System of Radiological Protection

International Commission on Radiological Protection (ICRP) is a non-governmental scientific organization founded in 1928 to establish basic principles and recommendations for radiation protection. The recommendations are being revised periodically taking into account new developments and research inputs. The current protection standards are based on the current 2007 Recommendations of the ICRP. The earlier basic recommendations (ICRP-60) were published in 1991.

International Atomic Energy Agency (IAEA), headquartered is in Vienna, came into being on 29 July, 1957, is an independent intergovernmental organization within the United Nations System. The objective of the agency is to facilitate peaceful and beneficial applications of atomic energy worldwide. It is also involved in developing internationally harmonized Basic Safety Standards (BSS) for protection of the people from the harmful effects of radiation and to ensure safety of the sources. The most recent IAEA document- International Basic Safety Standards (BSS), GSR Part 3, 2014 is the updated document providing requirements for all facilities and activities involving radioactive materials and radiation generators. The IAEA general safety guide document: Occupational Radiation Protection (ORP) GSG-7, 2018, provides required guidance to comply with the requirements of the BSS.

• Prevent the deterministic effects, and
• Minimize the probability of stochastic effects occurring among the occupational workers.

A linear relationship between the effect and the radiation dose is assumed by the ICRP as a conservative approach.

In the revised system of radiological protection, for practical control of radiation exposures of individuals in all possible exposure situations, the ICRP classified the situations as: Planned, Emergency and Existing exposure situations.

The exposures of persons are further classified into three types: occupational exposures, medical exposures and public exposures.

Planned exposure situations

These cover both normal exposures and potential exposures arising out of planned introduction and operation of sources, including medical exposures of patients. There are some situations, where unplanned exposures may occur due to loss of control over the sources from deviations from planned operating procedures. These exposures are referred to as potential exposures.

Emergency exposure situations

These situations occur during the operation of a planned situation, a malicious act or from any unexpected event. The situation calls for urgent action to avaoid or to reduce any undesirable consequences.

Existing exposure situations

The situations are already existing when a decision needs to be taken to control the exposures. Typially, high radiation background areas with high radon levels, and prologed exposure situations due to residual activity after taking mitigatory actions in emergency situations.

The key principles of the system of radiological protection remained as: justification of a practice involving radiation exposure; optimization of protection and limitation of individual doses. It should be noted here that, the dose limits do not apply for medical exposures since the exposure is only for the benefit of the patient. However, justification and optimization principles are applicable.

Implementation of the requirements is the responsibility of national regulatory bodies. In India, Atomic Energy Regulatory Board (AERB) has the responsibility of implementation of the safety regulations.

Justification

Any procedure involving radiation exposure should be justified. The procedure should result in net positive benefit to the person or group of persons.

Optimization

Once a procedure or use of radiation is justified, the resulting exposure or the dose to the occupational workers and the public should be optimized. Generally, cost benefit analysis can be carried out to optimize the dose, which gives maximum benefit. Radiation exposures should be maintained as low as reasonably achievable (ALARA), economic and social factors taken into account.

Dose limitation

No individual should be exposed to an unacceptable level of risk as a consequence of his exposure. The upper limits for the justified and optimized exposures are given by the system of dose limitation.

The primary dose limits are based on the stochastic risk (cancer / genetic risk) estimates, which are mainly based on the data from the survivors of nuclear bomb explosions over Hiroshima and Nagasaki. The compliance to the limits should be seen after adding doses received from external sources and internal dose component as estimated by bioassay and wholebody counting methods.

External dose + Internal dose ≤ Annual effective dose limit

ICRP (2007) recommended dose limits

For the control of internal dose due to intakes of radioactive materials, secondary limits, i.e., Annual Limit on Intake (ALI) and Derived Air Concentration (DAC) are derived for various types of radio-nuclides using the annual average dose of 20 mSv. The ICRP (OIR Series) document (ICRP- 130, 2015) provides the values for effective dose coefficients, i.e., the dose receivable due to the intake of unit radionuclide, through inhalation route and ingestion route. ICRP default value of the inhaled particle size is 5 micrometer dia. The ALI and DAC values can be calculated using these dose coefficients as:

ALI (in Bq) = 20 mSv divided by Effective dose coefficient for inhalation
DAC (in Bq /cubic meter) = ALI divided by 2400 cubic meter

Where, 1200 is the cubic meter of air breathed by the reference radiation worker in 2000 hours of his working in one year. The inhaled materials are classified into three absorption (dissolution) types in body fluids after intake, i.e., highly absorption (Type F), medium absorption (Type M) and slowly absorption type ((Type S).

Radioactive contamination of surfaces contributes for the airborne radioactivity in workplaces. Regulatory body also provides the derived levels for surface contamination to control the spread of contamination, which may result in contamination of the workers. The AERB Safety Manual (Rev. 4, 2005) gives the derived working levels for contamination, which are applicable in the Indian facilities. The dose records of all the occupational workers need to be maintained by the management of the facility for the duration as specified by the national regulatory body.

Personal decontamination

Soap and warm water, with mild scrubbing can easily remove the external skin contamination. In general, gentle washing for about a minute will remove 0ver 95 % of the contamination present on intact skin. For persistent contamination, other radionuclide-specific chemical decontamination agents, such as sodium bicarbonate, dilute EDTA solution need to be used. Expert guidance may be necessary under such conditions.

Temporary / itinerant workers

The BSS do not prescribe separate dose limit for temporary workers. In India, the national regulatory body has certain stipulations. Employment of Temporary workers in radioactive areas my be done only after giving them adequate training in radiation protection and safe work procedures under the guidance of concerned safety officer (RSO). Medically fit person above the age of 18 y can only be employed. Management need to maintain record of the dose (Dose passport) received by the workers.