Quiz 1: lec 1-3 Flashcards
Framework: Who is the regulator in Canada?
Canadian Nuclear Safety Commission (CNSC)
Framework: Who would be considered in the Research and Development Sector
CNL, Universities, Utility Partners, private eng, companies
-Advanced reactor designs
-candu reactor exports
Framework: Who would be considered in electricity generation sector
OPG, BP, NBP
-domestic elecrticity
60% ontario
12% canada
Framework: Who would be considered in uranium sector
CAMECO, AREVA
uranium exports (33% global)
candu fuel
Framework: Who would be considered IN commercial isotopes sector
MDS Noridion, CNL, McMaster
-nuclear medium (60% global)
-irradiation tech
Canadian Nuclear Industry: Approx ___ private companies
150
Canadian Nuclear Industry: ___Govt/Private R&D company
1
Canadian Nuclear Industry: ___electricity producers
3
Canadian Nuclear Industry: Over ____ jobs
Approx. ___/year electricity sales
Approx. ____/year savings
from foreign exchange
Approx. ____/year exports
26000
4 billion
1 billion
1 billion
Most recent new build?? - what, who and where
GE Hitachi and OPG - Darlington B - BWRX-300
What is a power plant
a power station where fuel is
“exploited” to produce energy/electricity
economically
what do power plants use fuel for
use fuel in unique systems and
processes to convert the heat energy produced
into mechanical energy, which then operates an
electrical generator.
purpose of nuclear plant
safe, efficient, reliable, cost effective, and
economic conversion of the fission energy of
fuel to electricity, with minimum impact on
the environment.
There are three main methods of generating
conventional electricity
- Thermal power plant
- Hydroelectric power plant
- Nuclear Power Plant
Nuclear Plant: how is steam generated
Fission in core
produces heat to
generate steam
Nuclear Plant: desired temperature conditions
want temp as high as possible
Thermal Plant
Burning coal/natural
gas/ oil/fossil fuels in a
combustion chamber, to
boil water in generating
steam
Hydro Plant
Hydro-electric dams
generate potential
energy. Hydraulic
head power to run
turbines
How do you make
electricity?
In each power plant,
the turbine turns a
giant magnet inside the
generator.
rankine cycle: nth always __1
<, most reactors 30%
energy lost in all directions
entropy
draw rankine cycle
write nth eq
show work on t-s diagram
above supercritical on t-s diagram
energy loss is small (more thermal efficiency)
carnot cycle eq
–
ideal - want Th as _____ as possible and Tc as ___ as possible
high
low
steam temp in current nuclear reactors is _____ than in fossil fuel plants
lower
most efficient fossil fuel plants have thermal efficiency of __%
40
thermal efficiencies in reactors are ___
PWR and BWR __
HTGR __
CANDU__
lower
32
40
30
carnot cycle review
slide 24
process of nuclear fission
use slow neutron (at <1 eV
~103 m/s) to combine with uranium 235. creates fission products and heat and fast neutron (>1 MeV
~107 m/s)
what does heavy water do and who uses it
used in CANDU
as moderator to slow neutrons
what is heavy water
D2O
difference in hydrogen and deuterium
d has extra neutron
difference between light and heavy water
heavy water has bigger mass (2 neutrons)
why does fission occur
high potential energy to low potential energy state
he total energy released in nuclear fission of one U-235 atom is on
average ___
200 MeV
most stable element
iron
typical candu: components
heavy water moderator
modular design
large heat sinks
simple fuel bundle(calandria tube, pressure tube, fuel)
on power fuelling
The core of CANDU is ____ diameter,___ length, with stainless
steel walls about ____
thick, and ends about ___ thick
~7.6m
7.6m
2.5cm (1”)
5cm (2”)
DESCRIBE CANDU PROCESS
The primary coolant flows
through hundreds of
individual pressure tubes,
each with a feeder at either
end leading to headers and
steam generators.
* The moderator that surrounds
the pressure tubes is at ~atm
pressure, so there is no need
for a large pressure vessel.
* Fueling is done on-line.
Typical CANDU uses ___ between calandria and pressure tubes
C02 - inert and wont react
delayed neutrons vs prompt neutrons
prompt come out right away - dont want
delayed can take up to 100 sec- want
Nuclear Fission is useful because we can control it by:
✓adding neutron scavengers;
✓keeping below critical mass;
✓moderating the chain reaction;
✓‘delayed neutrons’
review u-235 fission and products
slide 44
A nuclear reactor will not operate without ___
neutrons
neutrons role in reactors
induce the fission reaction,
which produces the heat in nuclear
power reactors
✓And fission creates more neutrons
that are used to sustain the chain
reaction.
review neutron cycle
slide 48
productive” absorptions
end in fission
“non-productive” absorptions
(in
fuel or in structural material), which
do not end in fission
* leakage out of the reactor
Neutron economy:
the very delicate balance between
fission reaction, neutron capture and neutron leakage
review why u-238 is bad
see notes
The neutron flux,
is the number of ‘balls’ per sec
per area: n/s/m2 or, n/(m2
s), or n/m2
s
review flux and cross section
page 51
types of fission
spontaneous, induced
Spontaneous fission
- The process in which an isolated nucleus undergoes
fission, “splitting” into two smaller nuclei, typically
accompanied by the emission of one to a few
neutrons - The fission fragments are typically unequal in mass
and highly radioactive - Energy is released in the form of kinetic energy of
the products and as excitation energy of the
(radioactive) fission fragments
Induced fission
- The process in which capture of a neutron causes a
nucleus to become unstable and undergo fission - The fission fragments are similar to those of
spontaneous fission
Atomic nucleus (“nuclide”) is specified
by the:
* number of protons (denoted ___)
and
* number of neutrons (denoted ___)
it contains
Z
N
protons and neutrons are both called ___
nucleons
atomic mass
The total number of nucleons in the
nucleus (N+Z ) is denoted A.
A neutral atom has a positively charged
nucleus with ___
with Z protons and N neutrons,
surrounded by a cloud of Z electrons
mass defect
refers to the
difference in mass between an atom and the sum of the
masses of the protons, neutrons, and electrons of the atom
mass is typically associated with
binding energy
between nucleons
“missing” mass ?
is the energy
released by the formation of the atomic nucleus.
binding energy per nucleon
slide 62 and 63 and 64
Where does the fission energy go?
The energy released by fission goes into the kinetic energy of the fission
products (≈168 MeV), and neutrons (≈5 MeV) and gammas and betas (≈19
MeV), which ultimately becomes heat. The rest goes into neutrino energy,
which does not become heat.
electron volts
kinetic energy gained by an electron passing through
an electric potential difference of 1 Volt (in vacuum). A Volt provides
one Joule of energy per Coulomb of electrons (6.24 x 1018 electrons).
The work done on a single electron accelerated through
1 V is given by the charge times the voltage:
How Nuclear Power Plants are Designed,
Built and Operated: With regard to technical design considerations, there are several key requirements that must be met:
✓ Requirement 4: Fundamental safety functions
✓ Requirement 5: Radiation protection in design;
✓ Requirement 6: Design for a nuclear power plant;
✓ Requirement 7: Application of defence in depth
✓ Requirement 8: Interfaces of safety with security and safeguards;
✓ Requirement 9: Proven engineering practices;
✓ Requirement 10: Safety assessment;
✓ Requirement 11: Provision for construction;
✓ Requirement 12: Features to facilitate radioactive waste management
and decommissioning
There are three main types of
reactors (PWR, BWR, and
CANDU) that can be
distinguished by their
Fuel * Neutron moderation * Cooling * Control and safety systems * Balance of plant systems
safety levels established by
stakeholders
customers
public reaction / experience
regulation
codes + standards
safety os function of
societal attitudes and values that change with time
with enough margin for human error
Pressurized Water Reactors (PWRs):
- the coolant/moderator is pressurized to15.8 Mpa;
T(inlet) = 289oC, T(outlet) = 325oC
Boiling Water Reactors (BWRs):
coolant/moderator is pressurized to 7.5 Mpa;
T(inlet) = 216 oC, T(outlet) = 288 oC;
Exit steam quality (wt % steam) = 14.7%
Heavy Water Reactors (HWR) - CANDU
The coolant is pressurized in the fuel channels to 11 MPa; T(inlet)=270oC and T(outlet)=305oC;
The moderator is at 70oC and roughly atmospheric pressure.
What is
Unique about
the Safety of a
Nuclear Power
Plant?
- There is a tremendous amount
of energy produced - There is a lot of heat produced
even after the nuclear reaction
is terminated - The core contains a large
inventory of radioactive material
that is hazardous to humans and
the environment - Things can happen very fast and
there are also considerations
over long periods of time.
Characteristics of
a Reactor Core
Control
* Greater than 2 billion watts at full power
* Equivalent to 30 747’s at full power
Cool
* 5 min. after shutdown - 1 747 (all engines)
* 5 hours after shutdown - 1 747 engine
* 5 days after shutdown - 1 747 engine at
half throttle
Golden Rules of
Reactor Safety
- Design should enable nuclear fission to proceed in a
controlled manner
CONTROL - Design should be capable to remove the heat produced
and convert the heat produced to useful work (power)
COOL - Design should have “a means” of ensuring that it’s
functions can continue over the expected plant-life in a
safe and reliable manner
CONTAIN
control
Keep Fission Products Within the
Fuel
✓ Control Reactor Power ▪ Control reactivity
additions
▪ Shutdown reliably
cool
Keep Fission Products Within the
Fuel
✓ Cool the Reactor and Spent
Fuel ▪ Maintain coolant
inventory
▪ Maintain coolant flow ▪ Maintain coolant heat
sinks
contain
Keep Radioactive Material Within Reactor ✓ Maintain Containment Integrity ▪ Prevent over-pressurization ▪ Prevent over-heating ▪ Prevent containment bypass ✓ Capture Material Within
Containment ▪ Scrubbing Deposition ▪ Chemical capture
88
Keep out of the Biosphere
Safety Design - What are We Trying to Do?
Contain fission products and other radioactive species under all
operating conditions, normal and abnormal.
* This is the goal of public protection activities.
Protect operating staff from harm.
* This is the goal of radiological and conventional safety
activities.
Prevent damage to plant equipment.
* This is the goal of financial loss control activities.
Elements of the Design Process
Nuclear design
* basic design of the reactor core and required shielding
Thermal and hydraulic analysis
* thermal analysis of the reactor core and fuel, design of the primary coolant system
Reactor control and kinetic analysis
* reactor control system
Mechanical design
* design of the fuel elements in conjunction with the nuclear and thermal analysis; design of
the primary containment system
Design
Methodology
Safety components and procedures area are NOT designed
independently
✓the design process is interactive
Exact design depends to an extent of the purpose of the
design
✓examining the design of a new concept (FOAK -First Of
A Kind)
✓construction of a prototype reactor of a new concept
✓examining the merits of a new feature added to a
developed reactor type (NOAK - Nth Of A Kind)
✓constructing a new size or improved model of a
developed reactor type (NOAK)
✓Constructing a well-developed reactor type in a size
previously designed but with a minor modification
needed to meet regulatory and/or customer
requirements
main components of reactor
core, control rods, steam to turb, water from condensor, reflector
Features of a Nuclear Reactor
- Neutron Source * The Reactor Core * Fuel Elements * Coolant * Moderator (depends on design) * Reflector * Pressurizer (depends on design) * Primary Containment * Containment * Heat Exchanger (depends on
design) - Safety Systems
Features of a Nuclear Reactor,Other Systems
- Safety Systems * Balance of Plant
Systems ✓Auxiliary Systems ✓Nuclear Steam Supply ✓Power Generating
Systems ▪Turbines ▪Generator
The Reactor Core
The heart of any nuclear reactor system
✓The fuel must be an ideal geometry
which allows nuclear fission to proceed.
✓Core geometry must allow the heat
generated to be readily and
economically removed by the reactor
coolant system.
✓Changes in the core lifetime must
maintain criticality
✓Provides for shielding of other
components from core radiation.
✓Low fuel costs.
fuel elements
few use molten salts
solid fuel used in all major concepts
must be able to contain fuel and products in config that can be properly cooled and handled
should retain integrity under expected op conditions
elements- must be compatible with core design, capable of high temp op, provide minimum parasitic neutron absorption
fuel fab and reprocessing costs should be low
basic fuel for reactor
uranium
pellets of uranium oxide uo2 arranged in tubes to form fuel rods
contained in reactor core
initially u-235 and u-238
during operation other fissile nuclides, partially pu-239, u-233 produced
Fissile material
refers to a nuclide that is capable of
being split by an interaction with a thermal neutron.
✓ e.g. U-233, U-235, Pu-239;
Fertile material
refers to a nuclide that may capture
a neutron to form a product that eventually decays
to become a fissile nucleus
✓ e.g. Th-232 which captures a neutron and becomes U-233 by
double beta decay; and U-238 which similarly becomes Pu-239
(Picture a fertile material as an atom ready to be turned into
useful fissile material through neutron capture. Like fertile
soil, ready to be used to produce harvestable crops.)
Initiating Neutron Source
In a new reactor with new fuel, a
neutron source is needed to get the
chain reaction going. Usually this is
beryllium (Be) mixed with polonium (Po),
radium (Ra) or another alpha-emitter.
* Alpha particles from the decay cause a
release of neutrons from the beryllium
as it turns to carbon-12.
* Restarting a reactor with some used fuel
may not require this, as there may be
enough neutrons to achieve criticality
when control rods are removed.
Primary Coolant system
Provide sufficient coolant circulation to remove the
heat generated within the core and transport the
energy to a prime mover or to a secondary system
which also transports it to a prime mover.
✓Coolant must be capable of sustaining high
temperatures
✓Must be compatible with the core design.
✓Should provide a minimum of parasitic neutron
absorption.
✓Must be designed to provide adequate for all
operational and shutdown states
✓Typical coolants: H2O, D2O, liquid Na (or Na-K alloy),
liquid organic compounds, air, CO2
, He, boiling H2O
Coolant and moderator concepts
✓ In light water reactors the water moderator
functions also as the primary coolant.
✓ In the PWRs, there is secondary coolant
circuit where the water becomes steam.
✓ A PWR has two to four primary coolant
loops with pumps, driven either by steam
or electricity
✓ A BWR has no secondary coolant circuit,
the water boils to steam directly
Reflector
The reflector surrounds the core
✓ Its Purpose
▪ To reduce the loss of neutrons from the core
✓ Reflector material
▪ Determined by energy distribution of neutrons in the
core
▪ Beryllium and graphite make good reflectors as they
can also act as a moderator. Steel and lead also work
but have less of an effect on neutron energy.
▪ Depleted Uranium oxide can also be used as U238 can
absorb neutrons and become new fissile material.
benefits of reflector
Benefits
▪ Lowers the critical mass
* Mass of fuel needed to sustain a chain reaction.
▪ Acts as thermal and radiation shield
Component of today’s thermal reactors:moderator
✓A moderator is a
material in the
core that slows
down neutrons
released from
the fission
process so that
they cause more
fission.
✓Used to slow down
high
-energy fission
neutrons (~to thermal
energies (~0.025 eV) ✓The best moderators
are elements of low
mass number with
small neutron capture
cross sections. e.g.
H2O,D2O, beryllium,
beryllium oxide,
graphite ✓Fast reactors don’t
need a moderator
Why not mix the fuel and moderator together
like a nuclear smoothie?
238U captures a neutron and
forms 239U, which eventually
decays to 239Pu after about 3
days.
239Pu is fissile, and
contributes to CANDU reactor
energy output, but we need
235U to undergo fission to
keep the reactor operating,
especially during start up or
after refueling, where 239Pu is
not yet present.
239Pu fission accounts for
about half of the heat
produced by a CANDU
reactor during critical
operation!
Primary Containment
✓Pressure vessel for PWR and BWR reactors,
Pressure Tubes for CANDU
* The reactor core, including the fuel, and primary
coolant must be contained in a leak-tight system.
* The containment must also serve as a reliable
barrier to the release of radioactivity:
✓From failed fuel
✓From coolant activation to the environment
* Must be designed to withstand shock loading
which could result from pipe severance or
earthquake/seismic activity
Primary Containment (Pressure
vessel or Tubes)
- The primary containment must
withstand the expected design
pressures.
✓robust steel vessel
containing the reactor core
and moderator/coolant in
some concepts
✓In others, e.g. CANDU, it
may be a series of tubes
holding the fuel and
conveying the coolant
through the surrounding
moderator.
Secondary External Containment
Structure
The structure around the reactor
and associated steam generators,
shown in green in the figure, is
designed to protect from outside
intrusion, and to protect those
outside from the effects of
radiation in case of any serious
malfunction inside.
* Newer Russian reactors, and some
other reactors, install core melt
localisation devices or ‘core
catchers’ under the pressure
vessel.
Steam Generator (Boiler)
- High-pressure primary coolant brings
heat from the reactor to a secondary
circuit to make steam for the turbine. - From high-pressure primary circuits in
some concepts, to a secondary circuit
where water turns to steam. - Essentially a heat exchanger like a car’s
radiator. Reactors have multiple
“loops’, each with a steam generator.
Steam Generator (Boilers)/Heat Exchangers
- The secondary water must flow through the
support structures for the tubes. - Tubes vibration and fretting is designed out
- Deposits build up impede the flow of steam
and must be avoided. - Chemically maintained to avoid corrosion.
- Tubes which fail and leak are plugged, and
there is margin designed to allow for this. - Detection of leaks by monitoring N-16 levels in
the steam as it exits the steam generator.
Reactivity Control System
- The reactor must be capable of safely bringing the reactor to
power, maintaining it there and shutting the reactor down - The control system respond to unexpected load variations and
rapid reactor shutdown (scram) during an emergency - Control rods
✓ Neutron-absorbing material (poison) inserted or withdrawn
from the core to control fission
✓ Control is achieved by varying neutron density.
✓ Poison materials: Boron (B), Gadolinium (Gd), Cadmium
(Cd), Dysprosium (Dy).
Neutron poison
✓Allows control of reactor
✓Insertion of poison results in a decrease in
reactivity (or neutron multiplication) of the
core, i.e., decreases neutron density. Hence,
reactor power level is reduced.
✓Withdrawing the poison increases the
neutron density and power level.
✓In some design concepts, special control rods
are used to enable the core to sustain a low
level of power efficiently. (Secondary control
systems involve other neutron absorbers,
usually boron in the coolant).
Pressurizer
- Used for Heat Transport System inventory (liquid state)
control and pressure control - The pressurizer is connected to the primary loop through
a surge nozzle at the bottom.
✓ Heaters are provided at the bottom of the pressurizer internals,
and
✓ a spray nozzle, relief nozzle, and safety nozzle are installed at the
top of the pressurizer head. - A “positive surge” of water from the primary loop due to
increasing loop pressure is compensated for by injecting
cold water from the top of the pressurizer to condense
steam - A “negative surge” of water empties the pressurizer,
reducing steam pressure at the top of the pressurizer and
thus loop pressure. - In this situation, the electrical heaters at the bottom of
the pressurizer are automatically activated, converting a
portion of the water into steam, resulting in a loop
pressure increase
Other Systems (Balance of Plant)
Auxiliary Systems
✓Removal of radioactive material and other contaminants
from the primary coolant.
✓The refuelling system
✓Removal of radioactive waste from discharged air and
water streams
Feed water systems
✓The feedwater system supplies demineralized and
preheated light water to the steam generators
Main Steam
✓The steam from the boilers is fed by separate steam
mains to the turbine steam chest via the turbine stop
valves, and its flow is controlled by the governor
valves;
✓Excess steam can be discharged to the atmosphere or
bypass the turbine by flowing directly to the
condenser;
✓Over-pressure protection is as safety relief valves on
each steam source
Power Generating Systems
✓The energy transferred from the primary coolant
must be transferred to a prime mover, which is
always the turbine
Safety Systems
✓Demonstration that no accident situation can
significantly endanger the health and safety of
workers and the public.
✓Systems to provide heat sink to cool under
accident conditions.
✓Enclosures to retain any radioactive releases must
also be provided
Reactor Power
The electric power used by equipment in
the plant is called the station service power.
* Station service power takes 5% or so of the
generator output.
The rest of the electric power is delivered
to the grid
* It is called the unit net electrical power.
Fission power (total power generated in
fission)
Reactor
Multiplication
Constant and reactivity
see 125-133
Concept of Critical Mass
- The minimum quantity of fissile material that is capable of sustaining a fission chain:
✓depends upon its nuclear material and properties
▪ nuclear fission cross-section
▪ its density, its shape,
▪ its enrichment, its purity,
▪ its temperature, and its surroundings. - Origin of issue
✓2 or 3 neutrons are liberated per fission, but only 1 is required to
maintain the fission chain. However, not all neutrons resulting from
fission are available to carry on the fission chain
▪ There are losses!!!!!
Concept of
Critical Mass
Neutron losses: non-fission reactions (e.g. radiative
capture:i.e., (n, ) ) with other nuclei, but also with
fissile materials; and escape from the system through
its physical boundaries (leakage)
Leakage: can be controlled/reduced by
increasing the size, i.e., mass of the fissile
material.
At critical mass, the chain reaction
becomes self sustaining
Concept of Critical Mass
- Critical mass depends on many factors, examples are:
✓physical form of fissile material and moderator type - Examples of critical mass of 235U:
✓<1 kg for a homogeneous solution in water of
uranium salt containing about 90% of fissile isotope
✓200 kg present in 30,000 kg of natural uranium
embedded in a matrix of graphite
Concept of
Critical Mass
- Because leakage of neutrons out of reactor
increases as size of reactor decreases
✓reactor must have a minimum size to work. - Below this minimum size (critical mass),
leakage is too high and keff cannot possibly be
equal to 1.
✓Less sustaining nuclear fission - Critical mass depends on:
✓shape of the reactor
✓composition of the fuel
✓other materials in the reactor - Shape with lowest relative leakage, i.e. for
which critical mass is least, is the shape with
the smallest surface-to-volume ratio: a sphere
Spherical Reactors and Geometry
- In CANDU reactors, fission neutrons travel about 50 cm before being absorbed by the fuel. The bigger the reactor,
the lower the chance that the neutron will leak out before it is absorbed. - Spherical reactors are not practical, so we use a cylindrical core with a diameter slightly bigger than the length.
Leakage and Critical Mass
- The larger the surface area is relative to the
volume, the larger the leakage. - Consider the area-to-volume ratio of a
sphere. Small reactors have larger leakage
and usually require higher enrichments. - Why?
- What is enrichment?
compare Reactor Sizes
- PWR (3400 MWTh)
✓ The vessel is ~12m high with a 4 m diameter,
✓ Core size ( 3.7 m high x 3.4 diameter) - BWR (3300 MWTh)
✓ The vessel is ~22m high with a 6 m diameter, with
✓ Core size ( 3.7 m high x 4.8 diameter) - CANDU-6 (2060 MWTh)
✓ The core is relatively “small”: ~ 7.6 m diameter, 6m
length
✓ But notice the diameter
Decay Power/ Heat
- Many fission products are still decaying long after the originating fission reaction.
- Energy (heat) from this nuclear decay is produced in the reactor for many hours, days,
even months after the chain reaction is stopped. - This decay heat is not negligible.
✓ When the reactor is in steady operation, decay heat represents about 7% of the
total heat generated.
✓shutting down the fission process brings core heat down to about 7 per cent of its
running temperature.
✓Even after reactor shutdown, decay heat must be dissipated safely, otherwise the
fuel and reactor core can seriously overheat.
Decay Power/ Heat % breakdwon
- 5% of fission heat goes to moderator
✓ 20% delayed
✓ 80% prompt - 95% of fission heat goes to fuel
✓ 6% delayed
✓ 93-94% prompt - 1% additional from pump heat
locations of heating
see 143
Decay
Power/ heat
Decay heat is an issue and must
be considered in two scenarios
during design
1.When used fuel is removed from the
reactor * Remedy is the fuel must be safely
stored, to cool it and to contain its
radioactivity.
* Design of spent fuel bays as part of
the Balance of Plant
2. During a transient when the reactor shuts down
* Remedy is to obtain adequate core cooling for
safe shutdown to prevent
✓ melt down of reactor and
✓ other safety issues
▪ release of radioactivity into the
environment, hydrogen/steam
explosions
▪ molten-concrete core interactions
* Design of emergency core cooling, emergency
mitigation equipment and other associated
safety and auxiliary systems for safe
shutdown
The main systems involved in
converting
✓heat energy of the steam in
the turbine (Primary mover)
✓ Flow of steam to turbine
✓Rotational energy (Rotor and
Stator)
▪ which turn and drives the
generator
✓Converts the mechanical
energy to electrical energy
All CANDU generating stations have turbine assemblies
consisting of
One double flow high pressure cylinder
* three double flow low pressure cylinders
with external moisture separators
* live steam reheaters between the high- and
low-pressure stages
* Called a tandem compound
* Note the size
✓ HP and LP Turbines
The governing system controls the turbine’s speed of rotation when the generator is not synchronized to the grid
Governor valve on a steam engine * the governing system determines
turbine/generator power
* Design features * Reliable and fast acting
governor valves
* Have emergency trip systems
which can detect and prevent
turbine overspeed and safely
unload the turbine.
Separator and Reheater
✓Steam exiting the high
pressure turbine has
about 10% moisture
content, which must be
removed prior to
admitting the steam to
the low pressure stages.
Separator
uses
mechanical means to
remove much of the
moisture content,
Reheater
live steam raises the steam
to superheated
conditions
The Generator
✓It is a three-phase four-pole machine
directly coupled to the turbine.
✓In the case of electrical system
operating at 60 Hz, the generator
typically operates at 1800 rpm,
▪ 50 Hz systems at 1500 rpm.
▪ # Poles = [(120 x Freq)/#rpm]
✓The output voltage is typically 24,000
volts, and is connected via forced air
cooled,
▪ isolated phase bus duct to the
step-up Main Output
Transformer.
✓Cooling of the rotor winding and stator core is by hydrogen
The Condenser: In order to extract
maximum energy from the
steam, it needs to be
condensed to a pressure
and temperature that is as
low as practicable.
can handle full steam bypass flow when the
turbine is not available.
Takes place in the
condenser,
✓ heat removed to the
environment by the
condenser cooling water
The Condenser;Consists of three separate
shells
one for each low-pressure
turbine cylinder;
Feed Heating
uses extraction steam to
preheat the feedwater in
order to optimize
thermodynamic
efficiency;
* consists of three low
pressure (LP), a
deaerator, and two HP
heaters;
* feed pumps return the
feedwater to the boilers.
REVIEW DIAGRAM IN LEC 3
3-11
REVIEW DIAGRAM DANDU CONNECTION TO GRID
13
- Groups of Power for Entire Operation
Normal (Group 1) and Emergency (Group 2)
The Group 1 power supplies are classified in terms
of their level of reliability (Non seismic qualified)
✓Class I Power
✓Class II Power
✓Class III Power
✓Class IV Power
The Group 2 power supplies is more reliable
than —— Group 1
than the highest in Group 1 (Class 1)
The Group 2
✓Seismic qualified – Can survive a seismic event
✓Emergency Power Supply
- Class I Power
- Uninterruptible direct current (DC) supplies for essential auxiliaries are obtained from the Class 1 power supply,
✓ Three independent DC instrument buses,
✓ These buses are each supplied from a Class III bus via a rectifier in parallel with a battery
✓ Buses provide power for DC motors, switchgear operation and for the Class II AC buses via inverters.
✓ typically 50V to 250 V DC.
- Class II Power
- Uninterruptible AC supplies for essential auxiliaries are obtained from Class II power supply, which comprises:
✓ Two low voltage AC three phase buses which supply critical motor loads and emergency lighting. These
buses are each supplied through an inverter from a Class III bus via a rectifier in parallel with a battery.
✓ If a disruption or loss of Class III power occurs, the battery in the applicable circuit will provide the
necessary power without interruption.
✓ supplies equipment and instrumentation essential to safe station operation;
✓ typically 50 V to 250 V AC.
- Class III Power
- Supplied by on-site Standby Generators if cannot be supplied from Class IV;
✓ AC supplies to auxiliaries that are necessary for the safe shutdown of the reactor and turbine are
obtained from the Class III power supply with a standby diesel generator
✓ These auxiliaries can tolerate short interruptions in their power supplies (may be unavailable for 3
minutes);
✓ Essentially to ensure reactor cooling is maintained
✓ typically 400 V to 5 kV AC
Class IV Power
- Supplies all major loads directly and all station equipment under normal operating conditions via the other
classes
✓ may be unavailable for extended periods (hrs);
✓ Essentially ensure heat sinks are operating
✓ Complete loss of Class IV power will initiate a reactor shutdown.
✓ highest distribution voltage within station(4 kV to 14 kV AC)
- Classes of Power
for Normal
Operation
The power supplies
are classified in
terms of their level
of reliability
* The lower the
number the more
reliable is the
power
- The Electrical Power Output Systems (EPOS)
consists of :
✓Main Output Transformer
✓The Switchyard
✓The Station Service and Unit Service
transformers
- The EPOS - link between
electrical power
generated by the station and the Power Grid
of the Utility that utilizes the energy.
✓It also produces the electrical power that
operates the station
Main Transformer
✓ The main transformer steps up
the generator output voltage to
the same level as the switchyard
transmission voltage.
✓ The transformer is rated to meet
the generator output
requirements and site
environment. It is equipped with
all standard accessories and the
necessary protective equipment.
Switchyard
✓ The switchyard, located near the
turbine hall, contains the automatic
switching mechanism, including the
breakers and disconnects, which is
the interface between the station and
the power grid transmission lines.
✓ There are at least two incoming lines
which are synchronized under normal
conditions. ▪ However, the switchyard
electrical equipment allows
transmission of full station
power through any one of the
incoming lines.
Unit Service Transformer
✓ During normal station operation the station services power is
supplied by both the unit service transformer and the system
service transformer.
✓ However, either transformer can provide the total service load
in the event of a failure of one supply. The transformer is fed
from the output system of the turbine generator.
✓ Its operation does not require that the unit be connected to
the grid
System Service Transformer
✓ The system service transformer is similar to the unit service
transformer. Fed from the electrical grid via the switchyard
✓ It supplies half of the plant services power requirements under
normal operating conditions and is able to provide the total
service load when necessary. This transformer is fed from the
switchyard and supplies all plant loads during the start
-up of
the plant, or when the turbine generator is unavailable
Computerized Reactor Control
✓Digital computers are used for
▪ station control,
▪ alarm annunciation,
▪ graphical data display and logging;
✓Two independent computers, both normally running,
but each capable of controlling the unit;
▪ only the ‘controlling’ computer’s outputs are
connected to the field devices;
▪ a fault in any essential part of one computer
results in automatic transfer of control to the
other computer
Main CANDU
Control Program
Because of the complex
interdependence of the
control systems in a
CANDU unit, Digital Control
Computers (DCC) perform
all 5 major control
functions
see slide 21
Each CANDU has four special safety systems
- The two independent shutdown systems
- Shut down system1 (SDS#1) and SDS#2
- The emergency coolant injection system (ECIS) and
- the containment system.
The plant design includes these four special safety systems to
protect the public from a harmful radiation release in the event of
failure, transient or accident
Special Safety Systems
✓ These systems do not take any part in normal power plant
operations, but are “poised” to act
✓ In other words, they are waiting and watching in case the
processes and their control systems cannot keep key operating
parameters within prescribed limits
✓ In such cases, when there is the potential for fuel failure to occur
with a risk of radioactivity release, these special safety systems
spring into action.
there are 4 Special Safety Systems in a CANDU:SDS1 and SDS2
✓ If the control of reactor power is not assured, one or both
Reactor Shutdown Systems will shut it down.
▪ SDS1 and SDS2 based on different physics
a. SDS 1- Shut off Rods to absorb neutrons
b. SDS 2 - liquid Poison (Gadolinium nitrate solution) to absorb
neutrons
Emergency Core Cooling
✓ If cooling of the fuel is judged to be insufficient,
Emergency Core Cooling will be implemented; using light
water to cool the core
Containment System
✓ If there is a risk, or perhaps an actual release of
radioactivity from any of the plant systems, then the
Containment System will ensure that no unsafe level of
radiation is released to areas outside the plant’s
boundary.
▪ Called boxed-up
▪ Containment is a leak tight structure
SDS1 and SDS2
SDS1 and SDS2
✓ there are two ‘full capability’ reactor shutdown systems, they
are functionally and physically independent of each other, and
each able to shut down the reactor;
* functional independence is provided by using different methods of
shutdown
✓ dropping solid neutron absorbing rods into the core for SDS#1,
✓ injecting liquid poison into the moderator for SDS#2;
* physical independence is achieved by positioning the shutdown rods
vertically through the top of the reactor, and the poison injection
tubes horizontally through the sides of the reactor;
✓ The two shutdown systems (SDS1 and SDS2) respond
automatically to both neutronic and process signals
Emergency Core Cooling
- ECC is initiated when the Heat Transport System pressure has dropped below 5.5 Mpa
✓Indication that Loss of coolant from the HTS has taken place
✓High reactor building pressure in case of break leaking coolant into the reactor
building
✓High moderator level in case of a fuel channel break leaking into the moderator
Containment
System
- The systems provide a sealed
envelope around the nuclear steam
supply systems if an accidental
release of radioactivity is detected:
✓ plastic lined pre-stressed posttensioned pressure-retaining
concrete containment
structure;
✓ Structure and supporting
systems which provide the
final barrier to limit radioactive
releases to the environment to
acceptable levels - Designed to withstand the maximum
pressure which could occur following
the largest possible LOCA - Containment is subdivided into:
✓ Containment envelope R/B
including extensions and
penetrations
✓ Containment penetrations and
isolation
✓ Atmospheric Control
- Atmosphere Control
✓ automatic dousing system-
▪ Dousing system for
pressure suppression
✓ R/B Air Coolers for heat
removal
▪ provide a long-term
containment
atmosphere heat sink
✓ Filtered Air Discharge (in the
long term if required)
✓ access airlocks;
✓ automatic containment
isolation system that closes
all reactor building
penetrations open to the
containment atmosphere
when an increase in
containment pressure or
radioactivity level is
detected;
Darlington, Bruce and Pickering Stations;CANDU
Containment
Designs
- Multi-unit plants with vacuum building:
China, korea, Argentina , Romania, Point Lepreau and Gentilly-2;CANDU
Containment
Designs
Single-unit plants with
no vacuum building
Site and Plant Arrangements
Land area sufficient to
provide the required
exclusion zone (500 -
1000 meters)
Source of cooling
water;
Large body of water
Connection to the
electrical grid;
Geology suitable for
foundations of the
required structures;
Known level of
seismic activity;
Transportation access.
