Dispersion of 222Rn from two New Zealand geothermal power plants

Abstract

The dispersion of 222Rn from 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 of 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 of radon hazardous to the population or to plant workers.