Patent Number: 056087675
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 2A and 2B, the source assembly comprises a metallic collector 26 in the form of a hollow flat disk of metal containing the .beta.-emitting isotope material 28, which is electrically isolated from the metallic collector 26 by ceramic stand-off 30 and ceramic feed-through 32. Preferably, the .beta.-emitting isotope material 28 is centrally arranged inside the collector 26. A first electrical lead 34 is connected to the .beta.-emitting material 28 and penetrates the ceramic stand-off 30. A second electrical lead 36 has an end connected to the metallic collector 26. In accordance with one preferred embodiment, the .beta.-emitting isotope material 28 is formed as a solid thin disk. Alternatively, the .beta.-emitting isotope material 28 is deposited on a substrate formed as a solid thin disk, which substrate may be made of material which is not a .beta.-emitter. Also, the ceramic material may be replaced by any other suitable electrically insulating material able to withstand the thermal and radiological conditions of the reactor environment. In accordance with the preferred embodiment of the invention, the .beta.-emitting radioisotope is derived from neutron-activated dysprosium, Dy.sup.164. The neutron-activated partial decay scheme of pure Dy.sup.164 is as follows: ##STR1## The notation used above is as follows: n.sub.th, thermal neutron; n.sub.f, epithermal neutron at the central energy of an absorption resonance; b, barns; h, hours; d, days; m, months; and y, years. All reactions subsequent to the decay of Ho.sup.166 have been neglected, although Er.sup.166 can be neutron-activated to produce some weak .beta.-emission. This chain displays promising properties, such as substantial activation cross sections and resonance integrals, moderate decay constants, energetic .beta.-particles and high .beta.-yields. There are four .beta.-particles emitted in the chain of each Dy.sup.164 nucleus, another favorable property. Dy.sup.164 is a naturally occurring, stable isotope of the rare-earth element dysprosium, found in the ratio of 28.1%. The pure metal is soft and malleable with density 8.55 gm/cc and melting point 1412.degree. C. It is stable in air at room temperature and non-toxic. These properties make it easily fabricatable as thin foils and ideal for use in devices contained in nuclear reactors. The .beta.-battery of the present invention is intended for use inside the reactor pressure vessel of a nuclear reactor, on the periphery or just outside the nuclear fuel core, where exposure to substantial neutron flux will activate the .beta.-emitting material to produce electrons. In accordance with an alternative preferred embodiment of the present invention, the cell shown in FIGS. 2A and 2B may be replicated many times and connected together ("sandwiched") to provide adequate current for conversion to voltage in practical applications. FIG. 3 is a schematic representation of this configuration for five unit cells, although 20-30 is a more typical number. In each cell, a thin emitter foil 28, whose thickness is typically 1.5 mm, is electrically isolated by the thin ceramic disks 30 (thickness typically 0.025 mm). In the example shown, each cell has a thickness of about 1.8 mm. Therefore, a 30-cell battery is about 5.4 cm thick, with an outside diameter of roughly 8 cm. The feed-through ceramics 32 and leads 34, 36 can be deposited by electrodeposition during fabrication. The plurality of cells composing a battery (see FIG. 3) are energized by the nuclear decay electrons that traverse the thin ceramic spacers and reach the collector electrodes. The output current from the common collectors is transmitted by mineral-insulated cable to a small circuit board used to convert variable current to constant voltage. For example, portions of the circuitry 20 shown in FIG. 1 could be placed on a circuit board for current-to-voltage conversion. The source current I.sub.s arising from the collection of nuclear decay electrons from all the emitters produces a voltage across a source resistance R.sub.s (see FIG. 1), which is a slowly decreasing function of time (because of the emitter decay). It should be understood that the source resistance R.sub.s represents the sum of the internal resistance of the current generator 22 and the resistance of a separate resistor. The Zener diode 24 and load resistor R.sub.L stabilize and limit the output voltage B.sub.+ to an appropriate design level, determined by the resistors. The conversion voltage is regulated, since large changes in diode current produce small changes in diode voltage. The resulting voltage across the load resistor is insensitive to the .beta.-emitter decay and can be used to power the active circuit components inside the reactor pressure vessel, without the necessity of external power. The electronic components are fabricated from radiation-hardened semiconductors (e.g., SiC semiconductor devices) capable of withstanding relatively high .gamma.-radiation fields encountered inside the reactor pressure vessel, but outside the core region. The circuit device is not intended for use in the core, where neutron fluxes are sufficiently high to destroy the electronic components. In accordance with the preferred embodiment of the present invention, the source of electrons is the decay of radioactive isotopes produced by neutron activation of Dy.sup.164, which is formed as a thin foil held in place between adjacent ceramic members that are very thin. The emitter foil is electrically isolated from the metallic collector by these ceramic insulators. The collector material could be nickel, or a nickel-base alloy, and the ceramics could be alumina to thermally match the metal. These are typical materials, but other possible combinations exist which would allow the device to operate reliably at reactor temperature. This low-voltage DC power supply has a finite lifetime, since the Dy.sup.164 eventually "burns up" to such a low level that it cannot produce a supply voltage sufficient to power the circuit connected to it. This lifetime is determined by the type of application, such as the design requirements of the operational amplifiers in a control circuit. Typically, it is about 3 years at full power in the reactor, if the lowest permissible current is 1 mA. The amount of current density j generated can be estimated from the following formula, which takes account of source decay and self-absorption: EQU j=N(.rho..zeta./A.tau..mu.)e.sup.-.mu..tau. [1-e.sup.-.mu.l ]amp/cm.sup.2 where l is the emitter thickness; .rho. is the emitter density; A is the emitter mass number; .tau. is the emitter mean-life (1.44 t.sub.1/2); .zeta. is the Faraday constant (96487 coulomb/gm-mole); .mu. is the electron absorption coefficient of the emitter/insulator; and N is the number of cells. For a .beta.-battery using dysprosium, the current generated, as a function of time after being placed in reactor service and taking account of source burn-up and self-absorption, is shown for a typical case in FIG. 4. This graph is the algebraic sum of the four separate .beta.-emitters in the decay chain of (n+Dy.sup.164). FIGS. 5-8 show the currents due to each constituent isotope, including the isomeric state of dysprosium, Dy.sup.165m. It is noteworthy that the early currents are due to Dy.sup.165, whereas the late currents are due to the eventual build-up and decay of the isotope Ho.sup.166. The contribution of Dy.sup.165 is negligible, since its .beta.-emission is heavily self-absorbed and its generation rate is low. The rise-time of the total current is very short (not resolved on the scale of the graphs). It turns out to take about a minute to activate the Dy.sup.165 output to a level of 1 mA. Therefore, for all practical purposes, the battery is prompt upon exposure to low levels of neutrons. The voltage produced by the collected .beta.-current is dependent on the resistance through which the current flows. This voltage varies with time in the same way the current varies, for a constant resistor. Using a typical resistance of 4 k.OMEGA., the voltage characteristic of a 30-cell battery is shown in FIG. 9. When the generated voltage is fed through a voltage regulation circuit, the constant voltage shown in the graph is produced. The level of the regulated voltage is determined by design; the values shown are for illustration only. As seen in FIG. 9, battery life for this example is about 3 full-power-years, using an end-of-life criterion of 1 mA output current, which corresponds to 4 volts. The rise of Ho.sup.166 (FIG. 8) peaks at about 32 full-power-months, which is the life extension mechanism. As the voltage sags below 4 volts, the voltage regulator becomes inoperative. The voltage gradually drops as the Ho.sup.166 current decays. FIGS. 5-7 show that all precursors have decayed to insignificant levels of current production when the Ho.sup.166 peak occurs. When reactor Outages occur, or power is reduced, battery life is extended. Battery life is clearly a variable subject to design, within broad limits. The limiting factor in life of this source is the current demand. Effective lifetime and/or peak current capability can be addressed in the emitter design by combining more than one isotope in the proper proportions to give the desired current-time characteristic. In accordance with an alternative preferred embodiment of the invention, the .beta.-battery has a very thin layer of low-density ceramic electroplated on every emitter surface, which is used as a substrate. Then, the ceramic surfaces are metallized and then electroplated with a metal having suitable electrical conductivity. The metal electroplated cells are then bonded together to form a multi-cell array, an example of which is seen in FIG. 3. In this array, the metallic collectors 26 separate each unit cell and form a bus to which electrical lead 36 is connected. The electrical leads 34 are connected to a bus 38. The feed-through ceramics and leads are also deposited by electrodeposition. Processes and techniques similar to those used in semiconductor device fabrication are available for manufacture of the device. The present invention can be used to power radiation-hardened circuitry located inside the primary pressure boundary of nuclear plants, without the necessity of electrical penetrations. The neutron-activated current generator is expected to be especially useful in operating plants where local protection of sensitized stainless steel components against intergranular stress corrosion cracking requires a long-lived, low-power, freestanding electrical source. The invention also has application in other reactor contexts, such as crud deposition suppression and monitoring of electrochemical corrosion potential. The preferred embodiments have been disclosed for the purpose of illustration only. Variations and modifications of those embodiments will be readily apparent to persons skilled in the art of battery design. For example, it will be appreciated that the .beta.-emitter 28 in FIG. 2A need not be electrically connected and that electrical conductor 34 can be eliminated, in which case the Zener diode 24 will not be connected to the current source via conductor 34, as shown in FIG. 1. Although the electrical conductor 34 prevents the build-up of a space charge which could suppress the ability of electrons to flow out of the emitter, this is not essential to the present invention. All such variations and modifications are intended to be encompassed by the claims appended hereto.