(BEING CONTINUED FROM 1/08/2011)
THE NUCLEAR FUEL CHAIN
Uranium mining is the messiest and most contaminating stage of nuclear
power generation. Yet, without it, the whole process cannot go ahead.
The cost to the global environment, and to persons, of this stage must be
factored into the cost of nuclear power generation.
Uranium mining, in particular open pit mining, which is what is currently
proposed in several locations in southern Ontario, involves digging
thousands of tons of radioactive rock out of a giant hole. (The Rossing
uranium mine in Namibia is 1 km wide, 3 km long and 1/3 km deep (28)).
Large quantities of this rock are dumped onto the earth’s surface. The ore
is then transported to a milling facility, usually nearby, and crushed to a
fine sand-like consistency, creating large amounts of radioactive dust and
a huge volume of finely ground mill tailings. The uranium is separated out,
usually with strong acids or alkalis. The sand-like tailings, containing about
85% of their original radioactivity, and often the chemicals used in the
extraction process, are deposited in large tailings ponds or containments nearby.
Dust containing uranium and its progeny is produced in large quantities
by rock-crushing operations. This particulate matter, containing long-lived
radioactive isotopes, can leave the site on wind. Wind erosion of tailings
piles can be significant as long as these remain exposed to weather.
Radon gas is continuously produced by the decay of thorium 230, a
radioactive decay product of uranium 238, through radium into radon.
Thorium 230 has a half-life of 76,000 years, and will produce radon gas
unabated for millennia.
In undisturbed uranium deposits, most of the radon gas is trapped within
rock formations until it decays into other radioactive byproducts.
However, crushed tailings on or near the earth’s surface allow
considerable radon to escape. In a 10 km/hr breeze, it can travel 960 km
within 4 days; much further in higher winds. Radon gas decays
sequentially into several other solid radioactive isotopes of polonium,
bismuth and lead, before finally becoming the non-radioactive lead 206.
These radioactive progeny of radon settle onto crops, bodies of water
and soil. Their patterns of accumulation in the biosphere, including our
food species, are not well known. The three isotopes of polonium
produced by radon, in addition to being radioactive, are among the most toxic naturally occurring substances on earth. The toxicity of lead is well documented.
Radon is a major contributor to the excess of lung cancer seen in uranium
miners (4, 5, 6). Radon at levels seen in some residences also carries a risk
(29). Radon emanations from bedrock in certain areas may be
unavoidable, however these can be greatly increased in the presence or
proximity of crushed mine tailings or abandoned mine workings which
provide pathways of migration to the surface. Some high residential
radon readings are being found by homeowners near old mine sites in the
Bancroft/Haliburton area (30).
Groundwater and surface water in the vicinity of uranium mining
operations frequently become contaminated (31). At the advanced
exploration stage of mine development, holes about 1-2” in diameter and
up to 1200 feet deep are drilled into rock, usually into the most
concentrated deposits. A hole of this depth is almost certain to penetrate
aquifers, giving water access to radioactive rock surfaces. Many uranium
compounds and decay products are soluble, toxic and radioactive. In an
area of fractured granite bedrock, as found in some uranium bearing
areas of Ontario, many of the aquifers interconnect and contamination
quickly becomes widespread.
Uranium in drinking water, at levels in excess of the safe drinking water
standard of .02 mg/L or 20 ppb, is principally toxic to the kidney, in
particular the proximal tubules (32). Uranium can also affect fertility, fetal
growth and postnatal viability (33). It may cause malformations in fetuses
and might be associated with reproductive cancers. It concentrates in
bone and may interfere with the activity of osteoblasts, possibly
contributing to bone cancers and osteoporosis (32).
Uranium in well water is often associated with some of its highly dangerous
daughter elements such as radium and radon (18). Their combined
radioactivity may be a limiting factor in water quality. Radon in well water
is a significant contributor to radon levels in houses (34).
During the operation of a mine, the use of copious amounts of water to
control dust, or to create a slurry for the extraction of uranium, can
contaminate large quantities of water, which then need to be disposed
of. Tailings impoundments containing liquid material can leach
contaminants into the soil and groundwater. Tailings dams can fail,
releasing massive quantities of radioactive material into local waterways
(35). Near the decommissioned mines at Elliot Lake, tailings piles were
covered with water to prevent the escape of radon gas, a standard procedure. Recent drought has caused serious difficulties with this maintenance protocol. A mere 15 years into the thousand-year period for
which it was designed, this environmental safeguard system is
underperforming (36). Over 100 million tons of uranium tailings are stored
in the Elliot Lake area (37).
Dry piles of uranium mill tailings are subject to erosion by wind and water.
Tree roots and plants take up this radioactive material, often
concentrating it (38, 39), and are eaten by biological organisms – birds,
insects, mice, deer, etc. – which disperse it in their feces or their bodies.
Root systems help to bring radon up to the leaves where it can be transpired into the air.
In Ontario, near Bancroft and Haliburton, there are about 5 million tons of
uranium mine tailings. Many of these were abandoned by mines which
closed before 1977, and as such they are under the jurisdiction of neither
the federal nor the provincial governments (40). In 1977, the federal
government created the Atomic Energy Control Board (AECB), later
replaced by the Canadian Nuclear Safety Commission (CNSC). Uranium
mines thus fell under a federal mandate, whereas before this they were a
provincial responsibility. Because of this shift, federal and provincial
agencies have been locked in a jurisdictional struggle over these older
mine tailings. As a result, according to a study by the Canadian Institute
for Radiation Safety (CAIRS) (40), many of the tailings “have not
undergone any remedial work designed to place them in a safe condition.”
Tons of radioactive rock are laying around unprotected, with
contaminants leaching out, wind blowing dust, radon gas escaping,
fencing and signage falling into disrepair and the area being used more
and more for hunting, hiking and recreation. It is possible that fill is being
taken for construction purposes from unmarked radioactive sites.
What are the risks from these tailings? According to the CAIRS study, a
person walking over a typical tailings pile for 1 hr every day will absorb a
gamma radiation dose of, on average, 0.73 mSv/yr (41). This would be in
addition to the ~1.0 mSv/yr of background gamma radiation we all
receive. Consider that doubling a person’s exposure will in general double
his/her cancer risk, and that this person will also be exposed to higher than
normal levels of radon gas near the tailings.
If a house were built on the tailings, or if substantial amounts of radioactive
fill were used near this house, or to mix concrete for the house, and a
person or family spent between 8 and 24 hrs/day in this house, their radiation exposure could be substantial. It might well be over the maximum of 1.0 mSv/yr above background recommended for the
general public (8). (In this scenario, it could be up to 0.73mSv/yr X 24
=17.52 mSv/yr per person.)
Use of contaminated materials in construction has been a problem not
only in the Bancroft area, but in Elliot Lake, in Port Hope, where there is a
uranium conversion facility dealing with highly radioactive material, and in
the United States in Navaho territory where there was intensive uranium
mining in the past (42).
URANIUM REFINING AND ENRICHING
After the uranium is mined and milled, it is refined. Canadian uranium from
all sources is sent for further processing to a refinery in Blind River, Ontario
or to a conversion facility in Port Hope, Ontario. The Blind River facility
produces UO3; in Port Hope uranium is converted to UO2 for use in fuel
rods for reactors requiring unenriched uranium or is incorporated into
uranium hexafluoride (UF6) in preparation for enrichment. The UF6 is then
sent to an enrichment plant in Kentucky where the isotopes U 238 and U
235 are separated from each other and remixed in more desirable
proportions. Uranium with an excess of the fissionable U 235 is “enriched”-
this leaves a stockpile of extra U 238 or “depleted” uranium.
Uranium ore, yellowcake (the milled uranium destined for Port Hope or
Blind River for refining), and uranium fuel rods for use in reactors are all
transported by rail or truck to their destinations. This carries with it the risk
of an accident or major spill, with further risk of air, water and soil
Canadian CANDU reactors use unenriched uranium. Until 1965, all
Canadian uranium was used exclusively for American nuclear weapons,
including the Hiroshima and Nagasaki atomic bombs. After this, the
Canadian government decided that Canadian uranium was only to be
used for civilian purposes, such as electricity generation (25).
Unfortunately, there is no effective way to track or enforce this once
uranium leaves our borders.
Canada does not reclaim the leftover depleted uranium after the
enrichment process. The American military now uses some of it in the
production of armour for tanks and for armour-piercing bullets. Bullets
made from this material combust on impact, producing a fine radioactive smoke which, when inhaled, damages lung tissue. This aerosolized uranium, and the contaminated spent shells remaining on the ground,
expose the local population, as well as soldiers, to this radioactive waste
for many years (the half life of U 238 is 4.46 billion years). These weapons
have been used in Serbia, Afghanistan, Iraq and other theatres of war.
This material, and its radioactive daughter products, will remain mobile in
the environment for a very long time. Canada is implicated indirectly in
this situation, as it supplies the U.S. with uranium.
NUCLEAR POWER GENERATION
CANDU reactors, designed and extensively used in Canada, use heavy
water (deuterium oxide) as a moderator and coolant. This material helps
prevent the build up of excessive heat and acts to regulate the flow of
neutrons involved in the fission process. Because deuterium can easily
become tritium by absorbing a neutron, CANDU reactors produce many
times more tritium than reactors using light water. Tritium is a radioactive
isotope of hydrogen. Like hydrogen, tritium can become incorporated
into water molecules, organic carbon-based molecules, and indeed most
molecules relevant to living tissue, including DNA. It can therefore
become pervasive in the natural environment, and incorporate itself into
human tissue. Tritium is a carcinogen, a mutagen and a teratogen. It has
been involved in testicular and ovarian tumours, chromosome breaks and
aberrations, fetal death and malformations, and in mental retardation
after in utero exposure (43, 44, 45). Presently the acceptable level of
tritium in drinking water in Canada is 7000 Bq per litre. This contrasts with
other jurisdictions such as the U.S., which has an acceptable level of 740
Bq per litre. The E.U. limit is 100 and the public health goal in California is 15 Bq per litre.
Tritium escapes continuously from all operating reactors built to current
designs. Most of the escaping tritium is released as steam into the air from
the chimneys of the reactors; some is released into the cooling water, and
from there into local bodies of water, such as Lake Ontario. Tritiated
steam, or water vapour, can be absorbed through the skin or by the
lungs. Some tritium can become bound into organic molecules, and
incorporated into animals and plants. Elevated tritium levels have been
measured in soil and in fruits and vegetables grown in proximity to nuclear
reactors (46). This can be an important component of human tritium
exposure. Because the half life of tritium is 12.6 years, it will continue to
accumulate in the environment until it reaches equilibrium in about 72
years, as long as nuclear reactors continue to produce it at present rates.
During this time it will be free to disseminate itself throughout the biological
kingdom. Darlington is the only reactor that has a tritium extraction facility
which removes most of the tritium, and sells it for use in luminous dials and
in airport runway lights. Much of this tritium will ultimately be released into
the environment as well. Tritium is a necessary component in the
hydrogen bomb. This raises major security concerns, especially with
respect to terrorism and less stable regimes.
In addition to tritium, all functioning reactors routinely release many other
radioactive substances to the air and into the cooling water. The noble
gases xenon 137 and krypton 90 decay relatively quickly into the deadly
cesium 137 and strontium 90. Cesium 137 accumulates in muscle,
including the muscle of our food source animals such as cattle, pigs and
sheep; strontium 90 accumulates in bone. Other radioactive isotopes of
xenon, krypton and argon are also released. Iodine 131 is mostly trapped
by filters, but can escape in accidental releases. It is highly toxic to the
thyroid, particularly in children. The highly radioactive primary coolant in
the reactor core is supposed to be kept separate from the secondary
coolant, which circulates in and out from a nearby river, lake or the
ocean. In reality, particularly in older reactors, there are many leaks and
defects which allow these to mix. Tritium, fission products and isotopes
from irradiated structures in the reactor can escape (47).
Recently there have been some reported intentional controlled releases
of tritium from the Chalk River nuclear facility into the Ottawa River
upstream of the city of Ottawa. The AECL (Atomic Energy of Canada
Limited) and the CNSC (Canadian Nuclear Safety Commission) claim that
after these releases, levels of tritium in the Ottawa River, the source of
drinking water for over a million people, did not exceed safe limits (48).
However, as we have seen from past experiences, humans have
succeeded in polluting huge volumes of the earth’s water, air and soil, by
considering each small (or large) release of a contaminant as “safe” or
trivial. There is certainly risk that repeated releases of small amounts of a
carcinogenic, mutagenic and teratogenic substance such as tritium into
the drinking water of a large population will have some health effects.
At the end of its useful life in a fission reactor, the spent fuel contains
hundreds of different fission products, many of them not found in nature.
Collectively, the fuel rods are so radioactive as to be lethal in seconds to
anyone near them, and so thermally hot they must be kept in pools with circulating water for 10 years to prevent them from overheating and releasing their radioactive contents into the environment (49). After 10
years or more, the spent fuel can be placed in large containers for dry
storage, where circulating air continues to cool it.
Apart from their own inherent risks, these cooling pools are alarmingly easy
targets for sabotage. Interference with water circulation could result in
overheating of the fuel rods. The possibility of an intense fire involving the
zirconium cladding of the fuel rods could lead to release of massive
amounts of radioactivity.
One of the most critical issues facing the nuclear energy industry is its
inability to permanently and safely dispose of spent fuel, which remains
radioactive and highly toxic for many thousands of years. In Canada,
there are plans for so-called “geologic storage”, which involves burying
the waste in containers a quarter mile down into the bedrock of the
Canadian shield, in hopes that it will remain undisturbed for thousands of
years until it is no longer a danger. There are many potential risks inherent
in this type of storage. The spent fuel will remain thermally hot for many
years and in fact does not reach ambient temperature for 50,000 years
(25). The effect this enormous heat will have on the surrounding rock is
unknown. The integrity of the rock will also have been disturbed by the
drilling of an entry hole, and radioactive material could conceivably
make its way out again by this route. Quite possibly deep aquifers will
have been disturbed and could become contaminated by the
radioactive waste. The earth’s crust is a dynamic entity. To assume it is
possible to predict how it will interact with stored material over thousands
of years is unjustified. For this reason deep geologic storage may not ever
be a satisfactory solution to the disposal of waste fuel.
Reprocessing is at best a temporary answer to fuel disposal. This
procedure allows the removal of many highly radioactive unwanted and
deleterious fission products from used fuel so that it can be used again.
These must then be disposed of. Reprocessing also involves handling
highly radioactive and toxic materials, creating elevated risks for both
workers and surrounding communities (50). One such reprocessing plant
in Ireland expels some of these highly dangerous radioactive waste
products directly into the Irish Sea, making it one of the most radioactive
bodies of water in the world (25). Reprocessing also allows the extraction
of plutonium, used in some power reactors outside Canada and in
nuclear weapons. There are serious concerns about terrorist groups
acquiring this spent fuel for its plutonium content and weapons potential.
Presently there are no reprocessing plants in Canada although one is
being considered for northern Saskatchewan.
(TO BE CONTINUED )
Dr. Cathy Vakil M.D., C.C.F.P., F.C.F.P.
Dr. Linda Harvey B.Sc., M.Sc., M.D.
The authors would like to thank Gordon Edwards, David Martin and Terry Mauer for their help in preparing this paper.