The purpose of the study is to experimentally investigate driving mechanism of major instabilities simulated in a natural circulation experimental loop, under a predetermined range of system operating pressure and inlet subcoolings. Pressure range of 0.1 up to 0.7 MPa, input heat flux range of 0 up to 577 kW/m 2 , and inlet subcoolings of 5, 10 and 15 K respectively, are applied in the experiments. The objective of the study is to formulate a rational startup procedure, in which major thermohydraulic instabilities can be detected and prevented. The study clarifies that four (4) kinds of thermohydraulic instability might occur even up to a higher pressure of 0.7 MPa. The instabilities' sequence is as follows: (1) geysering induced by condensation accompanied by flashing, (2) oscillation induced by hydrostatic head fluctuation, (3) density wave oscillations, and (4) flashing accompanying those instabilities. The experiments confirmed that the geysering region gets narrower and suppressed with the increased system pressure. With chimneys, natural circulation can be achieved reliably and more easily. However, the flashing in the chimney cannot be avoided at low system pressure. Stable two-phase natural circulation can be established if the system pressure is increased beyond 0.7 MPa, after the high frequency density eave oscillation thoroughly suppressed. The experiments were analyzed based on frequency domain of each instability phenomenon.
Summary
Conventional energy systems in both developed and developing economies are currently run by fossil‐fueled power plants. Many of them are ageing and are now being considered for the replacement to reduce the carbon intensity in electricity generation. One of the main routes to achieve this goal is by scaling‐up the deployment of the renewable energy system (RES). However, the increasing share of variable RES tends to affect the electrical grid operation. Hence, the need for reliable and flexible energy sources has emerged to cope with the variability in electricity and energy markets. Small modular reactors (SMRs) are emerging as alternatives to baseload fossil fuel systems and retiring large nuclear plants, primarily due to their small capacity and less‐capital intensive characteristics. The intermittency of power generation in the grid with a higher share of RES can be coped with in synergetic and effective way by adopting SMRs that form a nuclear‐renewable synergy in fulfilling the electricity demand. Furthermore, SMRs are designed to operate in load‐following mode adjusting the power output as demand of electricity fluctuates. SMRs feature modularity intended to enable enhanced constructability and phased‐deployment, adapting to the increased share of renewable energy in a distributed energy system, as energy demand increases. This paper will highlight the main outcomes of miscellaneous recent studies on the integrated energy systems carried out by international experts under the aegis of the International Atomic Energy Agency. The final publication of the study discusses the potential synergy of SMRs with RES for electric power production, as well as for other clean‐energy applications including seawater desalination, district heating, and hydrogen production.
This paper presents experimental study on transport mechanism of thermohydraulic instability, which may occur in natural circulation boiling water reactor during startup. The research was carried out using a natural circulation experimental loop featuring twin parallel boiling channels with chimney assembly. The experiments were performed with the pressure range of 0.1 to 0.7 MPa and maximum heat flux of 577 kW/m 2. The objective of the study is to formulate thermohydraulic stability maps required for determining rational startup procedure of the reactor, in which the instability could be prevented. The study clarified that the flow modes during startup consist of the following sequence: (1) single-phase flow, (2) geysering, (3) oscillation due to hydrostatic head fluctuation, (4) density wave oscillation, (5) transition oscillation, and (6) stable two-phase flow. The main findings of the experiments are as follows: First, low amplitude geysering still occurs at 0.7 MPa under lower heat flux and high inlet subcooling. Second, stable twophase natural circulation is achieved with system pressure as low as 0.2 MPa, under medium heat flux, and subcooling lower than 5 K. Third, oscillation due to hydrostatic head fluctuation only occurs under atmospheric condition. Finally, thermohydraulic stability maps and rational startup procedure are formulated.
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