Establishment of PCP Composition Diagrams for Cementations of Borate Wastes Using Response Surface Methodology

Abstract

For the purpose of quality assurance, it is requested by the regulatory authority in the solidification of low-level radioactive waste (LLRW) to implement the process control program (PCP), in which the condition of solidification should be decided in advance of solidification according to the so-called PCP composition diagrams, to assure that the condition of solidification is within established process parameters and that the quality of solidified LLRW meets quality criteria. In this paper, PCP composition diagrams for the cementation of radioactive liquid borate wastes were established with response surface methodology, including using the simplex centroid design to allocate experimental conditions and statistical curve-fitting techniques to construct quality models. The constructed models were verified to have a confidence level higher than 95% by doing the lack-of-fit test on them. Quality contours of solidified borate wastes were thus established based on mathematical and statistical principles and have been used as composition diagrams in the process control program of radioactive borate wastes solidification at Ma-An-Shan nuclear power station of Taiwan Power Company. The solidification agent used in this study was a mixture of Portland type-II cement and lime powder with a weight ratio of 1/0.22. Quality contours for solidified borate wastes including free-standing water content, compressive strength, water resistance, thawing and freezing resistance, irradiation resistance, and leaching resistance were established. Characteristics of these quality contours were also discussed. With these contours, the performance of the final waste form can be assured and consequently a volume reduction will also be achieved, when the PCP is implemented.