Patent Number: 055374506
Section: summary

DESCRIPTION 1. Technical Field The invention relates to on-line analyzing of the integrity of individual fuel elements or bundles during operation of a nuclear powered electrical energy generating plant. More specifically, the invention relates to the use, without removal of samples, of gamma spectrography to analyze signals typical of failed fuel element gaseous fission products released to the primary reactor system. 2. Background of the Invention If fuel cladding failures should occur during the operation of the nuclear reactor of a nuclear powered electrical energy generator there is a release of fission products to the primary heat exchange system. These fission products not only contaminate the reactor system, but the gaseous fission products can also be released to the environs via the plant off-gas system. If the failed fuel elements are allowed to remain in the reactor core and are used in the power production mode, the release of fission products typically keeps increasing and the plant must reduce power by deactivating the failed fuel elements and/or eventually shutdown to remove the fuel elements that have failed. Catastrophic failure of the fuel cladding will eventually take place if the failure mechanism is not ameliorated. This latter condition, referred to as "un-zippering", leads to release of nuclear (uranium/plutonium) fuel to the reactor internal components where it will continue to fission without containment of the fission products anytime the plant is operating. This is referred to as "recoil release". Typically, when an operating plant first experiences a non-catastrophic fuel cladding failure, the plant power output is reduced to 60 to 80% of full power so that the plant can produce a guaranteed steady power output while shutting down some of the control cells so as to be able, by the process of elimination, to locate the control cell which contains the problem fuel bundle. A typical 1,000 Megawatt electricity producing plant would have of the order of 180 control cells each of which has four fuel bundles. To locate the problem, damping rods are inserted in selected of the control cells and samples of the reactor off-gases are drawn into containers and analyzed via gamma spectrography to determine the Xe-133 count until the cell (or cells) responsible for the problem is identified. Current operating plants have the capability of sampling the off-gas and determining the composition and characteristics of the fission gases that may be present. However, the process involves grab sampling and a timed decay of about 40 minutes before analysis. Significant and complicated calculations must be performed to back calculate to the initial amounts of radiation due to various nuclides because of their very different half-lives. Normally only about six such analyses can be run in a typical twenty-four hour day. Thus, it would generally take 30 days to test all of the control cells in such a power plant reactor. The number of analysis per shift can be increased by using several technicians but the costs of training so many technicians and having them available as needed can be quite significant. Also, the possibility of random errors is increased since not every technician will generally perform the task in an identical manner. A relatively long test period leads to increased occupational exposure of plant technicians and is not conducive to making more than a hundred analyses in a short period of time. This of course represents very significant cost penalties (on the order of thousands of dollars per hour as the plant continues to operate at a reduced power output) as well as creating radiological hazards within the plant (Replacement cost is about $10.00 per Megawatt hour. If the plant is only operating at 80% capacity then the power replacement must be 200 Megawatt hours which is equal to $10.00 times 200 or $2,000.00 per hour. If the plant is operating at only 60% capacity the power replacement cost is proportionately higher, namely, $4,000.00 per hour.) A typical catastrophic fuel cladding failure will result in about 30,000 .mu.Ci of fission product gases being released per second. At these levels, it is almost imperative that the plant shutdown and perform a long and arduous task of examining the fuel bundles external to the reactor vessel. Once the leaking fuel bundle is found, it is then replaced and the fuel reloaded into the reactor. The economic impact on a typical 1,000 Megawatt electric utility is on the order of a million dollars per day under these circumstances. DISCLOSURE OF THE INVENTION In accordance with an embodiment of the invention an on-line method is set forth of detecting failed nuclear fuel elements. The method is useful with a nuclear reactor which has a plurality of control cells which contain nuclear fuel bundles and into which damping rods can be reciprocated to start, stop and control the rate of nuclear chain reaction. Such reactors producing an off-gas stream which includes, O-19, N-13, N-16, Kr-85m, Kr-87, Kr-88, Xe-133, Xe-135, Xe-135m and Xe-138 nuclides along with other gaseous species including other nobel gas isotopes. The method comprises flowing the off-gas stream from the reactor to a detecting cell of a gamma spectrograph. The flowing is for a time sufficient to reduce gamma radiation produced by O-19 and N-16 nuclides and to reduce Compton scattering produced by O-19, N-13 and N-16 nuclides sufficiently so that the magnitudes of the gamma radiation from at least one of Kr-85m, Kr-87, Kr-88, Xe-133, Xe-135, Xe-135m and Xe-138 nuclides can be determined in the gas cell. The spectrograph is of sufficiently high resolution to allow such determination. In accordance with another embodiment of the invention a method is set forth of continuing to operate a nuclear reactor which exhibits one or more leaks indicative of non-catastrophic cladding failure. The method comprises determining the control cell or cells which are leaking in an on-line manner as set forth above while keeping the reactor in operation. The rods are then reciprocated into the leaking cell or cells sufficiently to alleviate the leak or leaks and reactor operation is continued. The total power output of the reactor is preferably increased to full operating capacity by increasing the withdrawal of rods from non-leaking control cells. The present invention provides a method for rapidly analyzing the characteristics of minute amounts of fission products. The method involves utilization of on-line gamma spectrographic measurement of the off-gas stream. For the recoil situation wherein fissionable uranium is not contained within fuel elements (tramp from manufacture or from previous catastrophic fuel failures) a characteristic pattern with regard to the amount and type of fission products is observed. Similarly, for large releases from fuel pellets within the clad through larger openings (diffusion through the cladding) and pin-hole leakage (equilibrium release) both have different but distinct characteristic concentration patterns. These patterns are conventionally determined by plotting concentration (or counts), A, vs. concentration divided by .lambda. times the fission yield, Y, wherein .lambda. is the natural logarithm of 1/half-life. For recoil the slope is zero while for diffusion and for equilibrium release different non-zero slopes result. FIG. 6 illustrates such a plot. The method is described in "A COMPARISON OF FISSION PRODUCT RELEASE STUDIES IN LOOPS AND THE VBWR" by F. J. Brutschy, which was presented in February, 1961 at the Tripartite Conference on Transport of Materials in Water Systems and was predistributed (by General Electric Company, Atomic Power Equipment Department, Vallecitos Atomic Laboratory, Pleasanton, Calif.) on Jan. 21, 1961. Once the characteristics of the fission products in the off-gas stream is known, the source control cell can be identified by conventional analytical procedures typically used in the nuclear industry, namely, by determining the effect on the gamma spectrograph of the reciprocation of known control rods within the reactor. In order to achieve the required accuracy and detection capability, an appropriate sample flowing and spectrograph measuring methodology is disclosed as is an appropriate gamma spectrograph cell. Neither of these currently exist in operating power reactor stations. The present invention for the first time provides for a methodology to quickly and accurately analyze fuel bundle integrity for early on-line detection of small leaks while the plant is operating. Over fifty samples can be analyzed on-line in a typical twenty-four day. The working example set forth below demonstrates the analysis of 185 samples in a 60 hour period which would correspond to over seventy samples being analyzed in a twenty-four hour day. In this way, defective control cells can be quickly located and then isolated from the power generation sector of the core (by insertion of the corresponding rods) and the plant can remain on line at full power (the power output of the remaining cells can usually, in practice, be increased sufficiently to make up for the power lost by deactivating only a few of the control cells) for an extended period of time. Furthermore, once the failed fuel bundle is isolated from the power regime, it is cooled to the primary water temperature rather than the very high fission temperatures and the cladding stress is much relieved. Therefore, the probability for "un-zippering" and cataclysmic release of fission products is nearly eliminated. The defective cell or cells can be replaced during the normal downtime which is needed to replace spent and defective fuel bundles with fresh fuel bundles. Spent and defective fuel bundles are PG,8 replaced on the average about once every eighteen months.