Hafnium control rod for nuclear reactors

A neutron absorbing control device for service in nuclear reactors utilizing fissionable fuel. The control device includes hafnium metal as the neutron absorbing material which is employed in a unique structure which maximizes the advantages of hafnium while minimizing its disadvantages as well as providing other benefits.

FIELD OF THE INVENTION 
This invention relates to an improved control rod construction, utilizing 
hafnium metal as the neutron absorber, for service in nuclear reactors 
having a core of fissionable fuel. 
BACKGROUND OF THE INVENTION 
Commercial nuclear fission reactors for generating power normally comprise 
a core of fissionable fuel wherein the fuel material is sealed within 
tube-like metal containers. These tubular containers with the fuel are 
arranged or grouped in discrete bundles or units, which frequently are 
enclosed within an open ended housing known as a "channel" in the nuclear 
fuel industry. The discrete fuel bundles are assembled for service within 
the nuclear reactor to provide the core in predetermined patterns The 
assembled bundles are spaced apart from each other so as to provide 
intermediate gaps between each bundle, forming a surrounding area for the 
flow of coolant thereabout and also the insertion of reactor control means 
comprising neutron absorbing material. 
Nuclear reactor control means typically consist of components containing 
neutron absorbing compositions which are reciprocally movable in relation 
to the core body of neutron emitting fuel undergoing fissions. The rate of 
the fission reaction, and in turn heat generated, is regulated by 
governing the availability of fission produced neutrons for furthering the 
fission reaction and determining the magnitude of the reaction. 
In a conventional nuclear reactor, fissionable atoms such as uranium 
isotopes and plutonium absorb neutrons in their nuclei and undergo a 
nuclear disintegration or splitting. This fission produces on the average 
of two products of lower atomic weight and greater kinetic energy, and 
typically two or three neutrons, also of high energy. 
The fission neutrons thus produced diffuse through the core containing 
fissionable fuel and they are either utilized or lost in several distinct 
competing mechanisms. Some neutrons may migrate to the boundaries of the 
core and escape whereby they are lost from the system. Some neutrons 
undergo nonfission or radiative capture in the fuel material. Other 
neutrons undergo fission capture within the fissionable fuel and thereby 
produce additional fission neutrons, the so-called chain reaction. Namely, 
fast neutrons are captured in the uranium 235 and 238, while thermal 
neutrons are captured in uranium 235. Still other neutrons undergo 
parasitic capture in the various extraneous or nonfissionable compositions 
of the core and adjoining components such as the moderator, coolant, 
various structural materials, fission products produced within the fuel, 
as well as the reactor control elements. 
The balance between the fission production of neutrons and the various 
competing mechanisms for neutron consumption determine whether the fission 
reaction is self-sustaining, decreasing, or increasing. When the fission 
reaction is self-sustaining, the neutron multiplication factor equals 
1.00, the neutron population remains constant, and on average there is one 
neutron remaining from each fission event which induces a subsequent 
fission of an atom. 
Heat produced by the fission reactions is thereby continuous and is 
maintained as long as sufficient fissionable material is present in the 
fuel system to override the effects of fission products formed by the 
reaction, some of which have a high capacity for absorbing neutrons The 
heat produced by the fission reactions is removed by a coolant such as 
water, circulating through the core in contact with the tubular containers 
of fuel and conveyed on to means for its utilization, such as the 
generation of electrical power. 
The neutron population, and in turn the heat or power produced, of a 
nuclear reaction, depends on the extent to which neutrons are consumed or 
wasted by capture in nonfissionable material. Neutron consumption of this 
nature is regulated by governing the relative amount of neutron absorbing 
control material imposed into the core of fissionable fuel undergoing 
fission reactions. 
Control devices comprising elements containing neutron absorbing material, 
are commonly provided in the form of rods, sheets or blades. The elements 
are provided with mechanical or fluid operated means for reciprocal 
movement into and out from the core of fissionable fuel to any appropriate 
extent or depth for achieving the desired neutron population, and in turn, 
level of reaction 
Common neutron absorbing materials include elemental or compound forms of 
boron, cadmium, gadolinium, europium, erbium, samarium, hafnium, 
dysprosium, silver and indium. 
Commercial nuclear reactors for power generation are of such a magnitude 
that the control means, or systems, comprises a plurality of control units 
or rods. Each individual control unit or rod is selectively and 
reciprocally insertable to variable degrees of penetration into the fuel 
core by movement intermediate the discrete bundles of grouped tubular fuel 
containers through the spaces or gaps provided throughout the assembly of 
multiple fuel bundles. A common design for control rods, as shown in U.S. 
Letters Pat. No. 3,020,888, consists of an element having four blades, 
comprising sheaths containing neutron absorbing material, having a cross 
or cruciform cross section, whereby the four blades radially project at 
right angles to each other. With this design configuration, each control 
rod element is insertable into the spaces between four adjacent fuel 
bundles of the core assembly, and regulates the neutron flux or density 
emitted from the fissioning fuel of the four bundles. 
The construction designs, materials, operating mechanisms and functions of 
typical control mean for water cooled and moderated nuclear fission 
reactors for commercial power generation are illustrated and described in 
detail in the prior art, for example, U.S. Letters Pat. No. 3,020,781; No. 
3,020,888; No. 3,217,307; No. 3,395,781; No. 3,397,759; No. 4,285,769; No. 
4,624,826; and No. 4,676,948, and elsewhere throughout the literature 
dealing with nuclear reactors. The contents of the foregoing prior art 
patents are incorporated herein by reference. 
SUMMARY OF THE INVENTION 
This invention comprises an improved control device for nuclear fission 
reactors comprising a combination of a unique construction and application 
of a material composition for service in water cooled and moderated 
nuclear fission reactors. 
OBJECTS OF THE INVENTION 
It is a primary object of this invention to provide an improved control 
device for nuclear fission reactors which provides for the effective 
utilization of hafnium metal through reduced weight and costs. 
It is also an object of this invention to provide an improved control 
device for nuclear fission reactors comprising components which enable 
easy construction of units having variable neutron absorbing capacities 
across their length and breadth that selectively match or equate the 
absorptive capacity of the device with the uneven neutron densities in the 
region of service and thereby provide a saving in eliminating unneeded 
absorbent material. 
It is a further object of this invention to provide an improved control 
device for nuclear fission reactors which provides for flow of coolant 
within the control device means whereby the presence of water in liquid 
form is maximized to enhance neutron moderation and entrapment, comprising 
a so-called neutron "flux trap". 
It is a still further object of this invention to provide an improved 
control device which overcomes the adverse effects of structural 
distortion or induced stresses caused by differences in thermal expansion 
and/or irradiation growth resulting from different material and/or 
regional variations in radiation levels. 
It is another object of this invention to provide a control device which 
provides for maintaining tolerances in the structure of the neutron 
absorbent material for consistent performance.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1 of the drawings, this invention is hereinafter 
described and illustrated with reference to a common commercial design for 
nuclear fission reactor control devices, wherein the control elements are 
of cruciform cross section, and the preferred embodiment. Control devices 
with cruciform control elements and their utilization with fuel core 
assemblies are shown and described in the prior art comprising the above 
cited U.S. Letters Patents. 
Control device 10 comprises a base 12 which is coupled to a suitable 
control device drive mechanism (not shown), and supports a frame 14 of the 
neutron absorbing element including an upper support member 16 and a lower 
support member 18 and an elongated central spine support 20, or tie rod, 
connecting said upper and lower support members. Upper support member 16 
can also function as a handle to facilitate transportation and 
manipulation of the device. 
In the preferred cruciform embodiment for the element of this invention, 
the upper and lower support members 16 and 18, respectively, each comprise 
four radially extending arms projecting at about 90 degrees with respect 
to adjacent arms to form the cross. Central spine support 20, connecting 
the upper and lower support member 16 and 18, is preferably also a 
cruciform configuration with four abbreviated arms of relatively short 
radial extension in relation to the arms projecting from the upper and 
lower support members 16 and 18. 
The four radially extending arms projecting from the upper and lower 
support members 16 and 18, and the four abbreviated radial arms of the 
central spine support 20, are each respectively aligned in a plane with 
their counterpart to provide a cross configuration. The four arms of the 
upper and lower support members 16 and 18 are also substantially 
counterminous with respect to each other. 
A metal sheath 22 extends from each arm of the upper support 16 to each 
respective counterpart arm of the lower support 18 and adjoins the central 
elongated spine support 20 along its length. Sheath 22 typically comprise 
a U-shaped sheet metal housing of blade-like configuration and an internal 
width comparable to the thickness of the arms of the upper and lower 
supports. Preferably, each sheath is secured to its respective adjoining 
arms of the upper and lower support members 16 and 18, and also to the 
central spine support 20, by suitable means such as welding. Sheath 22 is 
also provided with a plurality of orifices 24 for the passage of coolant 
water. 
The foregoing structure of the control device of this invention is typical 
of common commercial control means in service in operating nuclear 
reactors. The structure of the foregoing frame 14 and its components are 
normally constructed of stainless steel or similar corrosion resisting 
metals. 
In accordance with this invention, the neutron absorbing component of the 
control device element comprises the combination of hafnium metal utilized 
in the specific form of a plurality of flattened hollow tubes provided 
with orifices in the tube walls substantially as illustrated. The 
flattened hollow tubes comprise a structure of two substantially parallel 
sides of various predetermined thicknesses in close proximity which are 
joined together along their length with arcs of small radius. 
Referring to the drawings, in particular FIGS. 1, 2 and 3, a plurality of 
flattened hollow tubes 26 of hafnium metal are arranged parallel with each 
other and vertically aligned with the central spine support 20 within each 
sheath 22 of the control device, such as tubes 26, 26.sup.I and 26.sup.II. 
The flattened hollow tubes of hafnium 26 are preferably supported within 
the sheath housing by suitable attachment to the respective arm of the 
upper support member 16. Measures for affixing the hafnium tubes to the 
upper support member 16 are disclosed in U.S. Letter Pat. No. 4,676,948, 
and one advantageous means is illustrated in FIG. 5. As shown, a 
complementing hooking unit 28-28.sup.I is provided by securing, such as by 
welding, one hook component 28 to the top of the hafnium tube 26 and the 
other hook component 28 to the upper support member 16. Such an 
arrangement enables easy replacement as well as initial assembly. 
The plurality of hafnium neutron absorber tubes 26 affixed to the upper 
support member 16 within the sheath 22 extend downward substantially to, 
but preferably short of contact to the lower support member 18. Thus free 
hanging, the hafnium tubes can elongate due to thermal expansion and/or 
irradiation growth without imposing any stresses or other distorting 
forces upon the frame members. 
As shown in FIGS. 2 and 3, in accordance with this invention, the wall 
thickness, and in turn mass, of each flattened side of the tube of hafnium 
metal neutron absorber 26 assembled within each sheath 22 can be varied 
and adjusted to selectively match or equate the neutron absorption 
capability of the element extending along the outward reach of the 
radially projecting absorbing blade to the uneven neutron flux conditions 
encountered along its surface in service. Generally, the neutron flux 
density is greatest at the outermost extremity or periphery of the blades, 
and least in an intermediate area of their outward reach. Thus, the 
neutron absorbing mass is designed to correspond to the needs of the 
varying neutron flux field. This aspect of the invention provides for 
customizing the neutron absorbing mass of the control element to provide 
ample neutron absorbing capacity to perform its designed function without 
over applying an excess of costly and heavy hafnium. 
Similarly, as shown in FIG. 4, the side wall thickness, and in turn mass, 
of each tube of hafnium metal neutron absorber 26 assembled in each sheath 
22 can be varied and adjusted to selectively match or equate the neutron 
absorption capability of the absorbing blade along its length from top to 
bottom of the element for the same objective. Sequentially varying or 
gauging the absorber tube side wall thickness in any effective arrangement 
to accommodate variable neutron flux conditions along the length of the 
element can be provided for by uniting several sections of flattened 
hollow tubes of hafnium having different wall thickness. Suitable means 
for achieving this aspect of the invention comprises welding a series of 
two or more segments of tubing abutting end to end to join the segments 
into a continuous unit. This vertical customizing to match or equate the 
neutron absorption capacity to the neutron flux or density pattern 
encountered over the sheath or blade surface also avoids the unnecessary 
application of an excess of costly and heavy hafnium metal beyond the need 
of the variable neutron flux field. 
The control device sheath 22 and the hafnium metal tubes 26 are both 
provided with a plurality of openings or orifices 24 and 30 through their 
walls, providing for passage or flow of ambient fluid into and out from 
their interior area. Enabling entry and the presence of coolant water 
within the flattened hollow tubes 26 of hafnium metal provides an arranged 
combination of substance media which forms a so-called neutron "flux 
trap", a more effective means for reducing the energy of and capturing 
neutrons. 
Long service life control rod devices in water cooled and moderated fission 
reactors utilize a principle of operation wherein the neutron material is 
concentrated in a small region to provide for neutron self-sheilding. The 
self-shielding effect delays absorption of neutrons inside the absorber 
until significant absorptions have occurred at the outer surface regions. 
This delay increases the service life of the control device. 
The principle underlying the so-called "flux trap" is the removal of some 
of the absorber material and to replace it with moderator, and the 
absorber material must be removed from within the absorber unit. 
This invention incorporates and enhances the neutron "flux trap" system by 
providing for the flow of water coolant/ moderation into and through the 
hollow interior of the hafnium metal tubes. In the "flux trap" of the 
construction of this invention, the higher energy neutrons which pass 
through the metal absorber wall initially without any interaction are 
subsequently moderated, i.e., slowed down in energy level, by the internal 
water moderator and then are absorbed in the inside surface area of the 
hollow absorber tubes. This "flux trap" effect compensates for the 
reduction in absorber material due to the hollow interior of the hafnium 
tube, and provides for the unit to maintain the same reactivity worth as 
control device of solid metal design. Thus, there is a significant benefit 
in reduced weight and costs, especially with hafnium metal which is 
extremely heavy and expensive.