Status and Future Direction of the MCCI Programby: M. T. Farmer and S. LomperskiPresented at: The 2nd European Review Meeting on Severe Accident Research (ERMSAR-2007)Forschungszentrum Karlsruhe (FZK), Germany 12-14 June 2007
ACKNOWLEDGMENTS • Work sponsored by the Organization for Economic Cooperation and Development (OECD). • Participating countries include Belgium, Czech Republic, Finland, France, Germany, Hungary, Japan, Norway, South Korea, Spain, Sweden, Switzerland, and the United States. • This support is gratefully acknowledged.
Presentation Outline • Summary of MCCI-1 Program (from January 2002 until December 2005). • Background and Objectives • Approach • Summary of Key Findings from Various Test Series • Data Utilization for Model Development and Code Validation • Scoping Calculations for Debris Coolability at Plant Scale • Summary • Objectives & Status of MCCI-2 Program (October 2006 to December 2009).
Background & Objectives • OECD-MCCI program is an international project investigating ex-vessel debris coolability and 2-D core-concrete interaction. • In the first phase of the project (MCCI-1), reactor material experiments and associated analysis were conducted to achieve the following technical objectives: • Assess the effectiveness of various mechanisms for cooling ex-vessel core debris, thereby arresting the accident progression and minimizing the potential for radiological release. • Address remaining uncertainties related to long-term 2-D core-concrete interactions, which may lead to containment failure by over-pressurization or basemat penetration resulting in fission product release to the environment. Achievement of these objectives supports the technical basis for improved SAMGs for existing plants, as well as better containment designs.
Approach • Debris Coolability: Separate effects tests carried out to investigate various coolability mechanisms, thereby providing data for development and validation of models and codes for extrapolation to plant scale. • Ex-vessel codes (e.g., WEX, COSACO, MEDICIS, TOLBIAC, MELCOR) generally do not have debris coolability models. • Experiment results used for code development, assessment, and improvement. • 2-D Core-Concrete Interaction: Conduct realistic integral tests to provide direct data for code verification and validation purposes. • Reduce modeling uncertainties in lateral/axial power split; resolve differences between codes in calculated cavity erosion behavior. • Test types and parameter variations selected to validate models over the range of anticipated conditions in plant accident scenarios so that the codes can be used to extrapolate to plant conditions.
Debris Cooling Mechanisms • Bulk Cooling: Gas sparging initially high enough to preclude stable crust formation. Thus, efficient heat transfer occurs across the agitated melt/water interface. • Water Ingression: Cracks/fissures in solidifying corium form pathways for water to ingress, thereby increasing the heat transfer rate above the conduction- limitation. • Melt eruptions: Sparging gases entrain corium through perforations in the crust to form an overlying porous particle bed. • Crust Breach: Crust failure events lead to rapid water flooding beneath the crust, thereby providing a pathway for renewed bulk cooling, water ingression, and melt eruption cooling mechanisms.
Separate Effects Tests Designed to Provide Data on Debris Cooling Mechanisms
Corium melt generated through a thermite reaction. Melt is flooded at the top of apparatus by four injection tubes that impact upon a baffle plate. SSWICS Tests • Multi-junction Type C thermocouple assembly used for in-situ measurement of water penetration rate. • Type C TCs at melt bottom surface used to detect arrival of saturation isotherm. • Posttest: Ingots sectioned and load-tested to determine corium mechanical strength.
Trends indicated by heat flux data: Quench rate decreases with increasing concrete content in the melt. Quench rate does not increase substantially with system pressure. Quench rate is a weak function of concrete type. Same trends deduced based on posttest water permeability measurements. Objective 1: Quantify Corium Dryout Heat Flux vs. Composition and System Pressure Heat Fluxes Measured During Quench Tests Heat Flux, Arbitrary Units Heat Flux, Arbitrary Units
SSWICS Posttest Debris Configuration (SSWICS-3) After Sectioning at Axial Midplane Top Surface of Solidified Debris
Data Utilization for Validation of Dryout Heat Flux Model Permeability-Based Heat Flux Data Quench-Based Heat Flux Data Heat Flux, Arbitrary Units Heat Flux, Arbitrary Units • Dryout heat flux model constant C adjusted according to experiment:
Objective 2: Obtain Crust Strength Data to Confirm that a Floating Crust Boundary Condition is Applicable at Plant Scale. • Data shows that crust load at plant scale >>ultimate strength. Thus, the crust will continually fail and maintain melt/crust contact. • Gives coolability mechanisms (i.e., water ingression and melt eruptions) the chance to proceed to their full physical limits.
CCI Test Procedure • A 400 kg core meltis formed in-situ by a thermite-type chemical reaction. • The melt is then resistance heated through two banks of tungsten electrodes to simulate decay heat. • CCI proceeds to 30 cm ablation in either radial or axial directions. • Objective is to quantify radial-axial power split. • Melt is then flooded to provide coolability data following late-phase flooding. • Crust formed at the melt/water interface is then failed with a lance to obtain data on the crust breach cooling mechanism.
CCI-2 Concrete Ablation and Melt Temperature Data Ablation Data Melt Temperature Data
CCI-2 Concrete Ablation and Melt Temperature Data Trends indicated by heat flux data: • Initial fluxes for all tests are quite high. • Long-term heat fluxes after initial transient are significantly higher than fluxes observed prior to water addition. • Crust breach causes significant transient increase in debris cooling rate.
Posttest Debris Configuration Corium and concrete walls Solidified corium over basemat
Summary of Principal Coolability-Related Model Findings from MCCI-1 Program
Simple MCCI code (CORQUENCH) upgraded to include the coolability modeling results shown on preceding viewgraph. • Code used to scope out a preliminary range in which coolability can be achieved for two principal containment concrete types. Results shown below: Coolability Envelope Predictions – PWR Conditions Limestone/common sand concrete Siliceous concrete
MCCI-1: Summary • MCCI-1 program completed January 1, 2006. • 11 large scale tests completed over 4 year timeframe. • Results documented through 25 tech. reports, 5 conf. papers, and 2 journal articles. • Tests provided information on relative effectiveness of cooling mechanisms in augmenting the overall debris cooling rate, and provided data for model development and validation. • Phenomenological models were developed on the basis of the experiment findings, & the models were incorporated into a simple analytical tool to evaluate the effectiveness of top flooding on accident mitigation at plant scale. • The results, although promising, indicate that cooling augmentation may be necessary to completely quench melt depths that must be considered for a full range of severe accident conditions. • Experimental investigation of engineering features to enhance coolability is one of the key topics that is being addressed in the MCCI-2 project that was started 10/06.
MCCI-2 Program: Objectives • MCCI-2 program initiated to help bridge data gaps that were not fully covered in the MCCI-1 program. The planned testing falls into the following four categories: • Combined effect tests to investigate the interplay of different cooling mechanisms, and to provide data for model development and code assessment. • Tests to investigate new design features to enhance coolability, applicable to new reactor designs. • Tests to generate two-dimensional core-concrete interaction data. • Integral tests to validate severe accident codes. • Aside from these tests, a supporting analysis task initiated to further develop and validate debris coolability models that form the technical basis for extrapolating the experiment findings to plant conditions. • Code with embedded debris cooling models supplied to all participants
MCCI-2 Program: Status • To date, two tests have been conducted as part of the follow-on program. • SSWICS-8 was a category 1 separate effect experiment to investigate the effect of gas sparging on the water ingression cooling rate during quench. • Utilized the SSWICS test facility, but with the addition of a gas sparging system to provide a controlled gas flow through the melt. • CCI-4 was a category 3 test to provide additional data on 2-D CCI. • Test section redesigned to provide more concrete for longer-term ablation (i.e., up to 7 hours during dry cavity ablation). • Design also modified to increase metal content (i.e., ~ 14 wt % Zr and Fe) as an initial condition for the onset of ablation. • Both tests operationally successful. • CCI-4 is noted to be complimentary to core-concrete interaction tests that are being conducted at the VULCANO test facility, where melt compositions with metal contents much closer to prototypic levels are being addressed.