Fundamentals of radiation protection

Transcription

1 Fundamentals of radiation protection Kamel ABBAS European Commission, Joint Research Centre Institute for Transuranium Elements, Nuclear Security Unit Via E. Fermi, 2749, I Ispra, Italy tel , 6 th International Summer School on Operational Issues in Radioactive Waste Management and Nuclear Decommissioning Ispra, 8-12 September 2014

2 Basic radiation physics - Some definitions: element, isotope, uranium enrichment - Radiation types and sources - Interaction of radiation with matter - Principles of radiation detection (gamma and neutrons) - Radioactive materials of interest in nuclear security - Definition of some units used in the nuclear field

3 Some definitions A chemical element is a type of atom that is distinguished by its atomic number (= its number of protons in the nucleus) electrons: negatively charged orbiting around the nucleus protons: positively charged neutrons: neutral nucleus atom An atom is neutral: it has the same number of electrons and protons An atom which has gained or lost electrons is call an ion

4 Some definitions Uranium (U): 92 protons; average mass: 238

5 Some definitions

6 Some definitions Isotopes of a chemical element have the same number of protons as it is the same element but have different numbers of neutrons. They have then different atomic masses. Examples: U-235: uranium 92 protons; atomic mass 235 Number of neutrons = = 143 neutrons U-238: uranium 92 protons; atomic mass 238 Number of neutrons = = 146 neutrons Representation: 235 U or U U or U-238 The U-235 is fissile, the U-238 is not

7 Uranium enrichment The proportion of U-235 defines the enrichment of uranium Natural uranium (NU) contains 0.72% U-235 and 99.27% U-238 Slightly enriched uranium (SEU) contains 0.9% to 2% U-235 Low enriched uranium (LEU) contains less than 20% U-235 Highly enriched uranium (HEU) contains more than 20% U-235 and is qualified weapon-usable ; if the enrichment is higher than 85%, it is qualified weapon-grade

8 Typical U enrichments The enrichment of the fresh fuel used in the Light Water Reactors is between 3% to 5% The enrichment of U reprocessed from LWR spent fuel is around 1% and thus still slightly enriched

9 Unstable Atoms Too many or too few neutrons in the nucleus Seek to become stable by breaking and emitting energy as Radiation. The process is called Radioactivity and the atom is said to be Radioactive Isotopes of elements which are radioactive are called RADIONUCLIDES Isotopes Hydrogen Deuterium Tritium Stable Atom Stable Isotope of hydrogen Radioactive Isotope of hydrogen Legend: = Electron (- charge) = Proton (+ charge) = Neutron (no charge)

10 Radiation types and sources What is Radiation? Radiation is the flow of energy through space and matter. Some examples of radiation are visible light, radio waves, and radiant heat. Radiation can be in the form of particles or waves. Ionizing radiation is radiation that produces ions in matter. It is able to disrupt chemical bonds of molecules and cause biologically important changes.

11 Types of Ionizing Radiations Two classes of ionizing radiations: WAVES PARTICLES Alpha particles helium nuclei Beta particles fast electrons Cosmic rays Assorted particles from neutrons and protons to massive nuclei NON-IONISING IONISING

12 Penetration of radiations Alpha particles can usually be stopped by a very thin barrier like a sheet of paper. Betas (electrons or positrons) can pass through the skin, but are usually stopped by a modest barrier such as a few millimeters of aluminum, or even a layer of clothing. Gammas can be very penetrating and can pass through thick barriers. Several meters of concrete would be needed to stop (attenuate) some of the more energetic gammas. A natural gamma source found in the environment (and in the human body) is K-40, an isotope of potassium. Neutrons are also very penetrating. Some elements, like hydrogen, slow down and capture neutrons. Water is commonly used as a neutron radiation shield.

13 Interaction of radiation with matter Alpha particles 1g 241 AmO 2 for the production of detectors Double positive charge Ionization process. Very easy to shield Very difficult to detect. The energy with which they are emitted is always distinct (signature!). For example Am-241 emits alpha-particles of 5.49 MeV (86%) and 5.44 MeV (13%).

14 Interaction of radiation with matter Beta Particles Negative (electron) or positive (positron) electrical charge Ionization process. Easy to shield Direct detection is difficult. The energies of the beta-particles from a radioactive source forms a spectrum up to a maximum energy - see figure below. Not a signature.

15 Interaction of radiation with matter Gamma Rays The energies of gamma-rays emitted from a radioactive source are always distinct (signature!). For example, 235 U emits gamma-rays, it has a characteristic peak at kev

16 Interaction of radiation with matter Neutrons No electrical charge No direct ionization No direct detection Conversion: neutrons Charged particles Detection

17 Radiation detection aspect As alpha particles can travel only a few cm in air and are readily stopped by a sheet of paper, they are no good candidates to reveal the presence of nuclear/radioactive material. As beta particles can travel only a few m in air and are readily halted by light material (Al, plastic), they are no good candidates to reveal the presence of nuclear/radioactive material. detection of nuclear/radioactive material by stand-off equipment is based on the detection of gamma radiations and neutrons.

18 Radiations of interest in nuclear security Radiological Dispersion Device Any radioactive sources used in industry, medicine, These are mostly beta/gamma emitters, few pure beta (T, Sr-90) Generally high energy gamma (detectable, difficult to mask) Nuclear Weapon based on HEU HEU is an alpha/gamma emitter. Low energy gamma (easily shielded) Nuclear Weapon based on Pu Pu is an alpha/beta/gamma/neutron emitter. Low energy gamma (easy to shield) + neutron

19 Characterisation of a radionuclide Remaining activity (%) - Type of radioactivity - Half-life: time taken for half of a radioactive material to decay 100 After 1 half-life After 2 half-lives Time (years) K-40 (1.3E9 y) Cs-137 (30 y) Co-60 (5 y) For the same quantity of material, the radioactive substance having the shortest half-life will have the highest activity.

20 Some definitions Activity: number of decays (transformations) per time unit undergoes by the radioactive source. The becquerel (symbol: Bq) is the SI derived unit of radioactivity. 1 Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. Analogy: you would say that a source has a radioactivity of 20 decays per second as you would say that a machinegun can fire 20 bullets per second. The curie (Ci) is an older, non-si unit of radioactivity 1 Ci = 3.7 x Bq activity of 1 g 226 Ra

21 Some definitions (radioprotection) The gray (symbol: Gy) is the SI unit of the absorbed radiation dose due to ionizing radiation. 1 Gy = 1 joule/kg Analogy: you would say that some of the bullets but not all reached the target and transmitted their energy. The rad is an older, non-si unit of the absorbed radiation dose 1 Gy = 100 rad

22 Some definitions (radioprotection) The sievert (symbol: Sv) is the SI derived unit of dose equivalent. Dose equivalent = absorbed dose x Q (depends on radiation) x N (depends on body part) To reflect the biological effects of the radiation. Analogy: you would say that some bullets might hurt more than others and that some parts of the human body are more sensitive than others (e.g. bone marrow). The rem is an older, non-si unit of the dose equivalent 1 Sv = 100 rem

23 Natural Sources of Radiation Humans have always been exposed to radiation. The major components of naturally occurring radiation are illustrated below. It is important to compare man-made radiation exposure levels to these natural radiation levels.

24 Man-Made Sources of Radiation Humans are exposed to man-made radiation as well. The major sources are illustrated below. By far, most of the dose comes from medical x-rays.

25 Background Radiation The worldwide average background dose for a human being is about 2.4 msv per year

26 Background Radiation (2) Every food has some small amount of radioactivity in it. The common radionuclides in food are potassium 40 (K-40), radium 226 (Ra-226) and uranium 238 (U-238) and the associated progeny. Here is a table of some of the common foods and their levels of K-40 and Ra-226. Food 40 K pci/kg 226 Ra pci/kg Banana 3,520 1 Brazil Nuts 5,600 1,000-7,000 Carrot 3, White Potatoes 3, Beer Red Meat 3, Lima Bean raw 4, Drinking water

27 Effects of radiation on human health (1) Radiation and living cells When radiation ionizes molecules in living cells it can damage them. If the DNA in the nucleus of a cell is damaged, the cell may become cancerous. The cell then goes out of control, divides rapidly and causes serious health problems. The greater the dose of radiation a cell gets, the greater the chance that the cell will become cancerous. However, very high doses of radiation can kill the cell completely. We use this idea to kill cancer cells, and also harmful bacteria and other microorganisms.

28 Effects of radiation on human health (2) The degree to which each different type of radiation is most dangerous to the body depends on whether the source is outside or inside the body. If the radioactive source is inside the body, perhaps after being swallowed or breathed in: Alpha radiation is the most dangerous because it is easily absorbed by cells. Local deposition of the whole energy (short range). Gamma and neutron radiations are not as dangerous because they are less likely to be absorbed by a cell and will usually just pass right through it. Beta particles can create health problems when inhaled. If the radioactive source is outside the body: Alpha radiation is not as dangerous because it is unlikely to reach living cells inside the body. Beta and especially gamma and neutron radiations are the most dangerous sources because they can penetrate the skin and damage the cells inside.

29 Effects of radiation on human health (3) There are three ways to minimize the risk of radiation exposure: Time: reduce the time of the exposure as much as possible. Distance: the further away from the source of radiation, the better. Shielding: In an exposed area, choose the appropriate shielding!

30 Response to a detection event Nuclear security or safety event If a radioactive source is discovered, appropriate protection measures are required to protect individuals from exposure: 1. Protection of the first responders 2. Protection of the public and the environment Usually, expose to radiation can be reduced to an acceptable minimum by application of proper shielding.

31 Health Effects of radiation Stochastic effects Associated to exposures to low levels of radiation over a long time. The effect (usually cancer induction) is uncertain but its probability to appear is increased. The higher is the (low) dose, the higher will be the probability of the effect to appear. e.g. for a dose update of 9 msv, the probability increase of a deadly cancer is + 0.5/1000 people. The usual risk of deadly cancer in Germany is 80 / 1000 people in 30 years

32 Health Effects of radiation Deterministic effects Associated to exposures to high levels of radiation over a short time. The effects will not appear under a dose threshold. The higher is the (high) dose, the higher will be the severity of the effect Sv: nausea, persistent fatigue, partial epilation, fatality 10% after 30 days Sv: nausea, vomiting, loss of hair, massive lost of white blood cells, fatality 50% after 30 days 6 10 Sv: bone marrow destroyed, fatality close to 100% after 14 days 2000 Ci 60 Co at 1 m distance unshielded : 25 Sv/h

33 Radiation dose uptake: some values 2.4 msv/year: world average dose due to background 1 msv/year: maximum dose uptake (in addition to the background) for the non-exposed workers (public, FLO) 20 msv/year: maximum dose uptake (in addition to the background) for the exposed workers A week in mountains at 2000 m: 0.03 msv Flight Paris New York: 0.02 msv Scanner (whole body): 150 msv

34 Is it dangerous or not? In order to stay below the dose uptake limit it is recommended to fix an intervention threshold: If the dose rate at 1 meter from the source is lower than 0.1 msv/h, it is safe to approach the source for localization and categorization Above 0.1 msv/h at 1 meter do not intervene, but establish a protection boundary such that nowhere the dose rate is >0.02 msv/h and call expert responders 0.1 msv/h = 100 µsv/h

35 Irradiated vs. contaminated Irradiation: a body exposed to radioactive material is exposed to radiation. The longer you stay, the closer you get, the higher your radiation dose. How to reduce the irradiation? reduce time exposure (as for a sunbath, UV are radiations!) stay away from the radiation source use the appropriate shielding Note: irradiation by α, β, γ-ray does NOT cause contamination!

36 Irradiated vs. contaminated Contamination: if you or an object gets in contact with radioactive material, you/it might become contaminated. Contamination can be external (e.g. skin) or internal the human body (lungs, bones...). How a contamination could become internal? By inhaling radioactive dust By ingesting radioactive material By contact with an injury (blood circulation) Note: irradiation by α, β, γ-ray does NOT cause contamination, contamination causes irradiation!

37 Thank you! Questions?