An Out-of-Pile Method to Investigate Iodine-induced SCC of Irradiated Cladding

Abstract

Pellet Cladding Interaction (PCI) during power changes can be a limiting factor for nuclear reactors operation as it can cause fuel cladding failure by Iodine-induced Stress Corrosion Cracking (ISCC) [1]. In the as-fabricated condition, there is a gap between the fuel cladding and the pellets. During reactor operation when fast power transient changes occur, the fuel experiences power ramp and the pellets expand more than the cladding, such that the gap is reduced and eventually closed. When the fuel and the cladding are in direct contact, further fuel swelling and thermal expansion produces tensile stresses in the cladding. Moreover, pellet cracking occurs causing the stress to increase locally in the cladding at the location of the cracks. ISCC initiates at locations where both the stress and the iodine content locally reach critical levels. The cracking initiates at the inner surface of the cladding and propagates towards the outer surface [2, 3].ISCC can be investigated out of pile by means of an expanding mandrel test technique [4, 5]. In this design of the mandrel test equipment, the fuel pellet is simulated using a ceramic sheath with 4 longitudinal slits to facilitate the transport of iodine inside the cladding, which contains a Zr slug (mandrel) inserted in the center. The Zr slug is axially compressed and expands radially forcing the surrounding ceramic sheath to crack. The ceramic fragments can strain the cladding with further compression of the Zr slug.Different versions of the mandrel technique have previously been used to test unirradiated cladding. However, previous studies show that ISCC is sensitive to the condition of the cladding inner surface as well as the material strength, thus the irradiated cladding is expected to behave differently with respect to ISCC than unirradiated cladding.To evaluate the susceptibility to PCI failure of a given fuel rod, power ramp tests are performed in experimental reactors [2]. However, a good out of pile test to complement single ramp tests is crucial to improve the understanding of the mechanisms behind the PCI failures as well as the effect of different parameters.Within the SCIP program<sup>3<sup/>, in addition to ramp testing, a mandrel testing technique that enables out-of-pile testing of irradiated cladding has therefore been developed. The equipment is designed to investigate critical parameters that are known to affect PCI failures such as: irradiation, the strain rate, pellet performance (expansion and cracking), manufacturing (missing pellet surface) and cladding properties (texture, hardness, liner composition etc) [2].The tests are conducted under carefully controlled environmental conditions. Ar gas (99,99%) is passed through a temperature-controlled bath with iodine crystals (reagent grade 99,8%).The effect of the following parameters can be investigated:- Strain rate,- Iodine concentration,- Oxygen potential,- Temperature,- Different crack pattern of the pellet or missing chip,- Cladding properties.The mandrel equipment at Studsvik is specially designed and installed in a hot cell to enable testing on irradiated cladding. The mandrel test technique has been qualified within SCIP using unirradiated cladding. The effects of stress/strain rate, iodine concentration and oxygen potential, on the ISCC behavior were investigated by mandrel testing coldworked non-liner Zircaloy-2 cladding materials.After qualification of the test equipment, several tests have been performed on irradiated non-liner recrystallized Zircaloy-2 claddings. The tests were performed at a temperature of 320°C and iodine partial pressure in the range of 0 to 200 Pa. The tests were run by controlling the strain rate with an initial fast strain rate (21.6%/h) up to 0.6% strain from the initial diameter followed by a slow strain rate (0.36%/h) up to a strain of 2% and with a holding time of 20 hours or until failure. After the mandrel tests, the samples have been characterized by visual inspection, profilometry, LOM and SEM<sup>4<sup/>.The results of the irradiated recrystallized Zircaloy-2 claddings showed that in the presence of iodine partial pressures in the range of 60 to 200 Pa, ISCC occurred at 0.7-0.78% total (elastic + plastic) diametral strain. The low strain to failure as well as typical ISCC features like fluting, observed at the fractured surfaces, are consistent with previous hot cell examinations of PCI failures, and thus the new expanding mandrel technique seems promising.