Laser ablation: becoming an option for nuclear decontamination

High power laser ablation systems designed for industrial decontamination applications are proving effective at nuclear sites around the world. Disaster sites Chernobyl and Fukushima have successfully evaluated this technology to improve worker safety and eliminate radiation efficiently.

Systems have come a long way since 2006, when the Electric Power Research Institute (EPRI) documented the successful testing of laser ablation to remove radioactive coatings, surface contaminants and oxides in power plant applications. Today, these industrial laser systems are commercially available, powerful and field-ready to support the global nuclear industry, in power generation as well as military and medical applications. They have features designed to optimise the benefits of a unique method that uses only focused light to quickly remove fixed low-level contamination from 80% of metallic nuclear waste that has only surface contamination.

Why process improvements are needed

Commonly used processes for removing low-level contamination include grit blasting, water blasting, CO2 pellet blasting, chemical cleaning and power tool grinding. While these methods can be effective, they typically produce a significant volume of mixed hazardous waste which is difficult and time consuming to collect, with high cost of disposal.

Operator safety is another significant concern with traditional methods. Concerns are excessive dose rates, repetitive stress, risk of serious injuries, excessive noise and the inhalation of hazardous airborne contaminants.

Abrasive grit and CO2 blasting are generally not suitable for use near other operations. During set-up, operation, and clean-up, on-going plant operations staff have limited access to work areas.

Abrasive blasting risks damage to adjacent surfaces and may remove excess base metal. Often costly custom tooling becomes unusable due to lack of safe, damage-free decontamination technologies. These abrasive media-based processes are time consuming and costly due to the large amounts of hazardous waste material that must be contained, collected and removed.

While traditional decontamination methods can be useful, commercially available laser-light-based processes have been tested and proven to be a cost-effective solution for a growing number of nuclear sites and industry service providers world-wide.

What is advanced laser ablation?

Laser ablation is a non-abrasive cleaning method that uses no consumable media, chemicals or gases. It may be used in close proximety to other activities, near sensitive controls and on or near operating equipment.

Typical systems consist of a portable laser source with a fibre optic beam delivery to a hand-held or robot-mounted laser end-effector. Ablated materials are collected at the target surface by a laser fume extractor, with multi-stage filtering, thereby preventing release.

The capabilites and benefits for laser ablation include:

Removal of contaminated coatings, oxides and rust and hydocarbons
Significantly minimising the volume of radwaste
Radwaste reduced to a less costly waste disposal classification
Dose risk reduction from potential contamination sources
Recovery of costly tooling
Recycling of valuable metals that would otherwise be costly radwaste
De-painting of critical weld seams for NDT

Case study: using lasers to decontaminate dump trucks

In Oak Ridge, Tennessee, laser ablation proved effective in reducing time required, worker stress and radwaste. The application involved the decontamination of carbon-steel from large dump truck beds used to haul contaminated materials from sites around the decommissioning of K-25 in Oak Ridge. Contamination comprised varying isotopes, primarily U-238. The contaminants were largely Beta emitters, found in conjunction with low Alpha contamination.

Testing was conducted by Adapt Laser and Philotechnics Ltd, which processes and transports radwaste. The latter provides radioactive and mixed waste management solutions, including decontamination and decommissioning services, radiological health physics staff augmentation, health and safety plan development, and radiological qualification training. Adapt and Philotechnics worked together to prove laser ablation as a viable source for removing radiological contamination with potential time savings.

The current decontamination method involved manual grinding of contaminated metal using large heavy hand-held power tools. This process required 200-man hours and two technicians to complete a single dump truck bed. The laser ablation process only required 7% of that time – one technician for four hours – to achieve similar results.

Other materials were also laser ablated. These were contaminated lead objects from various locations around the country, all with Alpha contamination (specifically radium). Lead sheets, lead bricks, lead “pigs” and even lead-lined gloves were decontaminated to 100% effectiveness in seconds with only one or two passes under the laser beam.

During these tests, continuous air found no detectable radiological airborne contaminants or the migration of contaminants to personnel or the laser equipment.

Pre- and post-testing of the contaminated surfaces revealed 100% efficiency in removing Alpha contamination and a significant reduction in beta particles. Areas with denser beta particles deeply imbedded into the base metal were not as easily removed by the laser, which does not damage the surface.

Ultimately, the laser’s greatest success came from its overall efficiency and a 93%-time savings compared to the manual grinding method. It is not surprising the technicians loved using the laser process compared to hours of stressful grinding.

Study: decontaminating equipment using laser ablation

In a recent study by Hi Tech Solutions and Reactor Services Inc radiological decontamination using laser ablation was carefully tested over several weeks. The applications consisted of an aluminium intermodal container lid, a painted and raw steel intermodal container and a steam turbine flexure housing section, all with fixed contamination.

The surface of each component was surveyed and marked-off to identify the contaminated areas to measure the contamination levels pre- and post-laser ablation.

The aluminium intermodal container lid had already been chemically decontaminated. Following that, and two days of power tool grinding, it retained fixed contamination that could not be removed.

After a sweep of the laser beam, the beta particle concentration was reduced by an average of 79%. Fixed alpha contamination up to 119 counts per minute was removed from each contaminated area in less than a minute, allowing to free release.

Contamination that could not be removed with conventional cleaning and grinding was removed from the lid by laser ablation in less than one hour.

Alpha and beta particle reduction was measured after each pass of the scanned laser beam. The alpha particle measurements taken from the painted and bare steel intermodal container were on average 91% lower following one sweep of the laser beam. In each case, a secondary pass of the beam eliminated nearly all detectable levels of contamination.

Decontamination using the laser ablation process

Adapt Laser has worked with partners including CleanLASER to develop its laser systems. The portable cleanDECONT CL1000 and CL2000 laser ablation systems use an Nd:YAG, high power laser source. During operation, the laser source is protected by placing it in a clean location outside the radiologically controlled area.

In the process, a hand-held, or robot mounted laser optic, is used to move an intense, focused and pulsed laser beam across the target surface. The end effector and fiber optic umbilical, up to 100m away, are wrapped to remain clean and avoid contamination.

The laser vaporises organic coatings, rust or oxides and hydrocarbons – which normally contain fixed, non-smearable, radiological contaminants. Vaporised residues are collected immediately after ablation by a strong point-source vacuum system that captures process residues. They pass through multi-stage filtering to be scrubbed free of particles and vapours, preventing hazardous airborne contaminants and minimising clean-up.

Laser surface preparation by mobile robot delivery is another option.

Recent system improvements and new functionality, includes:

Laser ablation integrated with Laser Induced Breakdown Spectroscopy, a process control to analyse the target surfaces before and immediately after laser cleaning to verify the absence of unwanted residues
Simple means for end users to decouple and exchange fiber optic cables
Nuclear-rated glovebox to safely decontaminate small parts
Gantry-based robotic work cells to treat large parts inside a Class 1 Laser Safety enclosure

Laser ablation decontamination will become more capable in future through automation, robotics and AI.