Patent Number: 043326390
Section: summary

The present invention relates generally to nuclear reactors and more particularly to a failed element detection and location (FEDAL) system and method for use in a nuclear reactor and specifically an LMFBR. The particular nuclear reactor disclosed herein is one which utilizes a number of fuel assemblies housed within an active core which, in turn, is housed within a sealed vessel. Each fuel assembly contains the reactor's fuel or fission products, that is, the active substance making up the reactor, such as plutonium oxide. This active substance is sealed within a relatively large number of elongated hollow pins (cladding) located within an opened container having an inlet and outlet. Each of these containers and associated fuel pins comprise a single fuel assembly and all of the assemblies are located within the active core. The reactor also includes liquid metal cooling fluid such as liquid sodium and means for circulating a stream of the fluid along a path, a section of which passes through the containers from their inlets to their outlets. Obviously, this particular type of reactor includes other components, which may be conventional like those thus far described but which are not necessary to an understanding of the present invention. Accordingly, these other components will not be discussed or even mentioned herein, unless to do so would be helpful to an understanding of the present invention. In nuclear reactors of the type described, it is often desirable, if not necessary, to monitor for cladding failures, that is, breaks in the hollow fuel pins comprising part of the fuel assemblies. If this break is relatively small, the passing fluid, specifically the sodium, may not come in contact with the active substance within the pin, for example the plutonium oxide. However, inert gases including specifically Kr-85, Kr-88 and Xe-133, Xe-135 will escape into the fluid stream, emitting gamma rays therefrom. On the other hand, if the break is relatively large, that is, sufficiently large to cause the passing sodium to actually enter the faulty pin and contact the plutonium oxide, the sodium will be contaminated with I-137 and Br-87 which are two of a number of by-products of the fissioning process taking place in the pin and which decay rather rapidly, giving off neutrons. The detection of small breaks is relatively conventional and typically accomplished by detecting for gamma rays emitted from the escaping gases Kr-85, 88 and the like as the latter surface from a central pool of sodium within the reactor vessel. However, accurate and reliable detection of the larger breaks in a reliable manner is not as simple, as will be seen hereinafter. One typical way of monitoring for large breaks heretofore has been to place a neutron detector at some entry point in the internal heat exchanger (IHX) which also comprises part of the overall reactor and which is located within the reactor vessel for receiving liquid sodium after the latter passes out of the fuel assembly and into the central pool. There are several problems with this approach. First, it may not be possible to locate the monitoring apparatus in a position to collect samples of sodium which have passed through all of the core assemblies because of the size of the IHX and the diverse ways in which the sodium enters the latter. Second, the time it takes for the sodium to reach the IHX bulk sodium pickup point within the central pool is relatively long. This means that by the time these contaminants are detected for the emission of neutrons, the level of neutrons being emitted will be relatively low thereby raising the question of reliability. For example, the half life for I-137 is approximately 55 seconds and for Br-87 it is approximately 22 seconds. In contrast to this, it may take as long as 150 seconds for a particular sample of sodium to reach the selected entry point of the IHX from the reactor core. Another way in which the relatively larger breaks in fuel pins have been detected in the past has been to individually sample each fuel assembly, one at a time, which can certainly be reliable. However, it is time consuming and costly to provide continuous individual monitoring of all of the fuel assemblies since a given core may be made up of as many as 600-700 such assemblies. As will be seem hereinafter the present invention provides for a particular FEDAL approach for use in a nuclear reactor of the type described without the previously recited drawbacks. Rather, as will also be seen, combined samples of liquid sodium are collected as soon as the latter passes through selected groups of fuel assembly containers while the neutron emission level of any collected contaminants is still relatively high, thereby making this approach reliable. Should there be an indication of a break, individual samples are then and only then taken to isolate the fuel assembly or assemblies responsible for the break. In this way, individual samples do not have to be continuously collected and detected as in the past. In view of the foregoing, one object of the present invention is to provide a reliable and yet economical FEDAL technique for use in a nuclear reactor of the type described above. A more particular object of the present invention is to provide a reliable technique of detecting for relatively large breaks in the previously described fuel pins wherein combined samples of sodium are collected at the outlets of selected fuel pin containers while the neutron emission level of the contaminants, if any, are still relatively high. Another particular object of the present invention is to collect individual sodium samples only if the combined sample indicates a break in one or more fuel pins. A further object of the present invention is to provide a FEDAL system which utilizes an uncomplicated and reliable valve assembly for collecting both combined sodium samples as well as individual samples. As stated previously and as will be seen hereinafter, particular FEDAL technique disclosed herein is one which is especially suitable for use in a particular type of nuclear reactor, specifically an LMFBR. As also stated, this type of reactor has a reactive core housing within a vessel and a plurality of fuel assemblies housed within the core. Each of these fuel assemblies includes an open container having an inlet and outlet and active substance such as plutonium oxide sealed within a relatively large number of elongated hollow pins located within the container. This reactor also includes liquid metal cooling fluid such as liquid sodium and means for circulating a stream of the fluid along a path, a section of which passes through the containers from their inlets to their outlets. As will be seen hereinafter, the particular technique disclosed is one which detects breaks in the hollow fuel pins of sufficient size to cause at least one predetermined contaminant to pass into the liquid metal cooling stream as the latter passes through the containers. In accordance with this technique, a combined sample of the liquid metal fluid is collected at the outlets of at least a group of the fuel assembly containers and detected for the presence or absence of the contaminant. If this combined sample indicates the presence of a break, individual samples of the fluid are selectively collected, one at a time, at the outlets of the fuel assembly containers in the same group and these individual samples are also detected for the presence or absence of the contaminant, thereby indicating the particular fuel assembly or assemblies responsible for the break.