Calculation capability of NETFLOW++ code for natural circulation in sodium cooled fast reactor

The present paper describes the simulation of the natural circulation in the secondary heat transport system (HTS) after an intentional plant trip of the experimental fast reactor 'Joyo' at 140MWt power using the plant dynamics analysis code NETFLOW++. This code is an integrated network code to calculate the nuclear steam supply system (NSSS) and the balance of the plant (BOP), i.e., turbine/feedwater system developed by the author. Up to now, the code has been validated using transient data of the experimental sodium facility PLANDTL, experimental fast reactor 'Joyo' and the prototype fast breeder reactor 'Monju'. These validations are steps to evaluate the natural circulation of a large-scale fast breeder reactor because it is important that the code can simulate not only a large plant but also a small plant or apparatus in order to have versatility. Therefore, the former validation results are introduced to show the degree of agreement. In order to consolidate the applicability of the code to the evaluation of the natural circulation, the present simulation was selected. Natural circulation tests were conducted at the experimental fast reactor 'Joyo' for the two reactor cores. The latest natural circulation test was conducted from the rated power of the Mark-II irradiation core at 100 MWt, and the NETFLOW++ code was validated using the measured data. At this occasion, the importance of the inter-subassembly heat transfer was recognized by the author in order to predict the core outlet temperature although the whole plant behavior was little effected. The 'Monju' reactor conducted some natural circulation tests in the primary and secondary heat transport systems utilizing the heat of primary pump operation. The NETFLOW++ code was validated using these data. Although these were not the complete natural circulation expected in the power plant,these results were important because basic factors relating the natural circulation such as the buoyancy force,the static head produced by cooling, pressure losses and so forth were included. An intentional scram test at 140 MWt at 'Joyo' was conducted to check the function of the heat transport system after the modification in order to increase neutron flux. This result was summarized by Kawahara in an unclassified report and calculated by Takamatsu et al. using the Mimir- N2 code for exclusive use. When the reactor was tripped intentionally, the primary motor was tripped and driven by a pony motor instead. The flow rate in the primary HTS leveled off at approximately 15% of the rated flow rate. Meanwhile, the secondary HTS was circulated by the natural circulation after the secondary pump trip. This event is simulated using a calculation model. The reactor core is divided into 10 subassembly groups from the center to the peripheral. All major components in two loops are taken into account. At time zero, the reactor power is tripped in the calculation and decreased along the decay heat characteristic. At the same time, the pumps in the primary and secondary HTSs in the computer code are tripped and run down based on the coast-down characteristics given by inertias. After a certain period, the primary pumps are operated with the low speed in the same manner as the test using a pump model which can trace the given flow rate automatically. While, the pumps in the secondary HTS are not operated after the trip. Comparison between the measured result and calculation result are shown. The DHX in the figure stands for a dump heat exchanger. The word DHX is used in 'Joyo' instead of the air cooler. Trends with closed symbols and solid lines stand for test result. The open symbols stand for the calculated results. In this case, the same shaped symbol means the same parameter as in the test result. The agreement is the same order as the Mimir-N2 exclusive code for 'Joyo'. As a result, it raise expectations that the NETFLOW++ code is firmly applicable to the natural circulation analysis of sodium-cooled fast reactors with the scale of the prototype reactor 'Monju'