The eerie blue glow of Cherenkov radiation given off as charged particles moving faster than the speed of light are slowed in water, in a light analogy to the sonic boom. (photo: Oak Ridge National Laboratory via Wikimedia Commons)

With reports mid-day Tuesday Japan time that high levels of radioactivity were being observed and that spent fuel rods could possibly be on fire, the prospect of complete loss of containment from one or more of the nuclear reactors and/or dispersal of much of the radioactivity from large numbers of spent fuel rods at the Fukushima Daiichi nuclear plant moved closer to being realized.  Because radioactivity and the effects of radiation on people are likely to be in the news for some time to come, now seems to be a good time to present a basic introduction to radiation and the effects of the large doses that could be delivered to people within the immediate zone of the disaster.

Radioactivity and Radiation

To understand radioactivity, it is most useful to consider the planetary model of atoms, where the large, dense nucleus of the atom is analogous to the sun and the small, quickly moving electrons are in orbit far away, just as is seen for planets.  The nuclei of atoms have protons, which have a positive charge, and neutrons, which have no charge, in them.  For chemists, the number of protons in an atomic nucleus is the atomic number.  The periodic chart of the elements arranges chemical elements by atomic number, starting with hydrogen with atomic number one (one proton in the nucleus), helium at two (two protons in the nucleus), carbon at six, etc.

While a chemical element is defined by the number of protons, for each element there can be multiple numbers of neutrons.  For example, there are three forms of hydrogen, with zero, one or two neutrons accompanying the single proton.  Hydrogen as we usually think of it has one proton and no neutrons in the nucleus.  When there is one proton and one neutron in the nucleus this form of hydrogen is called deuterium.  If you have ever heard of “heavy water”, that is water where the hydrogen is enriched for deuterium.  Atomic weight roughly correlates to the sum of protons and neutrons in the nucleus, so normal hydrogen is also called hydrogen 1 and deuterium can be called hydrogen 2.  The multiple versions of one element differing in their atomic weights are called isotopes.  Normal hydrogen and deuterium are called stable isotopes because they do not change over time.  However, hydrogen with one proton and two neutrons in the nucleus, tritium, is unstable.  Tritium can reach a stable state by radioactive decay.  When a tritium nucleus decays, one of the neutrons suddenly becomes a proton by ejecting an electron.  The half life of a radioactive element is the amount of time it takes for half the nuclei in a given sample to decay.

If we had an extremely small balance and could weigh both the tritium nucleus before it decayed and the resulting helium nucleus (since we now have two protons and one neutron in the nucleus) plus the electron that has been ejected, we would find that a small portion of the original mass is missing.  It was Albert Einstein who taught us how to bring this equation back into balance by finding that the missing mass can be accounted for as energy imparted to the electron as it departed the nucleus.  Einstein’s famous equation tells us that the energy imparted to the electron is equal to the “missing” mass multiplied by the speed of light squared: E = mc2, where “E” is energy, “m” is mass and “c” is the speed of light.

Electrons ejected in radioactive decay are one of the three basic forms of radiation, and are called beta particles. Alpha particles are essentially helium nuclei, with two protons and two neutrons, and come from more complex forms of decay of unstable isotopes of much higher atomic weight, such as uranium 235. The third basic form of radiation does not have an associated particle, but is instead a high energy X-ray, called gamma radiation. Gamma rays are ejected during radioactive decay of isotopes such as cobalt 60, cesium 137 and iodine 131.


Radiation Absorption in Biological Tissue

Another term sometimes used for radiation is ionizing radiation. That is because when high energy alpha particles, beta particles or gamma rays collide with matter, they impart their energy to the target they have hit. These collisions are of high enough energy that they break atomic or chemical bonds, creating ions.  Biological damage from radiation in living tissue is found as “broken” molecules.  Biological functions of important molecules can no longer be carried out when they are broken.  Accumulation of sufficient damage in structural molecules such as the cell membrane can lead directly to cell death.  Some forms of damage to DNA can lead to mutations, or changes in the information encoded in the DNA.  In some cases, these DNA changes can result in cancer or birth defects in the next generation.

When describing radiation doses to biological material, we have to think in terms of how much energy is absorbed per unit mass of the material.  Current standards of dose include the gray and the sievert.

One grey is an absorbed dose of one joule per kilogram.  A joule is a watt second.  You pay for electrical power to your home by the kilowatt (thousand watt) hour, which is 3600 watt seconds or 3600 joules 3.6 million watt seconds or 3.6 megajoules.  However, the different forms of radiation have different effects on biological tissue when they interact with them.  When doses are reported in sieverts, those different effects have been taken into account.  The dose is sieverts is the dose in grays multiplied by the “quality factor” of the radiation.  For beta and gamma radiation, the quality factor is one and the doses are the same.  For alpha radiation, however, the quality factor is 20, meaning that alpha particles do twenty times as much damage for absorption of the same amount of energy.  Alpha radiation is not a risk when outside the body since it cannot penetrate the dead layer of the skin.  It is only a risk when alpha-emitters have been ingested or inhaled.

Protection From Radiation: Time, Distance and Shielding

Physicists tasked with the responsibility to devise systems to protect people from harmful effects of high doses of radiation rely on the three concepts of time, distance and shielding.

For time, the idea is to minimize the amount of time that someone is exposed to a high dose rate.  As conditions deteriorate around the Fukushima Daiichi facility, look for workers to be limited in the amount of time they can spend in the immediate area.  In fact, some workers were evacuated from the area Tuesday because of those considerations.

Radioactive material that is not moving acts as a point source emitting radiation in all directions.  That means that as distance from the source increases, the dose of absorbed radiation would go down as the cube square of the distance.  For example, the dose rate two miles from the source would only be one eighth fourth the rate at one mile (two to the third power is eight squared is four), provided that the radioactive material is not being dispersed at the time.

Shielding allows radiation to be absorbed in material other than the biological tissue being protected.  That is why you are given a lead apron to wear when your teeth are X-rayed at the dentist’s office.  The apron is of sufficient thickness to stop the low energy X-rays that bounce off your teeth or jaw bone back toward your body.  Shielding is described in half value layers based on the energy of the radiation hitting it.  In this table, you see that it takes a little over two inches of concrete or a half inch of lead to stop half the gamma radiation emitted by cobalt 60.  A concrete bunker was erected over the remains of the Chernobyl reactor that failed in 1986 and a new concrete structure is now being built to replace the older crumbling one for further shielding from the remains of the radioactivity there.

Acute Effects of High Doses

Although any increased radiation dose has harmful effects on the population exposed, causing delayed effects such as cancer or birth defects, short term acute effects of radiation appear at higher doses.  There are three major syndromes of radiation sickness leading to death.  The lowest lethal doses destroy the bone marrow, leading to death in weeks to months from infection or internal bleeding.  At a higher level, the lining of the gastrointestinal system is destroyed, leading to death within about two weeks.  At the very highest doses, cardiovascular and/or neurological damage leads to death within hours to a few days.  Here are the descriptions and associated dose levels from the CDC:

  • Bone marrow syndrome (sometimes referred to as hematopoietic syndrome) the full syndrome will usually occur with a dose between 0.7 and 10 Gy (70 – 1000 rads) though mild symptoms may occur as low as 0.3 Gy or 30 rads4.
    • The survival rate of patients with this syndrome decreases with increasing dose. The primary cause of death is the destruction of the bone marrow, resulting in infection and hemorrhage.
  • Gastrointestinal (GI) syndrome: the full syndrome will usually occur with a dose greater than approximately 10 Gy (1000 rads) although some symptoms may occur as low as 6 Gy or 600 rads.
    • Survival is extremely unlikely with this syndrome. Destructive and irreparable changes in the GI tract and bone marrow usually cause infection, dehydration, and electrolyte imbalance. Death usually occurs within 2 weeks.
  • Cardiovascular (CV)/ Central Nervous System (CNS) syndrome: the full syndrome will usually occur with a dose greater than approximately 50 Gy (5000 rads) although some symptoms may occur as low as 20 Gy or 2000 rads.
    • Death occurs within 3 days. Death likely is due to collapse of the circulatory system as well as increased pressure in the confining cranial vault as the result of increased fluid content caused by edema, vasculitis, and meningitis.

For external radiation, the doses reported above in grays convert directly to the same numbers in sieverts and would only change if there are inhaled or injested alpha-emitters.

Potential Fukushima Fallout

It is beyond my training to describe or predict the composition of the potential fallout should the bulk of the fuel from one or more Fukushima Daiichi reactors or their spent fuel become airborne in the current crisis. Similarly, I am not qualified to estimate how much material could eventually find its way to the US.  I would, however, point interested parties to this description from the Health Physics Society of potassium iodide tablets and the current wave of buying now being seen on the US west coast:

Potassium iodide can provide important protection for one organ from radiation due to one radionuclide. It can only provide protection for the thyroid gland from an intake of radioiodine. It doesn’t have any value in protecting other organs of the body or in providing protection from radiation from other radioactive nuclides. For example, potassium iodide has no protective value from a “dirty bomb” or a dispersion of spent nuclear fuel. Here’s why.

/snip/

KI has been erroneously represented as a “magic bullet” of radiation protection. KI, if taken properly, only protects against internal radiation from radioiodine taken into the body. It will NOT protect against external radiation or internal radiation from radionuclides other than radioiodine. This salt, if taken either before or very soon after a radioiodine intake and if taken in the proper dose, will block the uptake of radioiodine by the thyroid. KI can be in the form of a pill or a supersaturated solution. The recommended daily dosage for an adult is 130 milligrams. If the thyroid absorbs all the iodine that it needs from the nonradioactive KI, then the radioactive iodine will not be absorbed and will be eliminated from the body mostly by way of the urine. Reducing the amount of radioiodine absorbed in the thyroid will reduce the dose received by the thyroid, thereby reducing the risks of thyroid cancer. Even though there have been minimal side effects (e.g., gastrointestinal effects or rashes) from the use of KI, this substance should only be taken on the advice of health care providers. Again, KI will only help reduce the effects of radioiodine taken into the body and not from other radionuclides.

The only possible sources of large radioiodine releases are from a nuclear weapons denotation and a catastrophic accident in an operating nuclear reactor. Therefore, KI has no protective value from a “dirty bomb” or a dispersion of spent nuclear fuel.

My thoughts and prayers are with the people of Japan as they face difficulties on a scale that was unimaginable only a few days ago.

Author note: My undergraduate degree was in Radiation Biophysics and my first year of graduate school was in Medical Physics before I transferred to Molecular Biology.  I took courses in these subject areas in the late 1970′s through 1980, so I take full responsibility for any errors I may have made in going back over topics I studied so long ago and for any errors in converting to the new terminology now used in describing radiation doses.  Please point out any errors in comments and I will note corrections the post.