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Page 767
Appendix G
Nuclear Energy
This appendix discusses the variety of options available for
generating energy using either nuclear fission or nuclear fusion.
The focus of this appendix is primarily on second-generation
nuclear fission reactors, as they are more likely than fusion power
plants to be introduced in the near term. Descriptions of some of
the proposed new technologies are provided below. These
descriptions are not intended to be a comprehensive and critical
analysis of the technological options for future development of
nuclear power nor an endorsement of particular technologies. Such
an analysis will be provided in a forthcoming National Research
Council Energy Engineering Board report.
Current advanced nuclear fission reactor development projects
worldwide are summarized in Table G.1. Any reactor development
project aims to improve performance in the categories of both
safety and economics; however, their individual improvement differs
substantially—reflecting differing evaluations of which types
of improvement are needed most. Reactor development projects can be
categorized according to whether they give primary emphasis to
economics or safety. The class of goals chosen for emphasis usually
guides formulation of the basic reactor concept.
The optimized reactor design is usually very similar to current
nuclear reactors, but reflects an approximate attempt to maximize
either economics or safety, while attempting to improve performance
in the other category of goals to at least the minimal extent
required by the safety authorities or the economics of competing
technologies.
Reactors emphasizing economic performance attempt to reduce the
costs of generating electricity—usually focusing on the cost
components of capital and plant operational availability. Reactors
emphasizing improved safety performance attempt to gain public
acceptance by decreasing the risk of core damage or radioactive
release. Reactor safety is improved by increasing
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Representative terms from entire chapter:
water reactor
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TABLE G.1 Worldwide Programs of Nuclear Power Technology
Development as adapted from Golay (1990)
PROGRAMS EMPHASIZING PASSIVE SAFETY
Federal Republic of Germany
• 100-MWe
Modular HTGR (Siemens, Brown Boveri)
United Kingdom and United States
• 300-MWe
Modular PWR (SIR Concept) (Rolls Royce & ABB-Combustion
Engineering)
United States
• 130-MWe
Modular HTGR (General Atomic)
• 130-MWe
Modular LMR (PRISM Concept, General Electric)
• 600-MWe LWRs
(Semi-Passive Safety)
ASBWR (BWR, General Electric)
AP-600 (PWR, Westinghouse)
• 20-MWe
Integral Fast Reactor (IFR Concept, Argonne National
Laboratory)
Sweden
• 500-MWe
PIUS-PWR (ASEA-Brown Boveri)
PROGRAMS EMPHASIZING ECONOMIC PERFORMANCE
Europe
• Joint European Fast Reactor (France,
Germany, United Kingdom)
• European 1400-MWe PWR (Nuclear Power International:
France, Germany)
Canada
• 450-MWe HWR
(CANDU 3) (AECL)
• 900-MWe HWR
(AECL & Ontario Hydro)
France
• 1400-MWe PWR
(N4 Project, Framatome, Electricité de France)
• 1200- to 1450-MWe LMFBR (Superphenix-1 Project, Novatome,
Electricité de France)
Federal Republic of Germany
• 500-MWe HTGR
(Successor to 300-MWe THTR
Project)
• 300-MWe LMR
(SNR 300 LMFBR Project)
Japan
• 1250-MWe
LWRs
ABWR (Tokyo Electric Power, General Electric,
Toshiba, Hitachi)
APWR (Kansai Electric, Mitsubishi,
Westinghouse)
• 714-MWe LMR
(Monju LMFBR Project)
• Successor to 148-MWe FUGEN LWR/HWR Project
United Kingdom
• 1000 to 1400-MWe PWR (Sizewell-B, Hinkley Point-C
Projects)
United States
• LWR Requirements Document Project (Electric
Power Research Institute)
• 1250-MWe
ABWR (General Electric)
• 1250-MWe
APWR (Westinghouse)
• System 80+ (ABB-Combustion Engineering)
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Soviet Union
• Emphasis upon Passive Safety
100-MWe Modular
HTGR
Chernobyl-Type RBMK Reactor Series
Discontinued
• Emphasis upon Economic Performance
950-MWe PWR
1250-MWe LMR (LMFBR
Type)
NOTE: Abbreviations are as follows:
BWR:
Boiling Water Reactor
CANDU:
Canadian Deuterium Uranium, Heavy Water
Reactor
HTGR:
High-Temperature Gas-Cooled Reactor
HWR:
Heavy Water Reactor
IFR:
Integral Fast Reactor
LMFBR:
Liquid-Metal-Cooled Fast Breeder Reactor (version
of LMR)
LMR:
Liquid-Metal-Cooled Reactor
LWR:
Light Water Reactor
PIUS:
Process Inherent Ultimately Safe (version of
LWR)
PRISM:
Power Reactor Inherent Safe Modular (version of
LMR)
PWR:
Pressurized Water Reactor
SIR:
Safe Integral Reactor (version of LWR)
the mechanical reliability of the post-shutdown reactor cooling
and post-fuel damage maintenance of containment integrity
functions. However,corresponding reductions in nonmechanical safety
function unreliability,subtlecommon-mode failures, and severe
external events (e.g., strong but rare earthquakes) appear to be
more difficult to achieve. Thus, from the safety-oriented concepts
substantial performance improvements can be expected, but hopes
that perfect or idiot-proof technologies will result may be poorly
founded.
The term often used for reactor concepts emphasizing safety is
passively safe. In a nuclear power plant, passive safety features
are those that perform a safety function using only sources of
motive force found in nature (e.g., gravity for cooling water
instead of pumps). There are developments on concepts that use
passive safety features only. In other cases, particularly in light
water reactors, a blend of passive and active safety features is
being pursued. These concepts could be termed semi-passively
safe.
In this appendix, the state of the art for the following three
fission concepts are reviewed briefly:
• Light Water Reactors (LWR)
• High-Temperature Modular Gas-Cooled Reactors (HTGR)
• Integral Fast Reactors (IFR)
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For the LWR, both passively safe and semi-passively safe
developments are included. For HTGR and IFR, only the concepts
using passively safe features exclusively are described. As noted
earlier, this is not intended to be a comprehensive or critical
assessment of all the nuclear technologies available or an
endorsement of particular technologies, but a description of a few
options to demonstrate some of the nuclear technologies currently
being discussed. For example, not all varieties of light water
reactors are discussed, nor are heavy water reactors, which are
currently not licensed in the United States.
Light Water Reactors
In most countries the focus of nuclear fission development
activities remains on light water reactors—emphasizing
economic-oriented designs. Among the more important elements are
plant operational availability and shortening plant construction
time. Even for those developments, there is always a serious
concern for safety. In the advanced pressurized water reactor (PWR)
and boiling water reactor (BWR) designs, automatic mechanical and
electronic devices have supplemented human operators in the
performance of many emergency duties—making safety systems
more redundant.
Light water reactors are divided into two types: large
evolutionary light water reactors and mid-sized light water
reactors. Large evolutionary light water reactors include the
advanced boiling water reactor, advanced pressurized water reactor,
and the system 80+ standard design pressurized water reactor.
Mid-sized light water reactors with passive safety features include
the advanced passive pressurized water reactor and the simplified
boiling water reactor. The large evolutionary light water reactors
are the most likely to be implemented in the time frame in the
Mitigation Panel's analysis. The cost of these reactors is the
basis for the calculation described in Appendix J.
One way to reduce risk is to build smaller reactors that contain
less radioactive material and, in principle, can be cooled more
easily in case of malfunction. The Electric Power Research
Institute (EPRI) has encouraged two efforts to develop smaller
advanced (LWRs)—the ASBWR and AP-600. These reactors utilize
semi-passive safety systems. They are designed to utilize a small
number of reliable ''active" components to initiate safety
operations, but do not require large AC power sources to run the
emergency equipment. All water required for heat removal in the
primary system drains by gravity to the reactor core. Large volumes
of water are stored above the reactor core. Typically, there is
sufficient water to flood the reactor containment and reactor
system. By so doing, some of the economic penalties of purely
passive design may be avoided.
The LWR concept that utilizes exclusively passive safety
features is the process inherent ultimately safe (PIUS) reactor
conceived by ASEA-Atom
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in 1979. Their criteria included safety from external events
such as earthquakes and terrorist attack. The intrinsic protection
can last a week or more without calling into action any active
safety equipment or human actions. Currently, coordinated
activities to develop PIUS are under way between ABB and companies
in Italy, South Korea, China, and the United States.
High-Temperature Modular Gas-Cooled
Reactors
The high-temperature gas-cooled reactor (HTGR) technology
utilizing pebbles for fuel was developed first in West Germany. In
the United States, the Fort St. Vrain reactor, which used General
Atomic's HTGR technology, was less than successful. The current
U.S. reference plant design developed by the Department of Energy's
HTGR Program uses four steam generating reactor modules and two
steam-turbine-generator sets to achieve a nominal plant rating of
550 MWe.
The modular gas-cooled reactor (MGR) is predicated on the
possibility of building reactors that will not, under any
circumstances that can be perceived at this time, release fission
products to the environment. In a reactor with localized fuel, this
is equivalent to the requirement that the fuel cladding retain its
integrity even if all coolant (not just coolant flow) is lost and
all control rods are fully withdrawn. This requirement is claimed
to be met in the MGR by limiting the power density and reactor size
in such fashion that the hottest location in the reactor core never
reaches temperatures capable of damaging the fuel. To achieve this,
it is essential both that the reactor possess a temperature
coefficient of reactivity sufficiently negative for the chain
reaction to be quenched before damaging temperatures are reached,
and that the processes of conduction and radiation are sufficient
to carry the after heat of the fission products to the environment
without exceeding the design basis temperature. The latter
requirement imposes important limitations on power density and
size.
Although it is possible in principle to meet these requirements
with a number of reactor types, the gas-cooled reactor with
ceramic-coated fuel has the combination of cladding strength and
temperature capability that holds promise in technical feasibility.
The challenge of the MGR program is not the usual one of trying to
make an economical reactor safe, but rather its inverse—that
of making a safe reactor economical. The issues are low power
density with its obvious cost penalties and small unit size with
its apparent cost in economies of scale. In addition, requiring a
containment facility (not in current plans) would increase the
cost. There are two different approaches to ameliorating the
economic disadvantages of the gas-cooled reactor: the first is
based on making the unit size as large as possible and combining
the output of several steam-generating units to achieve economies
of scale.
Another design concept, developed at the Massachusetts Institute
of Technology
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(MIT), utilizes a high-speed gas turbine in a direct-Brayton
cycle and generates electricity at a high frequency—higher
than 60 hertz—utilizing power electronics for frequency
conversions. The aim is to reduce the size of the balance of plants
and thus improve the economics.
The macroscopic properties of gas-cooled reactors are closely
coupled to the small-scale properties of the fuel. As a result,
incremental improvements in fuel quality can have substantial
effects on existing reactors and, more significantly, on the design
of commercially improved versions. This requires the most stringent
quality control on the fuel manufacturing process. Recent
improvements in fuel quality (a two-order-of-magnitude improvement
in fission product retention and a method for coating individual
pebbles with silicon carbide to render them fireproof) promise more
evolutionary improvement.
All modern gas-cooled reactors are based on microencapsulated
fuel in which submillimeter spheres of uranium oxide are surrounded
by a series of nested spherical shells of pyrolytic carbon and
silicon carbide. This fuel was developed by General Atomic and was
brought to its current level of capability in the West German
nuclear program, where the concept of the passively safe
small-scale modular reactor was first developed. In the West German
program, intensive series of tests of the fuel under a variety of
temperature and radiation environments have been conducted, and
several loss of coolant events were tested, and monitored and
compared with computer models. Potentially important advances would
follow from the development of higher-temperature coatings, which
would facilitate the deployment of higher power density systems,
with concomitant economic advantage, or the development of
1000°C systems for process heat applications.
Integral Fast Reactors
A new fission reactor concept developed at Argonne National
Laboratory is the integral fast reactor (IFR). If the IFR, which
uses a new formulation of fissile fuel material, performs as
claimed, it will breed its own supply of fresh fuel on site and
burn up the ultra-long-lived radioactive transuranic fission
products (the actinides: elements 89 through 103, including
plutonium (94)). This would leave a waste product that could
potentially decay in radioactivity to original ore levels in
several hundred years. The IFR features a very high fuel burnup
and, in two subscale experiments, has demonstrated the ability to
survive a total loss of cooling.
The IFR is a sodium-cooled breeder reactor with fuel in the form
of a metallic alloy. Conventional reactors employ a ceramic oxide
fuel form; the new fuel alloy is a far better conductor of heat,
which is one of the factors enhancing the safety of the IFR design.
The metallic fuel is reprocessed electrochemically rather than by
solvent extraction. Reprocessing may be carried out on site,
avoiding long-distance transport.
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The utilization of IFR facilities to reduce the radioactivity of
used fuel from conventional reactors has been suggested, but this
use is still in the early exploratory stage and controversial.
Nevertheless, despite all its benefits, one may still expect the
IFR concept to display vulnerabilities of its own. For example,
disposal of the radioactive inventory in the event of an early
reactor shutdown would be no less difficult than for a conventional
reactor. Also, loss of sodium coolant, as in an earthquake, could
produce a major disaster. Further, the cost for a full-scale
reactor is yet to be evaluated and may be quite high.
The safety of the IFR concept was demonstrated in two very
interesting experiments, both carried out in the EBR-II Idaho
reactor on April 3, 1986. Although small, with just 20 MW of
electric generating capacity, the power density of the EBR-II is
nevertheless typical of fast reactors. In the first experiment,
while the EBR-II was operating at full capacity, power was shut off
to the primary coolant pumps. Because of the reactivity feedback
characteristics of the IFR, the reactor shut itself down without
safety system or operator action. There was no damage to any part
of the system. Later that day, the reactor was brought back to full
power and a loss-of-heat sink without scram test was carried out.
Again, no damage was observed.
In summary, the IFR concept may offer energy and power from
nuclear fission while avoiding almost all of the major hazards
associated with conventional fission reactors. The reactor breeds
its own fresh fuel on site, features high fuel burnup, and burns up
the ultra-long-lived radioactive transuranic fission products,
leaving a waste product with lower radioactivity. Shipment of large
amounts of fissile fuel to and from a reprocessing plant is
obviated, as is the need to ship and store, for a geologic time
period, a highly pernicious radioactive waste. The on-site fuel is
unattractive for diversion to weapons use, and safe recovery in the
face of a complete cooling failure has been observed in pilot
experiments.
Nuclear Fusion
In sharp contrast to the existence of many operating fission
reactors, the feasibility of economic controlled nuclear fusion has
yet to be demonstrated. Research on this intensely difficult
problem has been under way since the very early 1950s, and several
orders of magnitude of progress has been made in temperature and
confinement time. Temperatures of 300,000,000°C have now been
achieved in fusion experiments, six times hotter than that
necessary for ignition under ideal circumstances. At lower
temperatures, confinement of such very hot gases has been extended
to periods as long as a second. Densities of these hot gases are
also close to those needed for fusion reactor operation.
Fusion experiments may achieve ignition of a deuterium-tritium
plasma by the end of this decade and thus provide a definitive
demonstration of
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fusion reactor feasibility. The first third of the twenty-first
century may then see a prototype fusion power plant in operation,
depending on the energy cost and environmental situation at that
time. The cost of nuclear fusion is expected to be very high in
comparison with alternative nuclear reactor designs. On the other
hand, nuclear fusion offers additional environmental protection
compared to nuclear fission. By midcentury, some fraction of energy
to the national electrical grids might possibly come from fusion
reactors.
The advantages of fusion power with respect to safety and the
environment follow:
• No danger from nuclear reactor runaway. The amount of
nuclear fuel in the reaction chamber at any given time is
minuscule, and a system failure of any sort can lead only to a
cooling down of the reacting plasma.
• Enormously reduced amounts of nuclear waste. The nuclear
ash from fusion is helium, a stable and totally benign gas. Almost
all of the neutrons coming out of the reacting gas will be absorbed
in a lithium 6 blanket, generating fresh tritium to replace that
used up in the deuterium tritium reactions. Also, although the
vacuum-chamber wall is expected to become radioactive due to
bombardment by these transiting neutrons, the material of the
vacuum chamber can be chosen to reduce the radioactivity level and
character of this radioactivity and problems associated with
storage or disposal. Further research is needed in this area.
• No production of gases deleterious to the environment
such as oxides of carbon and nitrogen; however, there is some
concern on the potential leakage of tritium into the water
supply.
• No inherent production of fissile materials.
The acid test of fusion power feasibility—achieving nuclear
ignition in a confined plasma—is anticipated no earlier than
the end of this decade, and a prototype fusion power plant should
not be expected before the year 2020. Further progress is clearly
needed in the science, technological development, and economics of
nuclear fusion before it can actually be implemented. Nevertheless,
in view of its minimal impact on atmospheric pollution and
greenhouse warming, and the very much reduced level of nuclear
hazard, controlled fusion still merits its reputation as a major
option for the future generation of electric power.
Reference
Golay, M. W. 1990. Testimony before the U.S. House Committee on
Interior and Insular Affairs. Washington, D.C., March 10, 1990.
