Both the fuel and cladding can swell . Cladding covers the fuel to form a fuel pin and can be deformed . It is normal to fill the gap between the fuel and the cladding with helium gas to permit better thermal contact between the fuel and the cladding . During use the amount of gas inside the fuel pin can increase because of the formation of noble gases ( krypton and xenon ) by the fission process . If a Loss-of-coolant accident ( LOCA ) ( e.g. Three Mile Island ) or a Reactivity Initiated Accident ( RIA ) ( e.g. Chernobyl or SL-1 ) occurs then the temperature of this gas can increase . As the fuel pin is sealed the pressure of the gas will increase ( PV = nRT ) and it is possible to deform and burst the cladding . It has been noticed that both corrosion and irradiation can alter the properties of the zirconium alloy commonly used as cladding , making it brittle . As a result , the experiments using unirradated zirconium alloy tubes can be misleading
SL-1
SL-1
Behavior of nuclear fuel during a reactor accident