[en] This paper presents a simulation model for accident analysis in reactors conceptually similar to a Westinghouse 600 MWe PWR type (AP600) reactor. The model utilizes the IBM version of the RELAP5/MOD2/CYCLE 36 code

[en] Highlights: • We demonstrate the idea of runaway zirconium–steam reactions in severe accidents in today's LWRs. • We predict the thermal-hydraulics conditions relevant to cladding oxidation in an exposed fuel channel of a partially uncovered core. • The Semenov theory of metal combustion is extended to define a criterion for runaway oxidation reaction in fuel cladding. - Abstract: A theoretical model based on simultaneous solution of the heat and mass transfer equations is developed for predicting the rate of thermo-chemical reaction between zirconium cladding and a hot steam environment. Ignition conditions relevant to cladding oxidation in an exposed fuel channel of a partially uncovered core are predicted based on the theory of metal combustion. A range of decay power, convective heat transfer coefficients, and initial temperatures leading to uncontrolled runaway cladding oxidation is identified. The model could be readily integrated as part of a fuel channel analysis code for predicting possible outcomes of different accident mitigation procedures in light water nuclear reactors under LOCA conditions

[en] The one-dimensional transient solution of thermal-hydraulic behavior of a coolant channel having a sinusoidal heat source is parametrically considered. The transient is initiated by imposing a positive step change in the channel inlet flow. At a certain distance from the entrance the liquid starts to boil, forming a moving boiling-boundary. A two-phase mixture region prevails above the boiling-boundary in which the vapor content increases monotonically until the liquid boils completely and a single-phase vapor flow is initiated. The present work focuses on the study of the two phase-flow region transient behavior and the simultaneous propagation of its top and bottom boundaries. The transient position of the boiling-boundary is calculated using the analytical method developed by the present authors in Reference 1. The initial (steady-state) level of the vapor region and spatial distributions of density and velocity in the two-phase region are estimated using the semi analytica procedure described in Reference 2. Results are obtained for a flow channel that simulates a representative coolant channel in a partially uncovered nuclear reactor core during a typical bottom-reflooding phase of a LOCA scenario

[en] A hypothetical accident is analyzed, in which an external (out-of-plant) natural or man-made event causes a loss-of-coolant accident after penetrating the containment wall. The computer codes CONTEMPT and RELAP4 have been used to study the containment thermal-hydraulic behavior during the accident. Results are given in the form of graphs showing the thermal-hydraulic response of the containment and the profile of radioactive release to the atmosphere. The physical model and input data are discussed. 13 refs

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[en] In the present study, the propagation in space and time of the two-phase region's boundaries are established following a step change in the coolant inlet mass flow rate. In addition, tracking the conditions at the dry region allows coupling of the thermal solution with the formation of oxide layer resulting from the chemical interaction between the cladding surface and the surrounding steam. This, in turn, facilitates the calculation of the time at which runaway clad ignition reaction starts in the uncovered fuel metal cladding

[en] The recent events at the Fukushima plants have underscored the importance of secondary heat generation sources, such as thermo-chemical reactions, to the overall safety of the reactor. The theory of metal oxidation and combustion has been extensively developed in the last century as part of the technological effort to develop efficient rocket propellants and later in conjunction with nuclear reactor safety. Extensive review of the basic theoretical and engineering literature is given in [1, 2]. This paper aims at surveying some of the classical models available for predicting the threshold ignition temperature related to Zr oxidation and outlines the possibilities for integrating chemical oxidation models in a full scale fuel channel transient analysis

[en] Criteria for runaway conditions in water-cooled reactor cores during hypothetical accident conditions were previously derived for various scenarios to determine whether for a given set of conditions the situation could lead to a runaway reaction or measures could still be taken to alleviate the situation and to mitigate the results. The present work extends this analysis by quantitatively accounting for the effect of additional parameters