Main Steam Line Break Benchmark Research Articles

Testing the NURESIM platform on a PWR main steam line break benchmark

Within the NURESAFE project, a main steam line break benchmark has been defined and solved by codes integrated into the European code platform NURESIM. The paper describes the results of the calculations for this benchmark. Six different solutions using different codes and code systems are provided for the comparison. The quantitative differences in the results are dominated by the differences in the secondary system parameters during the depressurization. The source of these differences comes mainly from the application of different models for the two-phase leak flow available in the system codes. The use of two different thermal hydraulic system codes influences the results more than expected when the benchmark was created. The codes integrated into the NURESIM platform showed their applicability to a challenging transient like a main steam line break.

Analysis of the Main Steam Line Break Benchmark (Phase II) Using ANCK/MIDAC Code

The three-dimensional (3D) neutronics and thermal-and-hydraulics (T/H) coupling code ANCK/MIDAC has been developed. ANCK/MIDAC consists of the 3D nodal kinetic code ANCK and the 3D drift flux T/H code MIDAC. In order to verify the adequacy of ANCK/MIDAC, the Phase II problem in the “OECD main steam line break benchmark (MSLB benchmark)” was analyzed. This MSLB benchmark has been defined in order to simulate the core response and the reactor coolant system response to a relatively severe steam line break accident condition. The Phase II problem has a conservative condition that the control rod with the maximum worth is stuck in a fully withdrawn position throughout the transient. The simulation was performed using the core inlet temperatures and flow rates for 18 different regions, which were provided by the PSU best-estimate TRAC-PF1/NEM calculations. The comparison of the ANCK/MIDAC results with other participants' results shows the excellent agreement on main core parameters. ANCK/MIDAC has good capability and reliability for the best-estimation code and a reasonable calculation time.

Analysis of the Main Steam Line Break Benchmark (Phase II) Using ANCK/MIDAC Code

The three-dimensional (3D) neutronics and thermal-and-hydraulics (T/H) coupling code ANCK/MIDAC has been developed. ANCK/MIDAC consists of the 3D nodal kinetic code ANCK and the 3D drift flux T/H code MIDAC. In order to verify the adequacy of ANCK/MIDAC, the Phase II problem in the “OECD main steam line break benchmark (MSLB benchmark)” was analyzed. This MSLB benchmark has been defined in order to simulate the core response and the reactor coolant system response to a relatively severe steam line break accident condition. The Phase II problem has a conservative condition that the control rod with the maximum worth is stuck in a fully withdrawn position throughout the transient. The simulation was performed using the core inlet temperatures and flow rates for 18 different regions, which were provided by the PSU best-estimate TRAC-PF1/NEM calculations. The comparison of the ANCK/MIDAC results with other participants' results shows the excellent agreement on main core parameters. ANCK/MIDAC has good capability and reliability for the best-estimation code and a reasonable calculation time.

Validation of Coupled Thermal-Hydraulic and Neutronics Codes for Safety Analysis by International Cooperations

Incorporating full three-dimensional models of the reactor core into system transient codes allows for a “best-estimate” calculation of interactions between the core behavior and plant dynamics. Considerable efforts have been made in various countries and organizations on the development of coupled thermal-hydraulic and neutronics codes. Appropriate benchmarks have been developed in international cooperations led by the Nuclear Energy Agency (NEA) of the Organization for Economic Cooperation and Development (OECD) that permit testing of the neutronics–thermal-hydraulics coupling and verification of the capability of the coupled codes to analyze complex transients with coupled core-plant interactions. Three such benchmarks are presented in this paper—the OECD/U.S. Nuclear Regulatory Commission (NRC) pressurized water reactor main steam line break benchmark, the OECD/NRC boiling water reactor turbine trip benchmark, and the OECD/U.S. Department of Energy/Commissariat à l’Energie Atomique V1000 coolant transient benchmark. To meet the objectives of the validation of best-estimate coupled codes, a systematic approach has been introduced to evaluate the analyzed transients employing a multilevel methodology. Since these benchmarks are based on both code-to-code and code-to-data comparisons, further guidance for presenting and evaluating results has been developed. During the course of the benchmark activities, a professional community has been established, which allowed our carrying out in-depth discussions of different aspects considered in the validation process of the coupled codes. This positive output has certainly advanced the state of the art in the area of coupling research.

Development of Pressurized Water Reactor Integrated Safety Analysis Methodology Using Multilevel Coupling Algorithm

The subchannel code COBRA-TF has been introduced for an evaluation of thermal margins on the local pin-by-pin level in a pressurized water reactor. The coupling of COBRA-TF with TRAC-PF1/NEM is performed by providing from TRAC to COBRA-TF axial and radial thermal-hydraulic boundary conditions and relative pin-power profiles, obtained with the pin power reconstruction model of the nodal expansion method (NEM). An efficient algorithm for coupling of the subchannel code COBRA-TF with TRAC-PF1/NEM in the parallel virtual machine environment was developed addressing the issues of time synchronization, data exchange, spatial overlays, and coupled convergence. Local feedback modeling on the pin level was implemented into COBRA-TF, which enabled updating the local form functions and the recalculation of the pin powers in TRAC-PF1/NEM after obtaining the local feedback parameters. The coupled TRAC-PF1/NEM/COBRA-TF code system was tested on the rod ejection accident and main steam line break benchmark problems. In both problems, the local results are closer than before the introduced multilevel coupling to the corresponding critical limits. This fact indicates that the assembly average results tend to underestimate the accident consequences in terms of local safety margins. The capability of local safety evaluation, performed simultaneously (online) with coupled global three-dimensional neutron kinetics/thermal-hydraulic calculations, is introduced and tested. The obtained results demonstrate the importance of the current work.

Consistent Comparison of the Codes RELAP5/PARCS and TRAC-M/PARCS for the OECD MSLB Coupled Code Benchmark

A generalized interface module was developed for coupling any thermal-hydraulic code to any spatial kinetic code. In the design used here the thermal-hydraulic and spatial kinetic codes function as independent processes and communicate using the Parallel Virtual Machine software. This approach helps maximize flexibility while minimizing modifications to the respective codes. Using this interface, the U.S. Nuclear Regulatory Commission (NRC) three-dimensional neutron kinetic code, Purdue Advanced Reactor Core Simulator (PARCS), has been coupled to the NRC system analysis codes RELAP5 and Modernized Transient Reactor Analysis Code (TRAC-M). Consistent comparison of code results for the Organization for Economic Cooperation and Development/Nuclear Energy Agency main steam line break benchmark problem using RELAP5/PARCS and TRAC-M/PARCS was made to assess code performance.

Methods and Results for the MSLB NEA Benchmark Using SIMTRAN and RELAP-5

The purpose of this paper is first to discuss the methods developed in our three-dimensional pressurized water reactor core dynamics code SIMTRAN and its coupling to the system code RELAP-5 for general transient and safety analysis. Then, we summarize its demonstration application to the Nuclear Energy Agency (NEA)/Organization for Economic Cooperation and Development (OECD) Benchmark on Main Steam Line Break (MSLB), co-sponsored by the U.S. Nuclear Regulatory Commission (NRC) and other regulatory institutions. In particular, our work has been supported by the Spanish “Consejo de Seguridad Nuclear” (CSN) under a CSN research project.Our results for the steady states and the guided-core transients, proposed as exercise 2 of the MSLB benchmark, show small deviations from the mean results of all participants, especially in core average parameters. For the full-coupled core-plant transients, exercise 3, a detailed comparison with the University of Purdue–NRC results using PARCS/RELAP-5, shows quite good agreement in both integral and local parameters, especially for the more extreme return-to-power scenario.

Analysis of the Pressurized Water Reactor Main Steam Line Break Benchmark by the Coupled Code System ATHLET-QUABOX/CUBBOX

The Organization for Economic Cooperation and Development (OECD) Pressurized Water Reactor Main Steam Line Break (MSLB) Benchmark has been calculated for all three exercises by the coupled code system ATHLET-QUABOX/CUBBOX developed by Gesellschaft für Anlagen- und Reaktorsicherheit (GRS). The results obtained are presented, and a detailed comparison with other solutions of the benchmark is discussed. An attempt is made to explain the differences observed in the solutions by the different modeling of physical processes in the codes. The sensitivity of results on modeling features is also investigated. In addition, the effect of different mapping schemes between fuel assemblies of the core loading and the thermal–fluid dynamics on the accuracy of three-dimensional (3-D) neutronics solutions is studied. The results for the MSLB transient are also evaluated to compare 3-D neutronics and point-kinetics solutions in view of integral and local parameters. Thus, the experiences with the coupled code system ATHLET-QUABOX/CUBBOX during the MSLB benchmark activity are summarized.

Analyses of the OECD Main Steam Line Break Benchmark with the DYN3D and ATHLET Codes

The Organization for Economic Cooperation and Development (OECD) Main Steam Line Break (MSLB) Benchmark was defined to validate the thermal-hydraulic system codes coupled with three-dimensional (3-D) neutron kinetic codes. The reference problem is an MSLB in a pressurized water reactor at end of cycle. The analyses were performed with the 3-D core model DYN3D, the thermal-hydraulic system code ATHLET, and the coupled code DYN3D/ATHLET. The results of the DYN3D and ATHLET simulations based on the specification are compared with the results of other participants in the final OECD reports. The effect of the thermal-hydraulic nodalization of the core, i.e., the number of coolant channels, and the influence of the coolant mixing inside the pressure vessel are studied in the paper. Calculations with a reduced number of coolant channels are performed often in coupled calculations for saving computational time. Results of a 25-channel model were compared with the 177-channel calculation (1 channel per assembly). The results for global parameters like nuclear power show only small differences for the two models; however, the prediction of local parameters such as maximum fuel temperatures requires a detailed thermal-hydraulic modeling. The effect of different coolant mixing within the reactor pressure vessel is investigated. It is shown that the influence of coolant mixing mitigates the accident consequences when 3-D neutron kinetics is applied. In case of point kinetics, coolant mixing leads to an opposite effect. To profit from the 3-D core model, a realistic description of the coolant mixing in the coupled codes is a topic of further investigations.

CEA-IRSN Participation in the Main Steam Line Break Benchmark

The Organization for Economic Cooperation and Development/Nuclear Energy Agency Main Steam Line Break (MSLB) benchmark provides a comparison of state-of-the-art and best-estimate models used to compute reactivity accidents. A comprehensive study has been carried out by Commissariat à l’Energie Atomique and Institut de Radioprotection et de Sûreté Nucléaire with the CATHARE, CRONOS2, and FLICA4 codes. The effect of mixing between primary loops in the core vessel is analyzed, then zero-dimensional and three-dimensional (3-D) kinetics are compared, and finally, the effect of the core thermal-hydraulic model is presented. The aim of this analysis is to assess the 3-D effects in the MSLB accident and to explain the return-to-power occurrence.

Using the OECD/NRC Pressurized Water Reactor Main Steam Line Break Benchmark to Study Current Numerical and Computational Issues of Coupled Calculations

Incorporating full three-dimensional (3-D) models of the reactor core into system transient codes allows for a “best-estimate” calculation of interactions between the core behavior and plant dynamics. Recent progress in computer technology has made the development of coupled thermal-hydraulic (T-H) and neutron kinetics code systems feasible. Considerable efforts have been made in various countries and organizations in this direction. Appropriate benchmarks need to be developed that will permit testing of two particular aspects. One is to verify the capability of the coupled codes to analyze complex transients with coupled core-plant interactions. The second is to test fully the neutronics/T-H coupling. One such benchmark is the Pressurized Water Reactor Main Steam Line Break (MSLB) Benchmark problem. It was sponsored by the Organization for Economic Cooperation and Development, U.S. Nuclear Regulatory Commission, and The Pennsylvania State University. The benchmark problem uses a 3-D neutronics core model that is based on real plant design and operational data for the Three Mile Island Unit 1 nuclear power plant. The purpose of this benchmark is threefold: to verify the capability of system codes for analyzing complex transients with coupled core-plant interactions; to test fully the 3-D neutronics/T-H coupling; and to evaluate discrepancies among the predictions of coupled codes in best-estimate transient simulations. The purposes of the benchmark are met through the application of three exercises: a point kinetics plant simulation (exercise 1), a coupled 3-D neutronics/core T-H evaluation of core response (exercise 2), and a best-estimate coupled core-plant transient model (exercise 3).In this paper we present the three exercises of the MSLB benchmark, and we summarize the findings of the participants with regard to the current numerical and computational issues of coupled calculations. In addition, this paper reviews in some detail the sensitivity studies on exercises 2 and 3 performed by the benchmark team using the coupled code TRAC-PF1/NEM. The purpose of these supporting studies was to aid participants in developing their models.

Effects of Secondary Circuit Modeling on Results of Pressurized Water Reactor Main Steam Line Break Benchmark Calculations with New Coupled Code TRAB-3D/SMABRE

All of the three exercises of the Organization for Economic Cooperation and Development/Nuclear Regulatory Commission pressurized water reactor main steam line break (PWR MSLB) benchmark were calculated at VTT, the Technical Research Centre of Finland. For the first exercise, the plant simulation with point-kinetic neutronics, the thermal-hydraulics code SMABRE was used. The second exercise was calculated with the three-dimensional reactor dynamics code TRAB-3D, and the third exercise with the combination TRAB-3D/SMABRE. VTT has over ten years’ experience of coupling neutronic and thermal-hydraulic codes, but this benchmark was the first time these two codes, both developed at VTT, were coupled together. The coupled code system is fast and efficient; the total computation time of the 100-s transient in the third exercise was 16 min on a modern UNIX workstation. The results of all the exercises are similar to those of the other participants. In order to demonstrate the effect of secondary circuit modeling on the results, three different cases were calculated. In case 1 there is no phase separation in the steam lines and no flow reversal in the aspirator. In case 2 the flow reversal in the aspirator is allowed, but there is no phase separation in the steam lines. Finally, in case 3 the drift-flux model is used for the phase separation in the steam lines, but the aspirator flow reversal is not allowed. With these two modeling variations, it is possible to cover a remarkably broad range of results. The maximum power level reached after the reactor trip varies from 534 to 904 MW, the range of the time of the power maximum being close to 30 s. Compared to the total calculated transient time of 100 s, the effect of the secondary side modeling is extremely important.

Analysis of the OECD Main Steam Line Break Benchmark Problem Using the Refined Core Thermal-Hydraulic Nodalization Feature of the MARS/MASTER Code

The refined core thermal-hydraulics (T-H) nodalization feature of the MARS/MASTER code is used to generate a high-fidelity solution to the OECD main steam line break benchmark problem and to investigate the effects of core T-H nodalization. The MARS/MASTER coupling scheme is introduced first that enables efficient refined node core T-H calculations via the COBRA-III module. The base solution is generated using a fine T-H nodalization consisting of fuel assembly–sized radial nodes. Sensitivity studies are performed on core T-H nodalization to examine the impacts on core reactivity, power distribution, and transient behavior. The results indicate that the error in the peak local power can be very large (up to 25%) with a coarse T-H nodalization because of the inability to incorporate detailed thermal feedback. A demonstrative departure from nucleate boiling (DNB) calculation shows no occurrence of DNB in this problem.