Effect of Hydrazine on Redistribution of Corrosion Product Deposits in the Primary Circuit of a Shutdown Reactor in Propulsion-Type Nuclear Power Plants

Abstract

A redistribution process of corrosion product (CP) deposits in the primary circuit of nuclear power plants (NPP) with propulsion service is examined. It is demonstrated that, after a shutdown of the NPP, the activity of activated corrosion products (ACP) and the concentration of stable corrosion products in the coolant increase considerably due to their entering the coolant from the deposits on surfaces of the primary circuit equipment. The corrosion products enter the coolant principally in the form of insoluble compounds, and the suspended solid particles above 0.4 μm in size account for a minimum of 90% of the total content of CPs in the coolant. This increases the CP concentration in the coolant by a factor of approximately 10, and the total activity of ACPs becomes more than 300 times the initial value determined during reactor operation. It was established that injection of hydrazine facilitated redistribution of the corrosion products and radionuclides associated with them between the surface of construction materials and the coolant: the corrosion products went into the coolant and the rate of their redeposition decreased by a factor of ten. This enables us to increase the fraction of highly active ACPs removed from the circuit by the normal cleaning system. In addition, a shift in the equilibrium on injection of hydrazine gives a higher activity of ACPs in the coolant after completion of the deposition process and removal of CPs that entered the circuit in the normal treatment system. The equilibrium activity of ACPs is approximately ten times the value specific for the installation that is cooled down without introduction of hydrazine. A decrease in this characteristic to stable low values is observed only after the reactor is put into operation.