Abstract:

A plant safety designer is expected to conceive all possible scenarios and sequences of probable initiating events including failure of action on the part of humans that may finally lead to a major disaster. This involve considering geographic location of plant and existence environmental conditions and of natural abnormal conditions that are likely to trigger a disaster. In India, in 1975 we had Chasnala mine disaster in which an explosion in mine followed by flooding resulted in 374 miners death. In 1984, we has Bhopal Gas tragedy or disaster in India which is considered as the world's worst industrial disaster in which resulting from an accident, the plant released 42 tonnes of toxic methyl isocyanate (MIC) gas, exposing more than 500,000 people to toxic gases as a consequence of existing local weather conditions. The first official immediate death toll was announced as 2,259.

The nuclear safety has been an important subject and is dealt and covered in depth by all concerned. To avoid conditions leading to possible disaster comprise the identification of worst scenarios of even least possibilities, difficult conditions of work, deteriorated access to the location of action, the assessment of human response under a distress condition etc. must be considered. For example, the European Stress Test of NPPs carried out in the aftermath to the Fukushima disaster; include the need to review Accident Management provisions considering the possible harsh working environment during a catastrophic condition.

Chernobyl nuclear disaster in 1986 is considered as the worst in the history in terms of cost and casualties in recent times and is classified as a level 7 event (the maximum classification) on the International Nuclear Event Scale. Fukushima is another level 7 event that took place on March11, 2011, in which three of the plant's six nuclear reactors had melt down. First commissioned in 1971, the plant consists of six boiling water reactors (BWR). These light water reactors allowed to produce an electrical power of 4.7 GWe, making Fukushima Daiichi one of the 15 largest nuclear power stations in the world. The plant was designed by General Electric (GE) and maintained by the Tokyo Electric Power Company (TEPCO). Units 2 through 6 were BWR-4, while Unit 1 was the slightly older BWR-3 design. At the time of an earthquake that occurred, Reactor 4 had been de-fueled and Reactors 5 and 6 were in cold shutdown for planned maintenance. The failure occurred when the plant was hit by a tsunami triggered by an earthquake of the magnitude 9.0. The plant started releasing substantial amounts of radioactivity on 12 March, becoming the largest nuclear disaster. In August 2013, it was felt that the massive amount of radioactive water is among the most pressing problems affecting the clean-up process, which may even take decades. As of 10 February 2014, some 300,000 people were evacuated from the area. The Fukushima Nuclear Accident Independent Investigation Commission found the nuclear disaster was "manmade" and that its direct causes were all foreseeable.

Tokyo University professor emeritus Kiyoshi Kurokawa, wrote in the inquiry report. "It was a profoundly man-made disaster -- that could and should have been foreseen and prevented. And its effects could have been mitigated by a more effective human response. The report also found that the plant was incapable of withstanding the earthquake and tsunami. TEPCO, regulators Nuclear and Industrial Safety Agency (NISA) and NSC and the government body promoting the nuclear power industry (METI), all failed to meet the most basic safety requirements, such as assessing the probability of damage, preparing for containing collateral damage from such a disaster, and developing evacuation plans. When the earthquake struck, units 1, 2 and 3 were operating, but units 4, 5 and 6 had been shut down for periodic inspection. Reactors 1, 2 and 3 immediately underwent an automatic shutdown called SCRAM.

When the reactors were shut down, the plant stopped generating electricity, cutting off power. One of the two connections to off-site power for units 1–3 also failed, so 13 on-site emergency diesel generators began providing power. The earthquake triggered a 13-to-15-metre maximum height tsunami that arrived approximately 50 minutes later. The waves overtopped the plant's 10 metres seawall, flooding the basements of the turbine buildings and disabling the emergency diesel generators at approximately 15:41. The switching stations that provided power from the three backup generators located higher on the hillside failed when the building that housed them flooded. Multiple unsuccessful attempts were made to connect portable generating equipment to power water pumps. The failure was attributed to flooding at the connection point in the Turbine Hall basement and the absence of suitable cables. In Reactors 1, 2 and 3, overheating caused a reaction between the water and the zircalloy, creating hydrogen gas. On 12 March, an explosion in Unit 1 was caused by the ignition of the hydrogen, destroying the upper part of the building. On 14 March, a similar explosion occurred in the Reactor 3 building, blowing off the roof and injuring eleven people. On the 15th, an explosion in the Reactor 2 building damaged it and part of the Reactor 4 building.

According to Naoto Kan, Japan's prime minister during the tsunami, the Japan was unprepared for the disaster, and nuclear power plants should not have been built so close to the ocean. He acknowledged flaws in handling of the crisis by the authorities, including poor communication and coordination between nuclear regulators, utility officials and the government. He said the disaster "laid bare a host of an even bigger man-made vulnerabilities in Japan's nuclear industry and regulation, from inadequate safety guidelines to crisis management, all of which need to be overhauled".

I like to thank the Guest Editors of this issue, Dr. Bernhard Reer, Prof. Oliver Str?ter and Prof. Kazimierz T. Kosmowski, who have worked hard to ensure quality papers. My thanks are also due to reviewers, who helped in maintain timeliness in reviewing process. I would also like to thank the authors whose contributions are included in this issue and for maintaining the dead-lines. It is hoped that this issue of IJPE will provide impetus to research in this important area. Incidentally, this is the last issue of IJPE in 2014 and we will continue to bring various interesting and important aspects of Performability Engineering to our readers as we have been doing for past 10 years.