Left to right: Mohamed Bourham, Leonardi Tjayadi, Jacob Eapen, K.L Murty, and Ronald Scattergood.

Leonardi Tjayadi successfully defends dissertation

On December 4, Leonardi Tjayadi successfully defended his PhD dissertation, Innovative Approach to SCC Inspection and Evaluation of Canister in Dry Storage. Leonardi’s committee consisted of his advisor, K.L Murty, and members, Mohamed Bourham, Jacob Eapen, and Ronald Scattergood.

 

Abstract

TJAYADI, LEONARDI. Innovative Approach to SCC Inspection and Evaluation of Canister in Dry Storage (Under the direction of Dr. K. L. Murty).

The spent nuclear fuels (SNFs) in the U.S. after removal from nuclear reactors and submersion in spent fuel pools are transported to Independent Spent Fuel Storage Installations (ISFSI) for storage in dry storage canisters usually made of austenitic stainless steels (SS) type 304, 304L, 316 or 316L for several decades. The dry storage canisters are then enclosed in a dry storage cask (DSC) equipped with a ventilation for natural cooling. Since the locations of the ISFSIs in the U.S. are generally near the ocean, the dry storage canisters can be exposed to ambient site conditions through the ventilation. The high level of chloride salts found in the ISFSI location can be deposited on the dry storage canister through the ventilation on the DSC leading to possible chloride-induced stress corrosion cracking (CISCC). Electric Power Research Institute (EPRI) conducted a study regarding the materials degradation mode in ISFSI and listed CISCC as the number one potential degradation mode for dry storage canisters. With the uncertainty of the projects on permanent storage facilities for the SNFs, dry storage facilities for the SNFs are in high demand and need to be given top priority. As the SNFs are stored in dry storage canisters for decades until a permanent storage facility is ready, the safety and long-term integrity of dry storage canisters are important. Understanding the SCC mechanism is key to the long-term integrity of the dry storage canisters and the mechanism can be investigated by performing crack growth experiments at different environmental conditions with fracture mechanics approach using a wedge opening loading (WOL) specimen and direct-current potential drop (DCPD) technique.

Prior to the crack growth experiments, calibration of the DCPD with crack length was established using a fatigue pre-cracked WOL specimen in air as well as in marine environments. This calibration chart was used later in the data analysis of the crack growth experiments.

Crack growth experiments were conducted using sensitized SS304H and sensitized SS304L at 22, 37 and 60 °C with substitute ocean water producing 0.975×10-10 ± 9.528×10-12, 3.258×10-10 ± 9.551×10-11 and 1.580×10-9 ± 2.593×10-10 m/s for sensitized SS304H and 3.064×10-11 ± 4.009×10-12, 1.945×10-10 ± 1.315×10-11 and 8.830×10-10 ± 9.863×10-11 m/s for sensitized SS304L. The obtained average crack growth rates as a function of temperature yielded activation energies of 60.9 ± 0.62 and 69.1 ± 8.96 kJ/mol for sensitized SS304H and sensitized SS304L, respectively which are associated with hydrogen diffusion in iron and steel. An experiment conducted with sensitized SS304L at 3x higher concentration produced an average crack growth rate of 1.985×10-10 ± 2.603×10-11 m/s which is similar to that of sensitized SS304L at a regular concentration suggesting that increasing salt concentration does not significantly increase the average crack growth rate in sensitized SS304L. Further data analysis shows that crack initiation time is also primarily dependent upon temperature. Therefore, hydrogen embrittlement is considered to be responsible for stress corrosion cracking as the main driving force since it dictates the hydrogen transport to the crack tip for crack initiation and hydrogen propagation for crack growth. In addition, microstructural characterization post experiments indicated that cracks propagate intergranularly due to the chromium depletion along the grain boundaries by forming carbides (M23C6).