1 - Introduction

Abstract

Today, there are about 440 nuclear power reactors operational around the world, which generate roughly about 370 GWe. In the entire history of commercial nuclear power so far, only three major accidents have taken place leading to damage of reactor core: Three Mile Island in 1979, Chernobyl in 1986, and recent Fukushima 2011. In the case of the Three Mile Island accident, even though the core had melted, there was no radiation exposure to any plant operator or any member of the public in spite of a series of human errors. This was all because of proper adherence to the defense-in-depth principles in the design of the plant constructed in the early seventies The Chernobyl nuclear power plant was one of the first Soviet nuclear power stations. The reactor design did not fully conform to stipulated principles of defense-in-depth applied to the reactors of such vintage constructed in the Western world. More specifically, the accident was a consequence of several design deficiencies compounded by a series of human errors. The recent Fukushima accident has been a concern for public as it happened especially with regard to safety of nuclear power plants. The accident in Fukushima occurred mainly due to extreme events such as an earthquake of magnitude of 9.1 on the Richter scale followed by a gigantic tsunami of height 14–15 m. These resulted in prolonged station blackout conditions with unprecedented devastation of on- and off-site infrastructures such as destruction of fuel tanks of the diesel generators, flooding of the diesel generator building, etc. which served as emergency power supply to the nuclear reactors. Of course, the accidents led to release of radioactivity due to probable melt down of reactor core and hydrogen explosion in the containment; however, there were no casualties in the accident. Of these accidents, the Chernobyl accident was definitely more severe in all its dimensions resulting in causalities. These accidents have reinforced the necessity for further improvement of safety in the design of nuclear power plants. The phenomenology of severe accident occurrence, its progression, and management are complex. To arrest the progression of a severe accident, new technologies have been developed which include in vessel corium retention and ex-vessel core catcher. The corium is a highly aggressive, high-temperature material and to retain it either inside the vessel or in the core catcher, it needs to be cooled down to sufficiently low temperature for a prolonged period because the corium continuously generates decay heat. Interaction of high-temperature corium with structural materials and cooling medium is complex; the occurrence of steam explosion may damage the structures and equipment. There are several studies on the above issues targeting to retain and cool the corium debris. This book addresses the corium cooling and retention aspects in pressurized heavy-water reactors (PHWRs) and light water reactors (LWRs) demonstrated through series of experiments and numerical analyses.