Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
When a pressurized containment system is used it is
often times desirable and sometirnes necessary to have inform-
ation of the pressure within such a system. This i5 parti-
cularly true in the case of nuclear reactor fuel rods where
such pressure measurements are desirable in order to detect a
possible leak in the fuel rod or to determine the status of
the fuel within the rod. In the case of nuclear reactors these
measurements rnay be made while the rods are in the fuel assem-
blies which are utilized in the reactor and it must be possibleto measure all of the rods, To be acceptable in the nuclear
field the measurement technique for pressure determination
must be radiation-proof and be capable of withstanding the
high operating temperatures of the reactor. Such a technique
also must be self-callbrating, or at least self-checking or
fail-proof and insensitive to changes in the outside surface
of the rod. There can be a severe economic penalty if the
pressure measurement is not accurate, since a low pressure
indication would cause what would actually be a good fuel
bundle to be discarded, while a high pressure readout where
the rod was actually failed could result in release o~ radi-
ation to the environment. Other fields where such a pressure
measurement system may be desirable are those which use highly
corrosive chemical vessels and high pressure, high integrity
systems.
It is apparent then that there is a very real need
for a pressure detection system which can function without ~he
need for any mechanical or physical penetration of the pressur-
ized system, while still obtaining the desired internal pressure
measurement. The prior art pressure sensing techniques such
as the differential balance system where small tubing pene-
trates the pressurized system, is pressurized, and connected
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to a differential gage, and the techniques which use a capaci-
tive diaphragm or a bellows device, have proven relatively un-
satisfactory, since they require expensive internal ins-tallation
costs and tend to develop leaks through the relatively thin
diaphragm.
SU~MARY OF THE INVENTION
Basically, the present invention relates to a pressure
measurement system including a transducer or sensor and elec-
tronic instrumentation, particularly for measuring gas pressure
inside a pressurized system and more particularly for measuring
fission gas pressure inside individual nuclear fuel rods, while
the rods are located in a fuel bundle within a nuclear reactor,
The present system makes use of primarily the well-known Villari
effect to perform pressure measurements without any mechanical
or physical penetration of the fuel rod cladding or end cap.
The system will respond to pressure changes as small as about
1 psi, and has a measurement capability of about O to 500 psi,
although the range obviously can be extended with relatively
minor modifications which will be within the skill of those
working in the art once they have knowledge of the present
invention. me present system requires very minimal and in-
expensive modifications to employ it in existing fuel rod
fabrication techniques and can be easily deployed in a nuclear
fuel reactor with relatively little effort. While in the pre-
ferred embodiment the system specifically is disclosed for
measuring fission gas pressure in a nuclear fuel rod, it is
generally applicable to virtually any sealed pressurized system
which has a pressure measurement requirement.
The pressure measurement can be made at a plurality
of locations by taking sequential measurements at each fuel rod.
The mea~urements are made by placing an external electro-magnetic
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search coil adjacent or proximate a sensor which is located
internally of the fuel rod preferably near an accessible end
thereof. The search coil is energized to develop an a-c mag-
netic field which penetrates the metallic pressure containment
wall and tests the magnetic properties of the sensor. The use
of the Villari effect is essentially relying upon the dependence
of the magnetic permeability (of the sensor) upon applied stress
(pressure of the system)~ This effect i5 associated with the
crystal or grain structure of the particular material employed
so that the magni~,llde and the sign (negative or positive) of
the change in permeability is a function of orientation of the
grain. The sensor made from a Villari effect material, such as
a nickel ferrite or nickel alloy, is located inside the press-
urized system such that the pressure within the system creates
a stress loading on the sensor. Changes in the initial or
virgin permeability of the sensor due to the pressure loading
are detected by the search coil located externally of the
pressurized system but sufficiently close to sense changes in
sensor permeability and are read out as pressure levels in a
suitable instrument,
Typically, a sensor according to this invention
would comprise a sealed hollow cylinder or sphere made of
material such asN-50 ferrite (approximately 50% nickel oxide)
which is a low temperature coefficient magnetostrictor com-
mercially available from Ceramic Magnetics of Fairfield, New
; Jersey. The Villari effect, which is a basic magnetos~rictive
effec~, in the case of the N-50 ferrite exhibits a very high
degree of sensitivity to applied pressure provided that proper
stress loading is achieved. Proper loading requires that the
internal volume of the sensor be sealed against pressure and
that the wall of the sensor be thin enough to obtain reasonable
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1 compressional stress at lts ou~er surfac~. Surface stress
2 ls required because electromagnetlc energy from the interro-
3 gation coil penetrates to only a very shallow depth below the
4 surface of the sensor. Typical dimensions for the sensor
would include an outside diameter of 0.125 inches, an inside
6 diameter o-f 0.075 inches, and length of 1 inch.
7 The electronic portion of the pressure detecting
8 system which will be described in detail hereinafter,
9 basically comprises a dual coil eddy current instrument with
phase sensitive dis~rimination and specifically designed for
1~ precisely measuring permeability. The coils comprise a
12 reference coil and a detector coil, the latter of which is
13 located close ~o the sensor during operation of the system.
14 The resultant voltages produced by each of the coils is a
product of current and impedance of each coil ~nd~ therefore,
16 is directly proportional to impedance. The coil voltages
17 then are applied to a differential amplifier wllose output
18 vo~tage is summed with a second voltage having adjustable
19 amplitude and phase. The adjustability of this second volt~
age permits obtaining a null balance. This balance i5 ob~
21 tained by adding the sum of ~he coil voltages to the second
22 voltage which is equal to but 180 out of phase with the sum
23 of the coil output voltage. By doing this, amplification of
2~ the signals is permitted without overdriving, that is without
saturating the ampliiersO The output from the summation
26 ampliier and the reference voltage signal are applied to
27 a standard analog multiplier which operates as a phase
28 sensitive ~ic-to-dc converter. The dc output of the multiplier
29 is proport~onal to the amplitude of the two inputs and the
cosine o the angle between them. The phase o~ the reference
31 signal, which is adjustable, estab~ishes the detection axis
32 (i.e. the sign of positive pressure). The output o~ the
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multiplier which can be calibrated directly in terms of press-
ure then is connected to drive a suitable meter or recorder
or other such readout device.
The foregoing described pressure detection instrument
operates basically by measuring the vector impedance of the
search or interrogation coil when it is placed over or near
the sensor. The instrument employs an impedance measurement
technique which is similar to that used in conventional instru-
ments, and the coils are driven at suitable frequency, e.g.
1-2KHz, to insure penetration of metal walls in the pressurized
system. Temperature compensation is provided by employing a
second sensor in operable association with the reference coil.
` The second sensor is maintained at ambient pressure but is ;
- relatively close to the detection sensor so as to be at the
same temperature which results in a cancellation of any tempera-
; ture changes.
Thus, it is apparent that it is an object of this
invention to provide a pressure detection system for measuring
the pressure of a pr~ssurized system without mechanical pene-
tration thereof and which system responds to relatively small
changes in pressure and has a substantially wide measurement
range.
In accordance with a specific embodiment o~ the in-
vention, there is provided a pressurized system including a
sensor located within said system comprising a material which
has a magnetic permeability which is dependent upon stress
~; applied thereto, and is responsive to the internal pressure
of said system, sensing means operably associated with said
pressurized system in proximate relation to said sensor within
said system for developing a magnetic field upon energization
which field penetrates the containment wall of said pressurized
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system and interrogates said sensor, said sensing means provid-
ing an output electrical signal which is proportional to the
stress applied to said sensor, and electronic circuit means
connected to said sensing means for processing said output
signal of said sensing means for providing a readout of said
pressure in said system.
In accordance with a further embodiment, there is
provided a nuclear fuel rod containing nuclear fuel and having
end plugs at opposite ends thereof and adapted for use in a
nuclear fuel reactor, wherein the improvement comprises:
means for detecting the fission gas pressure in said fuel rod
comprising elongated sensor means located within one of said end
-plugs, said sensor means comprising a material which has a
magnetic permeability dependent upon the stress applied thereto
by the pressure within said fuel rod, coil means operably
associated in proximate relationship to said sensor externally
of said one end plug of said fuel rod for sensing changes in
the magnetic permeability of said sensor upon energization of
said coil means, and means for processing the output signal
from said coil means for providing an output signal proportional
to pressure of said fuel rod.
From a different aspect, and in accordance with the
; invention, a method for detecting fission gas pressure in a
sealed nuclear fual rod having end plugs at opposite ends
thereof and a stress-sensitive sensor having a magnetic
permeability in one of said end plugs comprises the steps of:
(a) locating coil means in proximate relation to said sensor
and externally of said one end plug; (b) energizing said
coil means so that there is produced a magnetic field which
penetrates said sensor, (c) sensing a change in -the magnetic
permeability of said sensor by means of said coil, and
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(d) processing the output signal from said coil which is
proportional to sensed changes in magnetic permeability of
said sensor to provide an output proportional to the pressure
of said fuel rod.
~ aving in mind the foregoing objects that will be
evident from an understanding of this disclosure, the invent-
ion comprises the combination arrangements and devices as
demonstrated in the presently preferred em~odiment of the
invention which is hereinafter set forth in such detail as
to enable those skilled in the art readily to understand the ~ :
function, operation, construction and advantage of it when
read in conjunction with the accompanying drawings.
BRIEF DESCRIPTIO~ OF THE DRAWI~GS
Fig~ I is an exploded perspective view of a preferred
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embodiment of thepressure transducer according to the present
invention;
Fig. 2 is a cross-sectional view in elevation of the
pressure transducer of Fig. 1 in assembled relationship,
Fig, 3 is an elevational view of the pressure trans-
ducer of Figs. 1 and 2 accordiny to the present inventîon
asqembled in a nuclear fuel rod;
Fig. 4 is a schematic diagram of the electronics of
a pressure detection instrument according to the present inven-
tion: and
Fig. 5 is a response of a typical system constructed
according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
. _
As shown in Fig. 1, th~re is illustrated a pressure
transducer generally designated 10 in the form of an end plug
12 for a nuclear reactor fuel rod element 28 (see Fig. 3~.
~ The end plu~ 12 comprises a hollow elongated cylindrical end
; plug shank 13 sealed permanently at one end and at the opposite
end having an integral enlarged cylindrical portion 15 with
the central bore 17 extending ~rom the sealed end of and through
the shank 13 and the enlarged cylindrical portion 15, as shown
in Fig. 2. The end plug 12 receives an elongated magneto-
strictor sensor 14 in the bore 17, which although shown as a
single or unitary elongated tubular member also may comprise
a plurality of relatively shorter individual tubular members.
~he sensor is sealed at opposite ends by means of end caps 16
and 18 and is designed to be received within the hollow portion
17 of the plug 12. A typical material used for the sensor 14
is N-50 ferrite, which is a low temperature coefficient magneto-
strictor that exhibits the desired properties for responding toapplied stress. For the sensor it is desired to have any one
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1 oE a`number of materials which are characterizcd by having
2 a dependence of magnetic permeability on appli~d s~ress and
3 resistant to radiation damage. Suitable materials include
4 certain n;ckel ferrites and selected nickel alloys. The pre-
ferred material, a magnetostrictive ferrite, was found to be
6 satisfactory for purposes o this inven~ion. Preferably thc
7 sensor should be about 1.5 inches long in order to avoid any
8 edge effects that is, the search coil should be as close as
9 possible to the center of the sensorO It has been ~ound
that an open~ended h~llow sensor or a solid sensor ~uld not
11 be as satisfactory as the sealed ~ubular sensor o this ;n-
12 vention in terms of sensitivity. As previously discussed,
13 adequate sensitivity requires compressional rather than shear
14 stress loading in the area of the sensor being interrogated
by the search coil. It has been ~ound that this stress can
16 best be achieved with the sealed tubular con~iguration.
17 A suitable way of sealing the ferrite end caps 16
18 and 18 to the sensor 14 is to seal them with molten glass --
19 before ~inal sintering. ~ substantial amount o~ glass
normally is used as a binding agent in fabrication of ferrite
21 sensor so that no new mater:ial is introduced when the end caps
22 are sealed in place. Althougll it has been suggested to use
23 molten glass for sealing the end caps to the sen~or, any
24 suitable cementing teclmique will suffice. In the case of
fission gas sensors such as used in the preerred embodiment,
26 the technique used is particul~arly important since the sealing
27 agent (e.g., glass) has to withstand reactor irradiation.
28 A disadvantage of sealing the sensor, however, is that cooling
29 fr~m sintering temperature will create a vacuum in theisensor
necessitating a small hole in one of the end caps~l6 or 18
31 for pressure equali~ation. However, this equalization hole
32 (not shown) could subsequentl~ be sealed by various methods
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which would not cause reheating of the internal gas volume,
which would be within the skill of those working in the art.
Fig. 3 illustrates the assembled end plug with
sensor located inside and operatively connected to the nuclear
fuel rod 28 having a metal casing 30 and fuel elements 32 dis-
posed therein. The end plug 12 can he welded as shown in 34
to the uel rod casing 30. It should be recogni~ed, however,
that while shown in connection with a gas pressure system such
as a nuclear fuel rod, the present invention also can be used
in other instances such as measurement of small differential
pressures inside a high-pressure, thick-walled vessel. It is
also desirable in order to obtain repeatability of the measure-
ment, for the qensors to be identically located in each end
plug of a fuel rod. Another requirement for repeatability is
for precise location of the search~coil along the length axis
of the sensor. This can be accomplished by providing an index
on the shoulder 19 of the fuel end plug 12 against which the
base of the coil form is abutted before taking any measurement.
With the sensor 14 located primarily in the hollow end plug
shank 13, a positioning helical spring 20 is inserted into the
bore 17, whereupon a zircaloy retaining cap 22 having a central
pressure opening 24 is welded into place at the bottom of the
end plug 12. ~he outwardly beveled rim 21 of the cylindrical
portion 15 complements the beveled sides 23 of the retaining
end cap 22 to provide a groove for welding the retaining cap
22 to end plug 15.
r~he preferred sensor construction provides a seal~d
inner volume of air or other ~uitable gas at atmo~pheric pres-
sure. While the internal pressure will vary with tempera-
ture, this will be of no consequence as temperature compen-
sation tto be di~cussed in further detail hereinafter) is
provided by means of a second reference sensor, The desired
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effect which is the increase or decrease in compressional or
tensional stress at the surface of the sensor upon pressuriza-
tion thereof is achieved substantially irregardless of the
internal pressure of the sensor.
Turning now to Figure 4, which illustrates a block
diagram of the pressure detection instrument including its asso-
- ciated electronics, there is schematically shown the Villari
effect sensor element 14 which in actual practice for -the pre-
ferred embodiment would be found within the end cap of the
nuclear fuel rod as previously disclosed. Specifically, the
system includes a dual-coil eddy current instrument having
multi-channels 34, 36 including one which provides phase sen-
sitive discrimination. A reference coil 38 is employed in con~
junction with a reference transducer shown a-t 40, which is
maintained at ambient pressure and temperature. ~his reference
coil 38 when taken with the sensor coil 40 will provide suitable
temperature compensation for the system as discussed in detail
below. The use of phase sensitive discrimination allows mon-
itoring of energy aligned with the permeability axis (i.e.,
pressure information) while any signals corresponding to con- -
ductivity or coil wire resistance changes essentially can be
discarded because they are not needed. By way of example,
; coils which have been found suitable for use in the present
invention have comprises 312 -turns of No. 31 copper wire
wound on a 0.300 O.D. coil form, wi-th a length of 0.37~ inches,
and an inside diameter of about 0 25 inches. One requirement
of the search or sensor coil 40 is that it be substantially
smaller in its axial or longitudinal direction than the length
of the sensor slug 14, in order to avoid sensitivity to posi- -
tioning of the coil along the slug. If the axial length of
the coil were longer, end effects would become significan-t.
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1 The sensor coll will be sensitive to the type of
2 material which it is placed adjacent to. Thus, when the
3 coil is placed over tlle zircaloy shank of the end cap in
4 the preferred embodiment, there ~111 be a small increase in
coil resistance at a constant reactance, whereas ferrite
6 alone would cause a large increase in reactance at a con- -
7 stant resistance. ~en the coil, however, is placed over a
8 composite sensor such as in the presently discl~osed embodi-
9 ment (a fexrite sensor under a zircaloy enclosure), there is
an increase in bothtresistance and reactance. In this case
11 a change in ferrite (the sensor 14) pe~neability produces a
12 sensor coil impedance ~rhich is at approximately a 30D angle
13 relative to the resistance of the X axis, as compared to the
14 no~nally anticipated 90 directional change for a change
only in coil reactance. The instrument can be adjusted by
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16 ~he rotation control ~phase shit bridge 48) to detect im-
17 pedance changes along this locu~ (i.e., the 30~) or along
~8 either of the X or Y coordinate axes. In practice, however, -~~
19 it is preferable to read only the reactive or 90 component
of the coil vector so that the measurement will not be sensi-
21 tive ~o variations in conductivity and there~ore resistance
22 of the coil. This prccedure wil~ provide for a more than
23 ~adequate sensitivity as shown ~rom a typical response of a
24 prototype shown in Fig. 5. ~ach of the coils 38 and 40 are
driven e.g. at a 2 Khz frequency by suitable constant current
26 sources Il ard I2. The resultant coil voltages comprise the
27 product of current and impedance and therefore is directly
~8 proportional to impedance. The output coil voltages are
29 appli~d to a conventional differential amplifier 42 where they
are summed. ~he output voltage f~om diferential amplifier 42
31 is then applied to a further amplifier 43 to which is also
32 applled a further voltage w~ose amplitude and phase is
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adjustable and which is derived as discussed hereafter. An
oscillator 4~ of the LC feedback type or other similar conven-
tional circuit powers the coil drivers Il and I2, and also the
phase shift bridges 46 and 48. These bridges essentially have
unity gain and provide signals having adjustable phase angle
for driving the balance circuitry 50, 43, and the phase sensi-
tive discriminator 56, 60, 58. The output signal of bridge 46
can be adjusted by means of the amplitude balance control 50
which comprises a potentiometer. This results in being able
to make the second voltage applied to amplifier 43 equal in
amplitude but opposite in phase by 180 to the resultant coil
voltage applied from the output of amplifier 42. This produces
a nulled signal at the output of amplifier 43. The null balan-
cing of coil signals permits large amplification so as to ob-
tain sensitivity to pressure without overdriving the amplifiers
by applying excessive amplitude signals. Resistors 52 and 54
establish the voltage gain of amplifier 43. The bridge 48,
also driven by oscillator 44, produces an output reference
signal whose phase is adjustable by means of a suitable con-trol
on the instrument. By adjusting the phase, the detection or
readout axis is determined as seen from the following discussion. ,
The output from summation amplifier 43 is applied to analog
multiplier 56 and low pass filter 60, 58, which combination
operates as a phase sensitive ac-to-dc converter. The dc out-
put across capacitor 58 is proportional to the amplitudes of
the input signals received from channel 36 (amplified and
balanced coil signals) and the reference signal from conven-
tional phase shift bridge 48, and also to the cosine of the
angle between them. When adjusting the phase of the reference
signal control (bridge 48) in order to establish the appropriate
detection axis, it is preferable to set it so that the readout
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from converter 56, 60, 58 is along the reactance on 90~ axis,
whereby the output responds only to changes in sensor coil
reactance. To accomplish this the reference signal angle
should be set so that coil resistance changes have no effect
on converter output. The output from converter 56, 60, 58 can
- be calibrated to provide a readout directly in terms of pres-
sure by driving a suitable meter, recorder or other similar
readout device.
Fig. 5 illustrates response of typical prototype
system according to this invention. This data was taken when
nitrogen gas was used to pressurize an end cap with a sensor
pursuant to the present invention, While a non-linear response
is shown, this does not present significant problems for moni-
toring nuclear fuel rod pressure since the system transfer
function would be determined beforehand. By optimizing the
design of the sensor 14 (that is, proper selection of materials
and shape) the response could be made linear. The frequency
at which measurements were made was 2 KHz.
To measure the fission gas pressure of the fuel rod
the search coil first is calibrated with a standard calibra-
tion transducer. This is done by turning on instrument power
and placing both search and reference coils over -transducers
maintained at ambient pressure. The meter then is adjusted
for zero readout and the angle of the discriminator is ad-
justed by appropriate control so that there is no change in
readout when a small resistor (2-10 ohms) is momentarily in-
serted in series with the search coil by means of an un-
shorting switch. The reference transducer is pressurized to
ambient conditions, and the search or sensor coil is removed
and placed over the fuel rod end cap to measure fission gas
pressure. The reference transducer and its associated coil
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1 then are located in proximity to the search coil in order
2 to provide ~he desired temperature compensation which is ob-
3 tained since both transducers are at approximately the same
4 temperature. The fuel bundles would be removed from the
S reactor and transferred to a holding basin. There the
6 pressures in the room would be measured as indicated above.
7 l~hile a particular embodiment o the invention has
8 been sho~m and described and various modifications thereof
9 have been suggested9 it will be understood that the true
spirit and scope of~ the invention is set orth in the appended
11 claims which embrace other modifications and einbodiments
12 which will occur to those of ordinary skill in the art.
