Draft ISO standard
Methodologies for assessing the radioactivity of very low-level waste (VLLW) produced by nuclear facilities
Foreword
At the conclusion of the ISO TC85/SC5 subcommittee meeting held in June 2013 in Atlanta, Georgia, one of the resolutions issued by the WG5 subgroup and adopted by the subcommittee called for the establishment of a new ISO standard for assessing the radioactivity of VLLW produced in large quantities, as a supplement to the two existing ISO standards recalled below. This proposal for a new standard was mainly backed by France and Canada. The approach proposed by France consisted of sharing the radioactivity assessment methodologies currently applied for VLLW. The approach proposed by Canada also included certain types of low-level radioactive waste.
In the absence, to date, of exchanges with Canada, the following proposal is the French proposal.
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Recent ISO standards ISO 21238 (issued in 2007, confirmed in 2011) and ISO 16966 (2013) address the radioactivity assessment of waste produced by nuclear power plants.
-The first of these standards mainly concerns low- and intermediate-level operating waste contaminated with corrosion products activated in reactor coolant loops. It introduces a scaling factormethod based on key radionuclide (60Co, 137Cs), used to assess the activity of difficult-to-measure radionuclides.
-The second of these standards concerns intermediate-level activated waste (metallic components exposed to neutron flux in or near reactor cores). It provides guidelines for a basic theoretical methodology to assess the radiological impact of such waste by means of neutron activation calculations. These calculations are supplemented using the above-mentioned scaling factormethod and methods based on dose equivalent rate measurements.
These two complementary standards (certain components are both surface-contaminated and activated) cover a broad spectrum of radioactive waste generated by nuclear power plants during operation. Such waste is packaged and disposed of in surface, subsurface or deep repositories, in accordance with national radioactive waste management plans.
However, another category of radioactive waste is becoming internationally recognised: Very low-level waste. This is mainly due to the need to:
-sort such waste in order to not exceed the storage capacities of low- and/or intermediate-level waste disposal facilities,
-manage such waste separately so as to significantly decrease associated costs (very low-level waste has a specific activity generally below a few tens of becquerels per gram, i.e. low radiological impact). Waste packaging and design requirements for interim on-site storage and surface disposal facilities are therefore considerably relaxed.
It is therefore recommended to establish a specific ISO standard on radioactivity assessment methodologies for very low-level waste, taking into consideration the low activity levels to be reported and the significant volumes of waste to be generated (e.g. during NPP dismantling).Such a standard would be of particular interest for countries currently in the process of implementing separate management of very low-level waste (e.g. United Kingdom).Existing methodologies have been optimised and made more pragmatic (particularly by limiting the number of measurements) while maintaining the safety of interim storage and disposal facilities. In particular, these methodologies lead to a slight but controlled over-declaration of the activity of such waste. Furthermore, some of these methodologies also rely on the ratio method for difficult-to-measure radionuclides, thus making this new standard complementary and consistent with the two previous standards.
Note: These methodologies do not apply to waste exemptions. Controlled or uncontrolled clearancesinto the public domain, practiced by certain countries, obviously require more specific radiological characterisation.
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Content of proposed standard
Scope of application:
The proposed standard concerns the radioactivity assessment of very low-level (and low-level) waste generated by facilities during operation or dismantling.
The following considerations are excluded from the proposed standard:
-waste clearance/ exemption,
-cleanliness / waste zoning,
-waste subject to specific treatment (e.g. metallic waste melting),
-waste recycling in nuclear industries,
-chemical waste characterisation (e.g. hazardous waste, inert waste, etc.),
-waste packaging methods,
-specific management of radioactive waste storage capacities of interim on-site storage and final disposal facilities (e.g. IRAS index for French disposal site).
Structure of proposed standard
Definition of waste categories corresponding to the various activity assessment methodologies:
-Surface-contaminated metallic waste,
-Relatively homogeneous waste (i.e. finely divided waste): ion exchange resins, soils, sands, activated carbon effluents from iodine filter dismantling operations, etc.
-Heterogeneous non-metallic waste: filters, rubble, etc.
For each of these categories, the following is to be taken into consideration:
-Possibilities for sampling a given waste type,
-Stability and homogeneity of waste activity level or radionuclide composition (process-specific data).
For this purpose, as a supplement to the ratio method used for difficult-to-measure radionuclides, the proposed standard will introduce additional aspects and concepts not mentioned in the two previous standards:
-Concept of standard spectra for easy-to-measure beta/gamma ray emitters,
-Possibility of using these standard spectra and the physical characteristics of the waste (geometry, density, etc.) for assessing the activity based on dose equivalent rate measurements (easy to implement) and according to transfer functions (calculations), i.e. activity level dose rate,
-Concept of waste batches (produced and to be produced) for waste exhibiting highly stable activity levels and radionuclide compositions,
-Possibility of developing specific activity expressions based on surface activity assessment results in case of optimal arrangement (compactness) of surface-contaminated components.
The body of the standard will describe only the main methodological principles.It will be relatively succinct (see ISO 21238). The appendices will provide specific examples of radioactivity assessment. One of the appendices will address assessment uncertainties.