JC Ruppersberg and RT Dobson

1. Natural circulation closed loop Thermosyphon-type Heat Pipes for the nuclear Industry JC Ruppersberg and RT Dobson Department of Mechanical & Mechatronic Engineering, University of Stellenbosch, Stellenbosch, South Africa Telephone 27 (0) 21 808 4268 Email rtd@sun.ac.za

2. Preface and Background • Absolute safety and reliability is a desirable goal in the Nuclear Industry • Nuclear designers and Regulatory Authorities are now requiring that inherently safe (that is make use of natural phenomena) cooling and heating systems be considered a priori • Closed loop thermosyphons are such heat transfer devices requiring no circulation pump and active controls

3. Contents • Some systems that we have designed, constructed and tested • Theoretical simulation of the thermo-fluid dynamics of natural circulation system • Discussion, Conclusions and Recommendations

4. 2.4, 7.3 and 27 m closed loop heat pipes

5. Reactor Cavity Cooling System (RCCS) Reactor Concrete Water Loop Cavity Scaled down of a single loop (300 in total) of a reactor cavity cooling system for a high temperature nuclear reactor

6. Heat Pipe Heat Exchanger Continuous combustion chamber Flow control valves and pressure gauge Windows Hot tream Thermocouple wiring Cold stream

7. Natural circulation of the air in the space between a high temperature gas cooled reactor and the concrete

8. Proposed “passive” used/spent fuel cooling, heat recovery and utilisation system Natural convection air-cooled condenser Heat exchanger Expansiontank Vapour line Thermostatically controlled valve Energy recovery line Figure1: A two-phase flow waste heat recovery system Vapour Fuel Tank Fuel tank Cooling jacket Liquid Liquid line return

9. Thermocouple Air-cooled condenser Schraeuder valve Shut-off valve for air release Shut-off valve for air release Shut-off valve –100 to 500 kPa pressure gauge Heat exchanger 200kPa safety valve Vapour-liquid separation section Shut-off valve Expansion pipe Cooling jacket System shut-off valve 200 kPa pressure and expansion relief valve Waste heat recovery system experimental set-up

10. Expansion tank Air collector Schraeuder valve Schraeuder valve Shut-off for air released Air-cooled condenser Expansion pipe Shut-off valve Shut-off for air released Shut-off valve 200 kPa pressure safety –200 to 500 kPa pressure gauge Heat exchanger Fuel tank Shut-off valve Heating elements Cooling jacket Cooling water supply 200 kPa pressure and expansion relief valve System shut-off valve Drain shut-off valve Schematic of the experimental set-up

11. Assumptions • Quasi-equilibrium • Thermal equilibrium • One-dimensional control volumesAssumethe Bousinesqu approximation • Homogeneous two-phase flow model Simulate the system by a series of discrete-sized control-flow volumes Mass, Momentum and Energy [N}

12. Rearranging and expressing the conservation equations in a form suitable for an explicit finite difference numerical solution method (and use upwind differencing) • Conservation of mass: • Conservation of energy: • where • Conservation of Momentum: • where and • , • and

13. Typical set of results Mass flow rate, [g/s] Heat transfer rate, [kW] Temperature, T [°C] Time, t [h] Theoretically determined mass flow rate, heat transfer rates and Temperatures as a function of time for Single to single and two-phase modefor working fluid diverted to flow through the air-cooled condenserline Time, t [h]

14. CONCLUSIONS A number of natural circulation loop heat pipes suitable for the nuclear reactor technology A relatively simple theoretical technique for simulationd the transient thermo-hydraulic response of an entirely passive cooling system for a nuclear fuel tank has been presented.

15. Thanks JC Ruppersberg and RT Dobson Department of Mechanical Engineering,University of Stellenbosch,Stellenbosch, Western Cape Province, South Africa rtd@sun.ac.za