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Nuclear Reactors

Chapter 4

Nuclear Reactors

• Categories

– Breeder or Converter or Burner

– Coolant {water, heavy water, gases, liquid metal}

– Moderator {water, heavy water, graphite}

– Boiling water or pressurized water systems

Nuclear Reactors

• 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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Nuclear Reactors

Pressurized Water Reactors

Nuclear Reactors

Pressurized Water Reactors- Coolant Path

Nuclear Reactors

Pressurized Water Reactors- Coolant Path

�Components

Nuclear Reactors

Pressurized Water Reactors- Reactor compartment

Nuclear Reactors

Pressurized Water Reactors

1. One of 1st reactor designs

2. Standard for Naval vessels

3. Requires steam generator

(next slide)to produce steam

for turbines.

�Flow paths

�Components

Nuclear Reactors

Pressurized Water Reactors- The Steam Generator

�Steam flow path

�Feed water path

�General chemistry

(pH, phosphates)

�Components

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Nuclear Reactors

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)

Nuclear Reactors

Pressurized Water Reactors- The Pressurizer

Nuclear Reactors

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.

Nuclear Reactors AP 1000

Nuclear Reactors AP 1000

� Westinghouse Electric Design

� Generation III +

� PWR

� 2 Loops

� ~1000 MWe

Nuclear Reactors

Boiling Water Reactors

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Nuclear ReactorsBoiling 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

Nuclear Reactors Boiling Water Reactors

Nuclear Reactors Heavy Water Reactors

1. A heavy water (D2O) reactor 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

Nuclear Reactors Heavy Water Reactors

CANDU Reactor – Canada Deuterium Uranium reactor

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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.

Nuclear Reactors Breeder Reactors

Four Types

• 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

Nuclear Reactors Breeder Reactors

Four Types

• Use sodium coolant because• 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