Standards Development

Scope

This document describes a method for the determination of radionuclides ²³Np, ²³Pu, ²Pu, and ²¹Pu in soil by inductively coupled plasma mass spectrometry (ICP-MS) following chemical separation. The method is also applicable to the analysis of sediments, aerosols, and biological ash samples.

The method is applicable to any type of environmental monitoring or study, including both routine and emergency monitoring. It is suitable for the measurement of very low activity concentrations.

The mass of the test portion depends on the assumed activity concentration of the sample and the required detection limit, and can generally range from 0,1 g to 20 g of soil.

Purpose

Soils around nuclear facilities (such as fuel fabrication, fuel reprocessing, and waste management sites) may be contaminated by plutonium (e.g., ²³Pu, ²Pu, ²¹Pu) and ²³Np. Accurately determining the activities and ratios of these nuclides in soil is a crucial step in environmental radiation assessment, nuclear safeguards, and monitoring of nuclear facility decommissioning. In addition, ²³Np and plutonium isotopes, as characteristic nuclides of various nuclear reactors, are not only "tracers" for long-term pollution in the environment around nuclear facilities, but the ratios of these nuclides to isotopes such as ²³Pu/²Pu are also key data for nuclear activity source tracing (e.g., distinguishing between global nuclear test fallout and releases from nuclear facilities), and play an irreplaceable role in environmental tracing research and nuclear forensics (nuclear safeguards, nuclear accident source tracing).

ICP-MS has advantages over traditional radioactive analysis methods such as alpha spectrometry. It can accurately distinguish isotopes like ²³Pu and ²Pu, meeting the needs of nuclear forensic source tracing; and for long-lived nuclides such as Pu and ²³Np, it has a lower detection limit; moreover, it has a fast analysis speed and supports batch sampling, making it suitable for nuclear accident emergency monitoring and analysis scenarios involving large numbers of samples.

Although the ISO 18589 series of standards already covers the measurement of radioactivity in soil, there is no unified standard specification for the analysis of Pu and Np using ICP - MS. Among the existing standards, ISO 18589-4 specifies the alpha spectrometry method for the determination of Pu in soil, and ISO 20899 specifies the ICP-MS determination of Pu and Np in water samples. However, the soil matrix is extremely complex (containing a large amount of clay minerals, organic matter, and interfering elements such as rare earth elements, uranium, and thorium). Compared with simple liquid matrices, soil has higher difficulties in sampling representativeness, digestion, etc. during the sample preparation stage; in the separation and purification stage, the types of interference from rare earth elements, uranium, thorium, etc. in soil on target nuclides are more complex; in ICP-MS measurement, the signal suppression effect caused by residual soil matrix is more prominent, and it is completely different from the interference types and control requirements of alpha spectrometry.

In addition, without a unified ICP-MS analysis standard for Pu and Np in soil, different laboratories lack unified normative guidance such as sample pretreatment, separation and purification, and ICP - MS measurement, and can only carry out work relying on their respective experiences. This may lead to fluctuations in nuclide recovery rates for the same sample among different laboratories, inconsistent mass spectrometry analysis interferences, large control difficulties and interference correction deviations, and ultimately reduce the accuracy of detection results.

This standard will specify the general requirements for sample treatment when analyzing plutonium and ²³Np in soil using the ICP - MS method, the metrological traceability assurance measures during ICP - MS measurement, as well as quality control and quality assurance requirements. Through standardized constraints throughout the process, the impact of operational differences on results is reduced, and the repeatability and reliability of detection results are improved; it provides a direct technical basis for laboratories to pass quality system certifications such as ISO/IEC 17025, ensuring the quality of analytical testing in this field.