1c. Fission & Reactor Types Flashcards
What’s the difference between thermal and fast neutrons?
Fast neutrons: incident energy 10^6 - 10^7 eV.
• natural state of fission neutrons
Thermal neutrons: Incident energiy 10^-3 - 10^-2.
•slowed down by a moderator via interacting (colliding) with moderators atoms
Thermal neutrons have much higher cross-sections with U-235.
U-238 cross-section for fission (than capture) only higher for fast neutrons
What is neutron scattering CS and neutron absorption SC? What are the typical values for light, heavy water and graphite?
Scattering CS: probably of interaction between moderator and neutron
• want this to be high to slow neutrons down (for moderators)
Absorption CS: probability of material absorbing a neutron.
• want this to be as low as possible (for moderators)
Light Water: 49 SCS, 0.66 ACS
Heavy water: 10.6 SCS, 0.0013 ACS
Graphite: 4.7 SCS, 0.0035 ACS
What are the main classifications for nuclear reactors? What are the main features of light water reactors?
Main classifications:
• fuel - natural, enriched UO2, Pu
• moderator (thermal only) - water (L,H), graphite
• Coolant - Water, CO2, Na, etc
LWR
• use water (light) as both moderator and coolant
• reduces the cost and makes the reactor more efficient
What are fast neutron reactors and some of their features?
• Fuel - natural uranium
•breeder - generates its own fuel (Pu-239) and hence uses its fuel much more efficiently.
• can be more expensive to build and decommission
• typically use molten metal salts as coolant to avoid high neutron absorption cross-sections
AGRS - fuel, moderator, Heat Extraction, outlet temps, pressure, efficiency
Fuel - UO2 enriched to 4-5%, stainless steel cladding
Moderator - Graphite (high thermal inertia)
Coolant - CO2 gas extracts heat from reactor to convert secondary loop of water to steam driving a turbine.
Temp & pressure - 650C & 4.12MPa
Efficiency - 42%
New and improved version of the original Magnox reactors
CANDU - fuel, coolant, heat extraction, temps & pressures
Fuel: natural uranium (0.7%) held in Zr alloy cans loaded into horizontal Zr alloy tubes
•half of fission events are due to U-238->Pu-239
Coolant: heavy water pumped at pressure over fuel raising steam of a separate circuit of light water.
• heavy water results in a much higher neutron flux as less neutrons are being absorbed
• heavy water can be expensive
Temp & pressure: 305C & 8.85MPa
Efficiency: 30%
PWRs - fuel, coolant, temp, pressure, efficiency
Fuel: UO2 3.2% enriched, Zr alloy cladding
Coolant - pressurised light water which raises steam via a separate circuit.
Temp & pressure: 324C (outlet) & 15.4MPa
Efficiency: 32%
PWR Designs: EPR, AP1000, VVER, APR1400
BWR - fuel, coolant, temp & pressures, efficiency
Fuel: UO2 2.4% enriched, Zr alloy cladding
Coolant: pressurised light water boiling in the Pressure Vessel producing steam which directly turns a turbine
• secondary loop condenses the primary coolant back to a liquid phase
Temp & pressure: 286C & 7.24MPa
Efficiency: 32%
Doesn’t require heat exchanger but extra shielding of steam circuit and turbine.
