PSI - Issue 71

Tapan K. Sawarn et al. / Procedia Structural Integrity 71 (2025) 263–270

264

M

Maximum Bending Moment

µm mm min

Micro-meter

millimeter

minutes Newton

N

N-m Oxy

Newton-meter

Oxygen

PHWR

Pressurised Heavy Water Reactor

P max

Maximum Load in load-displacement curve

Pt-Rh RTT wppm Wt% s

Platinum-Rhodium Ring Tensile test

seconds

Weight parts per million Weight percentage

In pressurized heavy water reactor (PHWR) safety analysis, for a loss-of-coolant-accident (LOCA) scenario the fuel cladding is exposed to high temperature steam until the reactor core is reflooded by the emergency core cooling system (ECCS) water. The clad may not survive the thermal shock caused by quenching if they are embrittled significantly with severe oxidation and hydrogen absorption occurring prior to quenching. After the Fukushima-daiichi nuclear power plant accident, it became important to evaluate the post LOCA fracture resistance of the fuel cladding against various loads arising during post LOCA fuel handling and storage. Various studies have been reported on the post LOCA clad residual strength determination using various mechanical tests, like – ring compression, ring tensile and four point bend tests on the light water reactor (LWR) cladding (M. C. Billone 2012). Most of these tests were performed on un-ruptured cladding specimens (OECD, Report no. NEA/CSNI/R (2011)/7). However, fuel cladding can rupture under postulated LOCA conditions due to increasing temperature and rising internal fission gas pressure. Increasing temperature reduces the strength of the fuel cladding, so it may rupture due to increasing internal fission gas pressure. Once, the clad gets ruptured; it oxidises from the inner side also along with the outer side in a steam environment. This oxidation is accompanied by local hydrogen pickup, which can reduce cladding ductility (Uetsuka, et. al. 1981). Therefore, the use of ruptured cladding is preferable for the mechanical testing used to evaluate the integrity of fuel rods under LOCA and post-LOCA conditions (Yamato et. al., 2014). Four-point-bend-test (4PBT) is found to be the most suitable mechanical test for assessing the residual strength & ductility fuel cladding after experiencing the LOCA transient (M. C. Billone 2012). Very limited studies are reported on the assessment of residual strength and ductility of ballooned-burst and quenched clad (Billone, 2011; Flanagan, 2011; Yamato et. al., 2014; Yumura & Amaya, 2018; Okada & Amaya, 2020a; Okada & Amaya, 2020b). Four-point bend tests were conducted at Argonne National Laboratory (ANL) in order to evaluate the performance of the ruptured cladding (Billone, 2011 and Flanagan, 2011). Test results showed that bending strength decreased with increasing oxidation, irrespective of the extent of ballooning. In addition, it was also shown that the bending strength of the irradiated fuel cladding was lower than that of equally oxidized non-irradiated cladding. All these reported studies were performed on the light water (LWR) cladding, oxidised by steam up to a maximum temperature of 1200 °C. However, in case of a postulated LOCA accident in pressurised heavy water reactor (PHWR), cladding temperature may rise up-to 1600 °C or beyond under some critical break conditions (Sharma, et. al., 2015). So, it is necessary to evaluate the effect of high temperature oxidation on the embrittlement behaviour of cladding. The PHWR fuel cladding embrittlement criteria are based on the ring-tensile test (RTTs) results performed on the steam oxidised small sized ring specimens (Sawatzky, 1979). In order to assess the metallurgical condition of cladding at which embrittlement occurs, simulated LOCA integral tests were performed on the as received half-length clad tubes of Indian PHWR. Post-LOCA 4-PBTs were performed to assess the residual strength and ductility of the cladding. Residual strength and ductility were finally correlated with the evolved microstructure which governs the metallurgical state of the cladding. In the first series of experiments, clad tubes were oxidised up to maximum 1200°C and results obtained are discussed in this paper.