NUCLEAR SAFETY

    The fundamental objective of nuclear safety is to protect people and the environment from harmful effects of ionizing radiations. This objective applies to all circumstances that give rise to radiation risks. It implies the following:

• Control of radiation exposure of people and control of releases to the environment

• Prevention of events that could lead to a loss of control over nuclear reactor core or any other source of radiations;

• Mitigation of the consequences of such events if they were to occur.

      The fundamental safety objective and ten associated safety principles are stated in an IAEA publication (IAEA Safety Standards Series SF-1 ).

Nuclear Safety -1



Reactivity Control


1) Reactivity control requirements

      Reactor power level control in operation and shutdown under normal and off-normal conditions; Compensation for reactivity changes due to core configuration, experiments, burnup, or temperature changes; Compensation for transient poisoning effects; Rapid shutdown and sub-criticality in the most unfavourable conditions.


2) Typical reactivity control mechanisms

      Moveable control rods or blades. Moveable fuel. Chemical means, such as boric acid in coolant. Burnable poisons, such as Gadolinium. Removable poison plates.


3) Typical reactivity feedback mechanisms

      Fuel temperature: prompt effect, must be negative. Doppler broadening of resonance absorption in U-238 or other materials. Moderator temperature: normally subject to heat transfer delay. Coolant void formation: positive or negative.


Removal of Decay Heat from the Core


1) Radioactive fission products release energy in decay to a stable state.


2) The decay heat depends on:

      The fuel and fertile materials. The time of irradiation and the power density. The time after shutdown. The reactor neutron spectrum.


3) Since fission products are retained within the cladding, cooling must be sufficient to prevent cladding degradation or failure.


4) Fuel cooling considerations

      Adequate cooling must be maintained at all times to remove decay heat and prevent cladding failure in the reactor or in spent fuel storage.


5) Reactor cooling systems:

      Primary cooling system (light water, heavy water, CO2, liquid metals). Water is an excellent heat sink and its quality must be maintained to prevent corrosion phenomena. Secondary/tertiary heat removal system (pumps, heat exchangers, condensers, cooling towers). Emergency cooling system (light water).


Safety System Design Requirements


1) Systems

      Defense-in-Depth. Single failure criterion. Redundancy, diversity and physical separation. Emergency power supply. Periodic testing and reliability requirement.


2) Components:

      Qualification, including environmental qualification. Use of design and construction codes for nuclear field.


3) Systems and components

      Design against external and internal hazards. Application of QA rules.


Defense-in-Depth concept


• Is the key concept on which all of nuclear safety is based.

• Defense-in-Depth may be structured as five levels of defense.

• An implementation of Defense-in-Depth in design is multiple physical barriers to release of radioactive material to the environment.

• Independence of the barriers is a key factor.

Objectives of Defense-in-Depth:

• To compensate for potential human and component failures.

• To maintain the effectiveness of the barriers by avoiding damage to the facilities and to the barriers themselves.

• To protect the public and the environment from harm in the event that the barriers are not fully effective.

Nuclear Safety -2

Level of Defence-in-Depth



Nuclear Safety -3

Nuclear Safety -4

Technical Aspects of Safety


Site Selection

      The site selection shall take into account relevant features that might affect the safety of the installation, or be affected by the installation, and the feasibility of carrying out emergency plans.

      All aspects shall be evaluated for the projected lifetime of the installation and re-evaluated as necessary to ensure the continued acceptability for safety of site-related factors.


Design and Construction

• Reliable, stable and easily manageable operation of the nuclear installation.

• Application of the Defense-in-Depth principle.

• Proven or qualified technologies.

• Systematic consideration of the man-machine interface and human factors.

• Design against potential internal and external hazards:

• Internal hazards: fire, internal missiles, internal flooding, etc.

• External hazards: Earthquakes, aircraft impact, industrial environment, external flooding, extreme environmental conditions such as very low temperatures.

• Feedback from the Fukushima-Daiichi NPP accident should be taken into account (Design Extension Conditions).

• Exposure to radiation of site personnel and releases of radioactive materials to the environment shall be kept as low as reasonably achievable (ALARA).

• Performance of a comprehensive safety assessment and an independent verification of the design.



Design Principles

• No single equipment failure or human action should disable a safety function

• The possibility of common cause failure should be minimized by diversity of equipment;

• Redundant systems should function independently;

• Fail-safe design concepts should be used.

Minimize likelihood and impact of human error:

      Engineered systems;

• Automatic control, protection and alarm systems;

• Elimination of human actions that could jeopardize safety;

• Clear presentation of data and reliable communications.


Commissioning

• Specific approval by the regulatory body shall be required before the start of normal operation on the basis of an appropriate safety analysis (SAR) and a commissioning programme.

• The commissioning programme shall provide evidence that the installation as constructed is consistent with design and safety requirements.

• Operating procedures shall be validated to the extent practicable as part of the commissioning program, with the participation of the future operating staff.


Operation and Maintenance

• Definition of a set of operational limits and conditions (OLCs) is derived from the safety analysis, tests and subsequent operational experience.

• Conduction of the operation, inspection, testing, maintenance and supporting functions must be performed by sufficient numbers of properly trained and authorized personnel in accordance with approved procedures.

• Available engineering and technical support throughout the lifetime of the installation.

• Documents and procedures for operator response to anticipated operational occurrences and accidents.

• Reporting on incidents significant to safety and sharing of operating experience.


Verification of Safety

      The operating organization shall verify by analysis, surveillance, testing and inspection that the physical state of the installation and its operation continue in accordance with operational limits and conditions, safety requirements and the safety analysis.

   Systematic safety reassessments of the installation in accordance with the regulatory requirements shall be performed throughout its operational lifetime, with account taken of operating experience and significant new safety information from all relevant sources.


Radioactive Waste Management and Decommissioning

• The generation of radioactive waste, in terms of both activity and volume, shall be kept to the minimum practicable by appropriate design measures and operating procedures.

• Waste treatment and interim storage shall be controlled in a manner consistent with the requirements for safe final disposal.

• The design of a nuclear installation and the decommissioning programme shall take into account the need to limit exposures during decommissioning to ALARA.

• Prior to initiation of decommissioning activities, the decommissioning programme shall be approved by the regulatory body.


Principles of Safety

Responsibility for safety

      The prime responsibility for safety must rest with the person or organization responsible for facilities and activities that give rise to radiation risks.

Role of government

    An effective legal and governmental framework for safety, including an independent regulatory body, must be established and sustained.

Leadership and management for safety

      Effective leadership and management for safety must be established and sustained in organizations concerned with, and facilities and activities that give rise to, radiation risks.

Justification of facilities and activities

      Facilities and activities that give rise to radiation risks must yield an overall benefit.

Optimization of protection

      Protection must be optimized to provide the highest level of safety that can reasonably be achieved.

Limitation of risks to individuals

      Measures for controlling radiation risks must ensure that no individual bears an unacceptable risk of harm.

Protection of present and future generations

      People and environment, present and future, must be protected against radiation risks.

Prevention of accidents

      All practical efforts must be made to prevent and to mitigate nuclear or radiation accidents.

Emergency preparedness and response

      Arrangements must be made for emergency preparedness and response in case of nuclear or radiation incidents.

Protective actions to reduce existing or unregulated radiation risks

      Protective actions to reduce existing or unregulated radiation risks must be justified and optimized.