Using MELCOR Code Oleksandr Balashevskyi, Ph.D. Head of Department - - PowerPoint PPT Presentation
Main Approaches to Development of Spent Fuel Pools Models for Analysis of Severe Accidents Phenomena Using MELCOR Code Oleksandr Balashevskyi, Ph.D. Head of Department of Scientific and Technical Support SS Scientific and Technical Center
Oleksandr Balashevskyi, Ph.D. Head of Department of Scientific and Technical Support SS “Scientific and Technical Center” of SE NNEGC “Energoatom” Technical Meeting on the Phenomenology, Simulation and Modelling of Accidents in Spent Fuel Pools IAEA, Vienna, Austria, 2–5 September 2019
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The date, a significant amount of work has been carried out in Ukraine related to the analysis of beyond design, including severe accidents (SA). This activity is primarily aimed at implementing strategies for managing SA at all types of power units of Ukrainian NPPs, together with the implementation of additional reconstructive measures, will allow to qualitatively increase the level
At the same time, it is necessary to note the fact that, together with the reactor facility (RF), potentially dangerous from the point of view of the possibility of occurrence of SA, is related to spent fuel pool (SFP). The complexity of the work on the analysis of SA in SFP lies in the novelty of such works and the associated limited amount of information on this issue. 3
MELCOR has been developed by Sandia National Laboratories for the U.S. Nuclear Regulatory
severe accident processes at nuclear power plants with light water reactors - from the start of the initiating event up to the final state. 4
MELCOR applications estimate the initial conditions of a severe accident, and associated sensitivities and uncertainties in a variety of modes and scenarios. 5 Modeling a Severe Accident
✓ cylindrical structure of the core; ✓ the lack of simultaneous simulation of the events of SA in several compartments of SFP that physically separated by concrete partitions (separate COR and CAV models); ✓ lack of modeling of 2-tier racks of SFA; ✓ the need for SFA grouping in the COR model of the SFP, depending on their residual energy release; ✓ restrictions in the CAV model (no significant space between the lining and the concrete slab
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✓ The need of core partition when simulating the core of SFP, in accordance with the reactor type - PWR or BWR (no separate option for SFP). ✓ The absence of option for simulating separate racks of SFP (racks materials were included in addition to the steel FSA elements). ✓ The lack of the ability to simulate the presence of coolant between the absorption pipes (for the PWR model). ✓ Necessity of mass input (material only zirconium) and area of FA cover (for BWR model). ✓ The release of the fission products from SFA(occurs according to the distribution of residual energy release in the radial and axial direction). ✓ The lack of simulation of the flat metallic bottom of the SFP.
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➢ The type of power unit (VVER-440, VVER-1000) - SFP design, altitude marks of the location of the SFP in the containment; ➢ Racks manufacturer (Skoda, Izhora Plants); ➢ The type of storage racks installed and their characteristics; ➢ The presence of sealed canisters in SFA storage racks; ➢ The number of cells for SFA placement in SFP storage racks.
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9 General view of a SFP storage rack type "Izhora Plants"
10 General view of a fuel assembly
11 Nodalization scheme of SFP compartment CV101 - simulates the volume under the bottom base plate of the rack CV102 - simulates a part of SFP in which fuel assemblies are located. CV103 - simulates the descending part of the SFP between the walls of the SFP compartment and the rack CV100 - simulates the volume of water in the gaps between the absorption tubes of the rack CV104 - simulates part of SFP over FA FL110 - simulates SFP overflow
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Distribution of SFP compartment materials in the axial zones SFP compartment breakdown into radial rings
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Water level in the SFP compartment for PWR (left) and BWR (right)
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Total mass of steel and steel oxide in SFP for PWR (left) BWR (right)
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CAVITY model Nodalization scheme of SFP compartment
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In the DCH model, all nuclides are divided into classes according to their physicochemical properties. It is also possible for the user to supplement with additional classes or compounds (for example, a CsI compound). The main feature of the DCH package is: To determine the power of the residual energy release in the SFP; To determine and add to the DCH model the value of the initial masses of fission products (both active and inactive) and their specific residual energy release depending on the amount of SFA in the SFP and the duration of exposure (time after reactor scram) The use of these parameters by the default is not acceptable for modeling SA in SFP, since the values
the values at the time of reactor scram The correctness of taking these parameters into account in the DCH model has a significant effect on the chronology of the course of SA associated with changes in the power of the residual energy release
nonvolatile fission products.
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Differences in masses of fission products that left the core components for a model with a total volume for models COR and CVH and separate for a model CAV
21 ➢ Compiling the model by interconnection of all the MELCOR code packages performed using the MELGEN compilation program. ➢ Verification of compliance of the model of hydrodynamic volumes (model CVH) and the COR model : is checked in the file MELGEN.out with the keywords “CONSISTENCY CHECK ON VOLUME REPRESENTATIONS IN COR AND CVH”. ➢ Checking the correctness of the extrapolation of the entered data for the DCH model: comparison of the user-entered values of specific powers of the energy release and mass of nuclides according to the classes with the calculation results that were compiled by the MELCOR code at the initial moment of the calculation (checked in the MELCOR.out file using the keywords "POWER FROM FISSION PRODUCT CLASSES".
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Sample of input data in the RN package Sample of output data compiled with MELCOR code
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and higher.
✓ SA in several SFP compartments is actual for VVER-1000. ✓ SA in two tier racks of SFP is actual for VVER-440. 3 . SA Phenomena in SFP comes down to the following tasks: ✓ The possibility of recriticality. ✓ MCCI ( it is important to determine the actual chemical compositions of concretes) ✓ The relocation of the melt and the possibility of its cooling, etc.).
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