Japan's nuclear meltdown 2011: Lessons learned

Prior to the shutdown of the four affected Fukushima nuclear reactors last month, Japan had 54 nuclear reactors in operation, with a total installed generating capacity of around 49GW, housed in 16 main power plant locations (see Figure 1).1

Japan imported its first nuclear reactor from the UK in 1961.2Put into operation in 1966, it was a gas-cooled (Magnox) reactor with an installed capacity of 160MW built by General Electric and it operated for about 30 years to 1998.

Following this, Japan focused on building light water reactors (LWRs) which utilise enriched uranium dioxide as a fuel.3LWRs are either boiling water reactors (BWR) or pressured water reactors (PWRs).

Both are categorised as Generation II reactors. Japan has also developed Generation III reactors, which include advanced BWRs (ABWRs) and advanced PWRs (APWRs).4The advanced reactors contain numerous safety improvements based on operational experience.5Initially, the Japanese power utilities companies bought designs from US vendors and built them in cooperation with Japanese companies which, in turn, obtained licences to build similar plants.6There are currently 26 BWRs, 24 PWRs and 4ABWRs in Japan. The quake-affected Fukushima reactors were all BWRs.

Japanese companies such as Hitachi, Mitsubishi and Toshiba have been exporting Japanese nuclear designs and technologies, and collaborating with foreign nuclear companies since the late 1990s.7In Asia, Japan is recognised for its unrivalled nuclear plant performance and capabilities. In recent years, the Japanese have been working on fast breed reactors (FBR). The goal was to make these commercially available after 2050. Japan's first FBR was connected to a grid in 1995. After detecting a sodium leakage during performance tests in December that year, the FBR was shut down and was re-opened only in May 2010.8

Nuclear plants are designed to serve for 40 years. This is a standard imposed in the United States.9Figure 2 shows the age distribution of Japan's nuclear reactors, categorised by reactor types. More than half of Japan's nuclear reactors have been operating for at least 20 years, with two of them passing the 40–year mark. Due to the increasing number of aging nuclear plants, the Japanese Government introduced the Periodic Safety Review and Ageing Management evaluation of plants as a regulatory requirement in 2005.10The two 40-year-old reactors, Mihama-1 and Tsunga-1, were given approval in early 2010 for extension of their operating lives beyond 40 years. Tsunga-1 was granted extension to 2016 due to delays in the construction of new reactors to replace it.11Meanwhile, a feasibility study was initiated to study the possibility of constructing a new reactor at the Mihama site in Fukui prefecture to replace Mihama-1.

Fukushima-1, one among the four Fukushima reactors that were crippled by the March 2011 earthquake, was given a 10-year extension just one month before that fateful day. It had come into commercial operation in March 1971 and had just breached the 40-year period.

Ageing of the plants

The ageing of nuclear plants is a big dilemma for nuclear power companies. Due to the very high capital investment needed to build a nuclear plant and the challenges of repairs and equipment replacements, power companies have been requesting to extend the operating lives of their nuclear plants to amortise costs. Many companies have indeed extended their operations despite signs of premature ageing.12Given that the construction of new nuclear power plants incurs loud public outcry in Japan, nuclear power companies have been even more inclined towards extending operations rather than building new plants.

Two common ageing phenomena of nuclear plants are neutron irradiation embrittlement of reactor pressure vessel (RPV) steel and stress corrosion cracking (SCC). Neutron irradiation degrades the toughness of RPV steel, which weakens the integrity of the vessel structure. According to Japanese reports, "Japanese modern pressure vessel steels contain very low concentrations of copper (Cu) and other detrimental impurity atoms such as phosphorus (P), and hence have very low susceptibility to irradiation embrittlement". On the other hand, stress corrosion cracks have been detected in many of Japan's BWR plants, but BWR owners have since replaced stainless steel with low carbon stainless steel.13SCC occurrence is found to be positively related to carbon content. In general, careful monitoring and maintenance of nuclear plants maintain the safety of nuclear plants for daily operations. However, it is also important to note that while retrofits and component replacements incrementally increase safety of ageing nuclear reactors, installations also create opportunities for errors.

Nonetheless, Japanese companies like Hitachi and Mitsubishi had been developing new technologies relating to general maintenance, prevention of equipment deterioration and more robust inspection. France had, in fact, also started to order equipment from Japan to meet the country's rising demand for replacement parts.14The state of nuclear technology in Japan, be it nuclear reactor design, reprocessing or replacement parts, was and still is considered one of the best in the world.

Historical impact of earthquakes on Japanese nuclear plants

Despite being an earthquake-prone nation, Japan has the largest commercial nuclear programme in Asia. The Japanese Government revised its seismic guidelines for nuclear plant design and construction following the 1985 Kobe earthquake which had a magnitude of 7.2.15

Following this earthquake, the Japan Nuclear Safety Commission required that nuclear plants be able to survive a quake of magnitude 7.75. Seismic detectors trigger the tripping of reactors in the event of earthquakes that cause ground motions of a certain magnitude (specified as S1 or S2 in Gal units). The nuclear plants that are most vulnerable to high magnitude earthquakes are located in the Tokai region (see Figure 1) and these are are already designed to withstand earthquakes up to 8.5 in magnitude.16

A brief timeline of significant Japanese earthquakes is drawn up in Figure 3.

The Fukushima reactors were designed to withstand a magnitude 8.0 earthquake.17It was, of course, never foreseen that the reactors would be hit by a "once in 10,000 years" magnitude 9.0 earthquake coupled with a 10m-tall tsunami. The reactors were tripped according to design specifications during this earthquake. The failure of the reactors, notably Reactor 1, was largely due to the loss of electrical power, which was much needed for cooling the reactor core.

ce in 10,000 years" magnitude 9.0 earthquake coupled with a 10m-tall tsunami. The reactors were tripped according to design specifications during this earthquake. The failure of the reactors, notably Reactor 1, was largely due to the loss of electrical power, which was much needed for cooling the reactor core.

Implications

Engineers and nuclear scientists have been trying to ascertain the sequence of events that led to the meltdown. There is considerable discussion about the Fukushima reactors being "old" and "troubled".18

It was also reported that the power plant was designed to survive only a magnitude 8.0 earthquake. But investigations showed that the Fukushima reactors responded the way they were designed to, and actually survived the magnitude 9.0 earthquake until the tsunami hit.

Reactors are forced to shut down upon specified ground motions of a certain magnitude. This way of controlling reactivity, together with cooling the radioactive fuel and containing radioactive substances, is one of the three basic safety functions. Unfortunately, the tsunami that followed the earthquake crippled the generators that were powering the coolant pumps. Although there were backup batteries to provide electricity, the batteries ran out before adequate cooling was achieved. This was what led to the meltdown.

Essentially, two concerns are being raised here: The safety backup mechanisms and the age of nuclear facilities.

In a recent New York Times article, the US Nuclear Research Centre (NRC) commented that it suspects US reactors could withstand a similar event provided their backup batteries had as high a capacity as those of Japan. Currently, the capacity of backup batteries in American reactors is4 hours, which is half that of Japan's reactors.19However, the truth remains unknown, until the system is actually "tested". The NRC also commented that their reactors were able to withstand a total power loss and this safety mechanism was implemented in response to the 9/11 incident. Such a safety measure is expected to "mitigate conditions that result from severe adverse events".20

This is perhaps something that the Japanese overlooked--a safety backup system that can function in the event of power loss--and something that countries prone to tsunamis should be mindful of when designing and building their nuclear plants. Newer designs of cooling systems in BWR use gravity and natural convection to cool the core, thereby eliminating the use of electrical power.21

In addition, scenario risk planning has become more important in the face of such events. Worst-case scenarios must be seriously considered. Events with a remote chance of occurring are often omitted in scenario risk planning for economic reasons. However, with the present empirical evidence of such massive earthquakes and tsunamis, such omissions can no longer be made.

The answer to whether or not the age of a reactor plays a part in safety is inconclusive. Fukushima-1 had an old cooling system (utilised electricity, instead of gravity and natural convection as do the newer ones).22But reports showed that the other three reactors, which had not yet passed the 40-year mark (they were 33- to 37-years-old), were also severely damaged.23

There were also reports about Fukushima-1 being a "troubled facility", with accidents taking place before the earthquake.24There may also have been elements of human negligence. The NRC reported that the Japanese GE Mark 1 reactors had not undergone the retrofits that they were meant to. The retrofits--hardened vents--were supposed to "remove hydrogen that escapes the primary reactor containment shell and carry it outside the second containment building".25This maintenance issue is more a question of human operations and negligence than the age of the technology.

Conclusion

In retrospect, this terrible disaster, though costly, probably served as a timely wakeup call to the nuclear power-producing nations and those planning to build nuclear plants. It helped reveal lack of adequate and appropriate safety mechanisms in nuclear plants and cases of human negligence. All countries currently producing nuclear power will undoubtedly closely re-examine the current state of their nuclear technology. Such assessments could involve either more stringent approval requirements for extending the life of old nuclear plants, the use of more robust and encompassing safety designs, and/or the strengthening of safety measures at vulnerable sites.

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1. World Nuclear Association, "Nuclear Power in Japan", 2011 at <http:>.</http:>
2. Ibid. and K. Maize "A Short History of Nuclear Power in Japan", POWER Business and Technology for the Global Generation Industry Blog, 2011 at <http:></http:>.
3. World Nuclear Association, "Nuclear Power in Japan".
4. J-S.Choi, "Nuclear Power in Japan: A Gaijin Perspective", presentation at the Energy Studies Institute, National University of Singapore, 4 Nov. 2010.
5. World Nuclear Association, "Safety of Nuclear Power Reactors", 2011 at <http:>.</http:>
6. Paul J. Scalise, "National Energy Policy: Japan", Encyclopedia of Energy, vol. 4, 2004.
7. World Nuclear Association, "Safety of Nuclear Power Reactors".
8. World Nuclear Association, "Nuclear Power in Japan", 2011 at <http:>.</http:>
9. P. Brett, "The Dilemma of Aging Nuclear Plants", Special Report: Energy, New York Times, 19 Oct. 2009.
10. Japan Nuclear Energy Safety Organisation, "Aging of Nuclear Power Plants", 2007.
11. World Nuclear Association, "Nuclear Power in Japan".
12. P. Brett, "The Dilemma of Aging Nuclear Plants".
13. Japan Nuclear Energy Safety Organisation, "Aging of Nuclear Power Plants", 2007.
14. P. Brett, "The Dilemma of Aging Nuclear Plants".
15. WorldNuclearAssociation "NuclearPowerPlantsandEarthquakes", 2009at<http:>.</http:>
16. Ibid.
17. Y. Hayashi and M. Iwata, "Japans Struggles to Control Reactors, Wall Street Journal, 13 Mar. 2011.
18. R. Smith, B. Casselman and M. Obe, "Japan Plant Had Trouble History", Wall Street Journal, 2011.
19. P. Behr, "US Nuclear Plants are Safer than Japan's, but Operational Quality Needs Work", The New York Times, 21 Mar. 2011.
20. Ibid.
21. J. Tullock, "Radioactive Wave: Will Tsunami Lead to Meltdown", an interview with Professor Andrew Sherry, Japan Disaster, Allianz.com, 2011 at <http:>.</http:>
22. Ibid.
23. "The Risk Exposed: What Damage to the Fukushima Plant Portends for Japan – and the World", The Economist, 17 March 2011.
24. P. Behr, "US Nuclear Plants are Safer than Japan's, but Operational Quality Needs Work".

By : Catrina Yeo, ESI Energy Analyst