Japanese utility Chubu Electric Power Co has announced that it is restarting unit 5 at its Hamaoka nuclear power plant, which has been offline since a strong earthquake struck Shizuoka prefecture 18 months ago.
|Hamaoka (Image: Chubu)
On 11 August 2009, only two of the plant's five reactors - units 4 and 5 -were in operation when a magnitude 6.5 earthquake struck Suruga Bay on Hunshu, Japan's main island. Both boiling water reactors automatically shut down safely, as designed.
Hamaoka unit 3 was already shut down for a periodic inspection. Units 1 and 2 at the plant had earlier been permanently shut down after Chubu decided that upgrading the units to current earthquake safety standards was uneconomic.
It was possible to restart Hamaoka 3 and 4 within a couple of months of the quake, but unit 5 was found to have been subjected to greater seismic tremors leading to more serious damage. Ground acceleration during the earthquake was in the range of 100-160 gal for four of the reactors, although unit 5 experienced far more at 425 gal. Hamaoka reactors 3 to 5 are engineered to withstand up to 1000 gal while maintaining essential nuclear safety.
Restart of unit 5 has been repeatedly deferred while the company analysed why such high seismic acceleration was recorded, coupled with some planned maintenance being undertaken during the shutdown. In a report submitted to the Japanese government in December 2010, Chubu said that the primary reason unit 5 experienced more severe tremors was the bedrock beneath the reactor, in which seismic waves travel some 30% slower than in the surrounding bedrock.
However, the extended outage due to the earthquake provided Chubu a chance to fully repair its steam turbine set-up. Back in June 2006, the reactor had to be shut down due to excessive turbine vibrations, and it was found that a number of vanes in the 12th stage of its turbine had actually completely broken off. In fact, nearly all vanes in that stage showed fractures or cracking, while the majority of the vanes of the other two low pressure turbines also showed problems. Work to repair the turbines, which are of course critical to achieving the full 1380 MWe output of the power unit, was completed in May last year.
Having gained approval from Shizuoka prefecture, Chubu has today begun the process of restarting unit 5 and is steadily raising its power as a series of safety checks are conducted.
Earthquake regulations in Japan were revised after the Niigata-Chuetsu offshore earthquake which hit Kashiwazaki-Kariwa nuclear power plant in July 2007. The seven reactors at Kashiwazaki-Kariwa all stayed safe during the earthquake, but its strength came as a surprise to seismologists and led to a simplification and strengthening of requirements for nuclear plants. Four of the seven reactors are now back in operation.
Researched and written
by World Nuclear News