All of Switzerland’s nuclear power plants will cope with an extreme flood of the sort that may occur on average once every 10,000 years, in addition to their ability to cope with other hazards. Even if the external power supply fails at the same time as the flood occurs, the plants can be brought into a safe condition. All the plants are clearly in compliance with the applicable limits.
Following an in-depth review, ENSI has accepted the proof of ability to withstand flooding submitted by all Swiss power plants. However, the Mühleberg nuclear power plant can only be reconnected to the grid after the ongoing back-fitting measures for the cooling water intake from the river Aare have been completed and accepted by ENSI.
From ENSI’s requirements of the proof of ability to withstand flooding
On 1 April 2011, ENSI ruled that all Swiss nuclear power plants (NPP) must furnish deterministic proof of their ability to cope with the 10,000-year flood. All four NPP operators submitted their proof to ENSI by the required date of 30 June 2011. On the basis of the results from their analyses, all the plant operators conclude that their plants display a very high level of protection against an incident involving external flooding, and that the statutory requirements are met with large safety reserves. The dose value for the population in the area surrounding the NPP determined for this incident is significantly lower than the statutory limit of 100 mSv for persons in case of non-occupational exposure to radiation.
ENSI’s requirements for the proof of the ability to withstand flooding
Against the background of events in Japan and on the basis of the DETEC (Department of Environment, Transport, Energy and Communications, UVEK) Ordinance on the Methodology and Boundary Conditions for Reviewing the Criteria for the Provisional Decommissioning of Nuclear Power Plants dated 16 April 2008, ENSI ruled on 18 March 2011 that it was immediately necessary to carry out a new review of the design of nuclear power plants in Switzerland in respect of earthquakes and flooding.
Moreover, in its second ruling dated 1 April 2011, ENSI stipulated the overall framework of conditions and the scheduled dates for this review. In respect of the flooding hazard, a particularly detailed examination of consequential damage, such as the blockage or destruction of intake structures due to bedload and floating debris carried along by the flood, should be carried out.
ENSI’s review of the submissions by the power plant operators:
ENSI conducted a detailed review of the technical proof of safety submitted by the nuclear power plant operators. This in-depth review of the power plants’ analyses regarding the 10,000-year flood focused in particular on:
- impacts on the plants;
- the assumptions for bringing the plants into a safe condition;
- maintenance of this condition during the required period of 72 hours;
- the radiation dose for the population that would result from this incident.
Results for the individual nuclear power plants:
Beznau nuclear power plant
Based on the results of its analyses, the Beznau nuclear power plant (Beznau NPP) reaches the conclusion that both its units display a very high level of protection against an incident involving external flooding, and that it meets the statutory requirements with large safety reserves. The dose value for the population in the area surrounding the Beznau NPP determined in this context is over one hundred times less than the statutory limit for this incident.
During its review of the Beznau NPP’s data on the 10,000-year flood, ENSI established that the flow rate of 4,200 m3/s determined by the operator for this case is appropriate for an estimate of the average flooding level to be expected in the event of a 10,000-year flood on the site of the Beznau NPP. The flooding model used by the Beznau NPP allows a tenable determination of the expected water level in the area of the buildings at the Beznau NPP; the expected flooding levels of up to 0.37 m are relatively low as compared to the design of the Beznau NPP, which provides for waterproofing up to a level of 1.65 m.
The technical assumptions relating to the system and the overall framework of conditions are plausible; the same applies to the safe condition of the plant achieved by means of decay heat removal (DHR) via the steam generators, which was taken by the the plant as the basis for its analysis. In case of exclusive deployment of the special emergency systems to bring an incident under control, decay heat removal can be maintained for at least 96 hours with the available stocks of fuel for the special emergency diesel generators.
The review of the radiological analysis of the flooding incident has shown that the underlying assumptions are tenable. Beznau NPP’s calculations regarding dispersion and doses were carried out in accordance with the statutory guidelines, and the resultant incident dose was determined in a comprehensible and correct manner.
ENSI’s summary and conclusions:
The Beznau NPP has furnished proof of its ability to cope with a 10,000-year flood under the overall framework of conditions specified by ENSI. Even if conservative assumptions are made as regards the overall conditions, the radiation dose resulting from this flooding incident is significantly below the permitted value of 100 mSv for the population in the area surrounding the power plant as stipulated for such an incident by the Radiological Protection Ordinance (RPO/StSV).
Gösgen nuclear power plant
The Gösgen nuclear power plant (Gösgen NPP) submitted proof relating to two different flooding scenarios. In the first scenario, the Gösgen NPP assumes that the power plant site barely avoids being flooded in case of a 10,000-year flood, and that the incident can be brought under control solely with the safety systems that are fed by the emergency power supply. In the second scenario, the Gösgen NPP also proves that if a flood leads to inundation of the power plant site, this situation will also be brought under control thanks to the systems with special emergency protection.
For all the scenarios it considers, the Gösgen NPP concludes, based on the results of its analyses, that the resultant dose values are significantly below the applicable statutory limit.
During its review of the flow rate derived for the first scenario, ENSI determined that the Gösgen NPP’s assumption regarding the inflow into the Winznau weir is not sufficiently conservative in this case. Unlike the Gösgen NPP, ENSI assumes that there will be significant flooding of the power plant site in case of a 10,000-year flood. ENSI therefore bases its assessments on the second scenario (control of flooding with the help of systems that have special emergency protection).
ENSI accepts the plant’s proof of its ability to cope with an incident on the basis of this (second) scenario. The technical assumptions relating to the system and the overall framework of conditions on which the operator bases its analysis are plausible, as is the safe condition of the plant achieved by means of decay heat removal (DHR) via the steam generators targeted by the Gösgen NPP. In case of exclusive deployment of the special emergency systems to bring the incident under control, decay heat removal can be maintained for the required period of 72 hours with the stocks of fuel available on site.
The result of the radiological analysis submitted by the Gösgen NPP is accepted by ENSI. Some individual points of the Gösgen NPP’s radiological analysis do not conform to the requirements stated in ENSI Guideline A08, so additional documentation and amplification are required in these respects.
ENSI’s summary and conclusions:
ENSI accepts the result of the proof submitted by the Gösgen NPP of its ability to bring a 10,000-year flood under control. Even if conservative assumptions are made as regards the overall conditions, the radiation dose resulting from this flooding incident is significantly below the permitted value of 100 mSv for the population in the area surrounding the power plant as stipulated for such an incident by the Radiological Protection Ordinance (RPO/StSV). The Gösgen NPP has yet to provide additional documentation and amplification of its radiological analysis in accordance with Guideline A08.
Leibstadt nuclear power plant
The Leibstadt nuclear power plant (Leibstadt NPP) can rule out flooding of its site because it is located at 332.0 meters above sea level, about 18 meters above the crest of the flood wave for a calculated 10,000-year flood. The data in the hazard study and the proof were reviewed by ENSI and assessed as plausible. Flooding does not result in any damage to the plant. The only case which cannot be entirely excluded is blockage of the cooling water intake by floating debris carried along with the flood. In this case, however, the decay heat can be safely removed via the diversified groundwater wells.
Flooding with a loss of the external power supply results in the incident known as “Loss of Offsite Power (LOOP)”. If this case occurs, the three emergency diesel generators take over the supply to the necessary consumers and, if required, the two bunkered special emergency diesel generators. The decay heat is then removed via the redundant emergency and residual heat removal systems.
ENSI has reviewed the data provided by the Leibstadt NPP in its deterministic proof, and has assessed them as plausible. The flow rate range for the 10,000-year flood and the resultant flooding level deduced by the Leibstadt NPP are tenable. ENSI also agrees with the assessment that the flooding event would result solely in a Loss of Offsite Power of lengthy duration. The stocks of fuel available for this case are dimensioned so that each of the emergency diesel generators can be operated for 5.3 days at nominal load, so the plant can be brought into a safe and stable condition, and maintained in that condition, without the assistance of external emergency measures. Attainment of the protection objectives is therefore guaranteed.
In radiological terms, the “Loss of Offsite Power” incident is covered by the “Inadvertent closure of all main steam isolation valves” incident. The calculated dose for this incident is significantly below the applicable acceptance limit of 100 mSv for the combination of incidents considered here. Compliance with the dose limit as per the Radiological Protection Ordinance is therefore ensured for the 10,000-year flood.
ENSI’s summary and conclusions:
The Leibstadt NPP has furnished proof of its ability to withstand the 10,000-year flood under the overall conditions specified by ENSI. Even if conservative assumptions are made as regards the overall conditions, the radiation dose resulting from this flooding incident is significantly below the permitted value of 100 mSv for the population in the area surrounding the power plant as stipulated for such an incident by the Radiological Protection Ordinance (RPO/StSV).
Mühleberg nuclear power plant
The Mühleberg nuclear power plant (Mühleberg NPP) states – with conservative hazard assumptions – that protection of the relevant buildings and installations remains guaranteed in case of a 10,000-year flood; that it is proven that the plant can be brought into a safe condition; and that the dose value for this incident is significantly lower than the statutory value. According to the Mühleberg NPP’s assessment, this proof can also be furnished on the basis of the assumption that the main cooling water outlet, which is used as an intake by the special emergency system, is blocked by bedload.
The analyses to prove the ability to control the 10,000-year flood were based on the assumed hazard for the previously planned new Mühleberg power plant (EKKM). ENSI assesses the scenario regarding the maximum flow rate for the river Aare of 1,166 m3/s (PMF, Probable Maximum Flood) as appropriate, and this also applies to the maximum flood level of 466.25 meters above sea level at the site of the Mühleberg NPP, taking account of the inflow from the Saane. The procedure adopted by the Mühleberg NPP meets the requirements for proof.
Following its own review and after consulting the Dams and Reservoirs Section of the Swiss Federal Office of Energy (SFOE), ENSI concludes that the Mühleberg, Rossens and Schiffenen dams will withstand a 10,000-year flood. Consequently, a breach of the dams need not be assumed for the deterministic proof by the Mühleberg NPP of its ability to control a 10,000-year flood.
ENSI has reviewed the proof of the cooling water supply for the SUSAN special emergency building on the basis of the documents submitted and the documentation for the back-fitting measures approved in mid-August, which must be carried out and accepted by ENSI before the plant is restarted after the inspection that is currently in progress.
According to ENSI’s assessment, the main paths for the cooling water supply are the path via the main cooling water intake structure, the path via the main cooling water outlet with the back-fitted intake pipes, and the path via the back-fitted feed nozzles for the direct supply to the SUSAN cooling water system using mobile pumps. In terms of availability, the last two cooling water paths mentioned will provide reserves in relation to the expected level of the river Aare in case of a 10,000-year flood.
ENSI’s in-depth assessments have shown that the large number and the diversity of the back-fitted cooling water paths will guarantee the cooling water supply for the SUSAN special emergency system in every case.
Proof that the SUSAN special emergency system is able to function in case of a 10,000-year flood also constitutes proof that a cold startup of the plant is possible in case of any given single failure. No release of radioactivity due to an incident is to be expected under these conditions
ENSI’s summary and conclusions:
The Mühleberg NPP has furnished proof of its ability to cope with a 10,000-year flood under the overall framework of conditions specified by ENSI. Even if conservative assumptions are made as regards the overall conditions, the radiation dose resulting from this flooding incident is significantly below the permitted value of 100 mSv for the population in the area surrounding the power plant as stipulated for such an incident by the Radiological Protection Ordinance (RPO/StSV).
Acceptance of this proof is subject to the proviso that the back-fitting measures approved by ENSI in mid-August to upgrade the SUSAN intake structure are implemented, and that they are accepted by ENSI.