Clearance of BWR steam piping by off line chemical decontamination

Abstract

This paper deals with laboratory tests that analyze the acid attack of metallic samples, contaminated by Co60 in the Caorso nuclear power plant in Italy. The main aim was to establish the working parameters of the decontamination plant for metallic components.The study took into consideration the steam piping, located in the turbine building, that is, piping from the main header to the high pressure turbine stage, as well as other steam piping, connecting different turbine stages or that had other functions. The Co60 is produced in the reactor vessel by neutron capture in the iron nuclei of the materials located in the pressure vessel. The coolant erodes the steel surfaces and deposits these products along the piping. In the first phase of the activity the chemical decontamination process was simulated in the laboratory, in particular the acid attack and the subsequent high pressure water washing. For the various parts of the piping (straight lines, bends, intersections) smear tests enabled the radioactivity distribution to be determined. Metallographic analyses of the samples, core bored by the piping, determined the composition of the deposit (crud) on the internal surface of the components and the radioactivity along the thickness of the crud, and consequently the time of the acid attack in order to obtain the Clearance.Numerical simulations of the Co60 deposition by means of CFD codes are currently being carried out in order to compare the results to those obtained experimentally. This will enable us to classify the systems from a radiological point of view by estimating ‘a priori’ the time required for decontamination.