Stress distribution and flexibility of the suction bend of the primary sodium pump—LMFBR-SNR 300

Abstract

Hydrodynamic aspects lead to the design of a special smooth suction bend for the LMFBR-SNR 300 primary sodium pump. This bend is built up from two symmetrical halves, formed from plates by explosive forming. For the nuclear piping system design, the ASME Boiler and Pressure Vessel Code —Section III 1 gives formulae for the simplified stress analyses by using the stress indices and flexibility factors. The application of these formulae to the prediction of the stresses and flexibility of the suction bend considered does not guarantee the reliability of the results. To achieve a more accurate prediction of the bend's stresses and flexibility linear stress analyses are carried out based on two different methods. In the first method the bend is schematised by various toroidal segments with bend parameter λ ranging from 0·049 to 0·22. Each segment is analysed as described below. Internal pressure: Stress analyses are carried out using the KSHEL computer program developed by Kalnins and based on his theory. 2 In- and out-of-plane bending: Based on the energy equations of von Karman 3 and Vigness, 4 a theoretical solution was developed by Rodabaugh and George. 9 Applying the principle of least work gives the following equation: [D(λ,r,p,n,E)]{c n} = {−3,0,0,0,…} T where D is a symmetrical band matrix and c n is the modified unknown coefficient of the trigonometric series of the displacement field. The stresses and flexibility could be determined by substituting c n in the appropriate equations. The membrane circumferential stress distribution obtained by Rodabaugh and George was modified by Dodge and Moore 6 based on Gross's solution. 8 The computer program PIPEBEND, based on the above-mentioned solutions, is developed for calculating the stresses and flexibility of each toroidal segment. The second method deals with three-dimensional stress analysis. This is necessary in order to investigate the influence of change of curvatures and cross-sections (neglected in the first method). Modelling is carried out for the symmetry of half of the complete suction bend, applying the thick shell element QUABC9 of the ASKA system. Application of this element for thin shell problems requires the reduction of the number of integration points. For the considered model, a 2 × 2 reduced integration scheme is chosen for the transverse shear component only. The results of both methods were compared and it was concluded that, for internal pressure and pure—or nearly pure—bending load, adequate prediction of the stresses and flexibility can be obtained by the first method.