High Cycle Thermal Fatigue Issues in RHRS Mixing Tees and Thermal Fatigue Test on a Representative 304 L Mixing Zone

Abstract

In May 1998, a leak (30 m3 / h) occurred in the reactor heat removal system (RHRS) of the CIVAUX 1 power plant (PWR type N4 – 1400 MWe) which was then in a hot shutdown situation. A 180 mm through-wall crack was found in a 304 L austenitic stainless steel elbow in a mixing area of high and low temperature fluids [1, 2]. All mixing zones of main (␀10″) and minimum flow lines (␀4″) of the four N4 plants were affected by cracking [3]. After metallurgical examinations of these austenitic stainless steel components and an analytical damage evaluation, the major root cause for cracking was identified as high cycle thermal fatigue. The cracks were found in the mixing tees and at the roots of welds in mixing areas. The presence of ground surface finishes and geometrical discontinuities (weld roots and tapers) were identified as amplifier of fatigue damage. For the new RHRS mixing zones of N4 plants, decision was taken to suppress welds or locate them away from mixing area and to improve the surface condition (remove the weld root singularity, remove striations due to machining by polishing and reduce residual stresses). For the other 54 French PWRs (900 & 1300 MWe) with different design of RHRS mixing zones, the inspections showed that they were also all damaged by thermal fatigue with generally small cracks less than 3 mm excepted for the PWRs of Saint Alban 2 (5 mm) [4]. To reproduce the thermal fatigue phenomenon occurring in mixing zones, a representative endurance thermal fatigue test named “FATHER” was performed by CEA under an EDF, CEA and AREVA NP agreement [5, 6]. The test lasted 300 hours. It was performed on a 304L stainless steel mixing zone of 7 mm thick and 6″ diameter with a temperature difference of 160°C between cold and hot fluids. Different internal surface finishes were introduced in the test mock-up: coarse and fine grinding, industrial polishing, as extruded surfaces and as welded or flushed joints. Numerous NDE were performed during and after the endurance fatigue test like ultrasonic examinations or dye liquid penetrant inspections. They lead to the observation of many small thermal fatigue cracks located near as welded joints, on ground surfaces and on unpolished flushed welds. Cracks were not observed on industrially polished surfaces reproduced in straight piping sections or in flushed plus polished welds. After the test of 300 hours, the mock-up was axially cut in two symmetric half parts and sampling plates containing thermal fatigue cracks were machined from each of the half mock-up to perform detailed metallographic examinations. More than 50 thermal fatigue cracks with depths of 100 to 1000 μm were observed. Cracks initiate mainly on geometrical discontinuities like weld toes or grinding striations. Test results have also allowed to improve and to validate methods and tools for predicting crack initiation in mixing zones. The “FATHER” experiment can be seen as a significant contribution for preventing the risk of HCF in PWR equipment.