Radionuclides

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Radionuclides

• Topic 16

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Learning objectives

• To learn about characteristics of radionuclides
  as environmental pollutants what is special
  about them?
• To understand mechanisms of action and health
  consequences of radionuclides

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Natures alchemy radioactive decay

• Uranium-238 thorium-234 - -protactinium---
  bismuth lead
• 10,000,000,000 years
• 14 transformations
• Completely changes chemical and physical nature
  of the element
• E.g. radium (solid) transforms into radon (gas)

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Sources of radioactivity

• Natural
• Cosmic rays
• Natural radioisotopes
• Anthropogenic
• Nuclear reactions (weapon testing, energy
  production)
• X-rays

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Harmful effects to organisms are determined by
three factors

• Nature and intensity of the radioactive decay
• Half-life
• Biochemistry of the radioactive element

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Nature and intensity of the decay

• Electromagnetic photons (gamma (?) rays and
  X-rays)
• High energy
• High penetrating ability
• Beta (ß) particles
• Electrons or positrons ejected from the atom
• Lower energy
• Less penetrating (2-3 mm of aluminum or 2-3 cm of
  flesh)
• Alpha (a) particles
• Chunks of the nucleus ejected from the atom
• Lowest energy
• Stopped by a sheet of paper or outer skin surfaces

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Intensity of the radioactive substance

• SI unit becquerel (Bq)
• Number of atoms disintegrating per second
• Older units Curies (Ci)
• Number of disintegrations per second of 1 g of
  radium
• 1Ci3.7x1010 Bq

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Biologically meaningful units of radioactivity

• Take into account the nature of radioactivity
• Alpha less penetrating but most damaging due to
  large mass
• Beta more penetrating but less damaging
• Gamma highly penetrating, damage similar to beta
• SI unit Gray (Gy)
• Amount of radiation causing 1 kg of tissue to
  absorb 1 joule of energy
• SI unit sievert (Sv)
• Takes into account the RATE at which energy is
  delivered
• Safe annual dose 2 mSv

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Half-life of isotopes
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Biochemical nature of radioactive isotope
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Fate of the radionuclides in ecosystems

• Assimilation
• Varies for different nuclides (100-0)
• Bioaccumulation
• Depends on biological retention time
• Trophic transfer
• Biomagnification
• Isotopes of N, P, K, Na, Cs
• Biodilution
• Isotopes of Ca, Sr (calcium sinks shells,
  bones, cuticles)

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Biological effects of radiation

• Mechanisms of damage
• ROS production
• Breakage of chemical bonds
• Cross-linking molecules
• Damage to DNA, RNA and proteins

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Biological effects of radiation

• Tissue sensitivity
• ? with increasing rate of proliferation
• ? with increasing degree of cell differentiation

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Sensitivity of tissues and organs to radiation

• From most to least sensitive
• Blood-forming organs
• Reproductive organs
• Skin
• Bone and teeth
• Muscle
• Nervous system
• Embryos and young adults more sensitive than
  adults

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Health risks associated with exposure to radiation

• Acute
• Blood count changes 0.5 Sv
• Vomiting 1 Sv
• LD50 (with supportive medical care) 4.8-5.4 Sv
• LD100 (with the best medical care) 8 Sv
• Chronic
• Fetus development
• Cancer risks
• Genetic effects on progeny
• Mutations
• Chromosome aberrations

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Low occupational doses of radiation do not pose
extreme health risks
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Relative Risks (1 in a million chance death)

• Smoking 1.4 cigarettes in a lifetime (lung
  cancer)
• Eating 40 tbsp of peanut butter (aflatoxin)
• Spending 2 days in NYC (air pollution)
• Driving 40 miles in a car (accident)
• Canoeing for 6 min (drowning)
• Receiving a dose of 0.1 mSv of radiation (cancer)

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Summary

• Mechanisms of action and damage by radionuclides
  and radiation are based on their high energy,
  which they can transfer to cells and molecules on
  collision
• Radionuclides can be assimilated and stored by
  the organisms, which mistake them for essential
  elements
• Effects of radiation are particularly pronounced
  on rapidly dividing cells
• There are strong acute effects of radiation and
  small but perceptible risks associated with
  low-dose radiation exposure.

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Oklo fossil reactors natural reactors and
sources of radioactive waste
Uranium (U) ores Orefields several km wide with
pockets of enriched UO2 (70 pure)
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Remnants of the reactor 15
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Nuclear fission of uranium-235
Even numbered heavy mass isotopes negligible
rate of spontaneous fission (SF) Odd numbered
isotopes high SF rate (several fissions per sec
per kg of the isotope)
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Key elements (Pu, Am) need to be retained for ca.
100,000 years to reduce their toxicity
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Natural Oklo reactors Geological waste
containment experiment