Survey of natural radiation in France

The Science of the Total Environment, 45 (1985) 467--474

467

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

SURVEY OF NATURALRADIATION IN FRANCE A. RANNOU, C. MADELMONT, H. RENOUARD Commissariat a l'Energie Atomique, I n s t i t u t de Protection et de SOret~ Nucl~aire, D~partement de Protection Sanitaire, B.P. n° 6 F 92260 FONTENAY-AUX-ROSES (France)

ABSTRACT Survey of natural radiation is conducted in France since 1981, with the assessment of the components resulting from external sources (ground and b u i l ding materials) using thermoluminescent dosemeters. In addition, the internal exposure to Rn-222 and the potential alpha energy due to radon daughters, are estimated by passive track detectors in the f i r s t case and active dosemeters in the second one. This paper presents the French program methods and results with an analysis and discussion of the observed levels. INTRODUCTION Exposures from natural sources of radiation constitute a major part of the annual average doses received by individual of the public (ref. l ) . However the available data collected in different countries clearly show that exposures to natural radiation and specially those resulting from radon and radon daughters in houses, present a very large v a r i a b i l i t y which has to be assessed more accurately. In 1981, to this aim, the Commissariat ~ l'Energie Atomique started the measurements of external radiation and the estimations of radon and radon daughters concentrations in houses. OUTLINE OF THE SURVEY Field measurements The survey is s t i l l

in progress; I t is carried out with the support of the

Civil Defense and the help of local authorities. The choice of the locations in the studied area depends on the population density, with at least one measurement by town. putting-in

of

In agreement with the concerned inhabitants, the these dosemeters in houses is provided by the local autho-

r i t i e s . On an average, there is about one gamma radiation measurement per 150O persons (except in Paris and i t s suburb with only one measurement per 32500 persons) and per 20 square-kilometers At the present stage, this study has covered approximately 50% of the French population and 43% of the

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468

territory (Fig.l). In addition to the external radiation survey, radon concentration measurements in houses began in 1982 when using passive detectors. Measurements of potential alpha energy due to radon daughters have also been carried out in a few houses, trying to have a r e a l i s t i c sampling of the different types of houses. Measurement procedure and methods Because of the low levels to be measured and possible variations with time, long term integrating methods have been used for both components of natural sources. These methods have been described elsewhere (ref. 2, 3 and 4). Basically, gamma radiation is assessed by means of thermoluminescent dosemeters (Ca SO4, Dy thermoluminescent dosemeters have been chosen because of less potential t o x i c i t y when compared to Lithium fluoride based dosemeters) for approximately six months. The internal exposure in the dwellings is monitored using passive track detectors which provide the average Rn-222 concentration over one month, and with active dosemeter providing the integrated potential alpha energy due to the decay products of radon (Rn-222 and Rn-220). These two methods involve the use of cellulose nitrate as alpha detector (KODAK, LR-ll5). The readout of the passive detectors is performed by an automatic spark counter (ref. 5) and the film of the active devices is counted with an automatic optical system. The main characteristics of the houses where the measurements were carried out have been s t a t i s t i c a l l y analysed through questionnaries. RESULTS External exposure In order to estimate the t e r r e s t r i a l component and the one due to building materials, each chosen location has two detectors:an indoor

TLD and an o u t -

door TLD, the second one being used only in case of natural s o i l . The arithmet i c mean values obtained after substraction of the component due to cosmic rays (assumed to be 0.032 ~Gy.h- l ) are as follows : Indoors (5798 measurements) : 0.075 ~Gy.h-l • Outdoors (5142 measurements): 0.068 uGy.h-l The mean indoor exposure rates obtained in the studied areas are reported by classes on a map as shown on figure I. The distribution of the data is presented on figure 2. Close to 95% of the indoor measurement are found in the range 0.023 - 0.148 ~Gy.h- l . None of the studied locations shows an

"abnor-

mal"exposure rate; the highest value collected is 0.430 ~Gy.h- l . Furthermore this corresponds to the results that may be expected on the basis of geologi-

469 cal formations across France. The highest exposure rates have been obtained in granitic areas, whereas the lowest ones were observed in sedimentary areas. Analysis of these data leads to two observations. First, houses do not constitute a screen for t e l l u r i c radiation but on the contrary contribute to an increase of exposure (about lO%).Secondly,no clear significant difference could be observed among houses of the same area, when looking to the building materials. Comparison between houses using the same building materials but located in two different areas could perhaps provide a different conclusion. Nevertheless, the type and nature of the underground may be assumed to be the major factor of external radiation exposure indoors.

Fi~. 1- NATURAL RADIATION: Indoor gamma roy exposure rate ( mean value x jJ Gy. h - I cosmics substracted ) <0.057 o.o57- o.o o 0.080-0.114

~

>0.114 s,ud i. progress

!:-7D

Percent of dwellings 20%

Mean value = 0.075 p Gy.h-I

i

( N = 5798)

I I

15%

I 10%

I I

I I

5%

a !

I 0

0.05

Exposure rate 0.10

0.15

0.20

I

~ ' ~

0.25 pG,v.h - I

Fig. 2: Rela tire frequencies of exposure rate in houses

Internal exposure An important e f f o r t has been devoted to the radon measurement in the last few years. The comparison between the two devices used for this purpose giv~the practical

conclusion : l Bq.m-3 of Rn-222 estimated by the passive

detector is on an averageequivalent to an equilibrium radon concentration of 0.38 Bq.m-3 (or 2.1 x lO-9 J.m-3) measuredwith the active dosemeter. Rn-222 concentration

has been monitored in about 800 houses using the

passive method. The main characteristics obtained are presented in Figure 3 and Table I. The d i s t r i c t "Loire" presents on the average the highest Rn-222 concentrations in houses. This is a granitic area where Uranium mines were a few years ago s t i l l in operation. At the opposite the d i s t r i c t "AlpesMaritimes" located in south of France on the Mediterranean sea coast, gives the lowest values. Furthermore, a rather good agreement is found between the mean external radiation exposure rate in a given area (Ey) and the average radon concentration in houses of this area (Crn). These data may be f i t t e d to a curve by means of the least square method. The equation obtained is :

471 Crn (Bq.m-3) = 581 x E~ (~Gy.h - I ) - 5

(I)

with a l i n e a r c o r r e l a t i o n c o e f f i c i e n t r = 0.92. However, this equation obtained with the results of 9 studied areas cannot be applied to 3 others areas. This leads to some p r a c t ic a l considerations : -

in some s p e c i f i c cases, which have to be precised, external exposure measu-

rements may be of bad guidance f o r radon concentrations estimation -

but,

generally

in a given geographical area, the external exposure

rate is a good parameter to guide the implantation of radon measurement points.

Cumulative percent 100

/

"! -

N = 34

i"

N = 765

90 _

N= 47

10

lpes.Maritimes

,-illo17

I

I o,r. ( B q.m-3 )

10

5O

100

5OO

Fig. 3" Cumulative frequency distribution of R n . 2 2 2 concentrations indoors in 2 areas and for national data

472

TABLE l Characteristic values of the frequency distributions of Rn-222 concentrations indoors Radon-222 concentration (Bq.m-3) Alpes-Maritimes Median value

Loire

19

95% conf. range

7 -

National

ll6 36

44

20 - 541

9 -

Arith. mean

34

176

76

Most frequent value

15

41

25

Min - Max

4 - 451

9 - 905

202

3 - 1258

The analysis of the available data is summarized in table 2, which shows the influence of different parameters resulting in significant differences between houses. TABLE 2 Analysis of the data with regard to the characteristics influencing the Rn-222 concentration in dwellings

Characteristics of

Mean Radon-222 concentration (Bq.m-3)

the houses

Min

Building

Other than stone

Stone

material

(34)

(54)

Location of

Above ground level

the room Possible existence of a ventilation system

Max

(28) Yes

Groundlevel (40) No

(63)

(90)

Beside this analysis, no clear difference is seen when looking at the type of house (individual or multiple-dwelling), i t s age or i t s location (urban or rural). The potential alpha energy (PAE) of Radon daughters (Rn-222 and Rn-220) has been measured in 217 houses, with 173 of them in Western Brittany. This very old granitic formation shows rather high radon concentrations. The total potential alpha energy was found to range from 0.020 uJ.m-3 to 2.170 ~J.m-3 with a mean value close to 0.20 uJ.m-3. On an averagethe Rn-220 daughters

473

contribution is about 33% of the total PAE with the following relation : E220 = 0.417 (E222)0"428 (2) where E222 is the PAE due to Rn-222 daughters (~J.m-3) E220 is the PAE due to Rn-220 daughters (~J.m-3) however, with a rather poor correlation coefficient, r = 0.575. The number of measurements is not yet s u f f i c i e n t to have any serious statistical

analysis, but differences seem to appear between houses with d i s t i n c t

characteristics. DISCUSSION AND CONCLUSION At this stage of this survey, i t should be now possible to assess the contribution of exposure from natural sources in France. 0.075 ~Gy.h-l can be considered as a mean external radiation exposure rate in the dwellings and 40 Bq.m-3 as a mean radon concentration. However, these values are not

quite

representative for the whole t e r r i t o r y and geographical variations should be evaluated more precisely. Specially, radon measurements in dwellings have to be emphasized, for example in granitic areas where a strong probability of finding high levels exists. A reasonable knowledge of the existing exposure to radon and i t s daughters in home is necessary before considering any guidelines and furthermore regulations for l i m i t i n g exposure in existing or future situations. ACKNOWLEDGEMENT This work f i n a n c i a l l y supported by the Commission of the European Commun i t y (Contract No BIO-F-319-SI-F). The authors wish to thank G. TYMENand A. MOUDENfor t h e i r useful collaboration in the internal exposure survey. J,F. PINEAU and L. JEANMAIREare gratefully acknowledged for their technical assistance in this survey. The Direction of Civil Defense and all the local authorities making this survey possible are also

acknowledged.

474

REFERENCES 1

2 3

4

5

United Nations. Ionizing Radiation : Sources and Biological Effects. United Nations Scientific Committee on the Effect of Atomic Radiation 1982 report of the General Assembly, with annexes. United Nations sales publication No. E.82.1X.8. New-YorK, 1982. L. Jeanmaire et M. Verry Mesures de l ' a c t i v i t ~ alpha de l ' a i r au moyen d'un d~tecteur solide de traces, le LR-ll5. Radioprotection, GEDIM 1984, Vol.19, No 3, 199-209. L. Jeanmaire, A. Rannou, F. Posny Radon determination in dwelling : experimental comparison between active and passive detectors IRPA 6. Internation. congress, Berlin, May 7-12, 1984 (A. Kaul et a l . , eds) JUlich : Fachverband fur Strahlenschutz, 1984, Vol.3, 1205-1208. P. Duport, G. Madelaine, P. Zettwoog et J.F. Pineau Enregistrement des rayonnements alpha dans le dosim~tre individuel et le dosim~tre du site du Commissariat ~ l'Energie Atomique lO~me Conference Internationale sur les D~tecteurs Solides de Traces Nucl~aires, Lyon, 2-6 j u i l l e t 1979 (H. Francois et a l , , eds). Oxford : Pergamon Press, 1980. Le Thanh P., Nickpay P. Un d i s p o s i t i f de comptage a d~charges disruptives permettant d'att~nuer les d~charges disruptives multiples Proc. of the lOth International Conference, Lyon 2-6 July 1979, Pergamon Press, pp. 337-362.