Patent Number: 044951434
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a containment 10 enclosing a nuclear reactor 11 and a heat exchanger 12. A core 13 made up of a honeycomb array of open-ended passageways 14 (see FIG. 4) is in the reactor. Nuclear fuel "F" is designed to be located in certain of the core passageways according to some predetermined pattern or matrix, as is approximated in FIG. 4. Control rods or other conventional neutron absorbing control devices "C" are also designed to be fitted into other specific core passageways, again according to a specific matrix or pattern. The control devices "C" most typically are raised or lowered relative to the core 13 to establish a certain presence relative to the nuclear fuel "F", which allows fission reaction to proceed and generate heat in the core due to the bombardment of neutrons. To cool the reactor core as well as obtain heat useful for generating electrical power, molten sodium as a reactor coolant is circulated by pump 15 into the reactor 11 where it passes first downwardly in an annular space between the core 13 and the wall of reactor 11 and then upwardly through the passageways 14 of the reactor core 13, and via line 16 to the heat exchanger 12. The reactor sodium gives up heat to a second stream of molten sodium isolated in heat exchanger coil 17 and is circulated then back to reactor 11 through pump 15. The surface level of the molten sodium in the reactor 11 is indicated by dashed line 18, and a cover gas fills the space 19 over the sodium in the reactor. The heater second stream of sodium in heat exchanger coil 17 is circulated by pump 21 through steam generator 22 where in coil 23 it gives off its heat to water to produce steam. The water level in the steam generator 22 is indicated by the dashed line at 24, and the steam in the space 25 above the water surface passes through line 26 to turbine 27 which drives electrical generator 28. Steam leaving turbine 27 is condensed in condenser 29 and pumped by pump 30 through line 31 back to the steam generator 22. The reactor 11 and heat exchanger 12 have been greatly simplified in FIG. 1 for clarity, where in practice they are mechanically complex. From time to time it is necessary to open the reactor for refueling. In the liquid metal cooled reactor, these operations are difficult because the sodium must be maintained above its melting temperature and is a hazardous material to handle. Radioactive material leaking from the nuclear fuel in core 13 will contaminate the sodium coolant, heat exchanger 12 and associated pumps and piping; the radioactive contamination making these maintenance operations even more difficult. As noted, FIG. 4 shows a schematic representation of a horizontal section through the reactor core 13 of FIG. 1 including typical matrix arrangements of the nuclear fuel "F" and the control means "C" therein. The nuclear fuel "F" is generally housed in a fuel assembly 35 (see FIG. 2) shown here having an exterior hexagonal can 37, although any other suitable cross section, such as square rectangular, etc., might be used to fit within the correspondingly shaped core passageways 14. The fuel assembly 35 illustrated is comprised of seven generally parallel separate fuel rods or elements 39 supported in spaced parallel relation within the hexagonal can 37, one element being adjacent each corner of the hexagonal can and one element being at the center. At least two grid supports 40 (only one being shown) are used for holding the separate fuel elements 39 within the can 37, the grid support illustrated including a band 42 secured to the periphery of the can 37 and cross webs 43 which connect between the band 42 and separate collars 45 located on the individual fuel elements. The can 37 is open at its opposite ends (not shown) so that coolant can be readily passed axially through the can from one open end to the other within the core passageway, and the open spacing between the webs allow the coolant to pass axially along and over the fuel elements 39. FIG. 3 illustrates a typical fuel element 39, which in a commercial reactor would consist of cylindrical can or cladding 52 extended between two and six feet in length and having maybe only 0.25" to 0.5" inside diameter. A plurality fuel pellets 54, each likewise of generally cylindrical shape, are fitted within the can or cladding 52 stacked solid one against the other endwise for almost the entire length of the cladding. The cladding is closed by end caps 56 and 57 welded to the opposite ends thereof, where further there is provided a perforated intermediate wall 59 which is located adjacent the end cap 57. Spring devices 60a, 60b are interposed between the end cap 56 and the perforated intermediate wall 59, and the endmost fuel element pellets 54a, 54b operable to bias the fuel pellets snuggly against one another. The fuel pellets 54 themselves are of a smaller overall outer diameter than the inner diameter of the cladding 52 so that some radial clearance is provided and gas migration can occur axially along and within the fuel element 39 from one end to the other. The space 62 between the end cap 57 and intermediate wall 59 is known as the gas plenum and initially during fabrication this plenum 62 is pressurized with tag gases in a preprogrammed proportion. As noted, the fuel element 39 is sealed so that the tagging gas and all fusion gases generated by later reaction of the fuel pellets 54 are confined within the fuel element, but can migrate freely along the entire length of the fuel element. In accordance with the disclosed invention, a cover gas clean up system 64 is connected by lines 65 and 67 and pump 68 from cover gas space 19 and returned by line 69 back to the reactor space 19. A tag recovery and analysis system 72 is located downstream of the cleanup system 64, off tee 75 in line 69, and thus sees only the cover gas cleaned of the fission gases. A fission gas detector or alarm system 78 is included with pump 79, typically in a parallel hookup with the cover gas cleanup system 64, and is used continuously to detect for the presence of fission gases in the cover gas. Thus, operation of the pump 79 provides a small continuous sample of the cover gas for analysis by the detector system 78. The cover gas cleanup system 64 may operate continuously upon operation of pump 68, but otherwise it will be operated upon detection of fission gases in the cover gas. The tag recovery and analysis system 72 includes a means for recovering the tags and a means for analyzing the recovered tags. The recovery system is separated from the gas return line 69 by a valve 80, and is connected also then through valve 82 and line 83 back to the reactor via line 69. The analysis apparatus 84 is connected through valve 86 off of the recovery system. The tag recovery system is operated with the valves 80 and 82 open and valve 86 closed, and only when fission gases are detected in the cover gas and only when the cover gas cleanup system is operated. Thus all of the fission gases are removed in the cover gas clean up system 64, and a portion of the cleaned cover gas is continuously or intermittently passed through the tag recovery system 72. The tag analysis system 84 is operated generally with the valves 80 and 82 closed and valve 86 opened, and typically includes a mass spectrometer (not shown) to appraise of the specific tags present. The output of the mass spectrometer frequently is printed in graphic form. Only a small fraction of the cover gas passed through the tag recovery and analysis system 72 is consumed in the analysis, the remainder is returned to cover gas space 19 via lines 83 and 69. Both the cover gas cleanup system 64 and the tag recovery system 72 would include a bed of charcoal through which the gas including the impurities to be removed or recovery are passed. The specific construction of the charcoal bed is immaterial to this invention (being shown as 64a and 72a in the schematic), but each is operated at a specific cryogenic temperature. Thus the fission gases will be absorbed out of the cover gas in cryogenic bed 64a and the tag gases of neon and argon will be adsorbed out of the cover gas in the cryogenic bed 72a. In this invention, it is contemplated to use helium as the cover gas, and neon and argon as the tag gases. Specifically, the three stable isotopes of neon, namely, Ne.sup.20, Ne.sup.21, and Ne.sup.22, and the three stable isotopes of argon namely, Ar.sup.36, Ar.sup.38 and Ar.sup.40 are to be used where by concentrating the percentage of any one or more of such tag isotopes, it would be possible to define many separate, unique and distinct proportions of tagging combinations. The tag combinations can be comprised solely of the neon isotopes, solely of the argon isotopes, or blends of each. The separate combinations of tags will be injected into the fuel elements where all fuel elements of a specific fuel assembly would have the same tag, and the differently-tagged fuel assemblies would be at the different locations within the reactor matrix. Thus, the later specific identification of such a tag gas combination in the mass spectrometer would provide the unique identity and thus the location of the "leaker" fuel element. The separation by abundance of the varying isotopes in a mass spectrometer is all conventional. In the normal operation, the cover gas cleanup system cryogenic bed 64a would be operated between approximately 0.degree. and -25.degree. C., for example, while the tag gas recovery bed 72a would be operating in the range of -170.degree. to -185.degree. C. The cover gas along with any tag gases and fission gases would be directed through the cyrogenic charcoal bed 64a and via line 69 back to the reactor. Impurities including the fission gases would be adsorbed from the cover gas in the cover gas cleanup system bed 64a. Part of the purified cover gas (still including the tags) would then be passed through the tag recovery charcoal bed 72a, which is effective to adsorb from the cover gas the neon and argon tag gas isotopes used, while the cover gas of helium passes back to the reactor. In a preferred embodiment, the tag recovery and analysis system 72 is made up of three separate similarly arranged series of beds 72a and analysis apparatus 84 (only the one being shown) so that each series can be operated by itself and can be alternately in a batch format, while yet providing for continuous collection. Thus, while one series of apparatus is operating at cryogenic temperatures collecting the gases, the second is being heated to perhaps between 150.degree. and 200.degree. C. to drive off the collected gases for transfer to the analysis apparatus, and the third is being cooled down to the cryogenic temperatures for collecting again. In like manner multiple cover gas cleanup beds 64a can be used (only one bed being shown) to allow regeneration and/or even replacement of an individual bed while the others of the overall system are yet operating. Flow through each tag recovery and analysis system bed or series of beds, and/or through the cover gas clean up system beds will normally be controlled automatically by a timed/temperature sequence of valve operation. Also, a preferred tag recovery bed 72a might actually include a primary tag bed and a secondary tag bed (neither being shown specifically) located in a series flow connection, whereby the collected tag isotopes in the primary bed would be driven off by heat and recollected in a higher concentration in the secondary bed held again at approximately -170.degree. to -185.degree. C. Transfer from the primary tag bed to the secondary tag bed would begin when the valving had been shifted to redirect the cover gas to the next sequential primary bed and the cover gas flow into the original primary bed had been stopped. Clean helium gas could be admitted to backflush the charcoal in the primary tag bed while it is being cooled down. The effluent from the tag bed can be directed to a precooled evacuated sample vial or the like (not shown) in the analysis apparatus 84. The sample vial could either be removed from the system and transported in a shielded container to a laboratory for analysis, or the sample could be directed to an on-line mass spectrometer (not shown). Liquid nitrogen normally surrounding the sample vial would be boiled off to increase the vial temperature to about 50.degree. C. to drive the tag gases through the mass spectrometer. The identity of adsorbed tag gases can be used then with the matrix mapping of the specifically tagged fuel assemblies within the reactor to locate the leaking fuel element. Of particular interest to this invention is the fact that the cover gas cleanup system 64 and the tag recovery and analysis system 72 are operating independently of one another and do not compete with one another. This is of particular importance as the fission gases that escape into the cover gas are different from any of the tag gases. Moreover, the cover gas cleanup system bed 64a is in a series connection with and is operated upstream of the tag recovery system bed 72a and can be operated most efficiently toward removing the greatest percentage of such fission gases. The tag gases of neon and argon are not significantly adsorbed by the cleanup system bed 64a but pass with the cleaned cover gas of helium on through to the tag recovery bed 72a. On the other hand, the downstream tag recovery bed 72a, being operated at much cooler cryogenic temperatures (-170.degree. to -185.degree. C. versus 0.degree. to -25.degree. C.) collect the lighter tag gases of neon and argon but allow the cleaned cover gas of helium to pass through and back to the reactor. One major advantage of the disclosed reactor system having the helium cover gas and the unique blends of neon and argon tag isotopes is the overall lower cost of the system. In fact, it may be only 20-30% of the cost of an overall system having argon cover gas and the xenon and krypton tags, as is more conventionally used. Another advantage of this disclosed system is that the isotopes of neon and argon are very stable and thus are not significantly changed by neutron bombardment, so that tag ratio consistency and vertification can be more easily and reliably maintained. Another major advantage of this cover gas-tag combination is the ability to more accurately resolve the neon and argon isotopes, as compared to the resolution of the heavier xenon and krypton isotope tags. Also, the recovery and analysis efforts are performed on gases that have been cleaned of radioactive fission gases to reduce personnel exposure to the radioactive isotopes of xenon and krypton. Of perhaps greatest importance, however is the fact that the cover cleanup system and the tagging gas recovery system do not compete for the same gases and/or isotopes, but operate independently of one another, so that the sensitivity and accuracy of the tagging system and the effectiveness of the cover gas cleanup system each can be greatly enhanced.