Nuclear Metrology in Support of Characterisation of Radioactive Waste

A number of industries generate by-products containing naturally occurring radioactive material (NORM). Such materials must be characterised prior to re-use, for example, in building materials or for sentencing as waste. Reference materials, reflecting the variety of matrices and principal radionuclides encountered, are required to validate radioanalytical methods for accurate measurement and to demonstrate compliance. Currently, the only certified reference material available is a phosphogypsum matrix, distributed by the IAEA, in connection with phosphoric acid production.

A blast furnace slag reference material has been developed for use in the steel and cement production industries. The results of an inter-laboratory comparison exercise, involving 23 laboratories across 14 countries are summarised, focussing on measurement of ⁴⁰K, ²²⁶Ra and ²³²Th activity concentrations. The work addresses the need for additional NORM reference materials and provides an infrastructure for future inter-comparison exercises. A second material from an oil pipeline has been characterised in preparation for the next exercise.

Underpinning the accurate measurement of reference materials, and all applications of nuclear science, is the accuracy and precision of nuclear decay data. In some cases, the data are outdated and/or significant uncertainties remain, which limits the measurement precision achievable. This project presents a methodology for atom counting of long-lived radionuclides using isotope dilution mass spectrometry combined with absolute decay counting methods for measurement of half-lives. The methodology has been applied to ²³⁸U, for which limited half-life measurements exist, most recently from 1971. The methodology developed will be applicable to other naturally occurring radionuclides that suffer from similar decay data limitations, including ²³²Th.