Patent Number: 055374506
Section: description

BEST MODE FOR CARRYING OUT INVENTION The present invention is concerned with appropriate on-line sampling methodology and sample analysis by gamma spectrography with the nuclear electrical power plant remaining in service. This is accomplished by diverting a portion of the off-gas from the reactor, generally from downstream of the condenser, through a bypass line and through a sample cell which is scanned by a gamma spectrograph. The off-gas is then generally recombined with the system off-gas wherefrom it can be safely disposed of. It is also possible to dispose of the sampled off-gas separately but this would require an additional source of vacuum and extra hazardous materials handling. Reference to FIGS. 1 and 2 will illustrate operation of the invention. A portion of off-gas is diverted from the off-gas system 10 from an appropriate position 12 selected to provide a desired delay time before measurement. The off-gas portion passes via a line 14, a valve 16, a line 18 and a valve 20 to a sample (measuring) chamber 22 of a gamma spectrograph 24. The spectrograph 24 has a detector 26 and is shielded by a wall structure 28. A thin wall 30, the wall being thin enough so that at least about 30%, preferably at least about 40%, more preferably at least about 70% and most preferably about 95% of the Xe-133 gamma radiation will penetrate it, separates the off-gas from physical contact with the detector 26. The off-gas can be retained for a short time in the sample chamber 22 by manipulating an exit valve 32 but a flow through procedure is preferred as it is faster and equally accurate. The off-gas flows through the exit valve 32 and is returned via line 34, valve 36 and line 38 to an appropriate position 40 in the off-gas system 10. All of the off-gas is then decontaminated together and vented to the atmosphere via recombiner 42, cooler/condenser 44, desiccant dryer/short term holdup 46, low temperature vault/charcoal absorber section 48 and filter 50. Note that details of the off-gas treatment will vary from plant to plant. If desired the valves 20 and 32 can be closed to isolate the sample and the sample can be allowed to decay for a desired time period, for example, the eliminate any interference from short lived species such as N-13. Such a procedure might be used in the case of very small leaks if the indications from the flowing testing is inconclusive but questionable as to fuel integrity. The sample can be measured in this manner after two, or more, different decay times to increase detectability of small leaks. It is essential to the practice of the invention that the time of flow to the sample be controlled so that interfering gamma radiation from relatively rapidly decaying species be reduced to a sufficient extent so that it is possible to obtain a gamma spectrograph of the more slowly decaying Kr-85m, Kr-87, Kr-88, Xe-133, Xe-135, Xe-135m and Xe-138 nuclides so that the relative magnitudes of the gamma radiation due to at least one of these nuclides can be determined, with the emphasis being on the low energy Xe-133 gamma radiation. The time of flow from the release of the off-gas from the fuel in the core to the sample cell should be at least about 2 minutes, more preferably at least about 2.5 minutes, and more preferably yet at least about 3 minutes and is suitably restricted to fall within a range from about 3 to about 30 minutes, more preferably from about 3 minutes to about 15 minutes. Overly long flow times are not desirable since the resulting data will have to be corrected for the different half-lives of the constituent gases so as to back calculate to the original sample constituency. The appropriate flow rate for achieving this will vary depending upon tubing diameters, materials, sample cell volume and other factors. Typically, the flowrate will fall between about 1 and about 30 cubic feet per hour. To achieve this, a specially designed sample chamber as described above was developed which allows for the weaker gamma emitters such as the Xe-133, Xe-135, Xe-135m and Xe-138 nuclides to be measured with high resolution without sacrificing analytical accuracy. Thus, the sample analysis chamber must be made with strong yet low density material, such as aluminum, titanium, magnesium or alloys of these metals, which do not give off an interfering x-ray. Simultaneously, the sample should be taken at a location and at proper flowrates which allow an appropriate amount of hold up (The half-lives of high energy emitters and/or Compton scatterers such as the O-19 and N-16 nuclides are of the order of seconds, specifically, 26.8 seconds and 7.1 seconds, respectively while the half-lives of the Xe species are from hours to at least days. N-13 has a half-life of 9.97 minutes but is not a gamma emitter. N-13 does emit at 0.511 MeV but this is outside of the range of interest) so that the high energy, short-lived nuclides do not overshadow the analyses. A sampling chamber containing from 25 to 300 cubic centimeters of flowing off-gas sample has been found to work well in carrying out the method of the invention. For determination of the failure mechanism and location of the failed elements, samples are taken following reciprocating (either full or partial inserting or withdrawing) of the control or damping rod within a cell containing typically four fuel bundles. When a control rod is exercised (reciprocated) in a cell which contains a failed fuel element, the gaseous fission product release rate changes significantly. These fission products are primarily radionuclides of xenon and krypton gases. Specifically, the relative magnitude of at least one of the Kr-85m, Kr-87, Kr-88, Xe-133, Xe-135, Xe-135m and Xe-138 nuclides must be determined in order to assess fuel element leakage, with the emphasis on the low energy but high signal strength Xe-133 and on the ratio of Xe-133 to Xe-138 which provide particularly strong signals and/or signal ratio changes in such eventualities. The Xe-135 and Xe-135m nuclides are also useful for making the necessary determination. It should be noted that other nobel gas isotopes will also provide gamma radiation. Such radiation is non-interfering. FIGS. 3 and 4 may be compared to observe the very different gamma spectrographs which result in the case of no leaks (FIG. 3) (The spectrograph of FIG. 3 also results if a leak has been alleviated by reciprocating a control rod into the leaking cell) and leaks (FIG. 4). Note in particular the size of the signal attributable to Xe-133, i.e., that it is very large when a leak is present and relatively small in the absence of a leak.) Although it is not as obvious from the peak sizes of the Xe-135, Xe-135m and Xe-138 peaks there is a significant enough change in intensity of these peaks to use them for identifying cells which are leaking. Also, the ratio of the Xe-133 peak to the Xe-138 peak appears to provide an even larger change in signal than is noted with the Xe-133 peak alone. During the development for the present invention, a study was performed at an operating boiling water reactor (BWR) plant that was suspected to have leaking fuel. The existing off-gas sample lines were analyzed by gamma spectrography to determine which nuclides could be detected and which were interfering. In addition, hold up times, sample flowrates and sample volume parameters were determined during this feasibility testing. The main interfering nuclides were found to be activation products of oxygen and nitrogen which are inherently present in a BWR primary system. The oxygen activates under neutron irradiation to O-19 and N-16 and both have interfering gamma energies plus creating a great deal of Compton scattering (which leads to low energy high background counting levels) which can overshadow the measurement of low energy gamma rays. These nuclides are short-lived, however, and can be dealt with by selecting proper flowrate and sampling parameters. The sample piping itself also imposes limitations on detectability of the low energy nuclides of interest in that the wall thickness, even though quite thin, was found to attenuate most of the low energy Xe-133 gamma rays. The attenuation coupled with Compton scatter radiation and x-rays all within the same energy level of this nuclide make analysis very difficult. Once the above data was evaluated, a sampling point was selected which had a more favorable hold up time to allow for decay of the short-lived O-19 and N-16 nuclides. A corresponding sample flowrate was selected so that a reasonably short time would exist between release of off-gas from the fuel in the core and analyses. A long time would be undesirable as it is important to perform the analysis in as short a time as possible so as to allow the plant to be returned to full operation with the least loss in operation. A specially fabricated sample chamber with an appropriate volume and thin aluminum (magnesium, titanium or alloys of these metals would also have been suitable) counting window was used within the shielding of a high purity germanium or lithium drifted germanium gamma detector for analyzing the sample on a continuous basis. Thus, the problem with attenuation of the low energy gamma rays was minimized and an adequate, but not overburdening in terms of total activity, volume of sample was available for analysis. The system as developed incorporates a shielded high purity germanium detector for eliminating stray radiation from other sources in the power plant. This is necessary as the nuclides of interest also contribute to Compton scattering and increase the overall count rate. The shielding also serves to reduce external background radiation. The shield itself is suitably, but not necessarily, lined with a material such as copper, to absorb secondary x-rays from the shielding material. Suitably, the detector can have an attenuation factor of 500 or greater for gamma rays of 1.33 MeV. EXAMPLE The system was set up at the same BWR mentioned previously for analysis of the total core. The reactor in the plant had 185 control cells containing four (4) fuel bundles each. Therefore, a sequence was designed so that the cells more likely to have cladding failure in the high power central region of the core were sampled first on a cell-by-cell basis. Following this, pairs of control cells were analyzed and then groups of four cells from the low power perimeter of the core were analyzed. This scenario allowed for the project to be completed within about 60 hours. The fact that this could be accomplished in so little time is of extreme importance as the plant power had to be reduced to about 60% output during the testing to allow for the insertion and withdrawal of the control rods without major perturbations of the plant generator. By minimizing the time to perform the testing, the replacement power costs are minimized. The test was successful in identifying two (2) leaking fuel elements with a high degree of confidence based upon differences in the magnitude of the Xe-133 gamma signal and upon the ratio of the Xe-133 gamma signal to the gamma signal attributable to Xe-138. The signals observed from the leaking fuel elements were between one and two orders of magnitude higher than the baseline level. Even though the leaking elements were located almost immediately, the entire core was analyzed to confirm the accuracy and sensitivity of the methodology and equipment. FIG. 5 illustrates plotting of three types of count data as against the cell being measured. The data plotted are 1) the intensity of the Xe-133 peak, 2) the ratio of the intensity of the Xe-133 peak to the intensity of the Xe-138 peak and 3) the sum of six different peaks, namely, those attributable to Xe-133, Xe-135, Xe-138, Kr-85m, Kr-87 and Kr-88. Using the prior procedure and analyzing even twelve (12) samples per twenty-four hour day would have taken about 15 days (about 360 hours). At about $4,000.00 per hour replacement power costs this would amount to a difference of about $1.2 million dollars (360-60=300.times.$4,000.00=$1,200,000.00). The plant had been limited to approximately 80% power with the unidentified leaking bundles. Once the leaking fuel was located, the control rod pattern was appropriately adjusted so that the plant could resume full 100% power operation. The net savings to the operating utility is on the order of $2,000.00 per hour and the costs associated with an unscheduled forced outage of two to three weeks was avoided. INDUSTRIAL APPLICABILITY The invention is useful to reduce downtime and optimize power output of nuclear powered electrical power generators. While the invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification, and this application is intended to cover any variations, uses, or adaptations of the invention following, in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice in the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth, and as fall within the scope of the invention and the limits of the appended claims.