Ballooning in fuel clad: The first of its kind approach for its investigation in fast reactors

Ballooning in fuel clad and pressure tubes of light water reactors due to the Loss of Coolant Accident (LOCA) has been a topic of intense research for the past few decades. The possibility of the phenomenon in fast reactors has been ignored due to the preclusion of LOCA in the design relatively low operating pressure and better high temperature resistant structural materials. Albeit these merits, the high burn-up, high flux and therefore, high temperature in fast reactors poses an onus to the fuel clad. The key question is thus whether ballooning will take place in fast reactors and at what conditions. Experimental investigation of the behavior of the clad tubes at various operating conditions of pressure, temperature and heating rates suggest that ballooning takes place in fast reactor fuel clad at higher burn-up (which results in an increase in pressure and temperature of the clad tube) and lower heating rates. This paper gives an insight into the first of its kind approach for the investigation of ballooning in fast reactors using the Rupture And Ballooning In Tubes (RABITS) facility which is indigenously developed. It is observed from the experimental results that the rupture life of the clad at high temperature, i.e. 950 °C, is considerably lower than that predicted by the creep rupture equation available for the material. This reduction in life of the clad is attributed to the change in damage mechanisms at higher temperature (∼0.6 Tmelting point), which can be the damage due to the ballooning phenomenon.