Note: Descriptions are shown in the official language in which they were submitted.
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BACKGR3UND OF THE INVENTION
1. Field of the Invention
This inyention relates to a fuel asse~bly design
for use in a nuclear reactor and particularly to a fast
breeder reactor utilizing plutonium as a fuel and pres-
surlzed/hea~y water as a reactor coolant and moderator.
2,
The ad~antages of utilizing nuclear ~reeder
reactors which convert ~ertile material into fissile
ma~erial and generate hea~, e~g. for power generation,
ha~e been widely recognized in ~iew of the limited known
fis3ionable material resources of the world. Develop-
men~ o~ breeder reactors which convert the more abundant
fertile uranium-238 into fissile plutonium-239 utilizing
the latter as a fuel, possibly in conJunction wlth plu-
tonium generated in other known reactors, and breed more
~issionable material~than is consumed, is highly desirable.
Since extens~ve tech~ological development and experience
exists in the design and construction of pressurized light
and heavy water reactor plants, use o~ the pressuri~ed ~ -
water technology in a breeder application represents an
attractive alterna~ive to development of other breeder
options.
Heavy water, deuterium oxide (D2O), has essen-
tially the same physical and chemical propertles as ligh~
water~ H2O. Its nuclear properties, howe~er, are dlf-
ferent~ the neutron absorption cross-section and slowing
- down power of D2O being mar~edly lower than that of H2O.
Hence~ the use of D2O as a coolant in a fast breeder
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application ls desirable due to its nuclear characteristics
and the aprlicability of pressuri~ed water technology. In
a pluton~um-uranium-deuterium oxide(Pu-U-D20~ reactor system,
as the coolant to fuel atom ratio decreases~ it is known
that the conversion or breeding ratios increase. The breed-
ing ratio is the ratio of the number of fissile atoms
produced to those consumed. High breeding ratics~ approach-
ing a value o~ 1.40, may be realized in a Pu-U-D20 system
i~ a fuel latt~ce geomekry is d~veloped wherein moderator
to fuel volume ratios are ~d~us~ed to ~eld moderator ~o
fuel atom rat~os approaching 1.0 or le~s. As the selection
of a moderator to fuel atom ratio de~ines the volume of
coolant per unit mass of fuel, it can be appreciated that
difficulties arise in designing a ~uel lattice capable of
passing adequa~e cooling flow rate at low moderator to fuel
ratios. The high flow rates needed to assure adequate
reactor core cooling necessitate high velocities in ~low
:
channel3 tha~ are significantly restricted when achie~lng
a low moderator to fuel ra~io. In the tightly packed fuel
pin la~tices, the use of conventional spacer grids is dis-
advantageous owing to inherent limits in fuel pin packing
due to the interposed grids, a tendency to flow induced
spacer grid ~ibration, the parasitic absorption of the grid
plate ma~erial, and the increase in hydraulic pressure loss
resulting from introduction of grids within ~he restricted
flow passages.
The prior art teaches heavy water moderated and
cooled reactor des~gns for particular fuel "rod" dia-
meters and spacings wi~hin a moderator to fuel atom ratio
range from 0.35 to 4.0 and suggests ~hat a moderator to
fuel atom ratio of approxlmately 0.3 can be achieved
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in a fuel lattice utilizing touchln~ ~uel rods arranged
in a trian~ular pltch. Reduction of heat flux to the
de~ree necess~ry to avoid potentially destructive hot
spots at fuel pin contact polnks~ however, ~ould severely
limit the capability of operating such a core at pres- - -
surized water reactor conditio~s. ~urthermore, close
spacing of the ~uel pins may lead to plugging by solid
particles carried by the c~olant and prohibitiYely high
reactor coolant pumping power requirements Other dif~
~iculties become readily apparent On the one hand, elim-
in~tion o~ spacer grids ls desirable in order to permit
the higher coolant flow velocities needed to approach the
modera~or to fuel atom ratios yielding the high conversion
ratio of the touching fuel rod configuration. On the other
hand, elim~nation of spacer grids may result in impre~ise
fuel pin spacing, flow induced vibration and unequal cool-
ingi ~`~
SUMMARY OF THE INVENTION
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In accordance with the princlples of the inven-
tion, the dlsad~antages of the prior art, discussed above,
are effecti~ely surmou~ted by the practice of the in~
~ention. A fuel assem~ly, made in accordance with ~his
inYention, utilizes longitudinally flnned fuel pin clad- i;
ding tubes arranged to form an integral fuel assembly by
brazing together the continuous or interrupted ~ins of
one fuel pin to the fins of other fuel pins. The in~
te~rally brazed fln ~uel pin assem~ly is designed to
sat~sfy the thermal and hydraulic requirements of the
very tight lattice required to achieve high breedlng ratios. ~:~
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~ Case 4166
In an alternate embod.iment the fins o~ some fuel
pins may be connected directly to the tubular sect~on
of other fuel ~ns 50 that the resulting assemblies have
modera~or ~o fuel volume ratios which tend ko increase
the breed~ng ratio in a Pu-U-D20 reac~or core.
In a further embodlment of the invent~on~ the
core is fabricated from a solid mater~al ha~ng passages
which are alternative~ suit,ed for coolant flo~ and ~uel
retention.
Practice of the invention overcomes the dis~
advantages of the prior art by providing means ~or
obtaining moderator to fuel ratios whlch are conducive
to a high Pu-U-~20 reactor breeding ratio while assur-
ing accurate spacing of the fuel pins without the para-
sitic losses associated with the prior art~s use of
spacer grlds. Furthermore~ the arrangemen~s of the ~ ~
inYention eliminate hydraulic pressure losses assoc- ;
;~
iated with conventional spacer grids~and reduce the
- tendency o~ fuel pin~v~bration. ~he finned fuel pin
arrangements, moreover, increase the strength of the
pins, increase the available hea~ transfer surface and
improve the overall heat transfer coeff~cient.
The various ~eatures of novelty which charac-
terize the invention are pointed out with par~icularity ~-
in the claims annexed to and forming a part of this
specification. For a better understanding of the in-
vention, its operatin~ ad~antages and ~peci~ic ob~ects
` attained by its u~e~ ~e~erence should be made to the
.
accompanying drawings and descripti~e matter in which
3 there is illustrated and described a preferred embodi-
~; ~ . ment of the lnvention .
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Case 4166
BRIEF DESCRIPTION OF TH~ DRAWINGS
In the accompanying drawings, forming a part
of this specification~ and in which reference numerals
shown in the dra~ings designa~e like or corresponding
parts throughout the same~
Figure 1 ~s a partial section in plan of a fuel ?
assembly;
Figure 2 is an ele~ation VieW of part of a
number of finned fuel elements arranged ~n accordance
10with an alternate embodiment of the invention
Figure 3 is an elevation vlew of part of a ~
number of fuel elements arranged in accordance with ~ -
another alternate embodiment of the invention;
Figure 4 is a partial section plan of a ~uel
assembly having ~uel elements arranged in accordance
with still another embodiment of-the in~ention~ and
Figure 5 is a part plan of a block core arrange-
ment for a lo~ temperature reactor.
; ~
DESCRIPTION OF THE PREFERRED EMBODIMENTS :
Figure 1 shows part of a nuclear fuel asse~bly
10 of closely packed fuel plns 11 arranged in an array
: with their longitudinal axes in parallel. ~ach fuel :--
pin 1l consists of generally tubular cladding 12 which
has a pluralIty of lcngltudinally extending fins 13
formed as part of the outer surface of the cladding and
spaced circumferentially the~eabout. A nuclear fuel 14
consisting of a ~ixture of fisslle and fertile~material,
is contained within the cladding 12. The fuel pins 11
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`. Case 4166
in Figure 1 are arranged so that .the extremity of each
~in 13A ~buts ~h the extremity of a fin 13B of a ~uxta-
posed fuel pin; fins of peripheral fuel pins may abut the
fuel assembly can structure 15, ~he extremities of the
fins shown in Figure 1 are joined to each other and to
the reactor can structure by means of brazing at 16 and
17, respectiyely~ to form the integral fuel assembly 10.
The rins 13, in one embodiment, extend without
interrup~ion along the longitudinal surface of the ~uel
pin forming channels 20 in the interspaces o~ the fuel
pins which direct reactor coolant flow (not shown) there-
of the ~n
within generally in parallel with ~he longitudinal axis/
The fins 13, however, need not extend continuously:along
the len~th of the fuel pins but can be interrupted ~ins 21,
as shown in Figures 2 and 3,so as to allow transverse flow
and intermixing of the coolant through the fuel pin i~ter-
spaces. The axially interrupted fins 21 of juxtaposed
,
fuel p~ns may be brazed to each other at 22 (Figure 2)
org as shown in Figure 3 directly to the tubular portion
of the fuel pin at 23. An assembly utilizin~ a combina- ~.
tion of both arrangements shown in Figures 2 and 3, i.e.,
fin .to fin contact and fin to tube contact, is also
possible.
A finned fuel pin 26 design utilizing ~road rins
24 brazed to each other a~ 25 is shown in Figure 4. Broad .P
fins may be utilized to further 1imit the moderator volume
frac~ion at some sacrifice of specific core power.
Elimination Or conventional spacer grids and the
formation of fins ~s part of the tube. cladding:permits
: ~
3 reduction of the reactor core moderator volume fraction
Case 4166
3~
to ~alues cons~stent with the achievement o~ thé desired
moderator ~o fuel ato~ ratios, Illus~rat~e physlcal
design parameters are set forth in Table 1.
TABLE I
Example 1 2 3 ~
Fuel Pin Diame~er~ inches.35 .40 .40 : :
Fuel Pin Pitch~ inches ,39 - .43 .43
Clad Thickness, in~hes ,015 .020 .020
Clad Material Incoloy Type 316 Type 316
800 Stainless Stainless
. Steel Steel
Pitch - Diameter, inches.040 .030 .030
Number o~ ~ins per Pin 6 3 3
.~:
Fin height~ inches .020 .030 .030 ~:
Fin w~dth, inches .Q20 .030 .030
-.
Fin interruption, percent of 0 30
Fuel Volume Fraction - .6105 .6357 .6357
Structural ~olume Fraction .1381 .1659 .~1541
2C Coolant Volu~e Fraction.2514 .1984 .2102
Fuel/Coolant Volume Fraction Ratio 2.43 3.20 3.02 ~.
~ Moderator/Fuel Atom Ratio .82 .624 .66
: The fuel pins in the examples o~ ~able I are
- formed in the shapes of rods~ The fuel plns o~ examples
1 and 2 are pro~ided with continuous ~in~ along their
length, Example 3 ill~strates an alternate embodiment
of example 2 wherein the fins traYerse approximately
thirty percent o~ the }ength of the rods. The values
- for the moderator to fuel atom ratios shown ln Table I
3 approxima~e normal pressurized water reactor operating
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Case 4166
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conditions including primary coolant .temperature and
pressure~ fuel ~ellet shape~ clearances between the fuel
pellets and clad, and percent of theoretical U02 density
achieyed in the pelle~,
The fuel assemblies of Table I would be typlcally
formed by furnace brazing in a hydrogen atmosphere at
1950 to 2000F ~th a braæing alloy tradenamed "N~crobraz
~available ~rom the Wall-Colmonoy Corp.~ Detroit,
Michigan) usin~ ~igs, ~ix~ures and methods of braze alloy
placement known ln the furnace brazing art.
In still another embodiment, Figure 5 illustrates
a desi~n ~or low temperature reactors suitable for breed-
ing plutonium and low heat generation purpose, e.g.
residential heating. In this e~bodimen~ a fuel assembly
is fabricated from a block 32 of metal, e.g. aluminum
alloy. Parallel channels are formed for ~low passage
and for fuei 30. The surfaces of the flow ~hannels may
be roughened where needed to increase critical heat flux.
Illustrative design parameters ~or a block type reactor
:
are shown in Table II.
TABLE II
. .
Example 1 2
Fuel channel diameter, inches .40 .325 ~-
Fuel channel pitch, inches .500 O40
Coolant channel diameter, lnches .156 .125
Coolant channel pitch, inches .500 .40
Fuel Yolume fra~tlon ~ 0503 .518
Structure Yolu~e fr~ction ~421 .405
Coolant Volume fractlon ~o76 .0766
3` Fuel/Coolant Volume Frac~ion Ratio 6~62 6.76
Modera~or/Fuel Atom Ratio .44 ~43
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The moderator to fuel atom ratio o~ Table II
correspo~ds to a primary coolant water temperacure of
about 250F at low pressure. Other process parameters
are similar to those assumed ~or Table I.
The geometry of the coolant and fuel channels in
the block type fuel assembly will produce a degree o~ what
might be termed "moderator escape probab~llty" which wlll
ser~e to ~arden the neutron spectrum and improYe the core
conversion or breeding ratio. This occurs because each
fuel channel is not completely surrounded by moderator.
Hence, some neutrons produced in a fuel channel can pass
to another fuel channel without traversing a volume con-
- taining moderator, thereby lmproving the breeding or
conversion ratio since the average neutron energy at which
fission occurs is increased. This, combined w1th a mod-
erator to fuel ratio less than that which can be aohieved
with touching uel pins, should yield a uniquely high
breeding ratio for either H~O or D20 coolin~
By vlrtue of the moderator to fuel atom ratios
23 made possible by these approaches to fuel assembly design,
fast reactor physics can be applied to pressurized water
reactor tehnology. This combination has important ad-
vanta~es including:
a. A~oidance of gas or liquid metal coolants
- otherwise used for fast reactors.
. Reduced clad operating temperature.
c. AYailability of additional methods of
reactivity control, namely, chem~cal shim
and spectral shift control~
.~ 3a Availabllity of additional methods of reactivity control
... .
reduces the nor~.al dependence of ~ast reactors on control
rods. They allow a general reduction in required control
.
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Case 4166
rod worth and provide a means for continuous a~jus~ment
of e~cess react:lvity to a minimum value, thereby greatly
enhancing the safety of fast reactor cores. This would
lnclude operation with higher worth rods out of the core.
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