US3892969A - Neutron detector with gamma compensated cable - Google Patents

Abstract

An illustrative embodiment of the invention describes a technique for essentially eliminating the radiation induced background currents that are generated in the cable that connects an ''''in-core'''' neutron detector to an electrical terminal that is outside of the reactor''s radiation field. This undesirable radiation-induced cable current is suppressed through an appropriate selection of conductor and cable sheath materials and sizes that generally satisfy the equation:

Description

United States atet [191 Warren 3,892,969 July 1, 1975 Primary ExaminerHarold A. Dixon Attorney, Agent, or Firm-J. M. Maguire, Esq.; J. P. Sinnott, Esq.

571 I ABSTRACT An illustrative embodiment of the invention describes a technique for essentially eliminating the radiation induced background currents that are generated in the cable that connects an in-core neutron detector to an electrical terminal that is outside of the reactors radiation field. This undesirable radiation-induced cable current is suppressed through an appropriate selection of conductor and cable sheath materials and sizes that generally satisfy the equation:

z z s s where Z is the atomic number of the material; d is a characteristic of vthe size of the cable component; m and/n have values between 1 and 5 to express the electron emissivity of the cable component from photoelectric and Compton effects; 1 represents the conductor; and '3 represents the sheath. Thus, the radiation-generated electrons emitted from the conductor and the oppositely-directed electrons emitted from the inner surface of the cable sheath are mutually cancelled if thisequation is satisfied. A typical cable that does meet this criterion at low temperatures has a centrally disposed Zircaloy-2 inner conductor of 0.011 inch diameter, an annular insulation of magnesium oxide powder compacted to 100% density, and an Inconel sheath with an outside diameter of 0.062 inch and 0.011 inch wall thickness.

7 Claims, 1 Drawing Figure 1 NEUTRON DETECTOR WITH GAMMA COMPENSATED CABLE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electrical conductors, and, more particularly, to method and apparatus for reducing the radiation-induced electrical currents that are generated in the cables which transmit signals from the neutron detectors in a nuclear reactor, and the like.

2. Description of the Prior Art In order to operate a nuclear reactor at maximum safety and efficiency, the neutron distribution within the reactor core must be closely observed. To provide information with respect to these nuclear particles detectors are positioned in specific locations within the reactor core. This array of detectors should generate electrical signals that provide an almost instantaneous map or indication of the overall neutron distribution within the core, as Well as any local departures in this distribution from anticipated intensities.

Typically, a self-powered detector of this sort has a centrally disposed emitter that responds to neutron radiation by emitting relatively high energy electrons (beta particles"). The energy of these electrons is sufficient to traverse a sleeve or annulus of insulating material that encloses the centrally disposed emitter to enable these electrons to impact on an electrically conductive outer sheath or collector. Generally speaking, the intensity of incident neutrons is related to beta particle production. Consequently, the electrical current, which is a way of expressing electron flow through a conductor per unit time, provides a measure of the neutron population at the place of measurement within the reactor.

In any event,to carry the detector signal out of the reactor core, the detector is coupled to instruments in a control panel by means of a coaxial cable. Usually these cables have a centrally disposed leadwire that is electrically isolated, or insulated, from an encircling electrically conducting sheath. In order to avoid producing neutron-induced electrons, and thereby converting the entire cable-length into a neutron detector, the cable materials are chosen from a group of materials that do not exhibit neutron sensitivity and exhibit a low probability for neutron-electron reactions. Substantiallengths of this connecting coaxial cable are nevertheless exposed to the gamma radiation that is generated within the core.. This radiation, of course, produces further undesirable currents that are not related to the neutron population at the respective points of measurement through the Compton electron generation mechanism and other gamma ray-electron phenomena, of which the photoelectric" and thermionic emission phenomena are typical. a

A number of techniques have been devised to account for or to overcome these misleading signals that are induced in the cables. For example, in oneinstance the array of detectors that is lodged in a reactor core is provided with at least one cable that is not connected to a neutron detector. This unconnected cable produces a current that is subtracted from the other incore detector signals. This cable response. subtracted either manually or through automatic computation from each of the observed detector. signals, approximately eliminates that portion of the uncorrected detector signal that is attributable to gamma ray induced electrical currents in the detectors cable.

Another technique to compensate for these induced cable currents makes use of a single cable that has two insulated leadwires which are twisted about each other. One of the leadwires is connected to a neutron detector to provide a combined neutron detector and cable" signal. The other leadwire is not connected to a detector and thus generates only a background cable signal. Algebraic subtraction of these two signals should provide a result that is essentially an indication of the neutron population at the place of measurement within the reactor core. All of these correction systems require at least one extra leadwire and a provision for an algebraic subtraction, or its equivalent.

These approaches to the background signal compensation problem require additional materials, labor and computer capacity. By introducing these factors, the neutron population measurement system is rendered somewhat more vulnerable to fault. Clearly, there is a need for a more reliable and efficient background cable signal compensation technique;

SUMMARY OF THE INVENTION In accordance with the invention, the foregoing difficulties that have characterized the prior art are, to a large extent, overcome. More particularly, through a proper selection in accordance with the principles of the invention of the leadwire and sheath materials and sizes that are to be used in the detector cable, the gamma radiation induced cable currents are essentially cancelled.-

The signal from a neutron detector coupled: to a cable built in accordance with these principles generally reflects the reactor core neutron population at the place of measurement to the exclusion of any straycurrents induced in the cable through gamma ray interactions with the leadwireand sheath materials. More specifically, the electrons that are released from the sheath and pass through the interposed insulator to the leadwire produce a net negative current. Those electrons that flow in an opposite direction from the leadwire to the sheath produce inturn, a net positive current when measured at the sheath and leadwire terminals. ln accordance with a feature of the invention, it has been noted that this gamma radiation induced electron emission is almost proportional to the area of the emitting surface. It also has been observed that these electron emissions are proportional to some power of the atomic number (Z) of the element that is subjected to gamma ray irradiation. In this latter instance photoelectric reactions are proportional to the fifth power of Z and Compton electrons are proportional only to Z. Consequently, electron emissivity is related not only to the surface areas of the sheath and the leadwire but also to some power between I and 5 of the respective atomic numbers of these cable components.

.Thus, through an application of the invention, a relatively low Z material is chosen for the larger surface area sheath and a higher Z material for the smaller surface leadwire. In these circumstances the oppositely directed gamma ray induced electrical currents are brought into an essentially matching balance or mutual cancellation that approaches a zero net background current.

V Because these gamma ray effects are most marked in dense materialssteel, nickel, and the like, gamma ray interactions with the low density insulating material (e.g. magnesium oxide, alumina) are relatively infrequent and make a negligible contribution to background electron production within the cable. In this situation the current that is observed at the cable terminal is a measure of the neutron population in the vicinity of the detectorfto the automatic exclusion of background or stray currents that may have been induced through gamma ray interactions with the cable materials.

A number of cables that specifically embody these principles of the invention can be made from an lnco nel 600 sheath that has an outside diameter of 0.062 inch and an inside diameter of 0.040 inch. A Zircaloy-2 leadwire having a 0.01 1 inch diameter that is centrally disposed within and coaxial with the lnconel 600 sheath in an insulating matrix of magnesium oxide, for example, has been found to produce a background current of no more than 7 X 10 amperes per neutron per square centimeter per second per inch of cable (7 X 10 amps/nv-inch) in an average neutron flux of about 1.6 X 10 nv, where the unit nv represents neutron flux in terms of neutron/(cm -sec). This current is less by ,a factor of 20 than the current induced in a cable of identical characteristics (save for an lnconel 600 leadwire) in a similar neutron flux.

Similar stray, or background current reduction also has been observed through experiments conducted with a 0.053 inch outside diameter lnconel 600 sheath (0.007 inch wall thickness) a compacted magnesium oxide insulator and a 0.0075 inch diameter, coaxial zirconium leadwire.

From the result of experiments conducted to verify the principles of this invention, it can be concluded that for any given sheath size and material there should exist a particular diameter leadwire of higher atomic number material'that will result in gamma radiation induced background current cancellation. Naturally, the converse of this observation also should be true.'Thus, it 40 appears that for any given leadwire diameter and material, there should exist a particular size sheath of lower atomic number material that will result in a mutual cancellation of the gamma radiation induced background BRIEF DESCRIPTION OF THE. DRAWING The sole FIGURE of the drawing shows in broken section a portion of a cable in accordance with the invention.

' DESCRIPTION OF THE PREFERRED EMBODIMENTS For a more complete appreciation of the invention,

'it will be recalled that the two dominant physical char acteristics that bear on the production of gamma radiation induced background currents in neutron detector cable are:

where the subscript 1 identifies the leadwire, d represents the leadwire diameter, and the exponent n de- 15 pends on the relative contribution of the photoelectric and Compton effects to the background current. In this circumstance the exponent n must be some number between 1 and 5. Z, of course, is the atomic number of the leadwire. v

The number of electrons leaving the cable sheath, moreover, also are proportional to:

wherethe subscript s identifies the sheath and m depends on the relative contributions of the photoelectric and Compton effects'toward electron emission.

To establish the mathematical equivalent of equal 30 and oppositely flowing currents between the sheath and the leadwire expressions I and II are equated:

' 0 z," d, z," d III In these circumstances, it is clear that a zero condition can be produced for the case of a necessarily small diameter leadwire within a larger diameter sheath through some manipulation of the respective atomic numbers (21, Zs) and the associated exponents (n, m). Specifically, the net effect of the atomic number and the electron emission exponent that characterizes the sheath material must be low, relative to the atomic number and electron emission exponent of the leadwire in order to mutually cancel the background currents that are generated in response to gamma radiation.

As in almost every practical improveme nt,' ther e are other phenomena that tend to modify the simple physical relation that is expressed in equation lll. For example, some electrons are almost certain to be produced as a result of neutron interactions with the cable materials. Electrons generated through neutron reactions, however, are beyond the terms of the gamma radiation phenomena that are described in equation Ill. Conse- 55 quently, in accordance with an additional featureof the invention, cable materials should be chosen not only to satisfy equation III, but also with a view toward those materials 'that enjoy low probabilities for neutronelectr'onneactions. Because, after equilibrium'condit ions arereached in the cable, neutron reactions are almost'vcertain to make some contribution toward the over-all electron emission in spite of the most careful materials selection. Accordingly, it is desirable to use a leadwire diameter that is about one thousandth of an inch larger than the theoretical optimum diameter that is' predictedthrough equation III. If the cable is to be used at higher temperatures that characterize power reactor systems e.'g. 600 F. thermionc'allyemitted electrons also add to the undesired cable curren,t;., To compensate for this -fufthersource of error, anadditibnal thousandth of an inch should be added to the units leadwire diameter. This slight increase in the leadwire diameter will pro'ducea positive gamma radiationindu'ced electron current th'at'will' tend to offset'the oppositely 'flowing" negative current of thermionicallyemitted electrons' from the sheath. I

In addition to the need to depart from the ideal relation that is defined in equation III by reason of the inherent nuclear characteristics of the potential cable materials, the're isa further need to select materials that are metallurgically and mechanically compatible. For example, the properties of the sheath and leadwire materials may be so different that it could be extremely difficult to anneal drawn or swaged cable.

In order to verify the principles of the invention, a number of cables were built in which a sheath formed of Inconel 600 having a 0.062 inch outside diameter and a 0.040 inch inside diameter was used in conjunction with a leadwire 11 formed from zirconium (Zircaloy-2) and having diameters in the range of 0.0085 inches up to 0.025. An insulator 12 of magnesium oxide was compacted in the annular space between the leadwire 11 and the enclosing sheath l0. Naturally, neutron reactions with the insulation mate-. rial are also to be avoided 'as much as possible. Consequently, materials'of which alumina (A1 0 and magnesiumoxide (MgO) are typical, frequently are chosen for this purpose because, among other reasons, the chance for neutron absorption. (macroscopic cross section) is low, being on the order of 0.01 1 cm and 0.0034 cm", respectively, when based on the theoretical densities of these materials.

lnconel 600. has, a typical, composition, by weight, of 76.5 parts nickel; 14.5 parts chromium; 8.2 parts iron; .19 parts copper; .26 parts silicon; .007 parts sulfur; .25 parts manganese; and .03 parts carbon; Preferably, the manganese concentration should be reduced to,.l part byweight or less and cobalt, although not shown in the foregoing illustrative, composition, also should be avoided in order to reduce to a minimum the major source of undesired neutron, induced electron emissions. Naturally,trace amounts of otherelements also can be present. 4

Zircaloy-2 typically comprises Zirconium with a balance of 1.5% tin; .12% iron; .ll% chromium; 06% nickel, and as many as ten other elements present in trace concentrations, of which aluminium, boron, carbon, copper, and hafnium are illustrative.

Each of the cables, of which the sole FIGURE of the drawing is illustrative, were tested in the neutron and gamma ray environment established within a one megawatt pool-type reactor'at less than 300 "F. The background currents produced in each of the tested cables as a result of this'ne utron and gamma ray exposure was observed during a period of about minutes. It was found that the 0.01 1 inch diameter leadwire cable, exposed to an average neutron flux of about 1.6 X 10 nvwithin the reactor environment generated not more than 7 X 10 amps/nv-inch. This induced current is a factor of about less than the background current generated in a conventional cable containing an 0.009 inch diameter Inconel leadwire. As hereinbefore mentioned, additional sources of undesired electron generation of which thermionic emission and neutron in 0. 0l3 inches has been, found appropriates? Y i l early, the principles of the invention apply to other materials ,a'nd cable shapes aswell as mother sheath and leadwire size combinations. As hereinbefore mentioned,. for, aparticular setof sheath and leadwire materials, there is a particular set of sheath and leadwire di-,

ameters that result in maximum background current cancellation. In this respect, it is illustrative toreiterate one of the examples described in. the introduction,.--in which a coaxial cable having an Inconel 600 sheath of 0.053 inch outsidediameter and an 0..007.inch.thick' wall witha zirconium leadwire of 0.0075 inch diameter. isinsulated from the sheath with compacted magnesium oxide. This cable was tested and exhibited a gamma radiation induced background electron sensi-, tivity similar to the cable with the 0.01 l inch-leadwire described above. As hereinbefore indicated, to.compensate for neutron effectsat equilibrium conditions within the cable in relation to its environment, a thousandth of an inch should be added to this leadwire diameter. Also, if operation at higher temperatures, is contemplated, an additional thousandth of an inch should be added to the diameter to overcome the influence of thermionically emitted electrons. Thus, a .0095 inch diameter leadwire would be appropriate to. these circumstances.

It should be noted further that different .neutron fluxes and associated gamma ray activities are likely-to produce, minor second and third order background-current effects of varying significance. The general relationship, however, that is disclosed in equation III provides the broad, basic relation between sheath and leadwire sizes and materials which are subject to some optimization that can be identified through relatively minor testing. Withrespect to conductor shapesother than the coaxial configuration described above, .the factor d in equation III, instead of relating specifically to diameter, refers rather to a characteristic of the. size of the cable component, e.g. width, diagonal length; or the like. T he atomic number, Z, moreovenalthough being unique for each element in the periodic table, is,,

in the case of the alloys described herein as well as for other compositions, a weighted number that is basedon the individual atomic numbers and the relative proporlows:

1. An electrical conductor comprising a sheath havtions of all of the constituents of the composition under ing a material that has a low probability for neutronelectron reactions, a leadwire within said sheath, said leadwire being formed from another material that also has a low probability for neutron-electron reactions, said leadwire material also having anatomic number that is larger than the atomic number of the sheath material, in order to generally satisfy the equation in which I identifies said leadwire, s identifies said sheath, d is a size characteristic, Z is the atomic num- 5 A conductor according to claim 4 wherein said In- 'conel 600 sheath further comprises an inside diameter of 0.040 inches.

6.- A conductor according to claim 4 wherein said leadwire further comprises a diameter in the range between 0.01 l and 0.013 inches.

7. A conductor according to claim 4 wherein said Inconel 600 sheath further comprises an outside diameter of 0.053 inches and a 0.007 inch thick wall and said leadwire has a diameter in the range between 0.0075 and 0.0095 inches.

Claims (7)

1. An electrical conductor comprising a sheath having a material that has a low probability for neutron-electron reactions, a leadwire within said sheath, said leadwire being formed from another material that also has a low probability for neutronelectron reactions, said leadwire material also having an atomic number that is larger than the atomic number of the sheath material, in order to generally satisfy the equation O Zln dl Zsm d s, in which l identifies said leadwire, s identifies said sheath, d is a size characteristic, Z is the atomic number, and n and m are exponents that reflect the electron emissivities of said leadwire and said sheath respectively in response to photoelectric and ''''Compton'''' phenomena.

2. A conductor according to claim 1 wherein an electrical insulator having low neutron absorbtion cross section is interposed between said sheath and said leadwire.

3. A conductor according to claim 2 wherein said sheath material is composed of Inconel 600.

4. A conductor according to claim 3 wherein said leadwire is composed of Zircaloy-2.

5. A conductor according to claim 4 wherein said Inconel 600 sheath further comprises an inside diameter of 0.040 inches.

6. A conductor according to claim 4 wherein said leadwire further comprises a diameter in the range between 0.011 and 0.013 inches.

7. A conductor according to claim 4 wherein said Inconel 600 sheath further comprises an outside diameter of 0.053 inches and a 0.007 inch thick wall and said leadwire has a diameter in the range between 0.0075 and 0.0095 inches.

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