There are several types of nuclear power reactors that have been developed over the past decades. The most common types are light water reactors, which use regular water as a coolant and neutron moderator. These include pressurized water reactors and boiling water reactors. Other reactor types discussed include heavy water reactors, gas cooled reactors, breeder reactors, and graphite moderated light water cooled reactors. Each reactor type uses different coolants and moderators, and has distinct core designs and operating parameters.

1. Power Reactors
Prof. Debajyoti Bose

2. Reactor Issues
Of the different reactor concepts examined over the last
five decades, many were abandoned because of:
• economics,
• material
• considerations,
• design deficiencies, or
• poor thermal efficiencies.
“Reactor types using light water as a neutron energy moderator and heat
transfer medium together with slightly enriched uranium fuel have become the
dominant technology.”

3. Types of Light Water Reactors
1. pressurized water (PWR) and
2. boiling water (BWR)
PWRs prohibit boiling in the core and use secondary heat exchangers
or steam generators to produce steam, which powers the turbine-
generator
BWRs produce steam directly in the reactor core and pass the steam
to the turbine-generator.

4. Other Reactor Technologies
• Heavy water reactors: Uses deuterium oxide as the neutron
energy moderator and heat transfer medium together with
natural uranium fuel
• Gas-cooled reactors: Uses helium or carbon dioxide gas as the
heat transfer medium and graphite as a neutron moderator. The
fuel is either natural or enriched uranium.
• Breeder reactors: Uses high-energy neutrons to fission Pu-239
and produce additional Pu-239 from neutron capture in U-238.
Liquid sodium is the heat transfer medium.
• Water-graphite reactors: Uses light water as the heat transfer
medium and graphite as the neutron energy moderator.

5. Pressurized water (PWR)

6. • Light water, acting as the coolant and moderator, passes through the
reactor core under high pressure (2,250 psia [15.5 MPa])
• This removes the fission heat from the uranium fuel
• The reactor core consists of a large number of fuel rods, housed in fuel
assemblies
• A fuel assembly contains a square array of fuel rods—up to 17 x 17 or
289 fuel rods.
• Each fuel rod is a zirconium alloy-clad tube containing pellets of slightly
enriched uranium dioxide (2% to 3% U-235) stacked end-to-end inside
the tube to a height of 12 ft (3.6 m).
• The reactor is controlled by the use of control rods.
• The control rods enter from the top of the reactor and pass through the
fuel assemblies. Each control rod is a stainless steel tube containing a
neutron absorbing material such as boron carbide or an alloy of silver,
indium, and cadmium.

7. Boiling Water Reactor (BWR)

8. • Light water, which acts as the coolant and moderator, passes
through the core where boiling takes place in the upper part of the
core.
• The wet steam then passes through a bank of moisture separators
and steam dryers in the upper part of the pressure vessel.
• The water that is not vaporized to steam is recirculated through the
core with the entering feed-water using two recirculation pumps
coupled to jet pumps (usually 10 or 12 per recirculation pump).
• The steam leaving the top of the pressure vessel is at saturated
conditions of 1,040 psia (7.2 MPa) and 533° F (278° C).
• Fuel in fuel rod: 2% to 5% U-235

9. Heavy Water Reactors (HWR)

10. • Heavy water reactors (HWRs) use heavy water (deuterium oxide)
rather than light water for neutron energy moderating and fission
heat removal (coolant) functions.
• Since heavy water is a poorer neutron absorber than light water, it
can be used as a coolant-moderator with natural uranium fuel.
• The HWR design consists of a calandria reactor vessel that
separates the heavy water coolant for fission heat removal from the
heavy water moderator.
• The calandria reactor contains a large number of horizontal
pressure tubes.
• Each Zircaloy pressure tube contains 12 fuel bundles of natural-
uranium dioxide fuel. Each fuel bundle contains 28 or 37 Zircaloy-
clad fuel elements

11. • Heavy water is pumped through the fuel bundles within the pressure
tubes (where the fission heat is removed) and then sent to two or more
steam generators.
• The heavy water is not allowed to boil in the pressure tubes and is
maintained at 1,315 psia (9.1 MPa).
• After passing through the steam generators, the heavy water coolant
returns to the pressure tubes.
• The steam generators produce steam on the secondary or shell side
with ordinary water at saturated conditions of 683 psia (4.7 MPa)
and 500° F (260° C).
• Steam is expanded through a turbine-generator, condensed,
preheated in several stages of feed-water heaters, then pumped
back to the steam generators

12. • The calandria vessel is filled with heavy water, which surrounds the
fuel channels containing the pressure tubes and provides for
neutron moderation
• HWRs are manufactured and marketed by Atomic Energy of
Canada Limited (AECL).
• AECL currently markets its CANDU reactors (Canada deuterium
uranium) in sizes from 500 to 900 MWe, and AECL recently has
introduced its CANDU 3, which has a 450-MWe output.
• The majority of the CANDU designs are operating in Canada and
countries such as India, Pakistan , Korea, and Argentina.
• Heavy water moderated reactors comprise approximately 7% of
the world's nuclear capacity.

13. Gas Cooled Reactors
• The first series of GCRs used graphite as a moderator
and carbon dioxide as the coolant. (British Design)
• Had low thermal efficiency and low fuel lifetime because
of the low operating temperature and radiation damage
limits due to the metallic uranium fuel.
Integrated Model:
High-Temp Gas cooled Reactors (American Design)
• Superheated steam at 2,400 psig (15.4 MPag), 950° F (510° C)

14. High-temp Gas Cooled Reactor
• Produces a higher gas temperature, and thus, a higher steam
temperature and higher thermal efficiencies than those of the
LWR or HWR.
• Uses helium gas as the coolant and graphite as the neutron energy
moderator.
• This reactor consists of hexagonal graphite blocks in which
cylindrical fuel rods containing small spherical fuel particles of
enriched uranium and thorium are housed within fuel holes
interspersed with coolant holes for helium flow.
• The graphite blocks are stacked vertically to form the reactor core.
Helium flows through the graphite blocks, removing the fission
heat, and then passes to one or more steam generators that
produce superheated and reheated steam.

15. High-temp Gas Cooled Reactor

16. Breeder Reactors
• The breeder reactor has the capability of producing more fuel than
it consumes through the breeding of uranium
• The reactor core is surrounded by fertile material U-238, which
captures the neutrons not used for fissioning, and through a series
of nuclear decays produces Pu-239, a fissile fuel.
• Breeder reactors operate in the fast neutron energy range to take
advantage of the higher number of neutrons produced per fission
in uranium and plutonium fuel which result from the absorption of
the high-energy neutrons.
• Breeder reactors can produce additional plutonium fuel to support
several light water reactors, and thus have the potential to
increase nuclear fuel reserves.

17. • Liquid sodium is used as the coolant to remove the reactor fission
energy and transfer the energy to steam generators.
• Sodium is used because of its good heat transfer properties, low
neutron moderating characteristics, and low operating pressures.
• Since liquid sodium becomes radioactive in passing through the
reactor, a primary heat exchanger (sodium to sodium) is used to
prevent leakage of radioactive sodium into the steam cycle.

18. • The breeder core consists of a number of fuel assemblies of
stainless steel fuel rods that are packed with pellets of U-238
dioxide and Pu-239 dioxide material.
• The mixture is approximately 80% U-238 and 20% Pu-239.
• The core is also surrounded by a radial blanket of U-238 dioxide.
• Sodium passes through the breeder core and radial blanket,
where it removes the fission energy and then flows to the
primary heat exchanger, transferring the fission energy to the
intermediate sodium coolant.
• The intermediate sodium coolant passes to a steam generator
(sodium to water exchanger) that produces superheated steam
at 1,535 psig (104.45 atm.), 906° F (486° C).

19. Breeder Reactors

20. Graphite-Moderated, Light Water-Cooled Reactors
• Developed in the former Soviet Union, the GWR produced the first
commercial electrical energy.
• This particular design was developed prior to the PWR since
fabrication of major components such as pressure tubes could be
accomplished at existing manufacturing plants and did not require
specially built fabricating equipment such as that required for the
PWR pressure vessel.
• Later, the PWR became the dominant reactor manufactured and
sold in the former Soviet Union.
• The GWR reactor core consists of a large number of graphite blocks
in a cylindrical configuration.

21. • Each pressure tube is made of zirconium with steel ends and contains a
fuel assembly holding two fuel subassemblies, one stacked on top of the
other.
• Each subassembly consists of 18 zirconium-clad fuel elements containing
slightly enriched (1.8% to 2% U-235) uranium dioxide fuel pellets.
• Control rods enter the core in separate zirconium-clad channels from
the bottom. The control rods consist of boron carbide in an aluminum
alloy.
• Water is taken from one of four steam drums by the main circulating
pumps and sent to the bottom of the core, passing through the pressure
tubes where boiling takes place in the upper portion of the pressure
tubes.
• The 14.5% vapor-liquid mixture from each pressure tube goes to a
common steam drum (one of four) where the vapor is separated from
the liquid. The steam leaving the steam drum is at saturated conditions
of 1,015 psia (7.0 MPa)at 546° F, 286° С

22. Graphite-Moderated, Light Water-Cooled Reactors

23. URANIUM FUEL CYCLE

24. • More complicated than using coal
• Uranium requires several processing steps prior to use in the
reactor.
• The major steps in a uranium fuel cycle consist of mining and
milling, conversion to UF6, enrichment to several percent U-235,
fuel fabrication, then energy production.
• Following energy production, the fuel is transferred to a spent fuel
storage pool adjacent to the reactor in a separate spent fuel
building
• From this point, the spent fuel is treated either in an open or a
closed fuel cycle.

26. • The traditional concept is to use the closed fuel cycle in which the
spent fuel is reprocessed to obtain the unused U-235 and reactor
produced Pu-239.
• Typical fuels start with 2% to 5% U-235 and no Pu-239 and end up
with 1% to 2% U-235 and about 1% Pu-239
• Since natural uranium contains about 0.7% U-235, this spent fuel is
a rich resource.
• The reprocessed uranium is re-enriched in U-235 and fabricated as
new fuel.
• The fission products remaining are high-level waste and are
processed into a solid form for permanent repository storage. The
current concept is to use the open cycle in which the spent fuel,
after several years in water storage pools at the reactor site, will be
placed in a federally owned underground repository

27. Fuel Enrichment
1. Gaseous diffusion,
2. Gas-centrifuge, and
3. laser methods
• The gaseous diffusion method was developed during the
Manhattan Project

28. Gaseous Diffusion
• Prior to enrichment, uranium oxide must be converted to a
fluoride so that it can be processed as a gas, at low temperature.
• Following enrichment two streams of UF6 are formed:
The enriched ‘product’ containing a higher concentration of U-235
which will be used to make nuclear fuel, and the ‘tails’ containing a
lower concentration of U-235, and known as depleted uranium (DU).
• Feedstock may have a varying concentration of U-235, depending
on the source

29. Gas Centrifuge
• Like the diffusion process, the centrifuge process uses UF6 gas as its
feed and makes use of the slight difference in mass between U-235
and U-238.
• The gas is fed into a series of vacuum tubes, each containing a rotor
3 to 5 metres tall and 20 cm diameter (American Design)
• When the rotors are spun rapidly, at 50,000 to 70,000 rpm, the
heavier molecules with U-238 increase in concentration towards
the cylinder's outer edge.
• There is a corresponding increase in concentration of U-235
molecules near the centre.

30. • The countercurrent flow set up by a thermal gradient enables
enriched product to be drawn off axially, heavier molecules at one
end and lighter ones at the other.
• The enriched gas forms part of the feed for the next stages while
the depleted UF6 gas goes back to the previous stage.
• Eventually enriched and depleted uranium are drawn from the
cascade at the desired assays.
• To obtain efficient separation of the two isotopes, centrifuges
rotate at very high speeds, with the outer wall of the spinning
cylinder moving at between 400 and 500 metres per second to give
a million times the acceleration of gravity.

31. Laser Processes
• They are a possible third-generation technology promising lower
energy inputs, lower capital costs and lower tails assays, hence
significant economic advantages.
• Laser processes are in two categories: atomic and molecular.
• Atomic vapor processes work on the principle of photo-ionization,
whereby a powerful laser is used to ionize particular atoms
present in a vapor of uranium metal. (An electron can be ejected
from an atom by light of a certain frequency. The laser techniques
for uranium use frequencies which are tuned to ionize a U-235
atom but not a U-238 atom.)
• The positively-charged U-235 ions are then attracted to a
negatively-charged plate and collected

32. • Most molecular processes which have been researched work on a
principle of photo-dissociation of UF6 to solid UF5
+, using tuned
laser radiation as above to break the molecular bond holding one
of the six fluorine atoms to a U-235 atom.
• This then enables the ionized UF5 to be separated from the
unaffected UF6 molecules containing U-238 atoms, hence achieving
a separation of isotopes.
• Any process using UF6 fits more readily within the conventional
fuel cycle than the atomic process.

33. Nuclear Waste
• A major problem for the nuclear industry has been
the final, permanent storage of the spent fuel or
fission product waste.
• Fission product or high-level waste is highly
radioactive and thermally hot and must be stored
safely in the environment away from human contact.
• Storage times to allow for radioactive decay to
essentially background levels require thousands of
years, depending on the radionuclide makeup of high
level waste

34. • Deep underground storage appears to be the
most acceptable solution, both technically and
politically.
• Underground storage has following objectives:
1. waste isolation from people,
2. making accidental or intentional access highly
unlikely; and
3. waste isolation from circulating underground
water.