Transcript Nuclear Reactors - Department of Electrical and Computer

Nuclear Reactors
Chapter 4
Nuclear Reactors
• Categories
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Breeder or Converter or Burner
Coolant {water, heavy water, gases, liquid metal}
Moderator {water, heavy water, graphite}
Boiling water or pressurized water systems
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• Neutron Balance in Thermal Reactors
Notes:
1. 63 – 5 = 58 n produce Pu-239
2. 78-63 = 15 n produce U- 236
3. 59-32 – 17 n produce Pu-240
4. In steady – state ~ 32% of
energy is produced by
Pu fissions.
Nuclear Reactors
• Neutron Balance in Fast Reactors
Notes:
1. Fuel is typically 20% Pu and
80% depleted U.
2. Bulk of fissions from Pu
3. Pu created > Pu consumed
since: 116 n absorbed in Pu,
but: 134 – 13 = 121 produce
Pu-239.
4. Thus Rx “breeds” fissile Pu 239
Nuclear Reactors
Light Water Reactors
1. The most widely used electricity producing reactors in
the world today are thermal reactors that are
moderated, reflected and cooled by ordinary (light)
water (H20).
2. Two Main Types
-Boiling water reactors (BWR)
-Pressurized water reactors (PWR)
Nuclear Reactors
Light Water Reactors
2. Advantages
- Abundant supply of water.
- well known properties
- cheap cost
3. Disadvantages
- Water has high vapor pressure, requiring high
pressure operation.
- Water has a large cross- section of absorption for
neutrons. Therefore it is not possible to fuel a light water
reactor with natural uranium. The fuel must always be
enriched to some extent.
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Pressurized Water Reactors
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Pressurized Water Reactors- Coolant Path
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Pressurized Water Reactors- Coolant Path
Components
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Pressurized Water Reactors- Reactor compartment
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Pressurized Water Reactors
Flow paths
Components
1. One of 1st reactor designs
2. Standard for Naval vessels
3. Requires steam generator
(next slide)to produce steam
for turbines.
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Pressurized Water Reactors- The Steam Generator
Steam flow path
Feed water path
General chemistry
(pH, phosphates)
Components
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Pressurized Water Reactors- The Pressurizer
Components
Down power Tc (I), Coolant
Expands, Pzr level (I), Pzr
Pressure (I), Spray valve
opens, Steam condenses,
Pzr Pressure (d), limiting the
Pressure surge.
Up Power Tc (d), coolant
contracts, Pzr level (d), Pzr
pressure (d), water flashes
to steam, Pzr pressure (I)
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Pressurized Water Reactors- The Pressurizer
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Pressurized Water Reactors- The Fuel
-Slightly enriched 2-5 w/o U-
235
-Fuel pellets of UO2 black
ceramic looking
-Pellets 1cm x 2cm
-Loaded into Zircaloy tubes (low
Σa)
-Zircaloy is alloy of zirconium +
tin + iron + chromium
-Rods arranged as cluster or
assembly
Other arrangements possible,
such as fuel plates vice rods.
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AP 1000
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Westinghouse Electric Design
Generation III +
PWR
2 Loops
~1000 MWe
AP 1000
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Boiling Water Reactors
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Boiling Water Reactors
BWR Advantages:
• Direct cycle, no secondary loop
• Less mass flow rate since coolant water is permitted to
absorb latent heat and sensible heat.
• Can operate at lower pressure ~ 900 psi
{not zero/ atmospheric pressure since
1. high temp required to drive turbines
2. high pressure prevents wall dryout}
• Lower pressure mean thinner pressure vessel
and less expensive components.
BWR Disadvantages:
• Radioactive coolant throughout engine room
• Shielding and containment larger
• Lower power density – need larger core and PV then PWR
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Boiling Water Reactors
Nuclear Reactors
Heavy Water Reactors
1. A heavy water (D2O) where D = H2 , can operate on
natural uranium because the absorption cross section
of deuterium for thermal neutrons is very small.
D2O is also less effective in moderating neutrons than
H2O. Therefore neutrons lose less energy per collision
and travel farther before reaching thermal energies. The
core of a heavy water reactor is considerably larger than
a LWR.
Nuclear Reactors
Heavy Water Reactors
1. H2 is a rare isotope of H. ~ 150 ppm
2. It contains 1 n and 1 p, instead of just 1 p.
3. D2O was discovered by American Harold Urey in 1931,
for which he received the 1934 Nobel prize in chemistry.
• D2O is 10% heavier than H2O
• It is 3x worse at slowing neutrons
• It is 600x worse at absorbing neutrons
Can extract D20 from H20 is a multistage process.
Nuclear Reactors
Heavy Water Reactors
CANDU Reactor – Canada Deuterium Uranium reactor
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Heavy Water Reactors
CANDU Reactor – Canada Deuterium Uranium reactor
Nuclear Reactors
Heavy Water Reactors
CANDU Reactor – Canada Deuterium Uranium reactor
Nuclear Reactors
Heavy Water Reactors
CANDU Reactor – Canada Deuterium Uranium reactor
• To avoid a large pressure vessel it uses pressurized tube concept.
• Rx consists of a large tank called a calandria filled with D2O
moderator at atmospheric pressure.
• Tank is penetrated by hundreds of horizontal tubes containing
the fuel. The D2O coolant flows through the tubes at high pressure
(~1500 psi) and does NOT boil.
• Thus by pressurizing the coolant rather than the whole reactor
a large pressure vessel is avoided.
• Because of lower pressure (than conventional PV), the tubes
can not be raised to high enough temperature to steam at same
temp as light water reactors. The result is plants are less efficient
(~28-30%).
• Plant reactivity is controlled by absorber rods, and light water
compartments.
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Breeder Reactors
Four Types
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Liquid Metal Cooled Fast Breeder Reactor. LMFBR
Gas Cooled FBR
Molten Salt BR
Light Water BR
• Only LMFBR is only one significantly commercialized
anywhere in world.
• Operates on U -> Pu fuel cycle
fueled with Pu isotopes
blanketed with U 238 natural or depleted.
• No moderator since we want fast neutrons
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Breeder Reactors
Four Types
• Use sodium coolant because
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Excellent heat transfer
Non corrosive
High plant temperatures
High power density
• Disadvantage of Sodium coolant
Sodium reacts violently with water
High melting point (98 C) need heated piping.
Forms β and ϒ emitters (radioactive) most plants
use two loop system to prevent radioactive sodium
from entering S/Gs.
Nuclear Reactors
Breeder Reactors