Results of Validation and Cross-Verification of the ROK/B Design Code on the Problem of Loss of Cooling in the Spent Fuel Pool
The procedures of validation and cross-verification of the newly developed computational code ROK/B are described. The main problem solved using the ROK/B code is the substantiation by calculation of the coolant density in the spent fuel pool (SFP) (untight reactor) and the temperature regime of the fuel assemblies during a protracted shutdown of the cooling systems (break in the supply of cooling water). In addition to the above, it is possible using the computational code ROK/B to carry out calculation of an accident with the discharge of the coolant from the SFP with simultaneous long-duration shutdown of the cooling systems.
The ROK/B computational code allows carrying out calculations for various types of designs of the fuel assemblies and VVER reactors, in particular, VVER-1000, VVER-1200 and VVER-440 power units with single- and two-tiered fuel assemblies arrangement, with clad pipes in racks (for compacted assemblies storage) and pipes without cladding, with cased assemblies and caseless ones.
During fuel reloading, a high level of the coolant is maintained, which makes it possible to do “wet” transportation of the assemblies from the reactor to the SFP. The mathematical model for heat and mass transfer calculation, including the boiling coolant model, implemented in the ROK/B code, includes: the motion equation, equations for calculating the enthalpy along the height of the fuel section of a fuel assembly with natural circulation of coolant within the channel containing the fuel assembly (lifting section) and in the inter-channel space (lowering section), the equation of mass balance between the channels of the racks with assemblies and in the inter-assembly space and the amount of evaporated (and outflowed) water, the heat balance equation for a fuel rod in a steam environment. The system of equations is supplemented by closing relations for calculating the thermal physics properties of water and steam, fuel and cladding materials, as well as the coefficients of heat transfer from the wall to the steam, hydraulic resistance and density of the steam-water mixture in the channels, and the heat released in the reaction of steam with zirconium.
Validation of the computational code was carried out on the basis of the data of the ALADIN experiment performed by German specialists and the data of OKB Gidropress JSC. Cross-verification of the ROK/B computational code was carried out in comparison with the results of calculation using the KORSAR/GP and SOCRAT/B1 codes. Based on the results of the validation, it was concluded that the deviation of the ROK/B results from the experimental data is not more than 2 – 10% (10% for the option with a fuel rod power of 20 W). Based on the results of cross-verification, it was concluded that the discrepancy between the ROC/B results and the calculation results for the KORSAR/GP and SOCRAT codes is not more than 0.5% (for SOCRAT/V1) and less than 10% (for KORSAR/GP).
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