Laser sampling for characterisation

VIRIDISCOPE IS AN INNOVATIVE LASER sampling tool for materials characterisation in nuclear applications. It can be used in place of conventional drilling to remove material from a surface, to allow for fast sampling of materials such as concrete, plastic, wood, plaster and brick for a wide range of radionuclides and for elemental characterisation.

The scope was developed by UK-based Viridian Consultants, as a spin-off from a 2013 Innovate UK collaboration with Sellafield on a feasibility study.

The scope uses a laser to rapidly (around 120s) collect a 50-100mg sample from a surface under vacuum. The sample, without further preparation, is then transported away from the active area, along a 20m line to a collection pod. It is analysed for alpha, beta and gammas, or processed for elemental and radionuclide characterisation.

ViridiScope has been tested in active nuclear environments at five sites (the low-level waste repository at Drigg, the Socotec analyticial laboratory, the experimental Dragon reactor at Winfrith, a reactor undergoing decommissioning at Trawsfynydd and an operational nuclear power plant at Hinkley Point). The sites were selected to cover manual and remote sampling, alpha, beta and gamma activity, low levels and hotspots, on floors, walls and at heights.

Initial demonstrations

Prior to deployment on site, the performance characteristics of ViridiScope for concrete were evaluated in the laboratory. Concrete is a common material on nuclear sites but it was also envisaged that the tool could be used on a range of materials, potentially including steel and graphite.

Deployment for manual and remote operation was demonstrated in non-active areas at the Imperial College Consort Reactor (Berkshire), Sellafield (Cumbria) and Amec Foster Wheeler, and in an active field test at Sellafield.

ViridiScope was successfully demonstrated on a real decommissioning site in November 2016. It compared ViridiScope with a process of drilling, swabs and surface scraping. It permitted a preliminary assessment of radiation contamination issues, robustness and ease of use. During non-active and active demonstration, potential customers provided feedback to improve the design and functionality of the tool.

On-site trials

An important aim of the deployment project was to overcome resistance to change, a barrier that is endemic in the nuclear industry, and gain acceptance of this new approach. Field testing aimed to show companies that the tool was fit for purpose and could deliver in real-world projects with the performance needed for a wide range of application areas.

The team partnered with three site licence companies supported by the Nuclear Decommissioning Authority (LLW Repository Ltd, Magnox Ltd, Sellafield Ltd) and EDF Energy Nuclear Generation Ltd so it had access to five very different geographical sites.

“The pace of development was rapid, and the deployment on five nuclear sites in just nine months really tested the real-world operation and ruggedness of ViridiScope,” says John Williams, director at Viridian who was responsible for the design and build of the operating systems for ViridiScope.

The sampling and characterisation programmes tested functionality, speed of collection, robustness, contamination avoidance measures and reliability of ViridiScope in a real decommissioning site environment.

During three months of deployment, teams collected a total of 350 samples with a sample throughput rate of one every 10 minutes.

At Dragon, Winfrith, ViridiScope collected material at a height of 8m on an ROV, work which would previously have required scaffolding. Workers took 100 samples over 20m2 and demonstrated that the gamma activity for Am-241, Cs-137 and Co-60 over the area was below 0.2Bq/g. The process took under four hours. Hotspots were sampled and analysed to demonstrate that the activity was due solely to Cs-137 activity.

In less than two weeks of deployment at Drigg, over 100 samples were taken from walls and floor. Detection limits of 0.1Bq/g were achieved for alpha activity.

Working as subcontractors at Trawsfynydd, Viridian trained one of the contractor’s engineers to use ViridiScope to take samples in C2/C3 areas.

Finally, ViridiScope was used to sample a concrete core from Sellafield at Socotec’s analytical laboratories, a process that avoided the usual ‘sawing and grinding’ procedure for sample preparation.

Following the success of the deployment ViridiScope moved from technology readiness level (TRL) seven to eight.

The equipment worked well in various adverse conditions, such as temperatures of a few degrees Celsius and strong winds.

Much of the work on sites was to sample ‘clean’ areas so Viridian was driven to report low activities or detection limits by adapting the way samples are counted in situ to provide limits of detection acceptable to the sites. Modified sampling pods are now easier to open and acceptable to off-site analytical laboratories. Teams were also able to sample ‘hotspots’ and show there is no cross- contamination between samples.

Looking to the future

The deployments of ViridiScope on nuclear sites earned Viridian a prestigious award from the NDA.

Viridian director, Susan Parry, who project managed the work says: “We were really pleased when we won the NDA Minister’s Award for the work we carried out on nuclear sites. It increased our profile within the sector and we are in ongoing discussions with a number of SLCs and Tier 2s regarding forthcoming applications.”

The laser sampler will now be used at Trawsfynydd for the collection of material as part of an ongoing nuclear characterisation programme. “In addition to the sampling tool, the contract includes radiometric counting equipment for in situ measurement of radioactivity in the material collected,” notes Kym Jarvis, managing director at Viridian.

Looking to the future, Jarvis says Viridian has been working to develop a smaller sampling head with the capability to fire sideways for application in pipes, voids and other difficult to access areas. Supported through the Innovate UK REACH project, this involves working with partners Createc and UKAEA RACE, to develop a combined, robotically deployed sampling and gamma imaging sensor. “The system will be trialled in April, showcasing this unique capability. ViridiScope now reaches places previously unimagined,” Jarvis says.

Using ViridiScope
 

The scope takes samples using high energy laser light at 1064nm, with a pulse frequency of 10Hz. It is transmitted along an optical fibre to the sampling head, where it is focused onto the surface to be sampled. The interaction of the laser light with the sample surface causes small amounts of material to be ablated from the surface. Ablated material, as microparticulates, is immediately carried away from the sample surface, along a transport tube, to a sampling pod. Once collected in the sampling pod, the sample can be processed as appropriate.

The system control unit has been designed for operation in a decommissioning environment. The control system handles initial setup of the proximity sensors, operation of the vacuum system, sample line cleanout, laser firing and collection parameters. Safety is paramount and the laser cannot fire unless all of the interlock sequences have been performed.

The system readout shows the countdown time during sample collection. Sample collection times can be programmed from 30s to 5min in 30s increments.

The device uses a semi-disposable sampling head, through which an intense, high energy, focused beam of laser light is delivered to the sample surface. The sampling head can be located at some distance from the laser light source, control electronics and sampling pod. The device is currently fitted with a 20m sampling line and fibre optic, and analytical performance has been evaluated at this distance. The laser power supply, control electronics, vacuum pump, vacuum control and sample pod station require a single trolley.

Two operators are required. For manual deployment, one operator deploys the hand-held trigger on the sample surface while the second operator changes the sampling pods after each collection is complete. For remote deployment, all operations are performed from the control unit at a distance from the sampling face.

The sampling pods are weighed before sampling and immediately afterwards to determine the mass of sample collected, ready for in situ measurement.

The Ortec IDM-220-V, a large-area high resolution gamma detector with 50% relative efficiency, is used for in situ gamma scan. The detector is operated in a vertical configuration, and fitted with a steel collimator to reduce background. ViridiScope sampling pods are placed directly onto the end cap and counted for 10 minutes immediately after collection, to provide a detection limit of around 2Bq/g. When sampling in areas of very low activity, up to ten samples can be stacked and counted for 100 minutes to provide detection limits down to 0.4Bq/g.

The Mirion iSolo® or Ametek MPC-900-DP can be used for alpha/beta measurements.

The former is generally used for the analysis of air filters but is suitable for any other type of alpha/beta sample that fits into the drawer. It uses a solid state silicon detector for alpha and beta detection and can be operated for 10 hours or more with internal batteries.

The Ametek MPC-900-DP is a benchtop counter equipped with a dual phosphor scintillator-type detector that supports gasless operation and easy portability. This sample counter is designed for standard health physics applications such as smears and air filters and has been adapted for Viridian’s samples. The lid is removed from the sampling pod once it is placed in the drawer and the filter protected with a piece of Mylar film. The sample is counted for 10 minutes to give detection limits of 0.1Bq/g for alpha and 2Bq/g for beta. When counting is complete the sampling pod lid is replaced to give a tight seal for transportation if required.

(Images: Below left: Gamma counting, Below right: Alpha/beta measurements)