The regid element connecting from the center to surface of vessel is used for the following purpose:
(1) geometric consistency between vessel and piping and behave as rigid
CAESAR II's rigid element is taken 10 times of thickness defined previously defined in element.
So, the previous element is recommended to have a vessel diameter and thickness for its sufficient RIGIDITY,
(2) simulate vessel radial thermal elongation
So, it is necessary to have same temperature as vessel at connection