LU84158A1 - APPARATUS AND METHOD FOR MEASURING GRANULOMETRIC DISTRIBUTION AND MASS CONCENTRATION OF AEROSOL PARTICLES OF NUCLEAR FUELS - Google Patents

Description

The present invention relates to an apparatus for measuring the particle size distribution and the mass concentration of aerosol particles of nuclear fuels, as well as to a measurement method using this apparatus.

Various types of measuring apparatus are known in the prior art, but all have certain drawbacks.

a) The aerosol centrifuge device.

In this case, the aerosol enters the center of a cut portion of the spiral line in a rotor rotating at speeds 10 of up to 3000 rpm. Large particles are deposited on the wall of the pipe near the entrance, while smaller particles are deposited further in the pipe, where the centripetal acceleration is higher.

In this way, the particles are separated along a spectrum corresponding to their aerodynamic and inertial properties. (See W. Stöber, H. Flachsbart "Size separating precipitation of aerosols in a spinning spiral duct", Env, Sci. Tech 3 (1956) 1280-1296.) However, with this device, the mass concentration is not measured, particles larger than 5 µm cannot be separated, particle losses are large in the inlet of the centrifuge and depend on the size of the particles, and the user must design their own means of determination the particle size distribution of the particles; from the deposit on the wall of the pipe.

; B) The inertial spectrometer,. In this case, an aerosol stream, surrounded by a carrier gas, is sent into an abrupt 90 ° bend and is then sucked through a filter where the aerosol particles are j retained. During the passage through the 90 ° bend, the large particles slide relative to the gas flow more than the | small particles, so that a separation of particles appears on the filter which is a function of their inertial properties.

I (See V. Prodi et al. "An inertial spectrometer for aerosol particles", J. Aerosol Sci. 101 (1979) 411-419.) I 35 However, with this apparatus, the user must imagine. His own means of determination of the particle size distribution from the deposit made on the filter, the mass concentration is not measured, and the entry of very thin aerosol K is likely to be partially clogged, which causes I a distorted aerosol deposition figure.

I * c) The horizontal separator by entrainment • 5 In this case, a stream of aerosol is sucked into a narrow lateral slit K formed in the upper part of a horizontal duct I with a height of about 5 mm , with a width of! . about 50 mm and about 600 mm long. While the aerosol is transported along the pipe by a draft,! - 10 aerosol particles are deposited on the lower part of the I "pipe with speeds which are proportional to their aerodynamic diameters, which makes it possible to separate the aerosol particles according to their sizes. See VJ. Stöber I "Zur Bestimmung von Teilchengrößenverteilungen mit einem Horizontal- 1 15 Elutriator", Staub 24 (1964) 221-223.) 1 However, with this apparatus, the speed of aerosol sampling is very low , the whole apparatus must be thermally insulated to minimize the convection currents in the pipe, the user must imagine his own method of obtaining the particle size distribution from the deposit I ′ on the lower part of the pipe. driving, and the mass concentration K is not measured.

K d) The optical counter I In this case, an intense light beam 1 ”25 is focused on a stream of aerosol particles. The aerosol particles passing through the focused beam create pulses of scattered light I, which are measured by a photodetector, The size of each scattered light pulse and the signal amplitude of the detector are proportional to the particle size of § 30 l 'aerosol. (See K.ï, Whicby, K. vlilleke "Single Pareicle Opticaî" Counters: Principles and Field Use "p. 145-162," Aerosol Measurement "(1979), University of Florida Press.)

However, with this device, the amount of light that | is diffused by irregularly shaped particles depends on the orientation of the particles, the partially illuminated particles increase the apparent concentration in small particles, while Λ 3 that the coincident illumination of particles increases the apparent concentration in large particles, the apparatus being also expensive.

e) The depSt 5 microbalance In this case, we start by placing a sample of powder in suspension in a liquid, then it is left to deposit on the immersed tray of a microbalance. The particle size distribution of the particles can be deduced from the resulting cumulative time-weight curve using Stokes' law. (See S, Oden, Proc. Roy.

10 Soc. Edinb. 36 (1915) 219, A.E. Jacobsen, W.F. Sullivan, Ind. Eng.

Chem. 19 (1947) 855 and W. Bostock, J. Sei. Instr. 29 (1952) 209.)

However, with this apparatus, it is not easy to measure the particles of a size less than about 5 μπι, and a high mass concentration of the particles is required, so that the apparatus is better suited for measuring the powders than aerosols.

According to the invention, there is provided an apparatus for measuring the particle size distribution and the mass concentration of particles of a nuclear fuel aerosol comprising a container arranged vertically and intended for the aerosol, a means allowing the entry of the aerosol in the container, and an alpha particle detector placed or capable of being placed inside the container and having a detection window arranged horizontally and directed vertically, so as to allow the detection of the alpha activity of the particles of l aerosol that settle on the window.

The invention also provides a method for measuring the particle size distribution and the mass concentration of the particles of a nuclear fuel aerosol, this method being so much that the aerosol is entered into a vertically oriented container intended to contain it, placing an alpha particle detector inside the container so that its detection window is horizontal and directed vertically, stopping the admission of aerosol into the container, detecting the alpha activity of the particles aerosol cules which deposit on the detection window, and; 'h convert the measurement of alpha activity into measurements of particle size distribution and, or, of mass concentration of particles.

In the apparatus according to the invention, an aerosol cloud of nuclear fuel is allowed to settle on an alpha particle detector. When the aerosol particles deposit on the detector, the measured alpha activity increases. The analysis of the curve of the alpha activity as a function of time, carried out in a manner analogous to that used with the deposit microbalance, makes it possible to calculate the particle size distribution.

The use of an alpha activity detector greatly increases the sensitivity of the method compared to the microbalance deposition of the prior art, so that it is possible to make direct measurements on aerosol clouds of nuclear fuel containing particles as small as 1 ^ m.

The following description, intended to illustrate the invention, aims to give a better understanding of its characteristics and advantages; it is based on the appended drawings, among which: - Figure 1 is a sectional view of the apparatus according to the invention, taken along a plane perpendicular to the axis of articulation and along the axis of the tube , the apparatus being presented in its working position; - Figure la is a simplified view of the device in its cleaning position; FIG. 2 is a graph showing the variations in alpha activity over time as a result of the deposition of the aerosol; - Figures 3A and 3B are graphs showing the particle size distributions of aerosol particles of UPuO ^ ,. respectively in cumulative weight and in simple weight; and FIG. 4 is a graph showing the variatians for the production of aerosol of UPuG 4 from fuel pellets; tible.

We see in Figure 1 that the device comprises a circular tube 1, a height of about 400 mm and a diameter of-; 35 about 60 mm, through which the aerosol 2 is sucked by a pump \ (not shown) connected to an outlet 3. An alpha 4 activity detector, of the type available on the market, is mounted at the center 5 of the current aerosol on an arm 5, connected to the tube 1 by an articulation hinge 6. The detector 4 is kept in a housing 4 £ • resistant to alpha radiation and it is of the "surface barrier" type. The housing Aa has a surface barrier 4b, forming a detection window, which barrier is arranged so as to extend horizontally and to be directed vertically when | the detector is in its working position, so that the particles of the aerosol can settle on the window and their alpha activity can be measured. The arm 5 has a counterweight 7 10 ensuring that the detector 4 is normally held its face upwards of the tube 1. By lifting the counterweight 7, as shown in FIG. La, one can conveniently lower the detector 4 to perform a cleaning by means of a jet (compressed gas). The detector 4 is connected by a coaxial cable 8 to conventional equipment for counting alpha particles (not shown).

The measurement method using the apparatus of the invention is as follows.

The sampling pump (not shown) is started and the aerosol is aspirated in the tube 1 for approximately! 20 2 minutes with a flow rate of approximately 10 liters per minute. We stop I then the pump and the aerosol begins to deposit in the tube by gravity. The alpha count begins shortly before the pump stops, j and the alpha activity is recorded in short intervals of! b successive times (typically 1 to 10 s) until a constant level of alpha activity has been recorded (typically 15 to 20 min). Recording alpha activity in this way i makes it possible to efficiently obtain the derivative of the curve of cumulative activity as a function of time. The derivative of the cumulative activity curve as a function of time is equivalent to a plot of the cumulative weight [30 as a function of time, and the data can be evaluated using the techniques painted for the deposit scale, i by example the Oden process of tangential interceptions.

i The final activity measured on the detector is proportional to the mass of the particles on the detector, so that the mass concentration of the aerosol particles can also | be calculated if we know the isotopic composition of the aerosol! and detector performance.

! '6 i d

Advantages of the described apparatus and method are that the results can be obtained quickly, in particular if a computer is used to process the data, the weight distribution of the aerosol particles can be obtained. directly both as a function of the Stokes diameter and of the aerodynamic diameter of the particles, the mass concentration can also be obtained for materials of known isotopic composition, while the apparatus is simple, relatively inexpensive, robust and easy to use in the remote handling conditions which are necessary for nuclear fuels, for example during their manufacture, use and reprocessing.

Example

By means of the apparatus described, a test i is carried out in which the aerosol of nuclear fuel is produced by vibrating a perforated aluminum crucible containing fuel pellets of UPUO2 in a gas stream upstream of the device. The aerosol generator is started and the gas is sucked up by means of the pump at 19 mm / s so as to carry a stream of aerosol particles in front of the detector.

When a stationary current is created (3 minutes), the generator and the gas current are stopped and alpha counting is started. Since the aerosol particles in the tube above the detector are deposited, the alpha activity recorded by the detector increases as a function of time, as shown in Figure 2.

As long as there is no notable self-absorption! alpha particles inside the aerosol deposit on the detector, it can be assumed that the alpha activity recorded i 30 is proportional to the mass of the deposit. The curve of FIG. 2 giving the activity as a function of time is therefore equivalent to a curve giving the cumulative weight as a function of time, and one can apply the known mathematical treatments developed; for overhead scales.

i 35 The curves of the activity measured as a function of time ^ have treated tangential intersections using the Oden method, viz: 7 i i

(D

100 F (D) dD = 100 - tangential interception (1) where F (D) dD is equal to the mass fraction of particles having an i! diameter between D and D + dD and where the tangential interception! j denotes the interception of the tangent of the activity-time curve | 5 with the y axis ;, expressed as a percentage of the activity [. final.

j We obtain the correspondence between the instant of the deposit and the diameter of the particle by applying the friction force (Stokes law) to the gravitational force exerted on the 10 particles, namely: I b - [φφ (2) if i gt!

i OR

i | j D is the Stokes diameter of the particle, î j η is the viscosity of the aerosol gas, i \ 15 g is the constant of the acceleration of gravity, t is the density of the material of the particles of the aerosol, t is the deposit time, h is the deposit distance.

j jD

By plotting \ F (D) dD as a function of D, we obtain the f ^ | 20 cumulative weight distribution which is shown in Figure 3A,? Λ! from which we can deduce a distribution curve pon-! simple mismatch, if necessary, as shown on; Figure 3B. We simply obtain the weight distribution in | function of the aerodynamic diameter by multiplying the diameter of! 25 Stokes, that is D, by \ f \. Tangible interceptions of activity-time curves can be obtained with maximum precision when the curvature of the plots is maximum. The maximum curvature corresponds to the range of particle diameters for which the mass fraction is the highest, i.e. at the peak of the simple weight distribution curve and, therefore, to the most interesting part of the particle size distribution. On the contrary, the mass fraction corresponding to the upper and lower ends of the particle size distribution is low / I ^^^ R so that the precision of the weight distribution is weaker ^^ B than at the peak.

Since the alpha activity recorded is proportional to the mass of the aerosol particles deposited, it is possible ^^ B 5 to calculate the mass concentration of the aerosol particles ^^^ R as far as knows the isotopic composition of the aerosol material and the efficiency of the detector.

Deposition experiments were carried out using aerosols produced by vibrating UPuO ^ sintered pellets 10 at various amplitudes. The typical curve giving the activity alph ^^^ R as a function of time, which is presented in FIG. 2, is the average of four successive experiments carried out under identical conditions. It appears that the reproducibility of the data ^^ B is satisfactory. Using the Oden method of tangential interceptions ^^^ R 15, we find that the diameter of particles of average masses ^^^ R, that is, increases slightly with increasing vibration amplitude between 2.3 and 2.5 jim over the range studied. The lowest vibration amplitude (curve a in FIGS. 3A and 3B) leads to aerosols having a particle size distribution of ^^ B 20 particles which is approximately of the log-normal type (y = 1.4 pm ^ ^^ R However, when the amplitude of vibration increases, the proportion ^^ B of large particles increases, so that the distributions become frankly biased instead of being of the log-normal type (curves ^^ B and c of Figures 3A and 3B).

^ R ^ R 25 Amplitude of Concentration ^^^ B viorations à mass re- 1 ^ · 50 hz (mm) (pra) lative

HB

H b 1.8 2.35 2.15 HH 30 2.9 2.50 2.56 ^ H As can be seen in Figure 4, the quantity ^^^ R of aerosol produced, as it is measured by the alpha 1 ^ · final activity on the counter, appears to increase with the amplitude of vibration as soon as a threshold corresponding to maximum acceleration ^^ B 35 'of 5g has been exceeded.

9

In the description, the word "aerosol" was used. However, it should not be understood that the invention is limited to the use of a system of "colloidal" particles. Any dispersion of particles in a carrier fluid can be used, the preferred fluid being a gas, rather than a liquid.

Of course, those skilled in the art will be able to imagine, from the apparatus and the method, the description of which has just been given by way of illustration only and in no way limitative, various variants and modifications not departing from the 10 scope of the invention.

; ""

Claims (7)

1. Apparatus for measuring the particle size distribution and the mass concentration of particles of a nuclear fuel aerosol, characterized in that it comprises a vertically oriented container (1) intended for the aerosol (2), a means ( 3) 5 for entering the aerosol into the container, and an alpha particle detector (4) placed or capable of being placed inside the container and having a detection window (4b) arranged horizontally and directed vertically, so as to allow the detection of the alpha activity of the aerosol particles which deposit on the window. 2. Apparatus according to claim 1, characterized in that the container has the form of a tube (1) whose axis is vertical, and in that the means for introducing the aerosol into the container is a pump of suction connected to the upper end (15) of the tube. 3. Apparatus according to claim 1 or 2, characterized in that means are provided for cleaning the detection window of the detector between measurements. 4. Apparatus according to claim 3, characterized in 2. that the detector is movable inside the container between a measurement position and a cleaning position. $ 15. Apparatus according to claim 3 or 4, characterized • in that the cleaning means consists of a gas jet, 6. A method of measuring the particle size distribution and of the mass concentration of particles of a nuclear fuel aerosol, the method being characterized in that it consists in bringing the aerosol into a vertically oriented container. intended to contain it, to place a particle detector .¾ alpha inside the container so that its detection window $ 130 tion is horizontal and directed vertically, to stop the admission of aerosol in the container, è measuring the alpha activity of the aerosol particles depositing on the detection window, and converting the alpha activity measurements into particle size distribution and, or alternatively, mass concentration measurements of the 35 'particles. 'η F i, 7. Method according to claim 6, characterized in that I the operation for measuring the alpha activity begins before the introduction of the aerosol is stopped. 8. Method according to claim 6 or 7, characterized in that the alpha activity of the particles depositing is recorded over successive relatively short intervals until a constant level of alpha activity has been recorded, so as to "obtain the derivative of the curve giving the cumulative alpha activity! I as a function of time. i I 10 9. Method according to claim 6, 7 or 8, characterized in that the process is repeated several times so as to obtain an average measurement, the detection window of the detector! being cleaned by means of a gas jet between the measurements. jf s »jjli! I fiyy% i I '1" / · IH · i yxs fi