Nuclear fuel bundle with coolant bypass channel

A fuel bundle for a natural-circulation boiling-water reactor includes at least one coolant bypass tube which is at least partially open at both its top and bottom and extends about 2/3 of the bundle height. Water within the bypass tube remains liquid. As it exits the top of the bypass tubes, it merges with and "cools" the flow exterior to the tube. This arrangement reduces the pressure drop across the core, increasing coolant flow. In addition, the merging coolant helps improve heat transfer at the maximum heat flux levels within the core. Lateral holes through the bypass tube can further enhance with heat transfer distribution in the core. The reduced pressure drop in the core and the more uniform heat flux distribution both permit a reactor to operate at higher power ratings.

BACKGROUND OF THE INVENTION 
This invention relates to nuclear reactors and, more particularly, to a 
fuel bundle for a boiling-water nuclear reactor. A major objective of the 
present invention is to provide for greater power output for a boiling 
water reactor of a given size. 
Fission reactors rely on fissioning of fissile atoms such as uranium 
isotopes (U233, U235) and plutonium isotopes (Pu239, Pu241). Upon 
absorption of a neutron, a fissile atom can disintegrate, yielding atoms 
of lower atomic weight and high kinetic energy along with several 
high-energy neutrons. The kinetic energy of the fission products is 
quickly dissipated as heat, which is the primary energy product of nuclear 
reactors. Some of the neutrons released during disintegration can be 
absorbed by other fissile atoms, causing a chain reaction of 
disintegration and heat generation. The fissile atoms in nuclear reactors 
are arranged so that the chain reaction can be self-sustaining. 
To facilitate handling, fissile fuel is typically maintained in fuel 
elements. Typically, these fuel elements have a corrosion-resistant 
cladding. The fuel elements can be grouped together at fixed distances 
from each other in a fuel bundle. The fuel bundles include spacer grids to 
maintain alignment and spacing of the fuel bundles. A sufficient number of 
these fuel bundles are combined to form a reactor core capable of a 
self-sustaining chain reaction. Neutron-absorbing control rods are 
inserted into the core to control the reactivity of the core. The 
reactivity of the core can be adjusted by incremental insertions and 
withdrawals of the control rods. 
In a boiling-water reactor (BWR), heat generated in the core is transferred 
by water flowing up through the core. Some of the water is converted to 
steam which can be extracted from the reactor vessel. The extracted steam 
can be used to drive a turbine, which in turn can drive a generator to 
produce electricity. Water not converted to steam is recirculated back to 
the base of the core. 
In a BWR, water serves not only as a coolant but also as a moderator. In 
its role as moderator, the water slows the initially fast neutrons 
released during fissioning. The slowed or "thermal" neutrons have the 
appropriate energies for absorption by fissile fuel to produce further 
fissioning. Steam, because of its lower density, is a much poorer 
moderator than liquid water. As the water flows up through the core, the 
percentage of steam increases, so that moderation becomes less effective. 
Accordingly, some fuel bundles include coolant bypass channels which 
insulate 1%-2% of the water from the most intense heat generated at the 
fuel elements. These coolant bypass channels, which are generally in the 
form of a tube extending from the base to the top of the fuel bundle, 
provide moderation through the total vertical extent of the bundles. This 
insures sufficient liquid moderator at all levels within the fuel bundle. 
One problem with this bypass approach is that a percentage of the coolant 
flow is used exclusively for the moderator function. An alternative design 
uses a convoluted partial height bypass channel. Water flowing up a tube 
is partially forced into a second interior tube. The outer tube is closed 
at the top, so water emerging from the top of the interior tube is forced 
downward and out peripheral holes. 
Forced-circulation boiling-water reactors (FCBWRs) use pumps to promote 
water circulation, while natural-circulation boiling-water reactors 
(NCBWRs) rely on convection to promote water circulation without pumps. A 
typical NCBWR employs a chimney over its core to support a driving head. 
The driving head establishes a pressure differential between the region 
above the core and the downcomer. The downcomer is the annular space 
within the reactor vessel to the outside of the core and the chimney. The 
downcomer defines the path along which water exiting the chimney returns 
to the core. The pressure differential between the core and chimney on the 
one hand and the downcomer on the other determines the recirculation rate. 
The recirculation rate determines the maximum power that can be 
transferred from the core, and thus the maximum power output of the 
reactor. 
One way to increase the power capability of a NCBWR is to increase chimney 
height. A taller chimney supports a greater driving head, which in turn 
supports a greater pressure differential. The resulting increased coolant 
flow permits more power to be transferred from the core. 
However, increasing chimney height requires a larger reactor vessel. A 
larger reactor vessel requires a larger reactor containment complex. 
Reactor complex costs and complexity increase geometrically with chimney 
height. Basic changes, such as increasing chimney height, can only be 
applied prospectively. Such changes do not address increasing the 
performance of existing reactors of the forced-circulation type. 
What is needed is a design which permits increased power output without 
increasing reactor size and complexity. This design should be applicable 
to new NCBWRs. Preferably, the improvement should also be applicable, on a 
retrofit basis, to enhance the value of existing FCBWRs. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a fuel bundle for a NCBWR 
includes at least one partial-length coolant bypass tube which is open at 
the top. The bypass tube extends from near the bottom of the fuel bundle 
to near the critical heat flux level, which can be 40%-85% toward the top 
of the bundle. The alignment and spacing of the bypass tube can be 
maintained by the same spacer grids used to support the fuel rods. A 
single central coolant bypass tube can be used. Alternatively, coolant 
bypass tubes can form a subarray of a larger array including the fuel 
elements. For example, the coolant bypass tubes and the fuel elements can 
define a square array, with the bypass tubes constituting a square 
subarray of the overall array. Preferably, at least 4% of the water 
flowing through the bundle flows through the one or more bypass tubes. 
Water within a bypass tube remains liquid, as heat transfer between fuel 
elements through the main coolant flow and the bypass tube wall is limited 
due to a relatively small temperature gradient. Thus, the coolant bypass 
tube allows liquid water to merge with a water/steam mixture to improve 
moderation and heat transfer characteristics near the top of the core, 
where they are needed most. The water from the coolant bypass tube mixes 
with the exterior water, thereby "cooling" it in the sense of lowering its 
specific enthalpy. The benefits of the introduction of bypass coolant can 
be obtained to a lesser extent below the top of the bypass tubes using 
apertures along its vertical extent. These apertures slow the rate of 
boiling outside the tube. These apertures are located just above any 
nearby spacer grids to minimize turbulence-induced vibrations of the fuel 
rods. A constriction near the top of the tube can be used to force some 
water out of the lateral apertures. 
Water flowing through the bypass tube can flow vertically to the top level 
of the bypass tube without encountering major sources of turbulence, such 
as spacer grids. While the concentric tube coolant channel mentioned above 
also bypasses spacer grids, it introduces additional flow impedance by 
requiring water to reverse direction and then flow laterally before 
merging with the flow outside the bypass tube. By decreasing turbulence 
and flow reversals, the present invention reduces the pressure drop in the 
core. The reduction is increased by admitting a relatively high percentage 
(4% or more) of coolant into the bypass tubes. 
The reduced pressure drop translates into a greater pressure differential 
between the core-chimney region and the downcomer. Hence, coolant flow is 
increased and more power can be transferred from the core, thus providing 
for increased power output capability for an NCBWR. By embodying the 
present invention in a fuel bundle having the same form factor as existing 
fuel bundles, increased power output of existing reactors can be achieved. 
Moreover, the present invention provides for greater moderation and less 
flow impedance than is provided by fuel bundles without coolant tubes. 
Compared to fuel bundles with full-length coolant channels, the present 
invention provides for full utilization of coolant for heat transfer, 
rather than dedicating a portion of the flow for moderation only. As a 
result and relative to fuel bundles having full-length bypass tubes, the 
present invention provides for a smaller volume of steam adjacent to fuel 
elements. The reduced steam fraction increases the flow cross section 
available to the water. This also reduces the pressure drop through the 
core, enhancing power transfer. The coolant bypass channel also enhances 
the nuclear and thermohydraulic stability of the fuel bundle, as the flow 
through this channel tends to smooth out density fluctuations at the level 
where the coolant bypass flow enters the main coolant flow. 
Thus, the present invention provides increased power capacity relative to 
the various prior art NCBWR fuel bundles. Furthermore, this improved fuel 
bundle design can be incorporated in existing and prospective FCBWRs to 
obtain increased power density. These and other features and advantages of 
the present invention are apparent in the following description with 
references to the drawings below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiment of the present invention is designed for use in a 
nuclear reactor 100 comprising a reactor vessel 102 and its internals, as 
shown in FIG. 1. Heat is generated within a core 104 of reactor 100, which 
includes fuel bundles 106 of fissile material. Water circulated up through 
core 104 is at least partially converted to steam. A steam separator 108 
separates steam from water, which is recirculated. Residual water is 
removed from the steam by steam dryers 110. The steam then exits reactor 
100 through a steam exit 112 near a vessel head 114. 
The amount of heat generated in core 104 is regulated by inserting and 
withdrawing control blades 116. Control blades 116 are vertically 
extending elements with cruciform cross sections. They include rods of 
neutron-absorbing material, such as boron-carbide or hafnium. To the 
extent that a control blade 116 is inserted into core 104, it absorbs 
neutrons that would otherwise be available to promote the chain reaction 
which generates heat in core 104. Control rod guide tubes 118 below core 
104 maintain the vertical motion of control blades 116 during insertion 
and withdrawal. 
Fuel bundles 106 are supported from below by a fuel support casting 120 
mounted on a core support plate 122 located at the base of core 104. A top 
guide 124 helps align fuel bundles 106 as they are lowered into core 104. 
Vessel 102 is mounted on a concrete pedestal 126 which defines a space 
below where access can be had to control rod drives 128. 
As shown in FIG. 2, one of the fuel bundles 106 includes a housing 201, 
lower inlet end 202 and an upper outlet end 204. A grip 206 permits bundle 
106 to be manipulated into and out of core 104. Fuel bundle 106 includes 
sixty fuel pins 208 arranged in an 8.times.8 array, with the middle four 
positions of the array occupied by a coolant bypass tube 210, as shown in 
FIGS. 2 and 3. The vertical alignment and spacing of fuel pins 208 are 
provided by seven spacer grids 212. Spacer grids 212 include grid plates 
214 and springs 216. Springs 216 are mounted on plates 214 and flexibly 
support fuel pins 208. Bypass tube 210 is supported by grid plates 214, 
which are about 0.3 millimeters thick. Fuel pins 208 are coupled to a top 
plate 218 through coil springs 220 to accommodate thermal expansion. 
Moreover, fuel pins 208 and bypass tube 210 can slide relative to spacer 
grids 212 to accommodate thermal expansion. 
Fuel pins 208 are partially filled with fissile fuel 222. Near the top of 
fuel pins 208 are plenums 224 to accommodate gaseous fission byproducts. 
Collectively, fuel pins 208 define a lowest level 226 of fuel and a 
highest level 228 of fuel. The lowest and highest levels define a fuel 
extent 230 for fuel bundle 106. Typically, a critical heat flux level 232 
appears about 2/3 up along this fuel extent 230. The top 234 of bypass 
tube 210 is at about this critical heat flux level 232. 
The introduction of bypass coolant into the main coolant flow at the top of 
bypass tube 210 induces a sudden change in the steam fraction along the 
vertical extent of bundle 106. To achieve a more uniform distribution of 
steam fraction by height, apertures 236 are formed in tube 210. The 
diameter of each aperture 236 is empirically determined to optimize the 
steam fraction profile. Alternatively, diameters can be calculated using 
known techniques for a typical core configuration and typical operational 
requirements. Each aperture 236 is situated just above one of the spacer 
grids 212 so that minimal vibrations are induced by water exiting bypass 
tube 210 through the aperture 236. In order to force bypass coolant 
through apertures 236, a partial closure 238 is used to define a 
constriction near the top of bypass tube 210. 
An alternative fuel bundle 400 in accordance with the present invention 
includes a bundle housing 402, four spacer grids 404 (one of which is 
shown), seventy-two fuel elements 406 and nine coolant bypass tubes 408, 
as shown in FIG. 4. Collectively, fuel elements 406 and bypass tubes 408 
are arranged in a square 9.times.9 array, with bypass tubes 408 arranged 
in a 3.times.3 subarray. Spacer grids 404 include spacer plates 410 and 
springs 412. Springs 412 flexibly align and space fuel elements 406. 
Spacer plates 410 hold springs 412 and support and space bypass tubes 408. 
The vertical relationships between bypass tubes 408 and fuel elements 406 
are the same as in bundle 106. However, bypass tubes 408 do not include 
lateral apertures or a constriction. A major advantage of fuel bundle 400 
is a more uniform distribution of moderation and coolant merging across 
the bundle cross section. 
The specific dimensions of the bypass tubes in either embodiment depends on 
the heat flux profile of the including bundles. For a given coolant flow, 
heat transfer increases with increasing heat flux until a steam film 
develops which limits the conduction of heat from the fuel pins to the 
water. Increasing heat flux beyond this level dramatically decreases heat 
transfer. Operating a reactor at the peak heat transfer level is 
undesirable since a perturbation could cause fluctuations of hundreds of 
degrees Fahrenheit. These fluctuations can cause materials to oxidize and 
stress. To avoid these fluctuations, the reactor is operated on the upside 
of this heat transfer peak. Typically, a safety factor of 1.35 is provided 
for the maximum heat flux level. 
The present invention increases power capacity by increasing the coolant 
flow rate, which permits a higher maximum acceptable heat flux. In 
addition, the bypass tubes are dimensioned to introduce relatively cool 
liquid at the maximum heat flux level. This provides additional latitude 
at this level so that higher power generation can be handled. However, the 
merging of relatively cool bypass coolant with the main coolant flow at 
the top of bypass tube has the effect of moving the peak heat flux below 
the level at which merging occurs. Adding apertures, such as apertures 236 
in bundle 106, further smoothes the vertical heat flux profile, allowing 
higher operating powers without risking excursions beyond the peak heat 
transfer point. 
The present invention provides for fuel bundles of different dimensions, 
which can be selected as a function of the incorporating reactor. 
Different numbers and arrangements of fuel elements are provided for. One 
or more bypass tubes can be utilized. The bypass tubes can have a variety 
of cross-sectional shapes, including circular, square, and triangular. 
Coolant bypass tubes can have their inlets at or below the lowest level in 
the bundle containing fuel. 
The bypass tubes can extend to a level 50%-85% of the fuel extent of the 
bundle. For example, the top of a bypass tube extending to a 66% level 
would be twice as close to the highest level of the bundle having fuel 
than the top is to the lowest level of the bundle having fuel. Different 
types of contrictions can be used in the bypass tubes. Alternatively, the 
diameter of the coolant bypass channels can be reduced. In some cases, no 
constriction is required. Generally, constriction is required where 
lateral apertures are used. In addition, a constriction can be used when 
otherwise inadequate coolant would flow outside the bypass tube. In this 
case, the constriction is preferably at the base of the bypass tube. These 
and other modifications to and variations upon the described embodiments 
are provided for by the present invention, the scope of which is limited 
only by the following claims.