Fact Sheet: Radiological hazards associated with hot rock geothermal systems.
David L Battye and Peter J Ashman
School of Chemical Engineering, University of Adelaide SA 5005
Hot rock geothermal systems extract energy by circulating water within granites that are heated by natural
radioactivity. There is potential for radioactive elements to dissolve in the circulating water and be carried to
surface equipment. It is therefore natural to ask:
… Will hot rock geothermal power plants present any significant radiation hazards?
Radioactivity and radiation
A radioactive element, or radionuclide, is an element with an unstable nucleus. Radioactive decay describes
a transformation of the unstable nucleus whereby the following may happen:
Alpha decay: An alpha particle, consisting of two protons and two neutrons, is ejected from the
nucleus. Alpha particles may be stopped by a piece of paper.
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Beta decay: A neutron in the nucleus is converted into a proton and an electron is ejected. The
electron is known as a beta particle and may penetrate 1-2 cm into body tissue.
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Gamma radiation may accompany alpha or beta decay. Gamma radiation is a type of electromagnetic
radiation like light or radio waves, except that it has a far higher frequency and energy. It is highly penetrative.
Dense materials such as lead are used for shielding against gamma radiation, although any material can be
used provided the thickness is great enough.
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Alpha particles, beta particles and gamma radiation are all types of ionizing radiation, a class of radiation that
is hazardous to our health. Ionizing radiation has enough energy to knock electrons out of their orbits, ie. to
“ionize” molecules. When this happens to molecules in the cells of our body, those cells can become damaged.
If the genetic material (DNA) in cells is damaged, ionizing radiation can cause cancer. Damage to the DNA in
reproductive cells can cause genetic mutations that can be passed on to future generations.
Gamma radiation doses are the easiest to receive since gamma rays can travel further from their source than
alpha and beta particles. However, radionuclides emitting alpha and beta particles can be quite harmful if
inhaled or ingested.
Ionizing radiation doses are measured in a unit called the Sievert (Sv), where 1 Sv represents the biological
effect of 1 Joule of energy deposited in 1 kilogram of tissue by ionizing radiation.
The background ionizing radiation dose in Australia is about 1.5 millisieverts (mSv) per year and can be broken
down as shown below.
Source
Annual dose (mSv)
Cosmic rays
0.3
External, terrestrial sources (eg. rock, soil)
0.6
Internal sources (eg. potassium-40 in the body)
0.4
Radon
0.2
Total
1.5
Cosmic rays are highly energetic particles (protons, alpha particles and electrons) that bombard the earth from
outer space. External, terrestrial radiation comes from radionuclides present in our external environment,
mostly in rocks and soil. Internal radiation is caused by radionuclides occurring naturally in our body such as
the radioactive isotope of potassium, 40K. Radon is a radioactive gas produced by decay of radium isotopes
present in soils, rock and building materials such as bricks, concrete, mortar and tiles. The decay products of
radon can lodge in the lungs if inhaled, exposing them to ionizing radiation and increasing the risk of lung
cancer.
Australia’s recommended dose limits for ionizing radiation arising from activities that increase overall exposure
are 20 mSv per year for occupational exposure (averaged over 5 years), and 1 mSv in a year for members of
the public. These limits are based on the recommendations of the International Commission on Radiological
Protection (ICRP), and do not include natural background radiation or doses received from medical procedures.
However, the National Standard for limiting occupational doses requires that all doses should be kept “as low as
reasonably achievable” (ALARA).
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Decay chains
Abundance
(grams per tonne)
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Radionuclide
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The most abundant radionuclides in granites are uranium-238 (238U), thorium-232 (232Th) and potassium-40
(40K). These elements have existed in the granite since it was first formed and decay very slowly. Granites in
the Cooper Basin have an approximate radionuclide composition as shown below.
238U
232Th
40K
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and 232Th are each at the head of a decay chain of radionuclides, shown below. Another decay chain has
the uranium isotope 235U at its head, but this isotope represents only 0.72% of all naturally occurring uranium.
The 238U and 232Th in granites has been there long enough that an equilibrium has been reached in both decay
chains. In this radioactive equilibrium, the activity (number of atoms decaying per second) of each radionuclide
is equal.
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238U
uranium-238 decay chain
Half-life
Principal
radiation
uranium-238
thorium-234
proactinium-234m
uranium-234
thorium-230
radium-226
radon-222
polonium-218
lead-214
bismuth-214
polonium-214
lead-210
bismuth-210
polonium-210
lead-206
4.5 billion years
24 days
1 minute
246 thousand years
75 thousand years
1600 years
3.8 days
3 minutes
27 minutes
20 minutes
2 microseconds
22 years
5 days
138 days
stable
alpha
beta
beta
alpha
alpha, gamma
alpha, gamma
alpha, gamma
alpha, gamma
beta, gamma
beta, gamma
alpha, gamma
beta, gamma
beta, gamma
alpha, gamma
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Radionuclide
thorium-232 decay chain
Half-life
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Radionuclide
14 billion years
5.8 years
6.2 hours
1.9 years
3.7 days
56 seconds
0.15 seconds
11 hours
bismuth-212
61 minutes
polonium-212 (64%)
lead-208
thallium-208 (36%)
lead-208
0.3 microseconds
stable
3.1 minutes
stable
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thorium-232
radium-228
actinium-228
thorium-228
radium-224
radon-220
polonium-216
lead-212
Principal
radiation
alpha, beta,
gamma
beta
beta, gamma
alpha
alpha, gamma
alpha
alpha
beta, gamma
alpha, beta,
gamma
alpha, beta,
gamma
beta, gamma
Radionuclides in solution
The water circulating in a hot rock reservoir is a mixture of fresh water injected at the surface and water already
present in the natural fractures of the granite, which may be quite saline. In hot rock projects overseas, the
circulating water has typically had a total dissolved solids (TDS) content of less than 4 g/L and near-neutral pH.
Under these conditions the solubility of uranium, thorium, radium, radon and lead isotopes is expected to be
similar to that for dilute, granite groundwaters.
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Geothermal power plants often produce solid residues in the form of scales and sludges precipitated from the
geofluid. It is therefore reasonable to suspect that dissolved radionuclides may be included in these residues,
presenting a gamma radiation hazard. Experience in the oil and gas industry offers some insight on what can
be expected in this regard. Oil and gas reservoirs often also produce highly mineralized water together with the
hydrocarbons. When the produced waters are cooled and depressurized in surface production equipment,
radioactive scales and sludges are formed. The main source of radioactivity in these residues is radium. It has
been determined that the high dissolved solids content of typical produced waters stabilizes radium in solution,
preventing it from being scavenged by mineral surfaces in the reservoir. Furthermore, the produced waters are
usually saturated with barium, strontium and sulfate ions at the temperature of the reservoir, such that any
cooling of the water will cause the precipitation of barium and strontium sulfates. Being chemically similar to
barium and strontium, radium is readily incorporated into the precipitates. The low dissolved solids content of
typical hot rock geofluids means that we expect low radium levels in solution, and the geofluids are likely to be
under-saturated with respect to barium and strontium sulfates. Therefore, radium in solid residues is unlikely to
constitute a significant source of gamma radiation in hot rock power plants.
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The solubility of thorium and lead in groundwaters is low and so negligible levels are expected in hot rock
geofluids. The concentration of uranium isotopes in granite groundwaters can exceed 800 μg/L, and the
possibility of uranium deposition in surface residues cannot be discounted. However, the uranium isotopes only
emit alpha radiation and their decay products, which emit gamma radiation, do not become significantly active
for thousands of years. Hence, it appears unlikely that solid residues in hot-rock power plants will present a
significant gamma radiation hazard. However, given that all radionuclides can be hazardous if inhaled, and
without knowledge of the exact residue composition, workers involved with cleaning equipment will need to
avoid inhaling solid residue dusts.
Radon emissions
Radon enters solution in groundwaters predominantly by a mechanism called ‘alpha-recoil’, whereby a radon
atom is propelled into the geofluid by the alpha decay of a radium atom near the rock-fluid interface. Since
radon is an inert gas, it remains in solution until it decays or is emitted to the atmosphere.
In their most probable configuration, hot rock geothermal power plants will circulate the geofluid in a closed loop
in which it is never exposed to the atmosphere. In this context, dissolved radon gas will reach an equilibrium
concentration where the overall rate of emanation from the granite is equal to the rate of radioactive decay, as
is the case in many natural groundwaters. However, there are situations where essentially all radon in the
geofluid reaching the surface would be vented to the atmosphere. In open-loop circulation testing of newly
created reservoirs, all produced steam is vented. If a ‘flash’ power plant is operated, where steam for power
generation is produced by depressurizing the geofluid, then non-condensable gases including radon will be
vented from the steam condenser.
Based on experience gained in overseas hot rock projects, it is possible to estimate the radon emission rate
from reservoirs of the type being developed in the Cooper Basin. For a typical reservoir of approximately 5 km2,
an estimated 4 billion radon atoms per second may be emitted (8400 Bq s-1). If the radon was emitted from a
point 20 metres above ground and in light wind, atmospheric dispersion modeling suggests that downwind
concentrations of radon at ground-level would be almost identical to background levels. Higher concentrations
will be experienced during periods of calm or during the break-up of stable air layers near the ground.
However, the preliminary modelling suggests that concentrations are likely to be far below levels of concern
even during these periods.
Summary: Will hot rock geothermal power create any radiation hazards?
Considering the above arguments, the answer is no.
Concentrations of radionuclides in hot rock geofluids are likely to be similar to dilute granite groundwaters. Any
scales or sludges precipitating from the geofluids are very unlikely to contain radium, the main emitter of
gamma radiation in oil and gas industry residues. Should uranium be present in residues, it will not represent a
radiation hazard provided proper precautions are taken against inhalation of dusts during equipment cleaning.
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Preliminary modelling suggests that venting radon from a typical hot rock reservoir will increase downwind
ground-level radon concentrations negligibly in light to strong winds. In periods of calm or during break-up of
stable air, concentrations are still estimated to be low relative to levels of concern.