Patent Number: 042591520
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to welds and methods and apparatus for detecting development of a failure within a weld, particularly useful in nuclear reactor systems. 2. Description of the Prior Art In order to ensure reliability of any structural system including fastened components, such as those joined by welds, it is common practice for the welded area to be periodically inspected for evidence of crack initiation, metal separation or other failure. While many devices are available for such inspections, some provide indications not consistent with the degree of accuracy required, and some are not readily adaptable to remote use. An application where accurate and rapid indications of weld failure are particularly desirable is in nuclear reactors, particularly in relation to main structural components within the reactor vessel which support the nuclear core. For example, while the core of fuel assemblies is typically bottom supported by a lower support structure, control rods which are reciprocatingly insertable to control core reactivity are top mounted. In the unlikely event of failure of the lower support structure attachment welds, undesirable separation of, or interference between, the control rods and fuel assemblies could result. In-service inspection programs have been instituted by the nuclear industry for inspection of such welds, including remote visual indicators useful, for example, in water-cooled reactors. With liquid metal cooled reactors, however, such visual indicators are not readily adaptable as a result of the opaque nature of the coolant. And, typical inspection systems require costly items such as multiple access ports and equipment and time consuming inspections carried out closely adjacent to the welded area. In order to detect flaws in other reactor components, such as fuel rods, liquid metal reactors have been proposed which include the incorporation of tag gases within the fuel rods. The reactors include flow paths for directing tag gases released upon failure of a fuel rod to a monitoring and detection system which alerts the plant operator to such failure. Such reactors additionally are designed to prevent the accumulation of gas bubbles which could be swept through the core region. Accordingly, reactor internals are typically configured to vent any gas which enters the reactor system to a cover gas region above the core. The venting system thus permits gases released outside of the core to flow upwardly and eventually mix with any tag and fission product gases released from fuel assemblies within the core. It is desirable to provide arrangements which allow rapid and accurate weld failure indication. It is particularly desirable to provide such arrangements in nuclear reactors, and to take advantage of existing system designs and configurations. SUMMARY OF THE INVENTION This invention provides method and apparatus for detecting cracks or other failures in welds joining plural components, particularly beneficial to remote detection in liquid metal cooled nuclear reactor systems. In one embodiment a hole or chamber is drilled from a surface of one component, through the weld deposit and into, but not entirely through, the second component. A preselected tag gas is placed within the chamber, and the chamber is sealed at its outer end, such as by a threaded and/or welded plug. Any well-known manner is provided to direct the environment immediately about the welded area, which can be liquid or gaseous, to other well-known apparatus which detects the presence of the tag gas. A trigger gas can also be incorporated with the tag gas to actuate a detection or analyzer system which otherwise is maintained in a non-detecting condition. Detection of the tag gas indicates that a failure has developed which provides a release path between the chamber and the surface of the weld deposit or the joined components. In an alternate embodiment a plurality of chambers through a given weld area are each provided with different tag gases. Detection of plural tag gases ensures that an indication is not merely spurious, passed as leakage passed the sealing plug, and that a true failure has occurred. In another embodiment a plurality of chambers are incorporated in a single weld or in different welds, each with a different tag gas. Detection of a given tag gas thus evidences not only the existence of a weld failure, but also the location of the weld or the failed position within a specific weld. In addition to plural chambers, the volume through which a chamber passes can be varied, extending, for example, over only one weld deposit boundary and terminating within, or at a surface of, the weld deposit. A chamber can also extend from a surface of one of the joined components, through the weld deposit and to the surface of another component, requiring two sealing plugs, among other configurations. The weld failure detection arrangement is particularly beneficial in the primary structural component welds of a liquid metal cooled nuclear reactor, such as the weld joining the reactor vessel to the core support structures. Leakage from a tag gas filled chamber is readily directed, by the upward flowing coolant and the configurations of the reactor and internals components, to the cover gas at the top of the reactor vessel. The cover gas can be continuously or intermittently monitored, or monitored by a detection system actuated by a trigger gas, for evidence of the tag gas as in proposed systems for monitoring fuel rod failures.