Oxidation Runaway Criterion during LOFA

Introduction: The concept of runaway metal-steam reactions was introduced in [1, 2] using the Semenov [3] analysis to derive a conjugate set of heat and mass balance equations for the evolution of temperature and oxide layer thickness on metal in an oxidizing steam environment. This study extends the classical metal combustion theory to account for the special characteristic conditions taking place in a partially exposed nuclear fuel rod during Loss of Flow Accident (LOFA) in nuclear cores. A dimensionless ignition criterion is developed, which accounts for the effects of decay heat release, steam flow rate and the time varying steam temperature along the oxidizing cladding.

Methods: We consider the upper section of an exposed fuel channel in which the cladding undergoes a thermochemical oxidation reaction with steam. The cladding is assumed heated by both nuclear decay power and thermochemical oxidation reaction while simultaneously cooled by steam entering the exposed section with a temperature, T_in, and mass flow rate, m_in. The heat and mass balance equations for the steam, metal and oxide layer at the exposed section were non-dimensionalized to define a runaway criterion, ω that could lead to uncontrolled oxidation during a severe accident.

Results: For a given fuel channel configuration and system pressure, a criterion is derived that basically defines the range of m_in and heat transfer coefficients, H, for which runaway oxidation prevails. The critical conditions were conveniently expressed as a 2-D parametric plot describing the critical m_in vs. critical H for a range of characteristic temperatures that account for decay heat, exposed channel dimensions and material properties.

Conclusions: Ignition criterion is derived that yields a set of m_in and H leading to runaway oxidation. Representative preliminary results for LWR-like fuel-channel during LOFA have demonstrated the utility of the criterion for predicting runaway situations over a wide range of operational parameters.

Acknowledgments: Work supported by the Israel Ministry of Energy.

References: Khaikin, B. I., Bloshenko, V. N., Merzhanov, A. G., On the ignition of metal particles. UDC 536.46-662.215.1, Translated from Fizika Goreniya i Vzryva, No. 4, October-December, 1970, pp. 474-488, also in Combustion, Explosion, and Shock Waves, Vol. 6 # 4.

1. Elias, E., Hasan, D., Nekhamkin, Y., Zirconium Ignition in Exposed Fuel-Channel, Nuclear Engineering and Design 286, 2015, 205–210.
2. Semenov, N. N., ZhRFKhO, Ch. Fiz., 1928, 60, 241.