Register for Updates | Search | Contacts | Site Map | Member Login

news

View Comment

Submitted by Steve Walker, Syberad Limited
   Commenting as an individual
Document 2005 ICRP Recommendation
 
The section on "Protection of the Environment", although interesting, leaves the reader totally uniformed and unable to conclude anything about the type of regulation that should be developed. We have here the proposition that discharges to the environment may 'critically' expose some biota even when the consequence to man at the end of the relevant food chain are acceptable. The thesis, that if we protect human populations we will have protected all affected biota is rightly challenged here by ICRP and a plea for more research is made but, given that it has taken over 70 years for ICRP to develop the current frame-work for protection of people, would it be justifiable to do the same for other biota in the absence of any evidence that the biosphere is being harmed by existing levels of global radioactive discharge? I think not.

The definition of 'source' in the draft recommendations worries me a little because it could be interpreted mischievously by radiation employers who have to deal with inhomogeneous sources, like solid waste. For it allows the exclusion levels of Table S2 to be met by mixing discrete quantities of radioactive material with non-radioactive material. I therefore suggest that the values in Table S2 should relate to homogenous materials only or be qualified in some way in a footnote to the table.

Radiation Weighted Dose - I don't understand the ICRP thinking on this issue at all. Certainly it is true that many have confused dose equivalent with equivalent dose (including myself initially) but abandoning their use, in favour of a new quantity which has the same definition as equivalent dose, will not help and will in fact cause more confusion. To understand this, it is necessary to refer back to ICRP 26 and 60 where the dose equivalent and equivalent dose quantities are respectively defined.

In ICRP 26, dose equivalent is defined as a point kernel quantity where the dose at a point in a tissue, in Gray, is corrected by the applicable quality factor (Q). Whereas in ICRP 60, equivalent dose is defined as a tissue or organ quantity where the dose, averaged over a tissue or organ, is corrected by the applicable radiation weighting factor (Wr - similar to the quality factor). They are in fact quite different quantities; the former being useful for micro-dosimetry assessments of, for example, localized contamination of the skin or wound contamination, where it isn't appropriate to average dose over a whole tissue or organ, and the latter being used more generally. ICRP's proposal to replace both these quantities with a new single quantity of radiation weighted dose would therefore appear to be counter-productive.

Moreover, the proposal to give this quantity a new unit would run counter to established conventions in metrology. As a matter of convention, quantities that have dimensional equivalence should take the same units and so, radiation absorbed dose, in gray, should become equivalent dose, dose equivalent or effective dose in 'weighted gray'. But in 1979 the General Conference of Weights and Measures (CGPM) established the derived quantity of dose equivalent which took the sievert as its unit. Therefore, if there is to be any discussion of new units, it is that for the derived quantity of effective dose which needs to be something other than the sievert. However, in my opinion, all derived dose quantities, based on dimensionless 'detriment' adjustment factors, should take the sievert as their unit because there is now precedent for this.

ICRP should note that, in other areas of metrology, it isn't customary to assign separate units to quantities that have dimensional equivalence but differ only in relation to their use of specific dimensionless scaling factors. Usually in these circumstances, it is differentiation enough to standardize the name and symbol for the derived quantity whilst leaving the assigned unit unchanged.