January 22, 2024 (revised September 17, 2024)

Why is so little Chalk River waste suitable for near surface disposal? 

Extensive research work at the Chalk River Laboratories on nuclear reactor fuels, and in the early days, on materials for nuclear weapons, produced waste with large quantities of long-lived alpha emitters.  This waste is difficult to manage and can even become increasingly radioactive over time.  

According to the International Atomic Energy Agency, because of the presence of long-lived alpha emitters, waste from nuclear research facilities is generally classified as intermediate level, and even in some cases, as high level. This waste cannot be put in a near surface disposal facility because its radioactivity will not decay to harmless levels during the period that the facility remains under institutional control.   

Alpha emitters decay by throwing off an alpha particle, the equivalent of a helium nucleus, with two protons and two neutrons.  The external penetrating power of an alpha particle is low, but alpha emitters have extremely serious health effects if ingested or inhaled. They can lodge in your lungs and cause cancer.

Research at Chalk River and all other nuclear laboratories is ultimately based on three long-lived alpha emitters — thorium-232, uranium-235, and uranium-238. These are the “naturally occurring” or “primordial” radionuclides.  They were created by large stars and then incorporated into the Earth and the solar system when they formed some 4.5 billion years ago.  The waste inventory proposed by Canadian Nuclear Laboratories for the Near Surface Disposal Facility (NSDF) includes over six tons each of thorium-232 and uranium-238.

Each “natural” alpha emitter initiates a decay chain with roughly a dozen radioactive isotopes of other elements such as radium, radon, and polonium.  These elements also occur naturally, but in much smaller amounts because of their more rapid decay. 

When a radioactive element releases an alpha particle, the atomic weight of the product goes down by four.  Uranium-238 decays to uranium-234, with a 245,000-year half-life. Uranium-234 decays to thorium-230, with a 75,000-year half-life. Thorium-230 decays to radium-226, with a 1,600-year half-life.  Shorter half-lives mean greater initial radioactivity. Radium-226 decays to radon-222, with a 4-day half-life.  Radon-222, a gas, builds up in the basements of houses built over uranium-rich rocks.  When it is inhaled it decays into polonium-218, a highly toxic, cancer-causing substance with a 3-day half-life. “Naturally occurring” alpha emitters are clearly harmful.

Hazards increase when uranium and thorium are mined and concentrated from ores and used in their pure form.  Marie Curie, who spent much of her career isolating radium and polonium from uranium, died of radiation-induced leukemia at age 66. She was buried in a lead-lined tomb because her corpse emitted so much radiation.

When thorium-232, uranium-235, and uranium-238 are irradiated in a reactor, as at Chalk River, they absorb neutrons and produce significant quantities of new, man-made, long-lived alpha-emitters.  Irradiated uranium-238 absorbs a neutron and temporarily forms uranium-239.  Uranium-239 transmutes to neptunium-239, which quickly transmutes to long-lived plutonium-239, with a half-life of 24,000 years. 

Plutonium-239 is “fissile” – it can readily support a chain reaction.  It is what the early Chalk River researchers produced for the manufacture of U.S. nuclear weapons, by separating the plutonium from irradiated reactor fuel.  They also used the separated plutonium to make “mixed oxide” (MOX) reactor fuel, mixing it with fresh uranium.

Thorium-232, when put in a nuclear reactor, will absorb a neutron and transmute to uranium-233, with a half-life of 160,000 years.  Uranium-233 also can support a chain reaction, so it can be used in atomic bombs and reactor fuels as well. Chalk River researchers did a lot of work to separate uranium-233 from irradiated thorium-232.

All reactor fuel contains uranium-235.  It is the only naturally occurring isotope that readily undergoes fission and can sustain a chain reaction.  But not all uranium-235 atoms undergo fission in a nuclear reactor.  Instead they can absorb either one or two neutrons and form yet two more very long-lived, man-made alpha-emitters, uranium-236 (half-life of 23.4 million years) and neptunium-237 (half-life of 2.14 million years). 

Nuclear engineers don’t like uranium-236 because it acts as a “neutron poison”, absorbing neutrons instead of undergoing fission.  The longer that uranium-235 fuel remains in a reactor, the more uranium-236 and neptunium-237 are produced. 

Uranium-236 is certainly a part of the Chalk River waste. It is the longest-lived of all the man-made alpha emitters, but for some reason it was omitted from the NSDF inventory.

As noted above, thorium-232, uranium-235, and uranium-238 are the start of three naturally occurring decay chains.  A fourth decay chain starts with man-made neptunium-237 and ends with thallium-205 (the element before lead in the periodic table).  Neptunium and its “progeny” have all decayed away during Earth’s 4.5-billion-year history, but production of neptunium-237 in nuclear reactors (and uranium-233 by thorium-232 irradiation) has “resurrected” this hitherto extinct fourth decay chain.  

Americium-241, found in significant quantities in Chalk River waste, is another starting point for the man-made nepturium-237 decay chain.  Nuclear reactors have also greatly augmented the amounts of radionuclides in the uranium-235 decay chain by producing plutonium-239, and in the thorium-232 decay chain by producing uranium-236.

Early research done at Chalk River to extract (or “reprocess”) plutonium-239 and uranium-233 from irradiated fuel and irradiated thorium targets has created a legacy of buildings (e.g., the Plutonium Recovery Laboratory) and soils (e.g., the Thorium Pit) that are contaminated with long-lived alpha emitters.  Reprocessing was dangerous and caused several accidents. The resulting contamination has never been cleaned up.

Until 2018, highly enriched uranium-235 targets were irradiated in the NRU reactor at Chalk River, followed by dissolving the targets in nitric acid and extracting the fission product molybdenum-99, a “medical isotope”. After extraction of “moly-99”, the other fission products, and the long-lived alpha emitters uranium-236 and neptunium-237 (produced when uranium-235 atoms absorb neutrons instead of undergoing fission), remain in the medical isotope waste.  This waste resembles high-level spent fuel waste and represents one of Chalk River’s most dangerous legacies.

Fuel reprocessing, medical isotope production, and other research activities at Chalk River have produced very significant amounts of waste containing ­­long-lived alpha emitters.  This waste is unsuitable for near-surface disposal.  Much of it is mixed with shorter-lived fission products and cannot be separated from them.  This mixed waste should not be put in the NSDF. 

Detecting alpha emitters in mixed waste is expensive and challenging. Putting inadequately characterized waste in the NSDF would invalidate its safety case.

Unfortunately, the NSDF Project lacks adequate waste characterization procedures.  If the project is allowed to proceed, workers and future Ottawa valley residents could be exposed to unknown quantities of long-lived alpha emitters and suffer the serious health effects associated with them.

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