Zr.sub.2 Ni as a getter metal and nuclear reactor fuel element employing such

Zr.sub.2 Ni sorbs both oxygen and hydrogen from water making its use advantageous as a getter metal in a nuclear reactor fuel element.

Nuclear reactor fuel elements are well known. These nuclear reactor fuel 
elements generally comprise a sealed container in the form of a tube 
having two end caps one at each end of the tube. The tube is generally 
constructed of or is clad with zirconium or a zirconium base alloy. Within 
the chamber defined by the tube and the end caps are located pellets of 
fissionable material such as UO.sub.2. During operation of the nuclear 
reactor fuel element, water, generally in the form of water vapor, is 
released. This water vapor reacts with the components present in the tube 
with the undesirable release of hydrogen. Hydrogen is known to react with 
zirconium of the tube causing embrittlement which can proceed to eventual 
failure. In order to minimize this hydrogen embrittlement it has long been 
known to provide nuclear reactor fuel elements with a getter device 
employing a getter metal having a sorbtive capacity for water vapor. One 
such getter metal is a ternary alloy of zirconium, titanium, and nickel as 
disclosed for example in United Kingdom Patent 1,370,208 (1974). 
Unfortunately these ternary alloys selectively sorb oxygen from the water 
vapor with an undesirable release of hydrogen. Stated differently the 
ternary alloys do not sorb all of the oxygen and all of the hydrogen 
present in the water vapor. On the other hand, when sorbing water vapor 
there is an apparent undesirable release of hydrogen with a consequent 
danger of hydrogen embrittlement of the tube. 
It is therefore an object of the present invention to provide an improved 
getter metal and an improved nuclear reactor fuel element substantially 
free of the disadvantages of the prior art. 
Another object is to provide an improved getter metal which sorbs both 
oxygen and hydrogen from water vapor. 
A further object is to provide an improved getter metal which sorbs oxygen 
from water vapor without the release of hydrogen. 
A still further object of the present invention is to provide an improved 
method for reducing hydrogen embrittlement in nuclear reactor fuel 
elements. 
Yet another object is to provide an improved nuclear reactor fuel element 
employing a getter metal of the present invention.

According to the present invention there is provided a process for sorbing 
both oxygen and hydrogen from water comprising contacting the water with a 
getter metal consisting essentially of, and preferably consisting of 
Zr.sub.2 Ni. 
The water sorbed according to the present invention can be liquid water but 
is more often water vapor. The water vapor can be the sole gas or vapor 
present or can be admixed with other gases. Rare gases such as helium or 
argon are preferred. Helium is commonly present in nuclear reactor fuel 
elements. As is well known the helium in a nuclear reactor fuel element is 
generally present in an amount such that when considered with the other 
gases present will produce a pressure of one to thirty atmospheres and 
preferably 1 to 20 atmospheres. The getter metal of the present invention 
is active over a wide water vapor pressure range and generally over the 
water vapor pressure range existing within nuclear reactor fuel elements 
and generally less than 100 torr. 
The getter metal of the present invention can be employed in any physical 
form but is generally employed as finely-divided particles in order to 
provide a large surface area for sorption. The particles can vary widely 
in size but generally are between 1 and 300 microns and preferably are 
between 1 and 120 microns. The particles of getter metal can be employed 
in a loose form, coated onto a substrate or more preferably pressed into a 
coherent porous mass. 
The getter metal of the present invention is active over a wide temperature 
range generally from 150 .degree. to 700.degree. C and preferably from 
200.degree. to 500.degree. C. When the getter metal of the present 
invention has been activated by heating it to temperature of 800.degree. 
to 900.degree. C for 5 to 50 seconds, or at lower temperatures for longer 
times, then it is active over an even wider temperature range of 
20.degree. to 700.degree. C. 
Referring now to the drawings and in particular to FIG. 1 there is shown a 
nuclear reactor fuel element 10 of the present invention. The nuclear 
reactor fuel element 10 comprises a sealed container 11 comprising a tube 
12 into which is fitted a first end cap 13 and a second end cap 14. The 
end caps 13, 14 are held to the tube 12 by means of welds 15, 16. It is 
the process of forming the welds 15, 16 which frequently heats areas of 
the tube 12 adjacent to the welds 15, 16 activating the tube 12 and making 
the zirconium present in the tube 12 more receptive to hydrogen 
embrittlement. Within the tube 12 are a number of pellets 20, 21, 22 of 
fissionable material such as UO.sub.2. The space between the uppermost 
pellet 20 and the end cap 13 is generally referred to as the plenum 24. 
Within the plenum 24 is a spring 26. Within the spring 26 is a getter 
device 28 of the present invention. Alternatively the getter device could 
be in the position of the pellet 20 or could be present in a recess in the 
end cap 13. 
The Zr.sub.2 Ni in the getter device 28 is present as finely-divided 
particles having a size between 1 and 60 microns. The particles are 
pressed into a coherent porous mass. During operation of the fuel element 
10 the getter device 28 is generally maintained at a temperature of about 
200.degree. to 500.degree. C. In accordance with conventional 
manufacturing processes the container 11 is filled with helium to an 
extent such that the total gas pressure in the plenum 24 and in fact in 
the rest of the container 11 is between 1 and 30 atmospheres. As shown in 
FIG. 1 the end cap 13 is provided with a passage 29 which is closed prior 
to use of the nuclear reactor fuel element in a nuclear reactor. 
Referring now to FIG. 2 there is shown a system 30 useful for measuring the 
water sorption characteristics of getter metals. The system 30 is a closed 
system of a known volume and is provided with a micro balance 32, a 
pressure gauge 34, and a gas analyzer 36 such as a gas chromatograph or a 
mass spectrometer. The gas analyzer 36 is isolatable from the system by a 
valve 38. The system 30 is connected to a vacuum pump 40 isolatable from 
the system by a valve 42. The system 30 is also provided with a valve 44 
the function of which is explained below. Outside the system 30 is a 
heater 46 that can be employed to maintain the temperature of the getter 
device 28' at any given temperature. The system 30 also comprises a tube 
48 within which is an amount of water 50. Surrounding the tube 48 is an 
ice and water mixture bath 52. The micro-balance 32 comprises a magnet 54 
provided with windings 56 connected to an indicator 58. In operation as 
the weight of the getter device 28' increases the indicator 58 supplies 
additional current to the windings 56 in order to keep the balance beam 60 
of the micro-balance 32 level. The amount of current employed is converted 
electronically in the indicator 58 and can be read directly as the weight 
of the getter device 28' in excess of the weight of the tare 62. Any of 
the commercially available micro-balance available from Cahn industries 
division of the Ventron Corporation, Paramount, California. 
The invention is further illustrated by the following example in which all 
parts and percentages are by weight unless otherwise indicated. These 
non-limiting examples are illustrative of certain embodiments designed to 
teach those skilled in the art how to practice the invention and to 
represent the best mode contemplated for carrying out the invention. 
EXAMPLE 1 
Employing the apparatus of FIG. 2 the ice bath 52 is removed and the water 
50 frozen with a bath of liquid nitrogen not shown. The valve 38 is closed 
and the valves 42 and 44 opened. The pump 40 is operated until the 
pressure in the system 30 is reduced to 10.sup.-5 torr. The valves 42, 44 
are then closed and the water 50 permitted to reach 0.degree. C. The 
pressure in the system 30 is monitored by means of the pressure gauge 34. 
The ice and water mixture bath 52 is placed around the water 50 and the 
valve 44 opened. The pressure in the system 30 rises to 4.579 torr which 
is the vapor pressure of water at 0.degree. C. The temperature of the 
heater 46 is adjusted such that the temperature of the getter device 28' 
is raised to 300.degree. C. The pressure is monitored on the pressure 
gauge 34 while the weight increase in milligrams is read on the indicator 
58. These two values are plotted on FIG. 3 as the inventive line 66. By 
reference to FIG. 3 and the line 66 it can be seen that the pressure 
increases from a pressure of 0 to a pressure slightly above the 0.degree. 
C equilibrium water vapor pressure of 4.579 torr. It has been theorized 
that this pressure above the 0.degree. C equilibrium water vapor pressure, 
characterized by the line segment 68, is due to a partial release of 
hydrogen. However, if this is true the hydrogen is rapidly resorbed with 
the result that the pressure in the system remains at or near the 
theoretical pressure of water until the getter device 28' has increased in 
weight 30 milligrams. At this time the pressure in the system begins to 
rise above the theoretical pressure of 4.579 torr. Opening of the valve 38 
and analysis of the gas in the system 30 by the gas analyzer 36 indicates 
the presence of hydrogen. In this experiment the getter device 28' had a 
cylindrical shape with a diameter of 8.1 millimeters a height of 2.2 
millimeters and weighed 593 milligrams and was prepared from Zr.sub.2 Ni 
powder of size less than 60 .mu. as in Example 3. 
EXAMPLE 2 
This example is not illustrative of the present invention but discloses the 
undesirable results when employing a known ternary alloy. 
The procedure of Example 1 was repeated except that the getter device 28' 
was replaced by a getter device of a ternary alloy analyzing 5.03 weight 
percent nickel; 9.30 weight percent titanium and 82.0 weight percent 
zirconium, balance attributed to insoluble oxides of zirconium and 
titanium. This sample was made from powder having a size less than 60 .mu. 
received from the General Electric Company as a sample of their ternary 
alloy available under the tradename "Hipalloy." The same piston, 
cylindrical receptacle and compression forces were used as for the 
preparation of the sample of Example 1. The comparative getter device had 
a diameter of 8.1 millimeters a height of 2.3 millimeters and weighed 568 
milligrams. The results of this test are shown on FIG. 3 as comparative 
line 70. As can be seen by reference to FIG. 3 that segment 72 of the line 
70 rises above the theoretical value 4.579 torr and remains above this 
value indicating that a gas other than water vapor is present. Analysis by 
means of the gas analyzer 36 indicates that this gas is hydrogen 
indicating that the Hipalloy does not absorb all the hydrogen contained 
within the molecules with which the Hipalloy reacts. 
EXAMPLE 3 
This example illustrates the synthesis of a getter device useful in the 
present invention. 
Pure Zr.sub.2 Ni is produced as reported in Hansen Binary Alloys pages 1062 
and 1063 or by using the technique described by A. Barosi in Residual 
Gases in Electron Tubes, Edited by T. A. Giorgi and P. della Porta, 
Academic Press, London 1972. Zr.sub.2 Ni thus produced is mechanically 
broken with a hammer and then passed through a screen such that all the 
particles have a size less than 60 microns. A portion (600 milligrams) of 
this powder is placed into a cylindrical receptacle fitted with a piston 
and the piston compressed with a force of 3000 kilograms to produce a 
porous compacted mass of Zr.sub.2 Ni. The porous compacted mass can be 
employed according to the present invention to sorb water vapor and can be 
employed as a getter device 28 in the nuclear reactor fuel element 10 
shown in FIG. 1 of the drawings. 
EXAMPLE 4 
This example illustrates the synthesis and use of a further getter device 
useful in the present invention. 
Zr.sub.2 Ni is produced as in Example 3 and is mechanically broken with a 
hammer and then passed through a screen such that all the particles have a 
size less than 120 microns. A portion (100 milligrams) of this powder was 
placed in a 12 millimeter outside diameter U-section channel ring 
container of stainless steel and compressed with a force of about 4000 
kilograms to produce a getter device. The getter device is placed in a 
stream of argon at 25.degree. C saturated with water vapor such that the 
water vapor partial pressure is about 23 torr. The getter device is heated 
to about 300.degree. C for 10 minutes to cause it to sorb water after 
which it is allowed to cool and its hydrogen content is measured by known 
techniques. The hydrogen content is a measure of the water vapor sorbed. 
The hydrogen content is 0.5 cc. torr per milligram of Zr.sub.2 Ni. 
Although the invention has been described in considerable detail with 
reference to certain preferred embodiments thereof, it will be understood 
that variations and modifications can be effected within the spirit and 
scope of the invention as described above and as defined in the appended 
claims.