Reactor Safety Commission-report about nuclear power plants

As a result of the nuclear disaster in Fukushima, caused by the earthquake and subsequent tsunami that stuck Japan on 11 March 2011, the Federal Environment Ministry commissioned the Reactor Safety Commission (RSK) on 17 March 2011 with a safety review of the 17 German nuclear power plants. On 17 May 2011, Chairman of the RSK, Rudolf Wieland, presented the then Environment Minister Norbert Röttgen with the Commission's safety review of the German nuclear power plants.

Considering the information available and the scope of the topics considered, the following can be stated for German nuclear power plants when drawing a direct comparison with the causes and consequences of the accidents at Fukushima I:

According to current knowledge, initiating events that may lead to such tsunamis are practically excluded for Germany. At Fukushima I, the design of the plants was inadequate for a tsunami with an occurrence frequency of approx. 10^-3/a (meaning the event occurs on average once in 1000 years) on the basis of the literature available. In the area of external natural hazards, the effects to be considered according to the state of the art in science and technology in connection with occurrence frequencies of approx. 10^-4 /a for earthquakes and 10^-5/a for floods are taken into account throughout in the designs of German nuclear power plants. The electricity supply of the German nuclear power plants is more robust throughout than at Fukushima I. All German plants have at least one additional assured incoming supply and more emergency power generators, with at least two of them protected against external impacts.

Taking existing knowledge regarding Fukushima into account, the RSK focussed particularly on the safety margins (degree of robustness) of each individual plant in the case of impacts beyond the design basis and beyond assumptions thus far. The higher the safety margins that can be demonstrated against impacts on the plant regarding the fulfilment of the safety objectives, the higher the degree of robustness. The safety objectives are the prevention of chain reactions, the cooling of the reactor core and fuel elements and confinement of radioactive substances.

In the safety review the RSK differentiates between:

• Robustness levels regarding natural hazards, postulates (for example assuming full power outage), precautionary measures and accident management measures and
• degrees of protection for the man-made hazards to be additionally considered according to the RSK Catalogue of Requirements.

The assessment of the robustness of the plants is based on the fulfilment of basic levels defined specifically for each topic. As regards the assessment criteria, there are generally – specific to each topic – three levels or degrees of protection defined for each. At the highest level, for example Level 3 above the basic level, a violation of the safety objectives is practically excluded in the case of beyond design-basis scenarios as laid down by the RSK.

As a basis for the robustness assessment, the RSK presupposes that the plants correspond to their current licensed condition and that the improvement measures identified as safety-relevant in the safety reviews regularly carried out in accordance with the Atomic Energy Act (AtG) or as a result of other regulatory processes have been fully implemented (basis level).

As a priority, the design concept of German nuclear power plants is based on the prevention of events or of any safety-relevant consequences of events. This means that with regard to redundancy, diversity and barriers, the designs of more recent reactor generations tend to fulfil stricter requirements. This is why the technical realisations in the plants regarding the robustness below the assessment criteria described by the RSK are also different. This is not generally addressed in the assessment.

The RSK explained that within the time frame available, it was not possible to generate these assessment criteria with regard to the quantitative approaches on the basis of scientific limit analyses for this first statement, rather they could generally only be postulated.

The following areas were either assessed by the RSK for robustness level and degree of protection or alternatively evaluated generically by the RSK for example not plant-specific.

Natural hazards:

• Earthquakes
  – Level classification –
• Flood
  – Level classification –
• Other natural hazards (including climate influences)
  – generic –

Postulates:

• Station blackout (SBO) (complete power outage including emergency power supply)
  – Level classification –
• Long-lasting SBO
  – generic –
• Loss of service water supply
  – Level classification –

Precautionary measures:

• PWR
  – generic –
• BWR
  – Level classification –

Aggravating boundary conditions for the implementation of accident management measures (AMM) (wreckage, radioactivity,...):

• For cases observed
  – Basic level defined –
  – Recommendations drawn up –

Man-made hazards:

• Aircraft crash
  – level classification –
• Blast waves
  – level classification –
• Explosive gases
  – generic –
• Toxic gases
  – generic –
• Effects of an accident in one power plant unit on neighboring unit
  – generic –
• Terrorist attack
  – generic –

The RSK drew the following conclusion from the safety review:

• Compared to Fukushima, the design of these plants regarding electricity supply and the consideration of external flooding events, a higher level of precaution can be ascertained for German plants.
• Further robustness tests show that no uniform findings could be established regarding plant design or age. Older plants which originally had a less robust design were backfitted with partly autonomous emergency systems to ensure vital safety functions. This selectively leads to evidentially high degrees of robustness in older plants.

In its position statement, the RSK stresses that the assessment criteria established serve solely for a topic-specific differentiation with regard to the existing safety margins and do not represent any regulatory requirements. On the basis of the amount of information to be prepared in a short space of time, it was not possible to achieve a consistently reliable allocation of robustness levels or degrees of protection The results of the robustness assessment therefore often also include indications regarding the need for further analysis and assessment.