Patent Number: 048184705
Section: summary

This invention relates to the ultrasound testing of steam separator hold down bolts in a boiling water nuclear reactor (BWR). STATEMENT OF THE PROBLEM BWRs are periodically refueled. As part of this refueling procedure, the reactor dome is removed, the steam dryer assembly lifted out of the reactor vessel and the cylindrical steam separator is then lifted out of the reactor vessel. The dome, dryer and separator are placed in a holding pool so that water shields any radio activity from working personnel. This invention relates to a problem that is unique to the steam separator and provides for inspection of the shroud hold down bolts when the steam separator is in the holding pool. When the reactor is assembled and operating, the steam separator must be held in position to a shroud overlying the reactor core. This holding of the steam separator in position is accomplished by the shroud hold down bolts. The shroud hold down bolts are relatively complex bolt members. These bolt members are elongate, extending from the top of the steam separator assembly to the bottom of the steam separator assembly a distance of about 13 to 17 feet. The bolts are manipulated and operated from the top, and fasten the steam separator to the shroud overlying the reactor core at the bottom. Because these bolts are well known in the prior art, they will be set forth herein only in sufficient detail so that the problems that they generate with respect to periodic inspections can be understood. The shroud hold down bolts are typically spaced at equal radial intervals around the cylindrical steam separator. Each bolt includes a central tension member and an outer compression sleeve member with both members extending substantially the entire length of the bolts. Because of the relative proximity of the bottom of the bolts to the top of the radioactive reactor core, the bottom of the bolts are highly radioactive. The lower portion of the inner tension member has a lug which is rectangular in plan on the bottom for engagement with a bracket on the shroud at the top of the core. This rectangular lug rotates from a radial alignment where release of the bolt from a mating shroud bracket can occur to a position normal to a radial alignment where attachment of the bolt at the rectangular lug to the shroud bracket can occur. When the rectangular lug is engaged with the bracket, the outer compression member bears down on the lower portion of the steam separator, places the inner member under tension from the shroud bracket and holds the steam separator to the rest of the reactor during operation. All manipulations relating to the tightening or the loosening of the bolts occurs from the very top of the bolts. This enables engagement and disengagement of the bolts to be remotely manipulated, typically in a submerged disposition, so that radio activity is not a threat to operating personnel. Tightening of the engaged bolts can be easily understood. The bolts are tightened and loosened from the top of the steam dryer assembly. Assuming that the lug is engaged with the bracket, the outer compression sleeve of the bolt is forced downwardly over the inner tension member. The outer compression member bears downwardly on the lower portion of the steam separator. At the same time, the inner tension member pulls upwardly into the bracket on the shroud at the top of the reactor core. The result is that the steam separator is held to the top of the shroud overlying the core. Loosening of the bolts is more complex. Adjacent the bottom of the bolts, attached to the outer compression member below the attachment at the bottom of the steam separator is a sleeve. This sleeve has a window. The purpose of the window is to maintain the lugs of the bolts in the open position when the bolts are fully loosened. The lower end of the inner tension member is transpierced with a shaft normal to the sleeve. This shaft extends through the window in the sleeve. The purpose of the shaft is to co-act with a notch in the window of the sleeve to maintain the shroud bolt with its lug in the open position once it is fully loosened. When the shroud bolt is fully loosened, the rectangular lug at the bottom of the tension member falls below the shroud bracket. When the rectangular lug falls below the shroud bracket, it is free to rotate. Rotation of the rectangular lug and the attached tension member occurs to a position where the lug is radially aligned with respect to the shroud bracket. With such radial alignment, release from the shroud bracket can occur. Likewise, when the tension member and its rectangular lug rotate, the shaft in the window likewise rotates. This rotation of the shaft in the window continues until the rectangular lug is radially aligned and the transpiercing and protruding shaft engages the notch in the window of the sleeve. Once engagement to the notch in the window of the sleeve occurs, the rectangular lug at the bottom of the tension member is maintained in a position of radial alignment where release from the shroud bracket can occur. Assuming that all shroud bolts are fully loosened and their respective lugs retained in the radially aligned position, lifting the steam separator from the shroud overlying the core can occur. Movement of the steam separator to the holding pool takes place. Unfortunately, such shroud hold down bolts crack. The bolts crack on the inner tension member adjacent the lug, usually under the sleeve. They are subject to a metallurgical cracking phenomena known as inter granular stress crack corrosion. Simply stated, the oxygen in the water of the reactor combined with both the material and the tension on the bolts can cause cracking to occur. This cracking is intermittent and highly unpredictable. Further, the cracking defect is latent in at least two respects. First, inter granular stress crack corrosion (IGSCC) is hard to locate by observation--for example an under water TV camera. Second, the most susceptible location for the IGSCC has been found to be a location underneath the locking sleeve at the bottom of the bolt. Conventional inspection techniques are not desirable. Any inspection technique that involves removal and manipulation of the highly radioactive bolts is prohibitively expensive. Further, since the presence of the cracking is highly intermittent, routine inspection capability is desired for checking against this shroud bolt defect. The reader will understand that the complete understanding of the latent defect is relatively complex. Consequently, insofar as understanding of the problem to be solved constitutes invention, invention is claimed. SUMMARY OF THE PRIOR ART Ultra sonic testing is known. Typically, piezoelectric transducers are directly placed by hand upon a member to be tested. A sonic signal is imparted to the member. The transducer listens for a returned and reflected signal. Once the signal is returned analysis of the tested member can be made. Such ultra sound waves when transducing a rod such as the inner tension member have three types of waves traveling through the rod. These waves include longitudinal waves, refracted waves, and shear waves. It is by the analysis of these wave forms, that defects can be located. Since the analysis of such wave forms is well known in the prior art, further discussion will not be added here. SUMMARY OF THE INVENTION An apparatus for the remote examination of peripheral shroud hold down bolts on steam separators used in boiling water reactors (BWRs) is disclosed. The shroud hold down bolts surround the generally cylindrical steam separator at the outside circumference and are generally coextensive in length with the steam separator length, being in the order of 150 to 205 inches in length. During reactor outages, all bolts--between 24 and 48 in number--have rectangular holding lugs at the bottom of the hold down bolts radially aligned with respect to the steam separator to clear mating brackets on a shroud overlying the reactor core. When all bolts have their lugs radially aligned for release the steam separator with its attached bolts is moved to a holding pool and remains immersed in water to protect maintenance personnel from ambient radioactivity. The testing apparatus normally is used while the steam separator is in the holding pool with its respective lugs radially aligned. The testing apparatus includes a depending pole having attached at the bottom thereof an aluminum shoe. The shoe has a flat, upwardly exposed bottom and opens to one side at gathering surfaces to receive the lower ends of the shroud bolts. The upwardly exposed bottom of the shoe defines an aperture through which an upwardly exposed piezoelectric device is exposed for direct contact with the bottom of the bolt. An overlying clamp member is provided to clamp the lug onto the bottom of the shoe for test. The clamp member defines a receiving slot to receive the shaft of the bolt immediately overlying the rectangular lug. This clamp member is positioned at the slot on the shaft overlying the rectangular lug and thereafter moved down to and towards the lug by a pneumatic cylinder. The clamp member slides over the shaft at the slot into contact with the top of the lug. The clamp member clamps the lug end of the bolt securely onto the upwardly exposed piezoelectric device on the upwardly exposed bottom of the shoe. A piezoelectric ultrasonic test is then run from the bottom and radioactive portion of the bolt to and towards the top of the bolt. Testing for longitudinal sound waves, refracted sound waves and shear sound waves for inter granular stress corrosion cracking can occur despite the radioactive and remote under water location of the bolts. OTHER OBJECTS AND ADVANTAGES An object of this invention is to set forth a test protocol for a reactor steam separator shroud bolt without necessitating removal of the bolt. An advantage of the disclosed protocol is that regular testing of the bolts can occur concurrently with reactor outages concurrent to refueling. Further, no removal or disassembly of the steam separator is required for the disclosed test. Further, it has been found that the outer compression member, sleeved about the inner tension member, does not interfere with the desired testing. In short, a highly advantageous and economical ultrasonic test is disclosed.