Large BWR fuel channel design

A fuel bundle assembly for a boiling water nuclear reactor includes an open ended tubular channel subdivided into four quadrants by at least two interior partitions, each quadrant having a sub-fuel bundle assembly having a plurality of fuel rods extending between upper and lower tie plates. An inter-bundle support plate receives a lower end of the channel and has four flow openings at an upper end thereof, such that the lower tie plate of each sub-fuel bundle supported in a respective one of the openings in the support plate. In one embodiment, the sub-fuel bundles within a channel are separated by a cruciform shaped coolant passage. In a second embodiment, the cruciform coolant passage is omitted to thus provide a homogeneous sub-fuel bundle configuration.

TECHNICAL FIELD 
This invention relates to the fuel bundles for boiling water nuclear 
reactors, and more particularly to an improved and enlarged fuel bundle 
channel design. 
BACKGROUND 
A Boiling Water Reactor (BWR) generates steam in its core. This core is 
composed of an array of side-by-side, vertically upstanding square 
sectioned fuel bundles. These fuel bundles divide the core region of the 
reactor into the so-called core bypass region exterior of the fuel 
bundles, and the core region interior of the fuel bundles. The flow region 
interior of the fuel bundles is at a higher pressure than the bypass 
region. Typically, water is forced to circulate through the fuel bundles 
by pumping. The flow region exterior of the fuel bundles contains 
non-boiling water and is used to provide increased presence of water for 
the moderation of high speed neutrons to low speed neutrons so that the 
chain reaction in the boiling water reactor can continue. 
Fuel bundle construction can be summarized in a simplified format 
sufficient for the understanding of this invention. A fuel bundle consists 
of a group of fuel rods between an upper tie-plate and lower tie-plate. 
The upper tie-plate and the lower tie-plate and the fuel rods extending 
therebetween are provided with a polygon section, which section is 
preferably square. This section is surrounded by a water impervious 
channel which forms a water tight boundary from the lower tie-plate to the 
upper tie-plate. 
In a BWR, the fuel channels perform three distinct and separate functions. 
First, the channels form individual coolant cells in which the fuel rods 
or fuel assemblies are located, thus separating boiling coolant from 
moderator coolant within the core region. Second, adjacent channel sides 
form the control rod blade guiding annulus. Third, the channels position 
and laterally support the fuel assemblies. 
BWR fuel assembly size has remained basically the same for approximately 
three decades. A typical size is 5.518 by 5.518 inches square by 166.9 
inches long. Initially, bundle size was determined by the capability of 
the reactivity control system to keep the core in a state of cold 
shut-down with sufficient reactivity margin, considering one control rod 
projecting out of the core. This size was appropriate considering the 
known D-lattice, C-lattice and N-lattice designs in which one cruciform 
control blade is inserted between every tour bundles in the core. More 
recent advances in the use of burnable poisons and axial enrichment 
variation have made it possible to increase the fuel assembly size beyond 
the typical BWR size. Furthermore, by designing the core such that two 
control blades are adjacent to two opposite corners of the fuel assembly 
during shut-down, the bundle size can be increased even more from the 
viewpoint of reactivity control. This arrangement is called the K-lattice 
core. The bundle width for this application is approximately 6.375 inches 
or slightly bigger than the bundle width for other lattices as noted 
above. 
Recently, there has been interest in further increasing the size of the BWR 
fuel bundle for future BWR plant designs. The motivation for increasing 
the size is to reduce the total number of control rod drives (CRD's) 
required for reactivity control, and to reduce the amount of fuel handling 
and shuffling during the refueling outage. Bundle widths as much as two 
times the typical existing BWR bundle pitch are under consideration. The 
invention here explains how such an enlarged bundle can be constructed so 
that fuel design issues including channel bulge and availability of two 
phase flow thermal-hydraulic test data are addressed. 
DISCLOSURE OF THE INVENTION 
In accordance with this invention, a large fuel channel design is provided 
which consists of a large channel with inner channels or cross ties which 
divide the large channel into four quadrants. Each quadrant serves as a 
cell for a sub-fuel bundle. Each sub-fuel bundle is similar in size to a 
traditional BWR fuel bundle. By arranging four sub-fuel bundles within a 
single channel, and by arranging four of these enlarged channels about 
each CRD, it will be appreciated that the total number of required CRD's 
can be reduced significantly. 
The lower section of the large fuel channel has an attached inter-bundle 
fuel support to vertically hold the sub-fuel bundles. Thus, each channel 
and inter-bundle fuel support serves as a "basket" for the four sub-fuel 
bundles. 
Two embodiments of the invention are described herein, each varying only 
slightly in mechanical design from the other. One approach is referred to 
as the "water cross type bundle approach" while the other is referred to 
as the "homogeneous bundle approach". 
The water cross type bundle approach includes a large channel divided by 
inner channel pans or cross ties which divide the channel into four 
discrete quadrants. These inner channel parts or cross ties provide or 
define a cruciform passage centrally located vis-a-vis the channel 
(isolated from the sub-fuel bundles), and through which coolant is adapted 
to flow. One feature of this water cross type bundle is that water gaps 
that are maintained on all four sides of each sub-bundle similar to 
existing BWR lattice configurations in the core. Specifically, the water 
cross separates the sub-fuel bundles from each other, axially along the 
entire length of the channel. At the same time, the spacing between 
adjacent large bundles within the core is the same as the spacing between 
the sub-bundles of each large bundle, as defined by the water cross. It is 
this arrangement that provides substantially equal cooling flow along all 
four sides of each sub-fuel bundle. In addition, the arrangement of CRD's 
between the various large bundles is such that opposite corners of each 
large bundle are bracketed by a pair of CRD blades or wings. 
Four inlet nozzles are provided (one for each sub-bundle) in the 
inter-bundle support plate, simplified with coolant via a common inlet 
opening. This first embodiment has the ability to orifice each sub-bundle 
separately in order to assure good thermal-hydraulic-nuclear stability. 
The homogeneous type bundle has no water gaps and no water cross within its 
lattice structure, and instead is a "homogeneous" lattice of fuel and 
water rods. For structural support of the large fuel channel, however, 
relatively thin, vented partitions are used to provide strength to the 
channel, and to again divide the bundle into four quadrants or cells for 
the sub-bundles. Flow communication holes in the partitions provide 
pressure equalization among the sub-bundles to assure good stability 
characteristics. The partitions also assure good critical power ratio 
margin for the large bundle based on availability of thermal analysis 
basis data. 
Thus, in accordance with a broad aspect of the invention, there is provided 
a fuel bundle assembly for a boiling water nuclear reactor comprising an 
open ended tubular channel subdivided into four quadrants by at least two 
interior partitions, each quadrant having a sub-fuel bundle assembly 
comprising a plurality of fuel rods extending between upper and lower tie 
plates; an inter-bundle support plate receiving a lower end of the channel 
and having four flow openings at an upper end thereof, the lower tie plate 
of the sub-fuel bundle supported in a respective on of the openings. 
The channel design described herein reduces the time required for fuel 
handling by allowing roughly four times more fuel to be moved per lift 
compared to current BWR fuel channel designs. In addition to improving 
fuel handling, the large channel design has an increased load carrying 
capability under seismic events, and is structurally more resistant to 
channel bow and bulge deformation. It will also be appreciated that the 
new large channel design in accordance with this invention supports the 
three distinct and separate functions of conventional channels, discussed 
hereinabove. 
Advantages of the invention in addition to those described above, will 
become apparent from the detailed description which follows.

BEST MODE FOR CARRYING OUT THE INVENTION 
With reference to FIG. 1, part of a Boiling Water Reactor (BWR) core 10 is 
shown, including a cruciform Control Rod Drive (CRD) 12 projecting 
upwardly through a core plate P. The CRD 12 is centrally located relative 
to four fuel rod bundle assemblies, only one of which is shown at 14 in 
FIG. 1, but see also FIG. 2. 
The assembly 14 includes a large, open-ended tubular channel 16 provided 
with inner channels or cross ties 18, 20, 22 and 24 which divide the 
channel 16 into four corresponding quadrants A, B, C and D. The large 
channel 16 is substantially square in section, but with rounded corners. 
Each quadrant serves as a cell for a sub-fuel bundle 26 of otherwise 
conventional BWR bundle construction (including, in the embodiment shown, 
64 fuel rods including 6 which also serve as tie rods), except as noted 
below. Thus, unlike the conventional arrangement where each bundle 26 
would have its own fuel channel, the present invention utilizes a single 
large channel 16 to embrace as many as four sub-fuel bundles 26. Thus, it 
will be appreciated that the substantially square channel 16 may have side 
dimensions about twice that of the conventional channel. 
In this first embodiment, also referred to as a water cross type bundle, 
the inner cross ties 18, 20, 22 and 24 are arranged to provide a cruciform 
moderator passage (or water cross) 28 between the sub-fuel bundles 26, and 
specifically between the cross ties 18, 20, 22 and 24, with moderator 
introduced into the passage 28 via ports 30 located in the inter-bundle 
fuel support described below. Each cruciform passage 28 is provided with a 
plurality of reinforcements 29 in each section of the passage, and 
arranged at vertically spaced intervals along the length of the channel. 
The reinforcements 29 and cross ties 18, 20, 22 and 24 serve to reinforce 
the channel 16 and thus resist undesirable channel bowing or bulging. 
It should be noted that in the arrangement shown in FIG. 4, four large 
channels 16 and associated group of four sub-fuel bundle assemblies 26 are 
shown in place about the CRD 12 which includes four wings or blades in 
mutually perpendicular pairs. Additional CRD's are shown at 32, 34, 36 and 
38 to illustrate the manner in which two control rod blades are located 
adjacent two opposite corners of each large channel 16. At the same time, 
it may be seen that each sub-fuel bundle is cooled by in-channel flow on 
two sides and out-of-channel flow on the remaining two sides. By 
maintaining spacing between adjacent channels 16 substantially equal to 
the spacing between the inner cross ties 18, 20, 22 and 24, the flow gap 
or space on all four sides of each sub-fuel bundle is essentially the 
same, similar to existing BWR lattice configurations. 
Turning now to FIG. 3, it will be seen that the large channel 16 is 
supported on an inter-bundle fuel support plate 40 which, in turn, is 
supported on a cup 42 (FIG. 1). The latter is fixed to the core plate P. 
With regard to FIG. 4, it is initially apparent that each sub-fuel bundle 
26 comprises a plurality of fuel rods 46 supported at their lower ends by 
a fuel rod supporting grid 48 of a lower tie plate assembly 50. The fuel 
rods 46 are secured at their upper ends by an upper tie plate 51 (see 
FIGS. 1 and 2). The lower inlet nozzle portion 52 of each lower tie plate 
50 is circular in shape, and is seated within a corresponding opening 54 
in the inter-bundle support plate 40. With reference also to FIG. 6, it 
may be seen that each plate 40 is provided with four such openings 54 to 
accommodate the lower tie plates of the four sub-fuel bundles supported on 
each plate 40. Plate 40 is otherwise of generally square shape and of a 
cross sectional size similar to the large channel 16. 
The four openings 54 in plate 40 each taper downwardly and inwardly along 
surfaces 56 such that the vertical centerlines of the openings 54 are 
offset relative to the vertical centerline of a common inlet opening 62, 
such that coolant flow is caused to change direction as it flows upwardly 
through the plate 40 into the individual sub-fuel bundles 26. Upstream of 
the openings 54, a common inlet opening 62 as defined by the mounting 
sleeve portion 65 of the plate 40 supplies liquid coolant to the four 
openings 54. This sleeve portion 64 is formed exteriorly with a tapered 
annular seating surface at 66 and a horizontally oriented seating flange 
68 which engages complementary surfaces 70, 72, respectively, on the 
support cup 42, fixed to the core plate P and seated within the plate P in 
a similar manner. 
The openings 54 of the support plate 40 are surrounded by tapered seating 
surfaces 74 and a horizontal top surface 76 upon which mating surfaces 78, 
80, respectively, of the lower tie plates 50 are seated. The large channel 
16 is received over the upper edge 82 of peripheral side wall 84 of the 
support plate 40, as best seen in FIG. 4. 
With reference also to FIGS. 4-6, the top of the support plate is also 
formed with a cruciform groove or slot 86 which receives the inner 
channels or cross ties 18, 20, 22 and 24 and through which coolant is 
supplied via ports 30 (FIG. 2). 
Utilizing handles 90 secured to the upper end of the large channel 16 as 
shown in FIG. 1, the entire sub-fuel bundle assembly 14 including channel 
16 and four sub-fuel bundle assemblies 26 can be lifted from the support 
cup 44 (as shown in FIG. 5) during refuel or repair procedures. 
Turning now to FIGS. 7-11, a second embodiment of the invention is 
illustrated wherein a homogeneous type bundle approach is employed. In 
this embodiment, like reference numerals, but with the prefix "1" added, 
are used to designate components corresponding to those in the first 
described embodiment. A prime (') is used for corresponding alpha 
characters. In the homgeneous type bundle 114, no water cross 28 is 
provided between the individual sub-fuel bundles 126 within the large 
channel 116. Rather, a pair of mutually perpendicular, vented partitions 
118, 120 (which may be welded together as a cruciform unit prior to 
insertion within the channel 116) are utilized between the sub-fuel 
bundles 126 to provide structural support to the channel. Thus, while the 
vented partitions 118, 120 divide the bundle into four quadrants A', B', 
C' and D', no water cross is provided. Rather, the partitions 118, 120 are 
each formed with flow communication holes (preferably in the form of a 
regular array or pattern of holes 92 (see FIG. 9) along the entire length 
of each partition 118 and 120 in the nature of a perforated sheet) to 
provide pressure equalization among the sub-fuel bundles 126 and thus 
assure good stability characteristics. These cross ties or partitions 118, 
120 also assure good critical power ratio margin for the large bundle, 
based on availability of thermal analysis basis data. 
With reference to FIGS. 9 and 10, it will be seen that a cruciform groove 
or slot 186 is provided in the support plate 140 for the vented partitions 
118, 120. Otherwise, the construction of the plate 140 and support cup 144 
is similar to that previously described in connection with the first 
embodiment. 
While the invention has been described in connection with what is presently 
considered to be the most practical and preferred embodiment, it is to be 
understood that the invention is not to be limited to the disclosed 
embodiment, but on the contrary, is intended to cover various 
modifications and equivalent arrangements included within the spirit and 
scope of the appended claims.