Induced nuclear reactions: processes – systems – and elements – Nuclear transmutation – By neutron bombardment
Reexamination Certificate
1999-12-27
2002-09-17
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Nuclear transmutation
By neutron bombardment
C376S153000, C376S154000, C376S157000
Reexamination Certificate
active
06452992
ABSTRACT:
DESCRIPTION
1. Technical field
The present invention relates a method and device for measuring the relative proportions of plutonium and uranium in a body.
It applies notably to the differentiation of plutonium and uranium contained in large packages of radioactive waste. Large package means concreted waste barrels with a diameter of around one meter or more, or metallic containers whose volumes can be as much as several cubic meters.
It is in fact important, during the storage of radioactive waste, to know the nature and activity of the radioemitters and in particular of the actinides, that is to say essentially uranium and plutonium.
2. Prior Art
The known methods of determining the actinide content of a body can be classified into two categories. In fact the so-called “destructive” methods are distinguished from the so-called “non-destructive” methods.
When destructive methods are used, samples are taken by cutting the package of radioactive material. The samples taken are then analysed using different analysis techniques, amongst which chemical analysis, X-ray fluorescence, gamma spectrometry or neutron activation can be mentioned, for example.
The methods mentioned above are applicable only to samples with a small volume, from a few cubic centimeters to a few liters. They are therefore ineffective for non-destructive measurements of radioactive waste packages of large size.
Amongst the methods of determining the actinide content known as non-destructive, there are also two sub-categories comprising respectively the active non-destructive methods and the passive non-destructive methods.
The passive non-destructive methods are essentially methods of spectrometry of the gamma radiation emitted by the body, counting the neutrons emitted during the spontaneous fission of the actinides contained in the body, or calorimetry.
The method of gamma spectrometry of the radiation emitted is also applicable only to homogeneous samples of small size. This is because counting the neutrons emitted during the spontaneous fission of the actinides in larger samples requires sophisticated equipment, notably for being free of the influence of neutrons coming from sources other than the spontaneous fission of the body to be measured.
The calorimetry method makes it possible to evaluate the total quantity of heat released in a radioactive body, such as a package of waste. It does not however make it possible to determine the type of disintegration which gives rise to the heat.
Finally, active non-destructive methods are known for determining the actinide content of a body. These methods are said to be active because they use a radiation source known as interrogating radiation, external to the radioactive body.
Amongst these active non-destructive methods, there are the measurements of attenuation and the measurements of emitted radiation.
Measurements of attenuation, such as gamma measurement or tomography, measure the attenuation of an external radiation passing through the radio package to be examined. The measurements of emitted radiation, on the other hand, measure a radiation coming from the package itself and caused by an external interrogation radiation. The latter measurements are well adapted to an examination of packages of radioactive waste of large size such as concrete containers. However, the active non-destructive methods known at the present time make it possible only to determine the total quantity of actinides present in a package, without any distinction with regard to their nature or composition. In particular, they do not make it possible to differentiate the uranium from the plutonium contained in the body or radioactive package examined.
DISCLOSURE OF THE INVENTION
The aim of the present invention is an analysis method and device which does not have the limitations mentioned above.
Another obvious aim is to propose a non-destructive measuring method and device which make it possible to differentiate uranium and plutonium in radioactive waste packages.
In order to achieve these aims, the object of the invention is more precisely a method of measuring relative proportions of uranium and plutonium in a body, including the following steps:
a) irradiating the body with photons whose energy is sufficient to cause photofission of actinide elements,
b) counting the number of delayed neutrons emitted per unit time by fission products induced in the said body in response to the irradiation,
c) establishing a time decay function of the number of neutrons n
e
(t) emitted, characteristic of the actinide composition of the said body,
d) comparing the time decay n
e
(t) characteristic of the actinide composition of the said body, with time decays of emission of delayed neutrons n
u
(t), n
p
(t) characteristic respectively of uranium and plutonium, in order to establish the relative proportions of these elements in the body.
Thus the invention is essentially based on the photon activation of the actinides and on the measurement of decrease over time in the number of delayed neutrons.
Unlike neutron activation, which suffers from the short travel of the neutrons in materials containing hydrogen, such as concrete or bitumen, photon activation, used in the method of the invention, proves particularly adapted to waste packages of large size. The attenuation of the photons, notably of the photons with an energy greater than 10 MeV, is in fact low.
Moreover, the detection and counting of delayed fission neutrons has, compared with the counting of prompt neutrons, the advantage of freeing the measurement from the reactions on the nuclei of the matrix containing the radioactive waste. These reactions are in fact added to the phenomenon of photofission of the actinide nuclei during the photon activation.
The prompt neutrons are emitted during fission, and therefore during the gamma irradiation pulse period. The gamma photons also give rise to neutrons, through the neutron-gamma (n, &ggr;) reaction induced in the different elements. It is not possible to make a distinction between the prompt fission neutrons and the neutrons originating from the neutron-gamma (n, &ggr;) reactions.
The photonuclear reaction of the photons on the actinide nuclei takes place in two steps contained within an interval of time of around one picosecond. A photon is first of all absorbed by the nucleus. The absorbed energy then causes the emission of a photon or one or more particles. When the absorbed energy is greater than the fission threshold of the nucleus it causes on the other hand the fission of the latter.
The effective cross section of absorption of a photon by the atomic nucleus varies with the energy of the photon. For a photon whose energy is less than approximately 6 MeV, very narrow absorption resonances are observed, with a width of approximately 1 eV. On the other hand, for photons with energies greater than 10 MeV, a very broad absorption resonance of several MeV is observed. The absorption resonance energy for photons is respectively 12.26 MeV and 12.24 MeV for uranium 238 and plutonium 239.
The energy of the photons used for irradiating the radioactive body is chosen so as to be sufficient to cause photofission of the actinide elements. In particular, it is chosen so as to be greater than the is photofission threshold, which is situated at approximately 6 MeV. It is preferably chosen so as to be greater than 10 MeV and close to the absorption resonance energy.
The radioactive body is preferably irradiated for a sufficiently long irradiation time to accumulate a sufficient number of fission products and consequently to obtain a significant emission of delayed neutrons. However, it is not necessary to prolong the irradiation unnecessarily. Thus the irradiation time is preferably chosen so as to be around the mean period of the delayed neutrons, that is to say around 10 to 20 seconds, for example.
After the irradiation of the radioactive body to be examined, a counting of the neutrons is effected, preferably in a multiscale mode. The multiscale mode consists of counting the number of neutrons detected in
Carone Michael J.
Commissariat A. l'Energie Atomique
Keith Jack
Pearne & Gordon LLP
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