Assessment of the Residual Time to Rupture of Fuel Pins After Reactor Core Disturbances Using the Lebensanteil Rule

An important aspect of disturbances in the reactor core is the way in which they affect the service life of fuel rod cladding tubes. This factor also determines whether and how long the reactor core can be continued in operation, i.e. matters of safety and economy are involved. Potential disturbances of the reactor core affect the fuel rod cladding tubes as increases in temperature and, sometimes, as mechanical stresses for limited periods of time. As thermomechanical stresses acting on a cladding tube also give rise to creep events which may limit the service life of fuel elements, it is important to know how much creep life or time to rupture is consumed in the course of a core disturbance, and what the residual life is. For this purpose, the times to rupture before and during the accident must be added up and the balance calculated. As a rule of computation, the Lebensanteil rule is used in its form expressing the time to rupture of creeping solids. The simulation of accident with unirradiated cladding tubes revealed a drastic decrease of the residual time to rupture in those cases in which the cladding material (austenitic stainless steel) had recrystallized. Because of its structural stability, irradiated material turned out to be almost insensitive even under extremely severe accident conditions. The materials data so far available are sufficient for useful estimates. Presuming one of the damage accumulating processes of the creeping cladding material is predominant, there are no further validity limiting ranges concerning kind of accident, loading condition, cladding material and so on.