Radiant energy – Invisible radiant energy responsive electric signalling – Neutron responsive means
Patent
1997-04-15
1998-10-27
Glick, Edward J.
Radiant energy
Invisible radiant energy responsive electric signalling
Neutron responsive means
250394, G01T 300
Patent
active
058280693
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. TECHNICAL FIELD
The present invention relates to the field of dosing .alpha. emitters in solid waste, with an aim of non-destructively knowing the quantities of certain elements (e.g. actinides) contained in this type of waste. It more specifically relates to an enclosure for the irradiation of waste and counting of neutrons emitted following irradiation of said waste. It has a direct application in installations using active or passive, neutron and/or photon dosing methods. In particular, the dosing of .alpha. emitters in solid waste makes it possible to classify them, account for the stored .alpha. activity or carry out a fissile material balance. This analysis is a vital stage in any radioactive waste management or control program.
2. PRIOR ART
With a view to non-destructively knowing the actinide quantity contained in a waste, several measuring methods have been studied, including the photon interrogation method by bremsstrahlung photon bias. Thus, a bremsstrahlung photon beam generally emanating from a pulse electron linear accelerator is used. These energetic photons induce photofission reactions on the heavy nuclei present in the waste. The measurement is based on the detection of prompt and/or delayed photofission neutrons. This method makes it possible to determine the quantities of fissile and fertile elements contained in the waste to be dosed or assayed. It is described in the article by A. LYOUSSI et al entitled "Low level transuranic waste assay by photon interrogation and neutron counting", Institute of Nuclear Material and Management (INMN), 34th Annual Meeting, Jul. 18-21, 1993, Scottsdale, Ariz.
In more general terms, it is possible to use two measuring methods, namely low energy irradiation and counting the prompt photofission neutrons and high energy irradiation followed by counting of the delayed photofission neutrons.
The first method has the advantage of good statistics. However, the reactions (.gamma., n) for the production of photoneutrons on different elements, apart from the transuranic elements, produce a parasitic signal rising with the energy of the photons. This gives rise to the need to work at relatively low gamma energies. This has the effect of reducing the number of useful reactions, the effective photofission cross-section being a rising function of the incident photon energy. This limits the counting of the prompt signal, whose intensity decreases in a few hundred microseconds.
The second method is in principle insensitive to instantaneous parasitic photoneutrons. Thus, it is based on the counting of delayed neutrons.
Initial investigations with the second method revealed a neutron component persisting for a long time after the photon pulse. Thus, the neutrons or rather the photoneutrons formed within the conversion target are diffused by air molecules and then rebound on the concrete walls and the materials of the surrounding structure. The typical dimensions of a room in which such a measurement is performed are generally adequate to ensure that these neutrons undergo an effect identical to that of a ping pong ball on a ping pong table and consequently their life is non-negligible, being approximately 5 milliseconds. Thus, the useful delayed signal is drowned in this parasitic photoneutron signal.
A first apparatus has been proposed making it possible to improve the performance characteristics of the method. This apparatus is described in the aforementioned publication. In brief and as illustrated in FIG. 1, it has a target 2 placed on the path of an electron beam 3. The electron-target collision makes it possible to produce bremsstrahlung. The waste 4 is contained in an enclosure, whose walls 6, 8, 10, 12 are e.g. made from polyethylene, located between two cadmium coatings 14, 16. The wall also contains .sup.3 He detectors for counting the neutrons.
It is possible to identify the influence of the cadmium by carrying out a recording, as a function of time, of the neutrons (.gamma., n) by a counting unit with and without cadmium. These results are ill
REFERENCES:
patent: 5278417 (1994-01-01), Sun
"Low Level Transuranic Waste Assay by Photon Interrogation and Neutron Counting" Lyoussi et al.
Institute of Nuclear Material and Management (INMM), 34th Annual Meeting, Jul. 18-21, 1993, Scottsdale, Arizona.
"Determination of Alpha Activity and Fisstle Mass Content in Solid Waste by Systems Using Neutron Interrrogation" Dherbey et al Commissariat a L'Energie Atomique; Departmetn de Recherche Physique 1983; pp. 97-106.
Laine Frederic
Lyoussi Abdallah
Umiastowski Krzysztof
Commissariat a l''Energie Atomique
Glick Edward J.
Jiron Darren M.
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