Dose rate and contribution of the nuclides on the primary components during shutdown chemistry in Beznau NPP

The Nuclear Power Plant Beznau consists of two identical 380 MWe PWR units with two loops each, commissioned in 1969 and 1971. Westinghouse was responsible for the primary part of the plant and BBC, nowadays ABB for the secondary one. In the last 20 years the plant has undergone an extensive modernisation programme in which about 1.5 billion Swiss Francs have been invested. One of the main measures was the replacement of the steam generators with Inconel 690 tube material, realized at unit 1 in 1993 and at unit 2 in 1999. To keep the dose rate at permanently low levels, the minimisation of activity build-up on the surfaces of the primary components is always the main focus of the chemistry and radiation protection. A large number of measures to decrease the dose rates have been realized in the past, which are connected with permanent improvements of the primary coolant and shutdown chemistry. The shutdown chemistry, which has been introduced at the beginning of the eighties, was improved continuously during the nineties. A further optimisation was elaborated in the last years and has been proved to be a beneficial procedure. It is capable to remove activated corrosion products as well as iodine and noble gases during shutdown before opening the primary system. This process is supported by dosing hydrogen peroxide at 80°C. Since the replacements of the steam generators special attention has been paid to the behaviour and inventory of main nuclides and their contribution to dose rate. The contamination level and the deposited activities have been measured by in-situ gamma spectrometry at several locations, like steam generator and main circulation pipe surfaces. A crucial factor of the personal dose of the maintenance workers is the dose rate of the closure (crossover) leg during outages. Therefore, additional in-situ gamma spectrometric measurements have been performed during shutdown chemistry at the closure leg. The results of these measurements demonstrate an estimation of the behaviour of the nuclides on the surface and their contribution to the dose rate during shutdown procedure. The annual results of the in-situ gamma spectrometry and the dose rate measurements after shutdown are complemented by these measurements. (author)