Energy Research Integral System Test Research Articles

TRACE simulations of IIST cooldown experiments

Abstract IIST (Institute of Nuclear Energy Research Integral System Test) facility is a RHRP (Reduced-Height and Reduced-Pressure) facility for the three-loop PWR (Pressurized Water Reactor) NPP (Nuclear Power Plant) Maanshan. This test facility was established in 1992 for safety studies of the Maanshan NPP. TRACE is an advanced thermal hydraulic code which is still developed and updated by U.S. NRC. So, validation of specific models against experiments is still necessary. In this paper, cooldown experiments in IIST facility are used to validate the TRACE models. Additionally, the results of TRACE calculations are compared to results of RELAP5 calculations. This comparison shows that the calculations of TRACE are in good agreement to the data of IIST experiments and to the calculational results of RELAP5. So, TRACE can be used to simulate cooldown transients.

Assessment of TRACE CCFL Model with SBLOCA Experiment of IIST Facility

In this paper, the TRACE model for IIST facility is developed and verified with the Small Break loss of coolant accident (SBLOCA) experiment of IIST (Institute of Nuclear Energy Research Integral System Test) facility. By using the Wallis and Kutateladze correlations of countercurrent flow limitation (CCFL) model, the TRACE analyses results, such as break flow rate, primary pressure, and the temperature of cold-leg and hot-leg, are consistent with the IIST data. The results show the Kutateladze correlation of CCFL model can well predict the SBLOCA behavior and present good agreement with IIST experiment data in this paper. Besides, the sensitivity study results of Kutateladze correlation in CCFL model are verified and compared with the IIST data.

Adequacy of Power-to-Mass Scaling in Simulating PWR Incident Transient for Reduced-Height, Reduced-Pressure and Full-Height, Full-Pressure Integral System Test Facilities

A complete scheme of scaling methods to design the reduced-height, reduced-pressure (RHRP) Institute of Nuclear Energy Research Integral System Test (IIST) facility and to specify test conditions for incident simulation was developed. In order to preserve core decay power history and coolant mass inventory during a transient, a unique power-to-mass scaling method is proposed and utilized for RHRP and full-height, full-pressure (FHFP) systems. To validate the current scaling method, three counterpart tests done at the IIST facility are compared with the FHFP tests in small-break loss-of-coolant, station blackout, and loss-of-feedwater accidents performed at the Large-Scale Test Facility (LSTF) and the BETHSY test facility. Although differences appeared in design, scaling, and operation conditions among the IIST, LSTF, and BETHSY test facilities, the important physical phenomena shown in the facilities are almost the same. The physics involved in incident transient phenomena are well measured and modeled by showing the common thermal-hydraulic behavior of key parameters and the general consistency of chronological events. The results also confirm the adequacy of power-to-mass scaling methodology.

Experimental Study on the Performance of the Passive Safety Injection in an IIST

A passive core cooling system (PCCS) has been installed at the Institute of Nuclear Energy Research Integral System Test facility. It includes three core makeup tanks (CMTs), three accumulators (ACCs), a four-stage automatic depressurization system (ADS), a passive residual heat removal (PRHR) heat exchanger submerged into an in-containment refueling water storage tank (IRWST). The purpose of this research is to study the performance of the PCCS with passive injection during either a pressure balance line (PBL) break or a direct vessel injection (DVI) line break. Five experiments were performed simulating break area ratios of 0.5 to 2.0% (1.88 to 3.77 mm) at either a PBL or a DVI line. The general system response and the interactions of CMT, ACC, PRHR, and IRWST to the effect of core heat removal are observed and discussed. The experimental results show long-term core cooling can be reached for the cases of the PBL break and the DVI-line break following the PCCS actuation procedures.

IIST and BETHSY Counterpart Tests on a PWR Total Loss-of-Feedwater Transient with Two Different Bleed-and-Feed Scenarios

The thermal-hydraulic phenomena and recovery actions of loss-of-feedwater (LOFW) incidents in a pressurized water reactor were investigated experimentally at the Institute of Nuclear Energy Research Integral System Test (IIST) facility. To understand whether the physical phenomena observed in the full-height and full-pressure facility during an LOFW transient can be simulated in the reduced-height and reduced-pressure IIST facility, two counterpart tests based on the same scenarios as those of the BETHSY tests were performed. These two tests performed in BETHSY differ mainly at the initiation of the bleed-and-feed process on the primary side in order to examine the effectiveness of recovery measures on the processes of the pressurizer power-operated relief valves early and late opening. The initial and boundary conditions of the current tests were determined by scaling down the corresponding conditions of the LOFW experiments performed at BETHSY. In view of the inherent differences in design, scaling approach, and facility operation conditions in the systems, the consistency between the counterpart tests is examined by identifying key thermal-hydraulic phenomena and clarifying their differences. The results of the IIST and BETHSY tests showed the common thermal-hydraulic behaviors of key parameters, such as system pressure, void fraction in the hot leg, primary coolant inventory, pressurizer level, and discharged mass evolutions. The chronological events studied in the IIST facility are generally consistent with those studied in BETHSY. The results from the IIST facility may not be exact replications of the BETHSY response; however, the physics involved in bleed-and-feed are well measured and modeled.

Safety Evaluation of Primary-Side Bleed-and-Feed Scenarios by an Experimental Loss of Feedwater Event at the IIST Facility

The current emergency operating procedures (EOPs) for a three-loop pressurized water reactor on a total loss-of-feedwater (LOFW) incident are experimentally evaluated at the Institute of Nuclear Energy Research Integral System Test (IIST) facility. Four experiments were conducted with a different number of pressurizer power-operated relief valves (PORVs) and centrifugal charging pumps (CCPs) engaged in performing the primary-side bleed-and-feed operation. In addition, the effect of opening a pressure vessel venting-path was also evaluated. The focus was on the effectiveness and adequacy of current EOPs and the observation of associated thermal-hydraulic phenomena. The test results indicated that the current EOPs can function well in mitigating the consequences of a LOFW event provided that at least two fully opened PORVs and one actuated CCP are used. The opening of a pressure vessel venting path during the bleed-and-feed operation is helpful in the primary system depressurization but has little effect to increase the pressure vessel coolant inventory. However, by opening one PORV, precaution for the reactor coolant system (RCS) integrity should be taken for system overpressurization caused by insufficient heat removal capability. Furthermore, an effective control of restored feedwater to a low injection rate is favorable to minimize the thermal stress in the RCS pressure boundary.

Assessment and Application of the ROSE Code for Reactor Outage Thermal-Hydraulic and Safety Analysis

The currently available tools, such as RELAP5, RETRAN, and others, cannot easily and correctly perform the task of analyzing the system behavior during plant outages. Therefore, a medium-sized program aiming at reactor outage simulation and evaluation, such as midloop operation (MLO) with loss of residual heat removal (RHR), has been developed. Important thermal-hydraulic processes involved during MLO with loss of RHR can be properly simulated by the newly developed reactor outage simulation and evaluation (ROSE) code. The two-region approach with a modified two-fluid model has been adopted to be the theoretical basis of the ROSE code.To verify the analytical model in the first step, posttest calculations against the integral midloop experiments with loss of RHR have been performed. The excellent simulation capacity of the ROSE code against the Institute of Nuclear Energy Research Integral System Test Facility test data is demonstrated. To further mature the ROSE code in simulating a full-sized pressurized water reactor, assessment against the WGOTHIC code and the Maanshan momentary-loss-of-RHR event has been undertaken. The successfully assessed ROSE code is then applied to evaluate the abnormal operation procedure (AOP) with loss of RHR during MLO (AOP 537.4) for the Maanshan plant. The ROSE code also has been successfully transplanted into the Maanshan training simulator to support operator training. How the simulator was upgraded by the ROSE code for MLO will be presented in the future.

Reflux condensation behavior in a U-tube steam generator with or without noncondensables

A series of reflux condensation experiments on the mid-loop inventory were conducted in the Institute of Nuclear Energy Research integral system test facility with various steam flow rates and noncondensables contents. The focus of this paper is on reflux condensation behavior in a U-tube steam generator with or without noncondensables. In particular, this paper explains the effect of noncondensables on integral system responses, U-tube flow modes, control mechanism for liquid holdup/breakdown, and loop seal clearance/reformation phenomena.

Experimental Investigation of Early Initiation of Primary Cooldown by Secondary-Side Depressurization in a PWR Inadequate Core-Cooling Accident

The thermal-hydraulic phenomena of inadequate core cooling caused by a cold-leg small-break loss-of-coolant accident (SBLOCA) were investigated experimentally at the Institute of Nuclear Energy Research Integral System Test facility. The experiments were performed under the conditions of different break sizes (0.5 and 2%) in the cold leg followed by failure of the high-pressure injection system. The primary system cooldown is implemented by the secondary-side depressurization. The effectiveness of early initiation of the recovery action on reactor safety and related thermal-hydraulic phenomena are examined. The initiation criterion for recovery action considered here is determined by core water levels instead of core exit temperature based on the current emergency operating procedures. The impact of emergency core-cooling flow bypass phenomenon may significantly deteriorate the effectiveness of the recovery operation for a cold-leg SBLOCA. The results showed that the early initiation of secondary-side depressurization can effectively minimize the risk of core damage by preventing fuel rods from heating up throughout the transient. In addition, the core suffers a rather moderate thermal stress during the cooldown process.

Code Development and Assessment for Reactor Outage Thermal-Hydraulic and Safety Analysis – Midloop Operation with Loss of Residual Heat Removal

Although only a few percent of residual power remains during plant outages, the associated risk of core uncovery and corresponding fuel overheating has been identified to be relatively high, particularly under midloop operation (MLO) in pressurized water reactors. However, to analyze the system behavior during outages, the tools currently available, such as RELAP5, RETRAN, etc., cannot easily perform the task. Therefore, a medium-sized program aiming at reactor outage simulation and evaluation, such as MLO with the loss of residual heat removal (RHR), was developed. All important thermal-hydraulic processes involved during MLO with the loss of RHR will be properly simulated by the newly developed reactor outage simulation and evaluation (ROSE) code. Important processes during MLO with loss of RHR involve a pressurizer insurge caused by the hot-leg flooding, reflux condensation, liquid holdup inside the steam generator, loop-seal clearance, core-level depression, etc. Since the accuracy of the pressure distribution from the classical nodal momentum approach will be degraded when the system is stratified and under atmospheric pressure, the two-region approach with a modified two-fluid model will be the theoretical basis of the new program to analyze the nuclear steam supply system during plant outages. To verify the analytical model in the first step, posttest calculations against the closed integral midloop experiments with loss of RHR were performed. The excellent simulation capacity of the ROSE code against the Institute of Nuclear Energy Research Integral System Test Facility (IIST) test data is demonstrated.

An Evaluation of Emergency Operator Actions by an Experimental SGTR Event at the IIST Facility and a Comparison of Mihama-2 SGTR Event Record

The current emergency operating procedures (EOPs) for a three-loop pressurized water reactor on steam generator tube rupture (SGTR) incident are experimentally evaluated at the Institute of Nuclear Energy Research Integral System Test Facility. The focus is on the adequacy of EOPs to limit primary-to-secondary leakage with particular emphasis on the number of ruptured U-tubes on the severity of an incident and the response time available for operator actions. By comparing test data with plant records of a Mihama-2 SGTR event, the key thermal-hydraulic phenomena during SGTR transients can be successfully simulated. The test results indicated that the current EOPs can function well in terminating the break flow and maintaining adequate core cooling. However, the effectiveness in minimizing the radioactive release demands an early and substantial operator involvement. To mitigate the consequences of such an event, the timing to isolate the faulted steam generator (SG) and to terminate safety injection flow will be critical. Furthermore, to avoid overfilling, the water level in the faulted SG needs to be drained prior to the implementation of the cooldown process.

Using an IIST SBLOCA Experiment to Assess RELAP5/MOD3.2

The RELAP5/MOD3.2 code is used at the Institute of Nuclear Energy Research Integral System Test Facility to analyze a 2% cold-leg-break experiment that includes failure of the high-pressure injection system. The assessment code predictions include primary pressure, inventory distribution in the reactor coolant system (RCS), loop flow rate, break flow rate, and core thermal hydraulics. A comparison between the calculated results and the experimental data shows (a) a good match with the predictions of the RCS pressure and hot- and cold-leg fluid temperatures, (b) underprediction of the core and downcomer levels, (c) overprediction of the loop flow rates in single- and two-phase natural circulation, and (d) inadequate prediction of asymmetric coolant holdup in the three steam generators. Also presented are sensitivity studies of choked flow associated with the defaulted values of discharge coefficients in the simulation of the break flow, and of the core bypass area to evaluate the effect of core level depression.

Power-operated relief valve stuck-open accident and recovery scenarios in the Institute of Nuclear Energy Research integral system test facility

Four scaled small break loss-of-coolant accident (LOCA) tests simulating the pressurizer power-operated relief valves (PORVs) stuck-open accidents and the recovery actions in a pressurized water reactor (PWR) were performed at the Institute of Nuclear Energy Research (INER) integral system test (IIST) facility. The objectives of this study are to verify the effectiveness of emergency operating procedure (EOP) and emergency core cooling system (ECCS) on reactor safety. The break sizes were volumetrically scaled down based on one and all three fully-opened PORVs which is equivalent to 0.23% and 0.69% hot leg flow area of the reference plant. The experimental results indicate that in case of high pressure injection (HPI) system failure, the rapid depressurization of the steam generators is proved to be an effective way in the depressurization of the reactor coolant system and the core cooling. In contrast, if only one HPI charging pump operates normally, which injected half (or minimum) flow rate of normal cooling water, the core cooling can be adequately provided without operating the secondary bleeding during PORV stuck-open transient. This paper also presents the scaling methods for the reduced-height, reduced-pressure (RHRP) IIST facility and the test conditions. The validity of the present scaling methodology is confirmed by the results from previous IIST counterpart tests and comparison of the present results with those of the tests performed at the full-height, full-pressure(FHFP) stuck-open tests.

Investigation of Scale-Up Capability of RELAP5/MOD3 and Scaling Distortion in the IIST Facility Via Natural Circulation Experiments

During the past years, a number of reduced-scale test facilities have been constructed to investigate the physical phenomena of transients or accidents occurring in nuclear power plants. Since the behavior of a nuclear power plant is complicated, it is quite impossible for a small-scaled facility to simulate all the physical phenomena during the transient process. But, by way of proper scaling, most of the important aspects of transient behavior can be simulated. Calculations using RELAP5/MOD3 investigate whether most of the key thermal-hydraulic phenomena observed in the Institute of Nuclear Energy Research Integral System Test (IIST) facility can be expected in a prototype plant. When compared with experimental data, the calculated results of two different scale models show reasonable agreement with the natural circulation transients. The scale-up capability of RELAP5/MOD3 is demonstrated by simulating the single-phase and two-phase natural circulation transients. Also, the scaling distortions in the heat transfer areas of the IIST facility do not strongly distort the thermal-hydraulic behavior of experimental data.

Numerical simulation of natural circulation experiments conducted at the IIST facility

Abstract Natural circulation plays an important role in long-term cooling of pressurized water reactors (PWRs) under small break loss-of-coolant accidents. Recently, natural circulation experiments have been conducted at the Institute of Nuclear Energy Research integral system test (IIST) facility, which is used to simulate the Westinghouse three-loop Maanshan PWR. A numerical simulation is presented to investigate the natural circulation phenomena of the IIST facility with the RELAP5/MOD3 code. The calculated results are in good agreement with the experimental data of the single-phase natural circulation both quantitatively and qualitatively. The influences of power level and system pressure on natural circulation can also be predicted by the current model. Based on the two-phase natural circulation data, the calculated flow rate history is similar to that obtained from the experiment.