Modelling zinc behavior in PWR plant

¹ National Nuclear Laboratory (UK)
² EPRI (USA)
³ Consultant (UK)
⁴ Vandellos II (Spain)

Abstract

Zinc is added to PWRs to lower excore dose rates and to help prevent PWSCC with almost 80 reactors world wide now adding Zn. Despite this wide spread use our understanding of what happens to Zn when added to a PWR plant is still limited and plant to plant variations in behaviour are difficult to understand. For example some plants observe increases in dose rates immediately following a cycle of Zn injection while other reactors do not. Likewise, some reactors see increases in shutdown nickel releases following a cycle with Zn, while others do not. This variation in behaviour is due to differences in material corrosion behaviour at individual plants, differences in core duty and differences in operational chemistry and it is important to understand how these individual factors effect the behaviour of the plant during Zn addition. A major question is whether adding Zn leads to increased fuel crud and whether such crud significantly hinders heat transfer from the fuel. This will be the case if Zn leads to additional releases of Ni and Fe, and Zn compounds precipitate out in the steam chimneys of crud preventing wick boiling. Several plants adding Zn have also seen significant chromium levels within their fuel crud, which poses the question of how Zn affects the release of this element from excore surfaces.

Some understanding of the mechanisms for Zn behaviour in a PWR plant can be obtained from analysis of specific plant data. This paper therefore applies several models to analyse Zn behaviour at a number of PWR plants to rationalise the plant observations in terms of the chemistry and physics that is taking place. The thermodynamics of Zn compounds is first discussed, including the ferrite and chromite species that form the inner and outer oxide phases, as well the thermodynamics of ZnO and Zn-silicate, potential precipitates in fuel crud. This thermodynamics is then used in a number of models for Zn capture in the inner and outer oxides on the excore surfaces, for capture of Zn in fuel crud due to concentrating via wick boiling and for the effects of enhanced Ni and Fe release due to Zn uptake. All these models of Zn capture are considered and applied to understand the behaviour at a number of PWR plants, the predictions of the models being compared with the plant observations. The approach that is adopted gives a complete picture of what is happening when Zn is added to a PWR, but although the approach answers several questions there are still several uncertainties and these are also discussed.