'OptiClean' - Optimised laser cleaning for safe nuclear decontamination and decommissioning

Shortly after lunch on the 11th March 2011, a 15 m high tsunami triggered by the magnitude 9.0 Great Tohoku earthquake engulfed the Fukushima Daiichi Nuclear Power Plant (FDNPP) - crippling the site, its essential surrounding infrastructure and the multiple safety layers the provided emergency reactor core cooling. Resulting from this absence of sustained core cooling provision, over the following days and in response to critical temperatures and pressures within the reactors, seawater was injected as a final resort to provide emergency core cooling. However, this desperate effort was insufficient, and temperatures rose in each of the reactor pressure vessels (RPVs) to in-excess of 2,000C; causing the uranium contained within the nuclear fuel assemblies to melt (partially or fully) and corrode vertically downwards through the RPV, into base of the primary containment vessel (PCV) - as well as coating the internal volume of the extensive PCV.

In the UK, the Sellafield site includes the first commercial power-generating station (Calder Hall, with four Magnox reactors) and the plutonium-producing Windscale Piles 1 and 2, that fuelled the UK's original nuclear weapons programme. The haste to assemble led to little planning for end-of-life retirement, waste management and post-operational decommissioning, meaning that such facilities still present high hazards. The hazard removal challenges at Sellafield are, however, not unique in the UK, with the large number of Magnox stations also embarking on "accelerated decommissioning" ahead of long-term "care and maintenance". Many of the most pressing and complex decommissioning challenges across the NDA estate concern the decontamination of radiologically contaminated surfaces, with numerous methods having been considered to address this, and laser cleaning emerging as the promising candidate.

Laser ablation (or "laser cleaning") is an emerging decommissioning tool for the international nuclear industry to rapidly decontaminate surface-fixed radioactive materials, having recently been identified as a "promising technique" for use across the UK NDA Estate. It is particularly attractive for nuclear decommissioning as it is not only non-contact, but also produces much smaller volumes of secondary solid and aqueous wastes than alternative physical and chemical methods. However, some fundamental challenges remain which prevent widespread implementation of the technique:

- The ablative nature of the technique can generate localised atmospheric contamination.

- Waste collection and disposal are complicated due to the airborne particulate nature of the ablated, radioactive material.

- Additional 'before' and 'after' characterisation surveys are necessary to plan the decontamination activities and assess the quality of laser cleaning.

This timely, cross-disciplinary, and impactful proposal addresses these and other challenges by developing novel particulate containment and collection strategies, integrated with innovative optical characterisation, for planning and assessing cleaning activities. In this way, it will reduce the burden, risks and overheads of laser cleaning, leading to its broader international utilisation. This would be particularly applicable at FDNPP, but also at Sellafield and other legacy nuclear sites in the UK.

We will use knowledge from the laboratory assessments to make a prototype fibre coupled, 'OptiClean' system for integration onto our LBR-SuperDroid, as developed for the NNUF programme. The platform consists of a SuperDroid HD2, a large tracked robotic platform, with a KUKA LBR IIWA 14 robotic manipulator mounted on the top. Its tracked nature allows for remote doorway-scale accesses with additional stair-climbing capabilities whilst the LBR is a seven degrees of-freedom robotic manipulator with force feedback sensing for human-safe interaction.