When the conventional thermal stress analysis is made using simulation software, the thermal expansion is regarded to be completely limited on the fixed place. The zero displacement constraint is so applied. But in fact the structure can still expand in some degree, the boundary conditions of the simulation model are so strict seriously that the calculated stress will be far greater than the actual values, which is called “thermal stress distortion” in this paper.

In order to reduce the conservative degree some of the structure constraints should be set free according to the actual structure to make the structure extend appropriately. At the same time the integrity of the applied constraint should be noticed specially. The rigid motion shouldn’t be expected to exist because of the inadequate constraints.

The paper presents several constraint methods for avoiding the thermal stress distortion according to the typical structure of nuclear engineering: “extension constraint”, “column coordinates constraint” and “segmentation and alternate constraint”.

The “extension constraint” method is suitable for the structure simulation which distortion locates near the constraint place. But it can only eliminates parts of the distortion. The “column coordinates constraint” and “segmentation and alternate constraint” methods both can eliminate all of the distortion. The former is only applicable for the rotational structure, while the latter is suitable for frame and rotation structure, what is important is that the latter also provides an idea for structure design to eliminate the thermal stress distortion: setting expansion gap.

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