Flow instability of sodium-heated steam generators

For the purpose of developing the steam generators for the fast breeder proto-type reactor, a 1 MW steam generator and two 50 MW steam generators were manufactured and have been operated in the Power Reactor and Nuclear Fuel Development Corp., and various performance tests have been carried out. Since unstable flow occurred in the water side of the 1 MW steam generator at the time of load changed in normal operation, the flow stability test was added. Similar unstable flow has been reported in the U.S., Netherlands and others. It is important to prevent the occurrence of this phenomenon for securing the soundness of machinery and equipments over long period in view of the safety, economy and operation of steam generators. The exact evaluation of the flow stability at the planning stage is required, and the development of analysis codes concerning the unstable flow has been carried out. The instability test rig (ITR) was manufactured in the PNC, and various characteristic tests have been performed. In this paper, the conditions of occurrence, the state of phenomena and the characteristics concerning density wave oscilation and flow rate run-away phenomenon are reported from experimental and industrial viewpoints. The test apparatus and the results of test are described. The occurrence of flow rate run-away phenomenon can be predicted by the presence of negative gradient in the flow rate-pressure loss relationship. (Kako, I.)