Session: FSI-01-01 Thermal Hydraulic Phenomena with Vessels, Piping and Components

Paper Number: 61955

Start Time: Tuesday, July 13, 2021, 09:00 AM

61955 - Investigation on Leak Rates in Thin-Walled Structures 

Steam generators represent an important component in pressurized water reactors (PWR). Like all components of the safety relevant pressure retaining systems in nuclear power plants the steam generators undergo scheduled non-destructive testing according to the German KTA 3201.4 rule. In case of a leak in a steam generator tube, activated primary coolant passes over to the secondary water-steam loop. Measuring equipment in the secondary loop ensures that a plant can be shutdown at very low radioactive levels. In August 2018, during eddy current testing of a selected sample of the steam generator tubes (SGT) in two out of four steam generators of a German PWR, volumetric corrosion and linear indications were found. The flaws had a depth of up to 91% of the wall thickness of 1.2 mm. Recent tests in 2020 again showed linear indications on the hot side of some SGT. These flaws raise the question of possible types of failure as well as the expected leak rate in the case of wall-penetrating defects. While the determination of critical wall-penetrating cracks are well-covered in steam generator tubes, the assessment and prediction of leak rates caused by damage in SGT are subjected to unknown uncertainties. Therefore, experimental investigations are necessary to develop and verify leak rate calculation models for this kind of thin-walled leaks. The experimental program derived from these issues focuses on artificial defects like slits or boreholes with constant crack opening area (COA) but also includes a thin-walled fatigue crack.

In order to investigate such type of leaks, a modular test facility (fluid-structure-interaction test loop) that has been developed and installed at MPA University of Stuttgart within the framework of several research project sponsored by the German Ministry of Education and Research (BMBF) and the Federal Ministry for Environment, Nature Conservation and Nuclear Safety (BMU), is used. The test rig includes a leakage module, which carries artificial machined slits and fatigue through-wall cracks with a wall thickness l of about 1.2 mm. As a part of these investigations, the boiling delay in the flow regime at the outlet of the apertures was documented with a high speed camera. These experiments with boreholes cover a l/D ratio range from 1.5 to 12. Furthermore, leak rates with discharge into a water tank are compared to free discharge into air environment.

Leak rates were measured in a wide parameter range with temperatures ranging from 20 °C to 230 °C and a pressure range from 5 bar to 75 bar. The test series have been performed and the results will be used to develop a leak-rate model.  Within this paper an overview of the test facility, the testing procedure, and the results of the investigations will be presented and discussed.

Presenting Author: Fabian E. Silber Materials Testing Institute University of Stuttgart

Authors:

Fabian E. Silber Materials Testing Institute University of Stuttgart
Ludwig Stumpfrock MPA University of Stuttgart
Stefan Weihe MPA University of Stuttgart