Stress Tests

Objective and Scope of Stress Tests

The Fukushima Daiichi accident has forced the world to think about whether nuclear power plants are operating safely and whether their level of safety and reliability is adequate. Immediately after the accident, European politicians, ministers of the Member States, representatives of the nuclear industry and regulatory authorities agreed on the implementation of NPP safety reviews on European diplomatic ground. All 15 Member States of the European Union (EU) operating NPPs have been involved in the process, the neighbouring countries, Switzerland and Ukraine, have also joined.

To achieve equal access to safety reviews, the European Nuclear Safety Regulators Group (ENSREG) and the European Commission (EC) have issued a Declaration containing the Stress Tests specification.

The aim of the Stress Tests was to determine the degree of external hazard that the NPP can withstand without suffering severe damage to the nuclear fuel in the reactor core, or without significant leakage of radioactive materials into the environment. At the same time, the aim was to evaluate the existing capabilities of NPPs and their personnel in coping with extreme weather conditions, which may ultimately result in the loss of the plant’s basic safety functions. In other words, to demonstrate that NPPs operating in the EU have an adequate level of safety and do not represent a disproportionate risk to the population and the environment, even in the event of extreme externalities.

Prevention and management of the consequences of a malicious aircraft crash at an NPP are not part of the Stress Tests for nuclear safety. A special working group has been set up at EU level to assess the nuclear security, which includes assessing the risk of terrorist attacks.

Stress Tests in the Slovak Republic

Testing of Slovak NPPs (Bohunice V2, Mochovce 1&2, Mochovce 3&4) was conducted in the form of engineering analyses, calculations and assessments. The stress tests analysed extraordinary external events such as earthquakes, flooding and the consequences of other initiating events potentially leading to multiple loss of NPP´s safety functions. Combinations of events were also assessed, including power outages, long-term water supply interruptions, as well as power losses due to extreme weather conditions.

Stress tests of nuclear power plants in the Slovak Republic started on June 1, 2011, in accordance with the agreement between the EC and the ENSREG group. All operated Slovak NPPs, i.e. Bohunice V2, Mochovce 1&2, as well as Mochovce 3&4 plant under construction, were tested. The testing schedule was divided into several phases. In the initial phase, on 30 December 2011, the Slovak Republic submitted the National Report on the Results of Stress Tests performed at NPPs in the Slovak Republic to the Secretariat of Stress Tests, the function of which is performed by the Joint Research Centre in Petten, the Netherlands. However, this did not end the Stress Tests process, but continued with the phase of international review of results, which was organized in three steps.

Desktop Review

From 1 to 20 January 2012, there was a so-called desktop review, in which all participating national regulatory authorities of EU countries, as well as the EC itself, had the opportunity to study the submitted National Reports and formulate questions for the seventeen participating countries. The Slovak Republic received ninety questions on the national administration, to which ÚJD SR, in cooperation with the SE, a. s., prepared written answers.

Topical Peer Review

The second step of the review was a Topical Peer Review, which took place from 5 to 22 February 2012 at the EC premises in Luxembourg. As the name suggests, the review was divided into three main topics, namely:

  • earthquakes and extreme natural external conditions,
  • complete loss of power supply and loss of ultimate heat removal,
  • management of beyond-design (severe) accidents.

For each area, all EU countries had the right to nominate one expert to take part in the review. Almost ninety experts participated in the Topical Review together with EC representatives. The countries that performed the Stress Tests presented the main results of the review there. At the same time, they answered the crucial questions they had received in the previous phase (desktop review) and answered the direct questions of the experts present. The result of this review was a draft review report, for each participating country.

Country Peer Review

The country reports were finalized during the last phase of the Stress Tests, in March 2012, directly in the individual countries (Country Peer Review). The international team of experts, whose task was to verify the declared results of Stress Tests directly in the country and at the NPP, was in the Slovak Republic from 26 to 29 March 2012. Representatives of ÚJD SR and SE, a. s., answered the questions of foreign evaluators, who also verified the declared statements at the operated Units 1&2 of the Mochovce NPP, and at Units 3&4 of the Mochovce NPP under construction. They paid attention to the seismic resistance of safety-critical buildings, systems and components, measures against flooding of the power plant site, availability of standard and non-standard supplies of coolant at the power plant, power supply options from various sources, battery condition and other important aspects of operational safety. Based on the findings, the review team finalized the Review Report, which is published together with other reports from other EU Member States. In addition to a summary of the facts concerning the legislative requirements, the design and organizational measures related to the three monitored areas, the Report also lists the strengths of the NPPs in the Slovak Republic, as well as suggestions for further improvements.

Final Peer Review Report of EC

On April 26, 2012, the European Commission and the ENSREG Group issued the European Commission’s Final Report – Peer Review Report on the Stress Tests performed on European nuclear power plants. In their joint statement, ENSREG and the EC stated: “In terms of the principle of continuous improvement of nuclear safety, the stress tests identified specific improvements. Both the overall Peer Review Report and the Country Reports contain very practical suggestions aimed at achieving concrete improvements.“

The overall report highlights four principal areas for improvement that will be examined across Europe:

  • issuance of WENRA recommendations using the best available EU expertise for natural risk assessment considering the IAEA guides,
  • importance of the Periodic Safety Review,
  • implementation of recognized measures to protect reactor integrity,
  • minimizing accidents caused by natural factors, and limiting the consequences of such accidents.

Based on all the information gathered, the representatives of the EC review commission prepared the final report from the NPP Stress Tests covering all relevant countries. The Report is available for the public and has also been presented to the European Parliament (EP).

Public Hearing on the results of Stress Tests

On January 23, 2012, a public hearing on the results of Stress Tests was held in Bratislava. At the meeting, the representatives of ÚJD SR informed both the professional and lay public about the scope, content and results of Stress Tests performed at the NIs from the position of the regulatory authority. The public hearing was attended also by the representatives of SE, a.s., who together with the representatives of ÚJD SR answered all questions from the public. Slovakia was the first country within the EU organizing such public hearing on the results of Stress Tests.

Stress Tests Reports

Stress Tests Report prepared by ÚJD SR

ENSREG Reports (European Nuclear Safety Regulators Group)

Action Plan

On 13 December 2012, SE, a.s. submitted to ÚJD SR the Action Plan of SE, a. s., for the implementation of measures as lessons learned from events at NPP Fukushima Daiichi, and from Stress Tests of NPPs. By doing so, SE complies with the Atomic Act, according to which the nuclear operator is obliged to take corrective actions based on the results of analyses of the causes of operational events to increase the level of nuclear safety of NPPs in the Slovak Republic. UJD SR subsequently prepared the National Action Plan of the Slovak Republic, which was sent to the EC at the end of 2012. The Report contains the conclusions of the European Council of June 2012, the conclusions of the ENSREG group of July 2012, and integrates the proposed measures of the operator. SE´s Action Plan and the National Action Plan of the Slovak Republic are part of the Stress Tests process of European NPPs.

A workshop was held in Brussels from 22 to 26 April 2013, with the aim to evaluate the National Action Plans and to discuss the implementation of the recommendations. The result is a summary report from ENSREG workshop on national action plans.
A Workshop on Peer Review progress in the implementation of National Action Plans was held in Brussels on 20-24 April 2015.

Updated: 08.02.2024