Radioactivity, nuclear fuel cycle, radioactive wastes management

From history

1895: H. Becquerel - discovery of radiation

1896: W.C. Roentgen – X rays

1898: M. Curie-Sklodowska - discovery of radium (natural radioactivity)

1934: J. Curie – preparation of first artificial radioactive element

1942: E. Fermi – 1. controlled fission reaction

1954: Obninsk – 1. nuclear power plant

Radioactivity

• Feature of atoms (of given elements) to decay to other elements

Activity

• Amount of radioactive changes per time unit: 1 Becquerel (Bq)= 1 decay per second

Ionizing radiation

• Released by radioactive elements

• X rays

• Cosmic rays

Half time:

• Time period, one half of nuclei decay (from fraction of second up to bil. of years)

Izotope:

• Element is defined by number of protons, might differ by number of neutrons

How to detect and measure radioactivity (ionising radiation)

• We cannot use chemical procedures, but physical effects

  • Vaporous chamber

  • Geiger-muller

  • Photographs

  • Optical features of some matter

  • Changes in features of semiconductors

  • Thermoluminescence

Utilization of radioactive elements in practice

• Neutron activation analysis (detection of unknown elements with very low concentration – 10^-12 g/g

• Marked compounds – monitoring of chemical reactions, technological processes (e.g. blending), observation of metabolism

• radiopharmaceuticals – direct irradiation of cancer tumours

• spas

• sterilization and disinfection (e.g. old wooden objects)

• leak tests

• test of material quality

• material modification – e.g. colour of glass, polymers production

• fire alarms, and many other

Radioactivity and ionising radiation – health and other effects

Absorbed dose

• absorbed energy per unit weight in J/kg (1 Gray = 1J/kg), D_T,R

• effect differ with intensity and type of radiation

Equivalent dose

• respects “quality” of radiation – radiation weighting factor w_R

  • photons, all energies incl. gamma and X rays: 1

  • electrons, all energies: 1

  • neutrons: <10 keV 5

<100 keV 10

<2 MeV 20

<20 MeV 5

• protons: 5

• alpha particles, fission fragments, heavy nuclei 20

Effective dose

• respects type of organ affected, w_T

  • gonads 0.2

  • bone marrow 0.12

  • lung 0.12

  • …

  • skin 0.01

  • bone surface 0.01

Collective dose

• exposure of group of people or population (average dose times number of individuals affected)

Sources of radiation

• cosmic rays (magnetic field of Earth, absorption in atmosphere),

• radioactive background of Earth, differs according to location, typically 1-5 mSv

• nuclear test

• artificial sources (e.g. X ray apparatus, irradiation during cancer therapy, etc.)

Nuclear fuel cycle

1. Uranium mining

   • Classical way

   • Chemical way with use of sulphur acid – possible long term impact on underground water – Czech case

2. Chemical processing – “yellow cake” preparation (concentration)

3. Uranium enrichment (natural uranium consist of only 0.3% of U235 izotope)

• Conversion into gas UF₆, physical methods (slight differences in specific weight, or speed of diffusion)

• Conversion into U₃O₈

4. Fabrication of fuel cells

5. Nuclear power plant – e.g. 2x1000 MW, 30 years, 1826 tonnes of spent fuel

6. Temporary storage (app. 40-50 years)

7. Final disposal