BATCH WEIGHING fuels the reactors

Delivering material batches to an accuracy of +/-0.03 weight per cent might be routine for some batch weighing systems, but when the system also has to meet the stringent operating standards of the nuclear industry it becomes much more than routine.

`Their standards are high and for good reason,' says Kim David, sales and marketing director for Rospen Industries, of one of his recent customers, British Nuclear Fuels (BNFL). `For Rospen to have met or exceeded all BNFL's expectations is a reflection of the quality of design and manufacture we are capable of producing.' Clearly, Rospen has a satisfied customer here. So what were BNFL's expectations?

Ingredients of success

As with anything else to do with nuclear fuel processing, first and foremost was safety - followed closely by accuracy and reliability of a system that would be at the heart of a new facility at BNFL's Magnox reactor fuel manufacturing plant at Springfields near Preston, Lancashire.

Rospen's brief was to supply a batch weighing, mixing and dosing system to handle a range of ingredients. The three key ingredients are uranium tetrafluoride (UF4), magnesium and aluminium, which are combined in powder form to make pellets used in the casting and purification of the Magnox reactor fuel rods.

UF4 is a dense (around 3900kg/m3 bulk density), sluggish powder that needs special handling to prevent clogging or inaccurate dosing. Rospen's solution is based around a 100mm full pitch metering screw feeder to convey the powder from a storage hopper and deliver it in a carefully controlled dose to a spiral blade mixer mounted on a weigh frame. Here the UF4 is combined with the magnesium - fed independently from its own storage hopper via a 150mm wide loss-in-weight vibratory tray feeder - and the aluminium which is added, via a smaller tray feeder, during the mixing process to aid dispersion. Typical batch charges to the mixer are up to 600kg, with an approximate UF4:Mg ratio of 6:1 and a trace amount (0.1 per cent) of Al. Charging time is two minutes with the UF4 feeder running at a rate of 18 000kg/h.

All the feed hoppers to the mixer are fitted with capacitance-type level probes to ensure that sufficient material is present before the mixing process starts. These probes are linked to a Rospen Boss 3000 PC-based batch controller that has overall command of the process, taking signals from the weigh platforms and comparing them with preprogrammed recipes.

The batch controller is critical to the process as it provides core data that enables BNFL to maintain its stringent quality checks. The system has to be capable of monitoring and recording the integrity of the proportional mixing as well as giving feedback on pellet weights to the plant's data management system.

When the 20-minute mixing cycle is complete, the controller switches a discharge valve to send the mixed ingredients, via a vibratory tray feeder and a small weigh hopper, to the pellet press. The PC controller gathers and stores data from every ingredient weight added to the mixer to every pellet weight charged from the mixer to the press. This information is vital to BNFL for verification and traceability.

According to BNFL, since the Rospen installation was commissioned, productivity has been increased to such an extent that it has surpassed all of the company's initial objectives in terms of performance, reliability and accuracy. Pellets are now routinely within +/-0.03 per cent of optimum target weight as the press discharges pellets every 20 seconds.

To meet BNFL's stringent health and safety requirements on dust suppression, Rospen included high integrity bin seals and fully enclosed screw and tray feeders throughout the system. Another point of satisfaction for BNFL was that Rospen managed to design and install the system into the limited space previously occupied by an outdated and far less sophisticated plant.h