Tuesday, August 5, 2014

The (Radioactive) Decay of Western Civilization: Nuclear Physics and the Safety of Nuclear Power

          Nuclear physics and radioactive decay are probably among the most misunderstood scientific topics by the general public.  People have been terrified of nuclear energy since the creation of the nuclear bomb, fear which has only been enhanced both by legitimate disasters such as Chernobyl and the Fukishima Power Plant along with a 60+ year history of films that exaggerate the dangers and misrepresent the science.  Nuclear power plants have a better safety record than the fossil fuel plants that most of our energy currently comes from, while also being more environmentally-friendly.  Many scientists believe that worldwide energy problems could be improved by nuclear power, without the need for investing in newer less efficient technologies, if only they would be accepted by the public.  This post will explain the science behind nuclear power, atomic bombs and radioactive decay, including how they are so often misunderstood by society and the media.

Radioactive Decay
          Radioactive decay is the random breakdown of an atom with an unstable nucleus into a more stable form, releasing energy in the process.  Every radioactive substance has a predictable rate of decay, known as a half life.  The half-life of a radioactive substance is the time that it will take for half of its atoms to decay.  Radioactive decay releases energy, and it is the released energy that is known as radiation.  There are three major types of radiation released from decay: alpha decay, beta decay and gamma decay.  Alpha decay releases an alpha particle, which is essentially a Helium nucleus containing 2 protons and 2 neutrons.  It cannot penetrate materials very deep but is extremely damaging.  Beta decay releases an electron and in contrast to alpha decay, it has greater penetrance but is less harmful.  Gamma decay expels a high-energy photon, which is extremely penetrating while also able to cause DNA damage.  Radioactive decay occurs in all elements of atomic number (number of protons) 83 (bismuth) or greater.  Additionally, radioactive isotopes (varying number of neutrons) naturally exist for many elements under atomic number 83 at a specified ratio in nature.  By utilizing knowledge of the known ratio of these isotopes in nature and their half-life, they are useful for a multitude of processes including archaeological dating, medical imaging, and tracing of biological processes.

Nuclear Fission
          Nuclear fission is very often confused with radioactive decay.  While both involve the release of energy due to nuclear degradation, the two processes are unrelated.  Nuclear fission is usually instigated by bombardment with neutrons and results in the release of two large fragments of somewhat unpredictable size along with neutrons and massive amounts of energy. Unlike radioactive decay which is a controlled spontaneous process occurring at regular intervals that releases defined smaller particles (alpha, beta), nuclear fission only occurs spontaneously at extremely low rates in certain heavy elements (can theoretically occur in elements above atomic number 92, but only realistically observed above atomic number 231), and is typically induced through man-made reactions in only a select group of isotopes.  Isotopes that are able to undergo fission upon bombardment with a high energy neutron (even at low probability) are fissionable, while isotopes that can easily fission with lower-energy neutrons are fissile.  Nuclear fuel for energy reactors or bombs typically utilizes Uranium (U)-235, U-233, Plutonium (Pu)-239 and Pu-241.  These nuclides are all fissile isotopes that are capable of sustaining a chain reaction of neutron release and capture, are relatively abundant and are radioactively stable.

Radiation and Human Health
          People have been fearful of nuclear energy since the first atomic bomb tests in the 1940s.  “Radiophobia” has resulted in an extreme and often irrational fear of anything related to nuclear energy, enhanced by legitimate catastrophes such as the Castle Bravo hydrogen bomb test, the Chernobyl power plant disaster and the recent Fukushima power plant meltdown.  There are definite benefits to being fearful of radiation.  In addition to the production of both outlandish sci-fi movies and successful franchises, the Mutually Assured Destruction (MAD) strategy of the cold war ensured that nuclear war would not actually happen.  However, despite the previously mentioned disasters, nuclear power is overall a relatively safe method of energy production and has caused substantially less deaths than other energy industries, most notably coal mining. 
The concern over the safety of nuclear power plants likely stems from the long-term health risks from radiation exposure, along with a general fear of what we cannot see.  Several myths have been propagated about radiation, perhaps most importantly the linear no-threshold (LNT) hypothesis, which states that the relationship between radiation dose and cancer risk can be extrapolated even to extremely low doses, suggesting that any and all doses of radiation are dangerous.  Public policy concerning radiation safety has been largely based on this model, however evidence shows that this model is false, and extremely low doses of radiation may even help enhance cellular defenses against future DNA damage.
Health risks from power plants and nuclear bomb testing stem from the release of radioactive nuclear waste that is produced as products of the fission reaction.  Radioactive isotopes with short half-lives can deliver high doses and lead to radiation poisoning, while long-lived nuclides will release radiation slower but can contaminate the environment for several decades.  A nuclear power plant could never explode like a nuclear bomb because more enrichment of fissionable material would be required (meaning a higher percentage of fissile isotopes) and reactors also contain control rods to absorb excess free neutronsThe Chernobyl meltdown is the only recorded incident of nuclear accident-related deaths in history and can be blamed on both faulty design and improperly trained workers.  The Fukushima plant melted down due to damage from an earthquake and associated tsunami and was also blamed on both poor planning and improper responses in the aftermath. While several workers were exposed to radiation, there have not been any deaths or cases of radiation sickness due to exposure.  In both cases, the fear of radiation exposure was more damaging than helpful, with as many as thousands of deaths believed to have occurred due to evacuation and advised abortions in response to the threat of radiation damage.  The only nuclear power plant incident that occurred in the US was the partial meltdown of the Three Mile Island power plant in 1979, from which the only consequence was the release of some mildly radioactive gas without any injuries or long term health effects. The United States has extremely strict regulations for nuclear power plant construction, and even in case of an accident existing protocols would likely prevent any sort of disaster.

It is easy to see why the science of nuclear chemistry, radioactivity and fission is often misunderstood by the general public.  While it is debatable whether nuclear power is an worthwhile investment for future energy needs, it should definitely be more-utilized as an energy source as it is both safer and better for the environment than coal or natural gas power (Figure 1).  Nuclear energy does indeed carry risks, but with proper safety precautions it may be a better option than many existing forms of energy production.  Despite constant exposure to radiation on a regular basis from naturally occurring elements in the earth, cosmic rays from outer space and medical imaging, ongoing fear of the potential for even minuscule release of radioactivity from power plants has required creative thinking when proposing new nuclear plants.  Hopefully ideas like this will improve the safety, cost-efficiency and public support for nuclear plants, and they can be a part of our ongoing efforts for cleaner and more efficient energy production. 

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