Concepts and Solutions

Below is a process flow diagram for the scaled test bench.  Included in the diagram are the desired conditions for boiling verification and components critical to project realization.  These components include a cartridge heater, clear test section, line heater, pump, expansion tank, and heat exchanger.  Each of these components are essential to creating a scaled version of the conditions seen in a commercial boiling water reactor.
Scaled BWTL flow diagram
Figure 1.  Process flow diagram for proposed BWTL system showing critical components to satisfy desired system conditions.

Critical component selection:

1.   Heater Element
A Chromalox cartridge heater has been chosed to simulate the 18" nuclear fuel rod.
Cartridge heater
Figure 2. Chromalox cartirdge heater
The dimensions of the heater are 18" in length and 0.500" in diameter.  We expect the output of the heater to be 5.6 kW/ft.
2.   Viewing Section
The use of a sight glass tube will enable visual confirmation of boiling over the test section.  A glass pressure vessel has been selected.  The test section selected can be threaded or flanged directly to piping of loop and is rated to 500F and 500 psi.
Sight glass tube
Figure 3. Sight glass tube
Below is a proposed method for implementing  the cartridge heater into the test section.
Test section
Figure 4. Proposed test section
3.   Line Heater
A circular flow heater has been selected to maintain the conditions desired at the entrance of the test section.  The line heater will be accompanied with a control panel that will allow for the control of inlet and outlet temperatures.
Line heater
Figure 5.Schematic of ground heating tape
4.   Pump
The purpose of the pump is to pressurize the test section and circulate the water through the system.  The system specified is transporting a non-viscous fluid at a low flow rate while maintaining considerably high pressures and temperatures.  We have selected a positive displacement pump to satisfy these conditions as well as allow for the possibility of variable flow in the system.
Viking pump
Figure 6.Viking heavy duty internal gear pump
5.   Heat Exchanger
A heat exchanger is needed in the loop for the purpose of removing the energy added to the coolant by the line heater and fuel rod simulator. Even though the temperature of the fluid changes little, a large amount of energy transfer is required to condense the steam created.
To remove the required energy a shell and tube heat exchanger will be selected , having a capacity in excess of 20 kW.
Figure 7.Section view of a shell and tube heat exchanger

6.   Expansion Tank
The pressure vessel allows for thermal expansion of the water as well as water vapor for the boiling cycle.  It will have to rated for our specified operating pressure and temperature with an allowance for a safety factor.  At this point in the design process it is known that the tank in necessary but further analysis will have to be done to select sufficient sizing and the placement in the system.

Flow Analysis:

An exhaustive analysis of the system was completed to validate the conditions specified. The flow characteristics in the test tube must be examined to validate that boiling will occur and that heater burnout will not occur. Identifying the high velocity flow seen in the test section it was found that the critical heat flux of the heater is 629600 Btu/hr*ft2.   This corresponds to a linear heat rate of 24.15 kW/ft. This linear heat rate is much higher than the expected linear heat rates of the heater.  It is also less than the expected maximum linear heat rates found in the full sized BWTL.  Therefore film boiling will not occur in either the test tube or the reactor test section. 

Types of boiling?

The proposed clear sight tube will allow for a visual comfirmation of the nucleate pool boiling taking place in the test section.

Important dates:

Detailed Design Package   Jan. 29
Parts Ordered                   Feb. 5
Loop Built                         Apr. 9
Finish Testing                  May 7

 University of IdahoIdaho National Laboratory