Dose constraint

FIGURE 6.17 Planned constraints on the distribution of dose (left), and limits on the concentration distribution (right) that ensure dose constraints are not violated. 

The development of many standards that specify dose constraints for specific practices or sources at levels well below the annual dose limit of 1 mSv for all controlled sources combined (Kocher, 1988 Mills et al., 1988) is an important means of ensuring that the lifetime cancer risk from exposure to controlled sources normally will not... 

A prospective upper bound on the individual dose which is used in the optimisation of protection and safety for sources. For occupational exposures, dose constraint is a source-related value of individual dose used to limit the range of options considered in the process of optimisation. For public exposure, the dose constraint is an upper bound on the annual doses that members of the public should receive from the planned operation of any controlled source. The exposure, to which the dose constraint applied is the annual dose to any critical group, summed over all exposure pathways, arising from the predicted operation of the controlled source. The constraint for each source should ensure that the sum of doses to the critical group from all controlled sources remains within the dose limit. For medical exposure the dose constraint levels should be interpreted as guidance levels, except when used in optimising the protection of persons exposed for medical research purposes or of persons, other than workers, who assist in the care, support or comfort of exposed patients. 

Because there are so many factors involved in establishing dose pathways and which may be the critical pathway, various dose assessment computer based models have been developed. These models are also used to calculate the dose per unit release of a specified radionuclide for various pathways. The Release Upper Bound, RUB can be evaluated using a dose assessment model. This is done by varying the source term used such that the resulting dose equals the dose constraint or limit. The various exposure scenarios and pathways are also chosen for importance. 

Risk After Treatment of Thyroid Cancer Barrington et al. (1996) measured dose rates from 86 thyroid cancer patients receiving iodine therapy in the UK. The study comprised ablation patients (A) and follow-up patients (B). The faster clearance in the follow-up patients resulted in less stringent restrictions than those advised for ablation patients. The authors concluded that their data could be used to derive guidelines to restrict the annual dose to less that 1 mSv for children and members of the public, and to determine dose constraints for caregivers. 

In a study from eight centers in Belgium (Monsieurs et al., 1998), the measured doses in 94 relatives of thyrotoxicosis and thyroid cancer patients are presented. The relatives wore thermoluminescence dosimeters (TLD) on the wrist for at least 7 days. The authors propose the implementation of a nonrigid dose constraint for people who knowingly and willingly help patients treated with 1311. 

In a Norwegian study, Cappelen et al. (2006) measured radiation doses to 76 family members of thyrotoxic patients treated with I. The relatives wore TLDs on both wrists for 14 days. The patients received oral and written detailed EC recommendations (Radiation Protection 97, 1998) about restrictions of behavior. The time periods for restrictions varied according to the age of the family members (Table 100.5), and it was advised that children aged 0-2 years should be cared by some other person for the first days after the treatment. The radiation dose to family members was found to be well below the EC recommended dose constraints, except to one 2-year-old child, whose mother did not comply with the instructions given. 

Most studies show that radiation doses to the public, caregivers and relatives are below the recommended dose limits and dose constraints if proper instructions are given and compliance is adequate. 

Dose constraints recommended by ICRP is a few mil-hsievert per episode for caregivers and relatives. The... 

EC has recommended more detailed dose constraints depending on the age of household members. 

The ICRP-recommended dose constraint is a few mil-lisievert per episode. The ICRP-94 claims that restrictions following the release of patients should focus on the sensitive subgroup (i.e., infants and children). 

Dose constraints are not legally binding but act as ceiling levels, which should not be exceeded. They are recommendations with the aim of limiting doses to the patient s family, close friends and third persons. The dose constraints can be higher than the pubhc dose Emits for the family and close friends. Children (including unborn children) are regarded as members of the pubhc with an applied dose hmit of 1 mSv. 

The EC guidance document (Radiation Protection 97, 1998) has recommended more detailed dose constraints than those recommended by the ICRP, as shown in Table 100.7. The suggestions are for members of the public (sometimes called third persons), as well as for family members and close friends. The last group comprises all people living in the same house as the patient, or those who visit the patient at hospital or at home. 

Table 100.7 Dose constraints for different categories of oaregivers ...

Type of caregiver Reason for dose constraint (risks, habits) Dose constraint (mSv)... 

Fuel handling and storage facilities are designed to prevent inadvertent criticality and to maintain shielding and cooling of spent fixel as necessary to meet operating and off-site dose constraints. 

For each exposure situation an appropriate system of dose constraints and limits should be developed. 

In order to ensure that the dose limits for members of the public are not exceeded when die exposure of a critical group for a particular source is added to the exposures of tiiat group from all other sources, the concept of dose constraints has been introduced. Informatirm on the estd7lishment of source related dose constraints for members of the public are given in the IAEA-TECDOC-664 [8]. This document shows which considerations have to be taken into account when siting a dose constraint, and gives examples of the derivation of source related dose constraints. Several countries have already set dose constraints, especially for the nuclear fuel cycle facilities the values range between 0.1 and 0.3 mSv/a. 

It has been recognized (see section 3) that a disposal focility can be operated without large discharges of airborne or waterborne radioactivity. On the other hand the direct exposure from waste packages can easily be controlled. Therefore a low dose constraint for the effective dose to members of the public can and should be set for disposal facilities. A sensible order of magnitude for the dose constraint is 0.1 mSv per year. 

Even if no limit for potential exposure is set in the regulations, a low risk from the operation of disposal facilities should be strived at. An effective dose of 0.1 mSv per year, which is a sensible order of magnitude for the dose constraint, leads to a risk of fatality of five in a million per year. This indicates that the risk from the operation of a disposal facility with respect to potential exposures should not exceed the order of magnitude of one in a million per year. 

INTERNATIONAL ATOMIC ENERGY AGENCY, Establishment of Source Related Dose Constraints for Members of the Public, Interim Report for Comment, IAEA-TECDOC-664, Vieima (1992). 

For areas identified as being part of a practice, the clean-up would be subject to the same considerations as any other routine activity within the practice, that is, the clean-iq> operation would have to be optimized by weighing the costs of clean-iq) against the benefits. The optimized solution would be constrained by appropriate dose constraints. In principle, the concept of clearance is also relevant here although it seems likely that the environmental concentrations derived on the basis of trivial doses will be too low to be of practical application in this context. 

When monitoring data are to be used to assess the annual doses for a critical group and to verify comphance with the dose constraints in the case of practices or to check against the intervention level, the minimum detectable activity of the equipment concerned should be selected so as to enable measurements to be made at levels that are substantially lower than the established reference dose levels, with account taken of multiple pathways of human exposure. For every pathway that has to be checked, a certain fraction of the reference dose should be allocated the minimum detectable activities should be designed to guarantee the detection of these possible contributions to doses. 

Doses to persons shall be below the relevant dose limits. Protection and safety shall be optimized in order that the magnitude of individual doses, the number of persons exposed, and the likelihood of incurring exposure shall be kept as low as reasonably achievable, economic and social factors being taken into account, within the restriction that the doses to individuals be subject to dose constraints. A structured and systematic approach shall be adopted and shall include consideration of the interfaces between transport and other activities. 

Target 3 addresses the annual dose any persons off the site might receive from sources of ionising radiation on site. Section 12.4.7 of this PCSR shows that the API 000 design is below the Basic Safety Obj ective and below the dose constraint by a factor of 15demonstrating tiiat die anticipated dose is ALARP. 

This gives a total dose to the AP 1000 design critical group of 19 pSv per year. This is below the obj ective of 20 pSv and lies well below the UK dose constraint of300 pSv per year by a factor of 15 and below the proposed new constraint limit of 150 pSv per year, as given by the HPA advice on the Application of ICRP s 2007 Recommendations to the UK (Reference 12.10). 

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