BIOL354 – Environmental Toxicology I Spring 2012
Lecture 5: Radioactivity
- Review: PAHs can be anthropogenic or can be naturally emitted through volcanic activity & forest fires.
- Radioactive Elements & Isotopes:
o Naturally occurring radioisotopes @ low level
o Anthropogenic Sources: (CONCERN)
Mining of the reactor fuel - Uranium
Normal operational release of nuclear plants
Accidental releases from nuclear reactors & power plants
Medical or industrial effluents containing radioactive isotopes
Reprocessing of DU for Uranium weapons
Storage of DU @ nuclear cores
o Uranium (6 isotopes, all radioactive/unstable)
Naturally occurring Uranium consists of U-238 (99.2%) & U-235 (0.72%).
U-235 can undergo spontaneous fission; the predominant U-238 is not fissile.
Neutron of 1 U-235 strikes the nucleus of the 2 U-235, 2 nucleus splits into
fission fragments + 2 more neutrons + photons (energy) -> cycle.
U-235 Self-Sustaining Nuclear Fission Chain Reaction -> highly exothermic.
Uranium Enrichment: Removal of some U-238 [U-235 + U-238 = 100%]
Critical Mass: smallest amount of fissile material (U-235) needed to sustain NCR.
Slow & Long-lasting NCR (Nuclear Reactor Grade): Low enriched U-235 (3-4%)
Rapid & Short-duration NCR (Nuclear Weapon Grade): High enriched U-235 (90%)
Depleted Uranium (DU): U-238, by-products of Uranium Enrichment
Extremely dense, less radioactive than natural Uranium (removal of U-235).
Radiation Shield (Medical Clinic) & Armor Piercing Projectile (War).
DU has no biological role = Toxic Metal
o Adversely effects major organs & continual exposure -> birth defects.
o Radioactive t½ = 4.468 bil years, biological t½ = 15 days.
o Powder produced during impact/combustion of DU munitions (Uranium
Bullets) can spread & contaminate areas around.
Iraq: 2000 tonnes DU munitions -> inhalation -> elevated number of
cancers & birth defect s.
- Radioisotopes: Unstable isotopes undergoing spontaneous “decay” to lose excessive neutrons.
o Isotopes: Same proton # but different neutron # (Check atomic mass #)
Different proton # = Different element; Different neutron # = Stable or Unstable element.
o Ionizing Radiation: Sub-Atomic Particles (α & β) & Electro-Magnetic Radiation (γ-rays & X-rays)
Exponential Decay, rate is the isotopes’ half-life (t½ <<1s to > 1000yrs).
Rapid decline @ beginning & later slows down (Never 0 regardless of the # of t½s).
- The Electromagnetic Spectrum
o Non-ionizing Radiation (Grey area): Lower energy photons.
Longer wavelength = Lower frequency = Lower energy = Less dangerous
Radio waves & light (infrared, visible & aV )
Radiation generated by Human > Radiation generated y Microwave
o Ionizing Radiation: Enough energy per photon to ionize atoms or molecules (Higher energy photons)
Shorter wavelength = Higher frequency = Higher energy = Dangerous
Tissue penetration & DNA bond breakage (UV ,bX-ray, γ-rays)
The e & p leads to ROS (free radical) generation in biological systems. BIOL354 – Environmental Toxicology I Spring 2012
- Ionizing Radiation: First releases α or β particles & then γ-rays or x-rays
o After fission (release of Alpha or Beta particles)
UNSTABLE, EXCITED daughter nucleus - γ-rays = Lower energy Ground State
o Alpha (α) Particles = He (Ejected chunk of nucleus with a+charge)
Large subatomic particle (2x protons & 2x neutrons)
New Atomic Mass # (Proton + Neutron) = Original – 4, new element.
New Atomic # (Proton) = Original -2
o Parent Uranium-235 – Helium Nucleus (α) = Daughter Thorium-231
Highly ionizing, minimal penetrating depth due to its huge size
Protection: Stopped by few cm of air, skin, sheet of paper, etc.
Ingestion: Extremely dangerous
o Beta (β) Particles (2 types depending on whether electrons or positrons are generated)
Electron (β decay) (99.9%) Excess neutrons, 1 neutron converted to 1 proton & 1 electron
Positron (β decay) (0.1%) Excess protons, 1 proton converted to 1 positron
Positron (anti-electrons): + charged electron
High speed, very small, travels further
Less ionizing, can penetrate peripheral tissue & damage external organs due to small size.
Protection: Stopped by a layer