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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.

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