Is Ductile Cast Iron Ductile at -40C ?

Casks for transportation and storage of spent nuclear fuels, fabricated from ductile cast iron (DCI) have now attracted strong interest in Japan, with considerable effort expended toward their application. Such casks must maintain their structural integrity even when subjected to hypothetical accidents during transport to or handling at storage facilities. The International Atomic Energy Agency (IAEA) standards (IAEA 1985) require that type B casks, for instance, withstand a nine-meter free drop onto an unyielding target at a lowest service temperature of -4o·c. and they retain sufficient shielding efficiency. Such loading conditions envelope severe transportation collision accidents. Heavy-section DCl casks, with wall thicknesses of the order of 400mm, would be suspected to undergo a ductile-to-brittle transition that is a function of temperature and loading rate. Mechanical properties and fracture toughness at low service temperatures, under impact loading conditions, therefore, must be estimated accurately to assure the ductile behavior. Torsion tests and fracture toughness (initiation and arrest) tests were carried out on samples extracted from a prototypic cask and two model casks fabricated from ductile cast iron, to confirm that ductile cast iron has sufficient ductility and fracture toughness even at low temperatures and under tri-axial stress state conditions. For the assessment of the structural integrity of cask, a linear elastic fracture mechanics approach is appropriate. The integrity of the cask should be assured so that the brittle failure never takes place even if a crack-like flaw of fairly large size associated with detection sensitivity of non-destructive tests is postulated in the cask. Examples of safety analysis on the cask were also carried out based on a linear elastic fracture mechanics approach.