Natural Convection as a Passive Safety Design in Nuclear Reactors

ME 258

Ian Treleaven

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

Nuclear Power Plant Basics• Mass-Energy Relation: E=mc2

• ReactionEnergy: Q=(MA+MB-MC-MD)c2

• Missing Mass = Mass Defect = Energy Out

Nuclear Power Plant Basics

• Manufactured into pellets and inserted into a tube (cladding)

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

Nuclear Power Plant Basics• Main purpose is to provide heat to create steam and turn a

turbine• http://en.wikipedia.org/wiki/File:PressurizedWaterReactor.gif

Nuclear Power Plant Basics• Main purpose is to provide heat to create steam and turn a

turbine

Nuclear Power Plant Basics• Main purpose is to provide heat to create steam and turn a

turbine (Rankine Cycle shown below)

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)

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.

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.

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

Current Research• emergency passive residual heat removal system (EPRHRS)

Current Research• emergency passive residual heat removal system (EPRHRS)

Current Research• emergency passive residual heat removal system (EPRHRS)

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

Current Research• Spent Fuel Rod Storage - CFD

Current Research• Spent Fuel Rod Storage - CFD

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

Current Research• Containment Vessel Cooling• http://www.ap1000.westinghousenuclear.com/ap1000_psrs_pcs.

html

Current Research• In-Vessel Retention of Core Damage

Current Research• In-Vessel Retention of Core Damage• http://www.ap1000.westinghousenuclear.com/ap1000_safety_ir

cd.html

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.