|
[BACK] |
||||||||||||||||||||||||
|
Transition
to Large Scale Nuclear Energy Supply by ABSTRACT We can
expect to see the peak of world oil production very soon.
Some say that we can see that peak now in our rear-view mirrors as
we drive into an oil-poor future. Natural
gas already is in short supply in This paper examines, in very general terms, the implications of today’s shifting prospects for nuclear energy, as it exists today, and how those prospects might develop in the future. The time span under consideration is the remainder of the 21st century. INTRODUCTION During
the working lives of many professionals active today, the nuclear power
industry has hovered on the brink of extinction.
Will people accept the technology?
Who will buy the next plant, and if they buy, at what price?
Will the competition get the job?
Will government support pre-commercial product development? We
all have asked these questions. Now,
the questions are changing. Oil
supply analysts1 agree that world oil production must decline
at some time during the 21st century.
Huge imports of natural gas to Obviously, when looking at a 100-year time frame it makes no sense to propose the solution to energy supply questions. Rather than make such an attempt, the author has chosen to follow published projections as far as they go and then to make reasonable guesses at a series of development steps that can be taken to reach a defined goal at the end of this century. To some extent this process is based on a recent IAEA symposium2 with the declared goal of looking at the future of the world nuclear industry. HISTORY Uranium
was recognized as a vast potential source of energy from the first days
after the discovery of nuclear fission3.
Leo Szilard4 quickly recognized its potential for both
good and bad purposes. When
R&D for the nuclear-electric industry began in the 1940’s and 50’s
one of the main concerns was the potential shortage of uranium fuel.
At the time, exploration for uranium was limited and only low ore
concentrations had been found. This
apparent resource shortage led to intensive work on fast-spectrum reactors;
indeed, the first-ever electricity production from uranium was from a fast
reactor-- the Experimental Breeder Reactor (EBR1) in Dr.
W.B. Lewis5 was deeply involved with the uranium supply
question, both nationally and internationally.
Both uranium and thorium fuel cycles were studied in detail.
The latter element, of course, suffers from a lack of any naturally
occurring fissile isotope so that uranium must provide the initial fuel
supply. Atomic Energy of Successful
uranium exploration in the 1960’s and 70’s greatly increased known
uranium reserves. Large ore
deposits were found in This paper is not a “hard-and-fast” plan for the future. Further, it makes no pretence toward arguing that this future conceptual plan is the only one possible, or even that it may be the preferred plan. The objective of this paper is to illustrate some of the opportunities along with a few of the hurdles that must be passed along the way, in order to realize the vision of a secure and sustainable energy supply in the future world. THE
NEED FOR NUCLEAR ENERGY Today’s
world depends heavily on petroleum, both oil and natural gas.
It is still an open question as to what energy source or sources
can and will take over the burden once this resource is depleted. Many
options are available that can contribute to the solution, but it appears
that nuclear energy must be a major contributor6. There
has been much talk in recent years about the “Hubbert’s Peak”7
of world oil production. According to that model, once the peak has been
reached one should expect that only one half of the total resource remains
to be found. Now that prices have been high and rising for 5-10 years and
yet production has been decreasing over the same period, we can safely
conclude that we have passed the peak of world production. Conservation,
along with a number of alternate energy supply options, has been studied
for a number of years with limited success.
It has slowly become obvious that nuclear energy is the only
resource available today that could take over a large fraction of the
world demand for oil and gas, and yet remain neither capacity nor resource
limited – that is, to be “inexhaustible” or
“renewable”. There
is enough accessible uranium to supply the total present-day demands of
humanity for at least several thousand years8.
The
question “How much energy does the world need?” is the most important
and most difficult question of all, and well beyond this author’s
capability to estimate. As a
scale comparison, the total world energy demand of the World Energy
Council (WEC) ‘middle course’ scenario in 2050 is given as about 400
million barrels per day of oil equivalent9.
The WEC scenarios show a slowly decreasing role for coal, a large
role for natural gas, and a steadily increasing contribution from solar
and biomass at that time. As
noted briefly in this paper there is a real possibility for using coal or
biomass to manufacture synthetic transportation fuels by adding hydrogen.
Nuclear energy can be used in this way to produce fuels that are
fully compatible with today’s transportation, heating, and other
industrial systems. Since coal and natural gas do not seem to be scarce in a worldwide context, this work concentrates on substitution of nuclear energy for oil – a commodity that is rapidly becoming scarce. GENERAL
INDUSTRIALPLAN10,
11 Presumed
world oil demand in this study is shown in Figure 1. Up to 2030, the
estimate is taken from the 2005 International Energy Agency (IEA)
projection12. Beyond
that time demand is assumed, arbitrarily, to flatten out at 140 million
barrels per day. Figure
1: Presumed oil supply and demand vs. time The
supply curve for conventional oil also is taken from IEA figures up to
2030, without non-conventional supply and new discoveries.
After that time supply is assumed to flatten out at 50 million
barrels per day, or about half of the peak production in 2005.
(Note: There is no presumption that the long-term “Supply”
curve in Figure 1 is a prediction of what will happen.
The essence of the question is the timing of the expected supply
deficit and the fact that it will continue indefinitely into the future.)
Some fraction of the deficit between conventional supply and total
supply will be filled from other sources such as conservation,
introduction of hybrid vehicles, new oil discoveries, wind, solar, and so
on. It is important to note
that the particular timing of peak world oil production is quite
unimportant. It is necessary
only to agree that there will be a peak of production, at some
future time. In other words it
is necessary only to accept the fact that recoverable oil is a finite
commodity on earth. So,
what should we do about it? There
have been many studies conducted, and many proposals put forward.
The present-day situation has recently been summarized6, 8.
These modern assessments differ little from that described in the
International Institute for Applied Systems Analysis (IIASA) study carried
out more than 20 years earlier13.
The main change since the IIASA study is that the needed
replacement for fossil fuels is now urgent.
Nuclear energy using uranium offers the only practical answer for
filling in a major part of the gap between supply and demand shown in
Figure 1. Even then, the
enormous scale of the replacement task cannot be over-emphasized.
This is not to belittle contributions of other renewable resources
and conservation. The
statement is meant only to emphasize the central role of nuclear energy in
any sound plan, regardless of what other partial solutions are adopted. Substitution
of nuclear fuels for fossil fuels in the supply of primary energy is not a
simple task. For instance,
transportation requires a portable fuel of high energy density and low
weight – that is, if we choose to mimic today’s pattern of
transportation. The refining
and distribution of fossil fuels now embodies a massive infrastructure
that pervades nearly every corner of North American society.
A similar complex infrastructure is seen in the electricity
distribution system – it is difficult to scan the horizon in any
industrial nation today without seeing some evidence of this second system.
Could they/should they be combined in some way?
This might increase efficiency, but might at the same time increase
the system’s vulnerability. Substitution
of nuclear for fossil supply can be approached in different ways.
It is possible to expand the electrical distribution system and
then to provide local service either for battery-powered vehicles or for
some form of hybrid. The next
question is whether or not batteries can be developed that can match the
excellent characteristics of gasoline- and diesel-powered systems.
Today’s answer seems to be “not yet” though there is hope
that this will be possible soon14.
A
second method of substitution is to produce an intermediate energy carrier15
such as hydrogen that can be utilized in different ways such as local
night-to-day storage for peak leveling, in fuel cells, or as feedstock for
manufacture of hydrocarbons. Hydrocarbon
production is practiced today in South Africa16, using an
improved process relative to the basic method developed in Strategies for Installation of Large-Scale Nuclear SuppliesEven
though there is strong evidence that the long-term nuclear future must be
based on fast reactor technology, almost all nuclear plants operating in
the world today are powered by thermal reactors.
As a result we must consider the necessary steps in a transition
from today’s technologies to those appropriate for the long-term future
in which the predominant source of primary energy is nuclear fission. Figure
2 is adapted from an earlier paper18.
It is a concept sketch of an “energy park”, as several authors
have discussed over the past decades.
This version of the energy park concept includes all components
essential to production, from fuel input to waste disposal.
These components may of course, be either dispersed or concentrated.
It is useful to think of them as being co-located on an “energy
island”, either figurative or literal.
An actual island might be preferred if security and safeguards are
assumed to be dominant factors in this postulated future scenario. Figure 2 can be considered as a “target scenario”; that is, a future energy system toward which we could now aim, while recognizing that its actuality will be achieved only after several steps and stages collectively requiring several decades for implementation. The system includes fuel recycle facilities such as electrochemical reprocessing19 or direct use of PWR fuel in CANDU (DUPIC)21.
Figure 2 – Conceptual Arrangement of a Nuclear Energy Park An energy park such as this brings with it several advantages. First it is large, so that costs of perimeter security are distributed over a number of profit centers. In addition, this large scale permits the establishment of a large staff with diverse technical skills, and a revenue base capable of supporting effective waste management systems such as zeolite trapping of radioactive noble gases. Energy from such a facility may be distributed by electrical power
lines, via tankers in the case of synthetic fuel production, or in the form
of solid products such as industrial chemicals or fertilizer.
Given
the high probability of ongoing supply crises in world oil and gas supply
during the next couple of decades it is obvious that the only nuclear
technologies ready for immediate deployment in large numbers are the
pressurized water reactor (PWR), the boiling water reactor (BWR), and the
pressurized heavy water reactor (PHWR).
All of these reactor types produce electricity at mutually
competitive prices. Further,
if the authorities that must buy these power plants are conservative in
their choice of appropriate technology, these plants are likely to be the
same or very similar to plants operating today.
As decades pass, new improved designs based on similar technologies
will be chosen more frequently as their advantages come to be more strongly
assured. Recognizing
that hundreds of thermal reactor plants will be operating before a
significant shift toward fast-reactor powered plants comes onto the market,
it may be possible to choose some variant of thermal reactor that would make
the later transition easier. From
the point of view of fuel cycle sustainability, the most important
thermal-reactor characteristic is the amount of electricity that can be
produced per unit of natural uranium required to supply fuel to the plant.
By this measure, the PHWR is clearly superior. Table I
shows the energy produced per Megagram of mined uranium18. It
shows that a fleet of PHWR reactors can produce 30-60% more electricity than
can the same number of PWRs, from a given amount of mined uranium. Fuel
discharged from a once-through cycle in a PHWR can be sent to a reprocessing
plant to extract uranium 238 as well as some high-absorption fission
products and produce fresh, recycled fuel. Table I – Energy Output per Megagram of Uranium Mined
Another
advantage of the PHWR is illustrated by considering a simple equilibrium
steady-state ratio of thermal reactors to fast reactors in a combined system
where fast reactors provide fissile isotopes to thermal reactors.
Wade10 shows that this ratio is given by the equation
Number of thermal units/Number of fast units ≈ (BR-1)/(1-CR) The
approximate values of conversion ratio (CR) and breeding ratio (BR) are: FBR=1.4,
PWR, BWR=0.6, PHWR=0.8, PHWR(Th)=0.95. Table
II lists the consequent ratio of thermal to fast reactor plants for each
thermal reactor system.
When the
first fast reactor begins operation the actual ratio will be much larger
than this equilibrium value. As
more fast reactors are started up the actual ratio will decrease with time,
toward this equilibrium. New
fast reactors will be fuelled from processed PWR, BWR and PHWR materials
along with excess plutonium recovered from operating metal-fuelled fast
reactors via electrochemical (pyrometallurgical) processing19. Success
of the PHWR (Th) system depends entirely on the presumed capability for
reprocessing discharged Th-U233 fuel and utilizing the bred uranium- A serious restriction on the growth rate of the integrated nuclear generating system arises from the shortest-achievable value of compound doubling time of FBR reactors – which is about ten years. This figure sets an upper limit (about 5 percent per year) on the rate of increase of fast-reactor-powered nuclear stations, even if all the fuel produced is recycled into new units. Of course, if other recycled fuel is available from thermal reactors, this rate can be increased so long as such recycled materials are available. Y.I. Chang11 gives an excellent summary of these fuel-supply limitations. Clearly, the high cost of fuel would soon limit any system using only thermal reactors because they can utilize only about one percent of the potential energy in mined uranium. The
predominant area of need for new industrial capacity relates to fuel recycle.
The technology of the Integral Fast Reactor (IFR) is well established19,
and a viable commercial plant design is in hand20.
Pyroprocessing is known to work at the bench scale but still must be
demonstrated on a larger scale before qualifying fully as a commercial
process. Recycling
of used fuel from thermal reactors first requires extraction of uranium 238;
separation of some neutron-absorbing rare earth elements via an
oxidation-reduction process known as OREOX21 could be used to
improve the recycled product. The product then consists of transuranic
elements and some fission products. This
mixture is excellent as a fuel for fast reactors. In a
combined fuel cycle system such as this, the last step of the cycle will be
located in metal-fuelled fast reactors with integral reprocessing facilities.
During this final step (which will include a few recycles within each
plant’s reprocessing facility), essentially 100% of the transuranic
elements will undergo fission. The
processing facility output will, as a result, consist almost totally of
fission products – an important feature of this fuel cycle, because it
reduces the necessary time of waste isolation to five hundred years or less.
This eliminates the need for a special long-term waste repository –
final waste disposal probably can be located directly under the energy park,
in a deep borehole. Within a
static or slowly growing fleet of power plants, provided that more fast
reactor units are operating than the equilibrium number indicated above, the
configuration of some reactors can be adjusted to reduce the amount of
plutonium produced. However, in
a growing fleet with fewer than the equilibrium number of fast reactors
operating, the total inventory of fissile material will decrease steadily
unless more is added from an external source such as reprocessed LWR fuel or
newly mined uranium. The total
quantity of the first is known, and limited.
The quantity of uranium available is flexible and depends on the
price that buyers are willing to pay. This
‘demand price’ can be extremely high in an equilibrium system of thermal
and fast reactors because of the enormous amount of energy that can be
extracted from each unit of uranium8. Clearly,
in a system including a supra-equilibrium number of fast reactors every
fissile atom has a high value because it represents an opening toward
extraction of 100% of the potential energy in mined uranium.
Mining, even in very low-grade deposits, still benefits from a strong
economic incentive. Uranium
enrichment may be required in times of rapid energy demand growth; even in
that situation uranium tails may still have positive economic value because
of their eventual application as blanket materials in fast reactors. In
summary, the major new components yet to be established are:
Means other than isotopic separation may be feasible for sustaining the fissile isotope inventory in times of rapid electricity demand growth. Accelerator-driven spallation is one such possibility5; a fusion-fission hybrid concept also has been proposed22. POWER
PLANT SITES AND CHARACTERISTICS The
large scale of nuclear production facilities that may be required might
influence our consideration of options.
To get an impression of the scale involved, the total output of about
630 one-gigawatt-electric (GWe) nuclear units would be required to replace
the daily average energy released by burning gasoline in Today’s
worldwide fleet of nuclear plants comprises about 430 units that in total
generate less than 400 Gwe. These
plants are accommodated on more or less conventional sites.
However, if plants with a projected total capacity of 5,000-10,000
Gwe are to be installed over the next decades the choice of plant sites will
become a substantial problem. Very
large sites (up to ~50 Gwe each) will be preferred.
These sites would be large enough to sustain a broad array of
technical expertise as well as fuel cycle support and security facilities.
Comprehensive security systems would be a necessary and affordable
feature. Recycling, waste
management and disposal systems would be included.
Secondary industries such as hydrogen production and synthesis of
liquid transportation fuels could be established on the same site.
Distribution of energy from such sites will require a large infrastructure
– not unlike that surrounding large oil and gas production centers such as
those in the SITE
FACILITIES An
energy center should be built step by step, according to a broad but
adaptable overall plan. The
Bruce site on The next
step of site development could be addition of more generation capacity; if
this step is taken in the near future a good choice will be CANDU reactor
units, either of the type now operating or the new ACR type.
Later on, integral fast reactors might be added as a first move
toward a system with a closed fuel cycle.
These reactors could utilize the used CANDU fuel now stored on site,
given the addition of a processing plant.
(There is already sufficient used fuel on site to power an integrated
generation complex of ~15 Gwe for several hundred years.)
A U238 extraction plant could upgrade this fuel and supply the first
charge to each fast reactor as well as recycling mixed-oxide fuel to onsite
CANDU units. Depending on the
rate of capacity buildup it may be necessary to supply a limited amount of
enriched uranium or separated plutonium to the site from external sources. Depending
on circumstances in the external market, management of a mature site such as
this might choose to install a number of fast reactors above its equilibrium
level, and sell plutonium-bearing fuel to other similar sites still under
development under strict international control. The core and radial blanket
configuration of each fast reactor can be adjusted to regulate amount of
excess plutonium produced on the site. FUEL
SUPPLIES Fuel
requirements are very small at an integrated fast-thermal reactor site.
Basically, the amount of fresh uranium needed to sustain a
metal-fuelled fast reactor using integral pyroprocessing, located on an
ocean site, will be less than the amount of uranium dissolved in the
seawater required to cool its turbine condenser (seawater has a dissolved
uranium concentration of 3 parts per billion.)
In other words, only about 1/100th of the amount of fresh uranium now
required per operating megawatt of capacity will be sufficient to sustain
generation. The Bruce Energy
Centre is a fresh water site; this illustration simply indicates the very
small quantity of uranium needed to sustain such an integrated system. WORLD
NUCLEAR SYSTEM BY 2100 It is
possible to imagine a world energy supply system operating in about 100
years. That system could consist
of 10,000 Gwe of generation and associated peripheral systems, located on
100-200 sites worldwide. Some of
these sites might be dedicated to production of synthetic petroleum liquid
and gas as well as a wide range of other industrial processes.
At the low-temperature end of production cascades one might find
food-related installations such as fertilizer production and fish farming.
This network of large energy parks might be interspersed with smaller,
independent installations using sealed “nuclear battery” power systems10.
Reference 10a outlines an extension of this concept. There is enough uranium available for human use so that this large-scale world energy supply can be sustained for at least several thousand years8. CONCLUSIONSThis “Blue Sky’ concept paper shows that a sustainable nuclear system can be built up, step by step, from components and systems already proven and available today, and augmented by simple extensions of proven concepts – all well within the realm of known technology. Further work is required to guide the selection of reactor types and fuel cycle facilities during development of energy parks. Several prototype facilities must be established before commercial viability can be proven. It is important to emphasize, once again, that this paper presents only one possible pathway to sustainability of nuclear energy on a large scale. There may well be others. The central components of this pathway are thermal reactors with high conversion ratio and breeder reactors with high breeding ratio. This combination recognizes that thermal reactor installations will dominate for several decades, following which a shortage of fissile isotopes will require "manufacture" of more fissile material by one means or other. The proposed concentration of energy production and recycle components on a few large sites worldwide is intended to simplify nuclear safeguards as well as to reduce the magnitude of fuel trans-shipment. Reprocessing
of both breeder fuel and thermal reactor fuel also are essential. One major
component not considered here is waste management. It is obvious, however,
that elimination of most actinide elements in such a system will simplify
that process. References
|
||||||||||||||||||||||||
