cost recovery provision was to encourage the decommissioning of older and smaller units.
The Onagawa plant is on Japan's northeastern coast and was the closest plant to the epicentre of the 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 had been largely unaffected by the tsunami as it sits on an elevated embankment more than 14 metres above sea level.