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Radioactive elements in nature exist in rocks, soil, water, and living organisms, known as Naturally Occurring Radioactive Materials (NORM). These include primordial radionuclides present since the Earth’s formation – uranium (U238, U-235), thorium (Th232), and potassium-40 (K40, present in all plants and animals, including common foods like bananas), and products of radioactive decay radium (Ra-226), radon (Rn-222, a noble gas), and rubidium-87, and carbon-14 (C-14) formed by cosmic rays.
C-14 exists in all organic life and is used in carbon dating of once-living organic materials up to ~60,000 years old, as it has a half-life of 5,730 years. These elements are continuously decaying, producing other radioactive materials in the decay chain, as uranium leads to radium and eventually stable lead. Specialized civilian research reactors need less than 20% enriched uranium to produce vital radioisotopes for cancer diagnosis, medical imaging, and industrial tracers.
Fissile, and Fertile Isotopes: The revolutionary 3-stage Indian nuclear power programme requires less fissile U-235 as it uses fertile thorium, with radioactive plutonium waste needs.
- Since U-235 is the only naturally occurring isotope that is “fissile” (readily split to release energy), its concentration (only 0.7% in Uranium, the rest 99.3% is U-238) must be increased to 3 to 5% to sustain the controlled nuclear chain reactions required by most modern power plants.
- U-238 and thorium-232 are not fissile as they cannot sustain a nuclear chain reaction on their own, but are fertile as they can be converted into fissile Plutonium-239 or Uranium-233.
- The path-breaking Indian three-stage nuclear power programme uses fissile uranium-233 fuelled PHWRs in stage 1 (22 reactors in India); its radioactive waste fissile plutonium-239 and fertile thorium-232 are then used in a fast breeder reactor (FBRs) in stage 2 (Kalpakkam PFBR) to produce fissile Uranium-233, which will ultimately be used with thorium-232 in Advanced Heavy Water Reactors (AHWRs) of stage 3. It recycles roughly 97% of the radioactive waste material and provides clean energy at every stage.
- The final 3% waste will be immobilized in glass (vitrification) and stored in deep geological disposal facilities. Some high-level waste, such as Caesium-137 and Yttrium-90, is recovered from PHWR spent fuel reprocessing for use respectively in brachytherapy (internal radiation therapy) and radioembolization (SIRT) for certain cancers.
Enriched Uranium: Enriching Uranium-235 allows for greater technical efficiency by permitting the use of ordinary (light) water as a moderator to slow down neutrons. Without enrichment, reactors typically require more complex moderators like heavy water or high-purity graphite (e.g., the CANDU reactor).
- Low-Enriched Uranium-235 (LEU) 3-5% fuel is used in current commercial reactors.
- LEU+ (5% to 10%) is used to extend the time between refuelling for existing power plants.
- High-Assay Low-Enriched Uranium (HALEU) (15% to 19.75%) is used for advanced small modular reactors (SMRs) and modern research reactors, as it allows for smaller reactor designs and higher efficiency.
- High-Enriched Uranium (HEU) at 20% or higher is used primarily in nuclear weapons (often 90% or more), space-constrained naval propulsion and space reactors, and in about 74 legacy reactors worldwide for research and medical isotope production, where HEU usage is declining.
- 20% is the crucial threshold, as enriching beyond that to over 90% (the critical mass) is easier and faster to achieve, and makes the bomb smaller, lighter, and easier to deliver. Hence, any enrichment efforts beyond 20% is a matter of grave concern and cannot be justified as needed for civilian use.
Medical Isotopes: In healthcare, numerous isotopes are used for diagnostic imaging and cancer therapy. Fission-produced isotopes (Molybdenum-99, the precursor of Technetium-99m, Iodine-131, Xenon-133, Samarium-153) rely on the fission of low-enriched uranium targets in nuclear reactors. Many medical isotopes are produced in cyclotrons (PET isotopes, Thallium-201) or through non-uranium neutron activation (Cobalt-60, Lutetium-177).
India remains a significant importer of isotopes, alpha-emitting isotopes (like Actinium-225), and specific therapeutic radiopharmaceuticals, particularly from the Netherlands, Russia, and the United States. India is building a new dedicated reactor facility in Visakhapatnam for full self-sufficiency and potential export by the mid-2030s.
- Technetium-99m (Tc99m) is the most (about 80%) widely used diagnostic isotope globally.
- Iodine-131 (I-131) is used to image and treat thyroid cancer and hyperthyroidism.
- Xenon-133 (Xe-133) is used for lung ventilation studies.
- Samarium-153 is used for bone pain palliation in cancer patients.
- Thallium-201 is used for cardiac imaging.
- PET isotopes (Gallium-68, Fluorine-18, Carbon-11, Nitrogen-13, and Oxygen-15) are used for PET scans.
- Cobalt-60 is used for gamma radiation sterilization of medical disposables, pharmaceutical drugs, and biological materials, certain foods, and for radiotherapy.
- Lutetium-177, Yttrium-90, and Caesium-137 are used for targeted cancer therapy.
When even old highly enriched uranium stocks are being diluted (or “downblended”) to lower purity levels for civilian use, claims that 60% enriched uranium is required to produce medical isotopes are untenable, especially when the amount is centuries’ worth of civilian needs.
Dr P.S. Venkatesh Rao is a Consultant Surgeon, Former Faculty CMC (Vellore), AIIMS (New Delhi), and a polymath in Bengaluru, drpsvrao.com
