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Development of oxidation model for zirconium alloy cladding and application in the analysis of cladding behavior under loss of coolant accident [1]

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Date: 2022-04-01

During a hypothetical accident scenario such as Loss of Coolant Accident (LOCA) in Pressurized Water Reactor (PWR), the oxidation of zirconium alloy cladding is a very important phenomenon [1,2]. For a typical 1000-MW-electric-power PWR and an hour after the reactor shut-down, if cladding temperature rises to 1200℃, the ratio of the heat release rate of oxidation to the average decay heat power of the reactor might be larger than 0.5. Further, when cladding temperature rises to 1500℃, the ratio might be larger than 5, indicating a significant contribution of the oxidation of zirconium alloy cladding to the total heat source of reactor. Therefore, cladding oxidation will accelerate fuel assembly heating and promote core degradation, and the generated hydrogen becomes a key source term in explosion risk assessment.

The oxidation kinetics of zirconium alloy is usually described by parabolic rate correlation. Correlations proposed by different experiments [3], [4], [5], [6], [7], [8], [9], [10], [11], [12] are applied in accident analysis codes (e.g., MAAP [13], MELCOR [14], SCDAP/RELAP5 [15], MIDAC [16]) and fuel performance analysis codes (e.g., BISION [17], FRAPCON [18], FRAPTRAN [19]). However, parabolic rate correlations only provide oxide thickness obtained by the hypothesis of stoichiometric zirconia. The oxide behaviors under complicated conditions and the distribution of oxygen concentration which is very important for mechanical analysis cannot be obtained.

To cover the shortage of empirical correlations, oxygen diffusion equation with moving boundaries [20], [21], [22] was introduced for cladding oxidation simulation in previous studies. However, the cladding expansion due to oxygen uptake and oxide formation was not considered properly. The use of Pilling-Bedworth ratio [21] or the coordinate transformation based on plate structure and uniform mass density [20] cannot reflect the phase transformation process accurately. In addition, the neglection of velocity term after coordinate transformation is not suitable for the case of thick cladding and high extent of oxidation. Therefore, the zirconium conservation equation and the mass density varying with temperature and oxygen concentration are supplemented in this study for cladding oxidation simulation.

In this paper, mathematical model is firstly developed. Then, the improved model is validated by experimental data. Based on this model, the phase evolution of cladding oxidation during LOCA transient are analyzed through the simulation of QUENCH-05 [23] and QUENCH-SR [24] test.

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[1] Url: https://www.sciencedirect.com/science/article/pii/S0022311522000605

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