Standard Practice for Sampling Gaseous Uranium Hexafluoride using Alumina Pellets

Importancia y uso:

5.1 Facility operators and safeguards inspectors routinely take UF₆ samples from processing lines, isotopic enrichment cascades or storage cylinders to determine its uranium isotopic composition, most important the n(²³⁵U)/n(²³⁸U) isotope ratio, needed to calculate the amount of the fissile ²³⁵U in the sample. The current version of the “International Target Values for Measurement Uncertainties in Safeguarding Nuclear Materials” (3) contains recommended guidelines for these measurements.

5.2 The conventional sampling practice collects UF₆ samples in the range of 1 g to 10 g and requires the use of liquid nitrogen to condense them in sample vessels or metallic bottles. These samples must then be transported to external analytical laboratories for verification of the declared data, especially the isotope ratios. Transport includes, among other things, public roads and intercontinental air shipment. Due to the hazards of UF₆, air transport is becoming increasingly difficult, with many transport operators and regulators refusing to carry the material.

5.3 This sampling practice was developed to meet the following requirements:

5.3.1 Fit for Purpose: to enable the verification of the declarations of amounts of nuclear materials.

5.3.2 Simplicity: to ensure a simple and fast execution.

5.3.3 Flexibility: to be applied in a wide range of facilities.

5.3.4 Robustness: to ensure sufficient material is sampled even when operational parameters slightly change.

5.3.5 Reliability: to provide measurement results in agreement with those obtained using the conventional practice.

5.3.6 Confidentiality: to respect the facility’s operational procedure and confidentiality of data.

5.3.7 Safety: to reduce the risks associated with the sampling, handling and transport of radioactive and hazardous materials.

5.4 This sampling practice offers significant advantages over the conventional sampling practice because it allows handling non-reactive, non-volatile, solid UO₂F₂ instead of highly reactive and volatile UF₆.

5.5 A smaller UO₂F₂ sample can be transported with lower radioactivity level and reduced radiological problems in case of accident. Additionally, there is no risk of airborne uranium particle and HF release.

5.6 The uranium isotope ratios measured in UF₆ sampled by the conventional and this sampling practice provide measurement results which are in good agreement within the stated uncertainties (4, 5).

5.7 It is recommended to discard used sample vessels to avoid the possibility of isotopic cross contamination, mainly because this practice is associated with the processing of very small amounts of uranium.

5.8 In case recycled sample vessels are to be used, they must be decontaminated using a reliable procedure, to assure a removal of uranium adhering to the tube inner surface. A recommended procedure is presented in Appendix X2. The parties involved must agree on the use of recycled decontaminated sample vessels.

5.9 This practice provides guidance to obtain samples for determining the uranium isotopic composition for material nuclear safeguards as well as other applications.

5.10 The test methods describing procedures for subsampling, mass spectrometric, spectrochemical, nuclear, and radiochemical analysis of uranium hexafluoride are presented in Test Methods C761 and C1689. Most of them are routinely used to determine the compliance with Specifications C787, C996, and C1934.

Subcomité:

C26.02

Referida por:

C1913-21

Volúmen:

12.01

Número ICS:

27.120.30 (Fissile materials and nuclear fuel technology)

Palabras clave:

aluminum oxide pellets; enrichment; nuclear material safeguards; uranium hexafluoride sampling; uranium isotope composition; uranium isotope ratios;