Nuclear ReactorsChapter 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 ReactorsLight 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 ReactorsLight 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.

Nuclear Reactors

Pressurized Water Reactors

Nuclear ReactorsPressurized Water Reactors- Coolant Path

Nuclear ReactorsPressurized Water Reactors- Coolant Path

Components

Nuclear ReactorsPressurized Water Reactors- Reactor compartment

Nuclear ReactorsPressurized 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 pathsComponents

Nuclear ReactorsPressurized Water Reactors- The Steam Generator

Steam flow pathFeed water pathGeneral chemistry (pH, phosphates)Components

Nuclear ReactorsPressurized Water Reactors- The PressurizerComponents

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 ReactorsPressurized Water Reactors- The Pressurizer

Nuclear ReactorsPressurized 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 assemblyOther 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

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) where D = H2 , can operate on natural uranium because the absorption cross sectionof deuterium for thermal neutrons is very small.

D2O is also less effective in moderating neutrons thanH2O. Therefore neutrons lose less energy per collisionand travel farther before reaching thermal energies. Thecore of a heavy water reactor is considerably larger thana LWR.

Nuclear Reactors Heavy Water Reactors

1. H2 is a rare isotope of H. ~ 150 ppm2. 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

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 D2Omoderator 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 reactora 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 watercompartments.

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