A Mass Flow Rate Control System in the Novel Superheated Steam Supply System of NHR200-II

Abstract The previous version of the low-temperature nuclear heating reactor, rated at 200 MW (NHR200-II), was primarily dedicated to winter heating, hence limiting its applicability. Furthermore, the previous version of the steam generator within the secondary circuit was vertical and capable only of producing saturated steam. To expand its functionality, a novel superheated steam supply system has been integrated into the secondary circuit to generate superheated steam for an industrial park. This novel system comprises three crucial components: a preheater, a horizontal steam generator, and a superheater. The horizontal steam generator features effective fouling discharge, allowing the coolant in the secondary circuit to be softened water rather than the deionized water used in the previous vertical steam generator. NHR200-II comprises the primary, intermediate, and secondary circuits. The primary circuit operates via natural circulation under all operating conditions, accompanied by two passive residual heat removal systems designed to extract decay heat after reactor shutdown. These passive residual heat removal systems also rely on natural circulation, with reactor decay heat serving as the heat source and air-cooling towers acting as the cooling mechanism. However, the intermediate and secondary circuits are driven by pumps, generating superheated steam for industrial park use. The pressure in the intermediate circuit is slightly higher than that in the primary circuit, ensuring that no radioactivity would be released into the secondary circuit. The entire system is simulated by using RELAP5 code, achieving rated power conditions by employing design values. The heat transfer between tube and shell sides for the three important heat exchanger is modeled using the ‘Heat structure’, while a ‘Separator’ model is utilized atop the horizontal steam generator to separate water and vapor. Reactor core power adjustments are performed via the point reactor model, modified by introducing positive or negative reactivity through a logical control system. Additionally, a control system of the feedwater is implemented to regulate the mass flow rate within the secondary circuit, aiding in achieving different steady states with changing operational conditions. Simulation results demonstrate the effectiveness of the feedwater control system in efficiently adjusting the mass flow rate of the secondary circuit. Consequently, the entire system can attain a different steady state following alterations in operational conditions. Moreover, the temperature of the secondary circuit’s superheated steam exceeds 30°C, with a void fraction reaches to 1.00, meeting the specified requirements of the industrial park. The secondary circuit’s mass flow rate aptly corresponds to variations in reactor core power, ensuring the establishment of a new stable operational state.