7/16/2019 Knief Chapter 1 Final http://slidepdf.com/reader/full/knief-chapter-1-final 1/32 NUCLEAR ENGINEERING Theory and Technology of Commercial Nuclear Power RONALD ALLEN KNIEF Mechanicsburg, Pennsylvania American Nuclear Society, Inc. 555 North Kensington Avenue La Grange Park, Illinois 60526 USA
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After studying this chapter, the reader should be able to:1. Explain the two advantages and the two disadvantages of fission as an energy
source.
2. Arrange in sequence and describe the intent of each process step of the com-
mercial nuclear fuel cycle.
3. Explain the concept of and physical basis for recycling of nuclear fuel. Dis-
tinguish between open and closed fuel cycles.
4. Describe the role of each of the following support activities in the nuclear fuel
cycle: transportation, nuclear safety, and nuclear material safeguards.
5. Explain the following terms as they apply to classification of nuclear reactorsystems: coolant, number of steam-cycle loops, moderator, neutron energy,
and fuel production. State the full name and classify in these terms each of
the six reference reactor types: BWR, PWR, CANDU-PHWR, PTGR, HTGR,
and LMFBR.
6. Identify the four major elements of reactor multiple-barrier containment for
fission products. Describe the fuel assembly employed by each of the reference
reactor types and explain how it provides the first two of the barriers.
7. Perform basic calculations related to fuel-cycle material mass balance and
energy equivalence.
The current basis for commercial application of nuclear energy is the fission process.
Figure 1-1 shows a neutron striking an atom of uranium-235 [235U] to produce a
-----cJ-----? H -pNEUTRON 23S U ATOM FISSION FISSION FRAGMENTS
FIGURE 1-1
Fission of uranium-235 by a neutron.
fission, or splitting of that atom. From the standpoint of energy production, the reaction
has the major advantage that each such splitting provides nearly one hundred million
[100,000,000] times as much energy as the "burning" of one carbon atom in a fossilfuel. The production of more neutrons from fission allows the process to participate
in a chain reaction for continuous energy production in a device called a reactor. A
material that can produce a self-sustaining chain reaction by itself is said to be fissile.
Other fissionable and fertile materials can contribute to a chain reaction without being
able to sustain one by themselves. When the reaction is exactly balanced in a steady-
state condition, the system is said to be critical.
One major disadvantage of using the process as an energy source is the generation
of radiation at the time of fission. Another problem is the presence of the fission
fragments, which are radioactive and will themselves give off radiation for varying
periods of time after the fission events.These characterist ics each have major impacts on the design and operation of
nuclear fission systems. The six chapters in the second part of this book treat the basic
theories and principles that contribute to the ultimate utilization of fission energy. The
remaining parts then build on this framework to provide descriptions of the design
and operation of nuclear fission reactors, administrative aspects of nuclear energy, and
nuclear fuel cycle. The final part of the book considers nuclear fusion, which has
long-term prospects as a commercial.energy source.
Since theory and practice interact thoroughly, an overview of the current de-
velopment of commercial nuclear power can aid the understanding of the basic un-
derlying principles. Thus, the remainder of this chapter provides a brief overview ofthe nuclear fuel cycle and current reactor designs. The reader should note that only
basic understanding, and not thorough knowledge, is expected at this stage, since each
and every definition and concept is clarified and treated in greater detail in later
chapters.
NUCLEAR FUEL CYCLES
The production of energy from any of the current fuel materials is based on a fuel
cycle. Typical cycles, such as those for the fossil fuels, consist of at least the following
components:• exploration to identify the compositions and amounts of a resource available at
• mining or drilling to bring the resource to the earth's surface in a usable form
• processing or refining to convert raw materials into a final product
• consumption of the fuel for energy production
• disposal of wastes generated in all portions of the cycle• transportation of materials between the various steps of the cycle
The nuclear fuel cycle is substantially more complicated for the following
reasons:
l . 235U, which is the only practical naturally occurring fissile material, is less than I
percent abundant in uranium deposits (the remaining uranium is mostly non-fissile238U).
2. Two other fissile materials, 233U and 239pu [plutonium-239J, are produced by
neutron bombardment of 232Th [thorium-232] and 238U, respectively. (For this
reason, the latter two materials are said to be fertile.)3. All fuel cycle materials contain small to large amounts of radioactive constituents.
4. A neutron chain reaction [criticality] could occur outside a reactor under appropriate
conditions.
5. The same chain reaction that can be used for commercial power generation also
has potential application to a nuclear explosive device.
Each of these five concerns is considered in the following paragraphs and later in the
book as related to the structure of the nuclear fuel cycle.
Uranium Fuel Cycle
A schematic representation of a generic nuclear fuel cycle is shown in Fig. 1-2. Theuranium fuel cycle described here is used by the light-water reactor [LWR] systems
that dominate worldwide nuclear power. Variations, including the introduction of
thorium, are considered in the next section.
Transportation between the various steps of the fuel cycle is indicated by the
arrows in Fig. 1-2. Waste is necessary in all steps of the cycle, but is shown
explicitly only for the two major contributors-spent fuel and high-level reprocessing
wastes.
Nuclear safety, which is charged with protecting operating personnel and the
public from potentially hazardous materials in the fuel cycle, must be superimposed
on appropriate portions of the cycle. Also superimposed are material safeguards toprevent use of fuel cycle materials for nuclear explosives.
The steps preceding reactor use, which generally have little radioactivity, are
often considered to form the front end of the fuel cycle. Those steps that follow reactor
use are characterized by high radiation levels and constitute the back end of the cycle.
Exploration
The exploration process typically begins with geologic evaluation to identify potential
uranium deposits. Areas that have characteristics similar to those of known content
usually receive first consideration. The actual presence of uranium may be verified
by chemical and/or radiological testing.
Drilling into the deposit accompanied by detailed analysis ofthe samples providesinformation on uranium ore composition and location. Only after completion of a very
detailed mapping of the ore body will mining operations begin.
are consumed. An amount of 239pU is produced as 238U absorbs some of the extra
neutrons. The buildup of fission fragments and their radioactive products tends to
produce a "poisoning" effect by absorbing neutrons that could otherwise participate
in the chain reaction. Because the loss of 235U and the poison effect dominate overPu production, the fuel must eventually be replaced as it becomes unable to sustain a
chain reaction.
Traditional practice has been to replace one-quarter to one-third of the fuel
assemblies in the reactor core on a roughly annual cycle. More recently, some reactors
have begun to use 18- to 24-month cycles. By using careful fuel management, fuel
assemblies are shuffled to maximize the energy extracted from each during its 3-4
years in the reactor.
Interim Spent Fuel StorageSince the fuel assemblies are very highly radioactive when they are discharged from
the reactor, they are allowed to "cool" for a period of time in a water basin. Spent
fuel may be stored at the reactor site or in a special off-site facility for an indefinite
period of time (as is currently the situation in the United States). It may also be shipped
to a reprocessing facility, usually after at least 90 days of storage.
ReprocessingIn spent fuel processing [reprocessing], the residual uranium and the plutonium are
extracted for further use in the fuel cycle. The fission-product and other wastes produced
are handled in the waste disposal step.
In the initial steps of the reprocessing operations, the fuel assemblies are me-
chanically disassembled (i.e., chopped into small pieces) and dissolved in acid. Theuranium and plutonium are separated from the wastes, then separated from each other.
The large amounts of highly radioactive byproducts contained in the spent fuel ne-
cessitate very stringent environmental controls for the processing steps and the storage
of wastes.
RecycleThe residual uranium and the plutonium extracted from the spent fuel by the repro-
cessing operation may be reintroduced into the fuel cycle. Use of these recycled
materials can reduce uranium resource requirements by up to 25 percent.
The residual uranium is returned to the fuel cycle for reenrichment, as shown
in Fig. 1-2. The plutonium is transported to the fabrication operation where it is mixed
with natural or depleted uranium to produce a mixed oxide [PU02 + U02] with a
fissile content [effective enrichment] roughly comparable to that of slightly enriched
uranium. France, for example, is routinely recycling plutonium. Japan and countries
in western Europe also have plans to do so.
Waste DisposalAll steps of the fuel cycle (including the waste disposal step itself) produce some
amounts of radioactive waste. Near-surface burial of the "low-level" wastes from the
front end of the fuel cycle is generally appropriate.
Spent fuel assemblies are a waste form at the time of their discharge from areactor. I f reprocessing is implemented, the assemblies' waste contents are converted
to "high-level" liquid wastes. These liquids are stored for an interim period (nominally
above five years) and then solidified, usually in a vitrified, glass-like form. Final
disposal of spent fuel or solid high-level waste is most likely to be in a stable geologic
formation.
TransportationSince the various fuel cycle operations take place at a number of different locations,
transportation is a very important component. Effective transportation systems are
designed and operated to minimize the risks of:
• release of dangerous chemical or radioactive materials to the environment
• accidental nuclear chain reaction outside of a reactor core
• damage to expensive components
• theft of valuable and potentially dangerous materials
Based on the nature of these risks, specially designed containers and/or vehicles maybe used between various steps of the fuel cycle.
Nuclear SafetyNuclear safety in fuel cycle facilities is usually divided into categories of radiation
safety and nuclear criticality safety. The former includes shielding and containment
of radiation sources plus effluent control to minimize exposures to operating personnel
and the general public.
Reactors are designed to handle the effects of a fission chain reaction while fuel
cycle facilities generally are not. Nuclear criticality safety is charged with prevention
of such chain reactions in all environments outside of reactor cores. Because accidentalcriticality is not credible for natural uranium, these safety concerns begin at the en-
richment step (Fig. 1-2).
Material SafeguardsAll fissile materials have potential use for nuclear explosives and must, therefore, be
safeguarded against theft or diversion. Physical-security and material-accountancy
systems are designed to minimize the terrorist threat for theft by a subnational group.
International safeguards based on inventory verification have been developed to deter
proliferation, i.e., diversion by a nation for the purpose of acquiring nuclear weapons
capability.Safeguard measures should be commensurate with the risks perceived for given
materials. The slightly enriched uranium in the LWR fuel cycle, for example, could
only be used for a nuclear explosive if it were enriched further. The extreme complexity
of the enrichment technology makes implementation of the required clandestine op-
erations highly unlikely.
Because spent fuel contains fissile plutonium that can be separated chemically,
it is a somewhat more attractive target. Only a national effort, however, would be
likely to handle the complexity and hazard (as well as detectability) of reprocessing
operations.
By contrast, recycle with the presence of separated plutonium would offer thebest theft target for the terrorist or other subnational groups. Material safeguard mea-
sures, therefore, should be most stringent for this portion of the fuel cycle.
Other reactor concepts (e.g., as described in the next section) employ fuel cycles that
have many similarit ies to the LWR cycle just considered. The generic fuel cycle in
Fig. 1-2 encompasses the options.
The greatest differences occur for systems that use thorium. Because the main
constituent is 232Th, from which fissile mU is produced, the conversion and enrichment
steps are not required for thorium. The reprocessing step, of course, must be capable
of separating mU from 232Th and the wastes. As described later in this chapter, there
is a unique fuel assembly design for the high-temperature gas-cooled reactor [HTGR],
which allows 233U and 235U to be separated mechanically and recycled directly without
requiring enrichment technology.
Uranium enrichment requirements vary from use of unenriched, natural uranium
in the pressurized-heavy-water reactor [PHWR] to from 20 to 93 wt % 235U in the
HTGR. The liquid-metal fast-breeder reactor [LMFBR] uses depleted uranium-i.e. ,
enrichment tails-plus plutonium as its fuel material.
"Symbiotic" or "cross-progeny" fuel cycles are based on interchange of various
fuel materials among two or more different reactor types. One such possibil ity is the
exchange of plutonium between LWR and LMFBR systems.
Material safeguards are required for all separated plutonium and for separated
uranium whenever it has greater than 20 wt % enrichment in 233U and/or 235U. The
spent fuel usually may be protected at a somewhat lower level because reprocess ing
would be required to obtain the fissi le content.
NUCLEAR POWER REACTORS
All nuclear reactors are designed and operated to achieve a self-sustained neutron chain
reaction in some combination of fissile, fissionable, and other materials. The power
reactors use the fission process for the primary purpose of producing usable energy
in the form of electricity.
Common characteristics of power reactors, which are used for classif ica tion
purposes, include:
I. Coolant-primary heat extraction medium, including secondary fluids (i f any)
2. Steamcycle-the total number of separate coolant "loops," including secondary
heat transfer systems (if any)
3. Moderator-material (i f any) used specifically to "slow down" the neutrons pro-
duced by fission
4. Neutron energy-general energy range for the neutrons that produce most of the
fissions
5. Fuel production-system is referred to as a breeder if it produces more fuel (e.g.,
fissile 239pU from fertile 238U) than it consumes; it is a converter otherwise
The first two features relate to the current practice of converting fission energy into
electrical energy by employing a steam cycle.
Neutrons from fission are emitted at high energies. However, neutrons at verylow energies have a higher likelihood of producing additional fissions. Thus, many
systems employ a moderator material to "slow down" the fission neutrons. Neutrons
with very low energies (roughly in equilibrium with the thermal motion of surrounding
materials) are called thermal neutrons, with the slowing down process sometimes
called thermalization. Neutrons at or near fission energies are fast neutrons.
Any reactor that contains fertile materials will produce some amount of newfuel. The major distinction between breeder and converter reactors is that the former
is designed to produce more fuel than is used to sustain the fission chain reaction. By
contrast, the converter replaces only a fraction of its fissile content.
Six reference nuclear power reactor designs are currently employed in the world.
Important examples of these are identified in Table I- I and classified on the basis of
the reactor characteristics noted above. (A substantially expanded version of the table
is contained in App. IV.) The remainder of this chapter considers each of the reference
designs in some detail.
Steam Cycles
Most of the world's electric power is generated via a steam cycle. Water in a boiler
is heated to produce steam by burning fossil fuel. The steam then turns a turbine-
generator set to produce electricity. Condenser cooling water is used to condense
steam in the turbine back to liquid water and, thereby, enhance net conversion effi-
ciency.
Nuclear steam cycles have many of the same features as the fossil-fuel case.
The major conceptual difference is that the fission-energy heat source is a fixed-
geometry fuel core located physically inside the boiler, or reactor pressure vessel. In
two of the reactor designs, steam is produced directly in the core; in the others, heat
istransferred from the core to generate steam
ina secondary or tertiary system.
In the single-loop, direct-cycle reactor systems, water coolant flows through the
fuel core and acquires an amount of energy sufficient to produce boiling, and thus
steam, within the reactor vessel. Both the boiling-water reactor [BWR] and the pres-
sure-tube graphite reactor [PTGR] use a steam cycle similar to that shown in Fig.
1-3.
TABLE 1-1Basic Features of Six Reference Reactor Types
Pressure- High-
Boiling- tube Pressurized- Pressurized- temperature Liquid-metalwater graphite water heavy-water gas-cooled fast-breeder
reactor reactor reactor reactor reactor reactor
Feature [BWR] [PTGR] [PWR] [pHWRj [HTGR] [LMFBR]
Steam cycle/
coolant(s)
Number of loops I I 2 2 2 3
Primary coolant Water Water Water Heavy water Helium Liquid sodium
Secondary coolant(s) Water Water Water Liquid sodium/
water
Moderator Water Graphite Water Heavy waterGraphiteNeutron energy Thermal Thermal Thermal Thermal Thermal Fast
Fuel production Converter Converter Converter Converter Converter Breeder
Direct, single-loop steam cycle. (Adapted courtesy of U.S. Department of Energy.)
The indirect-cycle reactors maintain high-pressure conditions to prevent boiling
in the vessel. Instead, the heat acquired from the core by the coolant is carried to a
heat exchanger. Three of the reactor concepts-pressurized-water reactor [PWR],
pressurized-heavy-water reactor [PHWR], and high-temperature gas-cooled reactor
[HTGR]-employ a two-loop steam cycle, as shown in Fig. 1-4. As the name suggests,
the steam generator is a heat exchanger that produces steam for the turbine-generator.
It may be noted that steam generators in these systems play the same role of heat
sources as does the fossil-fuel boiler or BWR vessel.
As the name implies, the pressurized-water reactor relies on high pressure to
maintain water in a liquid form within its primary loop. Despite the steam cycledifference between the PWR and BWR, they have many design similarities resulting
from the use of ordinary water as their coolant and moderator. The two are, therefore,
grouped together as the light-water reactors [LWR] (whose nuclear fuel cycle was
discussed earlier in this chapter).
The PHWR uses heavy water as coolant in a cycle that is otherwise similar to
that of the PWR. The HTGR employs helium gas as its primary coolant.
REACTOR
PRIMARY LOOP
CORE
PUMP
STEAM LINETURBINE-
GENERATOR
CONDENSER COOLING
WATER
STEAM
GENERATOR
FIGURE 1-4
Two-loop steam cycle. (Adapted courtesy of U.S. Department of Energy.)
Number of fuel 748 1661 193-241 380 493 364 driver, 233 blanket
assemblies in radia l a rray
t More detailed data and references are contained in A pp . IV.
*The HTGR fuel geometry i s di ffe rent from tha t of the other reactors, leading to some slightly awkward classifications.
§All of the cores approximate right circular cyl inders. Fue l assembl ies a re loaded and/or s tacked lengthwise paral le l to the axis of the cylinder.
holding those products that do escape from the fuel. The integrity of the reactor vessel
and primary-coolant loop form a third barrier. One or more containment structures
form the final line of defense against the release of radioactivity. These last two barriers
located outside of the reactor core (along with associated safety systems and personnel-oriented administrative practices) are considered in Chaps. 10-14 and 16.
Since uranium dioxide [U02 ] and uranium carbide [UC] are relatively dense
ceramic materials with good ability to retain fission products, they are favored first-
barrier compositions. Encapsulation of the particles in metal tubes or in other coatings
provides the second barrier. Both features are incorporated in the fuel-assembly designs
for all of the reference reactors.
Light-Water ReactorsThe fuel assemblies for the two types of light-water reactor [LWR] are very similar.
Slightly enriched uranium dioxide is fabricated into the form of short, cylindrical fuel
pellets. The pellets are then loaded into long zirconium-alloy cladding tubes to produce
fuel pins or fuel rods. A rectangular array of the pins forms the final fuel assembly,
or fuel bundle.
The fuel assembly for the boiling-water reactor [BWR] is shown in Fig. 1-6.
The individual fuel pins consist of the clad tube, the fuel pellet stack or "active" fuel
region, a retention spring, and welded end caps.
Upper and lower tie plates plus interim spacers secure the fuel pins into a square
array with eight pins on a side. The fuel channel encloses the fuel pin array, so that
coolant entering at the bottom of the assembly will remain within this boundary as it
flows upward between the fuel pins, boils, and removes the fission energy.
URANIUM
DIOXIDE
FUEL PELLET
__ / FASTENER
ASSEMBLY
I I\I I
UPPER ...
TIE PLATE ' .
h ..
.",-- JJ- -- FUEL CHANNEL
- LOWER TIE PLATE
UPPER
TIE PLATE
FUEL ROD
INTERIM
SPACER
BAIL HANDLE
FUEL BUNDLE
FINGER SPRING
(TYPICAL of 4)
FIGURE 1-6
Fuel assembly for a representative boiling-water reactor. (Adapted courtesy of General Electric
The fuel assembly for the pressurized-water reactor [PWR] is shown in Fig.
1-7. The fuel pins are similar to those for the BWR.
The PWR assembly, typically of 16 x 16 or 17 x 17 pins, is larger than that
of the BWR. The fuel-pin array, and its interspersed non-boiling coolant, is not enclosed
by a fuel channel. (Additional detail on LWR fuel is provided is Chaps. 9 and 10.)
Pressure-Tube Graphite Reactor
The fuel assembly shown in Fig. 1-8 is that of the Soviet pressure-tube graphite reactor
[PTGR] known as RBMK-1000. The fuel pins consist of slightly enriched uranium
in U02 pellets clad in zirconium alloy quite similar to those in the LWR designs.
The cylindrical fuel assemblies are designed to fit into pressure tubes. Water
coolant is introduced to a fuel assembly as liquid, boils in removing fission energy,
and is discharged as steam.
The tubes are distributed throughout a core built up of the graphite blocks that
serve as the reactor's moderator. Coolant feeder pipes, valves, and the pressure tubesare arranged to allow refueling during full-power operation. (Additional detail is pro-
vided in Chaps. 9 and II.)
ALIGNMENT POST
UPPER
END FITTING
SGPRAICDER ;aJ. j;W;;; CEA--.I : GUIDE TUBE
11''"''"'\ '.:.:.: I_ASSEMBLY
_ FUEL RODS
116x16 ARRAYI
- . - .. - . -o C:: 0 Ij J G g 'J C C. _ ....
TOP VIEW
BOTTOM VIEW
UPPER
END CAP
SPRING
SPACER
URANIUM
DIOXIDEPELLETS
ZIRCALOY
FUEL
CLADDING
SPACER
LOWER
END CAP
I
FIGURE 1-7
Fuel assembly for a representative pressurized-water reactor. (Adapted courtesy of Combustion
Fuel assembly for a pressure-tube graphite reactor. (From NUREG-1250, 1987.)
Pressurized-Heavy-Water Reactor
The fuel assembly shown in Fig. 1-9 is that of the pressurized-heavy-water reactor
[PHWR] known as CANDU-PHW [Canada Deuterium Uranium-Pressurized Heavy
Water]. The terms PHWR and CANDU are often used interchangeably.The cylindrical CANDU fuel bundles are designed for insertion into pressure
tubes through which the primary coolant flows. These tubes penetrate a large vessel,
which contains the separate heavy-water moderator. (The specific design is treated in
some detail in Chap. II.)
The fuel pins consist of natural uranium in U02 pellets clad in zirconium alloy.
Since these short pins do not have to be free-standing (as is the case for the LWR's),
the clad is quite thin. The interelement spacers serve to separate the pins from each
other, while the bearing pads separate the bundle from the pressure tube.
High-Temperature Gas-Cooled Reactor
The conceptual design for a high-temperature gas-cooled reactor [HTGR] describedin Table 1-2 is comparable in thermal output to most of the other reference reactors.
It is one representative of a variety of gas-cooled, graphite-moderated reactors around
Fuel assembly for a representative high-temperature gas-cooled reactor. (Adapted courtesy of CA
Technologies.)
Liquid-Metal Fast-Breeder Reactor
A representative fuel assembly for a liquid-metal fast-breeder reactor [LMFBR] is
shown in Fig. I-II. Although the fuel pins have the same basic features as those for
the water reactors, they have a much smaller diameter, are clad with thin stainless
steel, are more closely spaced, and may contain two different fuel pellet types.
The primary fissile composition for LMFBR fuel is plutonium. The fertile com-position is 238U in the form of depleted uranium, the byproduct of the enrichment
process. Some pellets contain a PuOz-UO2 combination called mixed oxide. Typical
compositions have 10-20 wt % Pu with depleted uranium accounting for the remainder.
Other pellets contain only depleted uranium.
Basic driver fuel pins contain a central stack of mixed-oxide pellets with stacks
of depleted-uranium blanket pellets on both ends. Blanket pins consist entirely of
MIXED·DXIDEFUE L PELLETS
STAINLESS STEELFUEL CLADDING
FLOW CHANNEL
BLANKET
FUEL
BEARI NG
REGION
BLANKET
HANDLI NG SOCKET
LOAD PAD
" ,__-ABOVE CORE
LOAD PAD
SHIELDI
ORIFICE
REGION
INLET
NOZZLE
FUEL ASSEMBLY
CRDSS·SECTIONFUEL ASSEMBLY
FIGURE 1-11
Fuel assembly for a representative liquid-metal fast-breeder reactor. (Adapted courtesy of U.S.
depleted-uranium pellets. In both cases, the blanket material is employed to enhance
breeding by absorbing neutrons that would otherwise escape from the system.
The LMFBR driver assembly shown in Fig. 1-11 has a fuel channel that encloses
the hexagonal fuel-pin array. Liquid sodium enters at the bottom of the assembly, isdistributed in the orifice region, and then flows through the active fuel region. The
wall prevents the mixing of flow from adjacent assemblies.
Reactor Cores
Fuel assemblies in the reactor vessel form the core wherein the fission process produces
heat energy. Core configurations for each reactor are summarized in Table 1-2. The
fuel is loaded to approximate a right circular cylinder with the fuel assemblies placed
parallel to the axis. Coolant flow is axial (i.e., parallel to the axis).
The axis is vertical for five of the reactors and horizontal for the other. For each
system a plane perpendicular to the axis defines a radial cross section of the core.(Examination of the fuel-management patterns in Chap. 9 and the core drawings in
Chaps. 9-12 may aid visualization of the situation.)
In the LWR's, the fuel bundles are held in a vertical position in the reactor
vessel. A representative BWR has 748 assemblies, while the PWR has between 193
and 241 larger assemblies.
The CANDU has 12 bundles end-to-end in each of the horizontal pressure tubes.
An array of 380 such tubes makes up the reactor core.
The PTGR has two fuel assemblies stacked one atop the other in a "stringer."
The large PTGR has 1661 such stringers emplaced into pressure tubes in the matrix
of graphite moderator blocks.The HTGR fuel assemblies are stacked vertically. A typical core has 8 fuel
blocks along the axis and 493 horizontally.
The LMFBR core contains two basic fuel assembly types. Assemblies of driver
fuel pins constitute most of the central region of the core. The depleted uranium at
their top and bottom forms an axial blanket. The assemblies made from blanket fuel
pins are loaded around the outside of the central region to form the radial blanket.
Through the combination of the axial blankets with the radial blanket, the cylinder of
the central mixed-oxide core is essentially surrounded by a larger cylinder with depleted
uranium for 239pU breeding.
One current LMFBR design calls for 364 driver assemblies and 233 externalblanket assemblies. Although most of the blanket assemblies are placed around the
core periphery, some are interspersed among the driver fuel in the central core region.
EXERCISES
Questions
1-1. Explain the two advantages and the two disadvantages of fission as an energy
source.
1-2. Sketch the sequence of the process step of an open commercial nuclear fuelcycle. Describe the purpose and product(s) of each step.
1-3. Explain the concept of and physical basis for recycling of nuclear fuel in a
closed fuel cycle. Expand the sketch in the previous exercise to include the
added steps.
1-4. Describe the roles for transportation, nuclear safety, and nuclear material safe-
guards in the nuclear fuel cycle.1-5. Identify the reference reactor system(s) characterized by fuel cycles with:
a. slightly enriched uranium
b. 20-93 wt % 235U
c. required plutonium recycle
d. no enrichment facilities
e. thorium use
1-6. Identify each of the six reference reactors by full name and acronym. Complete
a data table that includes for each of the reactors the following:
a. number of loops
b. coolantc. moderator
d. neutron energy
e. fuel production
1-7. Explain the concept of multiple barrier fission-product containment and identify
the four basic components. Describe how the fuel assembly in each of the six
reference reactors is designed to provide the first two of the barriers.
Numerical Problemst
1-8. Using data from App. IV, make to-scale sketches of the outside cross section
for the fuel assembly from each of the six reference reactors. In a corner ofeach drawing include a circle the size of the outside diameter of a single fuel
pin for that reactor.
1-9. Calculate the masses of 235U and 238U per metric ton (t)f of uranium ore,
assuming the total uranium is 0.25 wt % of the ore.
1- 10. Coal has a heat content of 19-28 GJ/t of mined material. Uranium as employed
in an LWR has a heat content of 460 GJ/kg of natural uranium metal.
a. Calculate the heat content of a ton of low-assay uranium ore (see the previous
problem) and its ratio to that of the extreme coal values.
b. Considering that electrical conversion efficiencies [MW(e)/MW(th)] are about
32 percent and 38 percent for an LWR and a coal plant, respectively, calculateelectric energy ratios as in (a).
1-11. Repeat the previous exercise for a "giant" Canadian ore deposit with 12 wt %235U.
1-12. The 235U and 238U masses in natural uranium are split between enriched and
depleted streams as a result of an enrichment process. If the input masses andoutput enrichments are specified, mass conservation determines the maximum
(i.e., zero-loss) quantity of each isotope in the output streams. Considering a
1 kg input of natural uranium, a 3 wt % 235U enriched stream, and a 0.3 wt %
235U depleted stream:
t Units and conversion factors are contained in App. II.
:j:Metric ton is also sometimes abbreviated as teo
EPRI Journal-published monthly by the Electric Power Research Institute; feature articles on the
general status of nuclear power and alternative energy sources and on specific research projects;
brief abstracts from recently-issued EPRI reports (see "Reports" section below)
IAEA Bulletin-published by the International Atomic Energy Agency in Vienna, Austria; coversworldwide developments in nuclear energy at a level suitable for a general technical audience;
includes special attention to IAEA's primary mission, namely international safeguards and non-
proliferation and also addresses reactor safety and operating practices; announces new IAEA reports
(see "Reports" section below).
IEEE Spectrum-published monthly by the Institute of Electrical and Electronics Engineers [IEEE];
excellent feature articles on specific aspects of nuclear and other energy sources (usually several per
year) written for a general technical audience.
New York Times-Tuesday's "Science Section" frequently addresses current issues in nuclear and
other energy-related fields in well-illustrated articles aimed at a general audience; focus is generally
sociopolitical.
Nuclear Engineering International [Nucl. Eng. Int.]-published monthly in the United Kingdom;
excellent coverage of worldwide developments in the nuclear fuel cycle and reactor technology;
some issues devoted to a single reactor (often including a colored wall chart of the system), a reactor
concept , a national program, or a fuel cycle step (specific topics of interest are included in the
Selected Bibliographies for applicable chapters); "World Nuclear Industry Handbook" published
annually with the November issue includes reactor and fuel-cycle statist ics, station achievement
data, a reactor directory, and a buyers' guide to industry products and services.
Nuclear Industry [Nucl. Ind.]-published monthly by the Committee for Energy Awareness [CEA],
a U.S. organization representing electric utilities and other organizations involved in commercial
nuclear power; summary coverage of current issues and status, especially concerned with federal
regulatory policy and practice.
Nuclear News-published monthly by the American Nuclear Society [ANS], the major professional
organization for nuclear engineers; summary coverage of current issues and status, plus feature
articles on topics of general interest; updated "World List of Nuclear Power Reactors" appears in
each February and August issue.
Nuclear Safety-published bimonthly by the U.S. Nuclear Regulatory Commission; feature articles
on various aspects of reactor and fuel cycle safety and safeguards in the United States and worldwide.
Physics Today-published monthly by the American Institute of Physics [AlP], the major profes-
sional organization for physicists; summary coverage of current issues, plus occasional feature articles
on nuclear energy and other energy sources.
Power-published monthly as "the magazine of power generation and plant energy systems";
frequent overview articles on various aspects of nuclear and other power sources written for a general
engineering audience.
Power Engineering-published monthly as "the engineering magazine of power generation"; sum-
mary coverage of current issues in "Nuclear Power Engineering" section plus frequent overview
articles on nuclear and other power sources written for a general engineering audience (often includes
good color illustrations and pictures).
Science-published weekly by the American Association for the Advancement of Science [AAAS]
for the general scientific and engineering community; coverage of world issues and policies in science
and technology including nuclear power, energy sources, and related cost-risk-benefit evaluation.
Scientific American-published monthly; excellent feature articles on specific aspects of nuclear
energy and other energy sources (usually several per year) written for a general technical audience.
Technology Review-published monthly by the Massachusetts Institute of Technology; good review
articles on nuclear and other energy technologies with some emphasis on sociopolitical interactions.