Algirdas Kaliatka, Audrius Grazevicius, Eugenijus Uspuras Modelling of Heat Transfer Processes in the Spent Fuel Pool Using One and Three Dimensional Computer Codes Algirdas Kaliatka, Audrius Grazevicius, Eugenijus Uspuras NUTHOS-11: The 11th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Operation and Safety Gyeongju, Korea, October 9-13, 2016

Outline Introduction Modelling of SFP using one dimensional thermal-hydraulic computer code Influence of nodalization on the modelling results of water mixing process CFD analysis of loss of heat removal accident in the spent fuel pool Conclusions

Introduction The accident in Fukushima Daiichi NPP showed that a loss of coolant can occur with the resultant effect on the spent fuel in the spent fuel pools. The consequences of such an event can be very serious creating a possibility of significant amount of radioactive material release to the environment. This paper presents the comparison of two different modelling approaches (using three-dimensional CFD and one-dimensional system computer codes). Fuel assemblies at SFP in Fukushima Daiichi NPP Unit 4

Modelling of SFP using one dimensional thermal-hydraulic computer code (1) Spent nuclear fuel pool filling with different power assemblies SFP of Mark I boiling water reactor W.H. Jordan, Consequence Study of a Beyond-Design-Basis Earthquake Affecting the Spent Fuel Pool for a U.S. Mark I Boiling Water Reactor, pp. 369, Office of Nuclear Regulatory Research, US Nuclear Regulatory Commission, Washington, USA (2013).

Modelling of SFP using one dimensional thermal-hydraulic computer code (2) Initial and boundary conditions in SFP Spent nuclear fuel assemblies placed in pool, group characteristics Cross section of the fuel assembly Y. Naito, H. Okuno, Nuclide Composition and Neutron Multiplication Factor of BWR Spent Fuel Assembly, OECD/NEA, Burnup Credit Criticality Benchmark Phase IIIB (1996)

Modelling of SFP using one dimensional thermal-hydraulic computer code (3) SFP nodalization using RELAP5 code

Influence of nodalization on the modelling results of water mixing process (1) The presented SFP models were used for the analysis of water heat-up and mixing processes in the case of loss of heat removal. The analysis of water heat-up and mixing was performed until the water in the pool starts to boil. Water temperature at the outlet of fuel boxes (Nodalization 3) Water velocities in the fuel boxes (Nodalization 3)

Influence of nodalization on the modelling results of water mixing process (2) The comparison of results, received using different nodalization, demonstrates the influence of the selected nodalization to the simulation results Water temperature at the outlet of fuel boxes

CFD analysis of loss of heat removal accident in the spent fuel pool (1) For the analysis of processes in SFP, the ¼ model of SFP was created using the Computational Fluid Dynamics (CFD) Software ANSYS Fluent code. The 10.2 million elements mesh was created. The „porous media“ function was used, which let to simplified the complicated geometries and used values of viscous and inertial loses. For calculation are used standard  -  model.

CFD analysis of loss of heat removal accident in the spent fuel pool (2) The calculated using CFD ANSYS Fluent code water velocities are in reasonable agreement with results, received using one-dimensional RELAP5 code. Water velocities in the fuel boxes (RELAP5 - Nodalization 3) Water velocity after 20 hours

CFD analysis of loss of heat removal accident in the spent fuel pool (2) The good agreement between calculation results demonstrates what both modelling approaches (one-dimensional and CFD) are suitable for the analysis of processes in SFP. Water temperature after 20 hours Comparison of the water temperature behavior in the pool using different approaches

Conclusions (1) The water het-up and mixing in the Spent Fuel Pool processes in the loss of heat removal case was analyzed using one dimensional system thermal-hydraulic computer code RELAP5 and computational fluid dynamics tool ANSYS Fluent tools. The calculation results shows, that in Spent Fuel Pools the water is moving upwards through fuel assemblies occurs due to buoyancy force and the water velocity is warring in the range 0.005 – 0.02 m/s. The velocity of water depends on the heat generation in the assemblies.

Conclusions (2) The comparison of calculations performed using different computer codes and different nodalization demonstrated, the significant influence of nodalization in one-dimensional approach. Using one-dimensional codes it is necessary to develop the nodalization of pool in such way, that the water mixing in the upper part of pool would be modelled. The results of ANSYS Fluent calculation demonstrated that this three-dimensional CFD treatment avoids the need for many one-dimensional modelling assumptions.

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