Patent Number: 052415716
Section: summary

BACKGROUND OF THE INVENTION This invention relates to zirconium alloy burnable absorber materials which are suitable for use as fuel rod cladding tubes and as structural components in light water nuclear reactors (LWRs). Such structural components include guide tubes, or guide thimbles, in some pressurized water nuclear reactor (PWR) fuel assembly designs and channels in boiling water nuclear reactor (BWR) cores. More particularly, this invention relates to burnable absorber-containing zirconium alloys that do not displace fissile materials in reactor cores, while at the same time providing the strength and corrosion resistance required for the specific application at hand. DESCRIPTION OF THE PRIOR ART Many LWR fuel designs employ burnable absorber rods to control axial power peaking or moderator temperature coefficient. In some designs, burnable absorber rods are placed in fuel assembly lattice locations, thereby displacing fuel rods. Other designs employ burnable absorber rod inserts in guide thimbles of fuel assemblies. Still other designs involve the formation of burnable-absorber-bearing coatings on the inside diameters of cladding tubes, on fuel pellet surfaces, or involve the distribution of the burnable absorber within the fuel pellet. The use of erbium as a burnable absorber has been previously discussed wherein erbia (Er.sub.2 O.sub.3) is homogeneously dispersed in selected fuel pins..sup.1) Although such absorbers are advantageous over other burnable absorbers in certain fuel management schemes, such an arrangement for the application of burnable absorbers nonetheless displaces fissile material from the fuel rods and reduces the fissile material inventory available to produce power. Binary zirconium-erbium alloys with 10-90 weight percent (wt. %) erbium as well as Zircaloy-2 with 0.5-2.0 wt. % erbium have been suggested for use in control rod materials. Unfortunately, such alloys were found to have corrosion levels unacceptable for use as control rod materials in PWRs..sup.2) Accordingly, it is a continuing problem in this art to develop an absorber material that does not displace fissile material, while exhibiting adequate corrosion resistance. SUMMARY OF THE INVENTION In view of the foregoing, it is an object of the present invention to overcome these and other drawbacks of the prior art. Specifically, it is one object of the present invention to provide a burnable absorber that does not displace any fissile material from the fuel rods so that a higher fissile material inventory is available to produce power. It is a further object of the present invention to incorporate burnable absorber materials in cladding tubes and other zirconium bearing structural components, so that such components can be fabricated in facilities that do not require radiological materials handling capabilities or licenses. It is a further object of the present invention to provide cladding tubes for fuel rods constructed from such alloys. It is a further object of the present invention to provide cladding tubes of such alloys which include an outer layer of a more corrosion resistant material in the form of duplex tubing. It is a further object of the present invention to provide components such as guide tubes, or guide thimbles, in PWR fuel assemblies and channels in BWR cores constructed from such alloys. Such structural components may contain outer layers of a more corrosion resistant material. It is a further object of the present invention to use erbium as the burnable absorber for application in PWRs and gadolinium as the burnable absorber for application in BWRs. It is a main feature of the present invention to replace all or a portion of the naturally occurring erbium with purified erbium-167 isotope in PWRs and to replace all or a portion of the naturally occurring gadolinium with purified gadolinium-155, purified gadolinium-157, or both, in BWRs because fuel cycle economies are improved due to a reduction in residual concentrations originating from undesirable isotopes present in the natural materials. The present invention provides a zirconium alloy absorber material comprising a) a natural distribution of erbium isotopes, enriched erbium-167 isotope, or both, in a range of from 0.05 wt. % to 2 wt. % or b) a natural distribution of gadolinium isotopes, isotopically enriched gadolinium-155, isotopically enriched gadolinium-157, or any combination thereof, in a range of from 0.05 wt. % to 6 wt. %; tin in a range of up to 1.4 wt. %; iron in a range from 0.2 to 0.5 wt. %; chromium in a range from 0.07 to 0.25 wt. %; niobium in a range of up to 0.6 wt. %; vanadium in a range of up to 0.5 wt. %; 50 to 120 ppm silicon; 1000 to 2200 ppm oxygen; and the balance zirconium.