Getting a handle on hazardous materials

Tom Shelley reports on some of the techniques a small company uses to ensure the safe handling of some very hazardous materials



Using machines that handle radioactive and chemically hazardous materials clearly carries risks for those who have to operate them.
However, Gravatom, which designs and builds these, has developed its own methodology and techniques to keep people safe. The aim is to ensure that things cannot be dropped, cables snagged or dangerous fumes released. And, where operators do have to be exposed, they can undertake maintenance operations in very short timescales, without risk of things going wrong.
Designs are prototyped in software and then as full-scale mockups, whether they be equipment used in dismantling – or helping to dismantle – nuclear power stations or, increasingly, to handle radioactive materials used in various kinds of medical diagnosis and therapy.
Anyone concerned with handling hazardous materials could certainly benefit from studying their methods.
Based in Bishops Waltham, near Winchester, Gravatom has been making handling systems for the nuclear industry for more than 30 years. Its approach is illustrated by an interlocked nuclear fuel rod grabber with four fingers – which is inserted into narrow apertures in the advanced gas-cooled reactor at Dungeness B and used to extract the graphite containers carrying the fuel elements.
Design and technical manager Matthew Unwin uses a three-dimensional action – intriguingly referred to as ‘Lazy tong’ – which swings the fingers in to grasp the fuel containers in what he describes as a Pantogragh type motion. This involves the use of a pneumatic actuator, plus an additional sleeve, which slides down to lock the fingers, so they cannot be released, should the air pressure fail.
Managing director Dave Barker has been describing some of the challenges that had to be overcome in the design of a remote filter-handling module installed at Sizewell B. This uses a carousel of filters such that clean filters can be sequentially swapped with charged filters. The customer also wanted to be able to view the sealing faces of seal housings, in each case. Conventional solid-state cameras will not survive the radioactivity so, says Barker: “We had to use a good old fashioned periscope.”
The filters and shield blocks were quite heavy, so they had to be lifted by a hoist. This is equipped with load cells, so the operator can tell from the indicated weight whether it is lifting a fresh filter, wet, loaded filter or shield block. But what is really clever is how any possibility of the lift cable snagging or the load being brought to one side is avoided. This involves the winding reel moving from side to side, so that lifting cable is always on the centre line, rather than threading it through a guide.
Barker is particularly proud of a complete underwater machine tool, which the company made for a French nuclear client. It was built to cut up a large radioactive pressure vessel in a cooling pond. This used slitting wheels and actuators, all driven by water hydraulics, so that any leaks would only mingle with the water in the cooling pond instead of contaminating it with oil. It is always something of a challenge to sell services to a French government agency against French competitors – but the machine was so successful that it completed the work in five months, instead of the 14 months allocated.
According to Barker there has been rapid growth in the nuclear medicine side of the business of late. This means that, as well as functioning reliably, equipment has to be very well finished in order to keep any bugs out.
Instruments such as spectrometers have to be re-engineered and contained, if they are to be used for nuclear work. Gravatom’s Matthew Unwin showed Eureka the designs for a locking mechanism that allows a skid of parts – which has to be located inside a radioactive environment – to be swapped particularly quickly. In the conventional version of the instrument, it is fixed in place by bolts. However, apart from the risk of dropping them, as Unwin points out: “It is undesirable to have to handle M4 screws through leaded gloves.”
Instead, the whole assembly attaches through a bayonet fitting, with locating retention pins to ensure correct positioning. To lock it in place requires only the rotation of a captive ‘C’ spanner, which engages tapered slots under the heads of bolts and compresses an elastomeric seal to eliminate movement. Repeatability of positioning is to a few hundredths of a millimetre, he adds. The skid-swapping process now need only take minutes, rather than the previous four hours or so, as well as eliminating the possibility of dropped fasteners.
This particular piece of equipment has been designed in 3D in Pro/Engineer. The company use both 2D AutoCAD and four seats of Pro/E, because of different customer requirements. “The benefits of 3D are about the risks they eliminate,” explains Unwin. “In 2D, [at review meetings] everybody nods agreement without always knowing what is really going on.”
This is particularly relevant to the design of medical products, he says: “They are often medical professionals in charge, who don’t understand engineering drawings, so they need to see it in 3D.”
But the company does not rely solely on 3D virtual prototyping to get things right. After the design stage, it normally makes cardboard and plywood mock-ups for ergonomic and ventilation trials. According to Unwin, because of the nature of the hazards the company deals with, it could not rely on CFD to ensure avoidance of contamination.
“Most of the CFD we have looked at comes out at the wrong kind of scale [too small], so we have to prove it as well.”
Hence the mock-ups have to be full scale and tested with what Barker describes as “smoke and not quite mirrors”. Similarly, load-carrying designs are FEA analysed using Algor. However, and particularly in the case of the nuclear industry, these still have to be physically load tested. But perhaps most important of all is the integrity of the designers and Unwin puts great store by that.
“Most of our designers have done an apprenticeship on the shop floor, so they have a good appreciation of manufacturability,” he says.

Pointers

* Products are made primarily for the nuclear power industry, including decommissioning, but increasingly for use with radioactive materials for medical diagnostics and therapy

* Gravatom has developed a number of methodologies for dealing with high-risk materials handling

* While it uses 3D CAD, it does not rely solely on it, but also builds full-scale mock-ups of products for ergonomic and ventilation studies

Author
Tom Shelley

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