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Internal accident management

Briefly explained:

(Design basis) accidents …

… are possible technical malfunctions and events that have been investigated and assessed in accordance to the state-of-the-art of science and technology at the time of licensing of a nuclear facility. They can pose a danger to man and the environment. Reliably precautionary measures must be put in place to counter these scenarios (so-called design basis accidents), which are considered at the time of licensing, so that prescribed radiation exposure limits can be complied with.

Beyond-design basis accidents …

… are accidents in nuclear power plants, beyond the possible technical malfunctions and events (design basis accidents) considered during the licensing process. Beyond-design basis accidents may necessitate the use of emergency measures.

At German nuclear power plants internal accident management measures are in place for hypothetical events that were not considered a design basis accident at the time the license was granted and against which a nuclear facility is not designed.

Internal accident management comprises all technical and organizational measures which are taken within a nuclear facility (such as a nuclear power plant) to prevent, or at least minimise, any release of radioactive substances.

The operator of a nuclear facility is responsible

The internal accident management is stipulated in the Atomic Energy Act and is the responsibility of the operator of the nuclear facility. It refers to the plant itself. Thus, it distinguishes itself from the external accident management, which covers all measures outside a nuclear installation and is carried out by state authorities, e.g. the disaster control authorities of the Länder. For more information, please refer to the website of the Federal Office for Radiation Protection (BfS) "Responsibilities of the Federal Government, the states and operators in emergency radiological protection".

Internal accident management measures

Internal accident management includes all measures to detect and control beyond-design basis event sequences as early as possible and safely, and to end them with as little damage as possible.

The following measures are differentiated

  • preventive and
  • mitigative.

preventive

Preventive measures

First of all, preventive measures are taken to prevent severe fuel element damage. The main objective is to maintain or restore fuel cooling and to transfer the plant to a safe state.

Example

An example of a preventive measure is the concept of measures for secondary and primary bleed and feed at pressurized water reactors. At first, the pressure of the secondary side (i.e., in the secondary circuit) is relieved by opening valves. Then, it should be possible to supply feed water, e.g. with mobile pumps. If this does not succeed, appropriate measures for pressure relief and feeding of coolant in the primary circuit will be performed consecutively.

mitigative

Mitigative measures

If preventive measures fail and fuel element damage occur, mitigative measures will be performed in order to limit or mitigate severe radiological effects inside the plant and its environment. In this case the main objectives are

  • to maintain the integrity of the remaining barriers that retain the radioactive material, e.g. the intact reactor containment, which shall enclose the gaseous radioactvie substances which might be released of fuel assemblies damaged in the event of an accident, and
  • to ensure a controlled state of the plant for the protection of the environment in the long term.

Example

A mitigative measure is e.g. the filtered containment venting. In order to prevent a failure of the containment due to excessive pressure, the gases are discharged to the environment via a valve. The radioactive substances are largely reduced by a filter system.

Manuals for accident management

Each German nuclear power plant has an emergency manual in which the necessary organizational regulations and measures are described in order to enable situation-appropriate action by the staff. Beside organizational descriptions as well as task and activity descriptions, it also contains instructions, documents and tools required to handle a beyond-design basis event. These are pre-planned measures. The currently valid and up-to-date emergency manual must be easily accessible to the operating personnel in the control room of the nuclear power plant at all times. In addition, at least one copy each must be kept in the emergency control room and the place of action of an internal emergency response staff.

Severe Accident Management Guidelines (SAMG) supplement the emergency manual. This plant-specific guidelines support the internal emergency organization in taking mitigative measures in case of an accident. The SAMG serve the internal emergency response staff in case of event sequences or plant conditions for which no accident management measures have been pre-planned or for which the implemented accident management measures are not effective. The SAMG mainly describe strategies that limit the release of radioactive substances.

Development and legal classification of the internal accident management

Internal accident management is an integral part of defence-in-depth concept for nuclear power plants in Germany and stipulated in the Atomic Energy Act.

Implementation of the internal accident management began as early as the 1980s on the basis of recommendations of the Reactor Safety Commission (RSK) and took place essentially during the 1990s and 2000s as part of operator responsibility.

Following the reactor accident in Fukushima in 2011, further internal accident management measures were implemented, in particular based on a robustness analysis of the German nuclear power plants carried out by the RSK and recommendations from the EU stress test, for example the provision of mobile diesel generators for an additional power supply.

Since the publication of the "Safety Requirements for Nuclear Power Plants" in November 2012, internal accident management is a mandatory component of the nuclear regulations. Furthermore, it has been laid down in the Atomic Energy Act since June 2017.

State of 2018.07.27

© Federal Office for the Safety of Nuclear Waste Management