DOSE LIMITATION

The basic philosophy of the International Commission on Radiological Protection (ICRP) dose limitation system consists of three principal components:

(a) Justification. No practice resulting in human exposures to radiation should be authorized unless its introduction produces a net benefit, taking into account also the resulting radiation detriment.

IAEA-TC^53.5/18 21

(b) Optimization. All exposures should be kept as low as reasonably achievable economic and social factors being taken into account. This requirement implies that the detriment resulting from a practice should be reduced by protective measures to such a value that further reduction is less proportionate to the additional efforts and costs required to achieve the reduction.

(c) Individual dose limits. The effective dose equivalent to individuals from all practices (except those specifically excluded) should not exceed the applicable dose limits. Applying this requirement, it must be recognized that many present day practices give rise to effective dose equivalents that will be received in the future. This should be taken into account to ensure that present or future practices would not be liable to result in a combined overexposure of any individual.

4.1. Primary dose limits

The dose equivalent limits are not intended to be design or planning objectives.

Values above the limits are specifically not permitted, but values below the limits are a constraint for the optimization procedures.

The effective dose equivalent limits for workers based on ICRP recommenda-tions and specified in Safety Series No. 26 [8] are as follows:

(a) In order to prevent the occurrence of non-stochastic effects, a limit of 0.5 Sv (50 rem) in a year applies to all tissues except the lens of the eye; for the lens of the eye the recommended annual limit is 0.15 Sv (15 rem). These values apply irrespective of whether tissues are exposed singly or in combination with other tissues, and are intended to constrain exposures that fulfil the limitation for stochastic effects given below.

(b) For the limitation of stochastic effects, the limit on the annual effective dose equivalent (H^E) is 50 mSv (5 rem).

4.2. Secondary and derived limits

In practice it is necessary to use secondary limits for external and internal exposures when compliance with the primary dose limits cannot be demonstrated directly.

In the case of external exposure, secondary limits may be expressed in terms of dose equivalent index (deep dose equivalent index and shallow dose equivalent index).

In the case of internal exposure, secondary limits may be expressed in terms of the annual limit of intake (ALI). Internationally recommended values for the ALIs consistent with the dose equivalent limits are given in ICRP Publication 30 [9] and in Safety Series No. 9 [10]. An ALI value for radon daughters is given in ICRP Publication 32 [11] and Safety Series No. 26 [8]. Internal exposure may also be

22 AHMED and DORY

controlled by the use of derived limits, such as a limit for the annual time integrated concentration in air. Compliance with secondary or derived limits will ensure compliance with the annual dose equivalent limits.

4.2.1. Radon daughters

The ICRP recommends an annual limit for intake by inhalation of 222Ra daugh-ters of 0.02 J of inhaled potential alpha energy. This limit has been set based on both epidemiological and dosimetric approaches. This corresponds to a derived air concentration of 8.3 /J/m3 or 0.4 working level, and to an annual limit of exposure (ALE) of 0.017 J-h/m³ (5 working level months (WLM)). The working level is the sum of the alpha energies released by the decay of radon daughters which are in equilibrium with 3.7 Bq (100 pCi) radon. This amounts to 1.3 X 10⁵ MeV/L.

Therefore, one working level represents any combination of the short lived radon daughters in one litre of air that will result in the ultimate emission of 1.3 x 10⁵ MeV of alpha energy, taking no account of radon itself. If a worker is exposed to a radon daughter concentration of 1 working level (WL) for 1 working month (170 h) then the exposure is 1 WLM. The annual limit expressed in WLM is 0.4 X 12 = 4.8, i.e. 5 WLM.

4.2.2. Ore dust

The derived limit for respirable uranium ore dust is based on the total alpha activity of the long lived nuclides in the ²³⁸U decay chain, assumed to be in secular equilibrium with 238U. The internal doses and the ALI are highly dependent upon the ICRP inhalation classes for the individual nuclides. It is probable that the nuclides in ore are class Y. On this basis the ALI for uranium ore dust in terms of total long lived alpha activity is 1.7 kBq-h/m3. The corresponding ALE is 1.5 kBq-h/m3 for uranium. Should the ore body be seriously out of radioactive equilibrium, the ALI should be calculated using the actual radionuclide ratios.

4.2.3. Uranium

The ALI for uranium is based on the naturally occurring mixture of isotopes and on the assumption of an aerodynamic median activity diameter of 1 /an. Uranium concentrates vary widely in solubility, depending on the specific mill process. For class Y material, the uranium ALI is 1.5 kfiq and the derived air concentration is 0.61 Bq/m3. For class W material, the ALI for uranium is 28 kBq.

For soluble uranium compounds (class D), chemical damage to the kidney is more important than radiological effects. Based on a threshold concentration of 900 /ig of uranium in the kidney, the acute inhalation limit for uranium is 20 mg. For chronic exposure a daily intake of 1 mg could result in the kidney limit being reached in approximately 10 years.

IAEA-TC-453.5/18 23

4.3. Combined exposures

When external and internal exposures are received together, the individual dose limits will not be exceeded if the following two conditions are met:

j ALIj

500

where Hi d is the annual deep dose equivalent index in mSv, Hi s is the annual shallow dose equivalent index in mSv, i is the annual intake of nuclide j in Bq, and ALIj is the annual limit of intake of nuclide j in Bq.

In the uranium mines this additivity has the effect of requiring the inhalation of radon and its daughters to be kept below their recommended limits by an amount that depends on the exposure to external radiation and ore dust. The combination formula is:

Hj.d + Imd + Ipdu < j

50 0.02 1700 ~

whereHid = Annual deep dose equivalent index in mSv I^md = Intake of radon daughters expressed in J

lodu = Intake of uranium ore dust expressed in becquerels of total long lived alpha activity

In terms of exposure and practical units the formula may be expressed as follows:

50 ⁺ T 1500 " ^l

where Hi>d = Annual deep dose equivalent index in mSv

E^md = Exposure to radon daughters in working level months

Eodu ⁼ Exposure to uranium ore dust in total long lived alpha Bq-h/m³ Similar additivity considerations apply in milling operations. In cases where inhalation or ingestion of concentrates or other radioactive substances occurs the formula should be modified accordingly.

The ICRP recognizes that in some mining operations it may not be possible to operate within the combined limits. The competent authority will then have to take a decision on how best to deal with these situations.

For practical purposes, other limits are often set in addition to primary dose equivalent limits. Authorized limits are set by the competent authority or the management and are generally lower than the primary or derived limits. When

24 AHMED and DORY

authorized limits are specified by the management, they are designated as operational limits. Reference levels may be established by the competent authority for any of the quantities determined in the course of radiation protection programmes, whether or not there are limits for these quantities. A reference level is not a limit, but it is useful in determining a course of action when the value is exceeded.