Natural Circulation Test Facility Research Articles

Experimental investigation on flashing-induced flow instability in a natural circulation scaled-down test facility

With the deployment of nuclear technology and market demand, Small Modular Reactor (SMR) attracts a lot of attention of researchers and engineers which also was deeply studied and close to the wide application. The NuScale reactor is one of typical PWR-type SMRs which adopts the natural circulation system. The natural circulation is the main cooling method in the NuScale reactor which can simplify the nuclear reactor system. In this experimental investigation, a natural circulation test facility is scaled down from the prototype of NuScale reactor. This facility is operated at the low-pressure condition which is less than 0.5 MPa. The flow instability phenomenon was observed in the experiments which is regarded as the flashing-induced flow instability. The oscillation characteristics of the flashing-induced flow instability was observed and analyzed based on the results of flow rate, void fraction, and fluid temperature. The mechanism of the flashing-induced flow instability in the natural circulation system was analyzed based on the characteristics analysis. The effects of power and system pressure on the flashing-induced flow instability were also discussed. It is found that the flashing-induced flow instability can be suppressed by the increase of system pressure but induced by the increase of power.

Assessment of RELAP5/MOD3.2 for simulating density wave oscillation with a two-phase natural circulation test facility

The slight boiling mode (SBM) of the nuclear heating reactor (NHR) designed to operating under two-phase natural circulation with low vapor quality and PWR-type reactor design is rigorously constrained by the flow instability and the extent of saturated boiling in the core. The objective of this study is to evaluate the applicability of the RELAP5 code to simulate flow instability in natural circulation systems and then to quantitatively estimate the operating parameters space of the SBM. Based on a well-scaled facility of NHR200, a RELAP5 model is developed. After a general assessment, sensitivity analysis and frequency spectrum analysis of flow resistance coefficients, numerical method, and drift velocity are further conducted for better understanding the deviations existing in simulations. It can be found that the nature of density wave oscillation (DWO) can be revealed by the RELAP5 code from the reasonable agreements between the simulation results and experimental data. Furthermore, a reasonable calibration of the drift velocity can greatly improve the accuracy of both static parameters and oscillating characteristic frequency. Ultimately, quantitative calculation of the operating parameter space is achieved with the RELAP5 code assisted by the dimensionless number Nsub/Npch. This work paves the way for studying the start-up and operating characteristics of SBM.

Numerical investigation of the effect of inlet subcooling on flow instabilities in a parallel channel natural circulation boiling system

Some of the modern reactor concepts are based on natural circulation boiling systems. These systems are prone to flow instabilities and parallel channel systems can exhibit peculiar types of flow oscillations. In the present work, the existence and the nature of instabilities in a parallel channel natural circulation test facility (PCNCTF) has been investigated through numerical simulations using the system code RELAP5/MOD3.2. The numerical model is validated using experimental data of PCNCTF. The effect of inlet subcooling on flow oscillations in parallel channels is studied for different system pressure and input powers. Time variations of mass flow rate and void fraction in the channels are examined and the nature of oscillations is analyzed using phase portraits and Fourier power spectra. It is found that there exists a particular range of inlet subcoolings (at a fixed system pressure and input power), above or below which instabilities occur. Mixed mode of oscillations at low degrees of inlet subcoolings and superimposed density wave oscillations are observed at high inlet subcoolings.

Thermal hydraulic studies on cold start-up in a multichannel natural circulation system

Abstract Two-phase natural circulation has been adopted as the heat removal mechanism in the primary loop of Advanced Heavy Water Reactor (AHWR). For safe and reliable operation, it is required that the system should be operated in stable regions where no thermo-hydraulic oscillations are present. For the experimental study of thermo-hydraulic instabilities, a facility called Parallel Channel Natural Circulation Test Facility (PCNCTF) was erected and commissioned at the Indian Institute of Technology Bombay (IITB). The loop in this facility is geometrically similar to the primary loop of AHWR. The present paper focuses on the thermal hydraulics of cold start-up process in a Natural Circulation Systems (NCS) in general and AHWR in particular. Experiments were conducted in the three channel loop of PCNCTF as per the prescribed philosophy to be adopted in AHWR. The response of the system was then simulated satisfactorily using RELAP/MOD3.2 after some tuning. Subsequently, parametric studies were carried out with the developed numerical model to accelerate the cold start-up process by increasing power levels. Finally, a rational start-up methodology is developed which could further accelerate the cold start-up.

Flow pattern transition instabilities in a natural circulation cooling facility

Our research analyzed results from a two-phase natural circulation test facility designed to study the performance of reactor cavity cooling systems, and determined the presence of a flow transition instability. Reactor cavity cooling systems are passive safety system’s designed to remove decay heat from high temperature generation IV reactors. Water-based designs achieve this function by boiling-off a set water inventory held in a storage tank. During normal operation under accident conditions, the two-phase natural circulation facility undergoes a transition from stratified to intermittent flow regimes as the water level in the storage tank is lowered due to inventory evaporation. The transition is accompanied with an oscillatory flow rate due to the differing pressure drops between the flow regimes. The instability ceases after fully transiting into the intermittent flow regime. The presence of the transition was determined with a steady-state drift flux model and wire-mesh sensor data.

Assessment of RELAP5/MOD3.2 for startup transients in a natural circulation test facility

System level analysis codes have been proved effective tools for accident analysis and other transient analysis in nuclear reactors. With increasing interests in the natural circulation flow driven systems, flow instabilities occurring under low power and low pressure conditions have been receiving more research attention. This work aims to demonstrate the performance of one of the most widely used system codes, i.e., RELAP5, in simulating startup transients, as well as its capability to predict flow instabilities. The experimental database was acquired from a well-scaled natural circulation test facility, which was designed to investigate the startup flow instabilities for a BWR-type Novel Modular Reactor (NMR-50). The startup simulations by RELAP5/MOD3.2 code were compared with various thermal-hydraulics measurements including system pressure, temperature, natural circulation flow rate, and void fraction, etc. In addition, two power ramp rates were applied in the startup simulations to study heating rate effects on flow instabilities. In general, the RELAP5 code can consistently simulate the two startup transients and predict two flow instabilities, i.e., flashing instability and density wave oscillations, occurred in the natural circulation test facility at low pressures. The predicted natural circulation flow velocities had a good agreement with the measurements. However, the RELAP5 could underpredict void fractions in the heated section when the system is under thermal non-equilibrium conditions at low power and low pressure.

Modeling of flashing-induced flow instabilities for a natural circulation driven novel modular reactor

An analytical study based on frequency domain analysis is presented on the flashing-induced flow instability in a natural circulation test facility, which was designed to investigate the flow instability for a BWR-type novel modular reactor (NMR). To address the flashing phenomena at low pressure conditions, such as initial startup transients or accidents, the liquid enthalpy change in the P-T diagram due to reduced hydrostatic head in the riser or chimney was treated as an axially uniform heat flux. Based on the drift flux model, the system transfer function was obtained through small perturbations about the steady state in the frequency domain. The D-partition method was used to determine the neutral stability boundary in the dimensionless stability plane, which was constituted of the subcooling number and phase change number. From the frequency domain analysis, the flashing stability boundary and the density wave oscillations boundary could be predicted. Some parametric studies had been performed on the system pressure and the inlet flow resistance coefficients in the stability analysis. The results showed that the flashing stability boundary was more sensitive to the system pressure than the density wave oscillations. In addition, the theoretical stability boundaries were benchmarked against the experimental stability boundaries from quasi-steady state tests. Although the general stability boundary agreed well with the experiments, certain discrepancies still existed due to the assumptions of thermal equilibrium in current study. In the future, the thermal non-equilibrium conditions including subcooled boiling will be taken into account in the flashing induced stability analysis.

Investigation of Characteristics of Passive Heat Removal System Based on the Assembled Heat Transfer Tube

Abstract To get an insight into the operating characteristics of the passive residual heat removal system of molten salt reactors, a two-phase natural circulation test facility was constructed. The system consists of a boiling loop absorbing the heat from the drain tank, a condensing loop consuming the heat, and a steam drum. A steady-state experiment was carried out, in which the thimble temperature ranged from 450°C to 700°C and the system pressure was controlled at levels below 150 kPa. When reaching a steady state, the system was operated under saturated conditions. Some important parameters, including heat power, system resistance, and water level in the steam drum and water tank were investigated. The experimental results showed that the natural circulation system is feasible in removing the decay heat, even though some fluctuations may occur in the operation. The uneven temperature distribution in the water tank may be inevitable because convection occurs on the outside of the condensing tube besides boiling with decreasing the decay power. The instabilities in the natural circulation loop are sensitive to heat flux and system resistance rather than the water level in the steam drum and water tank. RELAP5 code shows reasonable results compared with experimental data.

Experimental observations of natural circulation flow in the NSTF

A 1/2 scale test facility has been constructed at Argonne National Laboratory to study the heat removal performance and natural circulation flow patterns in a Reactor Cavity Cooling System (RCCS). Our test facility, the Natural convection Shutdown heat removal Test Facility (NSTF), supports the broader goal of developing an inherently safe and fully passive ex-vessel decay heat removal for advanced reactor designs. The project, initiated in 2010 to support the Advanced Reactor Technologies (ART), Small Modular Reactor (SMR), and Next Generation Nuclear Plant (NGNP) programs, has been conducting experimental operations since early 2014. The following paper provides a summary of some primary design features of the 26-m tall test facility along with a description of the data acquisition suite that guides our experimental practices. Specifics of the distributed fiber optic temperature measurements will be discussed, which introduces an unparalleled level of data density that has never before been implemented in a large scale natural circulation test facility. Results from our test series will then be presented, which provide insight into the thermal hydraulic behavior at steady-state and transient conditions for varying heat flux levels and exhaust chimney configuration states.

Experimental investigation of natural circulation instability in a BWR-type small modular reactor

The Purdue NMR (Novel Modular Reactor) represents a BWR-type small modular reactor with a significantly reduced reactor pressure vessel (RPV). Specifically, the NMR is one third the height and area of a conventional BWR RPV with an electrical output of 50 MWe. Experiments are performed in a well-scaled test facility to investigate the thermal hydraulic flow instabilities during the startup transients for the NMR. The scaling analysis for the design of natural circulation test facility uses a three-level scaling methodology. Scaling criteria are derived from non-dimensional field and constitutive equations. Important thermal hydraulic parameters, e.g. system pressure, inlet coolant flow velocity and local void fraction, are analyzed for slow and fast normal startup transients. Flashing instability and density wave oscillation are the main flow instabilities observed when system pressure is below 0.5 MPa. And the flashing instability and density wave oscillation show different type of oscillations in void fraction profile. Finally, the pressurized startup procedure is recommended and tested in current research to effectively eliminate the flow instabilities during the NMR startup transients.

Experimental and Numerical Investigation on a Two-Phase Natural Circulation Test Facility

A geometrically similar model of the proposed Advanced Heavy Water Reactor being designed and built by Bhabha Atomic Research Centre has been conceived and built at Indian Institute of Technology, Bombay. The basic objective of setting up such a facility and conducting experimental studies was to simulate the thermal-hydraulic behavior during the startup and also during part or full power operation. Owing to hydrodynamic instabilities observed in natural circulation system, it is necessary to know the stability boundaries. In this paper, the details of the experimental facility, experimental studies, and numerical simulations carried out using RELAP5/MOD 3.2 are discussed. Type I and type II stability boundaries and stable two-phase operation zones have been experimentally obtained at various pressures and inlet subcooling and the results are correlated using appropriate nondimensional numbers. Further, the experimental facility is modeled using the thermal-hydraulic code RELAP5/MOD 3.2. The single-phase pressure drops measured in the loop are used to estimate the appropriate loss coefficients used in the model. The model is then used to predict stability boundaries, and the predictions are compared with the experimental findings. The details of the experimental model and simulation results are presented. The capability of RELAP5/MOD 3.2 to predict the flow oscillations during instabilities is discussed.

Experimental investigations on flashing-induced instabilities in one and two-parallel channels: A comparative study

Abstract In this investigation, experiments conducted in a natural circulation test facility at low power and low pressure conditions, in the one single and two-parallel channels configuration are presented and discussed in detail. The novel manner of visualizing the results allowed characterizing the facility at any time and position which helped to thoroughly understand the instability mechanisms. Different modes were observed for each configuration. In the case of having two-parallel channels, four different behaviors have been observed: stable flow circulation, periodic high subcooling oscillations, a-periodical oscillations and out-of-phase periodical oscillations. In addition, stability maps were constructed in order to clarify the region in which each mode is dominant. The results obtained from both the one and two-parallel channels configurations are thus analyzed and compared. As a result, some similarities have been observed between the intermittent flow oscillations found in the single channel experiments and the high subcooling oscillations found in the two-parallel channels experiments. Moreover, similarities have also been found between the sinusoidal flow oscillations existing in the single channel experiments and the out-of-phase oscillations from the two-parallel channels experiments. The experiments presented in this work can be used to benchmark numerical codes and modeling techniques developed to study the start-up of natural circulation BWRs.

Study of startup transients and power ramping of natural circulation boiling systems

Numerical models of a natural circulation test facility and its prototype have been developed with RELAP5/MOD3.4 code and verified for their grid independence by nodal sensitivity studies. The model of the test facility has been validated for its steady state as well as transient predictions with the help of experimental observations. The transient predictions and parametric trends obtained by the numerical model of the prototype have been compared with those of the numerical model of the test facility. Thus, the ability of RELAP5 code to predict the transients during startup of a natural circulation boiling water reactor is verified. A powering procedure for the test facility has been conceptualized with the help of its RELAP5 model and demonstrated experimentally. Based on this, a similar powering procedure for the prototype has been proposed and simulated numerically with its RELAP5 model.

Stability analysis of a two-phase test loop

This paper presents an analytical model developed for investigating the stability characteristics of a low-pressure natural circulation test facility. The approach is to convert the two-phase flow system into a linear feedback system with respect to a steady-state operating point and linear theory is then applied to determine if the system would be stable around the operating point. The analysis results show that the stability of the system is sensitive to its inlet temperature and power level.

Theoretical and experimental study on density wave oscillation of two-phase natural circulation of low equilibrium quality

A theoretical and experimental study of density wave oscillation (DWO) in natural circulation is presented in this paper. Experiments were performed on a natural circulation test facility. The influences of mass flow rate, pressure, inlet subcooling, heat flux and exit quality on DWO were analyzed. The marginal stability boundary (MSB) of DWO was obtained. A criterion of two-phase natural circulation, which predicts the stability thresholds, was developed by lumped parameter method. It is a function of non-dimensional parameters, such as phase change number N pch, subcooling number N sub, Froude number, Fr, geometry number N l and friction number τ. The geometry number and friction number are first defined in this paper. A correlation of DWO period was also obtained, it is also a function of the above non-dimensional parameters. The results of the present criterion and period correlation were compared with those of the experimental data and references. It is shown that they agree very well.