Disclosed is apparatus and method which, in cooperation with an electrical system, analyzes signals generated by detectors placed in a flux field of gamma rays and neutrons. A first detector with a first sensor is coupled to the electrical system. A second detector with a second sensor is coupled to the first sensor and the electrical system. A fissile material is housed in the first detector, and is heatable by gamma ray and neutron capture to in turn heat the first sensor, which generates a first signal which is delivered to the electrical system. A non-fissile material is housed in the second detector, and is heatable by gamma ray capture to in turn heat the second sensor, which generates a second signal. The first and second signals are analyzed in the electrical system to determine the power level, neutron flux, and gamma ray flux within the flux field. Also provided is a microtechnology apparatus and method for measuring neutron flux. A first thermocouple is formed on a substrate, and is covered by a fissile material heatable by gamma ray and neutron capture. This heats the hot junction first thermocouple and generates a signal. A second thermocouple is formed on the substrate, and is covered by a non-fissile material, heatable by gamma ray capture. This heats the cold junction second thermocouple and generates a second signal. The first and second thermocouples are electrically connected to each other in juxtaposition and to the electrical system, which determines neutron flux.

BACKGROUND OF THE INVENTION 
The field of this invention relates generally to measurement of the neutron 
flux within a nuclear reactor core. 
Neutron sensitive detectors have for some time been the primary sensors 
used as incore neutron monitoring systems in conventional nuclear fission 
Light Water Reactors (LWR). The gas filled fission chamber has emerged as 
the commonly used sensor for Boiling Water Reactor (BWR) incore 
instrumentation, whereas the self-powered detector (SPD) is used in 
Pressurized Water Reactor (PWR) cores. U.S. Pat. No. 4,121,106 to Terhune 
and Neissel, entitled "Shielded Regenerative Neutron Detector", discloses 
an ion chamber type neutron detector; this patent is hereby incorporated 
by reference into this specification. U.S. Pat. No. 3,760,183 to Neissel, 
entitled "Neutron Detector System", discloses a combination of ion 
chambers and self-powered detectors; this patent is hereby incorporated by 
reference into this specification. 
Alternatives to fission chambers exist in the art. One of these for example 
is the gamma sensitive ion chamber, now in use to calibrate the fixed 
incore power range sensors during power plant full power nuclear reactor 
operation. However, these sensors are expensive, difficult to manufacture, 
and somewhat delicate for general use. Another form of sensor, the 
previously mentioned SPD, is used in PWR's primarily for nuclear fuel 
management and steady state power distribution measurements. 
Unfortunately, SPD's do not have a prompt response, since the isotopes 
commonly used in the emitter electrode of the SPD typically have 
half-lives on the order of minutes. This precludes use of SPD's in BWR's, 
since the collective sensor signal is used for prompt safety functions, as 
well as fuel performance and power distribution monitoring. However, in 
those non-transient applications mentioned above, SPD's are simple in 
design and structure, reliable, inexpensive and long-lived. 
Other approaches are much less widely used. Thermocouple sensors, in which 
the output signal is a function of the local gamma ray flux and therefore 
a measure of local power within the reactor core, have been developed and 
applied in European PWR's. An example is the gamma couple sensor (gamma 
thermometer). Unfortunately, these devices suffer from a low signal level 
output, and the attendant noise problems that limit their accuracy. Their 
response is not prompt, but can be fast enough by design that electronic 
and computer deconvolution methods can be applied to derive the prompt 
component of the gamma ray flux. These methods are very noise sensitive, 
and are usually too inaccurate for use in LWR safety functions. 
Nevertheless, the gamma couple sensor is simple, reliable, rugged, and 
cheap to manufacture. And unlike the ion chamber sensor, the gamma couple 
sensor has no gas filled volume with seals which can fail in service. 
The fission couple is another thermocouple type sensor now in use. It is 
similar to the gamma couple, except that the source of heat for the 
fission couple is due to a fissionable isotope which is placed into 
intimate heat transferring contact with the thermocouple. As thermal 
neutrons induce fission in the fissile nuclei, energy is released in the 
form of heat, to thereby heat a locally placed thermocouple. This heating 
is a function of the local neutron flux. Additionally, the temperature of 
the fission couple is a function of the local gamma ray flux in a similar 
way as the gamma couple. 
Advantages of these devices are that they are rugged, require no seal, and 
are relatively cheap and simple to manufacture. They also do not have to 
be powered by a voltage source because they are self-powered in the sense 
that a thermocouple is self-powered. Additionally, low impedance 
electronics can be used. Unfortunately, the component of the fission 
couple signal which is a function of gamma ray flux manifests itself as 
non-linearities in the output signal, even though the signal is typically 
very much larger than in a comparably sized gamma couple. Also, 
sensitivity of the fission couple to gamma rays limits the life of the 
fission couple, due to the 5:1 criteria of prompt-to-delayed signal ratio 
applicable to BWR safety system sensors. Fission couple lifetime is also 
shortened by the small size of the fissionable element, which must be 
small in order to obtain a reasonably fast response by the sensor. 
Lifetime is further shortened due to the burnup of the U-235 isotope 
contained in the absorber element. 
Additional problems exist with present technology. Previous attempts to 
develop practical and useful fission couples have had limited success, 
principally because the sensors developed up to now have had slow response 
time, a short like, and produced a signal having poor linearity. 
Sensitivity has not been a problem, although it is known to be 
complementary to responsiveness. 
Therefore, new or improved sensors are needed to measure the neutron flux 
within a nuclear reactor. 
SUMMARY OF THE INVENTION 
In summary, this invention provides apparatus and method for measuring 
neutron flux. The invention acts in cooperation with an electrical system 
adapted to receive and analyze signals generated by detector instruments 
disposed in a flux field having both gamma rays and neutrons. A first 
detector, provided with at least a first sensor, is disposed in a flux 
field of neutrons and gamma rays, and electrically coupled to the 
electrical system. A second detector contains at least a second sensor, 
electrically coupled to the first sensor; it is disposed in an environment 
of neutrons and gamma rays, and is electrically coupled to the electrical 
system. A fissile material is housed in the first detector in heat 
conducting relation with the first sensor. The fissile material is 
heatable by gamma ray and neutron capture, and in turn is capable of 
heating the first sensor, to generate as an output a first signal which is 
delivered to the electrical system. A non-fissile material is housed in 
the second detector in heat conducting relation with the second sensor. 
The non-fissile material is heatable by gamma ray capture, and is in turn 
capable of heating the second sensor, which generates as an output a 
second signal. This second signal is capable of being analyzed with the 
first signal in the electrical system, to determine the neutron flux 
within the flux field. 
Additional features of this first embodiment include: using thermocouples 
as the sensors; disposing the sensors in metallic spheres; using a ratio 
of uranium isotopes of 79% U-234:21% U-235 to form the layer of the 
mixture of fissile material; matching the gamma ray absorption 
characteristics of the first and second sensors; providing substantially 
identical volumes for the detectors. 
To summarize a second embodiment, provided is a microelectronic apparatus 
and method for measuring the neutron flux in a flux field having at least 
gamma rays and neutrons. The apparatus comprises at least a first 
thermocouple, formed on a first substrate, covered at least in part by a 
fissile material capable of being heated by gamma ray and neutron capture. 
The heated fissile material in turn heats the first thermocouple to cause 
it to generate a first signal, which first thermocouple thereby functions 
as a hot junction. At least a second thermocouple is formed on a second 
substrate and covered at least in part by a non-fissile material capable 
of being heated by gamma ray capture. The heated non-fissile material in 
turn heats the second thermocouple to cause it to generate a second 
signal; this second thermocouple thereby functions as a cold junction 
relative to the hot junction. Also provided is a means for electrically 
connecting the first and second thermocouples to each other, and to an 
electrical system which is capable of determining the neutron flux within 
the flux field from the first and second signals. 
Additional features of this second embodiment include: combining a 
plurality of first and second thermocouples connected in series to form a 
thermopile; providing an insulating substrate formed from a high quality 
ceramic material; using the two tradenamed alloys chromel and alumel 
materials as the first and second wires to form the singular as well as 
the plurality of thermocouples; using a mixture of U-234 and U-235 to form 
the fissile material; housing the detectors in a sealed housing which is 
packed in a mineral insulated material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Broadly stated, the novel sensor offered by this invention and its 
associated electronics provides a gamma ray compensated and temperature 
compensated breeding fission couple sensor. The sensor produces an analog 
output signal which can be filtered, digitized, and deconvoluted by a 
digital microprocessor, to provide a real-time measure of the "prompt" 
neutron flux, such as is found within the core of a conventional nuclear 
fission reactor. The sensor is self-powered, utilizes conventional low 
impedance DC electronics, and desirably lacks the seals and gas of 
existing sensors. The sensor is desirably designed for long life and 
ruggedness, and for producing an output signal having low noise levels, 
and high measurement accuracy. The sensor can be designed to have a fast 
response time, in the order of twenty five milliseconds, which is adequate 
for use in existing safety related systems of LWR's. 
FIG. 1 is a cutaway schematic view of a generalized active detector element 
20 according to the first embodiment of the invention. A thermocouple 22 
is created at junction 24 by joining together in known ways a first wire 
26 and a second wire 28, typically consisting of thermocouple alloys such 
as commonly used chromel and alumel to form the preferred embodiment. Both 
wires have a very small diameter, on the order of from one to two mils. 
Thermocouple junction 24 is cast in the nominal center of a first sphere 
30, housing a filler material 32 preferably comprised of a metallic alloy, 
discussed below. 
A thin shell 34 surrounds filler material 32, and preferably is comprised 
of a thin layer of gold a few mils thick. Shell 34 may be selected from a 
group of materials including gold, platinum, and palladium; the preferred 
material is gold because of its superior thermal conductivity and because 
it can be made very thin, in the range of from 0.1 to 0.3 mils, and 
preferably 0.2 mils. Sphere 30 is typically in the range of from 35 to 40 
mils in diameter, and is either (1) the source of fission heating and 
gamma ray heating of junction 24 if filler material 32 has a fissile 
component, or (2) the source of only gamma ray heating of junction 24 if 
filler material 32 is composed of a non-fissile material. As the filler 
material 32 is heated, the thermocouple 22 operates in the conventional 
manner to develop a voltage across the output wires 26 and 28, which has a 
functional dependence on the temperature at the thermocouple junction. 
The voltage generated across wires 26 and 28 due to heating of the filler 
material 32 is composed of either one or two signal components, plus noise 
which is not directly related to the power in the gamma ray and neutron 
flux field. If filler material 32 consists of a non-fissile metal, the 
output voltage signal appearing across wires 26 and 28 is (1) noise plus 
(2) a voltage signal due to gamma absorption in (a) the sphere, (b) 
thermocouple junction, and (c) wires 26 and 28 where they reside within 
the flux field. As is known in the art, gamma ray heating is accomplished 
by absorption of gamma rays by a material such as filler material 32. The 
amount of thermal heating experienced by the filler material 32 is 
proportional to the flux of gamma rays which bombard filler material 32. 
If, on the other hand, filler material 32 consists of a fissile material 
such as a mixture of various uranium isotopes, in this preferred 
embodiment comprising U-234 and U-235 isotopes in the respective 
proportions of approximately 79% U-234:21% U-235, then the output voltage 
signal appearing across wires 26 and 28 represents the signal contributors 
of (1) gamma absorption, (2) U-235 fissioning and (3) noise, and (4) also 
again including heating due to (a) neutron and (b) gamma ray absorption by 
thermocouple junction 24 and (c) the portions of wires 26 and 28 residing 
within the flux field. As described in U.S. Pat. No. 4,121,106 to Terhune 
and Neissel entitled "Shielded Rejenerative Neutron Detector", which 
patent is hereby incorporated by reference into the specification, the 
purpose of U-234 is to increase the usable life of the detector element 20 
by the generation of U-235 through the process of epithermal neutron 
capture in U-234, a process having negligible affect on the temperature 
within detector element 20, and therefore contributing negligible unwanted 
signal voltage across wires 26 and 28. 
The foregoing discussion leads to the conclusion that, if (1) the gamma ray 
mass absorption properties of a FIG. 1 sphere 30 containing non-fissile 
filler material 32 are chosen to be approximately equal to, (2) the gamma 
ray mass absorption properties of a FIG. 1 sphere 30 containing a fissile 
filler material 32, then (3) the voltage signal across wires 26 and 28 
will have gamma ray absorption signals which are essentially identical for 
a sphere 30 containing non-fissile material and a sphere 30 containing 
fissile material as the filler material 32. 
Further, if two FIG. 1 spheres 30, one containing a non-fissile filler 
material 32 and the other containing a fissile filler material 32, are 
electrically coupled through the junction 24 residing within each of the 
two spheres 30, then the output voltage signal of the FIG. 1 pair of 
thermocouples 22 is essentially zero in the absence of a neutron flux, 
even though a gamma ray flux is present. Additionally, common-mode noise 
due to thermal fluctuations is rejected because the output signals of the 
two spheres 30 cancel due to juxtaposition, which common-mode noise 
rejection has the benefit of producing a low noise output signal. Properly 
chosen gamma ray mass absorption properties of the non-fissile versus the 
fissile sphere include consideration of such properties as density and 
atomic number, which fall in the range of from 74 to 92, but preferably 
approximately 92. 
FIG. 2 is a side cutaway view according to one embodiment of the invention, 
wherein two FIG. 1 detector elements 20 are used, one containing a fissile 
filler material 32 and the other containing a non-fissile filler material 
32; the detectors are cooperatively arranged to provide the inventive 
apparatus of the FIG. 2 sensor 36. First detector 38, essentially 
identical in structure to the FIG. 1 detector element 20, is filled with a 
first material 40, which in this case is a fissile material. Second 
detector 42, also essentially identical in structure to the FIG. 1 
detector element 20, is filled with a second material, which in this case 
is a properly chosen non-fissile material, selected for its gamma 
absorption properties. 
Detectors 38 and 42 preferably have a spherical shape. Positioned near the 
center of first detector 38 is a first thermocouple 46, formed in 
essentially the manner identical as the FIG. 1 thermocouple 22. In a 
similar manner, second thermocouple 48 is likewise positioned near the 
center of second detector 42. First and second shells 50 and 52 are formed 
on the surface of detectors 38 and 42 in a manner essentially identical to 
the FIG. 1 shell 34 formation around detector element 20. Thermocouples 46 
and 48 are electrically coupled to each other through first wire 54. First 
thermocouple 46 is connected through second wire 56 to the junction 58 at 
the base of first lead 60. Likewise, second thermocouple 48 is connected 
through third wire 62 to junction 64 at the base of second lead 66. 
Finally, an optional fourth wire 68 electrically couples first wire 54 
through junction 70 to junction 72 provided at the base of third lead 74, 
indicated by dash lines as shown. Fourth wire 68 and third lead 74 are 
optional, but preferably are included in order to provide a temperature 
sensing means for directly monitoring the nuclear reactor core (not shown) 
temperature when the nuclear reactor is in the shutdown condition. During 
shutdown, the non-fissile detector 42 generates a measurable voltage which 
is related to the temperature in the core. 
A cable 76 is comprised of a sheath 78 surrounding the previously mentioned 
first lead 60, second lead 66, and third lead 74. Cable 76 preferably has 
a circular cross-section. Disposed within the sheath 78 is first 
insulation 80 substantially surrounding leads 60, 66 and 74, and at the 
same time electrically isolating these three leads from the sheath 78. 
First insulation 80 is selected from a group of commercially available 
insulation materials, such as silica, alumina, or magnesia, or 
combinations of them. 
A jacket 82, preferably selected from materials including type 304 or 316 
stainless steel, is formed into a tube having a circular cross-section. 
Jacket 82 at one end (at its right end in FIG. 2) is provided with a lip 
84 which creates an opening having a diameter greater than that of jacket 
82 and capable of receiving cable end 86. Cable end 86 is securely 
fastened to jacket 82 by such means as weld 88, which in this preferred 
embodiment would be a single weld 88 encircling both the jacket 88 at its 
lip 84 and cable end 86. 
In the process of fitting jacket 82 around cable end 86, jacket 82 is also 
fitted to surround first detector 38 and second detector 42. Into the 
chamber 90 created by the joining of jacket 82 to cable end 86 is placed 
second insulation 92, capable of providing thermal contact and electrical 
insulation. Preferably this second insulation 92 is a fine powder of 
beryllium oxide (BeO), which provides thermal contact between the 
detectors and the interior walls of jacket 82, and further provides 
electrical isolation between the detectors and jacket 82. Insulation 92, 
preferably in the form of a fine powder, is compacted using known methods 
such as ultrasonic compaction. Second insulation 92 is the first layer of 
insulation placed within chamber 90. 
Next, powdered third insulation 94 is poured over second insulation 92 to 
substantially cover it. Third insulation 94 is selected from materials 
including magnesium oxide and silicon oxide; it provides a thermal barrier 
between first detector 38 and second detector 42. Third insulation 94 is 
disposed within the space between detectors 38 and 42, and does not 
contact either detector. Finally, fourth insulation 96 is put into place 
in chamber 90, to substantially cover third insulation 94 as well as 
second detector 42. This fourth insulation 96 provides the third layer of 
insulation material positioned within chamber 90. Fourth insulation 96, 
like second insulation 92, is designed to provide good thermal contact 
between detector 42 and the inside wall of jacket 82, while simultaneously 
providing electrical isolation between detector 42 and jacket 82. A 
suitable material for fourth insulation 96 is beryllium oxide, which in 
this preferred embodiment is the same insulation used for second 
insulation 92. All three layers, preferably fine powders, are compacted 
using known methods such as ultrasonic compaction. 
Finally, an end cap 98 is secured to the outside end 100 of jacket 82, by 
such attachment means as welds 102, which for a circular jacket 82 would 
be one continuous weld surrounding the circumference of jacket 82 and end 
cap 98. Chamber 90 is thus enclosed to form a water tight electrical 
voltage source whose voltage is a function of the local thermal neutron 
flux. 
FIG. 3 is a cross-section of the FIG. 2 cable 76, taken along the line 
3--3. To improve the perspective, the FIG. 3 cross-section of cable 76 is 
rotated 90.degree. counterclockwise with respect to the FIG. 2 cable 76. 
As previously stated, the FIG. 3 cable 76 preferably has a circular 
cross-section and includes the previously mentioned leads 60, 66, and 74, 
encapsulated in first insulation 80, all of the preceding housed within 
sheath 78 of cable 76. 
During operation, the sensor assembly is placed into a flux field which 
includes gamma rays and neutrons. The first (fissile) detector 38, in 
which is housed the first (fissile) material 40, is thermally heated by 
neutron and gamma ray capture, to generate in first thermocouple junction 
46 a voltage signal across wires 56 and 54. In a similar manner, second 
(non-fissile) detector 42 houses the second (non-fissile) material 44 
which is heated by gamma ray capture (but not neutron capture), to thereby 
be thermally heated. Heated material 44 in turn thermally heats the second 
thermocouple junction 48, to thereby generate a voltage across wires 54 
and 62. Because gamma ray heating is occurring in both detectors 38 and 
42, and because thermocouples 46 and 48 are electrically coupled through 
wire 54, the gamma ray heating occurring in each detector 38 and 42 
cancels out the electrical signal in response to the gamma ray heating. 
This cancelling action produces a resultant signal in wires 62 and 68 
which is indicative of essentially only the neutron flux, the gamma ray 
flux signal having been essentially eliminated. Because the neutron flux 
is directly related to the voltage generated by detectors 38 and 42, the 
signal emerging through leads 60 and 66 provide a direct measure of the 
neutron flux within the reactor core. This invention of sensor 36 thus 
provides a rugged means of power range neutron monitoring using 
regenerative neutron sensors and amenable to digital signal processing. 
Typical sensor 36 dimensions include approximately a 100 mil outside 
diameter, a range of from 200 to 300 mils active length 90 as measured 
from end cap 90 to cable end 86, and a length of cable 76 sufficient to 
exit the reactor core and vessel. Preferably, at least four of the sensors 
36 are combined in mechanical assemblies (not shown) to sense both the 
axial as well as the radial gamma ray and neutron flux distribution in the 
reactor core, as described in the above referenced U.S. Pat. No. 4,121,106 
to Terhune and Neissel. The output signal emerging through leads 60 and 66 
is a DC voltage in the range of from approximately 10 up to approximately 
1,000 microvolts in normal operation of the nuclear reactor at 
steady-stage. Under transient neutron and gamma ray conditions, the DC 
output transient signal lags in time behind the instant in time of the 
actual flux transient, and therefore requires signal processing to provide 
an accurate measure of the instantaneous flux at the instant of the 
transient. 
An additional advantage offered by the FIG. 2 configuration of first 
detector 38 and second detector 42 is that the output signals generated by 
thermocouples 46 and 48, travelling respectively through wires 56 and 62 
and on through leads 60 and 66, are essentially independent of the thermal 
temperature of the environment. According to this preferred embodiment, 
the thermal temperature of the environment will be the temperature of the 
nuclear reactor coolant in which sensor 36 is immersed, with the desirable 
consequence that sensor 36 is temperature compensated. In the context of 
this invention, the expression "temperature compensated" means that the 
interaction of the signals of first (fissile) detector 38 and second 
(non-fissile) detector 42, through the electrical coupling wire 54, 
essentially cancel out that signal component representative of the 
temperature of the environment in which sensor 36 resides. Therefore, the 
output signal through leads 60 and 66 does not include a substantial 
thermal temperature component due to this resident environment. This 
desirable feature of temperature compensation is in addition to the 
previously mentioned gamma ray compensation of the electrically coupled 
detectors 38 and 42. 
FIG. 4 is a top cutaway view according to a second embodiment of the 
invention. In FIG. 4, the sensor 130 is comprised of an apparatus 
manufactured according to conventional semiconductor manufacturing 
techniques. This second embodiment employs microtechnology in order to 
increase the overall signal output generated by sensor 130. 
Broadly stated, FIG. 4 and FIG. 5 show a second embodiment according to the 
invention. FIG. 5 is a cross-sectional side view taken along the FIG. 4 
section 5--5. In an insulator substrate 132, a plurality of hot junctions 
134 and cold junctions 136 are formed, all of which junctions are 
thermocouples. A plurality of first wires 138 and second wires 140 are 
joined as shown to produce junctions 134 and 136. Wires 138 are comprised 
of the same (first) metallic material, and wires 140 are comprised of the 
same (second) metallic material but which (second) metallic material 
differs from the (first) material used to form wires 138. 
Wires 138 and 140 are connected to form junctions 134 and 136 in an 
alternate fashion as shown, such that the junctions create thermocouples. 
Such an array is typically referred to as a thermopile. At opposite ends 
of the array of junction 134 and 136, junctions 142 and 144 are 
electrically connected to respective leads 146 and 148, which in turn are 
connected to suitable electronics. In use, the hot junctions 134 and the 
cold junctions 136 are placed in a source of neutron flux. The 
thermocouples become heated to generate a DC voltage which is output 
through leads 146 and 148 to differential amplifier 168. 
More particularly, with reference to FIG. 4 and FIG. 5, standard 
microtechnology is used in forming an insulator substrate 132 as a first 
step in fabrication of sensor 130. Next, the conductor layer 150 
(comprised of hot junctions 134, wires 138 and 140, and cold junction 136) 
is formed on the insulator substrate 132. A top layer insulator 152 is 
then formed over the substrate 132 and conductor layer 150, to thereby 
seal and electrically insulate the components of the conductor layer. 
Conventional photomasking techniques are used to etch a hot plane 154 and 
a cold plane 156 in top layer 152, which planes 154 and 156 are at a level 
below the upper surface 158 of top layer 152. 
Planes 154 and 156 are then metallized to form hot metallized layer 160 and 
cold metallized layer 162. Onto the hot metallized layer 160 is evaporated 
a mixture of U-234 and U-235 in the ratios as stated above. Onto the cold 
metallized layer 162 is evaporated an inert (i.e., non-fissile) metal. 
Fissile layer 164 is formed from a fissile material, and inert layer 166 
is formed from an inert metal. Typical dimensions of active element 
defined in FIG. 4 and FIG. 5 are 0.20 inches in length, 0.070 inches in 
width, and 0.040 inches in thickness. As shown in FIG. 4, leads 146 and 
148 are packed in a mineral insulated cable to provide electrical 
insulation. A principal advantage of the FIG. 4 and FIG. 5 construction of 
sensor 130 is that as many hot junctions 134 and cold junctions 136 can be 
formed as desired in order to increase the strength of the output signal 
across leads 146 and 148. 
An obvious alternate embodiment for the FIG. 4 planar projection is to form 
the FIG. 4 sensor 130 onto the surface of a cylinder upon which the 
various layers of the FIG. 5 sensor 130 are deposited to achieve a similar 
result. 
This invention, illustrated in the two above example preferred embodiments, 
offers numerous improvements and advantages over the prior art. Neither 
high voltage seals nor gas seals are required, in contrast to those 
required in the currently used ion chamber sensors. This provides for a 
simple, compact, rugged design that can be manufactured more cheaply than 
existing sensors. Reliability of performance is enhanced because there are 
no seals which can crack in service, thereby eliminating undesirable 
transient sensor behavior due to, for example, gas migration in an ion 
chamber sensor between the sensor and the cable. The FIG. 2 sensor 36 is 
gamma compensated, thereby increasing its neutronic life beyond that of 
existing designs. Sensor longevity will be dictated by electronic limits, 
and the ultimate burnup of the uranium content of the fissile material 
contained in the FIG. 2 detector 38 and FIG. 5 fissile layer 164 while in 
service. Furthermore, the sensor is an inherently low noise device, since 
common mode rejection is implicit in its design. 
This invention offers an opportunity to provide diversified methods of 
measuring core power distribution. The use of this invention in 
combination with the presently used ion chambers lessens the chances of a 
common mode failure of the neutron sensors and thereby increases reactor 
safety. The invention offers the potential of greater accuracy, better 
linearity, and operation closer to the actual power limits in nuclear 
power plants. 
The foregoing detailed description of the example preferred embodiments of 
the invention have been presented solely for purposes of illustration and 
description. This detailed description is not intended to be exhaustive, 
or to limit the invention to the precise form disclosed. Obviously, many 
modifications and variations are possible in light of the above teaching. 
The example preferred embodiments were chosen and described in order to 
best explain the principles of the invention and the invention's practical 
application, to thereby enable others skilled in the art to best utilize 
the invention in various other embodiments not described above, and with 
various modifications as are suited to the particular use contemplated. It 
is intended that the scope of the invention be defined solely by the 
appended claims.