Cs-137 in fuels and ash products from biofuel power plants in Sweden

J. Environ. Radioacfivify,

Vol. 31 No. 1, pp. 103-l 17, 1996

Copyright 0 1996 Elsevier Science Limited Printed in Ireland. All rights reserved 0265-931X/96 $15.00 + 0.00 ELSEVIER

0265-931X(95)00028-3

Cs-137 in Fuels and Ash Products from Biofuel Power Plants in Sweden Robert Hedvall* Department

of Radiation Physics, Lund, University of Lund, S-221 85 Lund, Sweden

Bengt Erlandsson Department

of Nuclear Physics, University of Lund, S-223 62 Lund, Sweden &

S&-en Mattsson Department

of Radiation Physics, Malm6, University of Lund, S-214 01 Malmii, Sweden (Received 25 November 1994; accepted 18 March 1995)

ABSTRACT The activity concentrations of G-137 in peat, wood chips and ash products from I3 Swedish district heating plants have been investigated during the winter seasons of 1986187, 1988j89, 1989190 and 1990191. There is a significant decrease in the activity concentration of Cs-137 in the fuel which was especially pronounced between the first two seasons after the Chernobyl accident. In spite of the varying deposition of Cs-137 over the country it has been possible to find a relation between the activity concentration in the peat and wood chips, and the deposition. The total number of Swedish heating plants of which 3540 are burning peat and 70-75 wood chips have been divided in three groups according to the activity concentration in the ash products. The mean Cs-137 concentration in ash and the total activity ‘produced’ per year in Sweden have been calculated. The maximum ground concentration and corresponding effective dose rate of inhaled G-137 as a function of the emission rates offlue gases from stacks with varying heights and during different Pasquill atmospheric stability conditions have been calculated. *Present address: Studsvik Nuclear AB, S-61 1 82 Nykiiping, Sweden. 103

R. Hedvall et al.

104

INTRODUCTION The use of biomass fuel has increased rapidly in Sweden during the last decade. After oil and coal, peat and wood chips are the most common fuels used in district heating plants. The energy production from peat burning was 1.1 TWh in 1986 and 3.2 TWh in 1992 and the energy production from wood chips was 3.1 TWh in 1986 and 5.1 TWh in 1992. The total energy production from the district heating plants was 46.9 TWh in 1986 and 47.7TWh in 1992. Peat and wood chips have thus increased their part in the energy production from 9 to 17%. A total of 3500 km2 of peat land could easily be used in Sweden which equals 2000 TWh. Potassium-40 and radionuclides belonging to the uranium and thorium series are the most prominent natural radionuclides in peat, wood chips and straw. Fission products from nuclear weapons tests since the 1950s are also found. After the Chernobyl accident in April 1986 the deposition of the caesium isotopes Cs-134 and Cs-137 has reached several hundreds of kBqmp2 in some areas of Sweden (SGAB, 1986). It is therefore of importance to decide if peat from contaminated bogs can be used as fuel without exposing workers handling the fuels or ash products or people living near the plants to high levels of external and internal radiation due to caesium isotopes. The aim of this investigation is to measure the activity concentration of Cs-137 in the biofuels peat and wood and their enrichment in the combustion process. The activity balance of the radionuclides in the heating plants will also be analysed. The amount of Cs-134 and Cs-137 in ash products produced in power plants per year in Sweden will be caiculated. Finally, a simple model will be applied for the calculation of the effective dose from inhalation of stack effluents to a critical group of persons living in the surroundings of the plants. There is also a component of external exposure. Other ways of increasing the body burden are, for example, ingestion of food products in areas affected by fall-out from stack emissions or by leaching elements from deposits of fly ash.

INVESTIGATION

SITES AND TIME SCHEME FOR THE SAMPLING

Samples of biofuel and ash products in the form of grab samples have been taken once a year from 13 heating plants in Sweden during the winter seasons of 1986/87, 1988/89, 1989/90 and 1990/91. After the first season of 1986/87 there was a break and the investigation was continued after a year. A map with the sites of the heating plants is shown in Fig. 1. The

es-137 from biofuel power plants in Sweden

105

CA-137 (kBq/m2)

Fig. 1. Heating

plants in Sweden studied in this investigation after the Chernobyl accident.

and the deposition

of Cs-137

R. Hedvall et al.

106

Localisation

TABLE 1 of the Biofuel Heating Plants, Type of Fuel and Sampling Location

Angelholm Vastervik Skiivde Eskilstuna Enkiiping Sandviken Hudiksvall Ham&and Gstersund Umel Skelleftea Boden Gallivare

56” 15’N; 57” 15’N; 58” 24’N; 59” 22’N; 59” 39’N; 60” 36’N; 61’44’N; 62” 38’N; 63” 11’N; 63” 50’N; 64” 45’N; 65” 48’N; 67” 08’N;

12” 53’E 16” 53’E 13” 51’E 16” 30’E 17” 05’E 16” 45’E 17”06’E 17” 55’E 14” 39’E 20” 15’E 2 lo OO’E 21’ 42’E 20” 39’E

Fuel P>W W W W W

P P P P P P P>W P

Time

Sampling time Mar Apr May Apr Mar Apr Apr Mar Apr Apr Mar Apr Apr

87 Dee 87 Dee 87 Dee 87 Dee 87 Dee 87 Dee 87 Jan 87 87 Jan 87 Jan 87 Dee 87 Jan 87 Nov

88 88 88 88 88 88 89

Jan Jan Feb Jan Jan Mar Jan

90 90 90 90 90 90 90

Feb Feb Jan Feb Jan Feb Jan

91 91 91 91 91 91 91

89 89 88 89 88

Feb Feb Feb Jan Jan

90 90 90 90 90

Feb Feb Feb Jan Jan

91 91 91 91 91

p: Peat. w: Wood chips.

exact locations and information on whether the plant is burning peat or wood chips are given in Table 1. Several of the heating plants are using fuel from places located up to 100 km from the plant where the deposition of radionuclides may differ considerably from the deposition at the plant itself.

INSTRUMENTATION The activity concentrations of various radionuclides in the samples were determined with a HPGe, or a Ge(Li) detector. The HPGe detector had a relative efficiency of 35.2% and a resolution of 1.78 keV (1.33 MeV). The Ge(Li) detector had an efticiency of 18% and a resolution of 188 keV (1.33 MeV). The detectors were placed in lead caves with wall thicknesses of 12&200mm and the lead caves were also lined with 1Omm of copper. The samples were first dried and homogenised and then packed into either 60, 123 or 180ml which were placed close to the front of the detector. The efficiency calibration was performed with the ion-exchange resin method (Bjurman et al., 1987) where the calibrated substance was packed in tubs with the same volume and shape as those used for the samples. With this close geometry, summation effects from cascading gamma rays must be taken into consideration. Other difficulties in analysing the gamma ray spectra with interfering peaks were also taken into consideration (for further details see Hedvall and Erlandsson (1992)).

107

G-137 from biofuel power plants in Sweden

RESULTS The activity concentrations for the following radionuclides have been measured: K-40, AC-228, Pa-234, Mn-54, Co-60, Zr-95, Ru-106, Ag-1 lOm, Sb-125, Cs-134, Cs-137 and Ce-144. The first three are naturally occurring radionuclides and the others are well known from the Chernobyl fallout. Further information about the activity concentration of K-40, AC-228, Ru106, Sb-125, Cs-134, and Ce-144 are given in Erlandsson et al. (1994). Caesium-137 does not entirely come from the Chernobyl accident but also from various atmospheric nuclear bomb tests mainly carried out in 1956 1958 and 1961-1962. The activity concentrations of Cs-137 in the peat, the wood chips and in the fly-ash are shown in Fig. 2. Notice that the time axis just marks the different winter seasons. It can be seen in Fig. 2 that there is a reduction in the activity concentration with time, but with some exceptions. For wood chips, both Eskilstuna and Enkoping show large variations which cannot be explained by the single sample technique, but most probably reflect variations in the contamination of those areas where the wood chips are coming from. In this case the deposition can vary between 3 and 40 kBq m-2. Also for peat there are great variations. For &tersund the activity concentrations are 360, 500, 130 and 240 Bq kg-‘. These variations are much larger than those found when a number of samples were taken during the same season. Ravila and Holm (1994) have found standard deviations of the activity concentrations for wood chips collected from 16 different plants during the 1990/91 heating season of about f8812%. For peat Hedvall and Erlandsson (1992) found variations of *8% (SD) for summer samples and f14% (SD) for winter samples at Sandviken. Peat is coming from peat bogs within a rather small area. The variations in the . Angelholm v Vlstervik 0 Skovde

0 Enkijping l Eskilstuna

0 Hlrniisand . bstersund h

o Sandviken l Hudiksvall

.a -..‘C .I “L

l Boden. peat x &den, wood 0 Glllivare

CJSkellefte& . umea

l.

. ..

.. ... peat

----Wood -Fly 1

I

l--l

1

I_-1

1

I_.1

1

.?5 .

ash

I_-1.

1

I_. ._

1 _.

I

R. Hedvall et al.

108

activity concentration may therefore not necessarily reflect variations in the deposition but primarily variations in the activity concentration due to changes in the harvesting technique. Surface layers used one year may be followed by deeper layers the next year and then by surface layers, etc. Variations in depth with 2&50 cm may result in variations in the Cs-137 activity concentration of 1O-20 times (Hedvall & Erlandsson, 1992).

ENRICHMENT

FACTORS OF Cs-137

There is an enrichment of most radioactive nuclides in the ash products because of the loss of organic matter in the combustion process (Table 2). This factor varies with fuel and year, but is on average 40 times for wood chips and 14 times for peat with an uncertainty of about f50%. These factors are also in agreement with the ash content which is 2-5% and 510%, respectively. For Finnish peat, Mustonen et al. (1989) have found an enrichment factor of 21 for the heating season 1986/87 which is in rather good agreement with an enrichment factor of 14 found in this investigation if uncertainties are taken into consideration. The statistics from the Swedish District Heating Association contain no information concerning

Enrichment

VPstervik Sk&de Eskilstuna Enkaping Sandviken Hudiksvall H&W&and astersund UmeH

Skelleftel Boden Ggllivare

wood wood wood wood peat peat peat peat peat wood peat + wood peat peat wood peat

nm: Not measured. nr: Not reliable. t: Total fly and bottom

TABLE 2 from Fuel to Fly and Bottom

Factors

Ash for Q-137

86187 Fly Bottom

SSjS9 Fly Bottom

89190 90191 Fly Bottom Fly Bottom

47 43 29

31 18 ~ 20 11 nm 5 nm nm 8 23 60 10

25 72 nr nm 11 nm 14 nm nm 18 12 53 8

15 10 12 nm nm 13 26 25 9

ash.

-

27 3 33 5 7 2 2 47t 21t nm 8 5 8 nm

1 16t 6 16 43 9 nm 3 nm nm 29t 3 10 6 nm

15 85 4nqr nm 5 nm 8 nm nm 58t nm 7 17 nm

20 25 14 23 11 nm 7 nm nm nm 12 29 113 18

17 18 2 31 11 12 nm 3 nm nm nm 1 8 51 nm

Mean 31+ 12 47 zt 24 20 I!Z8 19f4 llfl 9f5

13 f4 23 -i 7 63 +z 37 11 f5

G-137 from biofuel power plants in Sweden

109

TABLE 3 Conversion Factor for Total Produced Ash (kgMW_’ h-‘) which is Either Calculated or Adopted, Total Cs-137 Activity per Year (MBq) and Cs-137 Activity Per Produced Energy Unit (kBq MW-’ h-l) of Cs-137 Conversion factor (kg MW’ h-l)

Wood chips Angelholm Vastervik Sk&de Eskilstuna EnkGping Boden Peat Sandviken Hudiksvall Ham&and &tersund Skelleftea Boden Gallivare Mixed Umed (a) Adopted. (c) Calculated. - Information

Total activity (GBq) 86187 SSj89 89190 90191

7 (a) 7 (c) 7 (a) 9 (a)

0.08 3.57

0.22

1.39

0.02 0.64

0.38 0.46

6.42

7.61

8.11

9 (c) 9 (a)

11.0 0.90

2.02 0.65

Activity per energy unit (kBq MW’h-‘) 86187 M/89 89190 90191

5.65

3.9 25 8.0 31

3.0 36

0.69 0.86

2.49 1.33

161 0.4

13(c) 12(a) 12(a) 12(a) 12(a) 10(c) 12(a)

24.7 22.5 11.9 0.82 0.42 0.78 10.1 3.89 2.57 2.45 0.70 0.73 0.85 0.50 0.39 0.51 0.30 0.31

16.5 0.43 1.86 0.66 0.16 0.09

197 6.6 325 25 7.0 4.7

12(a)

26

7.0

2.6

12

147

2.7

0.4 4.4 6.6

3.4 2.9 2.7

45 7.7

31 30 6.1

19 51 8.6

196 3.6 ~ 29 9.5 4.1 2.8

130 7.1 18 10 5.2 2.7

130 3.8 18 8.1 2.4 0.7

45

89

26

10

missing.

fuel consumption or ash production. It only gives the produced thermal energy in GWh per year. To estimate the amount of produced ash, it is therefore necessary to calculate a factor that converts GWh to kg ash. Ash stands here for the total amount of ash products, both fly and bottom ash. From some of the plants we have obtained detailed information concerning the amount of fuel used, the amount of produced ash products (kg dry weight) and produced energy (GWh) for the four combustion seasons dealt with in this investigation. The calculated conversion factor is 6 kg ash per MWh for wood chips from Vastervik and 9 kg ash per MWh for wood chips both from Enkoping and Boden, respectively. The calculated conversion factor is 13 kg ash per MWh and 12 kg ash per MWh for peat from Sandviken and Boden, respectively (Table 3). Adopted factors for the other plants are also given in Table 3. The uncertainties for these figures are about 25% depending on the quality of the fuel and the condition of the burner. Based on these factors and the stated energy production, the measured activity concentration (mean value based on the

110

R. Hedvall et al.

information that the ash was composed of 75% fly ash and 25% bottom ash) the total outgoing activity and the activity per produced energy unit can be calculated (Table 3). The 13 investigated power plants are burning fuel from different parts of Sweden with greatly varying deposition of Cs-137 from the Chernobyl accident. It is not possible to calculate individually the activity per produced energy unit for each of all the 75 power plants. Both peat and wood burning plants have therefore been arranged in three ‘activity groups’. For peat, Sandviken and Ham&and have been arranged in group I with 250, 200, 150 and 150 kBq MW-’ h-’ for the four seasons, respectively, &tersund and Umea in group II with 150, 40, 50 and 20 kBq MW-’ h-l, respectively and Hudiksvall, Skelleftea, Boden and Gallivare in group III with 10, 5, 8 and 5 kBq MW-’ h-‘, respectively. For wood chips Enkoping and Eskilstuna have been arranged in group I with 50, 40, 30 and 35 kBq MW-’ h-‘, respectively, Vastervik and Boden in group II with 15, 10, 10, and 6 kBq MW-’ h-l, respectively and Angelholm and Boden in group III with 10, 5, 4 and 3 kBqMW-’ h-l, respectively. The grouping of the other 60 power plants was based on the total deposition of Cs-137 on areas from where the plant is obtaining its fuel. Wood chips were mostly coming from areas within a radius of about 80 km, whereas peat can be transported from bogs far away.

ACTIVITY CONCENTRATION OF Cs-137 AND ENERGY GENERATED The total generated thermal energy (GWh year-‘), the total yearly amount of ash products (kg), the total Cs-137 activity (GBq year-‘) and the activity concentration (kBq kg-‘) are given for different categories of more than 100 plants in Table 4. For peat ash the calculated mean values for the activity concentrations are 5.3, 3.5, 3.0 and 2.1 kBq kg-’ for the four sampling seasons and for wood chips the values are 2.2, 1.5, 1.2 and 1.0 kBq kg-‘, respectively. Although these values are adopted and have great uncertainties, they reflect the trend of decreasing activity concentration of Cs-137 in the ash products produced in Sweden. Very few power plants are obtaining their fuel from the same area year after year. For plants burning wood chips, a collecting radius of SO-100 km can include areas where the deposition can vary from 1 to 100 kBq m-2. For plants burning peat, not only the geographical distance to the bogs may vary, but also the manner in which the bogs have been harvested cause great variations in the activity concentration of Cs-137 in the fuel. Nevertheless, the mean value of the activity concentration of Cs-137 in the ash products show a reduction of about 50% both for peat

12.6 1949

23.4

13.5 1585

19.0

III

Total

3.20 1184

9.47 1501 12.0 3085 24.7

3.49 1239

9.91 1658 13.3 3333 26.7

IT 9.81 1809 14.5 3465 27.7

3.44 1226

430

31.7

17.6 2642

232 2.78 944 11.3 1466

(G Wh) cl@ kg)

12.6 2261 18.1 4419 35.4

4.63 1579

579

36.3

17.7 3021

286 3.43 1257 15.1 1478

fGWh) (@kg)

90191

1.0 17 1.2 58 2.2

6.3 19

22

5.3

50 20.9 40 12.6 11 0.8 101

(G&J Wq kg-‘)

86/S 7

1.3 8 0.7 36 1.5

5.0 12

16

3.5

50 16.5 26 3.4 5 0.4 81

(G&I W% kg-’ i

88189

1.2 7 0.5 32 1.2

3.8 12

13

3.0

35 12.6 47 4.2 12 0.7 94

(G&d Wq kg-‘)

s9/90

0.7 7 0.4 36 1.0

4.3 9

20

2.1

43 12.5 25 1.7 7 0.4 75

(G&J W% kg-‘)

90191

per Year and Cs-137 Activity

Peat Category I: Sandviken and Harniisand; II: ijstersund and Umei; III: Hudiksvall, Skelleftea, Boden and Gallivare. Wood chips Category I: Enkiiping and Eskilstuna; II: Vastervik and Boden; III: Angelholm and SkBvde.

Total

III

400

436

Wood chips I

II

252 3.02 645 7.74 1052

(GWh) (lo6 kg)

fGWh) (IO6 kg)

89/90

TABLE 4 Total Amount of Ash Products Produced per Year, Total Cs-137 Activity Concentration from 13 Swedish Power Plants

SSj89

Energy,

86187

Thermal

199 2.39 265 3.18 1121

I

Peat

Category

Total Generated

3 g

; z

2 S S

5 f G 2

5 3

n ? z

112

R. Hedvall et al.

and wood chips in the first 5 years after the Chernobyl accident. After that the reduction expressed as the ‘practical’ half-life would approach the physical half-life of 30 years.

RELATION BETWEEN THE DEPOSITION AND THE ACTIVITY CONCENTRATION OF Cs- 137 IN THE FUEL As already mentioned, the great variations in deposition within the collecting distance from the plant makes it very difficult to find a relation between the activity concentration of Cs-137 in the fuel and a caesium deposition. Of the ‘old’, pre-Chernobyl, Cs-137 from the atmospheric nuclear weapons tests, about 85% was deposited during the period 19611968. It was also much more evenly deposited than the fallout from Chernobyl. The deposition reflects the precipitation and the concentration in the air, which for Cs-137 from weapons tests fallout as a mean only varied with a factor of two from southern (highest) to northern (lowest) Sweden (DeGeer et al., 1978). Therefore the variations within a collection radius of 8&100 km for ‘old’ Cs-137 ought to be about 25%. In Table 5 the deposition of ‘old’ Cs-137 is given. Based on an activity ration of 0.58 f 0.01 for Cs-134/Cs-137 for the Chernobyl deposition (Arntsing et al., 1991) the amounts of ‘old’ (DeGeer et al., 1978) and ‘new’ Cs-137 in both fuel and ash products have been calculated. In Table 5 the activity concentration of Cs-137 in fuel from the first season 1986/87 and the calculated total deposition of Cs-137 are given. The smaller the old deposition was compared to the new deposition, the more uncertain the calculated total deposition is. As the sample values are from one sole grab sample, the uncertainties are also fairly large. Repeated sampling of wood chips gives an uncertainty of f12% (Ravila & Holm, 1994) and for peat the uncertainty is f12-15% (Hedvall & Erlandsson, 1992). Based on these figures, we have estimated an uncertainty of f25% to the total deposition. The activity concentration of Cs-137 in peat and wood chips as a function of the deposition of Cs-137 is shown in Fig. 3. To facilitate the presentation, both axes are exponential and to guide the eye a curve has been drawn. For wood chips the relation between deposition and activity concentration is rather well represented by the curve, whereas for peat no simple relation seems to exist which may be explained by differences in location of the peat bogs but also by differences in the harvesting technique. In Table 5 the deposition from the aerial survey is also given (SGAB, 1986). These values are based on reports from the different power plants concerning the area from which the fuel has come. The agreement between the calculated deposition and the measured deposition was quite good.

113

es-137 from biofuel power plants in Sweden TABLE 5

Activity Concentration of Cs-137 in the Fuel, Pre-Chernobyl (old) Activity of Cs-137, Calculated Old Deposition, Calculated Total Deposition and Estimated Deposition of Cs-137 from the Aerial Survey (SGAB, 1986) Activity concentration

f%)

Old deposition (kBq m-2)

Total deposition (kBq m-‘)

Deposition aerial survey (kBq mm2)

12 80 25 168 567 47

75 f 8 19h.6 34 i. 7 7zt5 4zt2 32 f 7

1.4 0.9 1.15 1.15 1.1 0.65

1.7 f 0.4 3.9 ItI 1.0 3.4 f 0.9 7.2 + 1.8 15.8 zt 4.0 3.8 III 1.0

o-2 2-5 O-2 3-10 5-30 O-2

1089 50 2921 367 191 39 54

2fl 32 f- 8 4 10&t 8zt1 43 f 8 62zt 15

1.05 085 0.8 0.8 0.7 0.65 0.8

75 f 3.9 f 2oi 4.7 It 7.0 + 1.2 It 1.6 f

19 1.4 5 1.2 1.8 0.3 0.4

2e-60 2-5 30-40 2-10 2-10 o-2 o-2

0.9

6.4 i 1.6

1O-20

Old

(Bq kg-‘) Wood chips

Angelholm Vastervik Sk&de Eskilstuna Enkoping Boden Peat

Sandviken Hudiksvall Harniisand ostersund Skelleftea Boden Gallivare Mixed

UmeH

277

7f4

Therefore it ought to be possible to get a rather good value at least for wood chips of the Cs-137 activity concentration of the fuel from an aerial survey and the curve in Fig. 3 for both peat and wood chips.

PROLIFERATION OF RADIOACTIVITY TO THE SURROUNDINGS OF THE PLANTS Proliferation of radioactive nuclides from the biofuel plant may occur in two ways, either as a deposition of the ashes in certain places or as flue gases from the stacks. The deposits of the ashes can have varying thicknesses and even if it is several meters thick it hardly constitutes any radiation problem (Ravila & Holm, 1994). It is not possible to calculate from how much Cs-137 that is released to the air, what is missing of Cs-137 in the balance between fuel and ash products based on single grab samples. Hedvall and Erlandsson (1992) have however shown that about 12 f 2% of the Cs-137 activity from a peat-fired heating plant escapes through the stack in the form of flue gases and is discharged into the environment. Mustonen et al. (1989) mentioned

R. Hedvall et al.

114 cs-137

/ 0 Har

0 Peat x Wood chips + Mix

San

x Enk

4 Ume X Esk

Y. VBS 0 Gal 0 bd

1

0 Hud x Bad / 0 Ost X Sko 0 Ske

/ x Ang

I

I

I

I

1

10

100

Deposition (k Bq/m*)

Fig. 3. The measured activity concentration of G-137 in peat and wood chips as a function of the estimated deposition.

that the long-term average of the collection efficiency for ash products of some Finnish peat-fired power plants was about 94%. For Sandviken we have also calculated an emission rate of 1 kg h-l. We therefore suggest that for Swedish biofuel tired power plants between 1.4% (case 1) and 10% (case 2) of the Cs-137 activity of the ash products is leaving the stacks in the form of flue gases and is then deposited in the surroundings of the plants. In Table 6 the emission rates which have been calculated for cases 1 and 2 based on the total Cs-137 activity in the ash products given in Table 4 and the number of plants in the different categories is shown. Using a Gaussian plume model (Slade, 1968) of the atmospheric dispersion of radionuclides, the maximum ground level air concentration has been calculated. The calculations have been performed for a wind speed of 4ms-‘, for effective stack heights of 20, 60 and 1OOm and for either Pasquill B or E atmospheric stability conditions. The results are presented in Fig. 4 which can be used to translate a given emission rate into a certain air concentration. As an example an emission rate of 33 Bqs-’ gives a maximum ground concentration of 2600 PBq rne3. An effective dose at this distance from the stack has also been calculated using ICRP 56 (ICRP,

G-137 from biofuel power plants in Sweden

Number

TABLE 6 of Plants and Air Emission Rates of Cs-137 (Bqs-‘) for the Three Different Categories of More Than 100 Plants and the Two Cases for Cs-137 Category

1986/87

1988189

1989/!M

19!IO/91

115

Number Emission Emission Number Emission Emission Number Emission Emission Number Emission Emission

of plants rate (Bqs-‘) rate (Bq s-‘) of plants rate (Bqs-‘) rate (Bq s-‘) of plants rate (Bq s-‘) rate (Bqs-‘) of plants rate (Bq s-‘) rate (Bqs-‘)

Case 1 Case 2 Case 1 Case 2 Case 1 Case 2 Case 1 Case 2

I

Peat zz

zzz

z

5 4.4 33 5 4.4 33 5 3.1 23 5 3.8 29

8 2.2 17 8 1.4 11 9 2.3 28 8 1.4 10

25 0.20 1.5 21 0.11 0.79 21 0.25 1.9 19 0.16 1.2

8 1.2 9.1 8 0.89 6.7 8 0.72 5.4 10 0.89 6.7

Wood chips zz zzz

20 0.42 3.2 20 0.27 2.0 21 0.25 1.0 22 0.18 1.4

37 0.20 1.5 38 0.09 0.7 41 0.80 0.57 41 0.08 0.57

1990) and ICRP 61 (ICRP, 1991) with the assumption that the normal inhalation is 20 litremin-’ (ICRP, 1975) of this concentration for 24 h (Fig. 4). If we assume that the total stack emission is 1 kg h-’ all year round from a 90m tall stack, the effective dose from inhalation would be

0.1

1

10

Emission rate (Bq/s)

Fig. 4. The calculated maximum ground concentration and maximum effective dose of Cs137 as a function of the emission rate for effective stack heights of 20, 60 and 1OOm for either Pasquill B- or D-atmospheric stability conditions.

116

R. Hedvall

et al.

2 $Sv year-’ for all important nuclides for a person standing 24 h day-’ at a spot where the ground concentration is at a maximum. The effective dose would be greater for smaller stacks. Although this is a maximum value from Swedish biofuel power plants the dose is very small compared to the dose from natural radiation, mainly the uranium and thorium series (Hedvall & Erlandsson, 1992). Earlier assumptions have suggested an emission rate of about 40 kg h-’ from smaller stacks from the Uppsala and Skelleftea plants during a 7 month period (Gyllander, pers. comm.), leading to a somewhat smaller maximum dose. Even if we use the inhalation dose factor (in SvBq-’ emitted) for a distance of 1 km from a 20m tall stack together with our own estimates of the emission rate, Cs-137 has a negligible effect on the effective dose. The absolute maximum from biofuel power plants is 2,&v per year, except for fuels with an extreme uranium and radium content (Hedvall & Erlandsson, 1992). The dose is negligible compared to the average effective dose contribution in Sweden from natural sources which is about 4mSv per year.

ACKNOWLEDGEMENTS This investigation was sponsored by the Swedish Radiation Institute under the grant SSI P397.91.

Protection

REFERENCES Arntsing, R., Bjurman, B., De Geer, L.-E., Edvardson, L., Finck, R., Jacobsson, S. & Vintersved, I. (1991). Field gamma-ray spectrometry and soil sample measurements in Sweden following the Chernobyl accident. A Data Report. FOA Report D-20177-4.3, December 1991. Bjurman, B., Erlandsson, B. & Mattsson, S. (1987). Efficiency calibration of Ge spectrometers for measurements on environmental samples. Nucl. Instrum. Meth. Phys. Res., A262, 548-50.

De Geer, L.-E., Arntsing, R., Vintersved, I., Sisefsky, J., Jacobsson, S. & Engstriim, J.-A. (1978). Particulate radioactivity, mainly from nuclear explosions, in air and precipitation in Sweden mid-year 1975 to mid-year 1977. Appendix II. FOA Report C 40089-T2(Al), November 1978. Erlandsson, B., Hedvall, R. & Mattsson, S. (1994). Radionuclide concentration in fuels and ash products from biofuel power plants. Report LUTFD2/(TFKF30A)/l-34 (1994). Department of Nuclear Physics, University of Lund, Lund, Sweden. Hedvall, R. & Erlandsson, B. (1992). Radioactivity in peat fuel and ash products from a peat-fired power plant. J. Environ. Radioactivity, 16, 205-28.

G-137 from biofuel power plants in Sweden

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