Is the accident at Fukushima Dai-ichi a super MCA (Maximum Credible Accident) like the Chernobyl disaster?
What happened at Fukushima Dai-ichi was a major nuclear accident, as was also the case at Chernobyl. However, there are significant differences between the two events: at Chernobyl, the nuclear chain reaction went out of control, and power increased abruptly by a factor of several hundred. A vapour explosion destroyed the roof of the reactor building and very large quantities of radioactive substances were released unimpeded into the surrounding area. Due to its design, the reactor contained large quantities of graphite, which ignited. The resulting fire caused radioactivity to be carried to great heights and over long distances.
At Fukushima Dai-ichi, all the reactors that were running at the time of the earthquake were shut down automatically, i.e. the nuclear chain reaction was interrupted. After a short time, however, the emergency power supply failed – due to the tsunami, according to reports. The cooling for the reactors, which continue to generate heat even after shutdown due to the strong radioactivity, ceased to function without power. Mobile pumps were used in an attempt to feed seawater into the reactors, but this was only partially successful. Overheating caused damage to the uranium fuel assemblies (partial core melt-down). Hydrogen gas, which forms under conditions such as these, ignited and led to the destruction of the upper sections of the reactor building. In the meantime, it must also be assumed that the containments around the reactors, which act as barriers to prevent the escape of radioactivity, have been damaged. Radioactivity is being released, but so far on a smaller scale than at Chernobyl.
Is the accident in Japan causing an increase in the values measured by ENSI’s MADUK probes?
Not as yet. If increased values are being measured at present, they are mostly due to precipitation, which causes naturally occurring radon progenies to be “washed” out of the air. On 28 March 2011, for example, higher values than the normal natural background radiation were measured at several of ENSI’s MADUK measuring stations in Switzerland. At the same time, precipitation was measured at MeteoSwiss weather stations in the corresponding areas. Precipitation causes dust particles in the air to be washed out and deposited on the ground. The air also contains radon, a natural radioactive gas in the uranium decay series. This decays to short-lived progenies which accumulate on dust particles and, in case of precipitation, may be washed out of the atmosphere with them and deposited on the ground. When precipitation occurs, it leads in each case to a brief increase in the dose rate values measured at the measuring stations. The measured values may increase to as much as twice the usual background value in these circumstances. The values drop back to the usual levels again within two to four hours (without precipitation) because the half-lives of the relevant radionuclides are less than half an hour. The radionuclides discharged into the atmosphere because of the accident at the Fukushima Dai-ichi nuclear power plant may also have been deposited on the ground on reaching Europe. No short-lived radionuclides are to be expected because the period needed to transport them from Japan to Europe is more than eight days. Radioactive iodine, which was partially measured in the air, has a half-life of about eight days. Consequently, any increase in the dose rate due to this radionuclide would also subside on expiration of the half-life.
Aren’t the nuclear power plants in Switzerland the same as those in Japan?
The structural design of the Mühleberg nuclear power plant is similar to that of unit 1 of the Fukushima Dai-ichi nuclear power plant, but not identical. The Mühleberg nuclear power plant has been continuously back-fitted, for example with an autonomous bunkered special emergency system of the type that is available to all the Swiss nuclear power plants. In particular, this system protects the nuclear power plant against external hazards such as earthquakes, flooding and aircraft crashes. The special emergency system has two independent emergency cooling trains with their own diesel generators to supply power. ENSI has no knowledge of the back-fitting status of the Fukushima Dai-ichi nuclear power plant.
What are the most severe earthquakes that could occur in Switzerland?
The most severe known earthquake in Switzerland occurred in Basel in 1356. It is likely to have been about one hundred times weaker than the tremor of 11 March 2011 off the coast of Japan. An earthquake of magnitude 9 (such as the one which struck off the coast of Japan on 11 March 2011) and a tidal wave on the scale of the subsequent tsunami can virtually be ruled out for Switzerland.
Independently of the latest earthquake, however, ENSI had already (in 1999) asked the operators to redetermine the seismic hazard to the nuclear power plant sites in accordance with the most advanced level of knowledge. Four years ago, ENSI published the results of this comprehensive study, which was the first of its kind in Europe at that time. The results show that the seismic hazard was underestimated in the past. However, hazards to nuclear power plants arise less from severe but very distant earthquakes than from moderate earthquakes of magnitudes (strengths) between 5.5 and 6.5 at distances of 10 to 20 kilometres. Based on these findings, ENSI defined new and more stringent seismic hazard assumptions for the safety analyses of Swiss nuclear power plants. The nuclear power plants were requested to review their seismic safety on the basis of the current data and to implement improvement measures as necessary.
The Japanese nuclear power plants were regarded as being well protected against earthquakes. Could a disaster of this sort also occur in Switzerland?
Because severe earthquakes are much more infrequent in Switzerland than in Japan – an earthquake of magnitude 9 such as the one that hit Japan on Friday and a tidal wave on the scale of the subsequent tsunami can virtually be ruled out for Switzerland – there are no grounds to assume an acute hazard. As all the Swiss nuclear power plants have autonomous bunkered special emergency systems, they enjoy a very high level of protection (measured on the international scale) against external events such as earthquakes and flooding.
Would the population also be evacuated in case of an accident at a Swiss nuclear power plant?
The overriding goal is to minimise the health risk to the population in case of an event involving increased radioactivity. The first measure to be implemented in Switzerland is for people to remain under protection in their houses, basements or protected areas. A horizontal evacuation will only be considered if sufficient time is available or if soil contamination after the accident is too high for people to remain in the area in the long term.
Why is ENSI not issuing immediate orders at least to shut down the older nuclear power plants in Switzerland?
The Decommissioning Ordinance (SR 732.114.5) stipulates the criteria that must be met for nuclear power plants to be shut down on a provisional basis. These shutdown criteria are not met by any of the Swiss nuclear power plants at present. Accordingly, there is no technical reason to shut down the nuclear power plants. This also means that ENSI does not currently have a legal basis to request the provisional decommissioning of a nuclear power plant.
- Questions on the subjects of radioactivity and the potential radiological effects of the reactor accidents on Switzerland are answered by the Swiss Federal Office of Public Health (FOPH).
- Questions regarding the safety of travel to Japan are answered by the Federal Department of Foreign Affairs (DFA).
- You can find ENSI’s latest media releases here in german and here in french.
- Report by the Japanese Government for the IAEA
- Swiss Federal Office of Public Health (FOPH)
- Federal Department of Foreign Affairs (DFA)
- National Emergency Operations Centre (NEOC)
- Nuclear and Industrial Safety Agency (NISA), Japan
- International Atomic Energy Agency (IAEA)