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 ” 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
Nuclear Energy in Ukraine Ukraine operates 15 power units: 13 power units - VVER-1000 design and 2 power units – VVER-440 design Map of Ukraine with the NPPs and ENERGOATOM headquarters • Total installed capacity - 13.8 GW(e) • The NPPs ’ share in the energy generation balance - 55 % (in 2019) 2
General Information 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 of safety of NPPs. 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 computer code background information MELCOR has been developed by Sandia National Laboratories for the U.S. Nuclear Regulatory Commission. This is a fully integrated computer code. It allows modelling the whole spectrum of severe accident processes at nuclear power plants with light water reactors - from the start of the initiating event up to the final state. MELCOR MELCOR Development 4
Melcor computer code brief description MELCOR applications estimate the initial conditions of a severe accident, and associated sensitivities and uncertainties in a variety of modes and scenarios. Modeling a Severe Accident 5
Development of the MELCOR simulation model for SFP Restriction of MELCOR code for SFP modeling ✓ 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 of SFP). 6
Development of the MELCOR simulation model for SFP Restriction of the MELCOR code version 1.8.5 for modeling SFP ✓ 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. 7
Development of the MELCOR simulation model for SFP Input data selection ➢ 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. 8
Development of the MELCOR simulation model for SFP Input data selection General view of a SFP storage rack type "Izhora Plants" 9
Development of the MELCOR simulation model for SFP Input data selection General view of a fuel assembly 10
Development of the MELCOR simulation model for SFP Hydrodynamics model 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 Nodalization scheme of SFP compartment 11
Development of the MELCOR simulation model for SFP Simulation of the SFP core ➢ Use of the BWR option (makes it possible to take into account the bypass part between the absorption pipes) ➢ The use of supporting structures for the base plate (PLATEB) of the rack and the supporting stands of the rack (COLUMN) ➢ supplementing the COR model with a separate radial ring to simulate the penetration of destroyed elements (debris) of SFP core into the near-wall part of the SPF 12
Development of the MELCOR simulation model for SFP Simulation of the SFP core SFP compartment breakdown into radial Distribution of SFP compartment rings materials in the axial zones 13
Development of the MELCOR simulation model for SFP Comparison of calculation results using various reactor options (PWR, BWR) - MELCOR 1.8.5 Water level in the SFP compartment for PWR (left) and BWR (right) 14
Development of the MELCOR simulation model for SFP Comparison of calculation results using various reactor options (PWR, BWR) - MELCOR 1.8.5 Total mass of steel and steel oxide in SFP for PWR (left) BWR (right) 15
Development of the MELCOR simulation model for SFP Comparison of calculation results using various reactor options (PWR, BWR) - MELCOR 1.8.5 Table of main differences 16
Development of the MELCOR simulation model for SFP Simulation package of the melt-concrete interaction (CAV) ➢ specifying the paths for the melt relocation at the ex-vessel phase of SA (it is recommended to link the CAV1 with the lower control volume of the SFP; ➢ The input of chemical compositions of concrete. CAVITY model Nodalization scheme of SFP compartment 17
Development of the MELCOR simulation model for SFP Simulation package of radionuclide propagation - RN ➢ redistribution of the energy release profile of fission products in the axial and radial direction; ➢ supplementing the RN model with additional pathways. 18
Development of the MELCOR simulation model for SFP DCH residual energy package simulation 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 of the masses and powers of nuclides depend on the nominal power of the reactor and correspond to 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 of volatile fission products and, accordingly, the energy release that will remain in the melt with nonvolatile fission products. 19
Development of the MELCOR simulation model for SFP DCH residual energy package simulation 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 20
Development of the MELCOR simulation model for SFP Verification of the developed calculation model ➢ 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". 21
Development of the MELCOR simulation model for SFP Checking the accuracy of the extrapolation of the entered data for the DCH model 22
Development of the MELCOR simulation model for SFP Verification of compliance with the model of hydrodynamic volumes and the model of the "active zone" BV Sample of output data compiled with Sample of input data in the RN MELCOR code package 23
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