Submission Title
Investigations into Fuel Subassembly Flow Blockage, Molten Material Relocation and Post-Accident Heat Removal in Sodium Cooled Fast Reactors
Presentation Type
Invited
Start Date
17-12-2018 3:00 PM
Abstract
Fuel pins in Sodium cooled Fast Reactors (SFR) are arranged in a tightly packed triangular pitch within a hexagonal sheath forming a fuel SA. Each fuel pin has a helically wound spacer wire. Tiny subchannels (hydraulic diameter, ~3.5 mm) combined with helical wire wrap enhance the possibility of flow blockages in fuel SA. Such blockages at the upstream of active fuel zone with adequate length for flow development can be detected by core monitoring thermocouples which are located at the top of the SA. But, large size blockages may not be detected by the thermocouples due to low velocity of sodium issuing from the blocked subassembly, eventually leading to core damage. The extent of damage propagation before reactor shuts down depends on the size of the blockage and its rate of growth. The thermal hydraulics phenomena involved during damage progression are very complex, involving phase change heat transfer with moving solid-liquid interfaces. After severe core damage, the core debris relocates to core catcher, after melting the grid plate. The time for this relocation decides the thermal load on core catcher. Thermal transients on core catcher also depend strongly on the amount of degraded core material that relocates to core catcher. Such transients can be mitigated by adopting multiple layer core catcher design with delay bed on top of core catcher. During an energetic CDA, there is large risk of significant quantity of core debris settling in the hot pool, especially, in the annular region between core periphery and inner vessel. Under this condition, the debris may eventually reach main vessel after damaging inner vessel which depends on the quantity of debris that settles in hot pool.
To understand these complex thermal hydraulics phenomena, decoupled mathematical models have been developed and are linked with commercial CFD tools. This includes development of a special purpose 3-D structured mesh generator and CFD code for fuel SA thermal hydraulics and enthalpy based melting / freezing models for fuel, clad and hexcan, and grid plate. Suitably using these models, (i) thermal hydraulics of fuel SA with partial blockage in active region and the associated risk of sodium boiling, (ii) consequences of total flow blockage in a single fuel SA, extent of core damage and detection of total flow blockage by core temperature monitoring, (iii) molten material relocation to core catcher during an energetic whole core CDA, (iv) potential of current designs of core catcher with single tray for post accident heat removal, (vi) capacity of single tray core catcher with an integrated delay bed to accommodate full core debris, and (vii) potential of inner vessel to survive the attack by core debris that can settle in hot pool, have been investigated. Major findings of these studies will be presented in this paper.
Recommended Citation
Velusamy, K. and Puthiyavinayagam, P. (2018). "Investigations into Fuel Subassembly Flow Blockage, Molten Material Relocation and Post-Accident Heat Removal in Sodium Cooled Fast Reactors," Symposium on Advanced Sensors and Modeling Techniques for Nuclear Reactor Safety.
Investigations into Fuel Subassembly Flow Blockage, Molten Material Relocation and Post-Accident Heat Removal in Sodium Cooled Fast Reactors
Fuel pins in Sodium cooled Fast Reactors (SFR) are arranged in a tightly packed triangular pitch within a hexagonal sheath forming a fuel SA. Each fuel pin has a helically wound spacer wire. Tiny subchannels (hydraulic diameter, ~3.5 mm) combined with helical wire wrap enhance the possibility of flow blockages in fuel SA. Such blockages at the upstream of active fuel zone with adequate length for flow development can be detected by core monitoring thermocouples which are located at the top of the SA. But, large size blockages may not be detected by the thermocouples due to low velocity of sodium issuing from the blocked subassembly, eventually leading to core damage. The extent of damage propagation before reactor shuts down depends on the size of the blockage and its rate of growth. The thermal hydraulics phenomena involved during damage progression are very complex, involving phase change heat transfer with moving solid-liquid interfaces. After severe core damage, the core debris relocates to core catcher, after melting the grid plate. The time for this relocation decides the thermal load on core catcher. Thermal transients on core catcher also depend strongly on the amount of degraded core material that relocates to core catcher. Such transients can be mitigated by adopting multiple layer core catcher design with delay bed on top of core catcher. During an energetic CDA, there is large risk of significant quantity of core debris settling in the hot pool, especially, in the annular region between core periphery and inner vessel. Under this condition, the debris may eventually reach main vessel after damaging inner vessel which depends on the quantity of debris that settles in hot pool.
To understand these complex thermal hydraulics phenomena, decoupled mathematical models have been developed and are linked with commercial CFD tools. This includes development of a special purpose 3-D structured mesh generator and CFD code for fuel SA thermal hydraulics and enthalpy based melting / freezing models for fuel, clad and hexcan, and grid plate. Suitably using these models, (i) thermal hydraulics of fuel SA with partial blockage in active region and the associated risk of sodium boiling, (ii) consequences of total flow blockage in a single fuel SA, extent of core damage and detection of total flow blockage by core temperature monitoring, (iii) molten material relocation to core catcher during an energetic whole core CDA, (iv) potential of current designs of core catcher with single tray for post accident heat removal, (vi) capacity of single tray core catcher with an integrated delay bed to accommodate full core debris, and (vii) potential of inner vessel to survive the attack by core debris that can settle in hot pool, have been investigated. Major findings of these studies will be presented in this paper.
