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Stack monitor testing by means of fluorescent
particles
Lars Ström
Summary
Nuclear installations have to monitor effluent air streams for radioactivity.
Typically a sample of the air flow is withdrawn, and brought through a pipe to
measuring instruments. The International Standard ISO 2889 sets rules for the
performance of such stack monitors. This report shows how the standard’s
conditions can be verified in a practical case. The demonstration took place in the
now closed nuclear power station Barsebäck 1.
The ISO 2889 distinguishes between singe-point sampling and multi-point
sampling. Single-point sampling is strongly recommended. For a location to be
suitable for single point sampling the air velocity shall be about the same over the
whole sampling cross-section, and the contaminants to be measured be well mixed
into the stream. If such favourable conditions do not exist, then multi-point
sampling is an alternative. But then the representativeness of a sample withdrawn
has to be proven.
The properties of the Barsebäck sampling site and sampling installation were
investigated by means of a fluorescent monodisperse test aerosol. The 10 µm
particles were introduced into the air stream a distance upstream of the sampling
location for the ordinary monitoring. The concentration of particles was measured
in the stack by means of a newly developed optical particle counter. Particles were
also collected on filters, and the particles counted with computerised image
analysis. Air velocity in the sampling section was measured in 16 places. From
these measurements the coefficient of variation (standard deviation/mean, COV)
was calculated for both air velocity and particle concentration.
The COV of air velocity turned out to be 20 %, on the limit for single-point
sampling. The particle concentration COV was 69 – 80 %. This very high COV
was created on purpose, by selecting the injection point for the test aerosol close to
the sampling area. With a longer way for the air to travel, the test aerosol should be
better mixed into the air flow and the COV lower.
The transmission efficiency of the sampling line between the stack and the
measurement equipment was measured. The ratio between the concentration
recorded by the collecting filter and the concentration in the stack was 6 – 7 %,
well below the standard’s requirement of 50 % for 10 µm particles.
This work was supported by the Swedish Radiation Safety Authority, project
number SSM2013-876.
Document1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
Instrumentinvest Aerosol AB
Tryffelstigen 12
SE-611 63 NYKÖPING
Sweden
Tel nat 0155-216862 int +46-155-216862
Org.nr 556559-7696
Bankgiro 5395-5639
E-mail lars.strom@instrumentinvest.se
www.Instrumentinvest.se
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Contents
1
2
3
4
Introduction ....................................................................................................... 3
1.1
The ISO 2889 rules ..................................................................................... 3
1.2
Control methods ......................................................................................... 3
Equipment used at the Barsebäck demonstration.............................................. 4
2.1
The Barsebäck 1 plant ................................................................................ 4
2.2
Generator for fluorescent tracer particles ................................................... 5
2.3
The turning carrier for sensors ................................................................... 6
2.4
Air velocity measurement .......................................................................... 7
2.5
Particle measurement with an optical counter ............................................ 7
2.6
Particle measurement by means of filter collection ................................... 8
2.7
Evaluation of filter samples ........................................................................ 9
Air flow ........................................................................................................... 11
3.1
Air velocity observations .......................................................................... 11
3.2
Velocity distribution and velocity coefficient of variation....................... 12
3.3
Calculation of the total air flow ................................................................ 13
3.3.1
Turning probe. ................................................................................... 13
3.3.2
Scilab velocity integration................................................................. 14
3.3.3
Flow calculated from the Barsebäck 2 formula ................................ 14
3.3.4
Facility statement of stack flow. ....................................................... 15
Flow of tracer particles.................................................................................... 15
4.1
4.1.1
Optical particle counter ..................................................................... 16
4.1.2
Filter array sampling ......................................................................... 17
4.2
Particle distribution and particle concentration coefficient of variation .. 17
4.2.1
Optical counting ................................................................................ 17
4.2.2
Filter array sampling ......................................................................... 18
4.3
5
Particle concentration observations .......................................................... 15
Calculation of total particle flow .............................................................. 19
4.3.1
Turning probe directly....................................................................... 19
4.3.2
Turning probe velocity and concentration interpolation. .................. 19
4.3.3
Filter array sampling ......................................................................... 19
Transmission efficiency of the sampling line ................................................. 20
5.1
Sampling efficiency in Barsebäck 1 ......................................................... 21
5.2
Contamination of a filter from Test 2 ....................................................... 22
6
Comments on the instrumentation .................................................................. 22
7
References ....................................................................................................... 23
8
Sammanfattning på svenska ............................................................................ 24
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1 Introduction
1.1 The ISO 2889 rules
The ISO standard “Sampling airborne radioactive materials from the stacks and
ducts of nuclear facilities” from 2010, reference 1, sets some rules for the
measurement of gases and particles in outgoing ventilation air. The standard refers
to the case when a sample must be withdrawn for radiometric analysis. The first
rule is that one-point sampling is to be preferred. Multi-nozzle samplers have
higher internal losses than simple ones. From this follows the second rule that the
sampled air stream should be well mixed, so the one-point sample is representative
for the whole stream of air. The third rule is that the air moves straight and even at
the sampling point. Finally it is required that 10 µm particles shall not undergo any
serious losses on its way to the measurement instruments. The standard expresses
these rules quantitatively.
1.2 Control methods
The purpose of this demonstration is to show how the ISO rules can be checked.
Some new instruments, especially designed to investigate compliance with the ISO
rules, are tried for the first time. The demonstration is limited to the control of
particles. The rules also apply for gases, but if an installation is good enough for
particles, it will also do for (inert) gases.
Stack monitor testing is done by generating test particles in the stack air stream. If
the particles are 10 µm and monodisperse, sampling of these particles will relieve
the distribution pattern of such particles. Alternatively a polydisperse test aerosol is
used, which is much easier to generate. Then a size-selective optical sampler is
used for analysing for 10 µm particles. Unfortunately optical particle counters
suitable for use in an industrial ventilation environment cannot measure 10 µm
particles because of inlet losses. At best the relative concentrations can be obtained.
This suffices for calculation of the COV, but is not enough for determining the
transmission efficiency of the sampling line. This transmission is often the weakest
link in stack monitoring. In the past Swedish monitoring installations have been
tested with a monodisperse test aerosol with a tracer substance and filter sampling.
The same tracer is used for determining the transmission efficiency of the sampling
line.
This technique works well, but is quite expensive in the steps of particle
fabrication, tracer marking and analysis. There is also the risk of contamination of
samples. The dilution from generator to sample is of the size of order 10-6. The
complicated preparation of the test aerosol and its injection must be well separated
from sampling and analysis. A better method is desirable.
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The idea of measurements based on particle counting rather than bulk analysis of a
tracer in a sample has been investigated. The main advantages with the new
technique was expected to be
1.
2.
3.
4.
Easy analysis of particles in the air, by means of a new optical counter
Easy analysis of particles collected on filters, by particle counting
Risk of contamination errors in samples much reduced
Easier manufacture of tracer particles
To realize such a test method Instrumentinvest Aerosol AB has developed
Generator for fluorescent monodisperse test aerosols, for field use
Optical particle counter for fluorescent particles with low inlet losses
Technique for counting fluorescent particles on filter
2 Equipment used at the Barsebäck demonstration
2.1 The Barsebäck 1 plant
The nuclear power station Barsebäck 1 was shut down in 1999. The ventilation
system and the stack monitor are still operating, and were used for this
demonstration. Niklas Wallgren represented the Barsebäck plant, Tommy Hansson
and Hannes Tingfors from ISS participated in the practical work. The operations in
the plant took place 19 – 23 August 2013.
Sections through stack are shown in Figure 2 and Figure 4. The stack is not
circular. The permanent stack monitor has a sampler of the multi-nozzle type, with
four nozzles of 50 mm diameter. The sampled flows are brought together in a
transport pipe of inner diameter 100 mm, carrying the sample down to the
measurement equipment, at the flow rate of 56 litres/second. There a secondary
flow of 0,60 litres/second is withdrawn, with particle collection on filters of 50 mm
diameter.
Above the sampler there is a grated gangway or bridge over the stack, which is
accessible over a ladder on the stack inner wall. From this bridge the stack
measurements of this report are made.
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2.2 Generator for fluorescent tracer particles
Based on earlier work (reference 2) a vibrating orifice aerosol generator for field
use was developed. Such a generator is deceptively simple; the central part fits
easily in the hand. However, for field use, in an environment where the equipment
can be contaminated with radioactivity and must stand rough handling and
decontamination, the equipment became rather large, Figure 1.
Barsebäck 2013 report Figure B
Figure 1. Generator for monodisperse fluorescent particles of 10 µm diameter.
On the front panel there are controls for the nozzle vibration frequency, amplitude
and wave form. To the right there are valves and meters for the flow of liquid. To
the bottom left there is room for a filter for the air pressurizing the interior of the
box, to prevent inside contamination. The generation rate is typically 115 000
particles/second.
The particle size at the Barsebäck tests was calculated to be 8,7 µm, based on liquid
flow and disintegration frequency. The corresponding aerodynamic diameter is 9,1
µm. The diameter measured by means of a microscope was 8,9 µm. The particle
diameter varied slightly during the tests, because the pressure, driving the flow of
liquid, had to be restored at intervals.
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2.3 The turning carrier for sensors
To measure air velocity and to sample particles on the same level as the ordinary
stack effluent monitor, probes was mounted in that level, hanging from the grated
bridge above, Figure 2. The probes were set on the ends of a cross bar, fixed to the
lower end of a vertical axis. The axis was rotated by means of a lever on the upper
end of the axis. This rotated the cross bar, so that the probes were moved on a
circle. There were four predefined settings for the lever, a, b, c, d. Readings taken
on the same letter are simultaneous, but the particle and velocity probes are
separated by the length of the cross bar, 600 mm.
Figure 2. Cross and longitudinal sections of the Barsebäck 1 stack at the sampling
level of the stack monitor.
The vertical axis was mounted in a stand hanging on the bridge handrail. The
whole setup could be moved on the rails to four different positions in the air
stream, 1, 2, 3, 4.
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2.4 Air velocity measurement
The velocity probe was a Prandtl tube, diameter 7 mm (also called Pitot – static
tube). The differential pressure output was measured with a micro manometer
Airflow PVM 100. Each record is the mean of five individual readings.
The manometer was calibrated against a laboratory standard Furness MDC FC00
0 - 100 Pa and an inclined tube alcohol meter, Sartorius 655. The Airflow tended to
show about 0,2 % too low. This small correction was not applied to the
measurements.
2.5 Particle measurement with an optical counter
The particle concentration was measured with a newly developed optical particle
counter. It has a shrouded probe inlet, with the same proportions as described in
reference 3. The inlet diameter is 34 mm, the flow 3 litres/second, inlet velocity 3,3
m/s. The outer flow in the shroud is filtered and used as a sheath of clean air in the
optical sensing section. The geometry predicts that 2/3 of the total air flow should
be filtered and form the sheath, the rest counted for particles. Unfortunately it is
quite difficult to demonstrate the sampling properties of a shrouded probe, so the
evaluation is based on the assumption that the free air concentration in the stack is
three times the concentration measured by the optical counter.
Barsebäck 2013 report Figure M
Figure 3. The optical particle counter.
The tracer particles are fluorescent. This discriminates against other particles in the
air. The fluorescence is exited with an axial illumination, in the direction of the
incoming aerosol flow. Fluorescent light is emitted in all directions, permitting a
simple concentric geometry with straight-through air flow.
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2.6 Particle measurement by means of filter collection
Particle distribution was also measured with ordinary 50 mm glass fibre filters,
mounted on a cross-shaped frame and hoisted to the level of the sampling section,
Figure 4 and Figure 5.
Figure 4. The cross-shaped support for filters A … G (blue) together with the
permanent stack sampler manifold with its sampling points R … U (green).
Barsebäck 2013 report Figure C
Figure 5. Filter holders A and B on an arm of the cross-shaped support frame. The
air inlets are here capped.
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The filters were connected to the suction side of a blower, equipped with an
accurately manufactured Venturi tube on the inlet. Inlet pressure and differential
pressure was indicated by manometers, controlled against an alcohol filled U tube
manometer. The venturi compared well with a sharp-edged orifice. The flow was
kept at 3,43 l/s.
The filters were mounted in filter holders with shaped inlets, Figure 5. The flow
through each filter in its holder at a given pressure drop was measured in advance.
The variation in flow was small, the standard deviation less than 2 % of the mean.
So the flow through each filter is assumed the same, 0,49 l/s, or 147 litres during
the sampling time of 5 minutes. – Inlet diameter was 0,93 cm2, inlet velocity 5,25
m/s.
The same kind of 50 mm diameter glass fibre filters were also installed in the filter
holders of the ordinary stack sampler.
2.7 Evaluation of filter samples
The fluorescent particles collected on the filters were photographed, and the
fluorescent particles appeared as “blobs” in the images. A computer program was
used to find particles, reference 4. Figure 6 shows a photo where the blobs
identified by the program have been coloured white.
Barsebäck 2013 report Figure E
Figure 6. Particle “blobs” from particles collected on filter 387 in position G in
Test 1.
Figure 6 shows a small area of a filter, processed by the JMicroVision program.
The image shows 14,9 mm × 9,5 mm of the filter paper. Most blobs are 11 – 15
pixels in area, or 70 to 80 µm across, much larger than the particles they represent.
The whole filter holds 745 blobs in the size range 10 – 96 pixels, of which 560 are
estimated to represent particles. Note that the number label contributes to the
background with some large blobs.
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The blobs from the image analysis are of different sizes, and can be displayed in a
histogram. Figure 7 shows such a histogram for Filter 385, used in position E in
Test 1. There is also plotted the histogram of the unused filter 388, spare filter in
Test 1. Only blobs in the range 10 to 96 pixels were regarded.
Number of blobs per class.
250
200
150
Filter 385 Cross position 1E Photo P1010987.jpg
Filter 388 not used Photo P1010989.jpg
100
50
0
1
11
21
31
41
51
61
71
81
91
Class number=blob size in pixel.
Barsebäck 2013 report Figure F
Figure 7. Histogram over blobs found in filters 385 and 388, Note that pixel here is
a unit of surface.
The high background was not expected. Theoretically the lower limit of detection
is one particle. In the present situation at least a hundred particles are necessary to
clearly stand out from the background. Test 3 was intended to demonstrate the
contribution to the background from the air in the stack. The contribution turned
out to be small. The background seems to stem mainly from the evaluation, so it
should be possible to bring the background down considerably.
Particle generation as a stream of droplets involves the risk of droplet collision and
the formation of double-sized particles, doublets. What impression a doublet would
make in an image is difficult to say, but a first guess is twice as many pixels as a
singlet. In histograms like the one above one can sometimes see a small increase in
blob numbers around 30 pixels.
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3 Air flow
The air flow in the stack should be straight and evenly distributed where the sample
is drawn. If one-point sampling is to be considered, ISO2889 requires that the
velocity in the central 2/3 of the cross-sectional area has a coefficient of variation
(COV) of less than 20 %. COV is the ratio of the standard deviation to the mean.
To find the velocity mean and distribution the standard recommends the
conventional geometry of two lines of measurements at right angles across the air
conduit. Such measurements cannot be taken in the Barsebäck stack, so a turning
carrier suspended from a grating above the sampling location was employed, as
described in section 2.3.
The standard also requires that that the air is not rotating in the stack. The average
resultant angle should be less than 20 degrees.
3.1 Air velocity observations
Air velocity was measured as described in section 2.4. Measurements were taken
in16 points distributed over the stack cross section at the sampling level of the
ordinary stack sampler, in Positions 1a … 4d, Figure 2. The following table
summarizes the observations.
Probe position
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
5c
5d
6a
6b
6c
6d
Test 1
4,73
4,40
3,96
4,36
4,12
4,32
3,61
3,83
5,29
5,48
6,16
5,72
6,14
6,90
7,21
6,27
Velocity m/s
Test 2
Test 3
4,73
4,73
4,28
4,12
4,40
4,08
4,65
4,47
4,40
4,69
4,32
4,24
3,72
3,70
3,74
3,83
6,63
7,12
7,35
6,83
5,48
6,30
6,78
6,90
Test 4
4,86
4,08
4,12
4,47
4,40
4,32
3,51
3,61
5,16
5,77
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In Test 2 the turning carrier was not correctly placed in the positions 4 and 5. The
unintended positions have been reconstructed by means of photographs taken
during the tests, and are called 5a … 6d.
In Test 3 the intention was to measure the “natural” background of particles in
ventilation air, and samples were taken only in eight positions. In Test 4 there was
a failure in the instrumentation, and the test was interrupted.
The observed velocities are in good agreement.
3.2 Velocity distribution and velocity coefficient of variation
By means of a computer program, Scilab, reference 5, the velocities observed in
Test 1 and Test 2 were converted to curves of equal velocity over the stack cross
section. Within the area covered by the observations the program interpolated with
credible results, but when extrapolating outside this area, out to the stack wall, the
program produced evidently erroneous lines of equal velocity. To rein in the
program a number of reasonable velocities were invented for the stack wall. The
result is shown in Figure 8. Test 2 is less reliable, because of the less favourable
positions of the measurement points, and the uncertainty in the reconstruction of
their position.
Figure 8. Curves of equal velocity generated by Scilab from data of Test 1 and Test
2. The grey area marks the central 2/3 of the whole stack area. The blue circles
indicate imagined measurement points on two orthogonal lines.
A measurement along orthogonal lines can be simulated by calculating the
velocities at the required measurement points by means of the Scilab. The resulting
COV should be reduced because of the smoothing effect of interpolation.
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In the table below the COV is calculated on four data sets, all from the central 2/3
of the cross section
Data set
Velocity from the suspended probe Test 1
Velocity from the suspended probe Test 2
Velocity on orthogonal lines, from Scilab, Test 1
Velocity on orthogonal lines, from Scilab, Test 2
Number of
data points
13
11
14
14
COV %
20
22
20
19
The coefficient of variation is near 20 %, on the limit of the acceptable for onepoint sampling according to ISO 2889.
The mean and the standard deviation are calculated on a limited number of
observations. Both numbers have a statistical uncertainty, and their quotient, the
COV, still more so.
3.3 Calculation of the total air flow
To calculate the total particle emission with the exhaust air the flow must be
known. The conventional way to calculate the total flow is to allocate a fraction of
the stack area ΔA to each velocity record. The whole area A is
∑Δ A= A
The whole air flow Q is
∑ΔA∙v = Q
The area allocation is a somewhat subjective task. Usually the measurement points
for velocity are so distributed that the area fractions are of equal size.
3.3.1 Turning probe.
Each measurement point is allocated the same area, A/16, although the distribution
of measurement points could be more even. This gives equal weight to each point,
and the flow is simply the mean velocity times the total area, 6,20 m2
Test 1 vmean = 5,16 m/s Q = 32,0 m3/s
Test 2 vmean = 5,48 m/s Q = 34,0 m3/s
A carefully performed measurement in the field, under favourable conditions,
should in most cases come within 3 % from the true value. In Test 1, and especially
in Test 2, the results must have larger error margins mainly because of too few
measurement points.
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3.3.2 Scilab velocity integration
The interpolation program used for creating the curves of equal velocity in Figures
6 and 7 can also be used for summing velocities. Because of the irregular shape of
the stack cross section, it was divided in 12 rectangular slices. The mean velocity of
100x100 points in each slice was multiplied with the slice area, and the flows of the
slices summed.
Test 1 Q = 30,7 m3/s
Test 2 Q = 34,3 m3/s
The result might be influenced a little by the three invented velocities, necessary to
keep Scilab within bounds outside the interpolation area.
3.3.3 Flow calculated from the Barsebäck 2 formula
Following the work in Barsebäck 2 in 2001, reference 6, a formula was devised for
the stack flow. It is based on measurements of velocity at two predetermined
positions, from the bridge across the stack, above the sampling area, REFÖ1 (-600,
-400) and REFÖ2 (-600, -1000). The data were taken with a hand-held thermal
flow meter, Compuflow GGA 665 P. The same instrument was now used in
Barsebäck 1. The stack in Barsebäck 2 has the same dimensions as the stack in
Barsebäck 1. Measurements were also taken in corresponding positions at the
lower grating, REFU1 and REFU2. The velocities are the mean of five individual
observations at each point
Monday 13:20
Tuesday 11:20
Wednesday 13:30
REFU1
8,60
6,62
6,90
REFU2
8,60
8,02
8,18
REFÖ1
7,32
8,44
6,94
REFÖ2
8,76
7,38
6,08
The Barsebäck 2 formula is
Q=5,11×(vREFÖ1+vREFÖ2)/2 [m3/s] , [m/s]
The velocity readings at the upper grating translates to the following flows
Monday
41,1 m3/s
Tuesday
41,4 m3/s
Wednesday 33,3 m3/s
The stack flow seems to vary considerably. The measurements at the lower grating
also show great variations. It is unlikely that the stack flow varies that much. The
Compuflow meter might not be suitable for flows of very high turbulence, even if
as here each flow is the mean over 2 × 5 individual observations.
It should also be noted that the I-I formula is devised for Barsebäck 2. In Barsebäck
1 the grating of the bridge across the stack is less permeable for the air flow
because of heavy deposits on the grating. This may increase the velocity in the free
space, where the readings are taken, giving a false impression of a higher flow.
The temperature in the stack was stable, 22,5 – 23,5 C.
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3.3.4 Facility statement of stack flow.
From the control room of the power station are reported the following flows,
measurement point K305 in the stack:
Min
m3/h
Monday
164742
Tuesday
164693
Wednesday 164698
Thursday
164728
Max
m3/h
165215
165193
165215
165175
Mean
m3/h
164966
164962
164964
164952
This flow is measured with a probe in the stack sampling area. The recorded flow is
extremely stable. The mean flow is 45,8 m3/s, much higher than the results
obtained in the manual measurements.
The total flow calculation is used as a check on the measurements. The following
table shows that the measurements could be better, or the stack flow more stable.
Measurement
3.3.1 Prandtl 16 points Test 1
3.3.1 Prandtl 16 points Test 2
3.3.2 Scilab integration Test 1
3.3.2 Scilab integration Test 2
3.3.3 Barsebäck 2 formula
3.3.3 Barsebäck 2 formula
3.3.3 Barsebäck 2 formula
3.3.4 Facility
Day
Tuesday
Wednesday
Tuesday
Wednesday
Monday
Tuesday
Wednesday
Mon- to Friday
Flow m3/s
32,0
34,0
30,7
34,3
41,1
41,4
33,3
45,8
The Prandtl tube measurements are regarded as the most reliable. For calculation of
tracer particle concentrations in the stack (see below), a constant flow of 33 m3/s is
assumed.
4 Flow of tracer particles
The purpose of this section is to describe two new particle counting techniques for
obtaining the COV for the particle distribution over the sampling section in the
stack.
The injection point for tracer particles was chosen to create a high COV in the
sampling section. The high COV observed is thus not a property of the Barsebäck
plant, but depends on the selected point for tracer injection.
4.1 Particle concentration observations
Particles were measured in two independent ways: with the optical particle counter
on the turning mount, and by means of an array of filters on a frame hoisted to the
sampling level in the stack. Booth methods were adapted to the peculiarities of the
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Barsebäck stack. The particle flow was also measured by the ordinary stack
monitor.
4.1.1 Optical particle counter
The optical particle counter was set to sample a few minutes in each of the 16
measurement points of the turning probe. The sampling time was made longer
where the particle concentration was expected to be low. The following
concentrations were observed.
Particle concentration, particles/litre
Test 1
Test 2
Test 3
Test 4
Tracer
Tracer
No tracer
Tracer
source:
source:
source:
Lower
Lower
Fan floor
grating
grating
Sampler
position
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
4a
4b
4c
4d
5a
5b
5c
5d
6a
6b
6c
6d
0,04
1,52
0,78
0,10
1,03
4,45
6,95
2,68
8,81
8,69
9,32
8,15
1,19
2,56
4,67
0,51
0,03
1,13
0,78
0,07
0,49
2,41
6,43
1,63
0,02
0,01
0,01
(0,08)
0,01
(0,21)
0,01
0,00
0,41
1,48
1,74
1,50
1,12
0,79
0,52
0,48
1,00
0,63
1,33
4,48
4,47
1,91
6,08
5,90
10,70
9,75
The Lower grating is about 9 m below the sampling area; the Fan floor further upstream the ventilation channel.
In the No tracer column there are two records within brackets. In those cases some
work was deliberately undertaken in the stack, liberating much dust from the
gratings and other dirty surfaces.
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4.1.2 Filter array sampling
The cross-shaped frame mentioned in 2.5 was used for obtaining filter samples
from the stack monitor’s sampling level. Sampling time was 5 minutes, sample
volume 147 litres/filter (Test 3 294 litres/filter). The filters were analysed for
particles with the photo - image analysis method described in 2.7. The following
blob counts were obtained.
Position
on
frame
Test 1
Number
of blobs
A
176
B
616
C
548
D
196
E
1191
F
1817
G
745
H Spare
189
Background 185
Conc
part/litre
-0,1
2,9
2,5
0,1
6,8
11,1
3,8
Test 2
Number
of blobs
198
813
210
100
519
1574
764
113
137
Conc
part/litre
0,41
4,59
0,50
-0,25
2,60
9,77
4,26
Test 3 Background
measurement
Number
of blobs
91
117
133
207
111
188
155
108
After subtraction of the background the particle concentration was calculated. The
background was taken from positions A and D, which were outside the particle
flow, and the spare filter H, which was never used. Test 3 was made especially for
investigating the background of particles in the air. Evidently the air contributes
very little to the background in comparison with other sources.
4.2 Particle distribution and particle concentration coefficient of variation
The COV:s are for particle concentrations, not particle flows.
4.2.1 Optical counting
The concentrations observed by means of the turning probe in Test 1 and Test 2
were converted to curves of constant concentrations by means of the program
Scilab. As for the velocity, the measured points had to be completed with some
invented points at the stack walls to keep the computer program’s extrapolations
within reasonable limits. The obtained level curves are shown in Figure 9 below.
The differences between the distributions depend on the sampling pattern; the
particle source was the same.
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Figure 9. Particle concentration levels in Test 1and 2.
4.2.2 Filter array sampling
The cross has only four sampling points within 2/3 of the stack area, B, C, F, G.
Four data points is very little for estimating the COV.
Test
1
2
Number of
data points
4
4
Mean particle
conc. part/l
6,72
4,78
Standard
COV
deviation part/l
%
4,66
69
3,81
80
The table below summarizes COV calculated on different data sets, based on
optical counting and filter sampling, all taken within the central 2/3 of the cross
section.
Table xx Particle COV calculated on different data sets
Data set
Concentration from the turning probe Test 1
Concentration from the turning probe Test 2
Two orthogonal lines Test 1
Two orthogonal lines Test 2
Filter array sampling Test 1
Filter array sampling Test 2
Number of
data points
13
11
14
14
4
4
COV %
72
67
76
90
69
80
The coefficient of variation is very high, and the ISO rules do not permit one-point
sampling in this situation. With the particle source further upstream, the COV
becomes smaller. The limited set of data collected during the interrupted Test 4,
when the tracer particles were injected from the Fan floor gives a COV of 49 %.
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4.3 Calculation of total particle flow
To investigate the consistency of the obtained numbers, the total flow of tracer
particles is calculated. This is also a demonstration of the use of data obtainable
with the particle counting technique. Each velocity record v and its corresponding
product of velocity and concentration v∙c is allocated a fraction ΔA of the whole
stack area A
∑ΔA=A
The whole particle flow P is
∑ΔA∙v∙c= P
The generator for tracer particles worked at 115 000 p/s. There are no known
particle losses (except a few per cent doublets) between generation on the lower
grating and sampling below the upper grating. The measured particle flow should
equal the generator output.
4.3.1 Turning probe directly.
Particle concentration was measured in 16 points. As with the air flow, each
measurement point was allocated the same area, A/16. The particle flow is
calculated to be
Test 1
Test 2
127 595 particles/second
133 875 particles/second
The particle flow is higher than the generation rate. The fault may lie in the area
allocation.
4.3.2 Turning probe velocity and concentration interpolation.
Based on turning probe data the Scilab program interpolates a nearly continuous
distribution of velocity and concentration, with 120 000 area fractions. Multiplying
each velocity with its corresponding concentration and doing the sums, the
following results were obtained
Test 1
Test 2
98 597 particles/second
89 393 particles/second
These particle flows are lower than generator output, 115 000 p/s. In this case the
error source could be the invented data points at the stack wall, necessary to
prevent the Scilab program from extrapolating to impossible values. In Test 2 the
unsuitable sampling points also contribute to the error. It is astonishing that the two
estimates of the particle flow, based on the same data set from the optical counter,
can be so different. This deserves further study.
4.3.3
Filter array sampling
For each filter on the filter array hoisted to the sampler level a concentration is
calculated. From the velocity distribution shown in Figure 8 the velocity of air at
each filter is obtained. Then each filter is allocated a surrounding area, for which
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the filter is assumed to be representative. The size of this area is an estimate. With
these numbers the total flow of particles can be obtained.
Test 1
Filter
position
1A
1B
1C
1D
1E
1F
1G
Concentration
particles/litre
-0,1
2,9
2,5
0,1
6,8
11,1
3,8
Area fraction
Test 2
Filter
position
2A
2B
2C
2D
2E
2F
2G
Concentration
particles/litre
0,41
4,59
0,50
-0,25
2,60
9,77
4,26
Area fraction
0,19
0,15
0,15
0,19
0,12
0,12
0,08
0,19
0,15
0,15
0,19
0,12
0,12
0,08
Air velocity
m/s
6,3
6,1
4,5
4,1
3,8
4,9
5,0
Total flow
Particle flow
particles/second
-724
16620
10325
350
19342
40465
9442
95821
Air velocity
m/s
7,0
6,5
4,9
4,1
3,5
4,4
6,8
Total flow
Particle flow
particles/second
3080
26088
2078
-1216
7347
35638
10578
85422
These particle flows should be the most reliable. The data points are admittedly few
in this very uneven flow field, but there are no invented concentrations. Maybe the
test aerosol generator output is not 115 000 particles/second; some particles might
be deposited inside the generator head because of the very turbulent air flow
surrounding it.
A further support for these low particle flows can be had from calculating the stack
air flow on the same numbers. One gets 31,1 respectively 32,9 m3/second, close to
earlier estimates, section 3.3.
5 Transmission efficiency of the sampling line
The ISO 2889 requires that the loss of 10 µm particles in the transmission line is
less than 50 %. Smaller particles have smaller losses. Larger particles have higher
losses, but are supposed to have little significance, if there is no special reason to
think otherwise.
Formulas have been devised for calculating the transmission losses; most
prominent the Deposition computer program, reference 7. Calculations were a help
when they predicted that a lower flow should give better transmission in the
sampling line of Oskarshamn 3 stack monitor, reference 8. A practical test proved
the prediction to be right, at least qualitatively. Generally calculations with the
Deposition program have given poor results when compared with measurements on
Swedish 100 mm transmission lines.
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The formulas are only as good as the assumptions that must be done about the
piping. If the pipes are leaky or dirty, or are not built as assumed the calculations
will be misleading. Still calculation is a good help in designing a new system.
5.1 Sampling efficiency in Barsebäck 1
Tracer particles were collected on filters in the filter holders of the ordinary stack
monitor. Analysis of the filters gave
Test
Test 1
Test 2
Test 3
Test 4
Sampled
volume
Conditions
Litres
High COV 5187
High COV 3801
Background 2205
Low COV
4330
Blobs
counted
1849
See 5.2
279
1596
Concentration,
background
subtracted p/l
0,23
0
0,24
Fraction of
stack
concentration
6,6 %
6,9 %
The stack concentration is calculated from the tracer generator particle output,
115 000 p/s, divided with the stack flow, 33 m3/s, page 14. The mean stack
concentration becomes 3,5 particles/litre.
The stack concentration presented to the multinozzle sampler can be also be
estimated from the filter array hoisted to the sampler nozzle level. As can be seen
in Figure 4, the filters A … G in the array form pairs surrounding the sampler inlets
R … U. By forming the proper means, the apparent stack particle concentration is
calculated to 3,9 particles/litre, Test 1. In lack of better information it is assumed
that the flows through the four nozzles are all the same.
The ratio between the concentration recorded with the ordinary monitor filter and
the concentration in the stack is used as the transmission efficiency of the sampling
line, including the nozzle arrangement. The transmission efficiency is low, 5,9 % 6,9 %. Using the apparent stack concentration, the transmission efficiency becomes
0,23×100/3,9=5,9 %.
The stack monitor in Barsebäck 2 was checked in 1999 (reference 6), and the
transmission was found to be 24% - 25%. There is a significant difference between
the sampling lines in Barsebäck 1 and Barsebäck 2. Maybe there are deposits in the
Barsebäck 1 pipe similar to those on the gratings. There are deposits also on the
Barsebäck 2 gratings, but not so solid.
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5.2 Contamination of a filter from Test 2
Filter F406 was used in cartridge number 2 for collecting particles in the regular
stack sampler, under Test 2.
Barsebäck 2013 report P1020005 Figure K.jpg
Figure 11. Image of a part of filter F406.
Figure 11 shows a portion (16 × 4 mm) of the filter. To the right the filter deposit
looks normal; to the left the blobs have different shapes and sizes, indicating that
they are not part of a regular sample. Collected samples can be screened for
contamination with a stereo loupe, and seen that way the strange deposit looks like
a fingerprint.
6 Comments on the instrumentation
The air flow and the COV of the air flow were successfully determined with
conventional technique, adapted to the circumstances. The observed stack flow did
not agree with the flow reported from the plant control room.
Tracer particle generation was only partly successful. The particle generation rate
may be lower than assumed. The generator is unreliable and stopped working
during Test 4.
Particle concentrations and the COV of the particle concentrations were
successfully measured with the optical particle counter, with some reservation for a
calibration factor. Particle flows from different evaluation methods differ
significantly. Internal contamination of the counter proved a problem, and some
parts of the detector should be made disposable. The electronics of the counter need
a makeover.
Particle concentration was measured successfully with the filter array and image
analysis of the fluorescent particle deposit. The sampling points were too few for
the very sharp concentration gradients.
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7 References
1. ISO 2889:2010
Sampling airborne radioactive materials from the stacks and ducts of nuclear
facilities
2. Lars Ström
The generation of monodisperse aerosols by means of a disintegrated jet of liquid.
The Review of Scientific Instruments 40 (6) 778-782. June 1969.
3. Chandra, S, McFarland, A. R. Comparison of aerosol sampling with shrouded
and unshrouded probes. American Industrial Hygiene Association Journal
56(5):459-466. 1995.
4. Roduit, N. JMicroVision: Image analysis toolbox for measuring and quantifying
components of high-definition images. Version 1.2.7.
http://www.jmicrovision.com (accessed 2 July 2006).
5. Scilab 5.4.1. www.scilab.com
6. Lars Ström
Kontroll av utrustning för skorstensmonitering av aerosoler, system 553.
Instrumentinvest Aerosol AB rapport I –I 00/01. 2000-02-20.
7. McFarland, A. R., A. M. Nagaraj, H. Ramakrishna, J. L. Rea, J. Thompson.
2002.
Deposition 2001.
Aerosol Technology Laboratory, Department of Mechanical Engineering Texas
A&M University College Station TX.
http://www.mengr.tamu.edu/research/AerosolLab/index.htm
8. Lars Ström
Kontroll av skorstensmoniteringens funktion vid sänkt provtagningsflöde.
Oskarshamn 3, oktober 2003.
Instrumentinvest Aerosol AB rapport I–I 04/02. 2004-05-18.
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8 Sammanfattning på svenska
Den internationella standarden ISO 2889 anger prestanda för utsläppsmonitering
vid nukleära anläggningar. Standarden (mer än 100 sidor med bilagorna) betonar
funktionen; det tekniska utförandet exemplifieras, men är inte del av standarden.
Standarden skiljer på enpunktsprovtagning i den moniterade luftströmmen, och
provtagning i flera punkter i strömmen. Enpunktsprovtagning rekommenderas
starkt, men innebär också att det ställs krav på att den provtagna luftströmmen är
välblandad. Har inte luftströmmen denna egenskap kan flerpunktsprovtagning
tillämpas. I ett sådant fall måste man på något annat sätt övertyga sig om att provet
är representativt.
För vissa ämnen, bland annat aerosolpartiklar, kan överföringen från provtagningen
till analysapparaten medföra betydande förluster. Standarden ställer här det ganska
stränga kravet att förlusterna för 10 µm partiklar och mindre får uppgå till högst 50
%.
Demonstrationen avser visa hur villkoren från ISO 2889 kan verifieras i ett
praktiskt fall. Delvis användes nyutvecklad teknik, anpassad till ISO-kraven.
En speciell testaerosol med partikelstorleken 10 µm infördes i luftströmmen, ett
stycke uppströms provtagningen för den fasta moniteringen. Partiklarna innehöll ett
fluorescent spårämne för att underlätta detektering. Koncentrationen av partiklar
mättes i ventilationskanalen, där samtidigt den ordinarie moniteringen tog prov.
Två olika tekniker användes, dels en nyutvecklad optisk partikelräknare, dels
insamling på filter, med datoriserad bildanalys för att räkna de fluorescerande
partiklarna på filtret. Även lufthastigheten mättes i ett flertal punkter.
För att en provtagningsplats skall vara lämplig för enpunktsprovtagning kräver ISO
2889 att lufthastigheten är någorlunda jämn; variationskoefficienten
(standardavvikelsen/ medelvärdet, COV) för mätningar fördelade över tvärsnittet
skall vara mindre än 20 %. Lufthastighetsmätningar utförda med ett Prandtls rör
och mikromanometer i 16 punkter gav COV på 20 %, och alltså just godtagbart för
enpunktsprovtagning.
Även fördelningen av partiklar över provtagningstvärsnittet skall vara jämn. För
demonstrationen hade dock generatorn för testaerosol placerats så, att en mycket
ojämn fördelning erhölls. COV låg i området 69 till 84 %. Partiklar från längre
uppströms i ventilationssystemet kan väntas ha betydligt lägre COV.
Överföringen av partiklar från provtagningen i skorstenen till insamlingsfiltret
mättes också. Den befanns vara endast 6–7 %, mycket mindre än ISO-kravet 50 %.