A new cladding embrittlement criterion derived from ring compression tests

摘要

It is of regulatory interest to prevent the breaking of fuel rods in LOCA transients. In current regulations this is accomplished by limiting the oxidation during LOCA to such an extent that still some residual ductility is preserved in the fuel rod cladding. The current oxidation limit in German as well as in US regulations is set to 17% ECR (Equivalent Cladding Reacted) which aims at maintaining a residual ductility for oxidized claddings. Recent ANL tests have shown that the combination of both oxidation and additionally hydrogen up-take affects the transition to zero-ductility. Furthermore, the oxidation during LOCA transient is accompanied by a significant up-take of hydrogen (secondary hydriding) if the fuel rod bursts during this transient. This secondary hydriding affects the cladding in the vicinity of the burst opening. These findings necessitate a new criterion for preserving cladding's strength. This paper describes a method how to derive a criterion which assures the required residual mechanical strength of the cladding for LOCA transients. This method utilizes the experimental results of 102 ring compression tests (RCT) conducted at ANL and KIT. RCTs of various cladding materials, oxidation levels and hydrogen content were considered. The basic approach was to compare the RCT test data with finite element analyses using the code ADINA. Starting with the cladding oxidation model of Leistikov, both the layer structure of the cladding and the distribution of the oxygen among these layers were determined. The mechanical properties of these layers were taken from MATPRO/FRAPCON models and adapted if necessary. For each RCT the calculated load/displacement curve followed the measured curve up to that displacement at which the first leaping drop appears which indicates the first crack in the test. For this displacement the maximum equivalent stress in the computational mesh is considered as burst stress. The burst stresses were determined for cladding materials Zircaloy-4, Zircaloy-4 HBR, ZIRLO and M5 at different levels of oxidation, hydrogen content and test temperatures. The ratio burst stress to yield stress (R) shows a linear dependency on the levels of oxidation and hydrogen up-take. The stress ratio R = 1 describes the transition from ductile to brittle behavior dependent on the levels of oxidation and the hydrogen up-take. This ratio can be applied for a conservative safety assessment of fuel rod claddings undergoing LOCA transients with fuel rod burst including secondary hydriding.