Japan's two-step nuclear stress tests are beginning as government and authorities seek to bring back nuclear power supply.
The move by Japanese safety authorities is inspired by the European Union's response to the Fukushima accident as well as the urgent need to reassure the public.
Most of Japan's nuclear reactors are offline. Some shut down automatically when the earthquake of 11 March struck, but others have not yet restarted after routine inspection and refuelling outages.
Restarts need the approval of prefectural governors - something that may be politically impossible until safety has been further demonstrated - and because the checks are mandated at fixed intervals, a failure to authorise any restarts would see all the reactors offline by the middle of 2012.
The stress tests come in two stages. In the primary tests, plant operators will assess whether main safety systems could be damaged or knocked out by natural disasters beyond the plant design basis. This should identify the sheer magnitude of events that could cause damage to nuclear fuel, as well as any weak points in reactor design.
The tests will start from an extreme plant condition, such as operating at full power while used fuel ponds are full. From there, a range of accident progressions will be simulated using event trees, addressing the effectiveness of available protective measures at each stage.
Every nuclear power company is to conduct these primary tests when one of its reactors is considered ready for restart, meaning that many are now beginning the work.
In the second stage even more severe events will be considered, with a focus on identifying 'cliff-edge effects' - points in a potential accident sequence beyond which it would be impossible to avoid a serious accident. This stage will include the effects of simultaneous natural disasters.
Of particular focus will be the fundamental safety systems that were disabled by the tsunami of 11 March, leading to the Fukushima accident: back-up diesel generators and seawater pumps that provide the ultimate heat sink for a power plant.
All power plants need to have been subject to this secondary analysis by the end of the year. The results will be considered by the Nuclear and Industrial Safety Agency (NISA) as well as the Nuclear Safety Commission. It is highly likely that a range of primary safety improvements will be required across Japan's nuclear fleet as the regulators revise their practices.
On 30 March NISA ordered all nuclear operators to ensure they could manage the loss of emergency back up power. Minister of Economy, Trade and Industry Benri Kaeida said "the level of safety required" had been "secured" by implementation of those measures. He wants to see nuclear power plants back in action to stabilise Japan's power supply and help support its recovery from the twin disasters that struck in March.
On his own initiative, prime minister Naoto Kan also asked Chubu Electric Power Company to shut down its Hamaoka nuclear power plant until sea defence and preparedness had been improved. Chubu recently announced its plans to meet that aim by the end of 2012.
Some 196 reactors in the European Union's 27 member states as well as its neighbours, including Russia, are the subject of similar stress tests. The addition of Japan's 50 utility reactors means the overall effort covers 55% of the world's nuclear power fleet.
A report that came from a US Nuclear Regulatory Commission (NRC) task force two weeks ago said a Fukushima-like accident was not likely at America' 104 power reactors, but the NRC should nevertheless work methodically review its frameworks to increase preparedness and reduce the risk further.