Sensitivity analysis for design factors on thermal stress near the sodium surface of reactor vessels

Ando, Masanori

A reactor vessel of fast reactor plants contains high temperature liquid sodium and its upper end is supported from a concrete structure of low temperature. Therefore, a sharp temperature gradient will arise at the vessel wall near the sodium surface and a large thermal stress will be generated around this part. From the economical viewpoint, a simply designed reactor vessel is desirable, but this usually tends to make the stress larger. For this reason, design evaluation methods that can permit large thermal stress are necessary. The purposes of this research are to grasp influence factors and the sensitivities of the thermal stress to those factors, and to identify important points of the structural design of the reactor vessels. The results obtained from this research are as follows. (1)Sn (range of primary+secondary stress intensity) is sensitive to the thermal boundary conditions in the reactor vessels. By adjusting the heat transfer between cover gas and reactor vessel, Sn decreased. (2)By considering variation of the sodium surface level during starting up, the sign of the thermal stress near the sodium surface is reversed, and Sn is thereby increased. (3)By changing thickness of vessel to 30mm from 50mm, Sn is decreased by 15%. These results can be utilized for the pursuit of improved design of the reactor vessels.