Japan readies for restarts

Power companies in Japan now know the regulations they are required to meet in order to restart nuclear reactors idled as a result of the 2011 Fukushima accident. The first units could potentially come back online before the end of this year.

New regulations were announced today and will be in force from 8 July, at which time several utilities are expected to immediately apply for permission to restart a limited number of units. This will include Ohi 3 and 4, which have been operating on special permission due to power demand in the Kansai region. The country's total nuclear fleet counts 50 reactors but utilities are yet to explain their expectations for most.

In order to operate, nuclear companies will have to show their reactor units are prepared for extraordinary external events comparable to the natural disaster on 11 March 2011. They need stronger and higher tsunami walls with waterproofing of key buildings. More cautious earthquake analysis will look further back in time when determining if a fault is active - at least the last 120,000 years but perhaps as far as 400,000 years in cases of uncertainty. Additional countermeasures are then required for accident situations: mobile power generators, secure sources of make-up water and methods to inject water.

The Nuclear Regulation Authority (NRA) said that it could take around six months to process an application, meaning that the first units could potentially come back online before the end of this year.

Accident mitigation

Beyond addressing the main issues that allowed the crisis to occur at Fukushima Daiichi, the NRA has also set requirements to address the issues that exacerbated the situation.

During the station black-out, operator Tokyo Electric Power Company (Tepco) struggled to open electrically powered valves that could have depressurized the reactors and helped it inject water early enough to avoid core damage. These valves will be required to be manually operable.

Assuming all of the above have failed and the plant is in a core-melt situation, operators should have sufficient countermeasures to cool and depressurize the containment, protect its integrity and keep radioactive materials confined. Filtered vents are specified for all boiling water reactors before restart, while pressurized water reactors will be allowed to restart without these but must install them within five years. Operators also need to have foam or water cannons on hand to help stop airborne releases of radiation spreading beyond the site.

Then, should corium fall to the bottom of the containment, units need a water injection system to cool this. Some new reactor designs feature 'core catcher' for this purpose, but Japanese units would have to retrofit a suitable analogue.

Hydrogen produced during an accident would have to be managed in order to prevent the reoccurence of the explosions that did so much to damage the Fukushima units and complicate the situation for workers on site. This will be achieved by hydrogen recombiners that operate without any power supply. Many plant owners have already put in orders for these components.

Turning its attention to potential terrorist attacks, the NRA has said that in the next five years all operators will need a secondary control room and sources of power and water away from the reactor building. This is to allow the remote control of the power plant in scenarios such as the deliberate crash of an aircraft that could cause large fires and physical damage on site.

Researched and written
by World Nuclear News