Gamma spectrometry application for steam generators radiological characterization

Steam generators (SGs) are heat exchangers used to convert water into steam from heat produced in a nuclear reactor core. They are used in pressurized water reactors between the primary and secondary coolant loops. The preservation of the complete separation between the primary and secondary fluid is of capital importance in order to avoid radioactive contamination of secondary fluid and small loss of coolant also. Most French PWR suppliers use the vertical U-tube design with inverted tubes. The heat exchange section consists of a vertical, inverted U-tube bundle with the tube plate and the channel head. The steam drum portion consists of the internal moisture separating equipment and the enclosing pressure shell. In operation, primary coolant from the nuclear reactor vessel is circulated through the U-tubes. During this passage, the coolant gives off heat to the secondary water on the shell side of the steam generator, causing it to boil to steam. This steam, in turn, is passed through the moisture separating equipment in order to reduce the entrained moisture content and produce essentially dry steam. These U tubes have an important safety role because they constitute one of the barriers between the radioactive and non-radioactive sides of the power plant. For this reason, the integrity of the tubing is essential in minimizing the leakage of water between the two sides. The integrity of the primary system must be assured in any case. When power plants approach the end of life, decontamination and decommissioning of Steam Generators must be planned. It implies many issues: strategic, technological and scientific, measurement, environmental, legislation, and economic issues. The first step is the decontamination which can be performed by several techniques such as: washing, heating, chemical or electrochemical action, mechanical cleaning, or others... Prior to performing the decommissioning, the appropriate knowledge on the presence, kind and distribution of radioactivity inside the SG should be known. This stage is the radiological characterization step. The radiological characterization and the radioactivity inventory for decommissioning purpose is an evaluation of internal contamination and activation in order to plan the best procedure for decontamination. As part of the decommissioning of Chooz A, after the decontamination of the steam generators, it is planned to classify these equipments as waste of Very Low Activity in accordance with the ANDRA acceptance criteria. In this context, our objective about the calculation of IRAS's (Index Radiological Acceptance in Storage) is double: - Determination of IRAS overall value of the steam generators - Determination of IRAS value of the plugged tubes in the SG (little or no decontamination) Based on a feasibility study for measuring the activity in the SGs thru gamma spectrometry, we developed a specific design of the measuring system and implementation to characterize SGs decontaminated on site. In this paper we present our approach which has been to: - Achieve a faithful model consisting of three term source including specific calculation method - Perform precise measurements thru: - design of 'a ring structure' to perform several measurements simultaneously - definition of the detectors and the measuring electronics We defined the operational procedures of the non-destructive measurements and interpretation of these results has been incorporated into the application for acceptance of ANDRA. (authors)