Gas-cooled reactors use gas like CO2 or helium as a coolant and graphite as a moderator. There are several types of gas-cooled reactors including Magnox reactors, UNGG reactors, Advanced Gas-cooled Reactors (AGR), Gas-cooled Fast Reactors (GCFR), Gas Turbine Modular Helium Reactors (GT-MHR), Very High Temperature Reactors (VHTR), High Temperature Gas-cooled Reactors (HTGCR), Pebble Bed Reactors (PBR), and Prismatic Block Reactors (PMR). Each type has distinct features related to the coolant, moderator, fuel type, and design configuration.

1. Gas-cooled reactors
By-
Dinesh Chandra Pant
M140207ME

2. Why gas cooled reactor ?
 Single phase behavior.
 Optical transparency and electrically non-conducting.
 Low power density.

3. Types of gas cooled reactor
CO2 cooled
reactor
Helium cooled
reactor
Magnox UNGG
AGR GCFR GT-MHR HTGCR

4. Magnox

5. Features
 Pressurized, CO2 cooled
 Graphite moderated
 Using natural uranium
 MAGNOX alloy for cladding
 Boron-steel control rods

6. 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

7. UNGG (Uranium Natural Graphite Gas)
(France)
• UNGG used a horizontal fuel rod orientation
• Requires heavily shielded facilities

8. Features
 Graphite moderated
 Cooled by CO2
 Fueled with natural uranium metal
 Cladding material- Magnesium-zirconium alloy

9. 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)

10. 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.

11. Gas-cooled fast reactor

12. Features
• 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.

13. Gas turbine modular helium reactor

14. 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.

15. Very high temperature reactor

16. High temperature gas cooled reactor

17. 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

18. 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

19. Figure : Pebble Bed TRISO Fuel Sphere Cross Section

20. 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

21.  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)

22. References
 en.wikipedia.org/wiki/Gas-cooled reactor
 Malcolm P. LaBar General Atomics3550 General
Atomics Court San Diego, CA 92121-1122