Confinement and containment for safety in nuclear power plants

According to the IAEA Safety Glossary⁽¹⁾, confinement is typically used to refer to the safety function of preventing or controlling the release of radioactive materials to the environment. This has to be ensured in any operational state and accident condition, but it could be also extended further to waste management.
In accordance with the concept of defence in depth⁽²⁾, this fundamental safety function is achieved by means of containment, that consists in methods and physical barriers or structures specifically designed to prevent or control the release or dispersion of radioactive substances.
In nuclear power plants, besides the containment building surrounding the reactor, associated systems are present able to guarantee the functions of isolation, control and management of mass and energy releases, control and limitation of radioactive releases, and control and management of combustible gases. A containment system shall be provided to ensure, or to contribute to, the fulfilment of three main safety functions:

• confinement of radioactive substances in operational states and in accident conditions;
• protection of the reactor against natural external events and human induced events;
• radiation shielding in operational states and in accident conditions.

First, the design of the containment shall be such as to ensure that any radioactive release from the nuclear power plant to the environment is as low as reasonably achievable, below the authorized

limits on discharges in operational states and below acceptable limits in accident conditions.
For operational states, the annual dose received by people living in the vicinity of a nuclear installation is expected to be comparable to the effective dose due to natural background levels of radiation (i.e. the levels that originally existed at the site). For public exposure in planned exposure situations, the proposed range of values for the dose constraint indicated by IAEA represents an increase of up to 1 mSv in a year over the dose received from exposure due to naturally occurring radiation sources.
The leak-tightness of the containment is essential to confine radioactive material and to minimize radioactive releases: leak-tightness is generally characterized by specified maximum leak rates that are not expected to be exceeded under accident conditions. Equipment are foreseen to ensure that the containment keeps its integrity and leak-tightness before and during the accidental event. Isolation of the containment is necessary to confine radioactive releases into the containment atmosphere caused by accident conditions. Multiple means are required to be implemented to remove heat from the containment in accident conditions. The structural integrity of the civil structures of the containment and of the associated systems necessary for the mitigation of accident conditions is required to be ensured with appropriate margins, taking into account the loads or combinations of loads originating from the hazards. In addition to the design provisions implemented to mitigate the

consequences of the postulated accident conditions, the use of non-permanent equipment is also to be considered, and adequate connection points and interfaces with the plant are required to be installed with the objective of avoiding large releases of radioactive material and unacceptable off-site contamination in accident conditions. In accident conditions, highly energetic phenomena that could jeopardize the structural integrity and the leak-tightness of the containment are required to be dealt with by incorporating adequate features to ensure that the possibility of such phenomena may be considered to have been ‘practically eliminated’.
Regarding the second safety function, the containment is required to be designed to provide protection of reactor against the effects of natural and human induced external hazards and against the effects of internal hazards originating from equipment installed at the site. Finally, in operational states and in accident conditions, the containment contributes to the protection of plant personnel and the public from undue exposure due to direct radiation from radioactive material within the containment. This means that the composition and thickness of the concrete, steel and other materials shall be such as to ensure that radiation doses to workers at the plant and to members of the public do not exceed the dose limits, that they are kept as low as reasonably achievable in operational states for the entire lifetime of the plant, and that they remain below acceptable limits and as low as reasonably achievable in, and following, accident conditions.