Induced nuclear reactions: processes – systems – and elements – Testing – sensing – measuring – or detecting a fission reactor... – Fuel assay
Patent
1998-10-21
2000-03-07
Jordan, Charles T.
Induced nuclear reactions: processes, systems, and elements
Testing, sensing, measuring, or detecting a fission reactor...
Fuel assay
376254, 376245, 376153, G21C 1706
Patent
active
060350105
DESCRIPTION:
BRIEF SUMMARY
The invention refers to a monitor for measuring both the gamma spectrum and neutrons emitted by an object such as a spent nuclear fuel pin or pin assembly or nuclear waste material.
In particular, this monitor is intended for the in situ measurement of neutrons and gamma radiation emitted by a spent nuclear fuel assembly to characterize the nuclear fuel in terms of burn-up, cooling time and inventory.
It could further be used for example to control at the customs containers which might contain fissile material, the traffic of which is forbidden.
Such measurements have been carried out until now in two different measuring apparatus, one equipped with a gamma detector and the other with a neutron detector. These devices must be associated to a mechanism for relative displacement of the measuring head with respect to the fuel pin or fuel assembly which might have a length of 3 meters or more.
Apart from the cost of providing two measurement apparatus intended to scan successively the fuel or from the problem of interchanging the measurement head of a unique apparatus in an highly dangerous environment, the precision of the measurement results depends also on the precision of the correlation between the two measurement cycles in order to associate to each scanned point of the measured fuel pin or assembly a couple of measurement values relating both to the gamma spectrum and the neutron emission.
It is well known in the art that gamma spectroscopy is performed by means of a gamma detector located in or behind a radiation protection such as lead or concrete, through which a tiny collimation channel is pierced. The open end of this collimator is intended to be close to the fuel pin or assembly to be scanned, whereas the other end is coupled to the gamma detector which might be a CdTe-crystal. Such a detector measures the gamma radiation spectrum which exists close to the open end of the collimator.
The neutron detectors commonly employed are cylindrical .sup.235 U fission chambers of about 70 mm length and 10 mm diameter, which are sensitive to thermal neutrons, but very insensitive to the intense gamma radiation emitted by the spent fuel. They should be placed with their axis perpendicular to the axis of the collimator and as close as possible to the point of measurement. A moderator material thermalizes the incident neutrons.
If a head combining both gamma spectroscopy and neutron detection was considered, then both detectors should be as close as possible to the scanned measurement point on the fuel pin or assembly. But the neutron detector should not be located too close to the collimator of the gamma detector in order not to unduly weaken the shielding effect of the material surrounding the collimator. Any compromise would yield in a distance between the two detectors which renders impossible a precise simultaneous measurement of both detectors at the same measurement point. The errors induced by this distance in the simultaneous measurement cannot be tolerated, if the values to be measured vary substantially along the scanning path, which is especially the case at the ends of the fuel pins.
The invention proposes a monitor which ensures simultaneous measurements of the gamma spectrum and of the neutron emission, which can be related to any particular scanning point on the fuel pin or assembly.
According to the invention, this aim is achieved by a monitor as defined in claim 1. As far as preferred embodiments of this monitor are concerned, reference is made to the secondary claims.
A preferred embodiment of the invention will now be described in more detail by means of the accompanying drawings.
FIG. 1 shows schematically and in perspective the monitor according to the invention.
FIG. 2 represents a cut view of the monitor of FIG. 1 in relation to a fuel pin to be monitored.
FIG. 3 represents a detail of an alternative realisation of the monitor.
Referring now to FIGS. 1 and 2, the monitor according to the invention comprises a lead block 1 in which a gamma detector 2 for gamma spectroscopy as well as two n
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George Nicolaou
Kamel Abbas
Lothar Koch
European Atomic Energy Community (EURATOM)
Jordan Charles T.
Keith Jack
LandOfFree
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