This case study simulates transient heat transfer in a packed pebble bed reactor core without forced convection, focusing on natural convection from an internal heat source to an external heat sink. Using Flownex, it models the temperature and heat removal during a 30 kW to 10 kW transient, validated against SANA facility data with nitrogen and helium cooling. This aids predicting reactor behavior post-cooling flow cessation.
CHALLENGE
The thermal hydraulic conditions of a typical high temperature gas cooled pebble bed reactor core are simulated. Figure 1 shows a schematic of the pebble bed with a single central long heating element. The top and bottom of the bed are well-insulated, while the outside of the furnace wall is open to atmospheric conditions. The heating is done with a graphite tube within a protection tube in the center of the bed.
BENEFITS
This case study demonstrates the transient simulation of the heat transfer through a packed bed with no forced convection. This case study applies to packed pebble beds where only natural convection is present in the bed due to a heat source on the inside of the bed and a heat sink on the outside of the bed.
SOLUTION
The transient simulation of a packed bed is demonstrated in this case study. The ability to model the correct transient removal of the heat through the bed will be important for predictions of when a nuclear pebble bed reactor will become recritical after the forced cooling flow was stopped or whether the bed will be able to remove the heat without any active heat removal systems present.
INTRODUCTION
This case study demonstrates the transient simulation of the heat transfer through a packed bed with no forced convection. This case study applies to packed pebble beds where only natural convection is present in the bed due to a heat source on the inside of the bed and a heat sink on the outside of the bed.
SYSTEM DESCRIPTION
The thermal hydraulic conditions of a typical high temperature gas cooled pebble bed reactor core are simulated. Figure 1 shows a schematic of the pebble bed with a single central long heating element. The top and bottom of the bed is well-insulated, while the outside of the furnace wall is open to atmospheric conditions. The heating is done with a graphite tube within a protection tube in the center of the bed.
OBJECTIVE OF SIMULATION
The objective is to simulate the natural convection in the pebble bed during a transient event where the heat source is reduced from 30 kW to 10 kW. The pebble surface temperature distribution is also predicted.
FLOWNEX MODEL
The Flownex model of the system is similar to the steady-state case study (Flow and heat transfer in a packed bed – Steady-state).
DESCRIPTION OF SIMULATION
In this case, the bed consisted of spherical graphite pebbles (60 mm diameter). The atmospheric conditions are at a pressure of 101 kPa and a temperature of 25.3 °C. During the transient event the heating power is reduced from 30 kW to 10 kW at a rate of 0.4 kW/h and thereafter held constant on 10 kW nominal heating power. The duration of the experiment was 90 hours. The pebbles were first cooled with Nitrogen and then the simulation was repeated with Helium as the cooling fluid.
RESULTS
The Flownex simulation results are compared to experimental data from the SANA test facility (Stöcker, 1998) at different heights through the bed at certain radial positions. In Figure 2 the results for the transient with Nitrogen and in Figure 3 for helium as cooling fluid are shown at different heights. From the Flownex results the user can determine what the maximum temperature in the bed will be and the amount of heat that can be removed from the bed during the transient.
CONCLUSION
The transient simulation of a packed bed is demonstrated in this case study. The ability to model the correct transient removal of the heat through the bed will be important for predictions of when a nuclear pebble bed reactor will become recritical after the forced cooling flow was stopped or whether the bed will be able to remove the heat without any active heat removal systems present.
REFERENCES
[1] STöCKER, B., 1998. “Investigations for the self-acting decay heat transport in high temperature reactors under special consideration of the natural convection” JüL-3504 dissertation, Forschungszentrum Jülich.