Precipitation washout of tritiated water vapor from a nuclear reactor

J. Environ. Rodiooctivif.v, Vol. 34, No. I, pp. 5!k68, 1997 Copyright 0 1996 Elsevier Science Limited

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SO265-931X(96)00002-1

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ELSEVIER

Precipitation Washout of Tritiated Water Vapor from a Nuclear Reactor

Hideki Tokuyama

& Masaki

Oonishi

Fukui Prefecture Environmental Radiation Research and Monitoring Harame-cho 39-4, Fukui-shi 910, Japan

Center,

(Received 21 November 1994; accepted 22 December 1995)

ABSTRACT Tritiated water vapor from a nuclear power plant is continuously released to the atmosphere and then scavenged by precipitation. Tritium concentrations above the natural level are often observed both in water vapor and in rainwater in the vicinity of a reactor site. In this paper, the washout coefficient of tritium and the validity of the calculation model are studied. The washout coefficient was calculated to be 7.3 x 10-ss-l from a rainfall intensity of 2 mm h-‘, based on the washout model and concentrations of anthropogenic tritium in water at the Tsuruga site. The validity of the washout coefficient was confirmed by comparing it with the values from two other sites, the Mihama and Ooi sites that have similar meteorological parameters to the Tsuruga site. Copyright 0 1996 Elsevier Science Ltd.

INTRODUCTION Japanese nuclear reactors, most of which are of the commercial light water type, are currently the major anthropogenic source of tritium in Japan. All tritium released from light water reactors to the atmosphere is chemically tritiated water vapor (HTO) since these reactors contain large amounts of water as coolant. Released tritium disperses in a manner governed by its own physical properties, which are essentially the same as those of atmospheric water vapor. This allows the exchange of Hz0 in raindrops with HTO when HTO vapor in air contacts with raindrops during atmospheric 59

60

H. Tokuyama,

M. Oonishi

diffusion and convection. The rainwater thus becomes tritiated. Rainwater samples taken downwind of a nuclear plant often contain tritium. Since the height of the exhaust stack is usually much lower than that of clouds, washout has a larger effect than in-cloud scavenging, i.e. rainout. Studies on the atmospheric dispersion of HTO vapor and tritium deposition by washout were made in the vicinities of tritium production facilities, reprocessing plants and heavy water reactors (Bander et al., 1979; Gorman & Wong, 1979; K&rig, 1979; Murphy & Pendergast, 1979; Brown, 1985; Papadopoulos et al., 1986). The tritium release amounts from the nuclear reactors in Japan are two-orders of magnitude lower than those from the above-mentioned tritium production facilities, and the tritium concentrations in environmental waters are at most several times higher than the tritium baseline. In a region with much rainfall in Japan, they receive an annual rainfall of around 2000mm (Japan National Astronomical Observatory, 1988). This is approximately twice the worldwide mean value. The removal of HTO vapor thus occurs primarily through the precipitation process. Therefore, although the tritium concentrations in environmental waters are low, consideration of the deposition by washout is indispensable for studying the behavior of atmospheric tritium in the vicinity of a source site. This paper describes the washout coefficient of tritium and the validity of the calculation model.

MATERIAL

AND METHODS

Sampling The tritium deposition by washout was determined from the tritium activity in rainwater samples. A collector with a surface area of 0.2 m2 was placed at a height of 1 m above the ground surface. The samples were gathered monthly for the years 19861992. The tritium determination was performed with a 100 ml aliquot of each rainwater sample. Samples of atmospheric water vapor at ground level were collected by using a sampler consisting of an aerosol holder, a condenser, a small electric pump and a water tank. This water vapor sampler was operated continuously for the years 1986-1992. Condensed water samples were gathered monthly. The ground-level tritium concentration was determined from the tritium activity ,of the condensed water sample. The measured values of the relative humidity and the density of the saturated water vapor in the sampled air allowed Bq liter-’ of water to be converted to Bqmp3 of air. These water samples were collected at a distance of about 2 km from a

Precipitation

nuclear plant in each area. The sampling points in Mihama areas are shown in Fig. 1. To evaluate the contribution of tritium released from a baseline concentration of tritium in environmental Samples of both water vapor and rainwater were also

Sea of

61

washout of tritiated water vapor

the Tsuruga

and

the nuclear plants, water is needed. collected at Fukui

Japan

"Fugenn-reactor

"-reactor -w-L-

Fig. 1.

Location of the sampling points and the reactor facilities in the Tsuruga and Mihama areas. 0 Reactor, 0 Sampling point.

62

H. Tokuyama, M. Oonishi

City as a reference point. Since the nuclear reactors in Fukui Prefecture are located more than 60 km from Fukui City (see Fig. I), the tritium releases from these reactors do not contribute to the tritium level at Fukui City. Sample preparation and tritium measurement Collected water samples were purified by single distillation to remove quenching materials and other radioactive materials. Each 40ml aliquot of the distilled water samples was taken in a lOOm1 vial (Teflon’” by E.I. Du Pont de Nemours & Co. Inc.) containing 60ml of a liquid scintillator (Insta-Gel@ by Packard Instrument Company). The mixed solution was stored in a cool and dark place for 48 h. The tritium activity was then measured with a liquid scintillation counter (Model LSC-LB3, ALOKA Co. Ltd, Tokyo). The counting efficiency was 26.5% and the background counting rate was 5.0c.p.m. in the counting region of 1.0-4.8 keV. The counting period was 500min. The minimum detectable limit was about 0.6 Bq liter-‘.

RESULTS AND DISCUSSION Washout coefficient Fukui Prefecture currently has 15 reactors located in four areas with a total generating capacity of 11.73 GWe. Table 1 lists these power reactors, along with their annual tritium release to the atmosphere (FERC, 1994). Most tritium reactors are originating from pressurized water reactors (PWR). As shown in Table 1, the annual tritium release reached 4.1 TBq year- ’ as the maximum. Other releases were usually much lower. The washout coefficient was first computed from available data of tritium concentration in water vapor and rainwater for the years 19861992 in the Tsuruga area, since much data were available from this area. The distance between the sample collection point and the reactors is 1.6 km. Figure 2 represents some of the data obtained for the years 19861992 with the monthly total release of tritium. As seen from Fig. 2, the water samples usually had higher concentrations than the tritium baseline, and these samples often contained tritium from the nuclear plants. The weak correlation between the tritium release and the tritium concentration was observed. This is because meteorological parameters, particularly precipitation, wind velocity and wind direction, strongly influence atmospheric diffusion and washout of tritiated water vapor released into the

1 2 3 4

PWR PWR PWR

PWR PWR PWR PWR

Ooi Unit Unit 23 Ooi Ooi Unit 4

Takahama Takahama Takahama Takahama

Unit Unit Unit Unit

PWR

FBR PWR

Monju Mihama Unit 1

Ooi Unit 1

PWR HWR

Tsuruga Unit 2 Fugen

PWR PWR

BWR

Tsuruga Unit 1

Mihama Unit 2 Mihama Unit 3

Typea

Reactor

826 826 870 870

1175 1180 1180

1175

500 826

280 340

1160 165

357

(MWel

Capacity

Mar. Dec. Apr. Oct.

1974 1974 1984 1984

Sep. May 1978 1991 May 1992

Dec. 1977

Apr. 1972 Jan. 1976

Apr. 1994 Jul. 1970

May May 1986 1978

Oct. 1969

Start of operation

0.6 0.6 0.5 0.5

0.6 -

2.4

0.9 1.1

1.0

0.5 3.7

1986

0.9 0.6 0.7 0.5

0.4 -

4.1

1.1 1.2

1.1

0.1 4.1

0.6

1987

0.3 0.3 0.4 2.6

0.3 -

2.3

2.0 2.0

1.2

0.1 1.3

0.6

1988

0.5 0.4 0.7 0.5

2.2 -

2.0

1.1 2.2

2.7

0.5 1.3

0.4

1989

0.5 0.7 0.9 0.6

0.1 -

1.8

2.0 1.7

2.2

0.9 1.2

0.3

1990

0.4 O-6 0.9 1.1

0.4 -

1.9

2.4 1.9

2.1

0.3 1.2 1.4

1991

Tritium release (TBq year-‘)

@BWR = Boiling water reactor; PWR = Pressurized water reactor; HWR = Heavy water reactor; FBR = Fast breeder reactor.

Takahama

Ooi

Mihama

Tsuruga

Area

TABLE 1 Nuclear Reactors in Fukui Prefecture and Annual Release of Tritiated Water Vapor

0.8 0.9 1.4 1.5

0.9 0.3 -

2.5

2.4 2.9

1.8

0.2 0.7 1.7

1992

1,O 1.0 1.4 l-6

0.3 0.2 0.1

4.1

1.8 3.3

2.9

0.2 1.4 1.3

1993

$

S z E q 2, 2. 9 % pB Y

$ z 3

h 2

64

H. Tokuyama, M. Oonishi

Jan

Apr JUI 1991

OCI Jan

Apr Jul 1992

OCt

l-

Fig. 2. The monthly variation of tritium activities in water vapor and rainwater for the years 1991-1992 in the Tsuruga area, with the monthly total release of tritium from Tsuruga Unit 1, Tsuruga Unit 2 and Fugen Reactors.

atmosphere. The Tsuruga area has frequent rainfall and a high frequency of northwesterly wind from November through March. Released tritium was often observed in rainwater and in water vapor during this period because the sample collection point at this area is located on the south side of the nuclear plants (see Fig. 1). The washout coeffkient (A, in SK’) is yielded by substituting the observed value of the tritium deposition rate (0, in Bqme2 s-‘) and the atmospheric tritium concentration at ground level (x0, in Bq mP3) into the washout model (eqn (1)). 0 = Axo&r

(1)

where HeK (in m) is an effective height calculated from the widely used dispersion formula (Chamberlain & Eggleton, 1964; Jacob, 1973; IAEA, 1980). The deposition rate is given as c x Is w=3600

(2)

where C (in Bq 1-l) is the tritium concentration in rainwater, and 1, (in mm h-‘) is the mean rainfall intensity during the sampling period. The ground-level tritium concentration (x0) was determined from the tritium concentration in atmospheric water vapor. The used tritium concentrations both in rainwater and in water vapor were obtained by subtracting

Precipitation washout of tritiated water vapor

65

the tritium baseline data at the reference point from the data obtained near the nuclear plants. The baseline data include tritium produced by cosmic rays and nuclear weapons tests. The calculations were based on the monthly data. Table 2 shows the results of calculated washout coefficients. The washout coefficients varied widely in the range of 1.3 x 1O-5-16 x lo-’ s-’ . The mean value of the 29 data is (7.3 f 4.1) x 10-5s-‘. It is reported that meterological parameters such as rainfall intensity, raindrop size and raindrop size spectrum influence deposition by washout (Chamberlain & Eggleton, 1964; Jacob, 1973; IAEA, 1980). Of these parameters, only monthly mean rainfall intensity data are available in this study. The mean value in Table 2 is considered to be representative of the washout coefficient and may be regarded as constant. Chamberlain and Eggleton (1964) made a detailed analysis of the exchange rate of HTO vapor with Hz0 in raindrops, and estimated that the washout coefficient of tritium should be of the order of 10F4 s-l. Inoue et al. (1985) reported that their washout model led to a washout coefficient of 4.6 x 10m4s-* at a rainfall intensity of 2mm h-i. Jacob (1973) obtained a washout coefficient of 3.6 x 10e4 s-’ from the theoretical considerations by assuming an average rainfall intensity of 4mm h-‘. In the present work, the rainfall intensity was observed to be 2 mm h-l, half of the Jacob’s assumed value. The calculated coefficient in this study is expected to be smaller than the Jacob’s value because of the lower rainfall intensity. The calculated mean washout coefficient is thus approximately one order of magnitude lower than those reported in the above-mentioned literatures. Since the washout coefficient depends on meteorological parameters as described above, a comparison of the coefficient data with each other requires detailed knowledge of these parameters. In addition, the calculations in this paper are based on the monthly mean values assuming the continuous tritium release, while some results in the literature were obtained from the data observed shortly after an accident. This may also result in variations in the washout coefficient values. The ddlty

of the model

To verify the usefulness of the washout model, which was based on the Tsuruga area data, the calculated tritium deposition rates were compared with those observed at the Mihama and Ooi areas (see Fig. 1). There are only pressurized water reactors at these two areas and the tritium release was in the range 0, l-4.1 TBq year-’ per reactor (see Table 1). Each sampling point is within 2 km from the respective reactors. The tritium concentrations in air and in rainwater were determined. The use of eqn (1)

H. Tokuyama, M. Oonishi

66

TABLE 2 Washout Coefficients Calculated for the Tsuruga Area Month

April 1986 May 1987 June 1987 August 1987 September 1987 November 1987 February 1988 March 1988 April 1988 May 1988 April 1989 May 1989 June 1989 September 1989 September 1990 October 1990 March 1991 November 1991 December 199 1 January 1992 February 1992 March 1992 April 1992 May 1992 June 1992 July 1992 September 1992 November 1992 December 1992

Monthly tritium release (x10” By)

5.9 5.4 4.2 2.6 3.6 2.9 4.8 9.7 3.5 2.1 1.6 I-8 2.8 2.5 3.4 1.9 1.6 2.0 1.6 1.5 2.0 2.8 3.0 4.2 3.8 2.4 1.7 1.6 1.3

xo%SD (mBqm_“)

34.4 zt 2.5 18.8 f 2.0 40.7 * 3.1 27.0 f 3.4 40.3 It 4.4 15.1 f 1.5 81.7 ok 1.6 115.0 f 3.0 22.6 zt 2.3 13.7 f 2.6 18.1 f. 2.1 16.8 i 3.1 14.6 k 2.1 15.7 f 2.4 21.0 + 2.7 9.0 f 2.0 9.7 It l-1 6.6 f 1.4 9.1 f 1.5 9.2 f 1.0 5.7 f 0.8 11.3 i 1.2 12.3 f 1.4 31.3 f 2.2 34.6 f 2.8 40.3 * 3.7 30.5 * 3.3 12.1 f 1.6 14.2 f 1.3

Herr (m)

1150 680 560 530 650 1250 1290 930 850 680 700 690 490 690 670 1050 1010 1050 1450 1240 1010 1050 750 700 680 680 620 1100 1680

wfSD (mBqm_‘s-I)

1.6fO.l 1.3 f 0.1 1.2 f 0.1 0.93 -f 0.09 0.97 f 0.13 0.30 l 0.05 1.4ztO.l 6.4 f 0.2 0.89 f 0.09 0.75 f 0.11 0.65 f 0.08 0.58 zt 0.06 0.98 xt 0.10 0.76 f 0.11 0~70fO~ll 0.64 f 0.09 0.99 + o- 19 0.92 f 0.09 1.8fO.l 1.2 f 0.1 0.67 f 0.07 1.5 + 0.1 1.5 f 0.1 1.8 f0.1 0.86 f 0.10 0.91 f 0.09 0.75 f 0.09 0.89 f 0.08 0.38 zt 0.06 Mean value

A (xIO-~S-‘)

4.0 f 0.4 10f 1 5.3 f 0.6 6.5 zk 1.0 3.7 f 0.6 I.6 XII0.3 1.3IkO.l 6.0 f 0.2 4.6 f 0.7 8.1 + 1.9 5.1 f 0.9 5.0 + 1.1 14f2 7.0 f 1.5 5.0 f 1.0 6.8 f 1.8 lOIt2 13f3 14f2 llfl 12f2 1312 16f2 8.2 f 0.7 3.7 f 0.5 3.3 f 0.4 4.0 k 0.6 6.7 f 1.1 1.6 f 0.3 7.3 rt 4.1

I!Ieffis given by:

where Q is the tritium concentration in the y concentration calculated refers to the atmospheric

emission rate, uys is the standard deviation of distribution of direction, U, is the mean wind speed, and xO,ca,is the mean from the ground-level formula (IAEA, 1980). The subscript S stability.

67

Precipitation washout of tritiated water vapor

m

Measured value Cal GUIat ed vat ue

4

0

Depasii Fig. 3.

Comparison

i on r a12e( w, i n3 mBq nh’)

of the observed deposition rates in the Mihama and Ooi areas with values calculated from the washout model.

leads to the deposition rate calculated from the observed atmospheric tritium concentration at ground level and the washout coefficient estimated from the results of the Tsuruga area. On the other hand, the substitution of the tritium concentration in rainwater into eqn (2) yields the tritium deposition rate. Since the Mihama and Ooi areas are 8 km and 40 km distant from the Tsuruga area, respectively, the two areas would not have exactly the same meteorological conditions as the Tsuruga area. Nevertheless, these two areas could be assumed as having almost the same conditions as the Tsuruga area. Hence, the value of washout coefficient estimated at the Tsuruga area was used for the calculations of tritium deposition at the Mihama and Ooi areas. The results are shown in Fig. 3. As seen from this figure, some calculated deposition rates of tritium differ largely from those observed. The ratios of the calculated values to the observed ones ranged from 0.4 to 2.2 with a mean value of 1.04 f 0.64. Hence, it is confirmed that, although there are a few exceptions, the calculated tritium deposition rates coincide with the observed ones.

CONCLUSIONS The value of washout coefficient was calculated and the validity of the washout model was verified in this study based on the comparatively longterm monthly mean value data. The results suggest that the washout model

68

H. Tokuyama. M. Oonishi

is applicable to the estimation of tritium deposition by washout, although

the validity of application of this washout model to an intermittent release or an accidental release still remains to be studied. Under conditions where HTO vapor can be considered as being continuously released at a constant rate and where the estimated point is situated near a reactor site, the washout model is reasonable and the obtained washout coefftcient is useful for estimating the deposition of tritiated water vapor by washout. REFERENCES Bander, T. J., Renne, D. S. & Sandusky, W. F. (1979). An analysis of tritium releases to the atmosphere by a controlled thermonuclear reactor. In Proc. Znt. Symp. on the Behaviour of Tritium in the Environment. San Francisco, 16 20 October 1978. IAEA, Vienna, Austria, pp. 125-37. Brown, R. M. (1985). HT and HTO in the environment at Chalk River. Fus. Technol., 8,2539-2543.

Chamberlain, A. C. & Eggleton, A. E. J. (1964). Washout of tritiated water vapor by rain. Znt. J. Air Wat. Poll., 8, 135-49. FERC (Fukui Environmental Radiation Council) (1994). The 1993 report of environmental radioactivity surveillance around the nuclear power plants in Fukui Prefecture. FERC Report 26. FERC, Fukui, Japan (in Japanese). Gorman, D. J. & Wong, K. Y. (1979). Environmental aspects of tritium from CANDU station releases. In Proc. Znt. Symp. on the Behavior of Tritium in the Environment. San Francisco, 1620 October 1978. IAEA, Vienna, Austria, pp. 623-34. IAEA (International Atomic Energy Agency) (1980). Atmospheric dispersion in nuclear power plant siting. IAEA Safety Series No. 50-SG-S3, IAEA, Vienna, Austria. Inoue, Y ., Iwakura, T. & Miyamoto, K. (1985). Environmental aspects of tritium released into the atmosphere in the vicinity of nuclear facilities in Japan. NZZ?S-M-52, 52, 2963 15. Jacob, T. (1973). Deposition of “Kr and tritium deposition released from a nuclear fuel processing plant. Health Phys., 24, 3742. Japan National Astronomical Observatory (1988). Chronological Scientific Tables, 2nd edn. Maruzen Co. Ltd, Tokyo. K&rig, L. A. (1979). Impact on the environment of tritium releases from the Karlsruhe Nuclear Research Center. In Proc. Int. Symp. on the Behavior af Tritium in the Environment. San Francisco, 1620 October 1978. IAEA, Vienna, Austria, pp. 59 I-611. Murphy, C. E. & Pendergast, M. M. (1979). Environmental transport and cycling of tritium in the vicinity of atmospheric releases. In Proc. Znt. Symp. on the Behaviour of Tritium in the Environment. San Francisco, 16-20 October 1978. IAEA, Vienna, Austria, pp. 361-72. Papadopoulos, D., KSnig, L. A., Langguth, K. G. & Fark, S. (1986). Contamination of precipitation due to tritium released into the atmosphere. Radiat. Prot. Dosim.,

16,95-100.