ZPPR Fuel Element Thermal Stress-Strain Analysis

Abstract

The design temperature of high plutonium concentration ZPPR fuel plates is 600˚C. Cladding integrity of the 304 L stainless steel cladding is a significant concern with this fuel since even small holes can lead to substantial fuel degradation. Since the fuel has a higher coefficient of thermal expansion than the cladding, an investigation of the stress induced in the cladding due to the differential thermal expansion of fuel and cladding up to the design temperature was conducted. Small holes in the cladding envelope would be expected to lead to the fuel hydriding and oxidizing into a powder over a long period of time. This is the same type of chemical reaction chain that exists in the degradation of the high uranium concentration ZPPR fuel. Unfortunately, the uranium fuel was designed with vents which allowed this degradation to occur. The Pu cladding is sealed so only fuel with damaged cladding would be subject to this damage. The thermal stresses that can be developed in the fuel cladding have been calculated in this paper and compared to the ultimate tensile stress of the cladding. The conclusion is drawn that thermal stresses cannot induce holes in the cladding even for the highest storage temperatures predicted in calculations (292˚C). In fact, thermal stress cannot cause cladding failure as long as the fuel temperatures are below the design limit of 600˚C (1112˚F).

Passive assay of plutonium metal plates using a fast-neutron multiplicity counter

“…The plutonium metal core is encapsulated in a thin 304 L stainless steel cladding. Based on the fuel element geometry [19], we estimate core densities of 15.08 g/cm 3 and 15.09 g/cm 3 , for PAHN and PANN.…”