Download Rapid procedure for determining radon

Rapid procedure for determining
radon-222 in drinking water
H-Rn-222-TWASS-01
Authors:
Th. Bünger
H. Rühle
Federal coordinating office for drinking water, groundwater,
wastewater, sludge, waste and wastewater of
nuclear power plants
(Leitstelle für die Überwachung der Radioaktivität in Trinkwasser,
Grundwasser, Abwasser, Klärschlamm, Reststoffen und Abfällen)
ISSN 1865-8725
Version December 1994
Procedures manual for monitoring of radioactive substances in the environment and of external radiation
(Messanleitungen für die „Überwachung radioaktiver Stoffe in der Umwelt und externer Strahlung“)
H-Rn-222-TWASS-01-01
Rapid procedure for determining
radon-222 in drinking water
1
Scope
The procedure described here for the determination of Rn-222 activity concentrations is suitable for use with clear and colourless samples of water irrespective of
their origin, and in particular for drinking, ground, spring and surface waters. A
result can be expected within several hours from receiving the sample of water. It
should be noted, however, that it takes about 3 hours for the daughter nuclides to
attain equilibrium with Rn-222 (see section 4).
2
Sampling
About 15 ml to 200 ml of water are required for the determination of the Rn-222
content. Prior to sampling, standard glass vials (volume: 20 ml) of the type commonly used for liquid scintillation counting techniques are filled with 5 ml of scintillation cocktail. Vials made of polyethylene are unsuitable for the purpose as both
toluene and Rn-222 will diffuse through their walls.
If samples are collected from water supply pipes, wells or groundwater measuring
points, the stagnating water in the piping system has to be pumped off or left to
drain for an appropriately long period prior to taking the sample. A good indicator
for whether the period of draining stagnating water is sufficient is the water temperature or its electrical conductivity having reached a constant value.
Using a syringe, 15 ml of the water sample to be measured is sucked from the
middle of the running jet of water and transferred to the vial that already holds
5 ml of the scintillation cocktail. It will be expedient to carefully inject the water
under the scintillation cocktail in the vial. Once capped, the vial is vigorously shaken. After the contents have segregated, the scintillation cocktail, which now contains the Rn-222, will float on the water surface.
If the water sample cannot be measured right away, an adequate volume (at least
100 ml) is filled to the brim (i. e., without air volume) into a glass bottle with an
interchangeable stopper made of glass. The partial sample used for measuring is
later taken from this sample bottle with a syringe.
Water samples containing radon tend to give off gases, in particular when stored
warm or ventilated. It is therefore necessary to work diligently when extracting the
sample. The possibility of losses of Rn-222 cannot be excluded.
If a low Rn-222 activity concentration or interfering radionuclides such as uranium
or Ra-226 at relatively high concentrations are to be expected, 200 ml of the sample water are shaken intensively with 20 ml of scintillation cocktail in a separation
funnel. Following the separation of phases, the (lower) aqueous phase is drained
and discarded. The organic phase containing the Rn-222 is then transferred to the
vial.
Collecting parallel samples and performing analyses at least in duplicate should be
standard in order to exclude the possibility of incurring errors during sampling and
handling.
Version December 1994
Procedures manual for monitoring of radioactive substances in the environment and of external radiation
(Messanleitungen für die „Überwachung radioaktiver Stoffe in der Umwelt und externer Strahlung“)
H-Rn-222-TWASS-01-02
3
Analysis
3.1
Principle of the procedure
The measuring procedure exploits the fact that radon dissolves about 17 times
more readily in toluene than in water. The radon is extracted from the water sample (aqueous phase) by liquid-liquid extraction to a scintillation cocktail based on
toluene and then measured in a liquid scintillation counter (LSC). Once radioactive
equilibrium is established, the alpha radiation emitted from the Rn-222 and from
the short-lived daughter nuclides Po-218 and Po-214 is measured together with the
beta radiation from the daughter nuclides Pb-214 and Bi-214. (The nuclides of the
U-238 decay chain are compiled in Figure 1.)
Fig. 1:
3.2
Decay chain of uranium-238. Decay processes with emission probabilities
of < 0,1 % are not included. The historical names of the individual isotopes are given in brackets.
Sample preparation
No further preparation of samples is required.
3.3
Radiochemical separation
No chemical preparation of the samples is necessary.
Version December 1994
Procedures manual for monitoring of radioactive substances in the environment and of external radiation
(Messanleitungen für die „Überwachung radioaktiver Stoffe in der Umwelt und externer Strahlung“)
H-Rn-222-TWASS-01-03
4
Measuring the activity
4.1
Calibration
A calibration source is prepared by covering 15 ml of a Ra-226 standard solution
with 5 ml of scintillation cocktail in a vial made of glass. This vial needs to be
capped well and stored in a refrigerator with the cap down. After allowing the radioactive equilibrium to establish over a minimum of 14 days (half-life of radon-222:
3,82 days), the bottle is shaken vigorously (time ts). After another waiting period
of 3 hours the sample measured in an LSC following another waiting period of
3 hours (cf. section 4.2).
In order to record the alpha emission of both the Rn-222 as well as those of the
daughter nuclides Po-218 and Po-214 and the beta-radiation of the daughter nuclides Bi-214 and Pb-214 (cf. Fig. 1), the energy range of the LSC should optimally
be set to record an energy range from 19 keV to 1710 keV. (In the case of a device
in which channels are not selected according to energy in keV, this energy range is
covered by calibrating the channels via recording the spectrum of a standard sample.)
The calibration factor, , is derived from equation (1):
A 
aS  VS
R S
 R0

 f1
(1)
with
f 1  exp

  tA


tA
exp   ln 2 

th





where:
A
as
Vs
Rs
R0

th
tA = tm - ts
calibration factor, in Bq·s;
activity concentration of the Ra-226 standard solution, in Bq·l-1;
volume of the standard solution used, in l;
gross count rate of the standard solution, in s-1;
background count rate, in s-1;
decay constant of Rn-222, in s-1;
half-life of Rn-222, in s;
period of time between shaking and the end of the measuring process
tm, in s.
The calibration may be verified by comparing the result with other approaches,
e. g., according to the emanation method described in procedure H-Ra-226TWASS-01.
4.2
Measurement
After preparing the counting source (water and scintillation cocktail) and before the
measurement, a waiting period of at least 3 hours must be ensured to establish
radioactive equilibrium. On the other hand, the measurement of the Rn-222 should
commence on the same day owing to its relatively short half-life of 3,82 days and
the ingrowth from Ra-226. It furthermore needs to be taken into consideration that
Version December 1994
Procedures manual for monitoring of radioactive substances in the environment and of external radiation
(Messanleitungen für die „Überwachung radioaktiver Stoffe in der Umwelt und externer Strahlung“)
H-Rn-222-TWASS-01-04
radon may be lost through diffusion through the cap even if the sample is kept cool
(the maximum storage period is 2 days to 4 days).
Notes
If it is impossible to measure directly after the samples have been prepared, the vials have
to be stored cap-down in a dark and cool place.
A counting source should be measured several (at least three) times in order to validate
the stability of the measuring configuration and discover possible influences by chemiluminescence and others (see also procedure H-C-14-AWASS-01). A blind source prepared from
de-emanated water (see section 7.1) must always be measured in the same measuring
cycle.
If the presence of interfering radionuclides is suspected and these cannot be extracted with toluene, a partial volume of the water sample should be de-emanated
and co-measured as a blind source.
To verify whether other alpha emitters are possibly present in a counting source as
well, the measuring process may be repeated about 3 days later, which should
then show a plausible decrease in the count rate that is in line with the half-life of
radon.
5
Calculation of the results
The activity concentration of Rn-222 is calculated according to equation (2):
c 
 A  R g  R 0

(2)
 f2
V
with
f 2  exp
where:
c
Rg
R0
V
tA
A

th

 tA


tA
exp  ln 2 

th





activity concentration, in Bq·l-1;
gross count rate, in s-1;
background count rate, in s-1;
volume of the sample, in l;
period of time between sampling and measuring, in s;
calibration factor, in Bq·s;
decay constant of Rn-222, in s-1;
half-life of Rn-222, in s.
The simple standard deviation of the activity concentration is derived from equation (3):
s 
A
V

Rg  R0
t
 f2
(3)
The uncertainty of the calibration factor, the volume, and the factor f2 are small
compared to the statistical counting uncertainty and are therefore negligible.
Version December 1994
Procedures manual for monitoring of radioactive substances in the environment and of external radiation
(Messanleitungen für die „Überwachung radioaktiver Stoffe in der Umwelt und externer Strahlung“)
H-Rn-222-TWASS-01-05
5.1
Worked examples
Inserting numerical values of:
A
Rg
R0
V

th
tA
tm
=
=
=
=
0,30 Bq·s;
1,76 s-1;
0,44 s-1;
0,015 l;
=
=
=
=
2,1∙10-6 s-1;
3,3∙105 s (= 3,82 d);
2,45·105 s (= 2,8 d);
600 s.
Equation (2) yields the activity concentration of a counting source of drinking water
at the point of time of sampling as:
c = 44 Bq·l-1
According to equation (3), the simple standard deviation amounts to:
s = 2,9 Bq · l-1
Thus, the results yields:
c = (44 ± 3) Bq·l-1
Note
Owing to the fact that this measuring technique records short-lived daughter nuclides as
well, a calibration factor of A < 1 is obtained for Rn-222, which corresponds to a detection
efficiency of  > 1.
5.2
Consideration of uncertainty
The total uncertainty of determining a mean radon concentration in drinking water
of ca. 5 Bq·l-1 ranges around 20 %.
Incidental errors may be recognised by performing repeated measurements or
working on parallel samples.
6
Characteristic limits of the procedure
For calculating characteristic limits, reference is made to chapter IV.5, section 2.1,
of this procedures manual. According to equation (2.6) of this section and applying
the numerical values given in section 5.1 with a factor of k = k1- + k1-β = 4,645,
the detection limit is obtained as:
g = 3,6 Bq·l-1
With respect to the point of time of sampling in the example above, a detection
limit of 6 Bq·l-1 has been attained. In practical operations, and given the conditions
specified above in conjunction with measuring periods of the background effect and
the sample of 6000 s each, a detection limit of about 2 Bq·l-1 can be achieved.
Version December 1994
Procedures manual for monitoring of radioactive substances in the environment and of external radiation
(Messanleitungen für die „Überwachung radioaktiver Stoffe in der Umwelt und externer Strahlung“)
H-Rn-222-TWASS-01-06
7
Catalogue of chemicals and equipment
7.1
Chemicals
All chemicals used should be of the purity grade “pro analysi”, if possible.
― Scintillation cocktail
(e. g., Packard-TOLUENE-SZINTILLATOR with 0,1 g·l-1 of POPOP and 5 g·l-1 of
PPO);
― De-emanated water:
may be produced, for example, by boiling tap water for about 10 minutes or
purging it with nitrogen (over a period of about 30 minutes);
― Ra-226 standard solution for calibration;
― Preparing the scintillation cocktail:
scintillation cocktail is prepared by dissolving 5 g of PPO and 0,5 g of POPOP in
1 l of toluene. Stored in containers protected from light, ready-to-use scintillator has a shelf life of at least one year.
7.2
―
―
―
―
―
Equipment
Liquid scintillation counter;
Syringes, net volume 20 ml;
Vials made from K-40-deficient glass, net volume 20 ml;
Separation funnels, net volumes 50 ml and 250 ml, respectively;
Basic equipment of a radiochemical laboratory.
References
(1)
Campisano, C. D.: Geochemical and Hydrologic Controls on radon 222 and
radium 226 in Ground Water. Proceedings of Ground Water Geochemics Conference, Denver, Colorado 23 (1988)
(2)
Davis, R. M., Watson, J. E.: Influence of Ra 226 concentration in surrounding
rock on Rn-222 concentrations in groundwater. Health Physics 58 No. 3
(1990), pp. 369–371
(3)
Dowd, R. M.: EPAs radon Study Results. Environmental Science & Technology
22 No. 1 (1988), pp. 28–36
(4)
Fukui, M.: Rn-222 concentrations and variations in unconfined groundwater.
Journal of Hydrology 79, 1/2 (1985), pp. 83–94
(5)
Gans, L., Fusban, H. U.,Wollenhaupt, H., et al.: Radium 226 und andere natürliche radionuclides im Trinkwasser und in Getränken in der Bundesrepublik
Deutschland. WaBoLu-Hefte 4/1987
(6)
Hewitt, C. N., Kelly, M.: Environmental radon - A Brief Review. Environmental
Technology 11 No. 4 (1990), pp. 387–392
(7)
Hoehn, E., Gunten, H. R. von: Radon in groundwater: a tool to assess infiltration from surfacewaters to aquifers. Water Resources Bulletin 25 No. 8
(1989), pp. 1795–1803
(8)
Lowry, J. D., et al.: Point-of-entry Removal of radon from Drinking Water.
Journal of American Waterworks Association, 79, 4 (1987), pp. 162–169
Version December 1994
Procedures manual for monitoring of radioactive substances in the environment and of external radiation
(Messanleitungen für die „Überwachung radioaktiver Stoffe in der Umwelt und externer Strahlung“)
H-Rn-222-TWASS-01-07
(9)
Lucas, H. F.: Improved low-level alpha-scintillation Counter for radon. Rev.
Scient. Instr. 28 (1957), pp. 680-683
(10) Mathieu, G. G., et al.: System for measurement of Rn 222 at low levels in
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(12) Michel, J.: Relationship of Radium and Radon with Geological formations. Radon, Radium and Uranium in Drinking Water. Ed. C. R. Cothern, Lewis Publication Inc. (1990)
(13) Muth, H., Schraub, A., Aurand, K., and Handtke, H.-J.: Measurements of normal radium burdens. Brit. J. Radiol. Suppl. 7 (1957), pp. 54–66
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(18) Prichard, H. M. and Gesell, T. F.: Rapid measurements of Rn-222 concentrations in water with a commercial liquid scintillation Counter. Health Physics 33
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(23) Gans, I., Kahl, T. and Rühle, H.: Radonmessungen im Rahmen der deutschdeutschen Zusammenarbeit auf dem Gebiet des Strahlenschutzes. Schriftenreihe Reaktorsicherheit und Strahlenschutz BMU-1993-384 (1993), Der Bundesminister f. Umwelt, Naturschutz und Reaktorsicherheit (ed.)
Version December 1994
Procedures manual for monitoring of radioactive substances in the environment and of external radiation
(Messanleitungen für die „Überwachung radioaktiver Stoffe in der Umwelt und externer Strahlung“)