A PROCEDURE TO MEASURE THE RADIOACTIVITY OF RADIOACTIVE MATERIAL ENCLOSED IN A CONTAINER

Abstract: The procedure comprises the steps of: weighing a known volume container (4) in order to determine the average density of its content; positioning the container (4) upon a support (1); positioning on respective supports (5) two gamma ray detectors (6) set symmetrically to the container; causing vertical movement (F2) of the container, so that its main longitudinal axis coincides with the axis (I-I) connecting the two detectors (6); causing movement (F1) of the two detectors upon their respective supports in order to stop them in a first tentative position; causing movement (F1) of the two detectors in order to stop them in a second symmetrical position that proves to be suited to providing countings of the radiations that are an optimum; measuring the gamma radiation and calculating the geometric mean of the counting ratios of the two detectors, such a geometric mean being linked to the intensity (I) of the corresponding gamma radiation by a formula (1-2) from which the radiation intensity is calculated.

Description: A procedure to measure the radioactivity of radioactive material enclosed in a container. The present invention concerns a procedure to measure the radioactivity of radioactive material enclosed in a container, more especially to measure the radioactivity due to radioactive sources and/or active or contaminated materials set within a sealed container During the performance of activities in the nuclear field (power plants, research laboratories, analysis laboratories, etc.) a considerable quantity of weakly radioactive solid wastes are produced that are normally gathered in containers that are then sealed. In order to correctly manage these wastes it is necessary to be able to rapidly and reliably check the nature and extent of the contamination present. Because we are dealing with sealed containers, the only technique applicable is that of externally measuring the γ radiation, from which, by means of appropriate calculations, it is possible to achieve the overall contamination ct + β +γ. The main problem in the interpretation of the results of the γ measurements is the assessment of the detection af iciency, owing to the fact that the sources are of large dimensions within which both the distribution and nature of the materials and the distribution of the activity is widely unknown. At the state of the art, the assessment of activity is generally conducted by means of the γ count in various lateral positions outside the container based on the hypothesis that the contents feature a homogeneous composition and that the distribution of activity is known, or else by means of the γ dose measurement performed along the sides of the container, measurements from which one then finds the value of the source activity by means of appropriate calculation procedures Laboratory measurements conducted on containers originating from nuclear power plants, some of which contained exhausted ion exchange resins and others with technological wastes, have shown differences also in excess of one order of magnitude between the values of activity calculated with such techniques and those of the true activity, as a consequence, the criteria f i radiation protection adopted in the handling and storage of low specific activity wastes appear to be excessively conservative The invention obviates the above mentioned drawbacks, it is a procedure to measure the radioactivity of radioactive materials enclosed in a container by means of γ ray detectors positioned outside the container and, as characterized in the claims, comprises the following phases i) weighing of a known volume container in order to determine the average density of its content, ii) positioning of the container upon a support, iii) positioning on respective supports of two γ ray detectors set symmetrically compared to the main tiansversal axis of the container, facing the smaller walls of the container, iv) vertical movement of the container, so that its main longitudinal axis coincides with the axis connecting the two detectors, v) movement of the two detectors upon their respective supports in order to stop them in a first symmetrical position compared to the container, usefully defined on the basis of experience, where the intensity of the radiation from the container is sampled in order to define the best counting geometry vi) movement of the two detectors upon their respective supports in order to stop them in a second symmetrical position compared to the container that proves to be suited to providing countings of the radiations that are an optimum in terms of accuracy of the measurement and counting time, the distance of such positions generally being greater than the largest dimensions of the walls of the container facing the detectors. vii) measurement of the γ tadiation and calculation of the geometric mean expressed as cps, of the counting ratios of the two detectors, such a geometric man being linked to the intensity of the corresponding γ radiation by the following formula when the activity is mainly concentrated in a limited region of the container, and by formula (C, β. J F 1 - ε ϊ (2) when the activity is distributed in a sensibly uniform manner within the container viii) calculation of the intensity 1 of the γ radiation by means of one of the above formulas It is understood that the container, generally in the form of a cylindrical drum, will be preferably positioned with its longitudinal axis horizontal so that each of the two circular walls faces one of the detectors. It is also understood that the positioning of the two detectors symmetrically compared to the main transversal axis of the container is not a compulsory condition; a non symmetrical positioning only involves a greater complexity of the mathematical procedure. In these formulas: μ * = (2/D) +μ where: μ is the mean attenuation coefficient for the material contained, expressed as cm " ', I is the intensity of the source, expressed as dps β is the branching of the γ line considered, FI is the absorption factor of the container base, F2 is the absorption factor in air; α is a parameter that depends on the energy of the photons and on the characteristics of the detectors, but not on the distance between container and detector, nor on the dimensions of the container, or on the nature of the wastes; D is the distance between a detector and the wall of the container facing the detector, expressed in cm., including the distance covered on average by the photons inside the container (in practice D is the distance between said wall of the container and the centre of the detector); L is the height of the container, expressed in cm.. The quantities β, D, L are known, while F2, very close to 1 , is calculated. The calibration of the apparatus hence requires the determining of α and Fl To this purpose a series of measurements are performed by setting calibrated sources, grouped in various ways, outside an empty container, as close as possible to its walls opposite the detectors, both on the side of the detector and on the other (distances Dl and D2, as shown in Fig. 2) The counts in the two positions are expressed as (X C ( Dι ) A /} - F ? ( Dι ) α and quantity FI is obtained by Next, α is calculated from the values of C(D1 ) and/or C(D2) Because the chemical composition of the wastes is not known, also the value of the attenuation coefficient required to calculate efficiency is not known However, within the energy interval involved in this type of measurement the attenuation coefficient at a given energy, expressed in cm 2 /g, is sensibly independent from the atomic number This leads to the ratio between the attenuation coefficients of two different substances, expressed in cm '1 , being roughly equal to the ratio between their densities It is therefore possible to adopt a conventional value for the attenuation coefficient μ coιn , that can be derived from the product of the water attenuation coefficient expressed in cm'/g, with the average density of the material gathered in the containers μ co n v= μ (H 2 O) p The values of μ (H 2 O) reported in literature are 0 085, 0 066, 0 062 cm 2 /g at 662, 1 173, and 1332 keV respectively The main advantages of the procedure invented are that said geometric mean is sensibly independent Horn the distribution of the material within the container and that the procedure offers a good sensitivity and accuracy of measurements in every imaginable practical situation featuring non homogeneous sources An optional detector of γ radiations is made to slide parallel to the greater dimension of the container in order to provide further information useful in deciding whether to choose formula ( l ) or formula (2) The performances of the measurement technique described have been theoretically evaluated for all possible combinations of non homogeneous materials and radioactivity within the containers To carry out the procedure invented very simple apparati may be used, as can fully automated ones both for the movement of the various parts (mainly containers and detectors) and for the detection and count of the radiations An example of embodiment of the invention that isn't simply the purely manual movement of the container and detectors, that is in fact to be avoided for the safety of personnel, is described below and comprises a) a supporting structure for a container associated to a positioning device suited to setting the container between two opposite detectors for γ radiations and above a device suited to weighing the container in order to measure the average density of its contents; b) a pair of supports for said detectors symmetrically positioned compared to the main axis of the container; c) a motor means controlled by a processor in order to produce equal and opposite translations of both detectors, in order to position the two detectors at a first distance from the detectors and perform a first count to gauge the radiation and hence rc-position them at a second distance in which the radiation count is at an optimum both from an accuracy viewpoint and considering the time of measurement. The following figures help illustrate the example: Fig. 1 is a plane view. Fig. 2 is a front view, according to arrow F in Fig. 1 , Fig s 1 and 2 together show a supporting structure 1 extended horizontally and bearing a loading plane 2 of a weighing device 3 suited to weighing a sealed drum 4; on each side of the structure 1 a horizontal support 5 is foreseen to bear a detector of γ radiations, shown with 6, that a motor means 7, controlled by a computer 8, longitudinally move along the two directions of arrow FI ; a motor means M also controlled by the computer 8 that vertically moves the loading plane 2, according to arrow F2, so that the longitudinal axis I - I of drum 4 coincides with that of detectors 6; the electric wires 9 connect computer 8 to the motor means 7 and M and the electric wires 10 connect computer 8 with detectors 6 (in Fig. 2 the wires are not shown); a third detector of γ radiations 6*, positioned upon a support 5* positioned at a convenient distance from and above drum 4, also connected to computer 8 by means of a connector that is not shown, that is made to move in the directions of arrow FI in order to provide data concerning the distribution of radioactivity within the container 4. In this example, the distance D l at which the detectors 6 are positioned compared to drum 4 so as to operate according to the procedure is greater than the dimension W of the circular walls of the drum 4, as stated in the above paragraph vi), because this condition is essential to obtain the best results from the procedure. The computer 8 is programmed also to process the data supplied by an operator, gather the γ spectra and solve the formulas ( I ) and (2)

Claims: Claims 1 A procedure to measure the tadioactivity of the radiactive material contained in a container (4) by means of γ ray detectors (6) positioned laterally, outside the container, characterized in that it comprises the following stages i)weighing of a container (4) of which the volume is know in order to determine the average density of its contents, ii) positioning of the container (4) upon a support ( I ), iii) positioning on restective supports (5) of two γ ray detectors (6) set symmetrically compared to a main transversal axis (II-II) of the container, facing the smaller (W) walls of the container (4), iv) movement (F2) of the container (4), so that its main longitudinal axis coincides with the axis (I-I) connecting the two detectors (6); v) movement (FI ) of the two detectors (6) on their respective supports (5) to be stopped in a first position symmetrical with the container, to be usefully delined on the basis of experience, in which to sample the intensity of the radiation from the container in order to define the best counting geometry; vi) movement (FI ) of the two detectors (6) on their respective supports (5) to be stopped in a second postion symmetrical with the container (4) that proves to be suitable to provide optimal radiation measurements compared to the accuracy of the measurement, the distance of such a position (Dl ) must be greater than the greater of the wall dimensions (W) of the container facing the detectors (6), vii) measurement of the γ radiation and calculation of the geometric mean expressed in cps, of the measurement ratios of the two detectors (6), such a geometric mean being linked to the intensity ( l ) of the corresponding γ radiation by formula ( l ): ( , C 2 ) 1/2 I β l 1< (al D 1 )- exp(-/ •— A)___/ .^. / ./ . ffj (1) 2 ' when the activity is mainly concent ιated in a limited legion of the container; viii) derive the intensity (I) of the γ radiation from the above stated formula (1). 2. A procedure according to claim 1 characterized in that the intensity (I) of the γ radiation is calculated by means of the formula (2) (C] βP; F 2 ε 2 (2) when the activity is distributed in a sensibly uniform way within the container. 3. A procedure according to claims 2, 3 characterized in that a third γ ray detector (6 *) is made to slide parallel to the main greater axis (I-I) of the container (4) and at a convenient distance in order to provide further useful information in deciding whether to use formula (1 ) or formula (2).

Inventors: BORIOLI ENRICO, CESANA ALESSANDRA, SANDRELLI GIANCARLO, TERRANI MARIO, BORIOLI, ENRICO, CESANA, ALESSANDRA, SANDRELLI, GIANCARLO, TERRANI, MARIO
Applicants: BORIOLI ENRICO, CESANA ALESSANDRA, ENEL SPA, SANDRELLI GIANCARLO, TERRANI MARIO, BORIOLI, ENRICO, CESANA, ALESSANDRA, ENEL S.P.A., SANDRELLI, GIANCARLO, TERRANI, MARIO
Application year: 1998
Main classification: G01T7
IPC codes: G01T1, G01T7