Some nuc. reactors

Nuclear reaction by Fission

• Nuclear fission: All commercial power reactors are based on nuclear fission.

• generally use uranium and its product plutonium as nuclear fuel, though a thorium fuel cycle is also possible.

• Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that sustain the fission chain reaction:– Thermal reactors– Fast neutron reactors

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TYPES of reactors

Boiling water R

• Boiling Water Reactors (BWR)

- Heat water in the core and allow it to boil into steam.

- The steam goes directly to the turbine outside the reactor.

Rods & Replacement• A modern BWR fuel assembly comprises 74 to 100 fuel rods, • and there are up to approximately 800 assemblies in a

reactor core, • holding up to approximately 140 tons of U.

Replacement: • The reactor fuel rods are occasionally replaced by removing

them from the top of the containment vessel.• Because they are hot both radioactively and thermally, this

is done via cranes and under water.

Pressurized Water Reactor (PWR)

• In a PWR, water is kept under pressure to keep it from boiling, even at 300 C.

• The pressurized water is pumped through a closed system of pipes called the primary circuit.

• Heat from the primary circuit warms up water in the secondary circuit.

• The water in the secondary circuit comes to a boil and its steam turns the turbine.

• The water in the primary circuit returns to the reactor core after giving up some of its heat.

Pressurized Heavy Water Reactor• A Candu reactor is an example of a PHWR. • Fuel assemblies are placed horizontally in a tank called a

calandria. • Heavy water coolant is pumped through tubes containing the

fuel assemblies to pick up the heat generated from the nuclear reaction.

• The coolant then moves to the steam generators to produce steam from ordinary water and back to the reactor.

Heavy water is virtually identical, except each of the hydrogen atoms have an extra neutron. This hydrogen isotope is called deuterium - D2O

Fast Breeder Reactor• FBR has a core of plutonium surrounded by rods of U-238. • The U-238 nuclei absorb neutrons from the core and are

transformed into plutonium (P-239).• For every four atoms of plutonium that are used up in the

core of the breeder, five new plutonium atoms are made from the U-238.

• Therefore, FBRs "breed" plutonium. • Fast breeder reactors work at such a high temperature that

they need a special coolant such as liquid sodium. • In addition, they are not equipped with a moderator to slow

down neutrons, and for this reason are called "fast" breeders.

High temperature gas cooled reactors

• HTGR offer an alternative to conventional light-water cooled and moderated reactors.

• They use graphite as the moderator and helium as the coolant.

• One design concept, called a pebble bed reactor, uses a fuel made of tennis-ball sized spheres known as "pebbles".

• Each pebble contains thousands of tiny "kernels" consisting of enriched uranium and graphite compressed together and coated externally with temperature resistant ceramic.

• The pebbles are stacked in the reactor and cooled by helium.

Nuclear Power Reactors: Typical Characteristics

Type Fuel Form Coolant ModeratorBWR Enriched Uranium Dioxide Water WaterPWR Enriched Uranium Dioxide Water WaterPHWR Natural Uranium Dioxide Heavy Water Heavy WaterGCR Natural Uranium CO2 GraphiteAGR Enriched Uranium Dioxide CO2 GraphiteLWGR Enriched Uranium Dioxide Water GraphiteFBR Plutonium Oxide & Uranium Dioxide Liquid Sodium None

•Boiling Water Reactors (BWR)•Pressurized Water Reactor (PWR)•Pressurized Heavy Water Reactor (PHWR)•Gas-Cooled Reactors (GCR)•Advanced Gas-Cooled Reactors (AGR)•Light Water Graphite Reactor (LWGR)•Fast Breeder Reactor (FBR)

Fusion reactor