1. Natural Convection as a Passive Safety Design in Nuclear Reactors
ME 258
Ian Treleaven

2. Nuclear Power Plant Basics
An increase in binding energy of a nucleus means decrease in rest energy of separated nucleons. This decrease in rest energy corresponds to released kinetic energy of fission products (aka heat)
A single fission reaction releases about 200MeV of energy
1lb uranium = 1 million gallons of gasoline

3. Nuclear Power Plant Basics
Mass-Energy Relation: E=mc2
ReactionEnergy: Q=(MA+MB-MC-MD)c2
Missing Mass = Mass Defect = Energy Out

4. Nuclear Power Plant Basics
Manufactured into pellets and inserted into a tube (cladding)

5. Nuclear Power Plant Basics
Chain reaction is kept under control with Control Rods
Their main purpose is to absorb neutrons to decrease the rate of reactions

6. Nuclear Power Plant Basics
Main purpose is to provide heat to create steam and turn a turbine

7. Nuclear Power Plant Basics
Main purpose is to provide heat to create steam and turn a turbine

8. Nuclear Power Plant Basics
Main purpose is to provide heat to create steam and turn a turbine (Rankine Cycle shown below)

9. Inherent Problem with Nuclear Reactors
A major problem with nuclear power is that the heat generated from the nuclear fuel is inherently unstable.
The heat generated wants to naturally accelerate unless the chain reaction is controlled.
Although the self-sustaining chain reaction in nuclear fuel can be stopped in the event of a shutdown (via control rods), decay heat is still present and must be dealt with to prevent meltdown of the nuclear rods or over pressurization of the containment vessel.
Control rods are typically reliable, but the major problem is a Loss of Coolant Accident (LOCA)

10. Inherent Problem with Nuclear Reactors
“When” a loss of coolant accident does occur, the rods will continue to heat up and need to be cooled by something to prevent them from melting and possibly breaching the reactor vessel.
The containment vessel will also need to be cooled to prevent overpressurization.

11. Passive Safety Systems
There is currently major research being performed on Passive Safety Systems.
Passive Safety Systems typically rely on physical phenomena such as pressure differentials, gravity, natural convection, material response, and phase change, to regulate a process.
Passive Safety Systems try to avoid moving mechanical parts, signal inputs, human inputs, external power or force, and a moving working fluid. (IAEA)
Modern-Day reactors require constant inputs to keep them operating as opposed to constant inputs to keep them from a meltdown.
“When” an accident occurs, passive safety systems are supposed to work with more reliability.

12. Natural Convection Passive Safety Systems
One of the most promising Passive Safety Systems in place for a Loss of Coolant Accident (LOCA) is using natural convection (natural circulation)
Density-Buoyancy is the driving force of natural convection

13. Current Research
emergency passive residual heat removal system (EPRHRS)

14. Current Research
emergency passive residual heat removal system (EPRHRS)

15. Current Research
emergency passive residual heat removal system (EPRHRS)

16. Current Research
Lead-Cooled Fast Reactor – cooled only by natural convection

17. Current Research
Spent Fuel Rod Storage - CFD

18. Current Research
Spent Fuel Rod Storage - CFD

19. Current Research Pebble Bed Reactor:
Piling Pebble allows for criticality
No piping, gas flows through pebble bed to extract heat
Shutdown is to change geometry and use natural convection; no need for control rods

20. Current Research Containment Vessel Cooling

21. Current Research
In-Vessel Retention of Core Damage

22. Current Research In-Vessel Retention of Core Damage

23. Conclusion
The use of natural convection is a useful tool to make nuclear reactors more passively safe because natural convection is a physical phenomena.
Many design parameters are inter-related and thus need lots of research before it can be properly implemented into next generation reactors.