1. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 1 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.zartd@sun.ac.za NATURAL CIRCULATION CLOSED LOOP THERMOSYPHON-TYPE HEAT PIPES for the NUCLEAR INDUSTRY

2. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 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. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 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. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 4 2.4, 7.3 and 27 m closed loop heat pipes

5. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 5 Scaled down of a single loop (300 in total) of a reactor cavity cooling system for a high temperature nuclear reactor Reactor Cavity Cooling System (RCCS) Reactor Concrete Water Cavity Loop

6. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS Heat Pipe Heat Exchanger Flow control valves and pressure gauge Continuous combustion chamber Thermocouple wiring Windows Hot tream Cold stream

7. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 7 Natural circulation of the air in the space between a high temperature gas cooled reactor and the concrete

8. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 8 Figure1: A two-phase flow waste heat recovery system Fuel Tank Vapour Natural convection air-cooled condenser Heat exchanger Vapour line Liquid line return Thermostatically controlled valve Expansion tank Cooling jacket Energy recovery line Liquid Fuel tank Proposed “passive” used/spent fuel cooling, heat recovery and utilisation system

9. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 9 Waste heat recovery system experimental set-up Schraeuder valve Shut-off valve Vapour-liquid separation section Cooling jacket Air-cooled condenser 200 kPa pressure and expansion relief valve Shut-off valve for air release Shut-off valve Thermocouple –100 to 500 kPa pressure gauge 200kPa safety valve Heat exchanger Expansion pipe System shut-off valve

10. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 10 Schematic of the experimental set-up System shut- off valve Cooling jacket Heating elements –200 to 500 kPa pressure gauge Shut-off valve Shut-off for air released Air collector Heat exchanger Expansion pipe Shut-off valve Schraeuder valve Shut-off for air released Expansion tank Schraeuder valve Cooling water supply Fuel tank 200 kPa pressure safety 200 kPa pressure and expansion relief valve Drain shut-off valve Air-cooled condenser

11. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 11 Simulate the system by a series of discrete-sized control- flow volumes Assumptions  Quasi-equilibrium  Thermal equilibrium  One-dimensional control volumesAssume the Bousinesqu approximation  Homogeneous two-phase flow model Mass, Momentum and Energy

12. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 12 Rearranging and expressing the conservation equations in a form suitable for an explicit finite difference numerical solution method (and use upwind differencing)

13. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 13 Typical set of results 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 mode for working fluid diverted to flow through the air-cooled condenser line Time, t [h] Temperature, T [°C]

14. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 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. SAHPA ® South African Heat Pipe Association Energy Postgraduate Conference EPC2013, 11-14 Aug 2013 iThemba LABS 15 Department of Mechanical Engineering, University of Stellenbosch, Stellenbosch, Western Cape Province, South Africa rtd@sun.ac.za Thanks JC Ruppersberg and RT Dobson