Idealized Thermal Nuclear Reactor

Idealized Thermal Nuclear Reactor

Fuel Rods:

• Fuel in form of small pellets
• Pellets held in bundles and several bundles make up fuel rod

Control Rods:

• Moveable pieces of neutron-absorbing material
• Used to control reactivity k of reactor
• CANDU reactors use light water in control rods
• Fine tuning of reactivity done with stainless steel neutron flux adjuster rods

Moderator:

• Slows fission neutrons to an energy where fission cross section of fuel high enough to guarantee k > 1

Neutron Reflector:

• Layer of neutron-moderating material surrounding the reactor core which causes fast neutrons emitted from core to undergo elastic scattering with some being reflected back into core
• CANDU reactors: D2O moderator surrounding core acts as a neutron reflector
• Greatly increases efficiency

Reactor Vessel:

• Outer boundary of reactor core
• CANDU reactors the calandria vessel is very big but isn’t exposed to high temp or pressure

Nuclear Reactor Fuels

Fissile Material:

• Material which undergoes fission when hit with low energy thermal neutrons

Fertile Material:

• Material that doesn’t undergo fission within a nuclear reactor however it absorbs neutrons and decays into new isotopes which are fissile

Light Water Reactors

• Use H2O as coolant and moderator
• Must use enriched uranium because H2O moderator absorbs too many neutrons to allow natural uranium to be used as fuel
• Thermal efficiency of 30-33%

Gas-Cooled Reactors

• Cooled with either CO2 or Helium (He) gas (not strong neutron absorbers)
• Moderated with graphite
• Most thermal efficient up to 50%!

Breeder Reactors

• Most efficient breeder reactors use Fast neutrons to accomplish the fission process
• Use 233U as fuel
• For molten salt breeder reactor, the salt solution is piped through a graphite moderated core

• No control rods in core
• k value controlled by flow rate of molten salt through the core
• inherently low efficiency but have high thermal efficiency (44%)

CANDU Reactor

• PWR reactor
• Pressure tubes are pressurized through the calandria
• UO2 fuel pellets are in tubes and pressurized D2O coolant runs through the tubes and carries away the fission heat
• Room temperature D2O fills spaces between pressure tubes in calandria and acts as primary moderator and neutron reflector of reactor core