Patent Number: 052672840
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Erbium, gadolinium and boron are three burnable absorbers that are typically used for LWR applications. In their naturally occurring state, each of these elements contains two or more distinct isotopes. Each of these isotopes has a different thermal neutron capture cross-section. Thus, the effectiveness of the burnable absorber can be increased by isolating those burnable absorber isotopes having a high absorption cross-section. For example, boron-10, gadolinium-157 and erbium-167 have desirable thermal absorption cross-sections. Specific methods are available for removal of a single isotope from an isotopic mixture. For example, desirable isotopes may be isolated by fractionation of the material using commonly known techniques such as gas diffusion, centrifugal separation, liquid chromatography, and fractional distillation. Another method is atomic vapor laser isotope separation (AVLIS). AVLIS was developed for large scale uranium enrichment applications at the Lawrence Livermore National Laboratory. AVLIS works by first heating and vaporizing a sample of interest followed by laser irradiation at a wavelength specifically selected to ionize only the selected isotope. Once ionized, the isotope is isolated using electric fields. Erbium, which has been isotopically depleted in the 166 isotope, and gadolinium, which has been isotopically depleted in the 156 isotope, were recently disclosed by Grossman et al. as favored additives for uranium dioxide fuel pellets in co-pending U.S. Ser. No. 07/761,438. The present invention relates to the fabrication and use of zirconium alloy structural components in LWR's which preferably contain isotopically purified erbium-167 as a minor constituent. Erbium has been under consideration as a burnable absorber because it was found, by calculation, to have certain advantages over boron and gadolinium. Very recent reactor test results using uranium dioxide fuel with erbium included in the fuel pellets have supported these calculations. Typically, erbium is included in the fuel pellets in concentrations of about 1.5 wt. % in about 20 wt. % of the uranium dioxide. This means that the overall weight ratio of erbium to uranium in the reactor core is about 0.3 wt. %. For a typical core design, the weight ratio of zirconium to uranium is about 30 wt. %. Thus, the weight ratio of erbium to zirconium in the core is about 1 wt. %. Furthermore, since erbium-167 comprises about 22.9 wt. % of naturally occurring erbium, the weight ratio of erbium-167 to zirconium is about 0.2 wt. % or 2,000 ppm. Therefore, to provide adequate reactor hold-down characteristics, erbium-167 should be introduced into the zirconium alloy components of the reactor core in an overall concentration of about 0.2 wt. %. At these low levels, any adverse effects that erbium-167 may have on the corrosion of the zirconium alloy will be minimized, while at the same time providing the desired burnable absorber benefits. Therefore, according to an embodiment of the present invention, erbium-167 is substituted for zirconium and/or other appropriate elements in the zirconium alloys in an amount ranging from about 0.1 wt. % to about 0.4 wt. %, preferably about 0.2 wt. %. According to an embodiment of the present invention, about 0.1 to 0.4 weight parts isotopically purified erbium-167 are homogeneously combined with about 100 weight parts of a zirconium alloy to form a zirconium alloy absorber material. As discussed above, the two zirconium alloys commonly used in reactor design are Zircaloy-2 and Zircaloy-4. Of course, other zirconium alloys could be used in the practice of the present invention as long as they meet the stringent requirement associated with reactor core conditions. The uniform use of isotopic erbium-167 in all or most of the zirconium alloy components in the core avoids the disadvantages associated with the use of more than one type of zirconium alloy. In addition to zirconium alloys containing erbium-167, equivalent levels of isotopically pure gadolinium-157, equivalent levels of isotopically pure boron-10 or equivalent combinations of two or all three pure isotopes can be used in concentrations appropriate for effective burnable absorber service. Thus, a zirconium alloy absorber material is provided which does not displace fissile material in the reactor core. Furthermore, since isotopically purified absorber materials are introduced into zirconium alloy components throughout the reactor core, the burnable absorber concentration is held to a minimum.