WO1998057194A1

WO1998057194A1 - Neutron modulation-activation for fissile material flow velocity and fissile content measurement

Info

Publication number: WO1998057194A1
Application number: PCT/US1998/011613
Authority: WO
Inventors: John T. Mihalczo; James A. Mullens; Jose A. March-Leuba
Original assignee: Lockheed Martin Energy Research Corporation
Priority date: 1997-06-13
Filing date: 1998-06-11
Publication date: 1998-12-17
Also published as: AU7818698A

Abstract

Systems and methods for neutron modulation-activation flow velocity and fissile content measurements are described. An apparatus includes a neutron source (110) located outside a conduit (120); a neutron absorbing material (130) removably located between the neutron source and the conduit; a gamma ray detector (150) located adjacent the conduit; and a gamma ray shield (160) located between the gamma ray detector and the neutron source. The systems and methods provide advantages in that no penetration of the conduit is necessary, the apparatus is readily attachable-detachable without modification of the conduit to which it is attached, and the measurement time is much shorter compared to penetrative techniques.

Description

NEUTRON MODULATION-ACTIVATION FOR FISSILE MATERIAL FLOW VELOCITY AND FISSILE CONTENT MEASUREMENT

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to the field of flow measurement within a conduit, such as, for example, a pipe. More particularly, the present invention relates to flow measurement in a conduit that does not require penetration of the conduit perimeter. Specifically, a preferred implementation of the present invention relates to neutron modulation/activation of the material that is being conducted within the conduit. The activity of the material is subsequently measured. The present invention thus relates to a flow measurement system of the type that can be termed nonintrusive. 2. Discussion of the Related Art Historically, it was known in the field of materials handling to measure flow rates within conduits. ^{(1 2)} However, all the prior art flow measurement methods, including the ultrasonic approach, are intrusive.⁽³⁾ All the prior art flow measurement methods are intrusive in that they require penetration of the conduit perimeter. Flow measurements of uranium hexafluoride (UF₆) gas within a pipe are performed by installing flow monitoring hardware inside the pipe. In the case of pressure differential measurement hardware typically includes an orifice that passes through a sidewall of the conduit with pressure measured on each side of the orifice. Of course, this approach requires penetration of the wall. In the case of ultrasonic measurement, the acoustic energy must be coupled into the gas. This is usually done by installing an ultrasonic transducer inside the conduit. This approach also requires penetration of the wall.

All penetrative measurement approaches have the disadvantage of breaching the wall of the conduit. This can be a significant disadvantage where the conduit functions as a containment vessel for hazardous materials. The need to penetrate the conduit wall can also be a significant disadvantage where the site of data collection is temporary (e.g., transient). Moreover, penetration of any conduit requires fabrication of the corresponding fittings and installation of the supporting apparatus (e.g., branch fittings, transducers, shut off valves). When the material is a radioactive gas, installation/maintenance of the penetrative fittings means radioactive contamination.

Within this application several publications are referenced by superscripts composed of arabic numerals within parentheses. Full citations for these, and other, publications may be found at the end of the specification immediately preceding the claims. The disclosures of all these publications in their entireties are hereby expressly incorporated by reference into the present application for the purposes of indicating the background of the present invention and illustrating the state of the art.

SUMMARY OF THE INVENTION Therefore, there is a particular need for a nonintrusive approach to measuring the flow characteristics of a material (e.g., UF₆) in a conduit. The invention uses a modulated thermal neutron source that is arranged in an external configuration, relative to the conduit. The thermal neutrons activate the material within the conduit (e.g., UF₆ gas). A detector of emitted radiation, which is also arranged in an external configuration, detects the residual activation of the material at a downstream location. Gamma ray detectors are used in this invention but detectors of of other emitted radiation can be used. For example, neutron detectors for counting delayed neutrons downstream could be used although the delayed neutron yield is lower than the delayed gamma yield. The invention provides an important advantage versus the prior art approaches because both the mass flow rate and the fissile content (e.g., ²³⁵U) of difficult to handle materials (e.g., UF₆) can be determined without penetrating the conduit. The fully external nature of the apparatus makes maintenance easier. Another advantage of the invention is that it can be attached and detached in a portable manner. Activation of the fluid in the pipe by external modulated radiation sources and correlating the signal downstream with the modulation also results in much shorter measurement times than are achievable by relying on the inherent radioactivity of a fluid and using two detectors on a pipe and correlating fluctuation produced by density or turbulence in the pipe. These, and other, aspects of the present invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the present invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS A clear conception of the advantages and features constituting the present invention, and of the components and operation of model systems provided with the present invention, will become more readily apparent by referring to the exemplary, and therefore nonlimiting, embodiments illustrated in the drawings accompanying and forming a part of this specification. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale.

FIG. 1 illustrates a schematic view of a system, representing an embodiment of the present invention;

FIG. 2 illustrates a plot of correlated component of a gamma ray detector resonse and a modulation of a source, representing an embodiment of the present invention; and

FIG. 3 illustrates a block diagram of an exemplary system, representing an embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The present invention and the various features and advantageous details thereof are explained more fully with reference to the nonlimiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well known components and processing techniques are omitted so as to not unnecessarily obscure the present invention in detail.

1. System overview The invention can measure both the flow velocity of a material (e.g.,

UF₆) in a conduit and the concentration of fissile isotopes (e.g., ²³⁵U) in the material. Thus, the invention can determine the mass flow rate of that fissile isotope in the conduit. The invention activates the material in the conduit with a modulated thermal neutron source and detects the residual activation downstream with a gamma ray sensitive detector. The gamma detector response is a function of delayed gamma rays resulting from fission induced upstream near the modulated thermal neutron source. The detector response is correlated to the modulation of the source.

2. Detailed Description FIG. 1 shows a high level schematic view of an embodiment of the invention. A thermal neutron source 110 is located near a conduit 120. In this embodiment, the material flowing through the conduit 120 includes UF₆. Throughout this application the term conduit is recited generically for a variety of materials handling structures whose species include, for example, ducts, hoses, tubes, pipes, and the like.

Although two thermal neutron sources are shown in FIG. 1 , they are substantially the same and only one will be described in detail. The source 110 includes sufficient ²⁵²Cf to produce a flux of, for example, approximately 1 ^x 10⁷ n/sec. Other sources of neutrons, or other particles, can be used to activate the material moving in the conduit 120. For example, Pu and Li (e.g., more than one) could be used to activate the material moving the conduit 120. In fact a plurality of sources could be provided near the conduit 120.

Coupling the activation energy from the source 110 to the UF₆ gas in the conduit 120 is not a problem. Neutrons pass through most structural materials. The thermal neutrons that reach the interior of the conduit 120 induce fission in the UF₆ containing material flowing through the conduit 120. The source 110 can be enclosed in a neutron moderator block 115 which is used to optimize the efficiency of the source neutrons to induce fission in the ²³⁵U of the UF₆. The neutron moderator block 115 slows down the neutrons and reflects some of those moving away from the pipe back towards the pipe where they can induce fission in the UF₆ gas. The neutron moderator block 115 includes polyethylene. Other neutron moderators materials and moderator blocks of alternative construction may be used.

The source 110 is modulated by moving a mechanism 130 that is located between the source 110 and the conduit 120. Although two such mechanisms 130 are shown in FIG. 1 , they are substantially the same and only one will be described in detail. The mechanism 130 includes a neutron absorbing material 140 (e.g., cadmium) that is located between the source of thermal neutrons 110 and the conduit 120.

To modulate the flux of neutron from the source 110 (i.e., provide a source modulation) the mechanism 130 can be moved between i) a closed position (shown in FIG. 1) between the moderator block 115 and the conduit 120 and ii) an open position (not shown in FIG. 1) away from the moderator block 115. This movement can be effected by an electric motor that is controlled by a processor that is part of a data acquisition computer. (The electric motor, the processor, and the data acquisition computer are not shown in FIG. 1.) Together, the mechanism 130, the electric motor and the computer compose a shutter that provides the source modulation.

In an alternative embodiment, the shutter can switch between i) a material that is opaque to thermal neutrons (e.g., cadmium) and ii) a material that is transparent to thermal neutrons. In either embodiment, the source modulation is a binary sequence that results in an input frequency spectrum associated with the source modulation frequency up to about 1 Hz.

The source modulation can be checked by independently detecting the position of the mechanism 130 which includes the neutron absorbing material 140. Alternatively, the source modulation can be checked by one or more neutron detectors (not shown in FIG. 1) that measure the transmission of thermal neutrons from the source 110 to the conduit 120. In either event, the source modulation is a first signal that is input to the processor.

Delayed gamma rays from the fission in the material within the conduit 120 are detected downstream with a gamma ray detector 150 that is located near the conduit 120. Detecting gamma rays from the downstream UF₆ gas is not made problematic by the presence of the conduit 120. The gamma rays of interest pass through most structural materials.

Although two detectors 150 are shown in FIG. 1, they are substantially the same and only one will be described in detail. The detector 150 can mounted directly on the conduit 120. The detector 150 can be a commercially available detector (e.g, Nal or BGO) set to count gamma rays with energies above 0.3 MeV. In this way, the proliferation of 186 KeV gamma rays from the alpha decay of ²³⁵U are not counted. The output from the detector 150 is a second signal that is input to the processor. In fact, there can be a collection of detectors spaced around the conduit

120. In the case where that are a collection of detectors, the resulting plurality of output signals are summed and input to the processor as the second signal.

Gamma ray shielding 160 (e.g., lead) is advantageously provided between the source 110 and the detector 150. The shielding 160 reduces the background noise from source 110 that is detected by detector 150.

Significantly, the source modulation produces a modulation of a delayed gamma ray signal from the material in the conduit 120 which is correlated to the source modulation. Cross correlation measurements between the detector 150 and the modulation of the thermal neutrons from the source 110 are measured. The time lag of the cross correlation function gives the flow velocity and the amplitude of the signal yields the active species (e.g., ²³⁵U) content of the material. The product of the ²³⁵U content and the flow velocity gives the ²³⁵U mass flow rate.

This activation of the fissile material by a modulated source of thermal neutrons can also be used to obtain flow velocity and mass flow from any gases or liquids, or slurries, which emit delayed gamma rays from neutron absorption. Although it is preferred that a device in accordance with the invention be mounted on (i.e., directly, or closely, adjacent) the outside of a conduit, the device does not necessarily need to be in contact with the conduit. Such a device can be installed on existing pipes without modification of the pipes. FIG. 2 is a plot of the correlated component of the gamma ray detector response and the modulation of the source, assuming that there is no detection background. The lag of the correlated component of the detector response is related to the time it takes the gas flow from the source to the detector. For turbulent flow there will be a single time lag and the ratio between the separation distance and this time lag gives the velocity. For laminar flow, there will be a dispersion of time lags and the average flow velocity can be obtained using a model of laminar flow in the pipe. The amplitude of the correlated signal is directly proportional to the fissile content and to other known parameters such as pipe diameter, source strength, detector efficiency, and fission fragments decay constants. Thus both the flow velocity and the fissile content can be obtained from these measurements. Although this example application of the invention uses the correlated component of the detector response, a similar data analysis yielding the same results can be performed using the cross correlation function or the cross power spectral density between the radiation detector and the source modulation signal.

FIG. 3 shows an electronic interface suitable for use with the invention. A position detector 310 for independently determining the neutron absorber (shutter) position inputs a first signal to a processor 320. The first signal is the source modulation signal. The processor 320 can be a personal computer (PC) based processor and is the controller of the neutron absorber (shutter) position.

The processor 320 outputs a control of shutter position signal to a motor 330 that adjusts the neutron absorber (shutter) position.

An inorganic scintillator 340 (e.g., Nal or BGO with lead shield) is connected to a photomultiplier tube 345. The scintillator 340 detects delayed gamma rays from the activated material. The photomultiplier tube 345 is connected to a base 350. The base 350 is connected to a power supply 355. Together, scintillator 340, photomultiplier tube 345, base 350 and supply 355 compose a gamma ray detector 360.

The gamma ray detector 360 is connected to a preamplifier 365. In fact, a plurality of detectors can be arranged around a single conduit and the summation of their signals sent to the preamplifier 365. The preamplifier 365 is connected to an amplifier 370. The amplifier 370 is connected to a discriminator 375. The discriminator 375 is connected to a pulse counter 380 that is part of the processor 320. Together, the gamma ray detector 360, the preamplifier 365, the amplifier 370, and the discriminator 375 compose a gamma ray detection channel. The gamma ray detection channel can be a standard commercially available detection-system electronics from ORTEC or TENNELEC. The gamma ray detection channel outputs a second signal to the processor 320.

Practical Applications of the Invention A practical application of the present invention which has value within the technological arts is in monitoring ²³⁵U mass flow in ²³⁵U enrichment plants (e.g., diffusion plants, gas centrifuge plants, etc.) in the U.S., Britain, France, and the former Soviet Union. Further, the present invention is useful in conjunction with other gases, liquids, and slurries such as, for example, natural gas, oil, and coal slurries where the material can be activated by thermal neutrons. The invention can be used as a maintenance tool to characterize blockage by measuring changes in flow velocities. There are virtually innumerable uses for the present invention, all of which need not be detailed here. Advantages of the Invention

A flow measurement system, representing an embodiment of the invention is cost effective and advantageous for at least the following reasons. The invention is nonintrusive (i.e., no penetration of the pipe required). The invention is easily installed and can be embodied in a portable package so as to be readily attachable-detachable without modification of the conduit to which it is attached. These external devices make maintenance easy, since all components are outside the pipe which contains the radioactive materials.

All the disclosed embodiments of the invention described herein can be realized and practiced without undue experimentation. Although the best mode contemplated by the inventors of carrying out the present invention is disclosed above, practice of the present invention is not limited thereto. It will be manifest that various additions, modifications and rearrangements of the features of the present invention may be made without deviating from the spirit and scope of the underlying inventive concept. Accordingly, it will be appreciated by those skilled in the art that the invention may be practiced otherwise than as specifically described herein.

For example, the individual components need not be formed in the disclosed shapes, or assembled in the disclosed configuration, but could be provided in virtually any shape, and assembled in virtually any configuration. Further, the individual components need not be fabricated from the disclosed materials, but could be fabricated from virtually any suitable materials. Further, although the apparatus described herein is a physically separate module, it will be manifest that the apparatus may be integrated into the apparatus with which it is associated. Furthermore, all the disclosed elements and features of each disclosed embodiment can be combined with, or substituted for, the disclosed elements and features of every other disclosed embodiment except where such elements or features are mutually exclusive.

It is intended that the appended claims cover all such additions, modifications and rearrangements. Expedient embodiments of the present invention are differentiated by the appended subclaims.

REFERENCES 1. Marks Mechanical Engineering Handbook, 10th ed., McGraw Hill, (Eugene A. Avallone et al. eds., 1996). 2. Perry's Chemical Engineers' Handbook, 6th ed., McGraw Hill, (Robert

H. Perry et al. eds., 1984). 3. Flow Measurement Engineering Handbook, McGraw Hill, Kingsport Press, (R. W. Miller, 1983).

4. The Electrical Engineering Handbook, CRC Press, (Richard C. Dorf et al. eds., 1993).

Claims

CLAIMS What is claimed is:

1. An apparatus, comprising: a neutron moderator block located adjacent an exterior wall of a conduit, said neutron moderator including a neutron source; a shutter including 1) a neutron absorbing material removably located between said neutron moderator block and said conduit, and 2) a motor for changing a position of said neutron absorbing material; a gamma ray detector located adjacent said exterior wall of said conduit; a gamma ray shield located between said gamma ray detector and said neutron source; and a processor connected to said gamma ray detector.

2. The apparatus of claim 1 , wherein said shutter includes a section of material that is substantially transparent to neutrons.

3. The apparatus of claim 1 , wherein said shutter includes a neutron detector connected to said processor, said neutron detector being adapted to detect neutrons entering said conduit from said neutron source.

4. The apparatus of claim 1 , wherein said shutter includes a position detector connected to said processor, said position detector being adapted to detect said position of said neutron absorbing material.

5. The apparatus of claim 1 , wherein said neutron source includes at least one nonstable isotope selected from the group consisting of Cf, Pu and Li.

6. The apparatus of claim 1 , further comprising an electronic interface including a preamplifier connected to said gamma ray detector, an amplifier connected to said preamplifier, and a discriminator connected to said amplifier, wherein said processor includes a pulse counter that is connected to said discrminator.

7. The apparatus of claim 1 , wherein said gamma ray detector includes an inorganic crystal scintillator, a photomultiplier tube connected to said inorganic crystal scintillator, a base connected to said photomultiplier tube and a power supply connected to said base.

8. A method for measuring a flow rate of a material in said conduit which comprises utilizing the apparatus of claim 1.

9. An apparatus, comprising: a neutron source located outside a conduit; a neutron absorbing material removably located between said neutron source and said conduit; a gamma ray detector located adjacent said conduit; and a gamma ray shield located between said gamma ray detector and said neutron source.

10. The apparatus of claim 9, further comprising a shutter located between said neutron source and said conduit, said shutter including said neutron absorbing material.

11. The apparatus of claim 10, wherein a position of said shutter is controlled by a motor.

12. The apparatus of claim 10, wherein said shutter includes a detector adapted to detecting a position of said shutter.

13. The apparatus of claim 10, wherein said shutter includes a material that is transparent to thermal neutrons.

14. The apparatus of claim 9, further comprising a neutron detector coupled to said source of thermal neutrons, said neutron detector being adapted to measure a transmission of thermal neutrons from said source of thermal neutrons to said conduit.

15. A method for measuring a flow rate of a material in said conduit which comprises utilizing the apparatus of claim 9.

16. A method, comprising: activating a material within a conduit at an upstream location with a source of thermal neutrons; and detecting gamma radiation from said material within said conduit at a downstream location with a gamma radiation detector.

17. The method of claim 16, wherein activating includes modulating said source of thermal neutrons to obtain a source modulation.

18. The method of claim 17, further comprising correlating the detected gamma radiation from said material with the source modulation by: determining a cross power spectral density function between the detected gamma radiation and the source modulation.

19. The method of claim 18, further comprising: determining a flow velocity based on the time lag of the cross power spectral density function; and determining a concentration based on the amplitude of the detected modulation.

20. The method of claim 19, further comprising calculating a product of the concentration and the flow velocity to obtain a mass flow rate.

21. An apparatus for performing the method of claim 16.

22. The method of claim 17, further comprising correlating the detected gamma radiation from said material with the source modulation by: determining a cross correlation between the detected gamma radiation and the source modulation.