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- 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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- 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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- 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.
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- 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.
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- 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.
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- Fission is more likely if neutron energy is low.
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- 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.
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- 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.
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- 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.
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- 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 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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- 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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- 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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- 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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- 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.
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- 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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- 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?
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- 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.
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- 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/
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- Introduction - a reality check
- The basics
- Fission
- The fuel: the source of energy
by the fission process
- The moderator: slowing down
those speedy neutrons
- The coolant: to take away the heat generated by fissioning
- Control: staying within desired and safe limits of power
- Shielding: providing protection from radiation
- The system that pulls it all together
- The engineering approach
- The Various Type of Reactors - CANDU vs PWR vs BWR vs ...
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