Patent Number: 048184772
Section: description

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT The present invention overcomes difficulties with the prior burnable poison coatings by providing a coating which is effective to prevent PCI without introducing excessive neutron absorbing isotopes into the reactor core. This is basically accomplished by adjusting the amount of neutron absorbing isotope in the coating. For example, if the coating were made of zirconium diboride, and since natural boron contains 18.4 weight percent of the absorbing isotope, boron-10, if the boron-10 in the boron were depleted by an order of magnitude to about 1.84 weight percent, it would permit coating 10 times as many pellets with the same layer thickness of boron as would have been possible if the coating were made from natural boron while introducing essentially no additional neutron absorbing material. Therefore, in accordance with the present invention all or nearly all of the fuel pellets in a reactor core can be coated with zirconium diboride of sufficient thickness to prevent PCI. Once it is determined how thick a pellet coating will be and how much absorbing isotope (boron-10) is needed to almost, but not quite, counteract all of the initial excess reactivity in the core, the only remaining varible is the weight percentage of boron-10 in the boron used to make the zirconium diboride. As detailed below, this number can be readily calculated. Turning now to FIGS. 1 and 2, there is illustrated a longitudinal cross section and a transverse cross section respectively of a typical fuel rod according to the present invention. The figures are schematic and not to scale. In FIGS. 1 and 2 like numerals refer to like components. A typical fuel rod 1 might be one-quarter to one inch in diameter and 8-15 feet long. A tube or cladding, typically about 20-40 mils thick, is represented by numeral 2. End closures or plugs, which may be of many shapes and sizes to facilitate handling, are represented by numerals 3 and 4 respectively. Coating fuel pellets, which usually comprise uranium dioxide or uranium plutonium dioxide, are represented by the numeral 5. The pellets 5 are preferably cylindrical and slightly smaller in diameter than the inside diameter of the cladding 2 in order to facilitate insertion of the pellets in the cladding. For ease of manufacturing and handling, the pellets 5 may be made with a length-to-diameter ratio of about 1.5. A boride coating, preferably zirconium diboride, is represented by the numeral 6. As illustrated in FIG. 3, from about 40-300 fuel rods of the type described above are collected and supported in a rectangular array to produce a fuel assembly 7. From about 40 to about 200 fuel assemblies may be collected and arranged in a generally cylindrical array to form a typical reactor core 10. Referring again to FIGS. 1 and 2, the coating 6 is disposed between the fuel 5 and the cladding 2 as a coating on the fuel. This prevents the cladding 2 from coming into direct contact with the fuel 5 thereby preventing PCI by preventing pellet clad contact. As alluded to above, the phenomena of PCI is believed to cause defects in fuel rod cladding during reactor operation which can be mitigated or eliminated by a layer of sufficient thickness to prevent such contact. It is generally considered that separating the fuel and cladding by a distance of about on the order of 10-100 microns (0.4 to 4 mils) is sufficient to mitigate or prevent the undesirable effects of PCI. The economical use of space and materials tends to provide an upper limit to the use of very thick layers between the fuel and the cladding. If a coating 6 is to function as a layer between the fuel and cladding to prevent the effects of PCI, it must be present in all of nearly all of the pellets in the core. As alluded to above, a typical large reactor might contain on the order of about 50,000 fuel rods each containing about 280 fuel pellets or roughly 14,000,000 pellets. If each of these pellets is coated with a zirconium diboride layer of only 10 microns thick and if the boron used to make the diboride is natural boron containing 18.4 weight percent boron-10, the reactor core would contain about 56,000 grams of boron. This is on the order of 3 to 5 times the amount needed to counteract the excess reactivity, k, of a fresh core. The present invention allows the use of zirconium diboride and other borides as a combination coating on the fuel pellets to provide the functions of PCI resistance and burnable poison by tailoring the isotopic content of the boron used to fit the needs of both functions. By tailoring the amount of boron-10 in the boron used to create the boride layer 6 it is possible to achieve PCI protection while effectively introducing the desired level of burnable poison in a reactor core or portion of a reactor core to effectively control the excess reactivity. Using boron-10 depletion or enrichment as a variable, many options for core design and for fuel pellet coating are now available. The following examples are illustrative, but not exhaustive, of possible coatings: EXAMPLE 1 Consider a reactor core containing 14,000,000 fuel pellets with a fuel enrichment that requires a total of 4500 grams of boron-10 to control its reactivity at startup. For PCI resistance, each pellet is provided with a 25 micron thick coating of zirconium diboride. Each pellet therefore requires 9.8 milligrams of boron, but only 0.32 milligrams of boron-10. Therefore, the boron used to make the fuel pellet coating will advantageously contain only 3.3 weight percent or 3.6 atom percent of the isotope boron-10. EXAMPLE 2 If the same 14,000,000 fuel pellets are coated uniformly with 4500 grams of boron-10, but the zirconium diboride layer is only 10 microns thick, each pellet requires 3.9 milligrams of boron and 0.32 milligrams of boron 10. The boron used to make the zirconium diboride coatings on the pellets should therefore contain 8.2 weight percent or 9.0 atom percent of the isotope boron-10. EXAMPLE 3 Consider a reactor core containing 14,000,000 fuel pellets divided into three regions I, II, and III (see FIG. 3), where one region or batch is highly enriched compared to the other two but where the pellets in all three regions are coated with a 10 micron thick layer of zirconium diboride. The core life is such that a total of 4500 grams of boron-10 are required for the entire core. A 10 micron thick coating of zirconium diboride can be applied to the approximately 4,700,000 pellets which make up the most highly enriched batch using natural boron containing 18.4 weight percent boron-10. These pellets would carry 3370 grams of boron-10 into the core leaving 1130 grams of boron-10 to be supplied by the remaining 9,300,000 pellets. The latter pellets require 3.9 milligrams of boron per pellet containing 0.12 milligrams of boron-10. The boron to make the coatings on these pellets would contain 3.1 weight percent or 3.4 atom percent boron-10. EXAMPLE 4 For a reactor core containing 50,000 fuel rods and requiring 4500 grams of boron-10 evenly distributed in zirconium diboride coatings on the pellets, each rod should contain about 0.09 gram of boron-10. This is only 0.009 gram molecular weight of boron-10 which produces the same number of moles of helium gas when the boron-10 absorbs a neutron. This is about 202 cubic centimeters of helium at standard conditions of temperature and pressure. Since the void volume in a typical fuel rod is on the order of about 20 cubic centimeters, the gas pressure due to this helium is on the order of about 10 atmospheres or about 150 psia at room temperature. Since helium is a good conductor (among gases) and since reactor operating pressures are on the order of about 1000 to 2200 psia, most fuel rods are initially pressurized with helium at pressures ranging from about 100 to about 450 psia to partially offset the reactor operating pressure. This internal pressure provides a good heat transfer from the fuel to the cladding and retards cladding creep down onto the fuel thereby delaying pellet clad contact and PCI. With the present invention, the 150 psia of helium pressure released by the boride coating may be taken into account for reducing the initial fuel rod pressurization by a proportional amount. The advantage of controlling the amount of helium gas released by the boron-10 in the fuel rod can best be realized by considering the following example: EXAMPLE 5 For a core design requiring about 3400 grams of boron-10 distributed amount about 5,000,000 pellets in about 17,000 fuel rods, each rod would contain about the 0.20 gram of boron-10. This amount of boron-10 would produce 450 cubic centimeters of helium gas per fuel rod in a fuel rod having about 20 cubic centimeters of void space and would result in a gas pressure of about 330 psia at room temperature. If this gas release and concomitant pressure is taken into account in fuel rod design, the initial pressurization can be reduced. Since gases are also released by the fuel itself and since it is not desirable for the internal pressure of a fuel rod to exceed the external pressure thereof at operating temperatures, no more than about 0.20 gram of boron-10 should belong in a single fuel rod having 20 cubic centimeters of void volume. By the present invention, the pressure of the helium gas can be accurately controlled and accounted for by allowing the boron-10 to be distributed uniformly among substantially all the rods in a reactor core. The uniform distribution of boron-10 among substantially all the rods in a reactor core permits more boron-10 to be added to the core to control excess reactivity at the beginning of a cycle. This in turn allows more excess reactivity to be added to the fuel to increase the life of the core and thus the length of a reactor cycle. In contradistinction, in reactors using only chemical shim reactivity control, reactivity is maintained by diluting the chemical shim as a function of time in the manner illustrated in FIG. 3. In accordance with the present invention, initial excess reactivity of the core is largely controlled by the boron-10 in the boride coatings on the fuel pellets in the fuel rods and where chemical shim is used in addition, the boron content of the chemical shim is at or near the minimum required to assure a margin of excess reactivity. As the boron-10 in the fuel rods depletes over a period of about 4 to 10 months, boron is added to the chemical shim in the manner illustrated in FIG. 4 thus assuring proper control of excess reactivity at all times. As detailed above, the coating 6 preferably comprises a boride. Borides in general are hard refractory compounds having a black metallic luster which are brittle and should be handled like ceramics. Advantageously, borides in a coating 6 are formed by reaction of the elemental materials although this is not essential. Boride coatings on material such as nuclear fuels or other metals and ceramics are readily applied by sintering compacts of the powdered boride to compacts of the powdered substrate material, or by mixing the boride with a low melting glaze and applying the mixture in the form of a frit to an object and sintering the mixture, or by plasma spraying the molten powder onto the object to be coated. These techniques are well known to those skilled in the art of ceramics. For applying thin coatings on fuel pellets, plasma spraying molten boride on the pellets is the technique preferred. The boride coatings 6 may also be applied by electroplating from salt baths. Where this method is used, the fuel pellet should be pre-coated with a very thin conductive layer and a boride coating electroplated over the thin conductive layer. Alternatively, the boride coating 6 may be applied by chemical vapor deposition. Because some chemicals used in conventional vapor deposition processes for the deposition of borides tend to react with oxide nuclear fuels, it is usually necessary to pre-coat the oxide fuel with an unreactive coating such as niobium metal as discussed in commonly assigned copending application Ser. No. 468,743 filed Feb. 22, 1983. The boride coating may also be applied by a technique variously known as ion sputtering, vapor plating, or vacuum metallization. This technique requires heating the coating material, for example zirconium diboride, to vaporize small amounts at a controlled rate into a vacuum chamber. The coating material may be heated by an electrical heater, an electric arc or an ion or electron gun. The object to be coated is maintained relatively cold in the vacuum chamber so that the coating material vapors condense onto it to form a coating. Since the coatings are deposited molecule by molecule from a vapor, the formation of thick coatings is relatively time consuming. However, this process has distinct advantages in that no pre-coating is necessary and the coating material need not be very dense nor accurately shaped and that the applied coating can be accurately controlled with respect to thickness. As should now be apparent, the present invention produces a fuel rod that is resistant to damage by PCI since it permits many or all of the fuel pellets in the reactor core to be coated. In addition, it produces a fuel rod that contains less boron-10 than the prior art rods and so generates less helium and lithium. This is important since excess internal pressure can adversely affect fuel rod lifetime. Moreover, the present invention controls excess reactivity in the core at the point of origin i.e. at the fuel so that excess reactivity can be controlled more effectively, more evenly, and without the use of large amounts of other types of reactivity control. This results in more efficient use of neutrons and fuel, and where chemical shim is used, greatly reduces the volume of chemical shim water which must be treated for recovery during each reactor cycle. Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is understood that numerous modifications may be made in the illustrative embodiments and that other arrangements may be devised without departing from the spirit and the scope of the present invention as defined by the appended claims.