Mechanistic modeling of water radiolysis in supercritical water-cooled small modular reactors.

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Together with hydropower, nuclear is the only low-carbon source of energy that can replace fossil fuels. The use of supercritical water (SCW) as the coolant in a nuclear reactor is the logical evolution of the current generation of subcritical water-cooled reactors (WCRs). The radiation-induced chemistry of water under the operating conditions of both current WCRs (~250-330 oC, 10 MPa) and small modular SCW-cooled reactor (SCW-SMR) concepts now under development (~280-500 oC, 25 MPa) remains poorly understood. One of the most significant water chemistry challenges for all SCW-SMR concepts is to predict the effects of water radiolysis on materials performance and develop a chemistry control strategy to mitigate these effects. This is because chemical species formed during radiolysis are highly reactive with most metal alloys at the elevated temperatures proposed for SCW reactors, increasing the rates of corrosion and stress corrosion cracking (SCC) and leading to fuel cladding failure. Direct measurements of the water chemistry are difficult (if not impossible) to perform due to the fact that this water is subjected to an intense mixed radiation field as it passes through the reactor core. As a result, theoretical modeling and computer simulations provide an important route of investigation to help elucidate mechanisms by which radiation interacts with SCW and the consequences for materials. Of a theoretical nature, our long-term goal is to develop a computer-based SCW-SMR radiolysis model using coupled molecular dynamics and Monte Carlo multi-track chemistry simulations, based on a detailed knowledge of radiolytic yields and their dependence on temperature, density, SCW structure and other parameters such as LET and pH of the solution. Our research activities are expected to generate exploitable results that will provide new insights into water chemistry in SCW-SMRs and thus provide benefits to Canada by supporting the program objectives. In addition, they will provide recommendations to nuclear regulatory agencies on key chemistry parameters that must be monitored to ensure that an SCW-SMR is operated safely.