Patent Number: 046577297
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Solid materials can be inserted into reactor elements before they are sealed that generate detectable gases on exposure to the neutron flux within a nuclear reactor to signal the occurrence of a leak or failure in the particular element. By using different solid tag materials and mixtures of different materials, particular reactor elements, such as specific tube assemblies, can be identified as being the source of a leak or failure. Elements such as the halogens, Group VIIA of the periodic table, generate radioisotopes when exposed to a neutron flux. These isotopes undergo beta decay to become noble gases. Examples of preferred embodiments of the process using halogens as solid tags are: EQU .sup.79 Br(n,g).sup.80 Br .beta..fwdarw..sup.80 Kr A. EQU .sup.81 Br(n,g).sup.82 Br .beta..fwdarw..sup.82 Kr B. EQU .sup.127 I(n,g).sup.128 I .beta..fwdarw..sup.128 Xe C. EQU .sup.37 Cl(n,g).sup.38 Cl .beta..fwdarw..sup.38 Ar D. EQU .sup.19 F(n.g).sup.20 F .beta..fwdarw..sup.20 Ne E. Although any solid material which produces a detectable gas may be used, the preferred embodiments of the invention are limited to those which result in detectable gases that are not produced to any significant degree during reactor operation. That is, materials which result in fission products cannot easily be distinguished from the background material found in the cover gas after a fuel element ruptures. For example, in a sodium cooled reactor, the use of fluorine may not be useful because sodium is also converted to neon. Similarly, .sup.127 I and .sup.81 Br may not be useful in every case as they may also be fission products. It is possible, however, in another embodiment of the invention, that a tag could be used which is similar to naturally occurring background constituents found in the cover gas as fission products if the background levels are recorded through constant or periodic monitoring. In such an embodiment, a significant departure from a recently measured or anticipated background level would indicate a component failure or leak. A procedure dependent on departures from known background levels used to indicate problems would be particularly useful in situations where there has been some build-up of gaseous isotopes from earlier tagged reactor component failures or other accountable sources. Gaseous tags are generated from solid materials according to the equation: EQU Atoms of tag gas=.phi..sigma.N where .phi.= ##EQU1## .sigma.=cross section (cm.sup.2 /atom N=atoms of target material For example, in the reaction of Br to Kr, neglecting bromine depletion, one gram of Br will yield somewhat more than 1/3 cc (STP) of Kr tag gas in 10 days in a typical liquid metal fast breeder reactor environment. EQU N=approximately 0.008.times.10.sup.24 atoms of Br/g EQU .phi.=approximately 10.sup.15 neutrons/cm.sup.2 .times.sec (typical fast breeder reactor) EQU .sigma.=approximately 10.sup.-24 cm.sup.2 /atom (typical fast breeder reactor) Approximately 10.sup.18 atoms of tag gas are generated each day. At STP one cc of Kr=2.76.times.10.sup.19 atoms. Therefore, approximately 0.37 cc of Kr at STP are generated from 1 g. Br in 10 days in this example. Although the above example is based on characteristics of fast breeder reactors, the generation rate of isotopic tag gases in a typical thermal reactor would be on the same order of magnitude. Although the neutron flux may be 10 to 50 fold smaller in a thermal reactor, its cross section would be larger by the same factor. Hence, the present invention may be used equally well in a thermal reactor. The production of detectable gaseous isotopes is linear with time, which is evident from the above equation. Starting with, for example, one gram Br, approximately 0.037 cc (STP) Br will be generated on the first day, and on every day thereafter. A leak in a reactor component will be detected within a fraction of the first day based on the use of one gram amounts of a tag material. Early detection can be assured, of course, by using slightly greater quantities of the tag material. The preferred method for testing the cover gas for tag gas is with a mass spectrometer. Conventionally, the cover gas is monitored by continuously passing a sample through a device for detecting fission products, i.e., radioactivity. When fission products are detected, a sample is assayed with a mass spectrometer to identify the isotope and determine the isotopic ratio. A mass spectrometer identifies identically charged ions having different masses by measuring differences in their deflection in electrical and magnetic fields. The quantity of gas generated from one gram of solid tag material in 10 days is easily detected. Detection of lesser concentrations can be enhanced using cryogenic techniques and concentrating the tag gas in an activated charcoal filter. Using a mass spectrometer with these methods, very small amounts of tag gases, present in amounts as low as 10.sup.-11, can be detected, identified, and accurately measured; thus enabling the early detection of a leak or component failure and identifying the source of the problem. In a typical reactor, a one gram amount of solid tag materials should generate sufficient isotopic tag gases to be detectable in the cover gas within a few hours. Requirements of a tag material, in addition to being detectable and measurable in contrast to the background, include the characteristics of not affecting the neutron flux adversely, not being further changed from the identifiable product on continued exposure to radiation, and not being significantly soluble in the reactor coolant. The solid tag materials of the present invention may be used in conjunction with the gas tags used in the prior art, if necessary, to obtain a greater variety of isotopic ratios. Moreover, failures which occur at the time a fuel assembly or other tagged reactor component is placed into the reactor would be signalled by the immediate detection of the gaseous tag. The solid tag alone requires a finite time to generate sufficient gaseous isotopes to be detected. The solid tag materials may be introduced as a salt of the desired element. For example, the sodium, potassium, calcium, iron or nickel salts of a halogen isotope tags. The salts may be introduced in the form of pellets or as powder or granules in containers which are fairly non-reactive in the fuel rod environment. The tag salts selected must consist of predetermined ratios of particular isotopes. For example, salts consisting of known ratios of .sup.79 Br.sup.- and .sup.81 Br.sup.- may be used to generate the tag gases comprising known ratios of .sup.80 Kr and .sup.82 Kr. Even though the element is in the form of an ion in the salt, it will be transmuted in the neutron flux to the noble gas. Similarly, salts having particular proportions of .sup.35 Cl and .sup.37 Cl will generate .sup.36 Ar and .sup.38 Ar in unique ratios quite different from the ratio of these isotopes found in natural argon. Mixtures of different elements may also be employed to establish unique tags. The same isotopic ratios of chloride could be used with or without iodine salts to double the number of different tags available. Any cation component may be used in the solid salt used for tagging as it is irrelevant to generating the isotopic gas. The commonly available sodium, potassium, or calcium salts may be used. Also, ion salts and similar salts of metals which are fairly unreactive in their elemental form may be preferred in some applications. The invention provides a method for tagging reactor components with solid salts that are inexpensive to use and are easily handled. It is believed that the cost of tagging reactor components will be reduced one hundred fold using these salts. Moreover, as the isotopic salts are stable, the solid tags can be produced on a large scale and inventoried at a central facility, then shipped to reactor sites as needed. In addition, as physically small amounts of solid salts quickly generate measurable quantities of isotopic gas, units of different salts can be combined at the reactor site as required to obtain a variety of different gaseous isotope ratios. The above embodiments are presented as examples to illustrate the invention without intending to limit the invention thereby. It will be understood that the present description is susceptible to various modifications, changes, and adaptations within the invention defined by the following claims.