NONDESTRUCTIVE EXAMINATION TO DIRECTLY MEASURE STEAM GENERATOR TUBE U-BEND VIBRATORY RESPONSE

ON FEBRUARY 9, 1991, AT THE MIHAMA UNIT 2 NUCLEAR POWER STATION, OPEARTED BY KANSAI ELECTRIC CORPORATION, A STEAM GENERATOR TUBE RUPTURED AND RELEASED APPROXIMATELY 9,000 GALLONS OF REACTOR COOLANT INTO THE SECONDARY SYSTEM. THE LEAKAGE CAUSED A RAPID DEPRESSURIZATION OF THE REACTOR COOLANT SYSTEM, WHICH RESULTED IN AN AUTOMATIC REACTOR TRIP AND ACTUATION OF THE HIGH PRESSURE SAFETY INJECTION SYSTEM. THIS WAS THE FIRST ACTUATION OF AN EMERGENCY CORE COOLING SYSTEM COMPONENT EXPERIENCED BY THE JAPANESE NUCLEAR POWER INDUSTRY. A STEAM GENERATOR TUBE RUPTURE HAS SPECIAL SIGNIFICANCE TO PLANT SAFTEY BECAUSE IT REPRESENTS THE SIMULTANEOUS LOSS OF TWO OF THE THREE BARRIERS TO THE RELEASE OF FISSION PRODUCTS TO THE ATMOSPHERE. WHEN A STEAM GENERATOR TUBE FAILS, BOTH THE REACTOR COOLANT SYSTEM AND THE CONTAINMENT BUILDING ARE BREACHED. THIS EVENT IS VERY SIMILAR TO A TUBE RUPTURE WHICH OCCURRED AT NORTH ANNA UNIT 1 ON JULY 15, 1987. LIKE THE MIHAMA 2 EVENT, THE PRIMARY-TO-SECONDARY LEAKAGE WAS THE RESULT OF THE COMPLETE SEVERANCE OF A SINGLE TUBE IMMEDIATELY ABOVE THE UPPERMOST (7TH) TUBE SUPPORT PLATE ON THE COLD LEG SIDE. ANALYSIS INDICATED THAT THE TUBE FAILED AS A RESULT OF HIGH CYCLE FATIGUE, WITH THE FATIGUE CRACKS ORIGINATING ON THE OD SURFACE. IT WAS CONCLUDEDTHAT THE TUBE HAD EXPERIENCED FLUID ELASTIC INSTABILITY WHICH HAD CAUSED LARGE U-BEND DEFLECTIONS AND AN ALTERNATING STRESS LEVEL SUFFICIENT TO INITIATE FATIGUE CRACKING. DESPITE THE IMPORTANCE OF NATURAL FREQUENCY AND DAMPING IN DETERMINING THE MARGIN TO FLUID ELASTIC EXCITATION, NOT MUCH IS KNOWN ABOUT ACTUAL TUBE FIXITY CONDITIONS IN AN OPERATING STEAM GENERATOR. IT IS SUSPECTED THAT DEPOSITS WHICH ACCUMULATE IN THE TUBE SUPPORT PLATE HAVE AN EFFECT ON TUBE FIXITY AND THEREFORE ON THE FLUID ELASTIC STABILITY MARGIN. HOWEVER, THE EXTREMELY LIMITED SUCCESS AND THE RADIATION FIELDS WHICH ARE PRESENT PRECLUDE THE USE OF EXISTING VIBRATION MEASUREMENT SYSTEMS. A SYSTEM CAPABLE OF PERFORMING DIRECT MEASUREMENTS OF STEAM GENERATOR TUBE U-BENT VIBRATORY RESPONSE WOULD HAVE TO BE REMOTELY DEPLOYABLE AND