CONTAIN Code Analyses of Direct Containment Heating (DCH) Experiments*

Transcription

1 CONTAIN Code Analyses of Direct Containment Heating (DCH) Experiments* D. C. Williams, R. O. Griffith, E. L. Tadios, and K. E. Washington Sandia National Laboratories INTRODUCTION In some nuclear reactor core melt accidents, a potential exists for molten core debris to be dispersed into the containment under high pressure. Resulting energy transfer to the containment atmosphere can pressurize the containment. This process, known as direct containment heating (DCH), has been the subject of extensive experimental and analytical programs sponsored by the U.S. Nuclear Regulatory Commission (NRC). DCH modeling has been a major focus for the development of the CONTAIN code. 1 Results of a detailed independent peer review of the CONTAIN code were published recently. 2 In support of the peer review, extensive analyses of DCH experiments were performed in order to assess the CONTAIN code's DCH models and improve understanding of DCH phenomenology. The present paper summarizes this assessment effort. EXPERIMENTS ANALYZED High-temperature melts generated by the iron oxide/aluminum thermite reaction were expelled by high-pressure steam into scaled reactor cavities that were connected to pressure vessels simulating to various degrees the reactor containment building. Chromium metal was added to the thermite reaction mixture to enhance chemical reactivity of the melt with steam. Pressure rise (AP), temperature distributions, amounts of hydrogen produced and burned, and debris transport parameters were among the experimental results measured in these tests. This work was supported by the U.S. Nuclear Regulatory Commission and was performed at Sandia National Laboratories, which is operated for the U.S. Department of Energy under Contract DE-AC04-94AL DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

2 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document.

3 Experiments analyzed included the Limited Flight Path (LFP) Series (six experiments), 3 the Wet Cavity (WC) series (three experiments), 4 ' 5 the Sandia National Laboratories Zion geometry Integral Effects Tests (SNL/IET Zion) series 6 (seven experiments analyzed), the Argonne National Laboratory Zion geometry IET (ANL/IET Zion) series (three experiments analyzed), 7 and SNL IET Surry geometry (SNL/IET Surry) series 8 (three experiments analyzed). The IET experiments included detailed scale models of containment structures as well as scaled reactor cavities, while the others had nonprototypic containment geometries. The SNL/IET Surry experiments were performed at 1/6-scale, the ANL/IET Zion experiments were 1/40-scale, and all others were 1/10-scale. The three ANL/IET Zion cases analyzed were scaled counterparts of corresponding SNL/IET experiments and provide a test of model scalability. The Zion IET experiments all had a small amount of water in the cavity and one WC experiment (WC-2) had a wet cavity. Cavities were dry in all other cases. CONTAIN DCH MODELING DCH-related phenomena modeled in the CONTAIN code include debris transport and trapping, debris-gas heat transfer, metal-steam and metal-oxygen chemical reactions, atmospherestructure heat transfer, interactions between nonairborne debris and blowdown steam, potential effects of debris-water interactions, and hydrogen combustion under DCH conditions. Detailed mechanistic models are provided where phenomenological understanding is sufficiently advanced to justify a mechanistic model. For poorly understood phenomena, simple models together with input flexibility are provided to permit studying the effect of modeling uncertainties upon the results of interest. Models for nonairborne debris interactions and debris-water interactions fall in this category. In the assessment of CONTAIN DCH models, a standard input prescription for use of the DCH models was defined and "frozen" while the code was applied to analyze the experiments without varying the code input except as dictated by the experimental initial and boundary conditions.

4 RESULTS Figure 1 compares calculated and experimental results for AP and hydrogen production. Hydrogen results are plotted after scaling up to plant scale in order to facilitate comparison of experiments performed at different scales. Plot symbols distinguish experiments performed in open geometry, the LFP series, the SNL/IET (Zion) experiments with and without hydrogen combustion, the ANL/IET (Zion) experiments, and SNL/IET (Surry) experiments. The CONTAIN AP and hydrogen production results reproduce the overall trends of the experimental data reasonably well. Comparison of the ANL/IET and SNL/IET Zion results reveals no obvious scale distortion. Comparison of the SNL/IET (Zion) results with and without combustion of DCH-produced hydrogen illustrates the importance of hydrogen combustion to DCH loads, and also shows that CONTAIN reproduces this effect reasonably well. The least favorable comparisons are with some of the LFP experiments, in which the code overpredicted debris transport to the dome and hence overpredicted AP. Extensive sensitivity studies were performed to assess strengths and weaknesses of specific model features. Findings included: Sensitivity was fairly low to particle size and to variations within empirically established limits for debris dispersal fractions and for the time-dependence of debris dispersal. Contributions of nonairborne debris interactions were important; AP and hydrogen production were significantly underpredicted in many instances if nonairborne interactions were omitted. Co-dispersed cavity water appeared to contribute significantly to hydrogen production and to AP in the Zion experiments. 3

5 Delta-P Due To DCH is.--'' 0.4 (8 Q. 0.3 a. a a 0 4-* a 3 jd 15 O *. x =* /' X X / ' X,.-'' a,-.--' a,--''' Open Geometry X LFP SNL Zion w/o Burn SNL Zion w. Burn -A. ANL Zion ISl SNL Surry i i i i i i i i i i i i i - "r r" i 1 " Experimental Delta-P (MPa) m _«"o E 6) a a u 3 a o k. a. x D ra o v> o 0) 53 "3 _o <a O Hydrogen Produced by DCH? _./ - Open Geometry y - X y _ LFP A * 3E+05- a /''' SNL Zion w/o Burn 2E+0S- - SNL Zion w. Burn ANL Zion ISl /' x SNL Surry -,,-'X» 1E+05- v,.'' X «X *S 0E+O0- ' I I 1 I I "1 I 1 T T i 0E+00 1E+05 2E+05 3E+05 4E+05 Experimental Scaled H2 Produced, g-mole Figure 1. Comparison of CONTAIN predictions with results of DCH experiments.

6 Atmosphere-structure heat transfer combined with hydrogen holdup in oxygen-starved subcompartment volumes is a very important mitigation effect. CONCLUSIONS A standardized input prescription for CONTAIN DCH analysis has been developed that is useful in achieving consistency in DCH analyses. Using this standard presciption, CONTAIN analyses of DCH experiments reproduce the major trends of DCH integral test results, including containment pressurization and hydrogen production, reasonably well. Despite the progress made, there remain significant modeling uncertainties that can be important in some instances. Sensitivity studies can be performed in order to define a reasonable representation of the uncertainty range. References 1. K. E. Washington and D. C. Williams, "Direct Containment Heating Models in the CONTAIN Code," SAND , Sandia National Laboratories, to be published. 2. B. E. Boyack et al., CONTAIN Independent Peer Review, LA-12866, Los Alamos Scientific Laboratory, Los Alamos, NM, M. D. Allen et al., "Experiments to Investigate the Effect of Flight Path on Direct Containment Heating (DCH) in the Surtsey Test Facility," NUREG/CR-5728, SAND , Sandia National Laboratories, October M. D. Allen et al., "Experiments to Investigate the Effect of Water in the Cavity on Direct Containment Heating (DCH) in the Surtsey Test Facility - The WC-1 and WC-2 Tests," SAND , Sandia National Laboratories, March M. D. Allen et al., "Experiment Results of Tests to Investigate the Effect of Hole Diameter Resulting from Bottom Head Failure on Direct Containment Heating (DCH) in the Surtsey Test Facility - The WC-1 and WC-3 Tests," SAND , Sandia National Laboratories, March

7 6. M. D. Allen et al., "Experiments to Investigate Direct Containment Heating Phenomena with Scaled Models of the Zion Nuclear Plant in the Surtsey Test Facility", NUREG/CR- 6044, SAND , Sandia National Laboratories, May J. L. Binder, L. M. McUmber, and B. W. Spencer, "Direct Containment Heating Integral Effects Tests at 1/40 Scale in Zion Nuclear Power Plant Geometry," NUREG/CR-6168, ANL-94/18, Argonne National Laboratory, September T. K. Blanchat et al., "Experiments to Investigate Direct Containment Heating Phenomena with Scaled Models of the Surry Nuclear Power Plant," NUREG/CR-6152, SAND , Sandia National Laboratories, June DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. 5