Note: Descriptions are shown in the official language in which they were submitted.
WO 93/16477 212 8 51~1 PCI/USg3/01037
NONPROLIFERATIVE LIGHT WATER NUCLEAR REACTORWITH ECONOMIC USE OF THORIUM
BACKGROUND OF THE INVENTION
Although thorium is known to be at least three times as
plentiful as uranium in the earth's core, no economic method
of producing nuclear power from thorium, with or without
proliferative fuels, has been found. The term "economic" is
used herein to mean that most of the nuclear reactor energy
is generated from thorium without the very expensive process
of extracting the highly gamma-active U-233 and fabricating
it into fuel elements.
The fundamental difficulty in utilizing thorium as a
nuclear fuel is that it contains no natural fissionable
material. Thorium can be made to produce energy only by (1)
an initial addition of fissionable material, as is described
in the report entitled "Thorium Utilization in PWRS", .
Kernforschungsanlage Julich GmbH (1988), or (2) providing a
neutron current into the thorium regions of the core, using
a "seed-blanket" arrangement, as described in the "CRC
Handbook of Nuclear Reactor CalculationsN, 1986, Volume III,
pp. 365-448.
These two~known approaches are summarized briefly
below: .
1. In a Brazilian-German collaboration extending from
1979 to 1988 and reported in " m orium Utilization in PWRS",
supra, the entire reactor core was assumed to consist of
W093/16477 2 1 2 ~ J- ~: PCT/US93/01037
thorium with the uniform addition of fissile material. The
most favorable results in the study were for cases where the
thorium was initially enriched with plutonium, an element
which is well known to be proliferative. According to
calculations, gains over a conventional uranium reactor were
obtainable only by repeatedly extracting the U-233 formed in
the thorium, fabricating it into fuel elements, and
reinserting it into fresh thorium. Another possibility that
was considered was to begin with U-235/U-238 in the volume
ratio of 20:80 as the initial fissile fuel for the thorium.
However, so much of this fuel would be required to provide a
sufficient reactivity for the accepted time between reloads,
twe}ve to eighteen months, that the amount of plutonium
formed in the U-238 into the thorium would be appreciable.
Again, extraction, fabrication, and reinsertion of the
highly gamma-active U-233 into the thorium would be
necessary.
2. In the second approach referred to above, seed-
blanket core arr~angements have been used as described in the
~CRC Handbook of Nuclear Reactor Calculationsn, supra. Such
cores consist of seed regions which have multiplication
(cr:iticality) factorsi greater than one and blanket regions
with multiplication factors less than one. In the
arrange~ents which have been studied the blanket regions
have been constructed primarily of natural thorium and the
seedi have contained either U-235 or U-233 of weapons grade
W093~16477 ' 2 1 2 8 ~ 1 ~ PCT/US93/01037
quality. In these studies the cores have been controlled
typically by upward motion of each seed region from a
position well below the core. This method of control has
resulted in severe mechanical problems because of the heavy
weight of the seeds to be moved. Furthermore, heat removal
i8 difficult because of great variations in the power levels
throughout the length and width of the core.
- SUMMARY OF THE INVENTION
It is a principal object of the present invention to
provide a nuclear reactor which is "non proliferative"; that
is, a nuclear reactor for which neither the initial fuel
loading nor the discharged, spent fuel can be used to make
nuclear weapons.
It is a further object of the present invention to
provide a nuclear reactor which makes economic use of
thorium as a fuel.
It is a further object of the present invention to
provide a nuclear reactor which has an extra margin of
safety over co m entional reactors.
It is a further object of the present invention to
provide a nuclear reactor which discharges substantially
lecs high level nuclear waste than conventional reactors.
These objects, as well as other objects which will
become apparent from the discussion that follows, are
achieved, in'`accordance with the present invention by
21 2 ~ 51`~ : 4 PCT/US93/01037
providing a nuclear reactor core having one or more seed
regions containing seed fuel elements essentially comprising
U-235 and U-238 in the maximum ratio which is
nonproliferative; a blanket region surrounding the seed
region(s) containing blanket fuel elements essentially
comprising Th-232 with a small percentage of
nonproliferative uranium; and a nonparasitic mechanically
simplified control system, all of which are described in
detail below.
1. Seed Reaions: These regions contain fuel elements
of U-235/U-238, preferably in the ratio of 20:80, in the
shape of rods and/or plates consisting of uranium-zirconium
alloy. The water to fuel element volume ratio is in the
range of six to approximately ten, far above the accepted
norms of approximately two to one in conventional reactors.
The high water content results in a resonance escape
probability of above 0.95 in the U-238. The reduction of
plqtonium ou~put comes first of all from the change in
enrichment. A change in enrichment from the conventional
value of U-235/U-238 (3:97) to U-235/U-238 (20:80l reduces
plutonium produation by a factor of seven. See
Optimization of Once-Through Uranium Cycle for Pressurized
Light Water Reactors~, by A. Radkowsky, et al., ~uclear
Science and Enaineerina, 75, pp 265-274 (1980). The high
value of the resonance escape probability of the seed fuel
further reduces the rate of plutonium production by a factor
WO93/16477 PCT/US93/01037
2 ~ 2 ~
of six. The high value of the resonance escape probability
also results in a high value of the seed multiplication
factor, which increases the proportion of energy obtained
from the blanket to the range of seventy-five to eighty
percent of the total core power. Taking into account that
the seed regions produce only twenty to twenty-five percent
of the core power, it is evident that the rate of production
of plutonium in the seed regions is well below one percent
of that in a conventional reactor. The seed regions also
contain some blanket fuel elements and are referred to as
"composite seed-blanket regions".
2. Blanket Reaion: The blanket region contains fuel
elements of mixed thorium-uranium oxide rods and/or plates.
The uranium oxide volume content in the thorium-uranium
mixture is in the range of six to approximately ten percent.
The uranium oxide is U-235/U-238 in the ratio of 20:80. Tbe
water to fuel volume ratio is in the range of .8 to l.S.
With this choice of paramet-rs, the blanket multiplication
factor stays approximately constant during an irradiation of
about l00,000 MMD/T. An irradiation of this magnitude has
been dbown to be feasibIe by experiments in Oak Ridge,
Tenn-s~ee. See~Irradiation Behavior of Thorium-Uranium
Alloys and Compounds" by A.R. Olsen, et al., International
Atomic Enerov ReDort (1977). The approximate constancy of
the blanket multiplication factor is necessary for two
reasons: (1) so that the blanket will produce its
W093/16477 PCT/US93/01037
2128S:14 6
appropriate share of the core power from the beginning of
core life and (2) for the proper functioning of the control
system as explained below.
For economic power it is necessary that the blanket be
left in the core for a long irradiation. Otherwise, each
time that a new blanket is inserted, fissile uranium fuel
must be added to avoid a large expenditure of seed neutrons
to build up the thorium reactivity. The U-238 inserted in
the thorium serves a further purpose by being mixed
uniformly with and thus denaturing the U-233 remnant in, the
thorium at the end of the blanket life. The plutonium
production rate will be, at most, 0.6 percent of that of a
conventional core (eight percent U-238 content times
seventy-five percent blanket power share divided by ten to
twelve years of the blanket residence in the core).
The blanket fuel elements may be of solid cylindrical
shape or of annular shape with the center hole open to the
water. For the same fuel volume the annular shape has
superior nuclear and heat removal characteristics, but this
shape requires internal as well as external cladding.
In addition to the blanket region internal to the core,
the term "blanket" is also used to describe the regions in
the reflector around the core which are utilized primarily
to reduce neutron leakage from the core. Such blankets will
have fuel compositions and fuel element shapes similar to
those described above, except that depleted uranium would be
W093~16477 PCT/US93/01037
7 2 1 2~ 51ll
used instead of the U-235/U-238 (20:80). The purpose of the
depleted uranium is to ensure that any U-233 formed in these
reflective blanket regions will be denatured by U-238.
3. Nonparasitic Control System: A nonparasiti~
control system is provided to increase safety and maximize
the amount of core energy obtainable from the thorium. This
control system ensures that all neutrons available from the
seed are utilized usefully in the core blanket region, thus
minimizing the number of fissions required in the seed
regions. Th$s is in contrast to conventional cores in which
all excess neutrons are wasted by absorption in parasitic
control materials.
The control system requires a uniform motion of the
control rods of only approximately forty-five centimeters
throughout the core length, as contrasted with the travel
over the whole core length, typically about twelve feet, of
conventional control rods.
The operating principle of the control system according
to the in w ntion depends upon the fact that the seed regions
h~ve a high multiplication factor, with correspondingly high
neutron leakage, such that the core reactivity is greatly
affected by small changes in effective seed dimensions.
The preferred embodiments of the present invention will
now be described with reference to the accompanying
drawings.
WOg3/16477 ~ PCT/US93/01037
5i4 ` 8 i~`
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic diagram of a pressurized water
reactor power system of the type to which the present
invention relates.
Fig. 2 is a schematic diagram of a seed/blanket core of
a nuclear reactor of the type to which the present invention
relates.
Fig. 3 is a diagram showing the neutron absorption
probability of U-238 over a spectrum of neutron energies.
Fig. 4 is a diagram showing the multiplication factor
of a natural thorium oxide blanket with respect to time as
compared to that of a thorium oxide blanket having some
initial fisæile fuel.
Fig. 5 is a diagram showing the blanket energy
production of various thorium and uranium blanketæ for given
inputs of seed neutrons.
Fig. 6 is a diagram showing the wasted neutrons over
time in-a nuclear reactor co # conerol}ed by conventional
8.
Fig. 7, comprising Figs. 7a-7d are schematic diagrams
:
a ~ingle seed/control/blanket asgembly illustrating the
principle of the nonpara~itic control system of the
i m ention. These Figs. show the maximum and minimum
reactivity po~itions, respectively, of the control system.
In Figs. 7a and 7b, the control system depicted
indicates the movement of both seed type fuel (20% Uranium-
WO93/16477 2 1 2 ~ PCT/US93~0l037
235, 80~ Uranium-238) and blanket fuel (thorium-uranium
oxide) in the operation of the control system. In Figs. 7c
and 7d, seed type fuel elements only are moved in the
operation of the control system.
Figs. 8a and 8b are horizontal sections (plan views) of
a portion of a nuclear reactor core according to the
invention showing respectively two equally preferred
embodiments, which will be referred to for convenience as
first and second preferred embodiments.
Figs. 9a and 9b are vertical sections (elevational
views) of one-half a nuclear reactor core showing the first
and second preferred embodiments of Figs. 8a and 8b,
respectfully, for the first seed cycle and each subsequent
odd numbered seed cycle. Similarly, Figs. 9c and 9d apply
to the second cycle and each subsequent even numbered seed
c.,vGle .
Figs. lOa and lOb, corresponding to Figs.~9a and 9b,
are repr sentational elevational views showing~a portion of
the control regions in their maximum reactivity positions.
Figs. lOc and lOd apply similarly to Figs. 9c and 9d.
Figs. lla to lld, corresponding to Figs. lOa to lOd,
are representational elevational views showing the control
regions in their minimum reactivity positions.
If the control scheme shown in Figs. 7c and 7d is
utilized, Figs. 9 to ll apply except that movable blanket
tyupe fuel-elements are omitted.
W093/16477 10 PCT/US93/0103~7
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The essential concepts as well as the preferred
embodiments of the present invention will now be described
with reference to Figs. l-ll of the drawings.
Fig. l schematically illustrates a pressurized light
water nuclear reactor power system (pressurized water
reactor or "PWR") of the type to which the present invention
relates. As may be seen, this system comprises two fluid
circuits between the nuclear reactor, which is the heat
source, and a steam turbine whi~h drives an electric
generator. The primary fluid circuit maintains ordinary
(light) water under pressure to prevent the formation of
steam. ~his water is heated in the nuclear reactor pressure
vessel and supplied to a steam generator which transfers
heat energy to ordinary (light) water of the secondary fluid
c~rcuit. The water in the secondary circuit is converted to
8team~which is used to drive the~steam turbine. Systems of
this type are well known and are described in detail, for
OXA ple, in Nuclear Fuei Nanagement, H.W. Graves, Jr., 30hn
Wiley & Sons, New York (-1980)~. ~
~ ~The~present invention relates~Qpecifically to the
naturo of the nuclear~rea~ctor core.l As is well known, the
reactor core is fueled by a fissionable (fissile) material
- ~uch~ the isotope uranium-235~(U-235). Since natural
- :
uranium contains only about 0.7 percent U-235, the rest
b-ing noDfissionable U-238, this natural uranium is
WOg3/16477 PCT/US93/01037
11 212S~'Iq`
"enriched" until the U-235 is about 3 to 4 percent of the
total. In a conventional reactor, a sufficient amount of
such enriched uranium fuel can provide enough energy for a
year to eighteen months of reactor operation.
Since the element uranium corrodes with almost
explosive force when coming in contact with the hot water
used for coolin~, the uranium cannot be used in metallic
form. Instead, uranium oxide is used, usually in the form
of l cm. diameter rods clad in zirconium, a metal which has
good corrosion resistànce and very little neutron
absorption. It is also possible to use a metallic alloy of
uranium and zirconium, either in the form of rods or plates.
There are two possible arrangements for the uranium
oxide fuel elements in the nuclear reactor core. The most
co D on arrangement is for all t~e uranium rods or plates to
have the same enrichment. Another arrangement, which i8
illu~trated in Fig. 2, includes a number of small islands of
moderately enriched uranium, having a reactivity greater
than one, ~urroundéd by regions of fertile material which
have a reactivity less than one: for example, natural
uranium or thorium.
Thi~ type of arr~ngement has come to be called a "seed-
blanket" core, the islands being called "seeds" and the
surrounding region the "blanketsn. Since the blanket
regions have a reactivity of less than one and the seed
regions a reactivity greater than one, the seeds supply the
W093/~6477 8 S I ~ 12 PCT/US93/01037
neutrons needed to keep the blanket neutron population at a
high enough level to generate the fissions necessary for the
rated power. Seed-blanket cores have operated successfully
for over 30 years at the world's first commercial nuc~ear
power plant at Shippingport, Pennsylvania.
There are several advantages of a seed-blanket core
over a conventional, uniform core: ~1) less total
enrichment is needed; (2) control rods are required only in
the seed regions since the blanket region is subcritical;
and (3) at each refueling (normally each year or 18 month
period) only the seeds have to be replaced. The major part
of the core - that is, the blanket region - can remain in
place for a number of years (normally 10 to t2 years). As a
result, there is a saving in fuel manufacturing cost.
So far, all efforts to utilize thorium economically
have been unsatisfactory, even without attempting to be
nonproliferative.
The aforementioned ten-year Brazilian-German
cooperation program on thorium utilization is typical of
past attempts. Since thorium has no natural fissionable
; content, the first remedy would be to add some U-235:
however,-much more U-235 would be needed than in natural
uranium because of the higher thorium absorption
prob~bility. Pure U-235 is undesirable because it is
proliferative; i.e., it can be used in nuclear weapons. A
low enrichment of uranium could~be used, but then so much
W093/16477 PCT/USg3~01037
13 212~S~
space would be needed for the accompanying U-238 that there
would be little space left for the thorium. (Thorium oxide
and uranium oxide have about the same density.)
Consequently, it was proposed to add plutonium Qxide to
the thorium oxide, since the plutonium has no accompanying
U-238. Plutonium can be obtained from con~entional reactor
discharged fuel. The German-Brazilian concept was to start
operation with plutonium for about a year, reprocess the
thorium to recover the U-233, which had been created in thë
meantime, fabricate the U-233 into fuel elements and then
use these elements with fresh thorium and a reduced amount
of plutonium. This operation could be continued and
gradually the reactor could be run almost entirely on U-233.
Such a procedure is, of course, (1) very expensive because
of the high cost of making the U-233 and plutonium fuel
elements, as has been explained above, and (2) proliferative
at every stage. Another aspect in the proposed program was
that no advantage would be taken of the high metallurgical
resistance of thorium ox~de, since the thorium was to be
melted down for reprocessing after each year or so of
operation. The Brazilian-German effort was eventually
di~continued because Brazil decided not to reprocess
plutonium from fuel discharged from its reactors.
With the present invention, first of all, the U-233
for~ed in the thorium is fissioned ("burntn) in place so
that it is not necessary to fabricate U-233 fuel elements.
W093/16477 PCT/US93/01037
2128~ 14
Second, for economic reasons as much energy as possible is
obtained from the thorium. Third, to fulfill both economic
and nonproliferative objectives, the thorium in the form of
oxide is retained in the core for its full metallurgical
lifetime. If fissionable material were added to the
thorium to make it critical (reactivity greater than one)
for such a long lifetime, so much would be required that
there would be no space for the thorium. The present
invention therefore employs a seed-blanket core arrangement,
as shown in Fig. 2, so that the thorium in the form of oxide
can be left as a blanket in the core for lO or more years,
and only the seed regions need be replaced at the end of a
normal refueling period. The blanket is always subcritical
with a reactivity of about 0.9, which is designed to be
nearly constant during operation. ~he seed regions must
therefore supply about 10% of the blanket neutron
population.
For the seed regions, an objective of the present
invention is to keep the plutonium production rate to a
minimum: to about l to 2% of that of a conventional reactor
core. The seed regions therefore utilize 20% enriched
uranium, (20% U-235 and 80% U-238); that is, approximately
the highest enrichment of uranium which is nonproliferative.
The enrichment in the seeds is made as high as possible
for two reasons. Pirst, every neutron absorbed in U-238
eventually results in plutonium. The high amount of U-235
WO93/1~77 2 1 2 8 ~i 1 1 PCT/USg3/01037
competes with the U-238 and reduces the number of neutrons
going into U-238. This also makes available more neutrons
for the blanket. Second, about four times as much cooling
water is used in the seed region as is used in a
conventional reactor core. Fig. 3 shows the neutron
absorption of U-238 versus neutron energy, evidencing that
U-238 has sharp lines, called resonances, at higher
energies, where the absorption of neutrons, to make
plutonium, is most intense. By providing a very large
amount of water in the seed regions and thus slowing the
neutrons, the high energy fission neutrons are reduced to
low energies, bypassing the resonances. In addition,
because thorium has resonances similar to those of U-238,
the low energy neutrons coming from the seed regions to the
blanket regions bypass the blanket resonances and are thus
used more efficiently. While the water to fuel volume ratio
in the seed regions is higher than in a conventional core,
that in the blanket regions is lower, so that over-all core
volu~e is no greater than that of a conventional core of the
s~é pow r output. ~ ~
To su~marize, two objectives are served by the
relatively high (20%) enrichment of the seed fuel: (l) the
reduction to a very low level of the amount of plutonium
created in the seed regions, and (2) (for a given power
generated in the eeed regions) maximizing the number of
'
WO93~16477 ~ 16 PCT/USg3/0103~7~
.
neutrons into the blanket so as to increase the amount of
energy generated from the thorium.
In regard to the blanket design, instead of using pure
thorium oxide, a few percent of 20% enriched uranium ~xide
is initially added to the fuel elements. This again has two
purposes. Without the uranium, the thorium would be "dead"
at the beginning, since it contains no fissionable material.
Consequently, all the power would have to be generated in
the small seed regions, and overheating would result. By
enriching the thorium, the blanket immediately starts to
generate power and, as the U-233 content builds up, the
blanket maintains an almost constant reactivity for very
hiqh burn-up, over a period of 10 to 12 years. This effect
is illustrated by the two curves in Fig. 4. The blanket
power is maintained by burning the U-233 as it is formed in
place. At the end of blanket life, the original U-235
content will have ?ong since fissioned, but the
nonrissionable U-238 will have remained and combined
unifor~ly with the remanent U-233 to make it useless for
weapons purposes. At the same time, there will be too
little U-238 to make any~appreciable amount of plutonium.
Thus, there will~be no incentive to reprocess the blanket,
and it will be discarded, like other nuclear waste.
As ~hown in Fig. S, for a given input of neutrons from
the seed, a thorium blanket produces nearly twice-as much
energy as does a natural uranium blanket. Also, the thorium
093/16477 2 1 2 ~ PCT/US93/01037
17
blanket with a small amount of U-235, as in the present
case, starts much higher and remains higher in energy output
than a natural thorium blanket.
An important aspect of the present invention is the
system of control which results in major gains in safety and
in reduction of costs, as well as advancing the objective of
nonproliferation. This control system actually overcomes a
basic defect in the control method of conventional power
reactors. First, it must be understood that in any reactor
it is necessary for practical purposes to add enough fuel to
the reactor core at the start of a cycle so that it will
last at least a year or 18 months, until shutdown for
refueling. Consequently, the core initially must contain
much more than the amount of enriched uranium needed to just
su~tain a chain reaction (reactivity of l.0). In order to
prevent the extra fuel from being operative until needed,
~control~ materials with high neutron absorption are
in~erted into the core. These materials simply absorb
neutrons wastefully, as is illustrated in Fig. 6. For
ex~mple, boric acid, which has a very high neutron
~b~orption, is added to the water in the core at the
beginning and gradually removed during the core lifetime.
Not only does the use of boron waste neutrons, but small
boron leaks cause safety problems, such as interfering with
the operation of vital valves. See NRC letter IN 86-108,
Supplement 2 of November l9, 198~. In addition,
212 8 ~1~ 18 PCT/US93/0103~
conventional control systems use control rods for rapidly
inserting a neutron absorber into the core. Such control
systems are subject to mechanical difficulties since the
control rods are commonly 36 feet long and must be able to
insert thin pins, about 1 cm. in diameter, a distance of 12
feet into the core. Any bending or distortion of the pins
can prevent a control rod from entering the core, thereby
causing a safety problem.
The control system according to the present invention
is mechanically simple and ensures that all neutrons
originating in the seed are absorbed usefully in the thorium
to make U-233. In particular, the control system is
entirely "nonparasitic"; i.e., nonwasteful of neutrons.
The control system according to the present invention
may be visualized as a kind of "Venetian blind" in which
each control element has to move only a small distance to go
from ~light to dark", from high reactivity to shutdown. In
contrast, the control rods in a conventional core are like a
"window shade~ in having to traverse the whole length of the
core to go from maximum to minimum reactivity.
Fig. 7 illustrates schematically the method of
operation of the nonparasitic control system. The seed is
divided into vertical layers each approximately 45 cm. long.
If we number successive layers as #14 and #15, each #14
layer has higher fuel density in the seed fuel elements than
in the #15 layer.
W093~16477 l9 2 ~ 2 S ~ 1 ~1 PCT/US93/01037
Flg. 7a shows the position of maximum reactivity.
Movable ~eed fuel elements in the center of the seed on the
#14 layers are connected by zirconium extensions, located in
the #15 layers. Movable blanket (mixed thorium uranium -
oxide) fuel elements in the center of the seed on the #15
layers are connected by zirconium extensions, ~ocated in the
#14 layers. The movable blanket fuel elements are
positioned on either side of the movable seed extensions.
Fig. 7b shows the position of minimum reactivity
(~hutdown). The movable seed elements are now located in
the #l5 layers, and the movable blanket elements are now
located in the #14 layers between the stationary seed fuel.
The reactivity of the core has been decreased because: (l)
the movable high density seed fuel has moved to a volume of
lower multiplication factor; and (2) The regions of
~tationary high density seed fuel elements are now separated
by blanket fuel, causing these regions to have a lower
effective thicXness and thus much higher leakage of neutrons
to blanket fuel.
It will be seen that all excess neutrons from the seed
~re u~efully ab~orbed in the thorium to create U-233, and
there i~ no parasitically absorbing control material. Since
no neutron~ are wasted, the necessary seed power is reduced
and the blanket power increa~ed, which fulfills the
ob~ective~ of the nuclear reactor. Seed power is expensive
W093~16477 2 1 2 8 5 1 20 PCT/~S93/0103
and produces a small amount of plutonium. Blanke~ (thorium)
power is inexpensive and does not produce plutonium.
The control system according to the present invention
is also much simpler mechanically than conventional ~ontrol
systems for nuclear reactor cores. In this connection, the
pressure vessel is one of the most expensive items in a
nuclear power plant. The present control system enables the
pressure vessel height to be reduced with consequent lower
cost. Thus, in addition to the nuclear gains, the present
control system both improves safety and reduces the initial
construction cost.
Fig. 8 shows two preferred geometries for the composite
~eed-blanket regions according to the present invention: In
Fig. 8a relatively small annuli and in Fig. 8b much larger
and relatively narrower annul i . Seed fuel elements 11 are
~urrounded by blanket fuel elements 12. The control
a~semblies 13 are located in the center of the annul i.
Figs. 9a and 9b show the vertical structures of the
~tationary portions of the composite seed-blanket assemblies
of Figs. 8a and 8b, respectively. These assemblies are made
up of alternating forty-fiv~e centimeter thick layers 14 and
15. Layer 14 consists primarily of seed fuel elements.
Layer 15 consists of blanket fuel elements and seed fuel
elements of reduced uranium content. Since it is necessary
to refuel the seed at intervals of twelve to eighteen months
while the blanket fuel remains in the core for ten to twelve
WO93~16477 2 1 2 ~ PCT/US93/01037
21
years, the following construction is adopted to permit
separate removal of the seed fuel. Advantage is taken of
the large spaeing of the seed fuel elements. As shown in
Figs. 10 and 11, stationary seed fuel elements 16 eon6ist of
a sequenee of forty-five eentimeter lengths of uranium-
zireonium alloy 17 alternating with forty-five eentimeter
lengths of redueed eontent uranium-zireonium alloy 18
throughout the length of the core. Thus all the seed fuel
elements 16 ean be removed from the eore and replaeed by
fresh fuel, while leaving all the blanket fuel elements in
plaee.
Figs. 10 and 11 also show the details of the
nonparasitie eontrol system. The movable seed fuel elements
19 of the eontrol assembly 13 eonsist of a sequenee of
forty-five eentimeter lengths 20 of uranium-zireonium alloy
alternating with forty-five eentimeter lengths 21 of pure
zireal}oy throughout the length of the eore. The movable
blanket fuel elements 22 of the eontrol assembly 13 eonsist
of a seguenee of forty-five eentimeter lengths 23 of
thorium-uranium oxide alternatinq with forty-five eentimeter
l-ngths 24 of pore zirealloy throughout the length of the
eore. These blanket fuel elements 22 extend b-tween the
seed fuel elements 16 and 19.~ The spaeing of uranium-
zirconium lengths 20, when opposite the layers 14, takes
-~ into account the water displaeed by zirealloy eonneetors 24.
~ 1 2 8 5 1~ 22 rCT/US93/01037
To operate the control system in order to reduce
reactivity to the minimum value, the seed fuel elements 19
of the control assembly are moved down forty-five
centimeters from layers 14 to layers 15. As the seed fuel
elements 19 move down, the blanket fuel elements 22 move
from layers 15 to 14. Just the opposite motion is used to
increase reactivity.
Both the blanket and seed fuel elements of the control
system have yoked drives 25 and 26 (Fig. 9), which move
together while the reactor is in operation. During shutdown
for seed refueling the drives can be unyoked and the seed
fuel elements removed and replaced without disturbing the
blanket fuel elements of the control system.
An important feature of the invention is the provision
of uniform axial depletion of the blanket fuel. It is
evident that, since the seed fuel is of lower density in
layer #15 than in layer #14, there will be lower seed power
in layer #15 and hence fewer neutrons supplied to the
blanket, resulting in lower blanket power on that level.
For the movable blanket fuel there is no problem. When
a fresb seed is inserted (seed~reactivity a maximum), the
moving blanket fuel will be located in layer #14. As the
~eed depletes, the moving blanket fuel will gradually
descend to layer #15. ~Thus in the course of a seed
lifetime, the moving blanket fuel will experience
.
W093/l6477 PCT/US93~01037
.~
23 2 1 2 ~
approximately equal exposure to the seed fuel on both
layers.
For the stationary blanket fuel, in order to ensure
uniform axial depletion, each successive seed ~as the. -
relative positions of the #14 and #15 layers reversed, as
shown in Figs. 9c and 9d, 10c and 10d, and llc and lld. For
the proper functioning of the control system, it is
necessary only to raise or lower the drive for the moving
blanket fuel by the approximately 45 cm. length of each
layer. Thus in the course of the blanket lifetime, which
will involve many seed replacements, each layer of the
stationary blanket will experience approximately equal
depletion.
For the annuli of Fig. 8a, either a separate control
drive 28 may be provided for each annulus, or a common
control drive may be provided for two or more annuli. In
the annuli of Fig. 8b, a number of separate control drives
28 may be provided as shown.
- It ~hould be no~ed that it is important that the
~ultiplication factor of the blanket fuel remain
~pproxia~tely constant throughout core operation. Otherwise
the effectiveness of the control system would have wide
variations ac the thorium multiplication factor increases
~ro~ nearly zero to a value close to one.
Typical dimensions for the preferred embodiments
illustrated in Figs. 8-11 are set forth in Table I below:
-
W093/16477 PCT~US93/01037.
21~8~1~ 24
TABLE I
~YPICAL DIMENSIONS
~g~re ~a: -
Seed Fuel Assembly
Distance Across Flats, cm 20
Number of Assemblies in core 69
Volume Ratio; Inner to Outer Region 25%
Inner Reflector, cm 7.5
Outer Reflector, cm 15
Fioure 8b:
Seed Fuel Annulus
Thickness, cm 14
Number of Seed Annuli 3
Inner ~eflection, cm 7.s
Fioures 9a:
Active Core Height, cm 360
Number of Axial Layers 8
Height of Axial Layer, cm 45
Number of Control Mechanism 69
Fioures 9b:
Active Core Height, cm 360
Number of Axial Layers 8
: Height of Axial Layer, cm 45
Number of Control Mechanisms 48
Ei~ 8 10 and ll:
Parts 16 and l9, Diameter = ,mm 7.2
Parts 23 and 27, Thickness = D 3.5
;: ~ Cladding Thickness, mm : 0.5
Composite Thickness, mm 2.5 min
Part 24 minimum required by
mechanical considerations
All of the above dimensions are to be considered
relatively important in the-respective embodiments since
they affect (l) the control characteristics and (2) the
neutron currents between the seed~and blanket regions and
2 1 2 ~ 5 1 ~ pcr/us93/olo37
the neutron leakage from the core, which in turn affect the
fraction of core power produced by the blanket.
The dimensions of each of the seed regions are set by a
compromise between minimizing the number of seeds so as to
simplify the core design, yet having enough seeds to provide
as uniform a power distribution as possible within the
blanket.
The height of the axial layers, which is also the
length of the stroke of the control mechanism, is set by a
compromise between making the control stroke as small as
possible, yet not having the sensitivity (change of
reactivity per unit length) so large as to cause problems in
the control drive mechanism.
Table II sets fo~th typical operating parameters for a
1300 megawatt electric pressurized water reactor employing
the principles of the present invention.
WO 93/16477 PCr/US93/0103,1.
2128514 26
TABL~E I I
OPERATING PARAMETERS FOR A TYPICAL 1300 MWe PWR
Seed Fuel Pin Diameter, mm 7 . 2
Blanket Fuel Pin Diameter, mm
Outer 14.4
Inner 0.5
Cladding (zircalloy) Thickness, mm 0.56
Moderator-to-Fuel Volume Ratio
Seed 8.0
Blanket 1.2
Temperatures K (-F)
Fuel 980 (1305)
Coolant 567 (560)
Cladding 630 (675)
Power Density
Kw/1 of core go
w/cm core height 250
Equivalent Core Radius, cm 186
Active Core Height, cm 360
Core Material Densities 95% theoretical
212~
WO93/16477 PCT/VS93/01037
27
MATHEMATICAL BASIS OF THE INVENTION
The mathematieal basis for the present invention is
deseribed in the ehapter entitled "Seed-Blanket Reactors",
CRC Handbook of ~uelear Reaetor Calculations, Volume III,
CRC Press, pp. 365-448 (1986). It should be recognized that
with the advent of high speed eomputers explieit
mathematieal formulae are no longer in eommon use today for
praetieal reaetor eore design ealeulations. However, sueh
formulae do provide physieal insights and are therefore
ineluded below where they may be helpful.
Instead of sueh formulae, elaborate reaetor codes of
high aeeuraey, eheeked by experiment, are in general use.
The prineipal eodes employed in the development of the
present invention were WIMS, RABBLE, DOT and ANISN. (See
J.R. Askew, F.J. Fayers, and P.B. Kenshell, "A General
Deseription of the Lattiee Code WIMS", J. B~ Nucl. Energy
Soe., 5(4) 571 (1966); P.H. Kier,~ and A.A~ Robbs, "RABBLE, A
Progra- for Computation of Resonanee Absorption in
~ultiregion Reaetor Cellsn, AN~-7326, Argonne National
Laboratory, Argonne, Ill. (1967); W.A. Rhoads, et al., ~ W T-
Two Di~ensional Diserete Ordinates~Radiat1on Transport
Coden, ORNL CCC-276, Oak Ridge Laboratory, Oak Ridge, Tenn.,
(1976)-and W.W. Engle, Jr., "ANISN - A One-dimensional
Diserete Ordinatesn, Transport Code with Anisotropie
WO93/16477 PCT/US93/01037
28
Sca~ ,8 ~ 99, Oak Ridge National Laboratory, Oak
Ridge, Tenn., (1967).
Seed Reaion:
a. The principal source of plutonium in the seed is
the capture of neutrons by the resonances of the U-238,
which forms eighty percent of the uranium fue} of the æeed.
Of all neutrons created by fission, the fraction of neutrons
which esca~e such capture by U-238 may be denoted by p, the
resonance escape probability. Then l - p is the fraction`~f
neutrons captured by the U-238, resulting in the formation
of plutonium. p may be written approximately as:
p = e~(^VF / 1~) '
where A is a constant depending on the fuel element
composition, VF is the fraction of fuel volume and V~ is the
fraction of water volume. It is evident that as VF/V~
decreases, p approaches the value of l. With the present
invention, with a range of VF/V~ between 6 to l0, the
minimum value of p is 0.95 so~that 1 - p = 0.05. Thus, the
production rate of plutonium in the seed region i8 extremely
low.
b. The ~eed multiplication factor, k~, is giYen by the
,, :
traditional four-factor formula:
kS = ~7 f P ~'
where ~ is 2.06, being the number of neutrons emitted per
neutron capture by U-235, ~ is the so-called "fast effect~ -
and i8 close to unity and f is the thermal utilization whose
WOg3/l6477 2 1 2 8 5 1 ~ PCT/USg3/01037
29
value varies with the amount of seed uranium and the
fraction of burnup p is the resonance escape probability,
as noted above Thus ks reaches a maximum as p approaches
unity
Elanket Region
a The water to fuel volume ratio in the blanket (in
the range of 0 8 to l 5) and the fraction (in t~e range of 6
to lO percent) of uranium oxide (U-235/U-238 in the ratio of
20 80) are chosen so as to keep the blanket multiplication
factor, ~, as high and as constant as possible over the
entire blanket lifetime o~ lO0,000 MWD/T The blanket
multiplication factor ~ is defined as usual as the number
of neutrons produced per neutron absorbed Many complex
factors are involved so that the optimum choices must be
determined by computer calculations Representative curves
are given on pp 384-5 in "Seed-Blanket Reactors~, CRC
Handbook of Nuclear Reactor Calculations, Volume III, CRC
Press, (1986) ~However, it is clear that th- water to fuel
voluoe ratio must~not be ~o sma1l as to~pr s-nt cooling
proble~and-not 80 larg- that too many n-utrons are
CAptur d by the water or protactinium
-~ b ~Th- ratio of b1anket to s--d power is of prime
i~port~nce in determining the en-rgy derived from thorium
A ~i~plifi-d for~ula~which is quit accurate for large
r actor~ that have only sma1l neutron 1-akage out of the
core i~ as ~ollows
WO93~16477 ! PCT/US93/01037
2 12 ~ 30
P~ k5 kB
Ps ks l - k~ - ~k~s---
Here PB is the power in the blanket, Ps is the power in
the seed, ~ is the multiplication factor of the bla~ket and
k, is the multiplication factor of the seed. ~kBS is related
to the current of thermal neutrons from the blanket to the
seed.
In the prior known seed-blanket reactors the sign of
~ks is negative; however, with the present invention,
because of the very high water content of the seed, the sign
of 6kBS is positive. The magnitude of ~kBS is about 0.25, but
it strongly influences the ratio of blanket to seed power,
a~ will be seen in the following numerical example. The
lowest value of ks (when the seeds are about to be
discharged) is about l.4. The average value f PB is about
O.93. Due to *he inclusion of the ~k~s term, the ratio f PB
to (PB ~ PS~ is over 0.8, 80 that;more than eighty percent
of the core pow r is derived from the blanket.
c. To calculate the~plutonium production in the
bl~nket,~it is assumed that the U-238 will absorb about as
,
~any neutrons as a similar a ount~of U-238 in a conventional
` uranium reactor~core. The maximu a~ount of U-238 in the
blanket is eight percent (taking the upper range of ten
percent uranium content~in the blanket). Since the blanket
: will 8tay in the core at least ten years, the plutonium
production rate per year~will be 0.8 percent of that of a
WO 93/16477 2 1 2 8 ~ 1 4 PCl`/US93/01037
31
conventional core. The rate of production is actually about
0.6 percent of a conventional core (i.e., 0.8 x 0.75) since
the blanket produces approximately seventy-five percent of
the power of a conventional core.
Nonparasitic Control System:
The control system motion of approximately forty-five
centimeters was calculated on the basis of highly accurate
codes ANISN and DOT 4.2, utilizing fifteen energy groups.
The neutrons in a reactor are distributed over a wide
spectrum of energies ranging from over a million volts to a
fraction of one electron volt. To make sure that all these
neutron energies are properly treated, the spectrum of
neutron energies is divided into a large number of groups.
In the present calculations, it was found that increasing
the number of groups above fifteen made no appreciable
difference in the results. Thus, it was concluded that the
u~e of fifteen neutron energy groups was adequate.
The calculation results showed that increasing the
rotion of the control system above forty-five centimeters
did not increase the amount of control available and would
merely add mechanical complexity. Reducing the stroke below
forty-five centimeters rapidly decreases the amount of
contxol available, and increases the change of reactivity
per centimeter. This necessitates finer control of the
control ~ystem motion and again adds to mechanical
WO93/16477 2 1 ~ ~ 5 1 4 PCT/US93/01037
32
complexity. Thus, approximately forty-five centimeters has
been found to be the ideal length for the control rod
motion.
THORIUM FUEL USED IN THE INVENTION
The nuclear reactor core according to the present
invention obtains about seventy-five percent of its power
from thorium or Th-232. Therefore, some words of
explanation about this fuel are appropriate.
Thorium is ~uite widespread in nature. The ores of
interest contain five to eight percent thorium, as
contrasted with one to four percent for uranium ores.
The thorium utilized in the present reactor core
blanket is in the form of oxide, just as uranium oxide is
utilized in conventional cores. The manufacturing processes
for thorium oxide and uranium oxide are very similar. Thus
no new techniques or tools are required for manufacturing
thorium fuel elements.
me important ways in which thorium differs from
uranium are: ~
! 1- Thorium i8 at l-ast three times as abundant as
uranium. m ere are ma~or supplies in India and Brazil.
Very little prospecting for thorium has been done since its
market price is very low.
2. Natural thorium contains absolutely no fissionable
material.
WOg3/1~477 2 1 2 8 ~ PCT/~S93/01037
3. Thorium has about three times the neutron
absorption probability of U-238.
4. When thorium absorbs a neutron, after about one
month it transmutes to U-233, a fissionable form of uranium.
The U-233 can be used for weapons, just as U-235 and Pu-239.
For reactor use, U-233 is superior since it emits about 10%
more neutrons per neutron absorbed than either U-235 or Pu-
239.
5. one disadvantage of U-233 is that it emits intense
gamma radiation. For this reason, fabrication of U-233 into
fuel elements must be done remotely, behind heavy shielding,
a very expensive process. In contrast, U-235 can be handled
without any special precautions~ The handling of plutonium
requires the use of face masks to prevent inhalation, so
that plutoni D fabrication is more expensive than for U-235,
but much less expensive than for U-233.
6. Thorium oxide has superior metallurgical properties
to ur~ani D oxide, in that thori D oxide can withstand 10% or
~ore of the atoms fissioned, more than twice a~ much as for
urani D oxide. This is because thorium oxide forms a
perf-ct cubic lattice, which is v~ery strong, while urani D
oxide ha~a structure with many irregularities. The present
invention takes advantage of this property of thori D.
7. Thorium oxide has~a~higher~melting te~perature, as
well as better thermal conductivity, than urani D oxide,
2 1 2 8 S 1 '~ 34 PCT/USg3/01037
which results in a greater resistance to meltdown in case of
a 108~ of coolant accident.
ADVANTAGES OF THE INVENTION
The principal advantages of the present invention over
conventional nuclear reactors may be categorized as follows:
1. Nonproliferation: Tbe United States Department of
Defense is understandably concerned about the tonnages of
plutonium generated by today's reactors. An even greater
danger is posed by countries like Japan, which are planning
to build sodiu~ cooled fast breeder reactors that will
produce vast quantities of weapons qrade plutonium, only few
kilograms of which are needed for a nuclear bomb.
2. Economics: The main item in tbe cost of operating
a conventional nuclear reactor today is tbe uranium fuel.
The cost of fueling a core construoted in accordance with
the present inv-ntion will be reduced by at lea-t 2/3 since
oDly 20 to 25% of the useable enerqy will be obtained from
uraniu~. The cost of fueling the core will also be reduced
bQcaus- 3/4 of the core (the thorium~blanket region) will
~last for 10 to l2~years instead of the three years of a
conv ntion~l core. Other substantial savings are also
available in the initial cost of constructing the core.
3.~ Safety: Co m ent~ional-nuclear reactor cores can be
de~cribed as ~waiting for an accident to happenn. Both the
.
WOg3~l6477 2 1 2 ~ ~ 1 '1 PCT/US93/0~037
3s
soluble boron control system and the mechanical control
system of conventional cores present quite obvious dangers.
4. Nuclear Waste: The nuclear reactor according to
the present invention discharges less than half the high
level nuclear waste than conventional reactors.
Each of these four categories will be discussed below
in detail.
1. Non~roliferation
The seed fuel employed in the reactor core according to
the present invention is 20% U-235/80% U-238. This is the
type of fuel which the U.S. Department of Energy specifies
for all research reactors, since even an infinite quantity
of this fuel could not produce a nuclear explosion. As this
fuel burns, the ratio of U-235 to U-238 is reduced.
The fuel discbarged from the blanket cannot be used for
nuclear bombs for two reasons:
a. The only fiss$onable fuel created in the
blanket is U-233, but it will be denatured by b-ing
unifor~ly mixed with relatively la~rge amounts of
non~i~sionable isotopes which are:~ the U-238 that was
included in the blanket at the start, and U-232 and U-234,
which are created during operation.
b. The U-233 discharged from the blanket will be
acco~panied by extremely intense gamma radiation. For this
reason alone it would be impracticable to build a useful
nuclear weapon from the U-233 because of the great weight of
. .
~ 1 2 ~ 5 1 ~ PCT/US93/01037
36
gamma shielding required for handling and personnel
protection.
After U-233 is created in thorium, its gamma activity
increases with time. This is an exception to the general
rule that radioactivity decreases with time. The reason is
that the gamma activity is not really due to the U-233
itself, but to the isotope U-232 which builds up by
secondary reactions leading to products which have high
gamma activity. The total amount of higk level nuclear
waste radioactivity discharged from the thorium is still
well below the proportionate amount from a conventional
core, as is explained below in connection with nuclear
wa~te.
2. Economics
The economics of nuclear power are made up of two
components: operating costs and capital costs.
As regards fuel costs at present, a conventional light
water nuclear electrical power plant spends about
S90,000,000 a year to replace one third of the reactor core.
In the core according to the present invention, where only
the relatively ~ all seed regions are replaced each 12 to 18
~onths, thi~ amount is reduced by half. The blanket will be
replaced once in ten to twelve years. Its cost will be much
le~ than that of the seed regions and will be ~pread over
mRny year~.
W093/16477 2 1 2 8 5 1 '1 ` PCr/vs93/olo37
37
The basic reason for this economic advantage is that
about seventy to eighty percent of the energy is obtained
from the thorium which, at present, is essentially "free".
This load factor is achieved by using a nonparasitic control
system, which greatly reduces the number of neutrons
required from the seed. Another point is that the fuel
discharged from a conventional reactor is discarded, because
most of its fissile content is plutonium which is too
expensive to use and the processing of which was prohibited
by the U.S. Government. With the present invention, the
discharged seed fuel will still have about ten percent U-235
content, and almost no plutonium. This fuel can be readily
stripped of fission products and reenriched to twenty
percent U-235 with very little cost in separative work.
Other costs of conventional cores that the present invention
avoids are the replacement of control absorbers and the
rearrangement of fuel assemblies.
In the area of capital costæ, the core design according
to the present invention results in a saving of about
fifteen to twenty pe-rcent of the total plant costs. This
saving is attributed to (1) the elimination of the soluble
boron ~ystem with thousands of feet of pipe, mixing tanks,
~ilters, injectors, etc.; (2) the reduction in the cost and
complexity of the control rod drives; (3) the reduced height
of the pressure vessel and (4) the resultant reduced size of
the containment.
38 PcT/us93/olo3?
As is well known, the so-called "load following" in a
conventional reactor core is both slow and cumbersome due to
the soluble boron control system. This is particularly so
at the beginning of an operating cycle when there is a lot
of boron in the core. In the core according to the present
invention, on the other hand, the so-called "throttle
control" technique can be used. This means that if there is
an increase in power demand, the throttle is opened allowing
more cold water to flow into the core increasina the
reactivity and then the power level. With a conventional
core the cooling water increases the density and the
concentration of the dissolved boron reducina the
reactivity. To overcome this difficulty in conventional
cores, additional special control rods ("half" rods and
"gray~ rods) are installed at considerable~extra expense.
Further, the slow response to power demand changes means
that some power is wasted, increasing operating expenses.
3. Safety
The reactor core concept according to the present
lnvention is superior from th- safety standpoint to
conv ntional light water reactor cores in the following
re~pe¢t~
In conventional light water reactors, control rods and
drive mechani~ms extend approximately three ti~es the core
h-ight of about twelve ~feet (i.e., a total of thirty-six
feet) for a 1000 MWe rating. Each typical rod terminates in
WOg3/16477 PCT/US93/01037
~ 39 212~
twenty-~even absorbing pins, each twelve feet long and one
centimeter in diameter, which must be inserted into holes in
the fuel assemblies. It is evident that driving such thin
pins from more than twenty-four feet away involves a risk
that the pins will suffer some distortion which could
prevent them from penetrating the core. Furthermore, to
shut down the core quickly, as in the case of a so-called
"loss of flow accident" (LOFA), the rods must go all the way
into the core.
In contrast, the control system according to the
present invention require~ a movement of only about forty-
five centimeters and therefore will shut down the core much
more quickly. The present arrangement is also such that
distortion i8 much less likely.
In case of a loss of flow accident (LOFA), the core
according to the present invention has several points of
superiority. me seed regions with their high neutron
leakage will behave much like small cores. The water in the
e ds will ~tart to boil first, resulting in a quick
reduction of reactivity. The fuel elements in the seed
region~ are preferably of metallic, uranium-zirconium alloy,
which have much less stor d heat than the~ceramic, uranium
oxide fuel elements of conventional reactors. Even the
blanket region of the present core has an advantage over
conventional cores, since thorium oxide has higher thermal
conductivity than uranium oxide.
W093/i6477 PCT/US93/01037
212S~14 40
Conventional light water reactors now utilize boric
aeid in the coolant to eontrol the reactivity and power
level of the core during operation. As previously stated,
it has been found that small boron leaks aeeumulate a~d
eorrode high strength steel parts such as those used in
eooling pumps and valves. The presenee of boron in the
eoolant interferes with effieient load following.
Nevertheless, the industry has not been able to eliminate
soluble boron eontrol, probably beeause sueh elimination
would entail the addition of many more eontrol rods with the
attendant meehanieal eomplexity deseribed above. In the
reaetor eore aeeording to the present invention, in whieh
soluble boron eontrol is not required, the reactor eontrol
system is nevertheless much simpler mechanically than that
of eonventional reaetors.
Another problem with soluble boron eontrol is that, in
ease of a LOFA, the emergeney eoolant supply might be left
unborated, thus pouring fresh water into the eore and
resulting in a 8evere reaetivity surge.
Although eonventional light water reaetors, if properly
designed and eonstrueted, present virtually no risk of
~preading radioactivity in ease of an aeeident, there are
still a number of weak points;whieh eould result in a
meltdown and a major eeonomie loss. With the reaetor eore
aeeording to the invention, the probability of sueh a
failure is greatly redueed.
WOg3~16477 4l 2 1 2 8 PCT/US93/01037
4. Nuclear Waste
There are two categories of nuclear waste to consider:
low level and high level waste.
For low level waste, the reactor core according to the
present invention has no advantage over conventional cores
since the quantity of such waste depends only upon the total
energy generated.
However, with regard to high level waste, the amount of
radioactivity the present core will discharge will be less
than half the amount from a conventional reactor core.
The explanation is as follows: The seed regions, which
are refueled every twelve to eighteen months, will discharge
high level-~waste at the same proportionate rate as a
conventional reactor, but only twenty to twenty-five percent
of the total energy is generated in the seeds. In the
blanket region, which stays in the core for ten to twelve
y ars, the radioactivity of the high level wastes will
decrease by at least a factor of seven, simply because these
wa~te~ di~int grate rapidly and form residues with much
~maller amounts of radioactivity. This process will be
- aided by neutron absorption in the high level waste while it
i~ in the core, which also results in transmutation to
nuclei which are less radioactive. Thus, the radioactivity
of the high level waste discharged from the blanket will be
les~ by at least a factor of seven than the proportionate
amount discharged from a conventional reactor core. If the
.
W093/16477 j PCT/US93/01037
2 1 2~r;1~ 42
amount of rad$oactivity produced from both the seed and
blanket regions is weighted by the amount of energy produced
from each region (twenty to twenty-five % from the seed
regions, eighty to seventy-five % from the blanket),.the
total radioactive waste discharged can be shown to be well
below half of the high level waste discharged from a
conventional core.
In conclusion, therefore, a novel nonproliferative
light water nuclear reactor has been shown and described
which fulfills all the objects and advantages sought. Many
changes, modifications, variations and other uses and
~pplications of the subject invention will, however, become
apparent to those skilled in the art after consid~ring this
~pecification and the accompanying drawings which disclose
the preferred embodiments thereof. All such changes,
modifications, variations and other uses and applications
which do not depart from the spirit and scope of the
i m ention are deemed to be covered by the invention, which
i~ to be li it d only by the clai~s which ~ollow.
.
