Nuclear power plants - Introduction

Lecture 11: Nuclear power plants - Introduction

1. Nuclear Power Plants
Introduction

2. Nuclear Power Plants

3. Nuclear Power Plants
* International Atomic Energy Agency (2009)

4. Nuclear Power Plants
REACTOR TYPES WORLDWIDE
*
:
Type No. of Units Total MW(e)
BWR 92 83656
FBR 2 690
GCR 18 8909
LWGR 16 11404
PHWR 44 22441
PWR 264 243121
Total: 436 370221
* International Atomic Energy Agency (2009)

5. Nuclear Power Plants
Types

6. Reactors are classified according to fuel type, neutron flux spectrum, coolant and
moderator.
LWRs are the most commonly used for power production. LWRs are divided into 2
types: Pressurized Water Reactor (PWR) and Boiling Water Reactor (BWR).
Types of Reactors
Neutron flux
spectrum
ModeratorCoolant Fuel material
Thermal Light water
Heavy water
Graphite
Light water
Heavy water
CO
2
Enriched Uranium
Natural Uranium
Natural or Enriched
Uranium
Fast Nil Liquid metal
(e.g. sodium)
Plutonium,
Thorium

7. Nuclear Power Plants

8. •Pressurized water reactors are the most common in the World (over 60%).
•2.5 –3% enrichment.
•The water in a PWR does not boil because it is under high pressure (~15 MPa).
•It has become the standard reactor of nuclear propelled vessels (submarines etc.).
•The Chashma Nuclear Plant(CHASNUPP 1; 2000) is also of this type.
•Disadvantage is the limitation of critical temperature.
Pressurized Water Reactor (PWR)

9. The primary loop contains the following:
•Reactor Vessel
•Steam Generator
•Pressurizer
•Coolant Pump
PWR: Primary Loop

10. The reactor vessel serves to contain the core, water,
and control rods. The core is composed of fuel
assemblies, moderator, and coolant.
•The coolant and moderator of a PWR is ordinary
water.
•Moderators slow down neutrons so that fuel will
absorb them and fission.
•Control rods are used for power control in the
reactor to control neutron flux levels.
PWR: Reactor Pressure Vessel

11. •Control rods are made of boron carbide, alloys of silver-indium-cadmium , or
hafnium. These rods are in the shape of cylinders or sheets.
•There are anywhere from 29 to 100+ rods depending on size of core.
Control rods are used to control reactor power by absorbing
neutrons that would otherwise fission and create more power.
PWR: Control Rods

12. The fuel pellets are put into rods of zircaloy or stainless steel.
About 250 rods are made into assemblies and 200 assemblies
make up a core.
PWR: Fuel Assemblies

13. The steam generator’s purpose is to exchange the
hot water’s heat into the cool water through an
inverted U-tube HX to generate steam.
•The lower section is called the evaporator section.
This is where the heat transfers to the cooler water
and it boils to produce moist steam.
•The upper section is called the steam drum section.
It contains a moisture separator that dries the steam
before it enters the turbines.
•Efficiency: 32-33%
•There also exists horizontal steam generators.
PWR: Steam Generator

14. In the PWR, pressure must be maintained fairly constant.
For that purpose is the pressurizer.
•There is just one pressurizer in the system.
•The pressurizer contains a tube full of water and steam
with a spray nozzle.
•When there is too much pressure (causes flashing), it
responds by spraying cool water on the steam to
condense some of it to reduce pressure.
•When there is not enough pressure (causes cavitation), a
heater activates and heats up the water.
PWR: Pressurizer

15. The secondary loop contains the following parts:
•Steam Turbine
•Condenser
PWR: Secondary Loop

16. The cooling loop is the final loop of a nuclear reactor system.
PWR: Cooling Loop

17. The safety systems are designed to retain
radioactive materials through the use of
layered and redundant protection systems:
•Fuel element Cladding
•Reactor Vessel
•Containment
PWR: Safety Systems

18. Barrier #2: Reactor Pressure Vessel
Reactor vessels are made from low carbon steels which make the vessel less brittle. The
duration of a reactor vessel is about 60 years.
•Thickness of vessel depends on pressure of the vessel.
•Radioisotopes used for quality control of a vessel.
There are many properties that fuel cladding must have. Some of these are as follows:
•Able to withstand high temperatures and transient temperatures.
•High corrosion resistance and low thermal neutron absorption.
•High thermal conductivity to minimize thermal stress and low expansion of metal.
•Zirconium, stainless steel, aluminum, magnesium, and beryllium have been used as cladding.
Cladding serves 3 functions:
•Provide structural support/strength for fuel & prevent distortion.
•Prevent release of radioactive fission products into the coolant stream.
•Sometimes also provides extended surfaces (i.e. fins).
PWR: Barriers
Barrier #1: Fuel Cladding

19. Barrier #3: Containment
The obvious picture of containment is the containment dome. It is a reinforced concrete dome
about so many meters high.
•Is six feet thick and composed of a lot of rebar.
•The plants are designed to withstand tornadoes, hurricanes, fires, floods and earthquakes
1
.
•Can be designed to survive impact of aircraft.
•Prevents the release of radioactive gases.
PWR: Barriers
[1] U. S. NUCLEAR REGULATORY COMMISSION

20. The boiling water reactor is similar to a PWRexcept that it boils the water directly into steam
from the reactor vessel.
•Second-most common reactor in the world.
•Coolant serves as moderator and working fluid as well.
•Steam separators is used to produce saturated steam.
•Water (moderator) is displaced and hence, reduces reactor power level.
Boiling Water Reactor (BWR)

21. There are several components that make a BWR
different from a PWR:
•Control rods insert from the bottom.
•There is no steam generator.
•Steam is separated by a steam separator on top of
the vessel.
•It is a little riskier, since there are more parts that
are exposed to radiation.
Boiling Water Reactor (BWR)

22. •Uses heavy water (D
2O) as moderator. It has a lower neutron capture cross-section than light
water and, thus, allows the use of natural Uranium as fuel. Can also use spent fuel from LWRs.
•10-11 MPa primary pressure; 4-5 MPa steam outlet pressure. No bulk boiling like PWR.
•Produces Plutonium and Tritium as by-products.
•The Karachi Nuclear Plant(KANUPP; 1971) is also of this type.
•Disadvantage is again the limitation of critical temperature. Also, higher construction cost.
Heavy Water Reactor (PHWR)
The primary loop is in yellowand orange, the secondary in
blueand red. The cool heavy water in the calandria can be
seen in pink, along with partially-inserted shutoff rods.

23. •Fuel assembly consists of a number of zircaloy tubes containing ceramic pellets (UO
2) of fuel
arranged into a cylinder that fits within the fuel channel in the reactor.
•In older designs, the assembly had 28 or 37 fuel tubes but the new bundle has 43 fuel tubes.
•It is about 10cm in diameter, 0.5 m long and weighs about 20kg.
PHWR: Fuel Assembly & Safety Features
*Atomic Energy of Canada Limited
Safety features:
CANDU reactors employ two independent, fast-acting safety shutdown systems. Shutoff rods
penetrate the calandria vertically and lower into the core in the case of a safety-system trip. A
secondary shutdown system involves injecting high-pressure gadolinium nitratesolution
directly into the moderator.

24. Magnox (UK) and UNGG (France) are obsolete. Advanced gas-cooled reactors (AGR) are
currently in use.
•Helium can also be used as coolant (Germany and USA).
•2.5 –3.5% enrichment as stainless steel tubes used for cladding.
•Online refueling at part-load can be done.
•Coolant reaches 640 ºC (& 4 MPa) compared to approx. 325 ºC for PWR.
Gas-Cooled Reactor (GCR, AGR)
Key

25. •FBRs usually use a mixed oxide fuel core of up to 20% plutonium dioxide (PuO
2) and at least
80% uranium dioxide (UO
2).
•Cooled by liquid sodium.
•Designed to breedfuel by producing more fissile material than it consumes.
•Disadvantages include inspection/repair difficulties, and depending on the choice of metal,
corrosion and/or production of radioactive activation products may be an issue.
Liquid Metal Cooled Fast Breeder Reactor (LMFBR)

26. Fast neutron reaction with U-238producing Pu-239is given below:
LMFBR: Reactions
238
U +
1
n
239
U + g
92 0 92
239
U
239
Np + e
0
92 93 -1
24 min
b
239
Np
239
Pu + e
0
93 94 -1
23 days
b
Sodium becomes radioactive as a result of non-fission reaction. The reaction is give below:
23
Na +
1
n
24
Na
11 0 11
24
Na
24
Mg + e
0
11 12 -1
14.8 h
b