Flaw Stability Considering Residual Stress for Aging Management of Spent Nuclear Fuel Multiple-Purpose Canisters

Abstract

A typical multipurpose canister (MPC) is made of austenitic stainless steel and is loaded with spent nuclear fuel (SNF) assemblies. Because heat treatment for stress relief is not required for the construction of the MPC, the canister is susceptible to stress corrosion cracking in the weld or heat affected zone (HAZ) regions under long-term storage conditions. Logic for flaw acceptance is developed should crack-like flaws be detected by Inservice Inspection. The procedure recommended by API 579-1/ASME FFS-1, Fitness-for-Service, is used to calculate the instability crack length or depth by failure assessment diagram (FAD). It is demonstrated that the welding residual stress (RS) has a strong influence on the results.

Bounding Surface Flaw Configuration Susceptible to Stress Corrosion Cracking Under Welding Residual Stress in a Multiple-Purpose Canister

“…These WRS distributions through the wall thickness are shown in Fig. 2 (measured from the outside diameter, OD) [1,2], from which it can be seen that the WRS components (RS2 and RS4) parallel to the welds (circumferential and longitudinal welds, respectively) are higher, as this is typically the case for the welded structures. In addition, near the canister surface, RS2 (the residual stress tends to open an axial surface crack) appears to be bounding.…”

“…As reported in previous publications [1,2], a typical dry cask storage system (DCSS), the Holtec International Storage Module (HI-STORM) [3,4], was selected for the present SRNL-STI-2017-00224 2 investigation on the stress intensity factors (K) of an external semi-elliptical surface crack under WRS. The canister is a cylindrical shell made of austenitic stainless steel (304/304L, typically) [3].…”

“…Because the HI-STORM welding information is proprietary, the WRS was estimated in previous work [1] using American Petroleum Institute (API) 579 Annex E (2007 Edition) [5] by assuming an average heat input of 1080 J/mm (based on a 4 mm electrode, 889 mm/min travel speed, with 500A-32V for stainless steel with SAW process). The yield stress for WRS estimation is 274 MPa (40 ksi) that is based on API 579 recommendations [5,6] to use the base metal code minimum yield stress [7] raised by 69 MPa (10 ksi) to account for actual material property that is usually higher.…”

“…The yield stress for WRS estimation is 274 MPa (40 ksi) that is based on API 579 recommendations [5,6] to use the base metal code minimum yield stress [7] raised by 69 MPa (10 ksi) to account for actual material property that is usually higher. Four WRS were calculated as the crack opening stresses for the surface cracks on the canister [1,2]: 1) RS1: Axial crack parallel to an axial or longitudinal weld with the residual stress perpendicular to the weld; 2) RS2: Axial crack perpendicular to a circumferential or girth weld with the residual stress parallel to the weld; 3) RS3: Circumferential crack parallel to a circumferential weld with the residual stress perpendicular to the weld; 4) RS4: Circumferential crack perpendicular to a longitudinal weld with the residual stress parallel to the weld.…”

“…The acceptable part-though-wall surface crack size under the influence of WRS and external loads has been estimated and proposed to Code Case N-860 [1,2]. The current work is focused on CISCC, in particular, to establish a threshold stress intensity factor (K ISCC ) and the acceptable initial flaw size below which the stress corrosion cracking will not take place.…”