Fatigue analysis of fuel bundle end plate

Natural Uranium Fuel Bundles for 220 MWe PHWR have 19 fuel elements, assembled together to form fuel bundle assembly, by end plates welded at both ends. The thermal neutron flux in different rings across a bundle diameter reduces from outer ring to centre being away from moderator. This results in unequal heat generation within the different rings of fuel bundle due to which the different elements of bundle expand differently in axial direction. These elements with differential expansion will try to bend the end plates. This gives rise to the bending stresses on the end plates of the bundle. There will be cyclic variation of these bending stresses because of bundle power cycling from zero to full power. It is observed that in actual practice 10 to 20 such power cycles are seen by fuel bundle in its life time inside the reactor. The analysis was carried out to calculate the number of fatigue cycles, which can be taken by fuel bundle. It was found that present bundle design qualifies the analysis. Analysis was also carried out varying the fissile content in bundle. Differential fissile content in different rings of fuel bundle will affect the thermal flux profile. Consequently, the end plate stresses and the no. of cycles the bundle can take will vary. This was analyzed by taking different fissile content combinations for different rings of fuel bundle. It was found that: 1. Number of power cycles that can be taken by bundle, decreases with increase in fissile content if all the elements have same fissile content. 2. For a specific bundle, if fissile content in outer ring is more than that in inner ring then the no. of permissible cycles decreases compared to that of a same fissile content for all the bundles. On the other hand, if the fissile content of inner elements is slightly increased compared to outer elements, it increases the no. of cycles the bundle can withstand. This paper gives the end plate stress analysis and fatigue cycle estimation for natural uranium bundle and bundle with different fissile content in different rings. (author)