Studies Needed to Prevent the Use of Expansion Bends in LMFBR Intermediate Heat Exchangers

摘要

The LMFBR IHX built in France consists in a vertical tube bundle welded on 2 tube sheets. The secondary sodium flows down a central pipe to the lower collector then up through the tube bundle where it is heated. During the preliminary studies for the PHENIX reactor, it was soon evident that a structure with expansion bends on tubes would lead to prohibitive sheet-thickness. Moreover, the presence of bends increased the total length, made vibration problems more complicated, and, finally made the structure more expensive. Therefore, a solution with straight tubes was studied, and, despite the pressure and diameter increase, the Phenix sheets had the same thickness as Rapsodie's ones. Evidently, the solution of the problems raised by the presence of thermal stresses needed thorough studies and led to the following theoretical and experimental developments: (1) A computer code was written for structural analysis. The structure was divided in annular elements that could be studied by means of the elementary theory of shells and plates; and reduced elastic coefficients were given to the tube sheets to account for the presence of drilled holes. (2) An experimental study was undertaken to determine the reduced elastic coefficients of the tube sheets. (3) A computer code was written to study the primary sodium flow around the tube bundle, and experimental studies were made on a mockup, the fluid being water. (4) The results of the previous code were used to determine, by means of a code for thermal analysis, the temperature field in the bundle both in steady state and transient regimes. Up to now, many transients were performed and the Phenix heat exchangers have been operating quite satisfactorily; this seems to prove the design assumptions were correct. (ERA citation 03:035081)