Gas-cooled reactors

By-

Dinesh Chandra Pant

M140207ME

Why gas cooled reactor ?

Single phase behavior.

Optical transparency and electrically non-conducting.

Low power density.

Types of gas cooled reactor

CO2 cooled

reactor

Helium cooled

reactor

Magnox UNGG

GCFR GT-MHR HTGCR AGR

Magnox

Features

Pressurized, CO2 cooled

Graphite moderated

Using natural uranium

MAGNOX alloy for cladding

Boron-steel control rods

Magnox is short for Magnesium non-oxidizing – an alloy used to

clad the fuel rods(covering to contain fission products) inside the

reactor

• This material has the advantage of a low neutron capture cross-

section

• Two major features of MAGNOX:

1. Limits the maximum temperature, and hence the thermal efficiency

2. Reacts with water, preventing long-term storage of spent fuel under

water

• MAGNOX Reactors used as

1. Power plant

2. Producer of Plutonium for nuclear weapons

UNGG (Uranium Natural Graphite Gas)(France)

• UNGG used a horizontal fuel rod orientation

• Requires heavily shielded facilities

Features

Graphite moderated

Cooled by CO2

Fueled with natural uranium metal

Cladding material- Magnesium-zirconium alloy

Later,

• Magnox was replaced by the Advanced Gas-Cooled reactor

(AGR) (an improved Generation II gas cooled reactors)

• And UNGG was replaced by the pressurized water reactor

(PWR)

Thermal, Hydraulic, Neutronic and

Material Considerations-

Mass flow rate of coolant gas.

Pumping power requirement of coolant.

Heat transfer performance of coolant.

Heat transfer area.

Neutronic considerations.

Material/chemical considerations.

Gas-cooled fast reactor

• Helium-cooled system

• Using a direct Brayton cycle gas turbine

• Produce electricity, while at the same time; producing (breeding)

new nuclear fuel.

• Fast-neutron spectrum.

Features

Gas turbine modular helium reactor

Features

Helium as a coolant.

Graphite is used as a moderator.

Cross vessel.

The reactor and power conversion systems are located in a

below grade concrete silo.

It gives an efficiency up to 48% - higher than any other reactor.

Very high temperature reactor

High temperature gas cooled reactor

There are two main types of HTGRs:

1. Pebble bed reactors (PBR)

2. Prismatic block reactors (PMR)

• Uses a graphite-moderated nuclear reactor

• The high temperatures enable applications such as process heat

or hydrogen production via the thermo-chemical sulfur-iodine

cycle.

• The prismatic block reactor refers to a prismatic block core

configuration, in which hexagonal graphite blocks are stacked to

fit in a cylindrical pressure vessel

Pebble bed reactor Graphite-moderated

Gas-cooled

Type of (VHTR)

Contain about 15000

TRISO particles

Each has a mass of

210g, of which 9g is

uranium

Figure : Pebble Bed TRISO Fuel Sphere Cross Section

Advanced gas-cooled reactor(Second generation of British gas-cooled reactors) 1. Charge tubes

2. Control rods

3. Graphite moderator

4. Fuel assemblies

5. Concrete pressure

vessel and radiation

shielding

6. Gas circulator

7. Water

8. Water circulator

9. Heat exchanger

10. Steam

Developed from the MAGNOX reactor

Coolant outlet temperature 650 0C

High thermal efficiency – 42%

Requiring stainless steel fuel cladding

The fuel is uranium dioxide pellets, enriched to 2.5-3.5%, in

stainless steel tubes.

Good quality superheated (and reheated) steam (comparable to

quality from a highly optimised coal plant)

References

en.wikipedia.org/wiki/Gas-cooled reactor

Malcolm P. LaBar General Atomics3550 General Atomics Court San Diego, CA 92121-1122