Selective flow path alpha particle detector and method of use

Details Selective flow path alpha particle detector and method of use Selective flow path alpha particle detector and method of use

2000-07-11

2002-09-24

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Including a radiant energy responsive gas discharge device

C250S374000, C250S376000, C250S394000, C250S386000, C250S387000

Reexamination Certificate

active

06455859

ABSTRACT:

BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention concerns improvements in and relating to monitoring and/or detecting and particularly, but not exclusively, to monitoring of multi-surface items, such as pipes, for alpha source contamination.
2. Present State of the Art
Any item which passes time within the active area of a nuclear facility may become contaminated by radioactive material from within that area. As a result, before the item can be removed and subsequently re-used, disposed of or recycled its potential contamination needs to be evaluated. The absence of contamination may allow an item to be reused, whereas the presence of contamination may call for the item's safe disposal.
Detection and monitoring of alpha contamination of such items presents a number of difficulties, principally due to the short distance over which alpha particles can be detected. Alpha particles are stopped by 10's of micrometers of solid material and within a few centimeters in air. Detectors further away than these distances cannot detect the alpha contamination. In certain scenarios this, therefore, makes the detection of the alpha contamination very difficult, calling for close proximity scanning of the article with a detector. In other scenarios such detection is physically impracticable as the inside surfaces of, for instance, pipes, scaffolding and ducts are not readily accessible to such detectors and the detectors cannot monitor the alpha contamination through the walls of such items.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
The present invention aims to provide apparatus and methods for the successful monitoring of items for alpha contamination, with particular emphasis on poorly accessible and/or inaccessible surfaces.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein, according to a first aspect of the invention we provide a method for detecting alpha sources on and/or associated with an item, the method comprising providing the item in an enclosed volume, providing a flow of gas from an inlet to the volume past the item to an outlet, the gas passing the item, at least in part, passing through an electric field formed by the application of an electrical potential to at least one of a plurality of electrical conductors, the current in at least one electrical conductor being detected.
In this way air passing over the item in the volume is partially ionized by any alpha contamination present and this can be detected when the ions are attracted to the conductors, due to the electric field, thus causing a current to flow.
The item may provide one or more discrete flow paths over its surface and/or surfaces. For instance, a pipe may have an external flow path separated from an internal flow path by the material forming the pipe. The method may further provide the division of the item's surface to form a plurality of discrete flow paths.
The method may provide for regulating the gas flow along one or more of the discrete paths.
Preferably the method provides for detecting alpha sources of one or more of the surfaces forming the discrete flow paths. The method may provide for monitoring of less than all the flow paths by blocking one or more of the flow paths to the passage of gas.
One or more of the discrete flow paths may be blocked by sealing that flow path. The seal may be provided between the item and the volume walls and/or across a flow path defined solely by the item. The flow path may be blocked by an inflatable seal. The flow path may be blocked by a variable aperture seal, such as an iris seal. The seal may be electrically non-insulating.
Where the item is of the general form of one or more walls defining a through passage, the method may provide for detecting alpha sources in the through passage and/or on the external wall(s) and/or both. Such items are typified by pipes, scaffold lengths, ducts, conduits and the like. The through passage may be blocked by a seal extending across the through passage. The external passage, between the item and the volume wall(s) may be blocked by a seal extending there between. The seal may be electrically non-insulating.
Preferably the discrete passages are blocked at, or towards, their end nearest the outlet.
The method may provide for detection using gas flow along all the discrete paths with further detection using gas flow along one or more of the discrete paths individually. All the discrete paths may be detected individually. A value for the total contamination level and/or contamination level of one or more of the discrete paths may be provided.
The gas flowrate through the instrument is preferably controlled. The flowrate may be adjusted in response to signals indicative of the flowrate. The flowrate may be adjusted by adjusting the speed of the fan. The signals indicating the flowrate may arise from an anerometer.
A gas flowrate of between 1 and 3 m/s is preferred. A gas flowrate of between 1.25 and 2.75 m/s, and more preferably between 1.5 and 2.5 m/s, may be provided, particularly where the inner seal is open and/or where the inner seal is open and the outer seal is shut.
A gas flowrate of between 1.25 and 1.75 m/s, and preferably between 1.4 and 1.6 m/s, may be provided where the outer seal is closed and/or where the outer seal is closed and the inner seal is open.
The optimum gas flowrate for the inner valve open may increase as the length of the item being monitored increases. A flowrate of 1.5 m/s+/−10% is preferred for a 1.75 to 2.25 meter length of item. A flowrate of 2 m/s+/−10% is preferred for a 3.5 to 4.5 meter length of item. A flowrate of 2.5 m/s+/−10% is preferred for a 5 to 7 meter length of item.
The enclosed volume may be elongate about an axis. A square, rectangular or circular cross-section volume, across the axis, may be provided.
Preferably only one inlet and/or one outlet is provided. Preferably the inlet and/or outlet oppose one another.
The electric field may be formed by the application of an electrical potential to an electrically conducting wall or portion thereof, of the volume and/or to a discrete conductor. The electric field may be formed between two or more conductors. Two or more conductors with applied potential and/or two or more conductors of a different potential, such as earthed, may be used. Applied potential conductors and different potential conductors may be paired with one another. The number of applied or different potential conductors may exceed the other, preferably by one.
Two or more of the applied potential conductors may be connected together. Alternatively or additionally two or more of the different potential conductors may be connected together.
The current is preferably detected by an electrometer.
The current in the applied potential and/or different potential conductors may be determined.
The method may include the filtering of gas entering the volume and/or exiting the volume. The gas may be filtered to remove particulate matter and/or ions. Preferably the gas is the ambient gas for the monitoring location, most preferably air.
The plurality of electrical conductors may include a wall of the volume or a portion thereof.
Preferably the item is supported within the volume to provide a flow of gas over substantially all the item's surface. The item may be supported on a frame or cradle. Preferably the item is supported in general alignment with the volume's axis.
Preferably the discrete flow paths are aligned with the electrical conductors and/or with the volumes axis. It is particularly preferred that the inlet, the item'discrete flow path(s), the electrical conductors and the outlet all be generally aligned along a common axis.
The item may be supported such that the axis of the volume passes along one of its discrete flowpaths. Where the item has an inner discrete path, preferably this path is aligned with the axis and/or center-line of the volume.
The first aspect of the invention may include any of the options, possibilit

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