Patent Number: 041727622
Section: claims

1. In a nuclear reactor control rod having a sealed cylindrical cladding of inside radius R1 within which are stacked a plurality of poison absorber pellets having outside radius R2 substantially equal to R1, wherein the improvement comprises: at least one of the pellets at the leading end of the rod having a radius R3 less than R2; and  a sleeve circumferentially wrapped around said end pellets, said sleeve having a thickness substantially equal to R2 minus R3 and a minimum linear compressibility of 1.0 percent per 1,000 psi continuous load. 2. The control rod of claim 1 wherein said sleeve is integral. 3. The control rod of claim 1 wherein said end pellets are made of B.sub.4 C. 4. The control rod of claim 3 wherein said sleeve has a thermal conductivity sufficient to keep the centerline temperature of said end pellets below the melting point of B.sub.4 C when said end pellets are located in a thermal reactor spectrum steady state total neutron flux of 3.0.times.10.sup.14 neutrons per second per square centimeter. 5. The control rod of claim 3 wherein said sleeve has a minimum thermal conductivity of 3.0 BTU per hour per square foot per degree F. 6. The control rod of claim 3 wherein said sleeve material is type 347 stainless steel having a density between 15% and 30% of theoretical. 7. The control rod of claim 6 wherein said sleeve material is type 347 stainless steel having a density of approximately 22.5 percent of theoretical.