Development Of Transient Analysis Code For S-CO₂ Brayton Cycle Thermodynamic System

The supercritical carbon dioxide(S-CO2)Brayton cycle system has good application prospects due to its compact structure,high efficiency of thermoelectric conversion and good economy.In the past decade,it is one of the important contents of the advanced nuclear reactor research in the world to use S-CO2 to replace helium as the core cooling working fluid of gas-cooled reactor and as working fluid of power conversion.CO2 has suitable critical parameters,can achieve satisfactory efficiency of thermoelectric conversion without high cycle temperature,and has some advantages such as good compressibility,rich reserves,non-toxic,natural existence,and stable properties within the temperature range of the reactor core coolant.The use of CO2 as the working fluid can reduce the cycle temperature and compression work,thereby improving the safety of the reactor and reducing the cost of the reactor.Therefore,CO2 is considered to be one of the most promising energy transfer and energy conversion working fluids in nuclear reactors.In this paper,significant improvements have been made to the RELAP5/MOD3.1 for the analysis of S-CO2 Brayton reactor system.These improvements consisted of adding carbon dioxide as a working fluid,simplifying the input settings of PCHE component model,developing a compressor component model,improving the turbine component model,and adding the Gnielinski heat transfer correlation.The improved RELAP5 code was used to establish steady-state model of the entire loop of the GFR system designed by MIT,and the code improvements were validated,generally through comparisons with independent design calculations.Then a MW S-CO2 recompressed Brayton cycle reactor was modeled as the research object.The calculation nodes division of the circulation system was described in detail,and a simulation model was established.Based on the model,the steady-state calculation was carried out to obtain the reasonable initial value of transient calculation.Then,the typical transients of the circulatory system,such as variable condition process,completing loss of electrical load resulting in shaft speed increasing significantly,and the temperature disturbance of cooling water,were simulated and analyzed..During the operation of the S-CO2 Brayton cycle system,it is easy to lead to the deviation of the overall parameters of the circulatory system due to the change of pressure,temperature,mass flow and other parameters.Therefore,the control objectives and the control methods of the recompressed Brayton cycle system were discussed.And three operation control strategy were discussed: the constant flow of the main working fluid;the constant core outlet temperature;the user defined compressor operation line.Then the simulation control effects of the three control schemes were analyzed.