13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- underneath shows that there exist the largest results in the middle location of fuel plate, that is, they locates at the right zone of the finite element model. It is induced by the fact that there exists the largest neutron flux which leads to the highest heat generation and irradiation swelling of fuel meat together with strengthened irradiation hardening in the cladding. This is the result of un-uniform irradiation conditions. In a word, it is necessary to optimize the constraint and irradiation conditions for fuel elements in order to assure their safety. (a) (b) Figure.5 (a) The Mises stress contours and (b) the stress concentration zone in cladding at 144.1days 0 3 6 9 12 300 330 360 390 420 450 480 Mises Stress(MPa) Distance along paths(mm) 29.9days-path1 86.9days-paht1 144.1days-path1 144.1days-path2 Figure.6 (a) The Mises Stress in different time along (b) the two paths in cladding 4.4. Deformation in the thickness direction It is well known that the exceeding deformation along the plate thickness direction will narrow the coolant water channel, which will further lead to degradation of the heat transfer coefficient between the fuel plate and the coolant water. This could even result in accident with rapid temperature rise. Thus, the deformation in the thickness direction of fuel plate under nonhomogeneous neutron flux is mainly investigated. Fig.7 (a) reveals the displacements in the thickness direction of fuel plate along the surface path depicted in Fig. 7 (b). It can be observed that (1) the displacements is very small at the part with no fuel meat underneath and they change little with time; (2) the large displacements mainly exist in the part with the fuel meat underneath, and they increase with time distantly; (3) as expected, the maximum deformation arises in the middle part along the plate length direction, and its increasing rate with time is highest. The fuel plate tends to be an arch. As can be easily understood, the main contribution of the above deformation is from
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