The UF6 produced by the enrichment process is subsequently converted into nuclear fuel. It is therefore important that the material supplied to the fuel manufacturer meets quality standards that allow the manufacturer to operate its process safely and successfully and to generate product that is suitable for onward use. Through the participation of industry experts, ASTM International has developed a series of standards relating to the nuclear fuel cycle, which have been widely adopted by the nuclear industry. There are standards relating to UF6 quality and others concerned with methods for sampling and analysis.

There are two main methods used to sample UF6:

1 As a gas during processing

2 As a liquid, from a batch

Gas sampling is relatively simple in that it involves tapping into the process line or container and diverting gaseous UF6 either to an on-line measurement instrument or to a sample point. The sample is withdrawn using low pressure (effectively sucking on the source) and may be collected by cooling a sample receipt container so that the UF6 freezes out. The operation of cooling also drives gas transfer and one common method for doing this is to immerse the container in liquid nitrogen. Sample containers can be of different designs and materials of construction depending on the analysis required. Stainless steel is perfectly acceptable for most analyses, including measurement of the proportion of uranium isotopes (the isotopic abundance). Nickel, monel (a high nickel alloy) or plastics such as polychlorotrifluoroethylene (PCTFE) may also be used.

Gas sampling works for isotopic abundance testing but it has limited value for checking the levels of contaminants in feed or product as they will not have the same volatility as the UF6 and may therefore be increased or decreased with respect to their true concentration during the sampling process. Proportional gas sampling is possible as a container is filled but requires considerable care in order to ensure that the sample is truly representative of the container contents. Measurement of contaminants in a batch of UF6 therefore typically requires liquid sampling to achieve homogeneity, which in turn requires that sampling be undertaken at pressures and temperatures above the critical point. The UF6 is sampled by pouring from a batch into a sample bottle. In a conversion plant the uranium hexafluoride is sampled at the end of the manufacturing process, where it is liquefied in large batches before being decanted into transport containers for delivery. In an enrichment plant the product container is heated to liquefy the contents in a purpose designed sampling autoclave and a sample decanted. In both cases the sample will be taken in industry standard sample containers known as 1S or 2S bottles. These are basically a monel or nickel bulb with a tube and valve arrangement fitted at one end. The 2S bottle is a little larger than the 1S bottle and may be used to take samples of up to 2.21 kg, while the 1S bottle is used for samples of up to 450 g.

The 1S or 2S bottle requires sub-sampling to provide samples that can be prepared for analysis. The bottle is again heated to liquefy the contents and to help ensure that the sub-sample is representative and a sample is tapped off into a smaller sample container, usually a P10 or P25 tube. These are again industry standard sample bottles made of PCTFE with a lid and seal arrangement. A P10 tube holds approximately 10 g of UF6 whilst a P25 tube holds around 25 g. Some sub-samples may be sealed at this point to be used for independent analysis or arbitration, whilst other samples will be immersed in water to produce a hydrolysed solution of uranyl fluoride and hydrofluoric acid. This solution is used for subsequent analysis.

The three main things that a customer is interested in is how much uranium they are getting, what enrichment it is and what contaminants are present that could interfere with the next stage of the fuel cycle. Measurement of uranium content is typically carried out using a redox titration while the proportion of 235U and 234U in the uranium is measured using a mass spectrometer. High precision is normally required for this measurement so that a sophisticated magnetic sector instrument is likely to be used. A number of different techniques may be used for measurement of contaminants depending on the equipment available within the laboratory and the contaminant being measured. Mass spectrometry can be used for many contaminants with a standard quadrupole unit likely to offer sufficient capability. ASTM International has published a number of standards for routine measurement methods.