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Патент USA US3042609

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July 3, 1962
Filed Oct. 3, 1960
2 Sheets-Sheet 1
l .l
. Russell L. CRowfher;
July 3, 1962
Filed 001:. 5. 1960
2 Sheets-‘Sheet 2
, “21W
Russel/ L. Cr-owz‘her,
United States a Patent
” ice
‘erties of the ‘?ssionable isotopes are‘ summarized in
6 (Ilaims. ' (Cl. 204-4931)
Russell L. Qrowther, Saratoga, Calif, assignor to General
" Electric Company, a corporation of New York
Filed on. 3, 1960, Ser. No. 59,993v
more particularly it relatesrto an improved nuclear fuel 10
er resonance
p neutron
element for use in a nuclear reactor in which such re_' '
v 7 Some Properties of Common Fissionablej' Isotopes
mass to energy th.ough certain nuclear reactions, and
actions may be maintained.
This invention ‘relates broadly to the conversion of
Patented July 3, 1 962
The process of nuclear ?ssion is now quite well known.
Brie?y, certain atoms such as U233, U235, and P1123‘? will
undergo disintegration following capture of a neutron in 15
1: 5
lower, molecular weight, and anuinber of neutrons greater
; The number of fast ?ssion neutrons which are emitted
'asa result of neutron absorption in U233 is greater than
than one.“ The great kinetic energy of the ?ssion products is quickly dissipated togvarying degrees in any ambient
‘for, any of the other common ?ssionable isotopes. It is
also important that this quantity is greater for absorption
their nuclei to produce two or more ?ssion products of ‘
material as v‘heat. ' The net generation of neutrons forms 20 of neutrons at both thermal and resonance, or‘slightly
the‘ basis for a self-sustaining -or chain ?ssion reaction.
The'several types of nuclear reactors all involve the disposition of a ~form~ of ?ssionable material as a nuclear
.epithermal, neutron energies.~There are, therefore, funda
. mental reasons for converting the fertile isotope, Thzsz,
to the ?ssionable isotope U233.
. -
‘fuel’’ ‘inwa reactorwith provision for removing the ‘heat
The nuclear reactions involved in the conversion of
liberated byv ?owing some kind of coolant through it, and’ 25Th2!” in a neutron flux‘ are as follows:
(mr) I’
.27 day
6.7 hr. Or’LZmin.)
(11f) ‘FL-T235
provision for controlling the nuclear reactionand the 50'Th232 by neutron capture becomes Th233 which under
energy liberation rate. As the; reaction proceeds, the
?ssionable material is gradually consumed and deleterious
?ssion products accumulate. Ultimately fresh fuel, must
he added, or reprocessing is required to separate ?ssion-
goesa ‘18 particle‘ decay to Pam. Pam'has a 27 day
half-life and therefore may either decay to U233 or ‘may
capture a neutron and go to PaZ34 during this period.
The rate at which Pa”3 decays, relative to the rate at
able material from the'?s'sion products.
The net addition of new fuel can be reduced, and in
some cases eliminated, if the nuclear reaction can be
made to produce from a “fertile” atom one not new
?ssionable atom ‘per atom used up in'the reaction. Th232
and U238 are such fertile atoms. Through neutron cap'ture and a double beta particle decay Th232 is converted
to U233 which is ?ssionable. Through the same mecha
nism U233 is converted to Pu238 which is also ?ssionable.
The presence of these fertile materials in the nuclear
‘fuel permits the conversion of non-?ssionable atoms to
55 which it absorbs neutrons, strongly effects the e?iciency
of the nuclear reaction. In order to understand the
process, and‘the disadvantages which are overcome by
the present invention,.it is‘necessary to follow each of
. the alternate paths of Pa233 destruction in detail.
If Pa233 decays to U233 then the ?ssionable isotope
U233’ may absorb a neutron and either ?ssion or give off
a gamma ray and transform to U234. Each time U233
of fuel can be- elfected simultaneously with consumption
If, instead of decaying, Pa233 absorbs a neutron, it is
then transformed to Pam. Because of the short half
absorbs a thermal neutron 2.28 fast ?ssion neutrons are
produced on the average. These are then free to‘be ab
65 sorbed in the fertile or ?ssionable isotopes or in the
structural materials in the reactor.
?ssionable atoms, thus varying degrees of regeneration
of the original ?s'sionable charge.
life of Pam, the probability that this isotope will absorb
‘Of the common ?ssionable isotopes, U233, U235, Bum,
‘P1125, which can be produced by neutron capture in‘ fer- 70 a neutron is very small and therefore virtually'all of the
tile isotopes, Th2”, U234, U238, rum respectively, U233 has
the most desirable nuclear properties. The nuclear prop
Pa234,*that'is produced, rapidly decays to U234. U2“ is
essentially non-?ssionable and therefore it captures a
neutron and is transformed to U235.
The isotope U235
neither decreases the resonance capture contribution to
the conversion of fertile to ?sionable isotopes which
may then undergo ?ssion; however, its nuclear properties
are not as good as those of U233.
occurs above ‘10.0 e.v. in the fuel nor interferes with
Thus, effectively two neutrons were lost when Pa”3 '
absorbs a neutron and, in addition, U235 which is less de#
the ?ssionable materials in the fuel which occurs below
0.05 e.v.
sirable than U233 is the end product. If the difference
The shielding material is preferably one, or a mixture
between the U233 and U235 isotopes is taken into con
of the isotopes of plutonium, speci?cally Pu239, Pum,
sideration then the total disadvantage when Pa?33 ab
Pug“. It is incorporated on or near the surface of the
sorbs a neutron instead of decayu'ng is approximately
ThZ32 fertile element, either in substantially pure form, or
2.2 neutrons. Obviously, when producing U233 by neu 10 as a mixture with Th232 or other isotopes.
tron irradiation of Th2”, it is highly desirable to reduce
A satisfactory alternate shield material comprises U235.
neutron absorption in Pa233.
[It may be either depleted of U235, or it may be the natural
Another problem associated with nuclear reactors in
mixture of uranium isotopes comprising 99.3 percent U238,
which ‘111232 is employed as a fertile material relates to
the large initial ?ssionable material requirements.
or it may be enriched with one of the ?ssionable uranium
The 15 or plutonium isotopes. The principle‘behavior of U238
thermal neutron absorption cross section of ThZ32 is
7 barns (7X1‘0—24 cm?) as compared to 2.75 barns for
U233, the other most common fertile isotope. U233 is
not available except as produced from irradiation of
in a neutron ?ux is as a precursor of Pu isotopes as
U338 (nnf)
Th232 and, therefore, in the past it has been proposed
that initially U233 'be produced by combining the fertile
Th232 with fully enriched U235. Because of the high cost
, 23.5mm.)
of producing U235 in a diffusion plant and the reason
ably large inventory which must be maintained before
sufficient U233 can be produced and recycled to reduce
(F. 2. 3 days)
the makeup of U235, this mode of operation imposes an
economic and nuclear disadvantage on the conversion
of Th232 to U233.
(m'f) P“24.0 (an Pu241 (mf‘ Pam
. Therefore the present invention is directed to an im
proved solid nuclear reactor fuel element containing Th‘m 30
as the fertile material and provided on at least one sur
face thereof with a neutron shield material serving to
effect a substantial absorption of neutrons in the energy
spectrum between 0.05 e.v. and 10.0 e.v. prior to the
Thus, although U238 itself will not provide the important
shielding of the Pa233 capture, the rapid buildup» of the
plutonium isotopes due to the neutron capture by U238
will promptly provide the necessary shielding.
irradiation of Th232 thereby. A neutron shield containing 35 The quantity of the above-indicated shield material
an isotope of Pu is effective, or if desired, a‘precursorv
employed per unit mass of Th232 in the fuel element is
of such an isotope such as U238 may be used. The un
somewhat variable. It depends upon the fertile and ?s
desirable neutron capture by Pa233 is substantially re
sionable isotopes which are present, the size of the fuel
elements, and the neutron spectrum. The neutron spec
It is accordingly a primary object of this invention to 40 trum depends on the ?rst two variables and, in addition,
overcome the above-discussed problems and disadvan
depends on the type of moderator, the spacing of the fuel
tages in nuclear reactors employing Th232 as fertile mate
elements, the size of the reactor, the type of control
rial for conversion to U233.
Another object is to improve the method of converting
Th232 to U233 by neutron irradiation.
> elements, and the structural and poison materials which
are in the reactor. Therefore, the neutron spectrum
45 varies with time.
It is a more speci?c object to provide an improved a
nuclear reactor fuel element comprising Th232 as a fertile
thermal neutron breeder or convertor reactors and with
element which is shielded to minimize neutron absorp
plutonium shields, the quantity of plutonium varies be
However, in general with thermal, resonance, or epi
tion by Pam.
tween about 0.5 percent and about 20 percent by weight
It is another object of this invention to shield Th232 50. of the Th232 present, the actual preferred amount depend
from irradiation by neutrons in the range of from 0.05
ing upon the variables referred to above, and in addition,
e.v. to 10.0 e.v., thereby substantial-1y ‘reducing Pa233
the temperature of the materials in the reactor and the
chemical form, i.e., elemental or compounds form, of
It is an additional object of this invention to improve
the ?ssionable and fertile materials in the fuel. The
the breeding e?iciency of U233 from 'I'h232 in a nuclear 55 subsequently described examples. indicate the degree of
reactor fuel element by shielding the Th232 fertile ma
improvement which is obtained under various conditions
terial with selected materials hereinafter de?ned which
with various quantities of the preferred plutonium shield.
are effective to reduce or eliminate neutron capture by
It has been found that although the thermal neutron
Pa233 generated in the fertile material.
cross section of Pa233 is only about 60 barns, its effective
Other objects and advantages of this invention will be 60 nuclear reactor cross section varies between 130 and 150
come apparent to those skilled in the art as the description
barns, depending upon the neutron energy spectrum of
and illustration thereof proceed.
the reactor. Measurements indicate that the resonance
Brie?y, the present invention comprises a Thzsz-con
integral of Pa233 is about 670 barns. Thus the Pa233
taining fuel element suitable for neutron irradiation to
resonances lie at very low neutron energies in order that
produce U233. The element is provided with an outer 65 such a relatively low resonance integral has such a large
shield layer comprising a material which is either fertile
effect upon the effective reactor neutron cross section.
or ?ssionable or a burnable control material, and which
The published measurements of the effective neutron
has a substantial neutron capture cross section for neu
capture cross section of Pa233 have all ‘been made in re-,
trons of energies in the range of from 0.05 to about 10.0
actors with relatively low neutron temperatures, i.e.,
e.v. In thermal power reactor spectrums the thermal 70 research or production reactors. Since power reactors
neutron energies are about 0.05 e.v. and below. The
must operate at high temperatures, the neutron tempera
important resonance capture of the fertile isotopes U238
tures in a power reactor are greater than in research or
and TH232 lies above 10.0 e.v. Thus the shield material
with resonances between 0.05 and‘ 10.0 e.v. according
production reactors and, therefore, the effective thermal
absorption cross section of Pam‘3 is greater in a power
to this invention substantially reduces Pal33 capture, but 75 reactor spectrum. Therefore, the absorption of neutrons
coolant. Zirconium, Zirconium alloys, stainless steel, alu
by Pa233 is more important in a power reactor than it is
in a research or production reactor.
It is estimated that in a high flux nuclear power reactor,
which has the most undesirable spectrum from the stand
minum are included as suitable cladding materials.
In FIGURES 4 and 5 are shown elevation and trans;
verse section views of an annular or tubular type fuel
element incorporating the neutron shield of this inven
tion. It is provided with a tubular ‘111232 element 39 hav
point of high Paz33 capture, the attainable integrated heat
release per unit mass of fuel can be tripled if complete
shielding of the Pa233 is achieved. However, complete
or maximum shielding of the Pa233 absorption is not neces
ing a central channel or opening 4t} which is left open
to reduce thermal stresses, or through which a non
moderating coolant such as sodium may be passed. On
sary as any amount of shielding is bene?cial when com
pared to the case Where no shielding exists. If ?ssionable 10 its outer and inner surfaces are provided cladding 42 and
plutonium is used as the shielding material, the maximum
44 respectively. Immediatelyinside the outer clad 42 is
shielding which can be effected is limited by the maximum
excess reactivity which can be incorporated in the reactor.
provided the neutron shield layer 46 which contains a
In the present invention, the Pu shield material adjacent
flux those neutrons in the 0.05 to 10.0 e.v. range which
In FIGURE 6 is shown a transverse cross section of
a fuel element of the tube type which is adapted to ?ow
of a moderating coolant such as water. The only dif
ference is that an ‘additional shield layer 46a. is provided
would otherwise be strongly absorbed by Pa233 in the fer
just inside the inner clad layer 44, other parts being iden
tile interior of the fuel element. They are thus captured
in the shield and produce fast ?ssion neutrons which are
not subject to useless resonance capture in Pam, but
tical to those shown and described in connection with
FIGURES 4 and 5.
substantial concentration of shielding material.
to the surface of the fuel element and surrounding the
Th232 fertile material effectively absorbs from the neutron
In FIGURE 7 is shown a transverse cross section view
of annular modi?cation of fuel element in which a com
bination of rod and tubular elements are used to form the
which are effective in the conversion of Th232 to U233.
The Th233 is thus permitted to decay through Pa233 to
U233 without substantial neutron capture by the Pam.
shielded fuel element. The inner Th232 rod 41 is shown
provided with clad 43. Channel 45 is open for ?ow of
a non-modulating coolant such as sodium. The tubular
The conversion of Th232 to a ?ssionable isotope of ura
nium, speci?cally U233, and the neutron utilization e?i
shield layer 47 containing plutonium, or U238, is disposed
coaxially with respect to inner rod 41 and‘is provided
ciency are both markedly increased. This increase is due '
to a substantial reduction in the Pa233 resonance capture
due to the shielding effect.
with inner and outer claddings 49‘ and 51. Coolant may
be passed around the outside surface of clad 51. This
element is satisfactory for cases in which there is in
The structure of fuel elements embodying the principles
of this invention will be more readily understood by refer
centive, such as simpli?ed fuel reprocessing, to keep the
' ence to the accompanying drawings in which:
shield material separate from the fertile Th232. Other
corresponding mechanical arrangements such as alternat
ing plates of fertile ‘and ?ssionable materials can be used
FIGURE 1 is a schematic elevation view in partial
cross section of a nuclear reactor vessel,
FIGURE 2 is an elevation view in partial cross section
to achieve this effect.
of a typical reactor fuel rod having the neutron shield
according to this invention.
In FIGURES 8 and 9‘ are shown detailed longitudinal
and transverse section views of a typical plate type fuel
FIGURE ‘3 presents a transverse cross section view of
element. The fuel plates 43 are secured at their edges
the fuel rod of FIGURE 2,
FIGURES 4 and 5 present the elevation and trans 40 between sides plates 50 and 52 and ?ow channels 54 are
provided between adjacent fuel plates for passage of
verse section views of a shielded annular or tubular fuel
coolant or moderator if used. The plates are disposed
parallel to one another, and although shown as ?at plates
in FIGURE 9, they may each be curved if desired. A
of the tubular fuel element having ‘an internal and an
handling loop 56 is provided to facilitate introduction and
external shield,
. 45
removal of the fuel element into ‘and from the reactor
FIGURE 7 is a similar view of another modi?cation
core. A lower end ?tting 58 with coolant ori?ce 59 is
of this fuel element,
provided for alignment and support of the element in
FIGURES 8, 9, and 10 present views of a typical plate
FIGURE 6 shows a transverse view of a modi?cation
the core.
type fuel element having a neutron shield as herein dis
closed, and
FIGURE 11 is a foreshortened vertical cross section
In FIGURE 10 is shown the structural detail of the
50 fuel plate embodying the neutron shield of this inven
tion. The innermost layer 60 containing Th232 is shown
between shield layers 62 and 64. The outer layers of
of a commercial scale fuel assembly using shielded ThOZ'
fuel elements.
Referring now more particularly to FIGURE 1, a typi
. cladding 66 and 68 are also shown. The edge 70 of the
cal nuclear reactor 10 is shown in simpli?ed form in? 55 picture frame typical of this type of fuel element is pro
vided and it is bonded physically, as by hot-rolling, to the
cluding the vessel head 12 attached by means of ?anges
cladding layers 66 and 68 to form a ?uid-tight enclosure
14 and 16. Coolant inlet and outlet 18 and 20 are also
surrounding the fertile Th232 layer and the shield layers.
provided. Supported from the inner surface of vessel 10
The fuel plate is ?nally secured to the side plate 52 by
by means not shown are upper and lower fuel element
support grids 22 and 24, and supported therebetween are 60 brazing or other suitable means, as indicated at '72.
There are several suitable procedures for manufacturing
fuel elements 26, here shown as cylindrical rods. The
the plutonium shielded Th232 fuel elements of the pres
fuel elementsare spaced apart from one another to per
ent invention, to some extent varying with the type of
mit coolant to flow between them. Control rod housing
fuel element to be produced.
28 is attached to reactor 10 and contains the means for
The rod-type fuel element of FIGURES 2 and 3 may
actuating control rods 30 which are movable into and out 65
of the core.
In FIGURES 2 and 3, elevation and transverse section
views of a rod type fuel element according to this in
vention are shown. This element corresponds to one of
the fuel rods 26 of FIGURE 1. The inner portion 34 70
' comprises the fertile material Thm, the next outer sur
rounding layer 36 is the neutron shield, speci?cally plu
tonium, or U238 which will produce plutonium on neutron
be produced by powder metallurgy techniques, by physi—
cal bonding of an annular cylinder of shield material to
an interior rod of Th2”, or by ceramic technology in
which both the Th232 and the shield material are in some
ceramic form such "as an oxide or carbide. These are
well-known techniques. The clad is then added and pro
vided with suitable end closures by usual methods of the
. The annular type fuel element of FIGURES 4-7 may
irradiation. Surrounding the shield is cladding 38 serv
be produced by similar methods modi?ed to provide the
ing to protect the fuel element from adverse effects of'the 75 mterior opening. For example, annular or tubular dies
may be used in conjunction with die sleeves and annular
liners to build up the respective layers prior to ?nal
heat treating. Again, the cladding and end closures may
?nally be added according to the conventional procedures.
The plate type fuel element shown in FIGURES 8, 9,
and 10 may be produced by hot rolling three plates, the
- This fuel assembly is typical of those in a reactor core
having 510 such assemblies disposed in a hexagonal lat
tice which is 9.5 inches across the opposite faces, and has
a 5.5 inch center-to-center assembly spacing. The mod
erator and re?ector are composed of 270 hexagonal graph
ite blocks, 10 inches across the faces and having semi
outer two of which are shield material, and the inner
circular indentations or recesses along the corners to pro
one of which is fertile Th2”, by powder metallurgy tech
niques, or by ceramic fabrication techniques. The inner
layer of Th2”, with the adjacent layers of plutonium, or
plutonium-enriched U238, or natural or depleted U233,
may then be inserted in the picture frame of cladding
vide space for insertion of the fuel assemblies. The mean
diameter of the core is 15.20 feet, the core height is-14.0
feet, the re?ector thickness is about 2.0 feet, and the mean
outer diameter of the core-re?ector assembly is 18.47
feet. The fuel loading is 45,300 pounds total, distributed
material. >Additional' layers of clad are then added on
each side of the shield layers, and then the entire as
as 88.9 pounds in each fuel assembly of seven fuel ele
ments each, each element containing 12.7 pounds of fuel.
sembly is hot rolled to bond the clad permanently to 15 Liquid sodium coolant is circulated through this reactor
the frame forming a ?uid-tight fuel plate.
at about 52,000 g.p.m., entering at about 700° F. and leav
Other procedures which may be conventional and
ing at about 10000 F. The reactor rating is 200 mw.
known to those skilled in the art may be substituted to
produce nuclear reactor fuel elements embodying the prin
The following data are given to illustrate the effect of
ciples of this invention.
20 the neutron shield of this invention upon the performance
It should be understood that although rod and tubular
of a nuclear reactor having Th232 as fertile material in
fuel elements of circular cross section and plate type fuel
the fuel elements. The reactor used as a basis for com
elements of rectangular cross section have been described
parison is a typical sodium cooled, graphite moderated
and illustrated, the principles of this invention are readily
thermal power reactor using oxide fuel. It is operated
applicable to other structural shapes of nuclear fuel ele 25 at an effective neutron temperature of about 0.084 e.v.
ments having different cross sectional con?gurations.
The fertile material in the fuel elements is rhwoz, the
For example, fuel elements having elliptical or oval or
?ssionable material being U233O2. The fuel elements are
other noncircular cross sections can be employed. Sim
the rod type, approximately 0.6 inch in diameter, 10 feet
ilarly, fuel elements having geometric shape of a prism
long, and clad with stainless steel. The initial fuel to
and bearing any polygonal cross section may also be em 30 mixed oxide atom ratio in the core is 0.04, and the mod
ployed. The invention is not limited to a reactor of the
erator to mixed oxide atom ratio is 20. The shield ma
type illustrated in FIGURE 1.
terial used in these examples is a mixture of natural
Referring ?nally to FIGURE 11, a vertically shortened
uranium oxide, which contains 0.72. percent U235, and
elevation view in cross section of a commercial scale fuel
element assembly utilizing the principles of this invention
plutonium oxides.
The additional plutonium isotopes
35 rapidly build up during irradiation in amounts which are
is shown. This fuel assembly is adapted to high tempera
sufficient to reduce the effective Pa233 capture cross sec
ture operation, in the range of 1000°—1100° F., with
tion in the reactor from about 190 barns to an equivalent
liquid metal coolants such as sodium, sodium-potassium
of about 46 barns. The effect of the neutron shielding of
eutectic (NaK), etc., and a graphite or berylliummodera
the Pa233 containing fertile material on the integrated
tor. Sections of the upper and lower fuel assembly sup 4-0 neutron multiplication factor at various neutron ?uxes
port plates 80 and ‘82 are shown respectively. Graphite
and various extents of fuel irradiation are shown below
moderator 84 provided with fuel channel 86 and cladding
in Table II.
88 surround the fuel ‘assembly which extends vertically
through the channel. The fuel assembly comprises lower
coolant ori?ce ?tting ‘90, seated in opening 92 in lower
grid plate 82, structural tube 94 which acts also as the
fuel assembly wall and as a coolant ?ow director sur
rounding the fuel elements, assembly lifting adapter 96
provided with lift ?tting 98, typical fuel element support
and spacer plates 100 and 102, and seven fuel elements
arranged in a bundle of six surrounding a central element.
The coolant enters through ori?ce 104 in ?tting 90, flows
through openings ‘106 in the lower spacer plate 100, up
wardly around the fuel elements, through openings not
‘shown in spacer 102 but similar to those in plate 100, and
outwardly through openings 108 in structural tube 94 and
openings 110 in lifting adapter 96.
Variation in Multiplication Factoigfk
Flux 1><1<>1a
Flux 1><1o1i
Extent of irradiation mwdJt.
Flux 4x10“
1. 128
1. 128
1. 128
1. 120
1. 116
1. 114
1. 112
1. 110
1. 11s.
1. 109
1. 103
1. 09s
1. 093
1. 110
1. 100
1. 092
1. 085
1. 080
,1. 116
1. 105
1. 091
1. 083
1. 073
1. 095
1. 080
1. 066
1. 051
At a flux of 1><1013, the Pa233 exerts very little in?uence
upon the change in multiplication factor k. However, at
The fuel elements in this modi?cation are approximate
higher neutron ?uxes the absorption of neutrons in Pa233
ly 15 feet long and are supported between lower end ?t
tings 112 and upper end ?ttings 114. The lower active 60 becomes increasingly important. The difference between
the k values for ?uxes of 1X1013 and 4X1014 is due to
fuel element is a solid rod 10 feet long. The exterior
the adverse effect of Pam. Comparison of the k values
heat transfer surface is provided with a stainless steel
for unshielded fuel elements with those for the shielded
cladding 120 which extends between lower end ?tting 112
fuel elements of this invention shows that the shielding
and upper end ?tting 114.
The inner fertile portion 122 of this fuel element is a 65 effect is relatively small at low thermal neutron ?uxes,
but is very appreciable at higher ?uxes. At a neutron
0.40 inch diameter solid rod of sintered high density
?ux of 4>< 1014, the reactor with a shielded fuel elements
T112320, This is surrounded by the shield layer‘124 0.12
has approximately a 2.0 percent reactivity advantage at
inch thick and consists of a mixture of U233_O2 containing
10,000 mwd./ t. over the same reactor with unshielded ele
4 percent by weight of Pu239O2. This in turn is sur
rounded by the clad 120 which is stainless steel and 0.015 70 ments. This amountmof reactivity. is particularly im
portant to increase the reactor lifetime at long fuel ir
inch thick. The outside diameter of the fuel element is
0.67 inch. The center-to-center spacing of the fuel as
radiations. The reactivity advantage of neutron shielding
sembly elements is 0.81 inch. The outside diameter of
the Th232 fertile material according to this invention is
clearly shown.
t?ow channel 94 surrounding each assembly of seven fuel
elements is about 2.50 inches.
75 The following data show the gain in discharge cycle
shield layer surrounding
rod is 0.12 inch thick and '
contains 18% (atom percent) U235O2, 74% U238O2, and
conversion ratio'i(D.C.R.)l i.e., the number of ?ssionable
‘atoms produced per ?ssionable atom destroyed at 10,000
8% of an oxide of Np23'7.
. mwd./t. irradiation, with’ variation in neutron?ux and
showing the ‘effect of-fertile material shielding according
to this invention; Again the shielding improvements are
more pronounced at higher neutron ?uxes.
on neutron irradiation of the fuel rod described above
in Example 1, the Np237 is transformed by neutron cap
ture to Npm. Although this neptunium isotope has a
fairly short half-life, it provides additional shielding of
Variation in D.C.R.
Pa”)3 according to the principles of this invention and
10 also serves as‘ a ?ssionable material.
0. 72a
0. 734
No shield
0. 007
0. 095
A nuclear fuel element utilizing Pu24° as the shield ma
terial according to this invention may be prepared having
15 a T193202 central core of 0.4 inch in diameter. This core
is surrounded by the shield layer, 0.08 inch thick, and
contains 19% U235, 80.5% U238, and 0.5% of Pu2‘10 pres
The improvement in conversion ratio at an average neu
ent as a metallic foil.
tron ?ux of 4><l0l4 is 5v percent at 10,000 mwd./t. Al
though the above data relate tofuel elements provided
A‘ nuclear fuel element utilizing Am241 as a neutron
with a shield composed of natural uranium enrichedwit‘h
plutonium according to this invention, an improvement
. also realized when the shield is made up of depleted
uranium and plutonium, depleted uranium and U233, or _
shield according to this invention may be prepared hav
ing a ThzazCz central core 0.4'inch in diameter surrounded
'/by a’ 0.12 inchrthick shield layer containing 18.5% U235
carbide, 79.0% U238 carbide, and 2.0% Am241 carbide.
natural uranium slightly enriched with U235.
Other satisfactory alternate, although temporary, shield
,The shielded Th232 fuel elements ofthis invention have
ing materials which can be used are burnable control
been. found to provide another distinct advantage over un- ~
poisons, These poisons burn out-as reactor irradiation
> shielded T112732 fuel elements, and this ‘involves the change
A U238 converter provides “an
continues and thus serve to control and shield the reactor
initial increase in reactivity after startup due to the rela
30 which haverne'utron absorption resonances just above
' in reactivity with. time.
tively rapid buildup of Pu239 with its high ?ssion cross
section. ‘This is followed, however, by a decline due to
saturation of the plutonium production, burnup of fuel,
The most suitable of these are poisons
thermal neutron energies which are large compared to
their thermal neutron cross sections. Applicable poison
and buildup of ?ssion products. , The Th232 converter
. materials which meet this requirement include In, Te,
- lower thermal neutron cross section of U238. Subsequent
the fuel element comprises a Th232O2 core 0.4 inch in
diameter surrounded by a shield layer 0.12 inch thick con
Ag, Cd, Cs, Sm, Eu, Dy, Ho, Er, Tm, Hf, Ta, Re,
initiallystarted on U235 exhibits an initial decrease in re 35 Ru,
11", and Au. However, it should be noted that this type
activity due to‘fuel bumup and buildup of ?ssion prod
ucts during the time in which the Pa233 is slowly building ' of shielding material’ is. not equivalent to the ?ssionable
or fertile U and Pu isotope shielding described above
up. This is followed ?rst by aslight'reactivity increase
because its shielding effect decreases with fuel irridiation
due to buildup ‘of U233 with its highervalue of 11, and
then a gradual decrease due to saturation of the U233 40 and because the Th232 conversion may be decreased.
Nevertheless, in some applications this type of combined
production and buildup‘ of ?ssion products. Therefore,
initial reactivity control and fertile elements shielding is
a blending of U238 and T1123.2 as fertile materials in the
required in the reactor.
initial fuel load e?ectively balances these reactivity
‘The following examples illustrate the utilization of
change effects against one another and permits longer
term irradiations. In addition, such blending decreases 45 burnable control poisons as Pa233 shieldstin fuel elements
embodying the principles of this invention. In each case,
the initial fuel inventory requirements because of the
fuel loadings each have successively higher Th232 con
centrations so that ?nally Th232 is the primary fertile ' taining the shield material noted in an appropriate form.
During the continuance of the full cycle 50 The 4% ThmOz is added to the shield layer to improve
its material properties, including its compatibility with
, the initial U235 is burned up and the U233 and plutonium
which are produced are ‘separated and recycled to supply
the required ?ssionable material. Thus, two additional
advantages are realized; the initial inventory require
ments of ?ssionable material which are required to reach
the equilibrium recycle condition are reduced, and the
initial cost of U235 is decreased since it is not required to
separate the U235 from U238 in a diffusion plant. The
shielded fuel element according to this invention provides
analogous improvements in reactivity and average con 60
version ratio to such mixed Th232-—U238 fuels as it does
to the Th232 fuels discussed above. '
In addition to U238 and the ?ssionable plutonium iso
topes described above, other ?ssionable or fertile iso
topes, including Npm, Npm, Pug“, and Am241 are effec
tive shield materials for the purposes of this invention.
The following examples illustrate the use of these ma
terials in a nuclear reactor fuel assembly and a reactor
of the types and dimensions described above in FIGURE
A fuel rod incorporating Np23" as shielding material
may be fabricated having a sintered high density solid
rod containing 92% Th232O2 and 4% beryllium oxide
and 4% U02. The rod is 0.4v inchtin diameter. The 75
the ThmOz core portion. The data are presented in
tabular form in view of the relatively large number of
poison materials.
Example Burnable Poison Shields
Shield layer composition-Atomic percent
Example No.
U 235 02
U m 01
Th 23% Or
16. 2
18. 0
18. 7
19. 2
19. 2
19. 2
18. 4
76. 0
74. 3
76. 6
64. 8
76. 76
76. 76
4. 0
4. 0
4. 0
4. 0
4. 0
4. 0
17. 0
68. 0
4. 0
0. 6
4. 0
3. 0
0. 2
0. 04.
0. 04
3. 7
18. 0
70. 0
4. 0
18. 9
19. 0
18. 8
75. 4
75. 2
75. 8
73. 4
75. 2
4. 0
4. 0
4. 0
4. 0
1. 7
2. 0
1. 2
4. 3
2. 0
It should be understood that either the shield ‘element
aeae, 598
or thefertile element can be a slurry such as a mixture
of .energiesiin this rangeand substantially reduce neutron
of sodium and a metallic ?ssionable or fertile. oxide, or a
capture in Pa233 present during the conversion.
?uid such as a molten alloy or salt of ‘a solution of ?ssion
3. In the irradiation of Thm in a neutron ?ux to pro
able or fertile material. Both elements may be such non
duce U233, the improvement which comprises passing the
solid materials in the presence of suitable structural or‘ $1 neutrons through a neutron shield material selected from
containment materials at the boundary between the ele
ments. The drawings show suitable geometries and in
dicate these boundaries.
This application is a continuation in part of appli
cant’s copending application, Serial No. 723,709, ?led
March 25, 1958 and'entitled “Shielded Thorium Fuel
Element,” now abandoned.
A particular embodiment of the present invention has
the class consisting of Npza'l, Np238, and Amz‘i1 and hav
ing a substantial neutron resonance capture cross section
for neutrons of energies in the range of from 0.05 to
about 10.0 e.v. to effect a substantial absorption from
said ?ux of neutrons in this energy spectrum prior to the
irradiation of the Th232 by said flux.
4. In irradiation of Th232 in a neutron flux to produce
U233, the improvement which comprises passing the neu
been hereinabove described in considerable detail by
trons through a neutron shield‘, material which is tem
Way of illustration. It should be understood that various 15 porary and burns out as irradiation of said fuel element
other modi?cations and adaptations thereof maybe made
continues and is selected from the class consisting of In,
by those skilled in this particular art without departing
from the spirit and scope of this invention as set forth
Te, Ru, Ag, Cd, Cs, Sm, Eu, Dy, Ho, Er, Tm, Hf, Ta,
Re, Ir, and Au and having a substantial neutron reso
nance capture cross section for neutrons of energies in the
I claim:
20 range of from 0.05 to about 10.0 e.v. to effect a substan
1. A fuel element for use in a nuclear reactor which
tial absorption from said ?ux of neutrons in this energy
in the appended claims.
comprises an inner fertile portion comprising Th232
which is converted at least in part to U233 by neutron ir
radiation, and an outer portion comprising a neutron
spectrum prior to the irradiation of Th2:32 by said flux.
5. A fuel element for use in a nuclear reactor which
comprises an inner fertile portion comprising Th232 which
shield material selected from the class consisting of Np237, 25 is converted at least in part to U233 by neutron irradia
Npm, and Am241 and having strong neutron resonances
tion, and an outer portion comprising Pu240 as a neutron
in the range of from about 0.05 e.v. to about 10.0 e.v. to
shield material having strong neutron resonances in the
absorb from the neutron flux neutrons of energies in this
range of from about 0.05 e.v. to about 10.0 e.v. to absorb
range and substantially reduce neutron capture in Pa233
from the neutron ?ux neutrons of energies in this range
present during the conversion.
30 and substantially reduce neutron capture in Pa233 present
2. A fuel element for use in a nuclear reactor which
comprises an inner fertile portion comprising Th232 which
is converted at least in part to U233 by neutron irradia
tion, and an outer portion comprising ‘a neutron shield
material which is temporary and burns out as irradiation
of said fuel element continues, and is selected from the
class consisting of In, Te, Ru, Ag, Cd, Cs, Sm, Eu, Dy,
Ho, Er,'Tm, Hf, Ta, Re, Ir, and Au and having strong
neutron resonances in the range of from about 0.05 e.v. to
about 10.0 e.v. to absorb from the neutron ?ux neutrons
during the conversion.
6. In the irradiation of Th232 in a neutron ?ux to pro
duce U233, the improvement which comprises passing
the neutrons through a neutron shield material compris
ing Pu24° having a substantial neutron resonance capture
cross section for neutrons of energies in the range of from
0.05 to about 10.0 e.v. to effect a substantial absorption
from said ?ux of neutrons in this energy spectrum prior
to the irradiation of the ‘H1232 by said ?ux.
No references cited.
Patent No. 3,042,598
July 3, 1962
Russell L. Crowther
It is hereby certified that error appears in the above numbered pat
ent requiring correction and that the said Letters Patent should read as
corrected below.
Column 1, line 63, for "Pu 238" read -‘- W239"; column 4.
line 2I for "fisonable" read —- ?ssionable -—,' same column 4.
line 3', after "with" insert —- thermal, neutron flux which
maintains thermal fission of -—; column. 11, line 3, for "of"‘
occurrence; read
~-. -
Signed and sealed this 6th day of November 1962.
Attesting Officer
Commissioner of Patents
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