|Comments to the ICRP-Draft 2006
KATOH Kazuaki, TATENO Yukio and TADA JunIchiro
(1) Philosophy of Radiation Safety
We are still disappointed to see the draft lacks clear statement how the Commission considers on safety. Hence it makes difficult to judge whether the proposed strategies meet the philosophy and/or policy of ICRP on radiation safety.
No skin damage was the goal of radiation protection in 1930fs. In 1960fs, the subject of radiation safety was replaced to suppressing damages to human gene pool. The Commission stated in the 1977 Recommendations that the target level of safety to radiation workers should be comparable with so-called desk works. In 1990 Recommendations, the target level of safety is discussed in relation with life-time exposure or total risk received in whole life.
We eagerly expect the Commission issues the philosophy of safety with dignity. At the same time, we think the main role of ICRP is to provide (or recommend) the technique of decision-making concerning radiation safety and the data needed to these works.
(2) Justification of Protection
The Commission seams to have changed the mind and remain the principle of justification in the Recommendations. Despite of effort, this principle would not live up to our society as the Commission expected. The supposed objective of radiation safety control is general, and in practice there are various conflicts of interest among the objectives.
As stated in former version of the draft, the judgment of justification does not stand only on radiological considerations, so the Commission should make a sharp distinction between what ICRP can do and cannot do. If the Commission prefers to remain the justification principle in the Recommendation, it must clearly indicate supposed persons in charge of decision-making as well as the method of judgment. Since the decision-making depends on so many factors such as surrounding conditions (economical conditions, sense of values of the concerned, etc.) and initial (or historical) conditions.
(3) Optimisation of Protection
The economical and social factors, though they are stated to be gtaken into considerationh, are often much more important than dose reduction in decision-making. It is because ionising radiation is merely one of the risk sources to human health, and contribution of each risk source varies society to society. As social resources, i.e. both human- and financial resources, to tackle various risk sources are finite and limited, expenditure to one risk source for risk-reduction will affect expenditure to either risk-reduction of other sources or benefit-decrease with other item, and as a result it can happen to detract the societyfs safety as a whole.
In principle, target of optimisation can be set by regulatory authorities at their choices. We think the Commission should not set the goal of optimisation in unified way, since it can be different each other among countries from under-developing to already developed. On the contrary, the upper bound of optimisation, i.e. dose constraints should be common among countries irrespective to the social- and economical conditions.
Moreover, we think the Commission should recommend the person(s) who take responsibility of ALARA or optimisation judgment.
(4) Dose Limitation
Draft 2006 recommends two kinds of individual-related dose restrictions, i.e. dose limit for occupational radiation workers and that for members of the public, following the former Recommendations 1990. However, it is desirable to set fundamental level of radiological protection for individuals be independent of gender, age, occupation, social status, nationality, race, religion, belief and so on. For this reason, the value of individual-related dose restriction, i.e. of dose limit, should be applied commonly to all human beings regardless of distinction between occupational or not. We strongly expect the Commission endorse our view and rewrite the role of the currently used dose limit for occupational radiation worker as stated above.
However, we must pay attention to the people who are especially susceptible to ionising radiation, such as infants and children, those having special genotype, etc. We think the protection of those against radiation should be made in supplement. The common protection level for human beings stated above should not applied to those people and this is the reason we need second type of controlling criterion. Currently used controlling criterion of radiation dose to members of general public should be reinterpreted in this way. To be emphasized is this additional protection can be realised through source-related dose restriction, i.e. dose constraint, taking the state of space where members of the public can access as the object of control and monitoring in cases.
(5) Paternalistic Considerations
Pursuit of benefit and avoidance of risk are nothing but fundamental human rights. As far as the possibility of causing harm to others is recognized as permissibly small by the society concerned, any practice should be approved ethically, even if it may actually harm the practitioner oneself. However, from the view point of paternalism, we believe that the society should not overlook the situation where any extremely miserable consequence is foreseen. For this reason, we propose a concept of paternalistic dose limit for such a category of practice. With the concept of paternalistic consideration, a new limitation can be introduced to the dose of astronauts or volunteers at emergency situation where currently used dose limits are difficult to apply.
The paternalistic dose limit is subject to be determined not by considering economic, strategic, medical, defence and so forth, but by common sense of good citizens. We think the Commission should take role of recommending the values of paternalistic dose limit based on scientific considerations. We propose the paternalistic dose limit be set aiming at preventing tissue reaction, e.g. 500mSv which is about one tenth of Hauterythemdosis.
(6) Concept of Single Source
The term gsingle sourceh plays important role in the proposed new system, but its concept is difficult to understand. Definition of the term should be stated clearly in the Recommendations.
The followings are our proposal:
i) Single source is to be interpreted as whole radiation sources controlled under a licensee in normal (planned) situation. In case where the licensee declares to divide the radiation source into several groups and treat each group separately and independently, each group of radiation sources can be regarded and treated as a single source. It should be noted that the radiation sources that have discharged into atmosphere or the terrestrial water pool by the licensee, should be regarded and treated as a single source, even if it is made in authorised way. Exceptional is the radiation sources transferred from a licensee to another licensee following legal procedures. They are single sources themselves, of course, but for the sake of safety-control, it should be included into the single source of the licensee.
ii) In emergent situations, such as radiological accidents, destruction of nuclear facilities by terrorism, war and natural disasters, radiation sources concerned are to be regarded as a single source. Here again, in case where the source can be divided into several groups, each group can be regarded as a single source as far as independent safety control is possible. .
For the practical conveniences, intentional bundling of radiation sources should be allowed under the condition that individual doses are restricted to be below the dose limit and impose the licensee set suitable edose constraintsf ( constraints in spatial dose rate for working place and for accessible general space) and monitor the state of the space concerned.
(7) Dose or Limit?
As time goes, quantity and quality of information and knowledge on specific risk of radiation increases. Here specific risk means amount of risk per unit dose. When drastic change in our knowledge on specific risk was brought, we must revise our system of protection. Since the system is constructed with fundamental dose (absorbed dose), modifying factors (wR and wT) and controlling criteria (dose limits), we have three degrees of freedom for the modification of the system. If we want to keep the absorbed dose as the fundamental dose, the number reduced to two.
Quantities for control, dose in broader and original sense, are subject of measurement or evaluation, and to be stable in time as far as possible. Therefore, it should be the limit to be changed in such occasion. Weighted doses are quantities for control of radiation safety and change in figures of weighting factors means change of controlling quantities.
The Commission has been fickle and unprincipled with this issue as seen in the taken measures for protecting eye crystals from radiation. Some time it changed the limit and some time it changed the weighting factor (quality factor of radiation).
We think it is important to realize the role of the quantity, dose, and the required property of the quantity, stability. From viewpoint of metrology, it is a matter of course to use a unit to various quantities, but it is not desirable to have similar but different quantities with same name and same unit.
(8) Significance of Suffix R of the Radiation Weighting Factor, wR
In the 2006 Draft, significance of the suffix R of the radiation weighting factor is changed. In the present system, it is understood that R stands for radiations impinging to the body or component of radiation field where the body is exposed. In the proposed system, it is to be interpreted as the direct local radiations contributing to each average organ dose, DT,R. This change might be welcome since the two operations, modifying with wR and modifying with wT, taken in the process of transformation from absorbed dose to effective dose, can be changeable in order. It is a serious defect of the current system that the two operations used in the definition of the effective dose are not commutative.
However it is a different problem whether the quantity for controlling risk associated with exposure to radiation to be measurable or not. Local radiation field inside and in the vicinity of various organs is also difficult to explore quantitatively.
(9) Divisibility and Rounding-Up of Figures
Divisibility and rounding-up of figures are inevitable to construct a system of radiation protection and the system shown in the Draft is not exceptional. However, such all must be made in harmony, i.e. qualities of evaluations or judgments made in the practice are expected to be approximately same in required level of quality. With this sense, provided data in the Draft seem in problem, especially the way of giving numerical figures to wR. The treatment with neutrons is intuitively difficult to accept.
(10) Value Assignment to wT
The Commission set the sum total of tissue weighting factors as unity and allot numerical figures to organs within this framework. This methodology would be introduced by the idea of keeping the dose limit unchanged, but progress in academic activities requires reshuffling of figures repeatedly and endlessly yielding change in definition of the quantity.
Assignment of values to organs must be made in comparison of its sensitivity with that of the standard organ, a fixed specific organ or tissue, whose value of wT is given as unity.
(11) Collective Dose
It is a respectful notice of the Commission that considerations to the cut-off of the dose in evaluating collective dose are important. Apparently it is desirable to have a common value of the cut-off, but in reality, it is quite infeasible. Moreover, as the Commission pointed out in the former draft, collective dose aggregates excessively the necessary information for decision-making and has often been used in irrational ways. Therefore, we think the quantity to be repealed.
(12) Controlling Quantity for Internal Exposure
Effective dose is useful in discussing or studying risk of external and internal exposure in unified way, since it is the quantity introduced as a measure of risk associated with exposure to radiation,
In spite of this fact, however, it is not the best choice to use the effective dose as the controlling quantity for the internal exposure. Complexity in the process of converting the measuring quantity, i.e. activity, into the effective dose, brings a big difference in levels of quality of evaluation between external and internal. From view point of practice, it is preferable to use more directly measurable quantity as controlling quantities.
We believe it is much more preferable to use intensity of radiation, or dose-rate as second best for the moment, for control of external exposure and radioactivity or amount of radioactive substance, for control of internal exposure.
KATOH, Kazuaki [email@example.com] : Professor Emeritus of KEK (High Energy Accelerator Research Organization) and Professor Emeritus of IPU (Ibaraki Prefectural University of Health Sciences)
TATENO, Yukiomyukio.firstname.lastname@example.org: Professor Emeritus of NIRS (National Institute of Radiological Sciences)
TADA, JunIchiromjun1tada51f@yahoo.co.jp; email@example.com: Safety Officer of Spring-8
September 15, 2006