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Dispersion of 222Rn from two New Zealand geothermal power plants
J. Environ. Radioactivity 2 (1985) 245--257
Dispersion of "Rn from Two New Zealand Geothermal Power Plants
N. E. Whitehead* Institute of Nuclear Sciences, DSIR, Private Bag, Lower Hurt, New Zealand
ABSTRACT The dispersion of e2eRnfrom emitted waste gases at Wairakei geothermal power station, New Zealand, is modelled. It is concluded that resulting concentrations in the nearby township of Taupo will never exceed the maximum permissible-in any meteorological situation. The greatest possible accumulation is calculated to be less than one eighth of the normal background radon concentration. A more realistic set of assumptions predicts long-term mean concentrations about 4% of background levels. A new geothermal power station, Ohaaki, a factor of three times more distant, is calculated to produce ten times lower concentrations than Wairakei. Measurements using a few passive solid-state radon detectors show that the natural variation of radon concentrations greatly exceeds any calculated contribution from either geothermal station; hence, much o f the radon is probably due to more local minor sources. Local sources have increased due to draw-off of ground water by the Wairakei station. Thus, there could be some indirect contribution to radon concentrations by its operation. The measured six-month integrated mean radon concentration at Taupo is a significant fraction of the maximum permissible concentration. It seems likely that natural sources alone may lead to levels in air which are above the maximum when temperature inversion exists. It is concluded that these two geothermal power plants are unlikely to produce concentrations o f radon hazardous to the population or to plant workers. *Present address: International Laboratory of Marine Radioactivity, Mus~e Oceanographique, Principality of Monaco. 245 J. Environ. Radioactivity 0265-931X/85/$03.30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
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INTRODUCTION Geothermal power plants are a polluting form of energy (Axtmann, 1975). The amount of carbon dioxide emitted is often much larger than that from a coal-fired power station of the same total power production. There is also concern about substances emitted in the steam (e.g. arsenic, mercury, hydrogen sulphide), possible leukaemia from trace quantities of benzene (Layton & Anspaugh, 1982) and lung cancer from radon. ~ R n is a well-known constituent of geothermal systems and volcanoes emit large quantities. From the figures of Pollian & Lambert (1979), for example, it may be calculated that Mt Erebus emits about 160 Ci (6 TBq) per year. This is about equivalent to 440 mCi (16 GBq) per day but geothermal power stations emit comparable quantities (Whitehead, 1980). The Wairakei geothermal power station, in the centre of the North Island of New Zealand, is about 7 km north of Taupo township (population about 14 000), the only major population centre nearby. A new power station is being constructed at Ohaaki, about 20 km north of Taupo. It is of interest to determine whether the radon emitted from these stations would cause any hazard to plant workers or to people living at Taupo. Figure 1 is a location map of the area near Taupo, with contours expressed in feet, and shows how far it departs from the ideal plane surface usually assumed in modelling. Investigations on possible hazards from Rn at the Wairakei station have already been published by Robertson & Matthews (1978), Matthews (1981) and Whitehead (1980). Robertson & Matthews (1978) measured a value of 0-06 pCi litre -t (2 mBq litre -L) of 22ZRn in air at Taupo. Levels near geothermal bores were similar and only in one case as high as 0.2 pCi litre -t (6 mBq litre-~). This observation showed that the radon was rising with the hot steam and contributing no significant radiation dose, even to workers standing beside the plume of steam. Even within the power-station turbine room, the concentrations were similar. In addition, there were no excess y-ray levels from deposition of radon daughter products on the inside of piping, even when the inside of the piping was examined (Whitehead, 1978, unpublished work). These values must be compared with the maximum permissible radon concentration for continuous exposure to the general population (ICRP, 1960) which is 1.0 pCi litre -I above any natural background present. Readers should note, however, that in view of the fact that radon exposure is now
Dispersion of 2eeRnfrom two New Zealand geotherma! power planta
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Fig. 1° Location of Wairakei and Taupo with height contours. Contours are in feet. Location of radon detectors, 0 ; underlined figures are measured radon concentrations in pCi litre- 1. The figure in brackets is from a detector exposed without membrane. The 0-06 pCi litre -t figure in the middle of Taupo is a single spot measurement taken from Robertson & Matthews (1978). The contours are taken from New Zealand topo~-aphical map NZMS 1 N 94. The Ohaaki station referred to in the text is 18 km north-east of Wairakei and hence is not shown on the map.
thought to be the major source of radiation exposure to humans ( U N S C E A R , 1982) changes in that ICRP recommendation could be possible a n d are currently under consideration. Radon accounts for a large dose to the body as a whole and is virtually the only significant source of exposure to the lungs. Matthews (1981) checked whether any long-term contamination from
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radon had accumulated over some years near the power station by measuring the -'~°Pb/L37Cs ratios but found that these were the same in geothermal and non-geothermal areas. :1°Pb would be produced from radon decay products, whereas the ~37Csresults from fallout and should be independent. The above authors agree that daily emission from the Wairakei station is of the order of 150 mCi (6 GBq) per day. The amount estimated to be emitted from the projected Ohaaki station through measurement of radon in individual bores (Whitehead, 1980) is almost identical, although the geochemistry of the two areas differs. Where does this rather large amount of activity end up in the environment? This paper reports the results of computer modelling of the radon dispersion along with a few radon measurements to determine whether the model is even approximately correct. The measurements were long-term to average out the effects of varying meteorological conditions.
MODELLING OF DISPERSION The computer model used here was constructed by Dr K. Lassey of this Institute (Lassey, 1979). It assumes a Gaussian distribution of a radionuclide in a cross-section of a plume emitted from a hot source. The model was originally intended to model the release of radionuclides from a reactor accident but it is easily adapted for modelling radon dispersion from a geothermal power plant. For the present calculations, the height of release was taken as that of the stacks on top of the Wairakei power station, which emit the waste gases through four narrow chimneys in a line, with spacing of more than 10 m, at a temperature of about 50°C (about 35°C above ambient). The height of gas release is about 31 m and the gas is almost entirely carbon dioxide. For the Ohaaki station the gas volume will be about ten times greater and the release will be 70 m above ground. In neither case is the gas derived from a single point source but this discrepancy had to be neglected in these calculations. The program (Lassey, 1979) gives results which depend on the meteorological conditions and these conditions are called Pasquill classes. For later calculations it was necessary to use the data of Wratt & Homes (1983); their USNRC categories were equated to the Pasquill classes following their advice (Wratt, 1983, Pers. comm.). It is intuitively obvious that the hot gases tend to rise up in a plume; the
Dispersion of "2:Rn frorn two New Zealand geotherrnal power plants
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height to which they rise depends on the air temperature and whether there is an inversion layer present. If the inversion layer is very low. local concentrations will be high. If more normal conditions prevail, the plume will rise, travel with the prevailing wind and then slowly descend. In such a case, the maximum concentration of the radionuclide is found some distance from the source, depending on the height of release. Like many programs of its kind, the one used for the modelling in this paper assumes that the terrain is flat and does not have features which markedly perturb the dispersion. The program also assumes that the mid-point of the Gaussian distribution, which gives the highest concentration, is of most interest; these values are therefore reported in this paper. The dosimetry of radon is quite complex and depends on the relative amounts of the daughter products present. This paper presents calculations based on only the amount of the parent ::2Rn but more complex treatments are certainly possible. In the following treatment, the effects of different conditions are considered. The wind speed and height of release are varied to determine sensitivity to these factors. In considering the results, account must also be taken of the natural radon levels which would be expected from exhalation from the ground; these vary considerably depending on temperature, pressure and rainfall. It was not very, clear initially what the background radon level at Taupo would be. Actual measurements could contain some contribution from geothermal sources and should be compared with measurements elsewhere in New Zealand. Many measurements at Wellington, a nongeothermal area, have consistently given results from 0.030 to 0-037 pCi litre -~ over five years (Ting, 1975). The lowest figures from the Taupo region (Robertson & Matthews, 1978) are less than 0-05 pCi litre -t but the lowest figure which is significantly different from zero in their report is 0-06 pCi litre <. This figure is low compared with a world mean of 0.17 pCi litre -~ (a combination of figures from Ting (1975) and UNSCEAR (1962)). If the mean background level is taken as 0-06 pCi litre -', as in this paper, it should therefore be noted that although in Figs 2 and 3 it is marked as a single line, there is substantial variation about that line, but mostly above it, to as high as 0"2 pCi litre -~. If a radon concentration calculated by the program is significantly smaller than 0-06 pCi litre -~, the large natural variation in background makes the calculated value even less important. Figure 2 shows the potential impact, at Taupo, of radon emissions from Wairakei. The curves for the various Pasquill weather conditions
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N. E. Wh/tehead 1-0 MPC
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78
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F'~. 2. Predicted concentrations of ~-~-Rnat Taupo with release at Wairakei under various Pasquill weather classes. ' B G D ' is the line representing the background radon concentration. 'MPC' is the Maximum Permissible Concentration, which is 1-0 pCi litre-l. The abscissa is the wind speed.
(specified in Table 1) demonstrate that there is no possibility that the m a x i m u m permissible concentration will be exceeded under any weather conditions. This is true even for classes E and F, in which the inversion layer is low, i.e. in the conditions which occur more often in winter and u n d e r low wind speed regimes. Figure 3 shows a similar set of curves for the proposed Ohaaki station; it also includes a set of curves to give the approximate impact on the Wairakei station from emissions at Ohaaki. Obviously the emission from the O h a a k i station will never cause the maximum permissible concentration to be exceeded at Taupo. In terms of the modelling, this result is p r o d u c e d because the distance to Taupo from Ohaaki is three times greater than that from Wairakei to Taupo and the radon has cor-
Dispersion of :~-2Rn from two New Zealand geothermal power plants
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m/s Fig. 3. Predicted concentrations of 2Z2Rnat Taupo (solid line) and Wairakei (dashed line) from release at projected Ohaaki site, under various Pasquill weather classes. The abscissa is the wind speed. TABLE 1 Model Predictions and Characteristics of Pasquill Classes*
Pasquill class A B C D E F
Wind speed (ms -1) 0.5-2-0 1-5 2-10 3-10 2-5 0-5-3
MLringlayer height (m)
Predicted distance of maximum radon concentration (m)
Maximum concentration (pCi litre -i)
1500 1500 1000 500 200 200
100-160 160--250 240--400 400--630 630-1000 1600--2500
0-085 0-051 0-034 0-017 0-02_6 0-068
*The data in this table are ones used for calculating the curves in Figs 2 and 3 but are not exhaustively definitive for the Pasquill classes.
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respondingly more time to diffuse in three dimensions and to be diluted. The model also predicts where the maximum concentrations will occur downwind of the Wairakei plant. Table 1 shows that some of the predicted maxima are only a few h u n d r e d metres from the geothermal power plant but in no case do they approach maximum permissible concentrations. Other calculations were carried out to check what the effects of changing the radon release height might be. The range considered was 20-60 m but, in the case of predicted values at Taupo, the resulting differences were only a few percent. Larger differences were calculated for much nearer the plant, the lowest release heights giving the highest concentrations. If the emission is from a stack height of only 20 m the calculated maximum at 1.6-2.5 km under Pasquill class F is 0-13 pCi litre -L, which is still very low.
METEOROLOGY A worst-case calculation for radon concentrations at Taupo can easily be performed by assuming that class F predominates. A wind-rose from meteorological data at Wairakei shows 30% of the winds were in the direction of Taupo, i.e. along the graben, and it can very pessimistically be assumed that in all these cases Pasquill class F prevails. If that is so, then the long-term mean radon concentration is predicted to be 0-034 pCi litre -~ due to Wairakei emissions alone during periods when the winds blow from that direction. However, if one assumes that this condition prevails only 30% of the time, the long-term mean radon concentration for all times is only 0-01 pCi litre -~. This value is low compared to the assumed background concentration of 0-06 pCi litre-~. From detailed investigation of the Ohaaki site, information is available on the percentage frequency of various meteorological classes as shown in Table 2 (Wratt & Homes, 1983). These figures were selected for wind directions in the quadrant north to east inclusive, which are assumed to reach Wairakei and Taupo. The Table 2 frequency data have associated absolute uncertainties of 3-7%; for example, the relative error on category B is 100%. As can be seen in Figs 2 and 3, the computer program does not calculate results for Pasquill class G. There will therefore be a resulting small error but it is of no practical significance in this particular case.
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Dispersion o f "-e-"Rn from two New Zealand geothermal power plants TABLE 2
Frequencyof Meteoroio~calConditionsat Ohaaki Pasquill class Frequency(%)
A 9
B 3
C 5
D 35
E 34
F 12
G
Using the data in Table 2 and assuming that there is no other contribution, an attempt can be made to calculate the long-term mean radon concentration at Taupo from Wairakei releases alone. One can integrate for all Pasquill classes when the appropriate north-to-east winds blow, assuming that the tabulated conditions for Ohaaki apply to Wairakei. Weighting each class by the frequency of occurrence, it is possible to calculate that the integrated radon concentration would be 0-003,4 pCi litre -1. This contribution is extremely small compared with typical radon concentrations in air and may be considered negligible. A similar calculation for the concentrations resulting from dispersion of radon from Ohaaki to Taupo, or from Ohaaki to Wairakei, gives values in the range 0.0002-0-0003 pCi litre-~, which are also negligible. Investigations in the Imperial Valley, California (Gudiksen et al.. 1979) and computer calculations similar to those of this paper suggested that the radon concentrations there would be five orders of magnitude below the maximum permissible. In the present paper the corresponding value is three orders of magnitude for Wairakei emissions and four for Ohaaki.
R A D O N MEASUREMENTS IN AIR Because of the limitations of the computer model and of the meteorological data, a few direct measurements were carried out. These were restricted because of equipment limitations but still give some useful information. Ideally one would like to monitor the long-term radon concentration at many sites at Wairakei and Taupo and, in addition, thoroughly record the meteorological conditions, using many instruments capable of recording changes on a time scale of hours. Since this was not feasible, the radon levels were integrated for six months during winter (to give a worst-case result--low level inversion layers should be common in winter) using passive solid-state Terradex Tracketch ®
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detectors. These detectors are in the form of cups with the detector element inside and on the bottom. They were placed outside at the positions shown in Fig. 1. In all cases the exposure was to ambient outside air and not to air inside houses for any significant time. The detectors have a thin membrane across the mouth which excludes other isotopes of radon and also radon daughter products. " R n has a half-life of 3-8 d and can diffuse through the membrane, but ~ has a half-life of only 56 s and decays before it passes through. The daughter products are electrically charged and do not pass through the membrane but deposit on it. The radon which has diffused through the membrane decays and emits c~-particles which strike the detector causing submicroscopic damage. After exposure, the detector is etched. This process enlarges the damage tracks to microscopic size and they are counted using a microscope or by other suitable techniques. In this study some of the detectors could not be counted after retrieval. Apparently they had been attacked by some alkaline vapour, possibly traces of ammonia from the small amounts of geothermal activity near Taupo township. This problem does not seem to have been observed by others so far and the present author would appreciate communication from other workers who have noted similar effects. No geographical pattern was obvious. The results are shown in Fig. 1. None is above the maximum permissible concentration but some approach it. The errors on the figures are --- 30% on average. To produce a long-term mean approaching the maximum permissible, but incorporating significant periods when the background concentration of 0.06 pCi litre -1 is present, there must be some times in the year when the concentration of radon must be very high--above the maximum permissible. The long-term means are more typical of values encountered in continental areas rather than oceanic islands such as New Zealand. The most southerly exposure site in Fig. 1 is at least 30 m above Lake Taupo. The house owner at this site observed that it was usually above any inversion layer, as indicated by the fog which usually spread southwards from the Waikato river over Taupo. In these circumstances it is quite reasonable that a low radon concentration was recorded. One detector without shielding membrane recorded a level of 0.6 pCi litre- ~; this detector was exposed by the river bank north-west of Taupo. Because no membrane was present, the value will also have a contribution from the short-lived isotope of radon, ;°Rn, which may typically be equal in concentration or even dominant. This value, therefore, can
Dispersion of 222R n from two New Zealand geothermal power plants
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only be taken as roumhly confirming the others. Its true m R n value is likely to be around 0-3 pCi litre -~. For comparison, a detector installed in another city, Rotorua, with substantial thermal activity, gave a radon concentration of 0.12 pCi litre-' and another in an energy-efficient house near Wellington (a non-geothermal area) gave a value of 1-75 pCi litre -~ which is quite typical of such houses and results from accumulation of radon from underlying soil. This observation suggests that the houses of T a u p o are the most likely places to anticipate the highest concentrations of radon but no survey has yet been performed. If the geothermal power stations do not contribute significantly to radon levels at Taupo, where does the observed radon come from? The probable source is local hot springs and releases of water from private geothermal bores. Also there is an area called Karapiti about 8 km north of Taupo, which is hot, bare and shown by a detailed survey (Whitehead et al., 1983) to emit large quantities of radon; it might contribute some radon, especially to the Waikato River site in Fig. 1, which recorded a concentration of 0-3 pCi litre-'. It is known that its activity is a direct result of the draw-off of steam by the Wairakei geothermal plant. This resulted in a lowering of-the water table and in less cooling of the steam and other gases which had previously been emitted. There was, therefore, a distinct rise in temperature (Allis, 1979) accompanied by minor hydrothermal eruptions. Other hydrothermal eruptions have occurred nearer Taupo. Therefore, although the direct effects of geothermal power stations may be negligible, the indirect effects through raising ground temperatures may be of more importance. These limited measurements do not contradict the outcome of the dispersion model. There appear to be only three papers which report measured radon concentrations near geothermal power plants (Stoker & Kruger, 1975; Anspaugh & Hahn, 1979; George etal., 1980) but even though in one case the emissions were 1.6 Ci d -~ (Rich & Szalinski, 1981), which is about 10 times the Wairakei emissions, and in the case of George et al. (1980) 50 times, no concentrations greater than background were noted.
DISCUSSION The above results suggest that the maximum permissible concentration of radon may be exceeded at Taupo for a significant amount of time, at least in winter, but that this is not due directly to the geothermal power
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stations. Such a condition also exists in the non-geothermal area. Christchurch (Robertson & Matthews, 1978). However, it should be r e m e m b e r e d that because there is no threshold for health effects to be seen, it is the integrated doses which are the critical values. In the present case these are below the maximum permissible values and are typical of continental areas. Lung cancer incidence would, therefore, be expected to be not greater than in the latter environments. There has been very little work on the health impact of radon concentrations associated with thermal activity. Maeda etal. (1973) studied populations near a radiationrich hot spring but found that the controls, who lived further away, actually had slightly worse life expectancy, though the differences were not statistically significant. Kruger (1979) states that, with 14.7 MW(e) installed geothermal generating capacity in the US by the year 2000, the overall radon addition would even then be only 0-01% of the natural background amount due to open-air exposure. All these findings taken in conjunction with those in this paper have yet to indicate a significant health hazard from radon emissions from geothermal power plants, even if the radon is emitted in an uncontrolled manner to the atmosphere.
ACKNOWLEDGEMENTS I would like to thank Dr Keith Lassey for discussions and for altering his computer program for my use, those Taupo residents and DSIR staff who exposed the Tracketch ® cups, the management of the Wairakei geothermal station for allowing access to their past meteorological data and Dr David Wratt of the New Zealand Meteorological Service who provided essential data and comments on the manuscript. I would also like to thank Dr J. Barton of IAEA, Vienna, for his valiant attempts to translate this manuscript from one machine-readable form to another.
REFERENCES Allis, R. (1979). Thermal history of the Karapiti area, Wairakei, New Zealand, Report 137, Wellington, New Zealand, DSIR Geophysics Division. Anspaugh, L. R. & Hahn, J. L. (1979). Human health implications ofgeotherrnal energy, UCRL-83382, University of California. Axtmann, R. C. (1975). Environmental impact of a geothermal power plant. Science, 187, 795-803.
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George, A. C., Scandiffio, G. & Breslin, A. J. (1980). The distribution of radon and radon daughter products in the geothermal region of larderello, Lawrence Berkeley Laboratory Report LBL-11555, 3-14, University of California. Gudiksen, P. H., Ermak, D. L., Lamson, K. C., Axelrod, M. C. & Nyholm, R. A. (1979). The potential air quality impact of geothermal power production in the Imperial Valley, UCRL-52797, University of California. ICRP (1960). Report of ICRP committee II on permissible dose for internal radiation with bibliography for biological, mathematical and physical data. Health Phys., 3, 1-380. Kruger, P. (1979). Radon release from geothermal resources. Trans. American Nucl. Soc., 33,152--4. Lassey, K. R. (1979). ERRANL; a system of computer programs to model the human consequences of a hypothetical reactor accident, Program INS-P-146, New Zealand, Institute of Nuclear Sciences. Layton, D. W. & Anspaugh, L. R. (1982). Health impacts of geothermal energy. In Health impacts of different sources of energy, 581-94, Vienna, IAEA. Maeda, K., Abe, S., Kurokawa, Y. & Minagawa, Y. (1973). Trialevaluation of the long-term survey of death causes in the population living near radiation rich hot spring, CONF-07-P1,491-4, Oak Ridge National Laboratory, Knoxville. Matthews, K. M. (1981). The use of lichens in a study of geothermal radon emission in New Zealand, Environ. Pollut., Series A, 24, 105-16. Pollian, G. & Lambert, G. (1979). Radon daughters and sulfur output from Erebus volcano, Antarctica. J. Volcanology & Geothermal Res., 6, 125-37. Rich, L. B. & Szalinski, P. A. (1981). Measurements of 3 environmental releases of radon from the Geysers geothermal power plant. Trans. American Nucl. Soc., 39, 81. Robertson, M. K. & Matthews, K. M. (1978). Measurements of air pollution by geothermal radon, Report NRL 1978/5, New Zealand, National Radiation Laboratory. Stoker, A. K. & Kruger, P. (1975). Radon in geothermal reservoirs. Proceedings of the 2nd United Nations symposium on the development and use of geothermal resources, San Francisco, 1975, 1797-803. Ting, S. B. (1975). Environmental natural radioactivity in the lower atmosphere. Unpublished M.Sc. Thesis, Victoria University of Wellington. U N S C E A R (1962). Report of the United Nations scientific committee on the effects of atomic radiation, New York, United Nations. U N S C E A R (1982). Ionizing radiation sources and biological effects, New York. United Nations. Whitehead, N. E. (1980). Radon measurements at three New Zealand geothermal areas. Geothermics, 9, 279-86. Whitehead, N. E., Gingrich, J. E. & Fisher, J. C. (1983). A New Zealand test of the Tracketch ® method of prospecting for geothermal steam. J. Volcat~ology & Geothermal Res., 15,339-54. Wratt, D. S. & Homes, L. F. (1983). Measurements on a seventy metre tower at Ohaaki, with applications to air pollution meteorology, unpublished report, New Zealand Meteorological Service.
Dispersion of "Rn from Two New Zealand Geothermal Power Plants
N. E. Whitehead* Institute of Nuclear Sciences, DSIR, Private Bag, Lower Hurt, New Zealand
ABSTRACT The dispersion of e2eRnfrom emitted waste gases at Wairakei geothermal power station, New Zealand, is modelled. It is concluded that resulting concentrations in the nearby township of Taupo will never exceed the maximum permissible-in any meteorological situation. The greatest possible accumulation is calculated to be less than one eighth of the normal background radon concentration. A more realistic set of assumptions predicts long-term mean concentrations about 4% of background levels. A new geothermal power station, Ohaaki, a factor of three times more distant, is calculated to produce ten times lower concentrations than Wairakei. Measurements using a few passive solid-state radon detectors show that the natural variation of radon concentrations greatly exceeds any calculated contribution from either geothermal station; hence, much o f the radon is probably due to more local minor sources. Local sources have increased due to draw-off of ground water by the Wairakei station. Thus, there could be some indirect contribution to radon concentrations by its operation. The measured six-month integrated mean radon concentration at Taupo is a significant fraction of the maximum permissible concentration. It seems likely that natural sources alone may lead to levels in air which are above the maximum when temperature inversion exists. It is concluded that these two geothermal power plants are unlikely to produce concentrations o f radon hazardous to the population or to plant workers. *Present address: International Laboratory of Marine Radioactivity, Mus~e Oceanographique, Principality of Monaco. 245 J. Environ. Radioactivity 0265-931X/85/$03.30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain
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INTRODUCTION Geothermal power plants are a polluting form of energy (Axtmann, 1975). The amount of carbon dioxide emitted is often much larger than that from a coal-fired power station of the same total power production. There is also concern about substances emitted in the steam (e.g. arsenic, mercury, hydrogen sulphide), possible leukaemia from trace quantities of benzene (Layton & Anspaugh, 1982) and lung cancer from radon. ~ R n is a well-known constituent of geothermal systems and volcanoes emit large quantities. From the figures of Pollian & Lambert (1979), for example, it may be calculated that Mt Erebus emits about 160 Ci (6 TBq) per year. This is about equivalent to 440 mCi (16 GBq) per day but geothermal power stations emit comparable quantities (Whitehead, 1980). The Wairakei geothermal power station, in the centre of the North Island of New Zealand, is about 7 km north of Taupo township (population about 14 000), the only major population centre nearby. A new power station is being constructed at Ohaaki, about 20 km north of Taupo. It is of interest to determine whether the radon emitted from these stations would cause any hazard to plant workers or to people living at Taupo. Figure 1 is a location map of the area near Taupo, with contours expressed in feet, and shows how far it departs from the ideal plane surface usually assumed in modelling. Investigations on possible hazards from Rn at the Wairakei station have already been published by Robertson & Matthews (1978), Matthews (1981) and Whitehead (1980). Robertson & Matthews (1978) measured a value of 0-06 pCi litre -t (2 mBq litre -L) of 22ZRn in air at Taupo. Levels near geothermal bores were similar and only in one case as high as 0.2 pCi litre -t (6 mBq litre-~). This observation showed that the radon was rising with the hot steam and contributing no significant radiation dose, even to workers standing beside the plume of steam. Even within the power-station turbine room, the concentrations were similar. In addition, there were no excess y-ray levels from deposition of radon daughter products on the inside of piping, even when the inside of the piping was examined (Whitehead, 1978, unpublished work). These values must be compared with the maximum permissible radon concentration for continuous exposure to the general population (ICRP, 1960) which is 1.0 pCi litre -I above any natural background present. Readers should note, however, that in view of the fact that radon exposure is now
Dispersion of 2eeRnfrom two New Zealand geotherma! power planta
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Fig. 1° Location of Wairakei and Taupo with height contours. Contours are in feet. Location of radon detectors, 0 ; underlined figures are measured radon concentrations in pCi litre- 1. The figure in brackets is from a detector exposed without membrane. The 0-06 pCi litre -t figure in the middle of Taupo is a single spot measurement taken from Robertson & Matthews (1978). The contours are taken from New Zealand topo~-aphical map NZMS 1 N 94. The Ohaaki station referred to in the text is 18 km north-east of Wairakei and hence is not shown on the map.
thought to be the major source of radiation exposure to humans ( U N S C E A R , 1982) changes in that ICRP recommendation could be possible a n d are currently under consideration. Radon accounts for a large dose to the body as a whole and is virtually the only significant source of exposure to the lungs. Matthews (1981) checked whether any long-term contamination from
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radon had accumulated over some years near the power station by measuring the -'~°Pb/L37Cs ratios but found that these were the same in geothermal and non-geothermal areas. :1°Pb would be produced from radon decay products, whereas the ~37Csresults from fallout and should be independent. The above authors agree that daily emission from the Wairakei station is of the order of 150 mCi (6 GBq) per day. The amount estimated to be emitted from the projected Ohaaki station through measurement of radon in individual bores (Whitehead, 1980) is almost identical, although the geochemistry of the two areas differs. Where does this rather large amount of activity end up in the environment? This paper reports the results of computer modelling of the radon dispersion along with a few radon measurements to determine whether the model is even approximately correct. The measurements were long-term to average out the effects of varying meteorological conditions.
MODELLING OF DISPERSION The computer model used here was constructed by Dr K. Lassey of this Institute (Lassey, 1979). It assumes a Gaussian distribution of a radionuclide in a cross-section of a plume emitted from a hot source. The model was originally intended to model the release of radionuclides from a reactor accident but it is easily adapted for modelling radon dispersion from a geothermal power plant. For the present calculations, the height of release was taken as that of the stacks on top of the Wairakei power station, which emit the waste gases through four narrow chimneys in a line, with spacing of more than 10 m, at a temperature of about 50°C (about 35°C above ambient). The height of gas release is about 31 m and the gas is almost entirely carbon dioxide. For the Ohaaki station the gas volume will be about ten times greater and the release will be 70 m above ground. In neither case is the gas derived from a single point source but this discrepancy had to be neglected in these calculations. The program (Lassey, 1979) gives results which depend on the meteorological conditions and these conditions are called Pasquill classes. For later calculations it was necessary to use the data of Wratt & Homes (1983); their USNRC categories were equated to the Pasquill classes following their advice (Wratt, 1983, Pers. comm.). It is intuitively obvious that the hot gases tend to rise up in a plume; the
Dispersion of "2:Rn frorn two New Zealand geotherrnal power plants
249
height to which they rise depends on the air temperature and whether there is an inversion layer present. If the inversion layer is very low. local concentrations will be high. If more normal conditions prevail, the plume will rise, travel with the prevailing wind and then slowly descend. In such a case, the maximum concentration of the radionuclide is found some distance from the source, depending on the height of release. Like many programs of its kind, the one used for the modelling in this paper assumes that the terrain is flat and does not have features which markedly perturb the dispersion. The program also assumes that the mid-point of the Gaussian distribution, which gives the highest concentration, is of most interest; these values are therefore reported in this paper. The dosimetry of radon is quite complex and depends on the relative amounts of the daughter products present. This paper presents calculations based on only the amount of the parent ::2Rn but more complex treatments are certainly possible. In the following treatment, the effects of different conditions are considered. The wind speed and height of release are varied to determine sensitivity to these factors. In considering the results, account must also be taken of the natural radon levels which would be expected from exhalation from the ground; these vary considerably depending on temperature, pressure and rainfall. It was not very, clear initially what the background radon level at Taupo would be. Actual measurements could contain some contribution from geothermal sources and should be compared with measurements elsewhere in New Zealand. Many measurements at Wellington, a nongeothermal area, have consistently given results from 0.030 to 0-037 pCi litre -~ over five years (Ting, 1975). The lowest figures from the Taupo region (Robertson & Matthews, 1978) are less than 0-05 pCi litre -t but the lowest figure which is significantly different from zero in their report is 0-06 pCi litre <. This figure is low compared with a world mean of 0.17 pCi litre -~ (a combination of figures from Ting (1975) and UNSCEAR (1962)). If the mean background level is taken as 0-06 pCi litre -', as in this paper, it should therefore be noted that although in Figs 2 and 3 it is marked as a single line, there is substantial variation about that line, but mostly above it, to as high as 0"2 pCi litre -~. If a radon concentration calculated by the program is significantly smaller than 0-06 pCi litre -~, the large natural variation in background makes the calculated value even less important. Figure 2 shows the potential impact, at Taupo, of radon emissions from Wairakei. The curves for the various Pasquill weather conditions
9_50
N. E. Wh/tehead 1-0 MPC
0"1 .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
p Ci/I
E
0.01
0
0.001
1
2
3
~
56
78
9
m/$
F'~. 2. Predicted concentrations of ~-~-Rnat Taupo with release at Wairakei under various Pasquill weather classes. ' B G D ' is the line representing the background radon concentration. 'MPC' is the Maximum Permissible Concentration, which is 1-0 pCi litre-l. The abscissa is the wind speed.
(specified in Table 1) demonstrate that there is no possibility that the m a x i m u m permissible concentration will be exceeded under any weather conditions. This is true even for classes E and F, in which the inversion layer is low, i.e. in the conditions which occur more often in winter and u n d e r low wind speed regimes. Figure 3 shows a similar set of curves for the proposed Ohaaki station; it also includes a set of curves to give the approximate impact on the Wairakei station from emissions at Ohaaki. Obviously the emission from the O h a a k i station will never cause the maximum permissible concentration to be exceeded at Taupo. In terms of the modelling, this result is p r o d u c e d because the distance to Taupo from Ohaaki is three times greater than that from Wairakei to Taupo and the radon has cor-
Dispersion of :~-2Rn from two New Zealand geothermal power plants
9_51
0.1
BOO
0"01
pCi/I
",,
~
0.001 N ' ~ ! " "
-.
B C
0.0001
1
2
3
/.
5
5
7
8
9
m/s Fig. 3. Predicted concentrations of 2Z2Rnat Taupo (solid line) and Wairakei (dashed line) from release at projected Ohaaki site, under various Pasquill weather classes. The abscissa is the wind speed. TABLE 1 Model Predictions and Characteristics of Pasquill Classes*
Pasquill class A B C D E F
Wind speed (ms -1) 0.5-2-0 1-5 2-10 3-10 2-5 0-5-3
MLringlayer height (m)
Predicted distance of maximum radon concentration (m)
Maximum concentration (pCi litre -i)
1500 1500 1000 500 200 200
100-160 160--250 240--400 400--630 630-1000 1600--2500
0-085 0-051 0-034 0-017 0-02_6 0-068
*The data in this table are ones used for calculating the curves in Figs 2 and 3 but are not exhaustively definitive for the Pasquill classes.
9-52
N. E. Whitehead
respondingly more time to diffuse in three dimensions and to be diluted. The model also predicts where the maximum concentrations will occur downwind of the Wairakei plant. Table 1 shows that some of the predicted maxima are only a few h u n d r e d metres from the geothermal power plant but in no case do they approach maximum permissible concentrations. Other calculations were carried out to check what the effects of changing the radon release height might be. The range considered was 20-60 m but, in the case of predicted values at Taupo, the resulting differences were only a few percent. Larger differences were calculated for much nearer the plant, the lowest release heights giving the highest concentrations. If the emission is from a stack height of only 20 m the calculated maximum at 1.6-2.5 km under Pasquill class F is 0-13 pCi litre -L, which is still very low.
METEOROLOGY A worst-case calculation for radon concentrations at Taupo can easily be performed by assuming that class F predominates. A wind-rose from meteorological data at Wairakei shows 30% of the winds were in the direction of Taupo, i.e. along the graben, and it can very pessimistically be assumed that in all these cases Pasquill class F prevails. If that is so, then the long-term mean radon concentration is predicted to be 0-034 pCi litre -~ due to Wairakei emissions alone during periods when the winds blow from that direction. However, if one assumes that this condition prevails only 30% of the time, the long-term mean radon concentration for all times is only 0-01 pCi litre -~. This value is low compared to the assumed background concentration of 0-06 pCi litre-~. From detailed investigation of the Ohaaki site, information is available on the percentage frequency of various meteorological classes as shown in Table 2 (Wratt & Homes, 1983). These figures were selected for wind directions in the quadrant north to east inclusive, which are assumed to reach Wairakei and Taupo. The Table 2 frequency data have associated absolute uncertainties of 3-7%; for example, the relative error on category B is 100%. As can be seen in Figs 2 and 3, the computer program does not calculate results for Pasquill class G. There will therefore be a resulting small error but it is of no practical significance in this particular case.
9_53
Dispersion o f "-e-"Rn from two New Zealand geothermal power plants TABLE 2
Frequencyof Meteoroio~calConditionsat Ohaaki Pasquill class Frequency(%)
A 9
B 3
C 5
D 35
E 34
F 12
G
Using the data in Table 2 and assuming that there is no other contribution, an attempt can be made to calculate the long-term mean radon concentration at Taupo from Wairakei releases alone. One can integrate for all Pasquill classes when the appropriate north-to-east winds blow, assuming that the tabulated conditions for Ohaaki apply to Wairakei. Weighting each class by the frequency of occurrence, it is possible to calculate that the integrated radon concentration would be 0-003,4 pCi litre -1. This contribution is extremely small compared with typical radon concentrations in air and may be considered negligible. A similar calculation for the concentrations resulting from dispersion of radon from Ohaaki to Taupo, or from Ohaaki to Wairakei, gives values in the range 0.0002-0-0003 pCi litre-~, which are also negligible. Investigations in the Imperial Valley, California (Gudiksen et al.. 1979) and computer calculations similar to those of this paper suggested that the radon concentrations there would be five orders of magnitude below the maximum permissible. In the present paper the corresponding value is three orders of magnitude for Wairakei emissions and four for Ohaaki.
R A D O N MEASUREMENTS IN AIR Because of the limitations of the computer model and of the meteorological data, a few direct measurements were carried out. These were restricted because of equipment limitations but still give some useful information. Ideally one would like to monitor the long-term radon concentration at many sites at Wairakei and Taupo and, in addition, thoroughly record the meteorological conditions, using many instruments capable of recording changes on a time scale of hours. Since this was not feasible, the radon levels were integrated for six months during winter (to give a worst-case result--low level inversion layers should be common in winter) using passive solid-state Terradex Tracketch ®
254
N. E. Whitehead
detectors. These detectors are in the form of cups with the detector element inside and on the bottom. They were placed outside at the positions shown in Fig. 1. In all cases the exposure was to ambient outside air and not to air inside houses for any significant time. The detectors have a thin membrane across the mouth which excludes other isotopes of radon and also radon daughter products. " R n has a half-life of 3-8 d and can diffuse through the membrane, but ~ has a half-life of only 56 s and decays before it passes through. The daughter products are electrically charged and do not pass through the membrane but deposit on it. The radon which has diffused through the membrane decays and emits c~-particles which strike the detector causing submicroscopic damage. After exposure, the detector is etched. This process enlarges the damage tracks to microscopic size and they are counted using a microscope or by other suitable techniques. In this study some of the detectors could not be counted after retrieval. Apparently they had been attacked by some alkaline vapour, possibly traces of ammonia from the small amounts of geothermal activity near Taupo township. This problem does not seem to have been observed by others so far and the present author would appreciate communication from other workers who have noted similar effects. No geographical pattern was obvious. The results are shown in Fig. 1. None is above the maximum permissible concentration but some approach it. The errors on the figures are --- 30% on average. To produce a long-term mean approaching the maximum permissible, but incorporating significant periods when the background concentration of 0.06 pCi litre -1 is present, there must be some times in the year when the concentration of radon must be very high--above the maximum permissible. The long-term means are more typical of values encountered in continental areas rather than oceanic islands such as New Zealand. The most southerly exposure site in Fig. 1 is at least 30 m above Lake Taupo. The house owner at this site observed that it was usually above any inversion layer, as indicated by the fog which usually spread southwards from the Waikato river over Taupo. In these circumstances it is quite reasonable that a low radon concentration was recorded. One detector without shielding membrane recorded a level of 0.6 pCi litre- ~; this detector was exposed by the river bank north-west of Taupo. Because no membrane was present, the value will also have a contribution from the short-lived isotope of radon, ;°Rn, which may typically be equal in concentration or even dominant. This value, therefore, can
Dispersion of 222R n from two New Zealand geothermal power plants
9_55
only be taken as roumhly confirming the others. Its true m R n value is likely to be around 0-3 pCi litre -~. For comparison, a detector installed in another city, Rotorua, with substantial thermal activity, gave a radon concentration of 0.12 pCi litre-' and another in an energy-efficient house near Wellington (a non-geothermal area) gave a value of 1-75 pCi litre -~ which is quite typical of such houses and results from accumulation of radon from underlying soil. This observation suggests that the houses of T a u p o are the most likely places to anticipate the highest concentrations of radon but no survey has yet been performed. If the geothermal power stations do not contribute significantly to radon levels at Taupo, where does the observed radon come from? The probable source is local hot springs and releases of water from private geothermal bores. Also there is an area called Karapiti about 8 km north of Taupo, which is hot, bare and shown by a detailed survey (Whitehead et al., 1983) to emit large quantities of radon; it might contribute some radon, especially to the Waikato River site in Fig. 1, which recorded a concentration of 0-3 pCi litre-'. It is known that its activity is a direct result of the draw-off of steam by the Wairakei geothermal plant. This resulted in a lowering of-the water table and in less cooling of the steam and other gases which had previously been emitted. There was, therefore, a distinct rise in temperature (Allis, 1979) accompanied by minor hydrothermal eruptions. Other hydrothermal eruptions have occurred nearer Taupo. Therefore, although the direct effects of geothermal power stations may be negligible, the indirect effects through raising ground temperatures may be of more importance. These limited measurements do not contradict the outcome of the dispersion model. There appear to be only three papers which report measured radon concentrations near geothermal power plants (Stoker & Kruger, 1975; Anspaugh & Hahn, 1979; George etal., 1980) but even though in one case the emissions were 1.6 Ci d -~ (Rich & Szalinski, 1981), which is about 10 times the Wairakei emissions, and in the case of George et al. (1980) 50 times, no concentrations greater than background were noted.
DISCUSSION The above results suggest that the maximum permissible concentration of radon may be exceeded at Taupo for a significant amount of time, at least in winter, but that this is not due directly to the geothermal power
256
N. E. Whitehead
stations. Such a condition also exists in the non-geothermal area. Christchurch (Robertson & Matthews, 1978). However, it should be r e m e m b e r e d that because there is no threshold for health effects to be seen, it is the integrated doses which are the critical values. In the present case these are below the maximum permissible values and are typical of continental areas. Lung cancer incidence would, therefore, be expected to be not greater than in the latter environments. There has been very little work on the health impact of radon concentrations associated with thermal activity. Maeda etal. (1973) studied populations near a radiationrich hot spring but found that the controls, who lived further away, actually had slightly worse life expectancy, though the differences were not statistically significant. Kruger (1979) states that, with 14.7 MW(e) installed geothermal generating capacity in the US by the year 2000, the overall radon addition would even then be only 0-01% of the natural background amount due to open-air exposure. All these findings taken in conjunction with those in this paper have yet to indicate a significant health hazard from radon emissions from geothermal power plants, even if the radon is emitted in an uncontrolled manner to the atmosphere.
ACKNOWLEDGEMENTS I would like to thank Dr Keith Lassey for discussions and for altering his computer program for my use, those Taupo residents and DSIR staff who exposed the Tracketch ® cups, the management of the Wairakei geothermal station for allowing access to their past meteorological data and Dr David Wratt of the New Zealand Meteorological Service who provided essential data and comments on the manuscript. I would also like to thank Dr J. Barton of IAEA, Vienna, for his valiant attempts to translate this manuscript from one machine-readable form to another.
REFERENCES Allis, R. (1979). Thermal history of the Karapiti area, Wairakei, New Zealand, Report 137, Wellington, New Zealand, DSIR Geophysics Division. Anspaugh, L. R. & Hahn, J. L. (1979). Human health implications ofgeotherrnal energy, UCRL-83382, University of California. Axtmann, R. C. (1975). Environmental impact of a geothermal power plant. Science, 187, 795-803.
Dispersion of "--"Rn from two New Zealand geothermal power plants
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George, A. C., Scandiffio, G. & Breslin, A. J. (1980). The distribution of radon and radon daughter products in the geothermal region of larderello, Lawrence Berkeley Laboratory Report LBL-11555, 3-14, University of California. Gudiksen, P. H., Ermak, D. L., Lamson, K. C., Axelrod, M. C. & Nyholm, R. A. (1979). The potential air quality impact of geothermal power production in the Imperial Valley, UCRL-52797, University of California. ICRP (1960). Report of ICRP committee II on permissible dose for internal radiation with bibliography for biological, mathematical and physical data. Health Phys., 3, 1-380. Kruger, P. (1979). Radon release from geothermal resources. Trans. American Nucl. Soc., 33,152--4. Lassey, K. R. (1979). ERRANL; a system of computer programs to model the human consequences of a hypothetical reactor accident, Program INS-P-146, New Zealand, Institute of Nuclear Sciences. Layton, D. W. & Anspaugh, L. R. (1982). Health impacts of geothermal energy. In Health impacts of different sources of energy, 581-94, Vienna, IAEA. Maeda, K., Abe, S., Kurokawa, Y. & Minagawa, Y. (1973). Trialevaluation of the long-term survey of death causes in the population living near radiation rich hot spring, CONF-07-P1,491-4, Oak Ridge National Laboratory, Knoxville. Matthews, K. M. (1981). The use of lichens in a study of geothermal radon emission in New Zealand, Environ. Pollut., Series A, 24, 105-16. Pollian, G. & Lambert, G. (1979). Radon daughters and sulfur output from Erebus volcano, Antarctica. J. Volcanology & Geothermal Res., 6, 125-37. Rich, L. B. & Szalinski, P. A. (1981). Measurements of 3 environmental releases of radon from the Geysers geothermal power plant. Trans. American Nucl. Soc., 39, 81. Robertson, M. K. & Matthews, K. M. (1978). Measurements of air pollution by geothermal radon, Report NRL 1978/5, New Zealand, National Radiation Laboratory. Stoker, A. K. & Kruger, P. (1975). Radon in geothermal reservoirs. Proceedings of the 2nd United Nations symposium on the development and use of geothermal resources, San Francisco, 1975, 1797-803. Ting, S. B. (1975). Environmental natural radioactivity in the lower atmosphere. Unpublished M.Sc. Thesis, Victoria University of Wellington. U N S C E A R (1962). Report of the United Nations scientific committee on the effects of atomic radiation, New York, United Nations. U N S C E A R (1982). Ionizing radiation sources and biological effects, New York. United Nations. Whitehead, N. E. (1980). Radon measurements at three New Zealand geothermal areas. Geothermics, 9, 279-86. Whitehead, N. E., Gingrich, J. E. & Fisher, J. C. (1983). A New Zealand test of the Tracketch ® method of prospecting for geothermal steam. J. Volcat~ology & Geothermal Res., 15,339-54. Wratt, D. S. & Homes, L. F. (1983). Measurements on a seventy metre tower at Ohaaki, with applications to air pollution meteorology, unpublished report, New Zealand Meteorological Service.