First phase of a study into the destructive sampling of radioactive waste material. Martin Jennings of De Beers Indusrial Diamond Review reports on how diamond core drilling has played its part.
Diamond Drilling For Nuclear Waste Quality Control
SPECIALISED diamond core drilling equipment could soon have a role to play in the safe disposal of intermediate level radioactive waste (ILW). Intermediate level wastes include irradiated fuel cladding, nuclear reactor components, chemical process residues, ion-exchange resins and filters.Before disposal the waste will be encapsulated in a suitable matrix, normally cement, and packaged in steel or concrete containers. Much of the ILW emits radiation at levels which require shielding to protect personnel In 1984 the Department of the Environment (DoE) in the UK published its guidelines on Quality Assurance in radioactive waste management:
Radioactive Waste Management, Information Note 2.Independent assessment
In Section 2.6 it was stated that, 'It is the intention of the authorising Departments that an assessment will be made of the product material independently of the waste processing organisations. In practice this appraisal will range from the scrutiny of plant records and documentation to a check on the quality of the product in a Waste Form Quality Checking Centre.' In layman's terms this means that ILW could be checked, prior to disposal, in an independent Waste Quality Checking facility.
With this in mind the DoE awarded a 3-year contract to Taylor Woodrow Management and Engineering Ltd (TAYMEL) to evaluate Quality Checking methods for packaged radioactive waste. Part of the TAYMEL programme was the development of destructive sampling techniques for cemented ILW. The test work was carried out at the TAYMEL Quality Checking Laboratory at the Atomic Energy Establishment (AEE) Winfrith, Dorset, UK.)
The objective of the destructive sampling work programme was to develop equipment to core and extract samples from cement-filled steel waste drums by techniques compatible with eventual remote-handling operations in a 'hot-cell'.
All coring tests carried out to date have been on simulant waste: 200 litre drums containing mixtures of Ordinary Portland Cement, Ground Granulated Blast Furnace Slag from concrete and other structures, and Pulverised Fuel Ash. No radioactive materials have yet been used for the coring trials.
Sampling rig
A sampling rig was designed and fabricated by Taywood Engineering Ltd; this allowed the waste drums and the coring equipment to be mounted in either vertical or horizontal positions.
Marcrist International Ltd of Doncaster supplied the coring equipment and the diamond coring bits.The coring equipment was hydraulically powered and as supplied, the machine had a drill rotation speed in the rang 400 rpm to 1000 rpm.
The coring machine was driven from a re-circulating hydraulic oil system in the power pack unit. The basic equipment was a Marcrist 24hp. hydraulic power pack delivering a maximum hydraulic flow of 46 LPM at a maximum pressure of 220 bar.
The drilling head was fitted with a Danfoss OMR hydraulic motor which could be operated at low speeds with a high torque output The core bit designed by Marcrist is shown in fig 1. It consisted of 16 diamond- impregnated segments brazed onto a 100mm diameter core barrel in a staggered arrangement. The segments are spaced to give an almost continuous cutting edge to minimise wedging of material between the gaps. To maintain the sharpness of the cutting edge, a sandwich type segment was employed, using a cobalt/bronze bond containing De Beers SDA100+ synthetic diamond abrasive. A 40/50 US mesh size in a 30 concentration was used for the outer faces and a 50/60 US mesh size in a 50 concentration for the inner.
Water containment
In established methods for taking cores from concrete and other structures, large volumes of water are used to cool the core bit and wash away sludge generated by the drilling operations. During the sampling of cemented ILW drums it would be necessary to contain all the water and sludge to minimise the spread of contamination in the shielded facility.
A water containment unit was developed by Marcrist to provide a water seal at the interface between the core drill and the top of the waste drum. The system is illustrated in Fig 2. The main design feature was that the unit was held on to the surface of the drum by a vacuum system. This eliminated the use of mechanical clamps which would be difficult to use under active handling conditions. The two 60 litre bins were each attached via two hoses to the water containment device and two vacuum pumps per bin provided the suction needed to evacuate the water containment device.
For the removal of core samples, a dedicated extraction tool was developed. This consisted of a simple internal wedge locking ring which tightened over the cut core as the complete tool was being removed. This unit comprised four screwed drill barrel sections, 100 mm in diameter and 1 m in total length. The first section contained the diamond impregnated segments, the two central sections each included an internal edge locking ring and the last section featured a solid back fitted with a female thread (Fig 3).
The sampling rig was used to demonstrate that conventional core drilling equipment can be modified for operation in a shielded facility. The equipment developed has confirmed that the water used to cool the core drill can be contained within a closed system.
Trials were carried out with the drum and coring equipment mounted in vertical and horizontal positions and, based on this work, the preferred orientation of the drum and equipment is the horizontal position.
Air-cooled Drilling
During the latter stages of the research, a programme of air-cooled horizontal drilling was undertaken. Compressed air was used instead of water and filter bags were fitted to the two 60 litre collection bins for dust collection. An air-flow of 400 litres per minute was adequate to cool the coring equipment The trials have confirmed that air-cooled tools can be used for drilling cores in cemented homogeneous wastes (liquids and sludges).
The substitution of air for water would ensure that the chemical composition of the core is not changed by the sampling process, for example leaching of radionuclides by the coolant water. Following the DoE's initiative in 1984, this research work was supported by the Commission of the European Communities (CEC) and similar studies are presently being undertaken in other European countries. Each country is conducting its own independent line of research and there is considerable transfer of information between them. At the Julich Nuclear Research Centre in West Germany, for example,
dry drilling tests have been successfully undertaken using drill bits mounted with polycrystalline diamond (PCD) segments.
At this stage it is impossible to predict exactly what system each of the European countries will adopt for the destructive sampling of ILW. This reflects the different waste forms encapsulation media and waste Quality Assurance regulations in force in each country. It is likely that there will be more than one answer to the problem.
One thing is certain, diamonds will continue to play a supporting role in the safe disposal of nuclear waste.
