2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill...

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2007-05-12 PEO/CNS Symposium 1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics, McMaster University, Hamilton, Ontario, Canada “One's view can get badly distorted if there is nothing to provide scale and perspective.” - Hans Tammemagi
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Page 1: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

2007-05-12 PEO/CNS Symposium 1

How and why is CANDU designed the way it is -

an introduction

Bill Garland, Professor, Department of Engineering Physics, McMaster University,

Hamilton, Ontario, Canada

“One's view can get badly distorted if there is nothing to provide scale and perspective.”- Hans Tammemagi

Page 2: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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Summary

• Herein, we introduce the CANDU reactor by – Looking at the broad social context to see why we

need nuclear power. – Then we take a look at the nuclear reactor design in a

nutshell.– The engineering approach is discussed to provide

some appreciation of the fact that CANDUs are engineered systems.

– The various types of reactors are briefly shown via a quick overview of CANDU vs PWR vs BWR vs …

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Introduction - a reality check

• We are here. Let’s make the best it.

• Quality of life requires energy.

• Nuclear power is the only existing option that transcends the limitations of nonrenewable alternatives and renewable alternatives.

• We conclude, then, that nuclear should be part of the energy mix now and in the future…

• …that is, we have a functional requirement for nuclear energy.

requires

Quality of life

Lots of Energy

Coal

Oil

Gas Solar

Wind

Biogas

Nuclear

CANDU

PWR BWRGas reactors

and other variations

Resource limited

Too dilute

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2

Bruce A & B Pickering A & BDarlington

Gentilly

Point LepreauChalk River

TRIUMF

Uranium Mines

Whiteshell Port Hope(Conversion Plant and Fuel Fabrication)Blind River

(Conversion Plant)

Canada’s Major Nuclear

Facilities

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Fission

• To make sense of nuclear reactor design in general, and CANDU design in particular, the reader needs to have some familiarity with a few key nuclear concepts and phenomena.

• In a nutshell, slow neutrons (called thermal neutrons) can initiate a fission of uranium 235 (U-235), an isotope of uranium that occurs in nature.

F iss io n

B e fo re A fte r

th e rm a l n eu tro n

U 2 3 5 n u c leu s

F iss io n P ro d u c ts

2 -3 fa s t n eu tro n s

~ 2 0 0 M eV in k in e tic en e rg y

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Fast vs. slow neutrons

• Natural uranium that is mined from the ground is 0.7% U-235 and 99.3% U-238.

• The result of fission is fission products that are radioactive, radiation, fast (or energetic) neutrons and heat.

• The fast neutrons have a low probability of inducing further fissions, and hence have a low probability of generating more neutrons and thus sustaining a chain reaction.

Page 8: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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Cross sections

• Fission is more likely if neutron energy is low.

Page 9: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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Basic functional requirements

We need: • a fuel such as U-235• a moderator to thermalize (i.e., slow down) the fast

neutrons • a coolant to remove the heat • a control system to control the number of neutrons • a shielding system to protect equipment and people from

radiation • a system that pulls all this together into a workable

device.• Let’s look at these requirements in turn to gain some

insight on how and why CANDUs are built the way they are.

Page 10: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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The fuel: the source of energy by the fission process • Most practical reactors are "thermal" reactors,

that is, they utilize the higher thermal cross sections.

• Possible fuels include some of the various isotopes of uranium (U) and plutonium (Pu). The only naturally occurring fuel with suitable properties of significant quantities is U-235, hence most reactors use this fuel.

• Naturally occurring uranium is composed of 0.7% U-235. The rest is U-238.

• One group of reactor types (PWR, BWR, HTGR) enrich the fuel (a costly task) and use a cheap moderator (ordinary water or graphite).

• In another class, natural uranium (relatively cheap) is used with an excellent but expensive moderator (heavy water). This is the CANDU approach.

• Which is better? There is no simple answer. Both work. In engineered systems, there are always tradeoffs and the final design has to be viewed in the overall context of the end-use environment.

Page 11: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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The moderator: slowing down those speedy neutrons

• The best moderator is something that is the same size as the neutron itself - hydrogen.

• However, hydrogen does absorb neutrons as well.

• The deuterium isotope of hydrogen, at twice the mass of hydrogen, is almost as good a slowing down agent but it has a very low absorption cross section -> better moderator

• D2O + natural fuel = CANDU• H2O + enriched fuel = PWR• Other possible moderators include graphite

and gases such as carbon dioxide and helium.

• A good moderator = high scattering cross section + low absorption cross section + slows the neutron in the least number of collisions.

Page 12: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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The coolant: to take away the heat

• The fissioning process generates energy, predominately in the form of vibrational kinetic energy of the fission products. Such vibrating molecules constitute a familiar phenomenon to all of us – the fuel heats up!

• If we don’t cool the fuel, it will melt and the radioactive fission products, now that they are mobile, may find a path to the environment.

• To prevent this, a coolant (water is commonly used) is passed over the fuel.

• So far, we have fuel, moderator and coolant. • We can conceptualize our CANDU as follows…

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Control: staying within desired and safe limits of power

• Control of the fissioning process is achieved most easily by simply adding or removing neutron absorbers.

• Materials such as cadmium readily absorb neutrons and can be conveniently formed into solid rods.

• So by having a number of these control rods partially inserted into the moderator tank the neutron population and be controlled.

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Shielding: providing protection from radiation

• Uranium isotopes are not very radioactive by themselves and do not constitute a direct radiation hazard.

• It is the fissioning process that creates the nasty radioactive fission products. These ARE dangerous and must be kept isolated from us.

• Radiation takes on a number of forms. – Alpha and beta particles – Neutrons are not charged and can penetrate solid walls. – Gamma radiation, essentially very energetic photons.

• Constructing good shielding is not an onerous task, but it is an important one. Like in the control systems, safety is enhanced by redundancy. In this case, this means layering the shielding systems, one inside the other like a Russian doll set.

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All this ‘just’ to make electrical power

• We typically use the 'heat engine' process to turn this heat into a more useable (that is, flexible, transportable, convenient, etc.) form of energy.

• As illustrated, this heat is used to boil water and the resulting steam drives a turbine which drives an electrical generator.

• Electricity is a very convenient form of energy - today it is so ubiquitous that it is hard to image life without it.

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The system that pulls it all together

• We pull together the various requirements related to – fuel, – moderation, – cooling, – control and – shielding

• Layered, defense-in-depth approach wherein the radioactive fission products are kept from the environment.

• Designing a nuclear plant is not a trivial exercise. There are many systems and sub-systems that interact.

• We’ll look at that issue next.

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CANDU

reactor coreheat transport

systemsecondary side safety systems

pipes pumps steam generator instrumentationpressure and

inventory control

shutdown cooling

pressurizer bleed condensor surge line D2O storage steam bleed feed level control

The engineering approach • Like all engineered systems, the best way to understand and

appreciate the CANDU reactor is to look at how it functions. • Engineered systems are designed by functional decomposition.• By breaking the big problem down into smaller and smaller

problems, we systematically define problems that we can solve. These solved pieces, however, must be integrated back into a whole if we are to have a successful solution.

Page 28: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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“One's view can get badly distorted if there is nothing to provide scale and perspective.”

Page 29: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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Designs evolve

• Implicit in such an approach is the question: "What does each subsystem have to do?".

• Thus the responsible engineer starts by asking "What are the functional requirements?".

• There are usually many ways to meet these functional requirements and it is the engineer’s job to find a workable solution that meets these requirements effectively – meaning an optimum of efficiency, cost and safety, taking society and the environment well into account.

• Past experience usually plays a large role in engineered systems, but this is especially true for nuclear reactors because of the stringent quality assurance requirements and high safety standards.

• Many person-years of effort and many millions in funding have been spent to 'get the bugs out'. As a consequence, progress is more by systematic evolution, not revolution.

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Resource Allocation

• We have finite resources.

• We ‘draw boxes’ and assign resources around our activities so that we can focus.

• How do we budget our resources?

• What are we optimizing?

• Where should we put our next dollar? Into nuclear safety or somewhere else?

Page 31: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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The Various Type of Reactors - CANDU vs PWR vs BWR vs ...

• Since the inception of nuclear power reactors in the early 1950's, many designs have been conceived, quite of few of these designs were actually built and a handful have been successful.

• The figure (next) maps the predominant types that have persisted over time. The types are distinguished by the major design features of fuel type, coolant type and moderator type.

• The current market share is held by the Pressurized Water Reactor (PWR) which uses enriched fuel, high pressure light water combined coolant / moderator in a pressure vessel. But CANDU, which uses natural uranium, high pressure heavy water coolant in pressure tubes, low pressure heavy water moderator, is a viable competitor.

• In the end, the choice depends on achieving a balance of technical issues such as cost, safety, operations, design infrastructure, etc. and non-technical issues such as electrical grid planning, institutional and societal preference, international politics, funding arrangements, indigenous resources, etc.

Page 32: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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Reactor Types: Prototypes and Successes

THERMAL REACTORSFAST

REACTORS

MODERATOR Graphite Water Heavy Water not applicable

COOLANTMolten

SaltCO2 H2O Helium H2O H2O H2O D2O Hydro-carbon CO2 Sodium / NaK

FUEL Natural U Magnox BLW CANDU OCR

Enriched U AGR RBMK HTGR PWR BWR SGHW Atucha KKN-EL4

Thorium-U MSBR THTR LWBR

Plutonium-U ATR LMFBR

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CANDU vs PWR

Accident Response

Void reactivity coefficient

Size of reactivity insertion

neutron lifetime

probability of rod ejection

Post-was national

infrastructure

choice of fuel and moderator

core sizediffusion length, L

Absorption cross section

Diffusion coefficient

pressure vessel vs pressure

tube

refuelling method

control rod penetration

rod worth

Page 34: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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Source: http://www.nrc.gov/reading-rm/basic-ref/students/animated-pwr.html

Page 35: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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Source: http://www.nrc.gov/reading-rm/basic-ref/students/animated-bwr.html

Page 36: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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Source: http://www.nrc.gov/reading-rm/basic-ref/students/animated-pwr.html

Page 37: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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Source http://www.cs.sbcc.net/~physics/flash/heatengines/Carnot%20cycle.html

Page 38: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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Further reading:

• The Virtual Nuclear Tourist - a very popular site run by Joe Gonyeau, a dedicated nuclear engineer. http://www.virtualnucleartourist.com/

• The CANTEACH website - be sure to visit the site library for extensive information on CANDU reactors. http://canteach.candu.org/

Page 39: 2007-05-12PEO/CNS Symposium1 How and why is CANDU designed the way it is - an introduction Bill Garland, Professor, Department of Engineering Physics,

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Take-away Points

1. Introduction - a reality check2. The basics

1. Fission2. The fuel: the source of energy by the fission process3. The moderator: slowing down those speedy neutrons4. The coolant: to take away the heat generated by fissioning5. Control: staying within desired and safe limits of power6. Shielding: providing protection from radiation7. The system that pulls it all together

3. The engineering approach4. The Various Type of Reactors - CANDU vs PWR vs

BWR vs ...