Options for the Removal of Contaminated Concrete From the Bore of the Windscale Pile Chimney

A legacy of the 1957 Windscale Pile reactor fire is the penetration of radioactive contamination into the internal surface concrete of the chimney bore. Gamma imaging of Cs-137 has shown that the contamination is widespread throughout the chimney, and core samples have shown that the contamination has penetrated to depths of around 5–25mm. The Pile chimney is 100m tall and has an internal bore diameter of 15m. It is constructed of a hard concrete comprised of Whinstone aggregate. The baseline decommissioning scheme is to remove approximately 5–25mm of the surface concrete from the entire bore of the chimney. The technology baseline in 2006 was to remove layers of contaminated concrete by mechanical means using shavers or scabblers. However, risks associated with mechanical technologies that may preclude their use include: the ability of mechanical devices to remove the hard concrete; clogging of the devices due to wet concrete; and deployment of the delivery systems. This paper discusses the options under consideration to reduce the risks associated with the removal of the contaminated concrete through application of alternative techniques. The present baseline technology is high pressure water jetting technique. Demonstrations have shown that this technology can successfully remove concrete without significant reaction forces. However, an inherent problem with this technology is the production of secondary liquid effluent waste, which would need to be treated by an appropriate conditioning process. To address the secondary effluent waste issue, technologies that produce little or no secondary waste have been considered. The technologies that have been considered are laser scabbling, microwave scabbling and nitrogen jet blasting. The paper discusses each technique in turn, highlighting their advantages and disadvantages. The results of an in-active laser scabbling and high pressure liquid nitrogen jetting trial are presented. The paper concludes with a discussion of the merits of each technology in support of the future strategy for concrete removal.