Five levels of defence

There are 5 independent layers of defence for nuclear facilities in Canada. The implementation elements for each layer are described below and are adapted from REGDOC-3.5.3, Regulatory Fundamentals.

Level 1 – Prevention of abnormal operation and failures

The first layer of defence encompasses items within the facility itself. The objective is to prevent any change from normal operation, and prevent failures of structures, systems and components (SSCs) important to safety.

Implementation of Level 1 includes the following:

Level 2 – Control of abnormal operation and detection of failures

The second layer of defence deals with detecting incidents and failures. The objective is to detect and intercept any change from normal operation, prevent failures from escalating to accidents, and return the plant to a normal state.

Implementation of Level 2 includes the following:

Inherent and engineered design features to minimize or exclude uncontrolled reactivity as well as system temperature and pressure changes to the extent possible, and return the plant to a state of normal operation after any deviations

Applicable regulatory documents:

Monitoring systems to identify any change from normal operation

Staff training to respond to changes from normal operation, if/when they occur

Applicable regulatory documents:

Level 3 – Control of accidents within the design basis

The third layer aims to control and minimize any consequences in the unlikely event of an accident. The objective is to control accidents within the design parameters, minimize the consequences of accidents, and prevent escalation to severe accidents.

Operations at this level are meant to maintain barriers and containment through sophisticated safety systems within the facility.

Implementation of Level 3 includes the following:

Inherent safety features, fail-safe design, engineered design features, procedures that minimize design-basis accident consequences, redundancy, diversity, segregation, physical separation, independent safety system channels, and protection against single-point failures

Applicable regulatory documents:

Instrumentation suitable for accident conditions

Applicable regulatory documents:

Design-basis accident guidance to manage accidents and mitigate their consequences as much as possible

Applicable regulatory documents:

Staff training for accident response

Applicable regulatory documents:

Level 4 – Control of severe plant conditions

With the defence-in-depth approach, measures taken at the first 3 levels ensure the structural integrity of the core and limit the severity of accidents.

The objective of Level 4 is to control severe plant conditions, mitigate consequences of severe accidents, and ensure that radioactive releases are kept as low as possible.

Implementation of Level 4 includes the following:

Guidance to manage accidents and mitigate their consequences as much as possible

Applicable regulatory documents:

Strong containment design that includes features to address containment challenges (e.g., filtered venting, hydrogen combustion, overpressure protection, core concrete interactions, molten core spreading and cooling)

Applicable regulatory documents:

Complementary design features to prevent accident progression and mitigate any consequences

Applicable regulatory documents:
- REGDOC-2.4.2, Probabilistic Safety Assessment (PSA) for Reactor Facilities
- REGDOC-2.5.2, Design of Reactor Facilities

Features to mitigate radiological releases (e.g., filtered vents)

Applicable regulatory documents:
- REGDOC-2.5.2, Design of Reactor Facilities

Staff training in accident response

Applicable regulatory documents:

Level 5 – Mitigation of radiological consequences

The fifth layer aims to mitigate consequences of a radiological release in the highly unlikely event of a severe accident not controlled through the preceding layers. Here, the focus is on comprehensive offsite emergency planning, training and response to address protective actions, interventions and coordination in order to protect the public and the environment. As with all onsite safety features of operating nuclear facilities in Canada, offsite emergency preparedness must also be exercised regularly. Stringent measures, controls and regulatory oversight are in place to ensure that there is a very low probability of approaching Level 5.

Implementation of Level 5 includes the following:

Emergency support facilities, established exclusion zone, and onsite and offsite emergency response plans and provisions

Applicable regulatory documents:

Staff training on emergency preparedness and response

Applicable regulatory documents:

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Date modified:: 2026-09-11

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