Fuel Densification:

Some uranium dioxide (UO₂) fuels have exhibited densification, the reverse of swelling, as a result of irradiation. Like  behavior  could  cause  the  fuel  material  to  contract  and  lead  to irregularities in the thermal power generation. A modification in fuel pellet dimensions have been small since the changes are localized in the central region of the pellet and are somewhat masked through other physical changes which occur at high temperatures in during the early categroies of the fuel cycle.

Fuel densification increases the percent of theoretical density of UO₂ pellets from a range of 90 percent to 95 percent to a range of 97 percent to 98 percent. Densification apparently arises from the elimination of little pores in the UO₂ pellets. Since densification takes place then axial and radial shrinkage of the fuel pellet results and a 3.66 m column of fuel pellets could decrease in length through as much as 7.5 cm or more.  While the column settles then mechanical interaction among the cladding and the pellet might occur, avoiding the settling of the pellet and those above it on the column below.   At one the gap has been generates, outside water pressure could flatten the cladding in the gap region, resulting in a flux spike. 

Since the thermal expansion of UO₂ is greater than in which of zircaloy, and the thermal response time for the fuel during power change is shorter than that of the cladding, the pellet temperature changes more rapidly than the temperature of the cladding during a power change? It  is possible  for  the  difference  in  thermal  expansion  to  cause  stresses exceeding the yield for the cladding material if creep (slow deformation) of the cladding has diminished the gap among the cladding and  the  fuel  pellets.  Since irradiation decreases cladding ductility, the differential expansion might lead to cladding failure.  The procedure of fuel densification is complete inside 200 hours of reactor operation.