The synergistic effects between radiation chemistry and carbon steel corrosion in the calandria vault and end shield cooling system of a CANDU® reactor

Abstract

Water radiolysis (radiation induced breakdown of water into hydrogen, oxygen and hydrogen peroxide) can lead to operational and safety issues in the water filled systems of a CANDU® reactor. The calandria vault and end shield cooling system is one such location where significant water radiolysis occurs. The purpose of this system is to act as a biological shield to protect against the high levels of radiation produced in the reactor core. The calandria vault surrounds the body of the reactor, while two end shields, filled with carbon steel balls, cover each reactor face. The vault is filled with light water that is continuously circulated through each end shield to act as a coolant for the carbon steel balls within. A nitrogen cover gas is circulated above the water level in the vault to allow for expansion during reactor start-up. When net water radiolysis occurs in this system, the hydrogen and oxygen produced can readily diffuse into the cover gas and, if not properly managed, may reach flammability limits. The calandria vault and end shield cooling system at the Point Lepreau Nuclear Generating Station had few issues with hydrogen production and migration into its cover gas until 1989 when purging of the cover gas began to be frequently required to keep the hydrogen concentration below the lower flammability limits. The root cause of the sudden excess production of hydrogen was traced to oxygenated make-up water, needed since plant start up in 1983 due to a small leak in the calandria vault. The use of hydrazine as an oxygen scavenger has been demonstrated to mitigate the problem by removing dissolved oxygen and returning the system to a net radiolytically suppressed state. Speculation on why it took seven years to reach net radiolysis conditions was focused on hydrogen production and oxidant consumption from the corrosion of the carbon steel end shield balls. Keeping excess hydrogen in an irradiated system is known to suppress net water radiolysis and limit production of hydrogen, oxygen and hydrogen peroxide. This study was implemented to better understand the synergies and interactions between carbon steel corrosion and water radiolysis. A test loop was built to simulate the conditions in the end shield cooling system, which contained a packed column of carbon steel ball bearings that was exposed to a gamma-radiation field in a Gammacell at the Chalk River Laboratories. Results demonstrated that oxygen consumption from corrosion processes can indeed place the system into net radiolytic suppression, subsequently, corrosion-generated hydrogen assists in maintaining this state. These corrosion processes minimize the production of radiolytically-generated hydrogen, oxygen and hydrogen peroxide. However, as the carbon steel passivates, the corrosion rate will slow, thereby diminishing the hydrogen production and oxygen consumption rates, after which net radiolysis can be re-established verifying the initial hypothesis. The study also explored methanol as an alternative oxygen scavenger to hydrazine and demonstrated that it could be effective in minimizing hydrogen production, but may increase carbonate loadings on ion-exchange resins in the system’s purification system.