Innovative Design and Analysis of Support Arrangement for Double Walled Tank Containing Primary Sodium

摘要

In Prototype Fast Breeder Reactor, an on line purification circuit for primary sodium is provided. The purification circuit comprises an electromagnetic pump, priming tank, economizer and cold traps. The priming tank of purification circuit is located in the down streamside of its electro magnetic pump. Since it contains radioactive primary sodium, it is not freely accessible. Priming tank is a vertical tank with torispherical dished ends on both top and bottom. The total weight of priming tank, including the sodium in it is 1.6 t. It is provided with double envelope to avoid release of radioactive sodium in reactor control building in case of leak in tank. Nitrogen is filled in annular space to provide an inert atmosphere. The material of the priming tank and the double envelope is SS 304 LN. The normal operating temperature of the priming tank is 400° C. During SGDHR the temperature rises to 540° C and reduces to 200° C during fuel handling condition. The tank is insulated to reduce the heat loss to surroundings. This paper aims at arriving at a support arrangement for the tank with the features (i) weight shall be transferred to support from main shell (ii) main shell shall be covered fully by double envelope, (iii) split flange with a conical shell in between the double envelope and support flange from thermal stress consideration, (iv) temperature at bolt location is less than 70° C and (v) easy manufacturing. To meet the above features, a conical support shell is designed to support the priming tank. The conical support shell is attached to the priming tank through a ring. The double envelope is also attached to the ring. Inside diameter and shell thickness of the tank main shell are 750 mm and 8 mm respectively. Based on parametric study, the height of the cone and height of insulated portion of cone from the junction are arrived at. This supporting arrangement is analysed for dead load and thermal loads during normal operation, fuel handling condition and SGDHR condition. Design check for the tank is carried out as per RCC-MR 2002 procedure for class-1 component. Two governing events namely loss of steam water system occurring 47 times in design life and the offsite power failure occurring 160 times in design life are considered for the analysis. The operating time for the tank at 540° C during SGDHR (corresponding to the two events) is ~1480 h. Since this exceed the creep cross over curve limit of RCC-MR, the hot junction between the cone and cylinder is analysed for creep fatigue damage. Parametric study is carried out using finite element model with axi-symmetric thin shell element in CAST-3M FE software. The cylindrical shell of priming tank, along with the double envelope and support shell and flange are modeled. After finalizing the height of the cone and insulation height, detailed analysis using 8 noded axi-symmetric solid elements is carried out to get the peak stresses at the junctions needed for creep-fatigue damage evaluation. The fatigue damage is found to be negligible. The creep damage at the junction of conical support shell to ring attached to main tank is found to be 0.074, which is acceptable. The structural integrity of proposed configuration is thus confirmed by analysis.