Pure and Simple

With the world facing an inevitable shift in the way it generates energy – nuclear power has once again come to the fore. Here we learn how water purification technology is playing a vital part in the enrichment of uranium  

URENCO'S Capenhurst site is in a valued sector of the fuel supply chain to the nuclear power industry. The continuous centrifuge enrichment plant takes in uranium hexafluoride which naturally contains about 0.7% U235. The hexafluoride, a solid at room temperature, is heated to form a vapour and passed through vertical centrifuges. The heavier isotopes – mostly U238 – are concentrated at the wall whilst the lighter U235 migrates to the centre. Repeated centrifuging in a cascade of units produces an enriched product containing about 3% U235. This is then cooled to produce a solid and then processed into fuel rods. Urenco leads the world in this kind of technology and supplies the nuclear industry not only with uranium enrichment services and advanced enrichment technologies but also with operational expertise.

Urenco is well aware of public attitudes to nuclear processing and continuously improves its environmental performance. The company is certificated to ISO140001 Environmental Management System and has received external recognition for its efforts in improving workplace safety and in this respect have been awarded a third consecutive RoSPA Gold Award. Part of the continuing improvement programme was the provision of new analytical laboratory facilities. The analytical techniques used in the laboratories are state-of-the-art and, working at the limits of detection means a need for ultrapure water. The water purification systems were the responsibility of ELGA Process Water.

“Each of the three new laboratories has a stand-alone water purifier to provide ultrapure water for a range of techniques including ICP-MS, ICP-OES and α and γ spectrometry”, says operations manager Katharine Hales. “As well as carrying out product-related work, we also monitor environmental discharges for toxic metals and so on. Accuracy is critical, so we use 18.2MΩ.cm resistivity ultrapure water for everything from making up standards to rinsing the glassware.”

This ultrapure water is produced by a two stage process. First, mains water is treated in an ELGA LabWater PURELAB Option S, which uses a combination of reverse osmosis and ion exchange to produce purified water with resistivity higher that 1MΩ.cm, which is significantly better than distilled water. This water is then polished by a Purelab Ultra Ionic to ultrapure standards.
 

“The analytical techniques used in the laboratories are state-of-the-art and, working at the limits of detection means a need for ultrapure water”

The Ultra Ionic is a sophisticated “polishing” unit combining a number of water purification technologies within a single bench-top cabinet. The process is shown schematically in Figure 1. First the pre-treated water is fed through a primary purification cartridge, which combines mixed bed ion exchange resins and an organic adsorbent to remove both ionic and organic impurities. It is then exposed to ultraviolet irradiation using a low pressure mercury lamp, which generates UV at a wavelength of 254nm to provide continuous bacterial control. The second purification cartridge removes the ions generated by photo-oxidation as well as any other trace residual ions, which remain after the primary cartridge. Resistivity monitoring between the two cartridges ensures that exhaustion of the primary cartridge is detected whilst most of the polishing capacity still remains in the secondary cartridge. This way there is no possibility of the treated water quality deteriorating. The water from the secondary cartridge is filtered to 0.05µm by a membrane ultrafilter and is then recirculated to the primary purification cartridge until required for use. An optional point of use filter is available to protect the outlet from bacterial contamination.

Maintaining water quality is vital to successful operation and this is confirmed by

Figure 1: Process schematic of purelab ultra ionic

resistivity monitoring after each purification stage and in-line real time measurement of total organic carbon (TOC). The treated water quality specification is set out in Table 1.

Table 1 – Ultra ionic treated water specification – Need redraw – can go anywhere on page.

Parameter Value Units
Resistivity 18.2 M.cm
TOC <3-10 ppb
Bacteria <0.1 CFU/ml*
Particle filtration 0.05 µm

(*with point of use filter, <1 CFU/ml without point of use filter)

An important feature of the unit is its dispensing system which automatically delivers a pre-set volume of ultrapure water rapidly, accurately and reproducibly straight into the flask. Rapid dispensing means that there are fewer queues, so common in busy laboratories. It also minimises the possibility of contaminating the water by collecting it in a beaker, measuring the required volume in a measuring cylinder and then pouring it into the flask; every transfer entrains airborne bacteria and contaminants. The dispense head is at the top of the unit to allow access for tall vessels, and is fitted with a 0.2µm filter as a final guard on water quality.

A microprocessor-controlled management system provides continuous monitoring of water purity, a warning when the fully traceable cartridges need to be changed and data collection capabilities via an RS232 interface in full compliance with GLP guidelines.

Another convenience is the ability to mount the units on the wall or bench. “Two of our units are wall mounted,” says Katharine, “whilst the other is on a bench-top. We had ELGA LabWater equipment in our old laboratories, so we knew it well, and it seemed the natural choice for our new facility.”