Automatic Measurement of Low Level Contamiantion on Concrete Surfaces

Automatic measurement of radioactivity is necessary for considering cost effectiveness in final radiological survey of building structures in decommissioning nuclear facilities. The RAPID (radiation measuring pilot device for surface contamination) was developed to be applied to automatic measurement of low level contamination on concrete surfaces. The RAPID has a capability to measure contamination with detection limit of 0.14 Bq/cm for 60Co in 30 seconds of measurement time and its efficiency is eva luated to be 5 m2/h in a normal measurement option. It was confirmed that low level contamination on concrete surfaces could be surveyed by the RAPID efficiently compared with direct measurement by workers through its actual application. INTRODUCTION In the final stage of decommissioning nuclear facilities, building structures are demolished to be green field conditions if dismantling is chosen as a decommissioning strategy. However, since building structures are generally made from massive concrete materia ls in nuclear facilities, it is not a rational way to treat all concrete materials arising from its demolition as radioactive waste. It is therefore necessary to segregate only radioactive parts from building structures to treat remaining concrete materials as non-radioactive waste. In order to separate only radioactive parts from building structures, accurate identification of radioactivity parts by radiological characterization may be an important process. In addition, confirmation measurement of radioactivity (final radiation survey) must be conducted so as to confirm that there is no radioactive parts in the remaining concrete materials in order to treat the remaining concrete materials as non-radioactive waste. For example, in the JPDR (Japan Power Demonstration Reactor) decommissioning program the final radiation survey was conducted to show that there are no artific ial radioactive nuclides produced by operation in the decontaminated area (1, 2). Gross of beta and gamma-rays was measured with handy type detectors in this process. In addition, it was found in some case that non-contamination was not easy identified counting rate is small with large fluctuation of background level. Moreover, the sensitive window area (164 cm) of the detector used was small so that a lot of time was spent in the final radiation survey in the JPDR decommissioning program because the area to be measured was large. Based on the lessons learned in the JPDR final radiological survey, R&D started to develop were efficient radiation measurement techniques (3). In development of radiation detector, new concept was created to identify only beta-rays by separating beta and gamma-rays from contaminants. Automatic measurement of radioactivity is also applied to reduce time required for WM’02 Conference, February 24-28, 2002, Tucson, AZ 2 final radiation survey. Then, the RAPID (radiation measuring pilot device for surface contamination) was manufactured to realize the automatic measurement of low level contamination on concrete surfaces (4, 5). This paper describes development of automatic measurement technique for low level contamination on concrete surfaces. EXPERIENCE IN JPDR DECOMMISSIONING PROGRAM The concept of classifying non-radioactive materials, which was studied by the Nuclear Safety Commission, is only applicable way to allow conventional demolition of building. The classification of non-radioactive materials was actually conducted in the JPDR decommissioning program based on the concept. Figure 1 shows the basic flow for identifying non-radioactive materials in the JPDR decommissioning program. Several steps were applied to demolish the building with conventional technology. At the first step, facility operation history was surveyed to identify contamination areas. Next, the contamination on building surfaces was characterized in detail by sampling and measurement. Samples were taken in 2 by 2 m block base; contamination depth and nuclides were checked in the process. Contamination maps were then drew on the basis of the measurement. The building surfaces were decontaminated; the surface layer in 2 mm depth was removed even in non-contaminated areas, the contaminated layer was segregated with twice of measured Fig.1. Basic flow for identifying non-radioactive materials Confirmation by regulatory body Final radiation survey -direct measurement (surface) -sampling and measurement Removal of double layers of contamination Identification of contamination in depth Removal of surface (2mm) Removal of contaminated layers +5mm depth Measurement for characterizing -distribution in layers -nuclide (sampling) Measurement Clearly non-contamination Doubts of contamination Surface contamination Deep contamination less than detectable level * less than detectable level * Sampling and measurement (Surface layer) Measurement of radioactivity (all surface) Survey of operation history and component installation status Measurement Characterization of contamination by sampling or measurement (2m x 2m unit)