ISO 21238:2007

Overview

ISO 21238:2007 - "Nuclear energy - Nuclear fuel technology - Scaling factor method to determine the radioactivity of low- and intermediate-level radioactive waste packages generated at nuclear power plants" - provides standardized guidelines for empirically deriving scaling factors used to estimate activities of difficult-to-measure nuclides in low- and intermediate-level radioactive waste (LILW). The standard focuses on waste streams from water-cooled reactors (and is also applicable to other reactor types, e.g. gas-cooled) and explicitly excludes purely theoretical (non‑empirical) scaling-factor derivations.

Key topics and technical requirements

• Principle: Use empirical correlations between easily measurable key nuclides (gamma emitters, e.g. 60Co, 137Cs) and difficult-to-measure alpha/beta or X-ray emitters to estimate activity in waste packages.
• Sampling: Emphasizes creation of a representative sample database. Two main sampling strategies:
  • Homogenized sampling for flowable or uniformly mixed wastes.
  • Accumulated sampling for heterogeneous or time-varying streams.
• Data management: Record sample ID, plant/reactor details, waste stream/type, sampling/analysis dates, measurement results (decay-corrected to waste generation date), detection limits and moisture content where relevant.
• Evaluation methods:
  • Linear relationship: scaling factor derived as the geometric mean of observed ratios (difficult-to-measure nuclide activity divided by key nuclide activity).
  • Nonlinear relationship: use linear regression on log-transformed activities (power-law model) when nuclide ratios depend on concentration.
• Statistical handling: Guidance on assessing applicability of correlations, outlier investigation/rejection, and combining datasets only when populations are statistically comparable.
• Key-nuclide selection: Criteria include measurability by non-destructive assay, activity above detection limits, correlation with target nuclides, and relatively long half-life; additional preference for similar production mechanisms or physical properties.

Applications and users

This standard is practical for:

• Nuclear waste characterization teams and waste management organizations estimating contents of LILW packages.
• Radiation protection and regulatory bodies assessing package acceptance and transport declarations.
• Plant operators, decommissioning teams, and analytical laboratories applying non-destructive assay and radiochemical analysis.
• Developers of site safety assessments who require reliable estimates of long‑lived radionuclide inventories.

Keywords: ISO 21238:2007, scaling factor, difficult-to-measure nuclides, low- and intermediate-level radioactive waste, nuclear power plants, sampling, key nuclide, non-destructive assay.

Related standards and guidance

ISO 21238 was prepared by ISO/TC 85 (Nuclear energy) and aligns with international waste-management practice (see IAEA Radioactive Waste Management Glossary and national regulatory criteria). Users should apply this empirical method alongside applicable national regulations and transport/disposal acceptance criteria.