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Safety of Nuclear Reactors
Safety of Nuclear Reactors

All nuclear facilities are sited, designed, constructed, commissioned and operated in accordance with strict quality and safety standards. The Atomic Energy Regulatory Board (AERB) frames the policies and lays down safety standards and requirements. It also monitors and enforces all the safety provisions. The AERB exercises regulatory control through a stage-wise system of licensing. This has resulted in India’s excellent record in operation of Nuclear Power Plants (NPPs).

Nuclear Power Plant

The safety of the reactors and their operations is ensured by:
  1. “Defence-in-Depth” philosophy at Design level
  2. Radiation exposures “As low as Reasonably Achievable (ALARA)” at Operations level
  3. Management of Radioactive Waste and
  4. Preparedness for nuclear emergency

Safety of Nuclear Reactors: Design level Safety

The basic design philosophy followed world-over for assuring nuclear power plant safety is called "defence-in-depth" approach with multiple safety systems supplementing the natural features of the reactor core. Key aspects of this approach which can be summed up as Prevention, Monitoring, and Action (to mitigate consequences of failures) are

  1. High-quality design and construction so that the reactor operates with a high degree of reliability. The prevention of accidents is through intrinsic design features and stresses on quality control, redundancy, testing, inspection and fail-safe design.
  2. Equipment which prevents operational disturbances or human failures and errors developing into problems,
  3. Comprehensive monitoring and regular testing to detect equipment or operator failures,
  4. Redundant and diverse systems to control damage to the fuel and prevent significant radioactive releases,
  5. Provision to confine the effects of severe fuel damage (or any other problem) to the plant itself.

The safety provisions include a series of physical barriers between the radioactive reactor core and the environment, the provision of multiple safety systems, each with backup and designed to accommodate human error. The barriers in a typical plant are: the fuel is in the form of solid ceramic (UO2) pellets, and radioactive fission products remain largely bound inside these pellets as the fuel is burnt. The pellets are packed inside sealed zirconium alloy tubes to form fuel rods. These are confined inside a large steel pressure vessel (for a light water reactor, LWR) or a pressure tube (for a pressurized heavy water reactor, PHWR) with walls up to 30 cm thick. The associated Primary Heat Transport system removes the heat substantially. All this, in turn, is enclosed inside a robust pre-stressed or reinforced concrete containment structure with walls at least 1 m thick. This amounts to three significant barriers around the fuel, which itself is stable up to very high temperatures.

Despite efforts to prevent accidents, it may be anticipated that one might occur. Therefore reliable protection devices are provided to prevent or minimise the effects of an incident. Such devices include an emergency core cooling system (ECCS) to provide adequate core cooling in event of a loss of coolant accident, engineered limits on the rate of power increase, a fast reactor shutdown system activated by redundant and independent instrument channels, an independent supply of off-site power etc.

An added level of safety is ensured by evaluating the design concept under conditions of severe hypothetical accidents. This adds design margin by assuring protection of the public even if seemingly remote and unlikely events occur. In this respect, several Design Basis Accidents (DBAs) are considered, such as the loss-of-coolant accident (LOCA) where a large pipe rupture is assumed to abruptly occur. Other design features include protection against seismic events, tsunamis, cyclones, floods, and component failures.

Safety of Nuclear Reactors: Radiological Protection during Operations

For radiation protection purposes, design values are prescribed for the radiation level at a specified distance from the equipment/components as well as for the general radiation fields in different areas of the plant. The plant layout is such that the areas are segregated according to their radiation levels and contamination potential. The maintenance approach and shielding are designed such that the individual and collective doses to the station personnel would be “as low as reasonably achievable”.

AERB has prescribed an effective individual dose (whole body) limit for a radiation worker as 20 mSv/year averaged over five consecutive years, calculated on a sliding scale of 5 years. (The cumulative effective dose in the same 5-year period shall not exceed 100 mSv). There is an additional constraint of a maximum of 30 mSv in any year. Dose limits have been specified for individual organs. Investigation limits are prescribed at which investigation of exposure cases exceeding these limits are carried out by an AERB Committee.

A limit on the collective dose is specified at the design stage of each NPP so that adequate provisions for radiation protection are made in the design of the plant to keep radiation levels in different areas below design levels. The collective annual dose to plant personnel is kept below the annual dose budget approved by the AERB. Constant efforts are made each year to reduce this progressively by devising measures for reduction at the plant level.

Each NPP has a Health Physics Unit (HPU), comprising of a group of trained and experienced radiation protection professionals, who are responsible for the implementation of the radiation protection program in the plant. They provide radiological surveillance by monitoring of areas, personnel, systems, effluents, exposure control and exposure investigations. The external and internal exposures of all the plant personnel are assessed on a regular basis. Following the ALARA principle, measures for reduction of both the individual and collective dose are devised. These include engineering and administrative solutions such as shielding, ventilation, use of protective equipment, procedure adherence, work permit system, access control, mock up, training, supervision, etc. Air contamination levels in different zones of the plant are maintained well below the prescribed limits by provision of proper ventilation and filtration systems. AERB carries out regulatory inspections frequently to check the adequacy of the radiation protection program and its implementation. For the radiological protection of the public, AERB has prescribed a dose limit of 1 mSv/y (which is 1/20th of that of an occupational radiation worker) during normal operation of all the facilities at a given site. This is about 40% of the average dose received from natural background sources. The sources contributing to generation of radioactive solid, liquid and gaseous wastes and their release to the environment are examined with respect to minimization of waste at the source at the design stage itself. The design analysis demonstrates that the calculated dose to the members of the public at the site boundary under a postulated design basis accident condition does not exceed the reference doses prescribed by AERB.

Safety of Nuclear Reactors: Management of Radioactive Waste

Gaseous wastes from reactor buildings are filtered using pre-filters and high-efficiency particulate air filters and released after monitoring through a stack. The release rate and integrated releases of different radionuclides are monitored and accounted for to demonstrate that the releases are within the prescribed limits. Similarly, the radioactive liquid wastes are also segregated, filtered and conditioned as per procedure and after adequate dilution (so as to comply with the limits of discharges) disposed to the water body. The activity discharged is monitored at the point of discharge and accounted on a daily basis. The radioactive solid wastes are disposed off on-site in brick-lined underground trenches, re-enforced cement concrete (RCC) vaults or tile holes, depending on the radioactivity content and the radiation levels.

AERB has prescribed limits on the annual volume and activity of discharge, daily discharges and activity concentration from each NPP, which are site-specific. The performance of the radioactive waste management system established at NPPs is reviewed to ensure that appropriate methods and management practices continue to be in place and the generation of radioactive waste is kept to as minimum as practicable in terms of activity and volume.

Safety of Nuclear Reactors: Preparedness for Nuclear Emergency

The prescribed standards ensure an adequate margin of safety so that NPPs can be operated without undue radiological risks to the plant personnel and members of the public. Nevertheless, it is necessary to develop emergency response plans so that in case of any eventuality, however unlikely, no undue radiological risk to plant personnel and public is incurred. All NPPs have established and documented emergency procedures by having an on-site as well as an on-site emergency preparedness plan. The role, responsibilities and action plans for various agencies required to act during an emergency are detailed in these plans. Specific requirements with respect to emergency preparedness in NPPs have been formulated by AERB.

NPPs are generally sited in a relatively low-population zone. An exclusion zone around the plant is established, which is under the exclusive control of the operating organization, and no public habitation is permitted in the area. The dose limits to a member of the public, under normal operating conditions and under design basis accident conditions specified, are applied at the boundary of this exclusion zone. A Sterilized Zone beyond the exclusion radius is also established. The Emergency Planning Zone is the zone defined around the plant up to a 16-km radius and provides for the basic geographical framework for decision making on implementing measures as part of a graded response in the event of an off-site emergency. This zone is examined in great detail while drawing up an offsite emergency plan and arranging logistics for the same. In case of an accident, the emergency measures consist of emergency actions in respect of notification, alerting personnel, assessment of situation, corrective actions, mitigation, protection and control of contamination.

The Environmental Survey Laboratories installed at all Nuclear Power Plant sites are well equipped for the assessment of radioactivity in various environmental matrices in and around the regions of the NPPS during normal operation. They play an important role during a nuclear emergency by assessing the impact of the emergency. Environmental monitoring is carried out with special focus on the affected sector so as to help decide the plan of action and suggest countermeasures, if required.