Radiation Protection Dosimetry Advance Access originally published online on July 22, 2008
Radiation Protection Dosimetry 2008 131(3):340-345; doi:10.1093/rpd/ncn179
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Reconsideration of the minimum dose constraint for public exposures in radiological protection
Radiation Safety Research Center, Central Research Institute of Electric Power Industry (CRIEPI), 2-11-1, Iwadokita, Komae-shi, Tokyo, Japan
* Corresponding author: thattori{at}criepi.denken.or.jp
Received October 10, 2007, amended May 20, 2008, accepted June 10, 2008
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODRESULTSDISCUSSIONCONCLUSIONFUNDINGREFERENCES |
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By using a probabilistic approach, the effects of the dose distribution of radiation due to man-made radioactive nuclides when added to those of natural background radiation have been studied. These results show that additional exposure to man-made radiation of up to 0.5 mSv y–1 (as a dose constraint) would not significantly change the distribution of total public doses. Taking into consideration such probabilistic analysis and rationales of derivations of exemption and clearance levels, it can be concluded that the minimum dose constraint that requires optimisation in radiation protection, should be set to 0.1 mSv y–1, which is one-order magnitude higher than 0.01 mSv y–1, the current dose criterion for exemption and clearance.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODRESULTSDISCUSSIONCONCLUSIONFUNDINGREFERENCES |
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The dose criterion used to derive clearance and exemption levels is of the order of 0.01 mSv y–1 based on the Basic Safety Standard (BSS)(1) of the International Atomic Energy Agency (IAEA), the use of which has been agreed upon by many countries. To provide guidance to national authorities, including regulatory bodies and operating organisations, in the application of the concepts of exclusion, exemption and clearance as established in the BSS, the IAEA published a safety guide, RS-G-1.7(2), and gave specific activity concentrations for radionuclides of both natural and man-made origins that may be used for bulk amounts of material for the purpose of applying exclusion or exemption. Two different approaches were employed to establish the activity concentrations given in RS-G-1.7 for use in making decisions on exclusion, exemption or clearance(3). The first approach applies the concept of exclusion to derive activity concentration suitable for radionuclides of natural origin. The activity concentration for radionuclides of natural origin in RS-G-1.7 was selected considering the upper end of the worldwide distribution of activity concentrations in soil provided by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)(4). It has also been stated that doses received by individuals as a consequence of these activity concentrations would be unlikely to exceed about 1 mSv in a year, excluding the contribution from the emanation of radon, which is dealt with separately in the BSS. On the other hand, the second approach uses the concept of exemption in order to derive activity concentration for radionuclides of man-made origin. The primary radiological basis for establishing activity concentration for radionuclides of man-made origin is that the effective doses to individuals should be of the order of 0.01 mSv or less in a year. To take into account the occurrence of low-probability events leading to higher-radiation exposures, an additional criterion was used, namely, the effective doses due to such low-probability events should not exceed 1 mSv in a year. This approach is consistent with that used in establishing the exemption levels for small amount of solid material, e.g. less than a few tons, which is provided in Schedule I of the BSS(1). These backgrounds around RS-G-1.7 show that there are a variety of dose criteria in the stage of deriving levels of exclusion, exemption and clearance.
It should also be noted that there must be large uncertainties in the final stage of clearance inspection. The estimation of target radionuclide concentration for clearance inspection is usually based on the direct measurement of the concentrations of readily monitored nuclides such as gamma emitters, but for radionuclides whose concentrations cannot be measured by gamma measurement, the radionuclide spectra (could be termed radionuclide vectors or ratios to gamma emitters) can be utilised in the estimation. In such estimations for clearance, a low level of radioactivity results in many uncertainties (errors) of measurement. Also, there may be a serious uncertainty (scattering) beyond the order of the activity concentrations and nuclide spectra used in the clearance inspection. This indicates that in the final stage of the implementation of clearance, a dose criterion higher than the order of 0.01 mSv y–1 has to be allowed for effective doses for the public, particularly in the case of low probability events(5,6).
On the other hand, the International Commission on Radiological Protection (ICRP) released drafts of new recommendations in June 2004, June 2006 and January 2007 and finally approved a new set of fundamental recommendations on the protection of humans and the environment from ionising radiation at its meeting in Essen, Germany, 19–21 March 2007. In the new ICRP recommendations, namely, Publication 103(7), dose constraint is considered an effective tool for the optimisation of radiological protection. In Publication 103, the numerical value for the minimum dose constraint for the public remains undetermined.
In this paper, a numerical value for the minimum dose constraint, which is required for the optimisation of radiological protection, is proposed, taking into account the fundamental concept used in the exclusion, exemption and clearance criteria and the practical effects of the minimum dose constraint on individual effective dose distribution of the public.
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METHOD |
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TOPABSTRACTINTRODUCTIONMETHODRESULTSDISCUSSIONCONCLUSIONFUNDINGREFERENCES |
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Dose distribution of natural background radiation
The UNSCEAR 2000 report(4) provides the annual dose distribution due to natural background radiation averaged over 15 countries. The arithmetic mean of the dose distribution is approximately 2.0 mSv y–1(4). The distribution has a peak in the dose region lower than the median value and can be redrawn as a lognormal distribution. On the other hand, the UNSCEAR 2000 report(4) also states that the annual worldwide mean dose is 2.4 mSv y–1 using the other approach than the 15 countries' data, which is frequently used when addressing natural background radiation level.
Taking these investigation results into consideration, in this study, the dose distribution of natural background radiation was assumed to be lognormal with 2.0 mSv y–1 for the geometric mean (EGM) and 2.0 for the geometric standard deviation (SG). A value of 2.5 mSv y–1 can be theoretically obtained as the arithmetic mean of the supposed distribution, which is almost consistent with the 2.4 mSv y–1 worldwide mean dose provided by the UNSCEAR 2000 report. This indicates that the assumption of the dose distribution for natural background radiation is sufficiently valid.
Dose distribution of radiation due to man-made nuclides
The first step in this study is to determine the additional effect of a dose due to a man-made source on the public is added. In this study, it is not important to exactly determine the dose of radiation due to a man-made source, but a probability distribution is required. It is therefore necessary to determine the standard deviation of the dose distribution of radiation due to a man-made source.
There are few, if any, studies that have actually investigated the distributions of external and internal doses that affect the public. Thus, no rationale can be found to consider a case study result as a representative dose distribution. However, there are some cases in which the dose distributions of radiation that workers in nuclear facilities were exposed to, where individual doses were not limited and controlled, were similar to the lognormal distribution(8), which is the same as that in the case of natural background radiation. For this reason, the dose distribution of radiation due to a man-made source that members of the public may be exposed to was assumed to be lognormal in the present study.
Lognormal distributions are determined by two parameters, EGM (median) and SG, which represent the degree of scattering. As described in the previous section, the SG of the dose distribution of radiation due to natural background radiation was assumed to be 2.0. From the point of view of external and internal exposures through various pathways, because both natural and man-made nuclides are distributed in the environment by the same way, the dose distributions of radiations due to natural and man-made nuclides would be similar each other. For this reason, the SG of a dose distribution of radiation due to man-made nuclides was assumed to be 2.0, which is the same as that due to natural background radiation.
On the other hand, the assumption of an appropriate EGM is closely related to the requirement for compliance with dose constraint using the concept of a representative person(9), which was published as Publication 101 by the ICRP. The representative person is a hypothetical person who receives a dose that is representative of the more highly exposed people in the population. In a probabilistic dose assessment, the ICRP recommends that the representative person should be defined such that the probability is less than about 5% that a person drawn at random from the population will receive a greater dose. This concept of the ICRP defines how the dose constraint should be complied with using a probabilistic approach, in addition to the previous approach of compliance with the dose constraint, namely, the deterministic approach, which is based on conservative and simple dose assessments. In other words, the magnitude of the conservativeness of the dose assessment is mathematically and more accurately given by the concept of the representative person.
When using the concept of the ICRP, it can be proven theoretically using the following equation that the 95th percentile of the dose distribution for the representative person is always lower than the dose constraint
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| (1) |
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If a value of 2.0 is given to the SG, then equation (2) can be expressed as
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From the above results, the maximum dose distribution of radiation that the public can be exposed to due to man-made nuclides can be determined from equation (3), on the basis of the ICRP concept of the representative person of ICRP. As an example, the dose distributions of natural background radiations and that due to man-made nuclides are shown in Figure 1 in the case of 0.5 mSv y–1 for the dose constraint. It can be seen from the figure that the dose distribution of radiation due to man-made nuclides ranges in the dose region lower than the dose constraint. It should be noted that this distribution is a hypothetical one for a representative person who receives a dose that is representative of the more highly exposed individuals in the population, and that the dose actually received by the public may be lower.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODRESULTSDISCUSSIONCONCLUSIONFUNDINGREFERENCES |
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Probabilistic distributions of the public dose obtained by adding a dose distribution of radiation due to man-made nuclides to that due to natural background radiation were calculated using the Monte Carlo technique. The dose distribution of radiation due to natural background radiation was lognormal with 2.0 mSv y–1 for the EGM and 2.0 for the SG. The dose constraints to be complied with, were selected as 0.1, 0.3, 0.5 and 1.0 mSv y–1. Using these values and equation (3), the dose distributions for man-made nuclides were chosen as lognormal with 2.0 for the SG, and the EGM were chosen as 0.032, 0.096, 0.16 and 0.32 mSv y–1.
Figure 2 shows plots of the dose distribution for natural background radiation and its summation with that for man-made nuclides.
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In Figure 2, the bold and thin solid lines are the dose distribution for natural background radiation and that of the total dose, respectively. It can be seen that at dose constraints of up to 0.5 mSv y–1, there is no significant difference between the two distributions. On the other hand, at a dose constraint of 1.0 mSv y–1, which is both the maximum dose constraint and the dose limit, there is a slight difference between the two distributions. However, the difference is only significant in the lower-dose region and there is no difference in the higher-dose region. This indicates that the effect of the additional dose is relatively higher for people who receive a lower-dose of radiation due to natural background radiation, but it should be noted that in such a case, the absolute dose level is remarkably low. Moreover, a more important point is the fact that the public is unaware of the dose they are actually exposed to and where they are located in the dose distribution for natural background radiation. From the above results, the additional dose resulting from setting the dose constraint to the order of 0.1 mSv y–1 can be regarded as trivial in the framework of the discussion of the radiation protection system.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODRESULTSDISCUSSIONCONCLUSIONFUNDINGREFERENCES |
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Dependence of dose distribution of radiation due to man-made nuclides
To obtain the results, the SG of the dose distribution for man-made nuclides was assumed to be 2.0. To determine the effects of the change in the SG on the summation of the dose distributions, the probability distributions of the summation of doses of radiations due to natural background radiation and man-made nuclides were calculated using the Monte Carlo technique, with values of 1.5, 2.0, 3.0 and 5.0 for the SG in the case of 0.5 mSv y–1 for the dose constraint. The results are shown in Figure 3. It can be seen that the summation distributions are almost independent of the SG. This indicates that the assumption of the dose distribution for man-made nuclides does not significantly affect the result that the additional dose resulting from setting the dose constraint to the order of 0.1 mSv y–1, can be regarded as trivial. It can be concluded that this result is robustly supported by the relation between the dose constraint and the 95th percentile of the dose distribution for man-made nuclides as shown in Figure 1.
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In this study, the dose distributions for man-made nuclides were only added to those for which the natural radiation levels were already well known by the UNSCEAR. However, the contribution to total public exposure due to medical exposure in addition to natural radiation should be considered. It might be possible that the effects of exposure to man-made nuclides can be regarded as trivial even in the case of the setting of a higher dose constraint, if sufficient quantitative knowledge of medical exposure is clarified hereafter.
Radiation workers are usually under the radiation control with personal dosimeters, and accurately know the external dose received at work. They work under radiation control so as not to receive internal exposure. However, when it occurs, the internal dose is assessed through appropriate radiological assays. On the other hand, members of the public know neither external nor internal exposure to radiation due to natural background radiation or man-made nuclides discharged in the environment. Therefore, it would be a reasonable approach to discuss about the lower-dose region that requires optimisation in radiation protection, based on the uncertainty expressed by the probability distributions of the dose received by the public.
Minimum dose constraint in new ICRP recommendation
Before approving a new set of fundamental recommendations on the protection of man and the environment against ionising radiation at the ICRP main commission held in Essen in March 2007, the ICRP released draft reports on its new set of recommendations in June 2004, June 2006 and January 2007 and openly discussed with many experts on radiation protection through a website consultation system. In the 2004 draft report, the minimum dose constraint was described to be 0.01 mSv y–1. In the subsequent 2006 draft reports, this description was removed, but a similar description was noted in the 2007 draft report as an expression of the range of dose constraints with a minimum value of 0.01 mSv y–1. Finally in ICRP Publication 103(7), the description of the minimum dose constraint of 0.01 mSv y–1 has been removed again. Thus, the numerical value for the minimum dose constraint for the public is still an unsolved issue in a radiation protection system.
The dose constraint is an effective tool for optimisation in radiological protection. If the minimum dose constraint is determined, no one has to set the dose constraint lower than the minimum dose constraint, which leads to an understanding that the minimum dose constraint is equivalent to a lower bound of optimisation in radiological protection.
Comparison with exemption and clearance criteria
As mentioned above, the ICRP has once proposed 0.01 mSv y–1 as a minimum dose constraint in previous draft recommendations. As an example, the dose distribution for the public, setting 0.01 mSv y–1 as a dose constraint, is compared with the dose criterion of exemption. The result is shown in Figure 4, assuming that the dose distribution is lognormal and the SG is 2.0. It can be recognised that setting 0.01 mSv y–1 as a dose constraint leads to a request for further dose reduction to make the public dose lower than the exempted dose region. It is clear that 0.01 mSv y–1 is too low and strict as a minimum dose constraint. It should be noted that the dose distribution obtained by setting 0.01 mSv y–1 as a dose constraint is hypothetical for a representative person who receives a dose that is representative of the more highly exposed people in the population and that, consequently, the dose actually received by the public in general would be considerably lower.
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In addition to the above result, to discuss how the numerical value for the minimum dose constraint should be set in the radiological protection system, the rationale and theoretical background of exclusion, exemption and clearance are needed; it has been agreed upon to adopt the IAEA safety guide RS-G-1.7, and it might be adopted in the revised BSS. In RS-G-1.7, the exclusion criterion for natural origins has been approximately 1 mSv y–1. Exemption criteria for man-made origins have been categorised into two types, the order of 0.01 mSv y–1 for normal situations and that of 1 mSv y–1 in the case of low-probability events. It should also be noted that RS-G-1.7 permits member states to exceed the relevant activity concentrations for natural and man-made origins by up to ten times (or of the order of 0.1 mSv y–1: 10 x 0.01 mSv y–1), because regulatory bodies' decisions would depend on the nature of national regulatory infrastructure.
To conclude the above discussions, it is proposed that the minimum dose constraint for the optimisation of radiological protection of the public should be set at 0.1 mSv y–1
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CONCLUSION |
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TOPABSTRACTINTRODUCTIONMETHODRESULTSDISCUSSIONCONCLUSIONFUNDINGREFERENCES |
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The effect of the addition of the dose distribution for man-made nuclides to that for natural background radiation on the distribution of the dose received by the public was studied using a probabilistic approach. The rationales of derivations of exemption and clearance levels in the RS-G-1.7 in the IAEA were reviewed. Although the dose criteria of exemption and clearance are currently of the order of 0.01 mSv y–1, the minimum dose constraint that requires optimisation in radiation protection should be set to one order higher than the current dose criterion of 0.01 mSv y–1, on the basis of the present reasonable probabilistic approach and theoretical backgrounds of exemption and clearance levels. It can be concluded that the minimum dose constraint for the optimisation of radiological protection of the public should be set at 0.1 mSv y–1.
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FUNDING |
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TOPABSTRACTINTRODUCTIONMETHODRESULTSDISCUSSIONCONCLUSIONFUNDINGREFERENCES |
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This study was carried out under the low dose radiation research project in the Central Research Institute of Electric Power Industry (CRIEPI).
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMETHODRESULTSDISCUSSIONCONCLUSIONFUNDINGREFERENCES |
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- International Atomic Energy Agency. Derivation of activity concentration values for exclusion, exemption and clearance. (2005) SR-44.
- United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and effects of ionizing radiation. (2000) UNSCEAR 2000 Report Vol. I.
- Hattori T. An approach to safety margin for measurement error and scattered nuclide spectrum on the monitoring for compliance with clearance level. Trans. Atomic Energy Soc. Jpn (2004) 3(4):363–368. (in Japanese).
- Hattori T. Approach to safety margin for uncertainty in measurement and nuclide spectrum in clearance level inspection. (2006) Proceedings of the 4th International Symposium on Release of Radioactive Material from Regulatory Control—Harmonization of Clearance Levels and Release Procedures: Hamburg. March.
- International Commission on Radiological Protection. The 2007 recommendations of the international commission on radiological protection. (2007) ICRP Publication 103.
- United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and effects of ionizing radiation. (1993) UNSCEAR 1993 Report.
- International Commission on Radiological Protection. Assessing dose of the representative person for the purpose of radiation protection of the public. (2006) ICRP Publication 101, Part 1.
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