Evaluation of Fluid Conduction and Mixing Within a Subassembly of the Actinide Burner Test Reactor

1. Evaluation of Fluid Conduction and Mixing Within a Subassembly of the Actinide Burner Test Reactor Author: Cliff B. Davis

2. Introduction • RELAP5-3D is being considered as the thermal-hydraulic system code to support the sodium-cooled Actinide Burner Test Reactor (ABTR) • An evaluation* was performed to determine if existing code models could be used to represent important features of the ABTR • Fluid heat conduction (axial and radial) • Radial subchannel mixing * This work was presented at NURETH-12

3. The EBR-II XX09 subassembly was used as a surrogate for the ABTR

4. Two RELAP5 models were used 1D 2D

5. The control system was used to simulate fluid heat conduction and mixing • The control system provides a generalized capability to evaluate algebraic and differential equations using standard mathematical operations and functions that can interact with the code’s hydrodynamic calculations • The control system was used to calculate the heat transfer associated with heat conduction and radial mixing • The calculated amount of heat was then added to or subtracted from the various control volumes in the model

6. Fluid heat conduction and mixing were represented as Heat conduction: D = f ( k, geometry) Radial mixing: = effective transverse mass flux / axial mass flux for the XX09 subassembly

7. The 1D model was used to determine the effects of axial conduction for a wide range of steady-state conditions

8. Axial conduction affected the temperature profile at very low flows Without axial conduction With axial conduction in the fluid • Results were consistent with theory • The effects of axial conduction in the heat structures were smaller than those in the fluid

9. The 1D model was used to simulate a loss-of-flow transient • Transient was for a loop-type reactor and simulated a loss of primary pumps, scram, and a pony motor trip near 330 s • Inlet flow was assumed to completely stagnate for 40 s to maximize the effects of axial conduction

10. The effects of axial conduction during the transient were small Maximum clad temperature Fluid temperature profile • The results of axial conduction were exaggerated by the assumption of complete flow stagnation and the lack of natural circulation in the 1D model

11. The radial variation in temperature was large at high flow rates Fluid temperatures at the top of the core • Results are from the 2D model without radial heat transport • The outer ring is cooler because the subassembly wall is unheated • Buoyancy effects flattened the temperature profile at low flows

12. Radial heat transport flattened the temperature profiles at high flows but did not significantly affect the temperature profiles at low flows 100% power and flow 1% power and flow • The effect of radial mixing was larger than conduction for 100% flow, but was smaller for <10% flow • The 1D model significantly underpredicts the maximum fluid temperature at high flows

13. The 2D nodalization and radial fluid conduction affected transient results Maximum cladding temperature Fluid velocity at the top of the core • The 2D modelspredicted significant internal recirculation which lowered the peak cladding temperature • Radial heat conduction reduced the flow at the top of the core • The effects of radial mixing were small because of the low axial flow rates during the transient

14. Conclusions • The effects of axial conduction in the fluid are not important for most ABTR applications • Subchannel effects are important in the calculation of cladding temperature • The 1D model underpredicted the maximum temperature during normal operation and overpredicted the maximum value during the loss-of-flow transient • The effects of radial conduction in the fluid are important in the calculation of cladding temperature • The effects of radial mixing in the fluid are important at high flow rates

15. Conclusions (cont’d) • The control system model can adequately simulate the effects of heat conduction in the fluid and radial mixing between subchannels • The use of the control system places a burden on the user in terms of the amount of work required to represent the phenomena • Internal code models would be much easier to use • Because of the finite number of control variables available, the approach can only be used at about 430 junctions • Internal code models that calculate the effects of heat conduction and mixing in the fluid should be added to RELAP5-3D to support analyses of the ABTR