ENHANCEMENT TO STEAM GENERATOR TUBING EDDY CURRENT INSPECTION BY APPLICATION OF THERMALLY INDUCED STRESSES

STEAM GENERATOR TUBES REPRESENT A UNIQUE PART OF THE REACTOR COOLANT SYSTEM (RCS). LEAKAGE FROM ANY OTHER PART OF THE RCS IS CAPTURED BY THE CONTAINMENT BUILDING, BUT STEAM GENERATOR TUBE LEAKAGE IS TRANSPORTED OUT OF THE CONTAINMENT BUILDING ALONG WITH THE STEAM, WHICH IS NORMALLY UNCONTAMINATED. STEAM GENERATOR TUBES, CONSEQUENTLY, DESERVE, AND RECEIVE SPECIAL CONSIDERATION. THE ASSURANCE OF STEAM GENERATOR TUBE INTEGRITY IS THE GOAL OF THE INSERVICE INSPECTION PROGRAM. THIS PROGRAM PROVIDES FOR PERIODIC TUBE EXAMINATIONS WHICH ARE TYPICALLY PERFORMED BY EDDY CURRENT TESTING (ECT). ALTHOUGH ECT HAS PROVEN TO BE VERY GOOD AT DETECTING AND QUANTIFYING GENERAL WASTAGE, SOME FORMS OF TUBE DAMAGE ARE BEYOND THE CAPABILITY OF THE STATE-OF-THE-ART OF ECT. THE RECENT EXPERIENCE OF PORTLAND GENERAL ELECTRIC AT THE TROJAN NUCLEAR POWER PLANT IS A GOOD EXAMPLE OF THIS. TUBING SAMPLES REMOVED FROM THE TROJAN STEAM GENERATORS WERE INSPECTED USING THE MOST UP-TO-DATE ECT EQUIPMENT AND PROCEDURES AVAILABLE. THIS TESTING DETECTED NO DEGRADATION OF THE TUBE WALLS IN THE SUPPORT PLATE INTERSECTIONS. WHEN THE TUBE SAMPLES WERE SUBJECTED TO METALLURGICAL EXAMINATION, HOWEVER, THEY WERE FOUND TO CONTAIN TIGHT, AXIAL CRACKS, WHICH PENETRATED AS MUCH AS 70% THROUGH WALL. SUBSEQUENT BURST TESTING VERIFIED THAT THE TUBING HAD MAINTAINED ADEQUATE STRENGTH DESPITE THE CRACKS, BUT THE FAILURE TO DETECT THEM BY ECT MUST BE ADDRESSED. THE POTENTIAL EXISTS TO SUBSTANTIALLY IMPROVE THE DETECTION AND CHARACTERIZATION CAPABILITY OF EXISTING ECT BY PERFORMING THE INSPECTION IN THE PRESENCE OF A THERMALLY INDUCED TENSILE STRESS. THE TENSILE STRESS WOULD SERVE TO OPEN THE TIGHT CRACKS SUFFICIENTLY TO PERMIT THEIR DETECTION BY ECT. THE SUBSYSTEM FOR INDUCING THE THERMAL STRESS WOULD PROBABLY REQUIRE INTEGRATION WITH THE ECT SUBSYSTEM TO SATISFY TIME REQUIREMENTS.