Building materials radon exhalation rate: ERRICCA intercomparison exercise results

The Science of the Total Environment 272 Ž2001. 109᎐118

Building materials radon exhalation rate: ERRICCA intercomparison exercise results N.P. PetropoulosU , M.J. Anagnostakis, S.E. Simopoulos Nuclear Engineering Section, Mechanical Engineering Dept., National Technical Uni¨ ersity of Athens, 15780 Athens, Greece

Abstract The Nuclear Engineering Section of the National Technical University of Athens undertook the organisation of a European building material radon exhalation rate intercomparison exercise in the framework of the European Research into Radon In Construction Concerted Action ŽERRICCA.. The intercomparison started in June 1998 and it was concluded in February 1999. Twenty participants from 13 countries took part. The exercise focused on the radon exhalation rate determination from a concrete slab, specially constructed to produce radon surface flux well below 10 mBqmy 2 s y1. This paper describes the measurement results obtained using different instruments and methods in order to assess ‘state-of-the-art’ low-level radon exhalation measurements, being performed around Europe. Results are compared to each other and they provide an indication of the collective precision of such measurements for low exhalation rates. The agreement, with a few exceptions, is satisfactorily good. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Radon;

222

Rn; Building materials; Radon exhalation rate; Intercomparison

1. Introduction The Commission of the European Union and the Building Research Establishment of the United Kingdom ŽBRE᎐UK., along with other

U

Corresponding author. Tel.: q301-7722939; fax: q3017722914. E-mail address: [email protected] ŽN.P. Petropoulos..

associated contractors from all over Europe, have agreed, in December 1996, to a concerted action called ‘ERRICCA ᎏ European Research into Radon In Construction Concerted Action’, to be carried out in the framework of the Nuclear Fission Safety Research and Training Programme. The overall objective of the action was the exchange of experience in research and development for radon and building construction among all participating countries, as well as to use the

0048-9697r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 1 . 0 0 6 7 4 - X

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N.P. Petropoulos et al. r The Science of the Total En¨ ironment 272 (2001) 109᎐118

expertise of the more experienced in this field to the benefit of the less experienced. Furthermore, the action had certain specific objectives, each one of them formed a topic. It was agreed that some of the associate contractors should coordinate the topics considered. The Nuclear Engi-

neering Section of the National Technical University of Athens ŽNES᎐NTUA. was appointed to coordinate the topic ‘radon emissions from materials’. Following this rationale and in the course of the March 1998 ERRICCA meeting at Gent, Belgium, NES᎐NTUA undertook the organisa-

Table 1 List of participants in the ERRICCA radon exhalation intercomparison Austria Austrian Research Centre Seibersdorf Radiation Protection Dept.

Austria Salzburg University Institut of Physics and Biophysics

Bulgaria St Kliment Ohridski University of Sofia Physics Faculty

Denmark Risoe National Laboratory Nuclear Safety Research Dept.

Germany SARAD GmbH Pesterwitz

Greece Aristotle University of Thessaloniki Physics Dept.

Greece National Research Center for Physical Sciences ‘Demokritos’ Institute of Nuclear Technology and Radiation Protection

Greece National Technical University of Athens Nuclear Engineering Section

Greece University of Athens Medical Physics Dept.

Italy ANPA ŽEnvironmental Protection Agency. Environmental Radioactivity Sector

Italy Universita Cattolica del Sacro Cuore di Roma Žin collaboration with ANPA.

Poland Central Laboratory for Radiological Protection Radon Research Group

Poland Central Mining Institute Radiometry Lab.

Portugal Instituto Teclologico e Nuclear Departamento de Protecca e Seguranca Radiologica

Romania Babes-Bolyai University Physics Dept.

Romania Public Health Institute Radiohygiene Laboratory, Cluj-Napoca

Romania ICPMRR Research Laboratory for Radiation Protection

Slovenia Occupational Safety Institute Ecology Toxicology and Radiation Protection Dept.

Spain University of Cantabria Faculty of Medicine, Medical Physics Lab.

Switzerland Swiss Federal Office for Public Health Radioactivity Surveillance Section

The Netherlands Kernfysisch Versneller Instituut, Nuclear Geophysics Division

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tion of a European building material radon exhalation rate intercomparison exercise. To this end it launched a participation call for most of the radon researchers in Europe, including nonERRICCA participants. Following the positive response from over 20 laboratories ŽTable 1. we: Ža. constructed three concrete slab specimens with dimensions 30 = 30 = 10 cm Ž9 l. and approximate weight 20 kg each, using the same aggregate; and Žb. organized the specimens dispatching scheme to the participating laboratories all over Europe. The intercomparison started in June 1998 and it was concluded in February 1999. Each participant was allowed for a 20᎐25 days measurement time. All participants reported back their results in March 1999, in the same format. It was requested to report the exhalation results in Bqsy1 along with the total associated uncertainty calculated by following the equation: u s Ž a2 q b 2 q c 2 .

0.5

Ž1.

where: as the counting Žrandom. error at the 1␴ confidence level; bs the systematic error Ždue to instruments used, techniques, calibration etc..; and c s the computation error Ždue to any algorithm applied to the raw data, i.e. least square fittings, etc.. A ‘three-character-code’ was assigned to each participant: the first character, being a letter of the Latin alphabet, is the participant code; the second is ‘1’, ‘2’ or ‘3’ depending on the concrete specimen analyzed by the participant; and the third is an integer ranging from 1 to n depending on the number of methods, used by the participant to determine the exhalation rate of the slab, or on the number of consecutive measurements to assess reproducibility, performed by participants H, K and L. The results are not being reported against a standard or reference measurement, since the intercomparison exercise was never intended to be used as an intercalibration of methods, techniques and instruments and should not be interpreted as such following the context of the 6th IRMP intercomparison test and workshop ŽIRMP6. held from 12᎐15 June 1995 in New York, USA ŽHutter and Knutson, 1998..

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2. Materials and methods The 226 Ra concentration of all the raw materials used for the construction of the three concrete slab specimens was determined by high-resolutionrhigh-efficiency ␥-spectroscopy ŽSimopoulos and Angelopoulos, 1987.. The aggregate composition and 226 Ra concentration ᎏ per mass of dry material ᎏ of its components were: Ža. 21% cement with 131 " 7 Bq kgy1 ; Žb. 28% sand with 4.0" 0.2 Bq kgy1 ; Žc. 21% gravel with 3.0" 0.2 Bq kgy1 ; and Žd. 19% water. To artificially enhance the 226 Ra concentration of the slab, 11% fly ash with 226 Ra concentration equal to 1000 " 50 Bq kgy1 was further added to the aggregate. Water was assumed to have negligible 226 Ra concentration. This resulted in concrete slab specimens with an estimated mean 226 Ra concentration equal to 140 " 6 Bq kgy1 . Two aggregate samples subsequently analyzed were found to have an 226 Ra concentration equal to 107 " 5 Bq kgy1 . This discrepancy between the estimated mean 226 Ra concentration and the measured 226 Ra concentration may be accounted for by: Ža. inadequate mixing of the aggregate from which the samples were taken; and Žb. the use of wet materials when forming the aggregate. After construction and following a short 4-week conditioning period at 25⬚C and ; 45% RH, all three concrete slab specimens were measured prior to shipment, in order to determine their radon exhalation rate and to ensure that there was no significant difference between the exhalation rates of each one of them. It was concluded from these measurements that specimen 1 presented somewhat higher radon exhalation rates than that of specimens 2 and 3; this result was further verified in the course of the intercomparison exercise. Radon exhalation measurements are mainly based on the principle of enclosing or covering the sample to be measured. Different techniques are then applied for measuring the radon concentration in the enclosure. As reported by the intercomparison participants the main methods employed for estimating the radon exhalation rate from the concrete slab specimens were:

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2.1. Closed-chamber-methods (CCM) 2.1.1. Enclosed sample method (ESM) According to the summary in Stranden Ž1988., this is the most common way to assess the exhalation rate of a building material. The sample is enclosed in an airtight container and the radon concentration growth in the air volume is being followed. Ideally the radon concentration in the free chamber volume V Žm3 . is described by the following equation: C s C0 exp Ž -␭ t . q E w 1-exp Ž -␭ t .xŽ ␭V .

y1

Ž2.

where: Cs

the total radon concentration ŽBq m -3 . in the container at growth time t Žh. Es exhalation rate ŽBq hy1 . ␭ s radon decay constant Žhy1 . C0 s the initial radon concentration ŽBq m -3 . in the container at time t Ž0h. ᎏ i.e. the background.

It is well understood that Eq. Ž2. is valid if: Ža. there is no leakage of radon out of the container; and Žb. the activity concentration in the container air is low compared to the activity concentration in the pore air of the sample. Leakage and backdiffusion effects may be taken into account if the effecti¨ e decay constant ␭U is further introduced, thus transforming the equation to: C s C0 exp Ž y␭U t . q E w 1-exp Ž y␭U t .x = Ž ␭U V .

y1

2.2. Open-chamber-methods (OCM) Measurements are performed in well-ventilated containers or accumulators under near-zero concentration conditions. The container or accumulator gas is continuously removed using a constant flow pump, providing a known gas-exchange rate. The steady-state radon concentration in the container gas is given by the following equation: C s EŽV ␭¨ .

y1

q C0

Ž5.

where: ␭ ¨ s air-exchange rate Žhy1 . C0 s the radon concentration ŽBq my3 . of the gas being used to flush the container ᎏ i.e. the background.

3. Problems encountered in exhalation rate measurements 3.1. Background C0

Ž3.

where ␭U s ␭ q ␭ b ,l and ␭ b,l is a decay contant correcting for first-order removal of radon by back-diffusion ‘b’ and leakage ‘l’. A participant reported that radon concentration vs. time data for short growth time might be also approximated by the quadratic polynomial formula: C s aq EtV y 1 q ct 2

2.1.2. The accumulator method (AM) Another common technique for radon exhalation rate measurements from concrete slabs, is to seal a container ᎏ called in this case an accumulator ᎏ to a part of the exhaling area and measure the radon growth inside the container. The same growth equations apply but in this method back-diffusion and leakage effects are even more pronounced.

Ž4.

High radon concentrations in a Laboratory environment result in a high initial radon concentration in the container or the accumulator air, thus preventing any precise exhalation rate estimations in short growth ŽSG. times for the closed-chamber-methods or the discrimination of the exhaling radon contribution to the total air radon concentration for the open-chamber-methods. Radon free air or nitrogen may be used as the carrier gas in either group of methods to ensure that background is minimized inside the

N.P. Petropoulos et al. r The Science of the Total En¨ ironment 272 (2001) 109᎐118

measurement containers Žnitrogen was used by participants A, G and H.. If the background C0 has not been measured, concentration vs. time data, fitted to Eq. Ž2. or Eq. Ž3. allow the estimation of C0 , as an unknown parameter of the fitting, just like E and ␭U . 3.2. Back-diffusion It is very important to keep the activity concentration in the chamber air low, compared to the activity concentration in the pore air of the sample. Almost immediately after enclosing a sample in a container or attaching an accumulator to an exhaling surface, the radon released from the exhaling material has a significant probability of diffusing back to the sample. Closed-chambermethods of small chamber volume compared to the pore volume of the sample are highly susceptible to such phenomena. The practical approach to overcome this difficulty is: Ži. to use short growth ŽSG. times and to assume that the radon growth follows Eq. Ž2. or its approximation Žfor non-detectable background.: C s E Ž ␭V .

y1

Ž6.

or Žii. to determine a growth curve similar to that resulting from Eq. Ž3., in case that long growth ŽLG. time is used. The theoretical and experimental work reported by Samuelsson and Petterson, Ž1984., Samuelsson and Erlandsson, Ž1988., Samuelsson, Ž1990. suggests that choosing a chamber free volume 10 times larger than the pore volume of the sample, may acceptably minimize the backdiffusion effect. Nevertheless, if for reasons of sensitivity one has to work with small chambers then it is further suggested that in the case of 10 cm thick samples ᎏ just like the intercomparison concrete slab, the use of Eq. Ž2. or Eq. Ž6., is only valid for a time less than 1.5 h ᎏ that is the suggested short growth ŽSG. time. Controlled

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leakage as in the open-chamber-methods reduces significantly the back-diffusion effect. 3.3. Leakage The practical approach to overcome the difficulty of uncontrolled leakage is again either Ži. or Žii. as described for the back-diffusion effect.

4. Radon concentration determination techniques 4.1. Acti¨ e techniques (AT) Most intercomparison participants used active measurement techniques for determining radon exhalation rates. These techniques mainly involved commercially available integrated instruments andror various types of scintillation cells and photo-multiplier assemblies. In many cases the instrumentation was placed either inside the measurement chamber Žinside-chamber-technique, ICT., or it was connected to it through suitable piping and a pump system forming a closed circuit Žoutside-chamber-technique, OCT., thus allowing for continuous Žor quasi-continuous. concentration estimation. In other cases using instrumentation set outside the chamber, grab sampling ŽGS. of the chamber air was regularly conducted. 4.2. Passi¨ e techniques (PT) Some participants reported the use of containers with activated charcoal ŽAC. sealed to the exhaling surface. The charcoal absorbs virtually all the radon entering the container, and thus the back-diffusion problems seems to be reduced. The radon accumulated on the charcoal may be determined by counting gamma emissions from the container after the end of the growth period. This exhalation rate measuring method is evidently characterized as a closed-chamber, accumulation method. A couple of participants reported that when using the enclosed sample method, they placed charcoal or solid-state nuclear track detectors ŽSSNTD-inside-chamber-technique. inside

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Table 2 The summary of exhalation rate measurement methods and radon concentration measurement techniques employed in the course of this intercomparison Participant code

Number of methods employed

Exhalation rate estimation method

A B C D E F

2 1 1 2 1 1

G H J

1 1 1

ESM,LGrOCM 45.18 ESM, LG 91 ESM, LG 91 AM, SG NRa ESM,LG 1000 ESM, SGr 36 ESM, LG OCM 71 ESM, LG 1000 ESM, LG 100

K L M N P Q R S T

1 1 1 1 1 2 1 1 4

U V

1 1 a b

Free chamber volume Žl.

ESM, SG 34.6 ESM, LG 1000 ESM, LG 111 ESM, LG NRa OCM 18 ESM,SGrLG 56 ESM, LG 258 ESM, LG 266 ESM,SGrESM, LG 47 NRr47 AMrESM, LG ESM, LG 47.9 ESM, LG 52

Radon measurement technique

Lower limit of detection Ž␮Bq sy1 .

Leakage correction and back-diffusion correction Ž%.

AT, ICT AT, GS AT, ICT AT, ICTrPT, AC AT, ICT AT,GSrAT, GS

- 2r30 NRa 3 NRa 50 50r3

5.3rNAb NRa 3 No, short growthryes, NRa 5 NRrNR

PT, OCT, AC 5 AT, ICT 200 PT, ICT, SSNTDrAT 25r14r60y 10ŽU . ŽU . depending on ICTrAT, GS growth time AT, OCT 50 AT, ICT 200 = 10-3 AT, ICT NRa AT, ICT NRa AT,OCT 17 AT, ICT NRa AT,ICT NRa AT NRa PT, ICT, ACrAT, ICT 27.5r44 PT, ACrAT, GS 27.5r36

NAb 10᎐20 1.4r0.8y 1.4r0.4y 2.1 ŽU . ŽU . depending on growth time No, short growth 10᎐20 No No NAb No, short growthr8.5 NRa Yes, NRa Yes 5 y 15r5 Yes 5 y 15r5

AT, ICTrAT, OCT AT, ICT

2.7 NRa

67 250

NR: not reported. NA: not applicable.

the measurement chamber. The radon accumulated in the charcoal is again determined by counting its gamma emissions after the end of the growth period. In the case of the SSNTDs the number of tracks recorded is directly proportional to the exposure ŽBqh.. One participant used an open-chamber-method and employed charcoal placed outside the chamber Žoutside-chambertechnique .; in-line flow through the charcoal canister which allowed the absorption of the exhaled radon. All methods and techniques employed in this intercomparison are being summarized in Table 2 using the abbreviations already introduced for each exhalation rate estimation method and radon concentration measurement technique. Table 3 presents the exhalation results reported by each

participant along with their associated uncertainties.

5. Results and discussion Fig. 1 presents the exhalation rate results reported for each one of the three specimens. Four of the reported results ŽU31, U32, B11, D12. lying outside the graph range, were excluded as outliers. The results, excluding the outliers, may be summarized as follows Žmean " standard deviation; coefficient of variation; range, in mBqs -1 .: specimen 1: 1.65" 0.21 Ž13%.; 1.36᎐2.05. specimen 2: 1.37" 0.09 Ž7%.; 1.24᎐1.61. specimen 3: 1.32" 0.40 Ž31%.; 0.44᎐2.00.

Participant code

A

11 12

1.72" 0.10 2.05" 0.21

Participant code 21 22 23 24 25 26 27 28

H

Participant code 31 32 33 34 a

B

7.1" 1.5

J

C

D

E

F

G

H

K

L

1.39" 0.14 1.57" NRa 1.74" NRa 1.34" 0.06 1.73" 0.07 1.66" 0.17 1.70" 0.16 1.74" 0.17 2.9" NRa 1.36" 0.07 1.85" 0.17 K

L

1.50" 0.15 1.33" 0.11 1.44" 0.09 1.61" 0.16 1.35" 0.13 1.35" 0.11 1.44" 0.09 1.24" 0.12 1.31" 0.04 1.33" 0.04 1.35" 0.10 1.36" 0.04 1.33" 0.02 1.36" 0.01 H

K

L

M

1.40" 0.14 1.66" 0.16 1.23" 0.12 2 " NRa 1.13" 0.11 1.47" 0.15

NR: not reported.

N

P

Q

R

11.7" NRa 1.49" 0.26 1.77" 0.06 1.5" 0.2

S

T

U

V

1.60 " 0.16 1.38" 0.03 0.44" 0.04 0.9" NRa 1.41" NRa 0.50" 0.05 1.14" NRa 1.34" 0.04 1.33" 0.04

N.P. Petropoulos et al. r The Science of the Total En¨ ironment 272 (2001) 109᎐118

Table 3 Exhalation rate measurements results ŽmBqsy1 ..

115

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N.P. Petropoulos et al. r The Science of the Total En¨ ironment 272 (2001) 109᎐118

Fig. 1. ERRICCA intercomparison radon exhalation results.

Two of the participants determined the exhalation rate of the concrete slab specimens used in the exercise, for reference purposes. From the above results it becomes obvious that ᎏ as already mentioned ᎏ specimen 1 presents a statistically significant higher radon exhalation rate than the other two specimens. A minor loss-ofweight effect Ž; 1 kg. was noticed in the course of the intercomparison run, obviously due to the loss of water. Nevertheless, it seems that none of the specimens presented any systematic aging effects wi.e. a decrease of the exhalation rate, see Yu et al. Ž1995.x during the cycle of the follow-up measurement procedure.

The low coefficient of variation of the measurements of specimen 2 is mainly due to the multiple evaluations by the same participant, which are in extremely good agreement. The relatively high coefficient of variation of the measurements of specimen 3 is due to the two outlying measurements of one participant. Excluding these two measurements the coefficient of variation becomes 18%, which is comparable to the coefficient of variation of the measurements of specimen 1. Therefore, it is concluded, that a coefficient ranging between 10 and 15% may be considered as reasonable and typical for such measurements.

N.P. Petropoulos et al. r The Science of the Total En¨ ironment 272 (2001) 109᎐118

Following the pore volume estimation of specimen 2 conducted by participant J Ž2.5 l., one can reasonably assume that the three concrete slab specimens constructed for this intercomparison have a 2᎐3 l pore volume. Therefore, it seems that most of the intercomparison participants used chambers with volume well adequate to fulfill the prerequisite of a free volume 10 times larger than the pore volume of the sample. Only two participants ŽD, K. reported thoron exhalation results. Participant K estimated the thoron exhalation for specimen 2 equal to 70 mBq sy1 , whereas participant D estimated the thoron exhalation for specimen 1 equal to 20 mBq sy1 Žclosed-chamber accumulator method, inside chamber active technique. and equal to 100 mBq sy1 Žaccumulator method, activated charcoal.. Although intended in the beginning of the intercomparison exercise no time was spared to allow for time apart duplicate measurements of the same slabs by the same participants. The outlying results of participants B and N are attributed by them to the limited time available for correct specimen conditioning and measurement ᎏ for reasons beyond the control of the intercomparison organizers.

6. Conclusions The agreement between the participants’ data, with a few exceptions, is satisfactory. The results are fairly well within the precision expected for such measurements. Given the fact that the exhalation measurement itself disrupts the very parameter that is being measured, this conclusion is very encouraging. Following this conclusion, it seems that there are currently available instruments, techniques and know-how, that meet the need for accuracy and precision for low-level radon exhalation rate measurements from building materials, thus permitting building material reconnaissance and screening regarding radon exhalation rate. However, no standard method has been applied for the measurements, most participants are in favor of the enclosed sample method, the radon concentration which was monitored by

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various techniques. Further intercomparisons are absolutely useful for researchers to evaluate their techniques and methods and isolate potential problems for such low-level exhalation rate measurements. In the follow-up of this intercomparison exercise, and since ᎏ to our knowledge ᎏ no other comparison between methods has been carried out in the past, it might be possible: 1. to get a better overview of the demands which should be met by the radon exhalation measuring methods Žincluding sample preparation.. 2. to evaluate measuring procedures from both practical and scientific point of view taking into account cost effectiveness, practical performance and reliability. 3. to influence future recommendations for standard radon exhalation rate measurements procedures.

Acknowledgements Thanks are due to DHL International Hellas S.A., for their sponsoring of the sample forwarding within the European Union member countries. Acknowledgements are further extended to Claus Andersen and Emiel van der Graaf for their useful suggestions and comments in the course of the intercomparison run. References Hutter AR, Knutson EO. An international intercomparison of soil gas radon and radon exhalation measurements. Health Phys 1998;74Ž1.:108᎐114. Samuelsson C, Petterson H. Exhalation of 222 Rn from porous materials. Radiat Prot Dosimetry 1984;7Ž1-4.:95᎐100. Samuelsson C, Erlandsson K. The implication of the time-dependent diffusion theory on radon-222 exhalation measurements, Radiation Protection Practice, II. Sydney: Pergamon Press, 1988:898. Samuelsson C. The closed-can exhalation method for measuring radon. J Res National Institute Stand Technol 1990;95Ž2.:167᎐169.

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Simopoulos SE, Angelopoulos MG. Natural radioactivity releases from lignite power plants in Greece. J Environ Radioact 1987;5:379᎐389. Stranden E. Building materials as a source of indoor radon. In: Nazaroff WW, Nero AV, editors. Radon and its Decay

Products in Indoor Air. New York: John Wiley and Sons, 1988. Yu KN, Chan TF, Young ECM. The variation of radon exhalation rates from building surfaces of different ages. Health Phys 1995;68:716᎐718.