Patent Number: 044951434
Section: summary

BACKGROUND OF THE INVENTION A typical nuclear reactor includes a core within which chain-reacting nuclear fuel material is located. The fuel material typically might be pellets of U.sup.235 or U.sup.238 or Pu.sup.239 which are encased in separate corrosion resistant heat conductive cans or cladding to form an elongated fuel element (also referred to as a fuel rod or fuel pin). A number of the fuel elements are grouped together and supported within a larger fuel assembly. The fuel assemblies are located then in a prearranged spaced matrix within the core of the reactor, with moderators or other form of control means being located in a different prearranged matrix within the core. The controlled presence of the fuel elements and control means regulate the extent of the nuclear reaction whereby neutron bombardment provides for thermal heating of the fuel elements and surrounding core structures. A reactor coolant is circulated through the core and fuel assemblies and over the fuel elements so as to cool them. The reactor coolant in turn is passed through a heat exchanger whereby a second coolant, commonly steam or water, is heated which second coolant is then expanded through appropriate steam expansion equipment for producing useful output typically for generating electricity. Each fuel element, as noted, has a sealed exterior can or cladding, typically of stainless steel or zirconium alloy, so that the fuel material itself is sealed therein and is isolated from the coolant. This is needed firstly, to chemically isolate the nuclear fuel material from the coolant, and secondly to prevent the release of any radioactive fission products that may be generated in the nuclear reaction. Failure of the cladding, such as by localized melting or cracking, may thus release such fission products which would radioactively contaminate the circulating coolant which then would interfere with plant operation and maintenance. Further, a leaking fuel element could be the result of swelling that in turn further might block coolant flow and cause more extensive or costly overheating damage to the reactor. Thus, it is desirable to identify and locate a leaking fuel element as soon as possible so that the situation can be appraised and that fuel replacement procedures can be quickly handled with a minimal degree of cost and effort during subsequent reactor shutdown. Most modern power reactors, particularly the breeder reactor where a liquid metal (sodium, for example) is used as the reactor coolant, have a sealed reactor system with an inert cover gas, typically argon that serves as a collector for any fission gases carried in the circulating coolant. To remove the fission gases, the cover gas must be removed from the reactor and processed in a cover gas cleanup system, such as in a bed of charcoal held at a cryogenic temperature, whereby the purified cover gas is then returned to the reactor. The fission gases commonly include the radioactive isotopes of xenon and krypton. This cleanup system can be operated continuously or only after a leaking fuel element has been detected. A gamma ray radiation detector is commonly used to examine the cover gas for the presence of any of the gaseous fission products. However, this has little accuracy in identification specifics, so that further identification of the leaking fuel element and evaluation of the severity of the leak must still be made by other means. Systems are being used to sample the cover gas and/or the coolant circulating in the reactor in an attempt to localize the leaking fuel element. The use of "sippers" has worked moderately well, whereby a portion of the coolant from selected fuel assemblies would be diverted to a remote sampling facility; and a multiple port valve would be shifted to periodically sample the coolant output from different proximate fuel assemblies. This system, however, does require prior assemblied clusters of coolant lines and valves, so it would not be practical in most existing power reactors not having the required hardware. The concept of gas tagging is also known, being taught, for example, in the U.S. Pat. No. 3,632,470 assigned to General Electric Company; and U.S. Pat. Nos. 3,663,363 and 3,746,614 assigned to the U.S. Government. In gas tagging, stable isotopes of a gas are isolated in proportioned percentages of concentration to one another as a means for establishing unique combinations of such isotopes. The unique combinations of such isotopes, along with a filler gas perhaps of helium, would then be sealed within the different fuel elements as they were manufactured. The filler gas might comprise perhaps 90% of the gas mixture and would provide effective heat transfer between the fuel material and the fuel element cladding. The different fuel elements with their unique tags would be cataloged according to some matrix in the reactor core. Upon a breach of fuel element cladding, the unique "tag gas" mixture would escape to the coolant and would ultimately be carried to the cover gas area. Mass spectrometric analysis of the cover gas would give the weighted presence of the isotopes, and therefore identify the unique "tag gas". The corresponding fuel assembly "leaker" might then be identified according to the matrix catalog. U.S. Pat. No. 3,632,470, proposes using the three stable non-radioactive isotopes of neon: Ne.sup.20, Ne.sup.21 and Ne.sup.22. However, the teaching has been inoperative in practice because the mass of the cover gas atoms (which typically is argon) to that of the neon tag gas atoms, is almost two to one. Thus, a doubly ionized argon isotope (Ar.sup.40++) looks almost identical on a mass spectrometer to a singly ionized neon isotope (Ne.sup.20+), and a precise analysis is difficult if not impossible to make. U.S. Pat. No. 3,663,363 proposed using the xenon 124-130 isotopes as the tags. Although these isotopes would not normally be among the fission products generated in the reaction, the xenon 131-136 isotopes and the krypton 83-86 isotopes are the most common gaseous fission products. Consequently, the tag detecting system must attempt to isolate tag and fission isotopes from the same xenon family, which in effect greatly dilutes the concentration of the tag isotopes. Where the cover gas cleanup system is operated continuously, this in effect means that is is competing with the tag gas recovery and detecting systems for the same xenon isotopes. When the cover gas cleanup system can be stopped and only the tag gas recovery system operated, the xenon tags are diluted by the background blanket of fission product xenon and it becomes increasingly difficult to separate the tags from the fission gas. Notwithstanding these shortcomings, tag detecting systems of this type have been used in several liquid metal fast breeder reactors with varying degrees of success for many years. While helium and argon have been proposed for possible use as the cover gas in the liquid metal fast breeder reactor, in practice argon has been used almost 100% of the time as the cover gas because of the relative ease of containment. The only gases suitable as failure "tags" in fuel assemblies are the noble gases of xenon, krypton, argon and neon. The argon cover has precluded the use of argon tags because of course, the tag isotopes could not be detected against the huge background of natural argon. As noted, neon tags cannot be used because the presence of doubly-ionized argon in the mass spectrometer interferes with the ability to resolve the neon tags. For these reasons, most tagging systems propose using either isotopes of xenon, or mixtures of isotopes of xenon and krypton. The high cost and complexity of xenon or xenon/krypton tags discourage their use in large-scale reactors where 600-800 unique tags would be required. Furthermore, xenon is very difficult to extract from air, and by far the most difficult to enrich isotopically by thermal diffusion columns. This is so since as the atomic mass increases, the fractional difference in mass between adjacent isotopes becomes smaller. While Xe.sup.128 can be produced in essentially pure form by transmutation of iodine in a thermal nuclear reactor (albeit at a large expense), there exist only a finite number of enrichments for the remaining stable isotopes. Thus, the maximum mole percent attainable in commercially available enrichments is 40% for Xe.sup.124, 16% for Xe.sup.126, and 70% for Xe.sup.129. Moreover, for any given enrichment of one of these isotopes, the relative ratios of the remaining isotopes are essentially fixed. These considerations impose severe physical constraints on the range of unique tag compositions that can be obtained with a xenon or xenon-krypton system of tags. Another and most significant drawback on the xenon/krypton tag system is that when a fuel element has ruptured and fission gases have been detected the cover-gas cleanup system must run continuously to strip out the radioactive xenon and krypton fission gases. This means that any released tags of xenon or krypton will be subject to this cleanup system and likely disappear completely within a short time, viz., less than a few hours. Thus, if a tag is missed for some reason during this brief time, it is likely that the leaker which released that tag will not be identified. Yet another drawback with xenon and xenon/krypton tags is that the principal tag ratios change substantially upon irradiation in the reactor. The most common two ways of dealing with these large changes, both of which increase the cost of tagging, are: (1) to track the compositions of every tag in the reactor by analytical and empirical means as a function of irradiation history; and (2) to provide sufficient spacing between adjacent tag ratios so that no tag can "burn into" a neighboring tag during irradiation. This required wide spacing of the xenon/krypton tags has the added drawback of using much more of the most expensive isotopes. U.S. Pat. No. 3,746,614 use the three stable isotopes of Au.sup.197, Sb.sup.121, and Pt.sup.198 in slightly different weight ratios to one another as part of the bond coating over sodium bonded fuel elements. Thus the unique tag ratios of the different fuel elements can be identified by gamma spectrometric assay, and a fuel element tagging catalog can be used to pinpoint the element location in the reactor core. However, the coolant must leak through a failed cladding and contact the fuel element coating before the resulting tag would be released to flow with the coolant past the detecting area; and as the tag is a solid, the system is almost completely insensitive to a gas leaker. SUMMARY OF THE INVENTION This invention teaches an improved combination of cover gas and tagging means for use in a liquid metal fast breeder reactor for monitoring a large number of separate fuel assemblies arranged in a matrix configuration in a common reactor core, the invention providing improved sensitivity and discrimination in specifically isolating and locating a leaking fuel assembly within the matrix. This invention uses separate charcoal beds respectively maintained at different cryogenic temperatures for isolating from the cover gas impurities including the fission gas and tagging gas isotopes respectively that escape from a sealed fuel element upon failure of the fuel element cladding, detecting means including gas spectrometry to identify the isolated tagging gas isotopes, and the invention specifically uses the unique combination of helium as the cover gas and neon and argon isotopes as the tagging gas isotopes. The invention specifically provides using the stable isotopes Ne.sup.20, Ne.sup.21 and Ne.sup.22 of neon and the stable isotopes Ar.sup.36, Ar.sup.38 and Ar.sup.40 of argon in preselected different ratios one to the other so as to define many unique combinations of such tagging isotopes. Each distinct tagging combination is thereupon sealed in a fuel element with the fuel material; although similarly tagged fuel elements could be made and located in a common fuel assembly. Breach of the fuel element cladding thereby allows the unique tagging gas combination contained within that fuel element and the generated fission gases to escape and to combine with the cover gas. The cover gas is directed initially to the cover gas cleanup system whereby the fission gas impurities are first separated from the cover gas and most of the cleaned cover gas can then be recirculated back to the reactor; and part of the cleaned cover gas is then directed to the gas tag recovery system whereby the tag gases are isolated from the cleaned cover gas. The isolated tag gases can then be directed to mass spectrometry or other equivalent analysis for identifying the unique tag combination of the leaking fuel element which, along with the mapped matrix of the fuel assemblies, locates the leaking fuel element in the reactor core. In this manner, the fission gases and the tagging gases of neon and argon are independently isolated and separated from the cover gas of helium by noncompeting systems to provide improved sensitivity and accuracy in identifying the tagging isotopes. The advantages of the helium cover, argon-neon tagging combination include low gas costs, high tag-ratio stability, improved mass spectrometer detection sensitivity, favorable heat transfer consideration, lack of competition between the cover gas cleanup and tagging gas isolation systems, and analysis of tagging gases free of radioactive fission gases. Another attractive feature of neon and argon isotopes is that both gases have very small neutron capture cross sections and essentially zero fission yields for either uranium or plutonium fission. Thus neon-argon tag ratios are expected to change very little during irradiation in reactor. With the high neutronic stability and low capture resonances for fast and intermediate energy neutrons, (1) no analytical corrections will probably be required for the argon/neon tag systems; (2) tag ratios can be spaced more closely together, with a considerable cost savings; and (3) the reliability of identifying failures should be tremendously enhanced, particularly when compared against present xenon/krypton tag compositions and neutronics correction procedures. Neon and argon isotopes are more easily resolved with a mass spectrometer than xenon/krypton isotopes, simply because the fractional difference in mass for two adjacent isotopes of these lighter gases is larger than for either heavier gas of xenon or krypton. Moreover, the background signal on the mass spectrometer due to non-tag noble gas isotopes is much smaller for neon and argon than for fission-produced and tramp-produced xenon and krypton "background" gases. As a result, greater sensitivity will likely be obtained with a mass spectrometer instrument when using a neon-argon tag system compared to using the xenon-krypton tag system. Moreover, neon and argon typically have significantly higher thermal conductivity values at all temperatures when compared to the higher-mass isotopes of krypton and xenon. Although the quantity of tag gas loaded into the fuel element generally has not been limited on the grounds of heat transfer, there is less restriction on the quantity of neon and argon that can be added to each fuel element, than for xenon and krypton. The cost per liter for neon and argon isotopes are in most cases substantially less than for the more conventionally used xenon and krypton isotopes, perhaps only 5 or 10% as much--particularly considering the same relative spacing between tag nodes. The cover gas cleanup system can be operated continuously or only after a leak has occurred in a subassembly. With the neon-argon tags, it is possible to have the cover gas cleanup system operated continuously where the radioactive fission isotopes will be removed from the cover gas, and without significantly removing or otherwise reducing the concentration of the tag isotopes. The tagging gas collecting system can then be operated on the cleaned cover gas to remove or recover the tag gas isotopes for analysis. Moreover as the tags are basically unaffected by the cover gas cleanup system, the tag gas concentration can be allowed to buildup in the cover gas before activating the tag recovery system, thus assuring that an adequate sample concentration is isolated for accurate identification. Moreover, if some malfunction (e.g., sample contamination, errant mass spectrometer or computer readout, etc.) did occur, subsequent tag gas samples can be taken until a satisfactory analysis is made. Only then need the tag gas be purged from the cover gas.