Plant MaterialsDOE-HDBK-1017/2-93NUCLEAR REACTOR CORE PROBLEMSOperational limitations such as rate of power increase and power for a given power ramp rate areimposed to lessen the effect of PCI. PCI appears to be more likely to occur during initial powerincrease and can be very costly if cladding failure occurs.FuelDensificationSome uranium dioxide (UO2) fuels have exhibited densification, the reverse of swelling, as aresult of irradiation. Such behavior can cause the fuel material to contract and lead toirregularities in the thermal power generation. The changes in fuel pellet dimensions have beensmall because the changes are localized in the central region of the pellet and are somewhatmasked by other physical changes that occur at high temperatures during the early part of the fuelcycle.Fuel densification increases the percent of theoretical density of UO2 pellets from a range of 90%to 95% to a range of 97% to 98%. Densification apparently arises from the elimination of smallpores in the UO2 pellets. As densification takes place, axial and radial shrinkage of the fuelpellet results and a 3.66 m column of fuel pellets can decrease in length by as much as 7.5 cmor more. As the column settles, mechanical interaction between the cladding and the pellet mayoccur, preventing the settling of the pellet and those above it on the column below. Once thegap has been produced, outside water pressure can flatten the cladding in the gap region, resultingin a flux spike. Because the thermal expansion of UO2 is greater than that of zircaloy, and thethermal response time for the fuel during power change is shorter than that of the cladding, thepellet temperature changes more quickly than the temperature of the cladding during a powerchange. If creep (slow deformation) of the cladding has diminished the gap between the claddingand the fuel pellets, it is possible for the difference in thermal expansion to cause stressesexceeding the yield for the cladding material. Because irradiation reduces cladding ductility, thedifferential expansion may lead to cladding failure. The process of fuel densification is completewithin 200 hours of reactor operation.The problems of cladding collapse resulting from fuel densification and cladding creep haveoccurred mainly with unpressurized fuel rods in PWRs. To reduce the cladding creep sufficientlyto prevent the formation of fuel column gaps and subsequent tubing collapse, the followingmethods have been successful: pressurizing the fuel rods with helium to pressures of 200 psigto 400 psig; and increasing fuel pellet density by sintering (bonded mass of metal particlesshaped and partially fused by pressure and heating below the melting point) the material in amanner leading to a higher initial density and a stabilized pore microstructure.There are three principle effects associated with fuel densification that must be evaluated forreactors in all modes of operation.a.an increase in the linear heat generation rate by an amount directly proportional to thedecrease in pellet lengthb. an increased local neutron flux and a local power spike in the axial gaps in the fuelcolumnRev. 0Page 23MS-05
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