PSI - Issue 60

A. Syed et al. / Procedia Structural Integrity 60 (2024) 195–202 A. Syed/ StructuralIntegrity Procedia 00 (2023) 000 – 000

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1. Introduction In case of severe accident scenario like loss of coolant accident (LOCA) in PHWRs reactor, the coolant flow through the pressure tube (PT) gets impaired. This causes the reduction of heat removal from the fuel clad containing the uranium oxide pellets. This leads to the increase in temperature of fuel clad above the operating temperature. Also, due to LOCA conditions, there will be loss in pressure inside the pressure tube. The pressure inside the fuel clad increases due to the release of fission gas. This creates a differential pressure on the fuel clad surface causing the fuel clad to deform and balloon leading to the increase in diameter of the clad tubes. The deformation of fuel clad depends on the creep behavior of fuel clad under internal pressure. Fuel clad continues to balloon till it reaches a critical damage parameter after which it burst. It is important to evaluate this damage parameter as it influences the operator response to reactor safety measures during severe accident scenario. In order to study the deformation and burst behavior of LWR fuel clad, several studies have been carried out by Chung et al. (1977), Fiveland et al. (1977), Erbacher et al. (1979) and Chapman et al. (1979). Ballooning and rupture characteristics of fuel clad of Indian PHWRs have been carried out by Khan et al. (2013), Sawarn et al. (2014, 2017). In these works, an integral fuel pins have been used under different internal pressure (3-73.5 bars) and heating rates (7 - 12ºC/s). Burst temperature and pressure has been obtained for different cases of fuel clad burst. Using the circumferential strain at fracture, burst stress has been obtained. An empirical correlation for burst stress as a function of temperature has been developed. Since burst behavior of the fuel clad depends on the properties of the material under different operating conditions, it is important to develop a critical material damage parameter that can predict the burst of the fuel clad. This material parameter represents the material degradation with time. A number of mathematical models are available in literature for modelling the void growth and fracture like McClintock (1968) model, Rice and Tracey’s (1969) model and Gurson (1977) model. Large number of work have been carried out by implementing these models for modelling the damage development during ductile fracture of the material (Schiffmann et al. (2000), Jia and Kuwamura (2014), Shakoor et al. (2019) and Pardoen et al. (1998)). In this work, Rice and Tracey’s model has been used to evaluate the critical value of mat erial damage parameter that can predict the burst of the fuel clad using the stress field developed under different conditions of pressure and temperature. Finite element simulations has been carried out to evaluate the critical damage parameter that can predict the burst of the fuel clad under different operating conditions. Experiments carried out on Indian clad tubes (Sawarn et al. (2014, 2017)) are used for predicting the burst time of fuel clad using the damage parameters. An exponential curve has been obtained for evaluating the burst temperature and time at any instant of internal pressure using the developed critical damage parameters. Nomenclature E quivalent plastic strain increment Dc Critical damage parameter ̇ Creep strain rate (/s) P Internal Pressure σ von-Mises stress (MPa) R Actual mean void radius σ m /σ eq State of stress triaxiality R 0 Initial mean void radius a,b,c Material constants T Temperature (in K) A Constant creep parameter t Time (s) n Hardening creep parameter T b Burst Temperature (in °C) C Activation energy parameter 2. Brief description of experiments carried out on Indian clad tubes and data taken from literature Experiments (Sawarn et al. 2014, 2017) were carried out on 220 MW PHWR fuel clad tube made up of Zircaloy 4 material. Single fuel pin was heated using direct electrical heating system through copper bus bars and copper clamps holding the pins. Argon gas was supplied through a cylinder for internally pressurizing the clad tube. Whole assembly was enclosed in quartz tube for providing the oxidizing steam environment on the surface of the tubes. Pressure transmitters and thermocouples was placed to measure the internal pressure and temperature respectively