Radiation sterilization of food and items
Ever wonder what actually takes place when things are exposed to ionizing radiation or whether things exposed to radiation become radioactive? What is ionizing radiation for that matter or perhaps simply, why should I care? Well to start off with, what makes ionizing radiation different from other forms of radiation (such as visible light and microwaves or radiowaves) is that ionizing radiation can ionize materials. To ionize something can be interpreted to say that you are removing electrons from the material leaving it locally charged. Ionizing gamma and x-ray radiation then has enough energy in each photon of light to literally knock electrons off of the atoms to which they were attached and then send them zooming off in some direction. When an atom has had its electron removed in this way, that atom has been ionized.
Ionizing radiation includes such things as the x-rays used in medicine and at the dentist office along with cosmic rays from outer space (among other things). If a sufficiently large dose of ionizing radiation is given to a living organism, it will die (this takes a very large dose). This property is sometimes used to sterilize items such as medical equipment. The radiation dose required to kill off any and all bacteria does not affect the mechanical properties of the sterilized items in any noticeable way and so is a very clean, fast and economical way of doing this work. Lower doses of radiation can be used to partially sterilize food items without using preservatives or other chemicals. When this is done, the food is neither left contaminated nor radioactive from the process. The process does break down some of the chemical bonds in the food in the same way that slightly cooking the food would do. The food is not left in a state that will never perish but rather, it will have a much longer shelf life. Because of this, many people feel food irradiation is an option for a healthy alternative to using chemical preservatives. The same chemical reactions take place in our bodies when we are exposed to radiation but our internal immune system is able to fix and correct for this as we are continually exposed to ionizing radiation from very many naturally occurring sources around us. It is for these reasons also that very large radiation doses can interfere with the operation of sensitive microelectronics (they cannot repair themselves). Ionizing radiation can break chemical bonds in the microelectronics. Likewise, ionizing radiation exposure generates ions in the semiconductors which typically reduce the performance of these devices in making calculations.
There are some exceptions to the general rule that exposing things to radiation will not make them radioactive. Some very powerful particle accelerators have enough energy that the ionizing radiation they generate can leave materials radioactive when exposed to their beams. In the same way, cosmic rays from outer space have enough energy to make exposed materials become radioactive. It is this very process of cosmic rays interacting with our upper atmosphere which generates the carbon-14 so commonly referred to in archeological dating applications. The naturally occurring nitrogen in the air up there gets transformed into the carbon-14 due to the charged cosmic rays continually bombarding the earth. This radioactive carbon then gets taken up into all the plants around the globe and into the rest of the food chain (including ours).
When it comes to more mundane sources of radiation, as a general rule, these are not able to make other things radioactive, they only ionize atoms from materials exposed (which then typically will very quickly neutralize their electron charge due to atomic interactions). One exception to this general rule of not making things radioactive would be industrial neutron sources such as those used in the oil field for well logging applications. The amount of radioactivity these generate in objects around them is typically so incredibly small; it is far below detection limits for many of the more sensitive instruments around. Industrial sources of radiation typically are photon emitters and are not able to make things radioactive by exposure. The other exception to this general rule would be due to powerful particle accelerators, these are able to make photons having energy much larger than industrial or natural sources from the earth.
Stay tuned for next weeks subject which will be an introduction to nuclear medicine.
How to safely handle radioactive contamination
Ever wonder what the different hazards are from being exposed to radioactive materials? There are many different hazards which depend on the type of radioactive material in question. This includes not only the particular radioactive material responsible for the radioactivity but also the chemical form of the material itself. This includes radioactive material concentrated from naturally occurring sources as well as man made radioactive materials. Radioactive materials can be gas, liquid and solids ranging from fine powders to solid metal items.
If the radioactive material gets into your body, the hazards can be completely different than if it remains outside of your body. The amount of ionizing radiation you are exposed to and the rate at which this occurs can also change the physical effects you might expect from large radiation doses. For example, an external dose of 100 rem (a very large radiation dose) can effectively give you a sun burn on your skin if given in a matter of minutes or hours. If this dose is given over a matter of months, no immediate external physical effects would be noticed, only an increased probability of getting cancer later in life.
The key to safety when handling any radioactive material (as with any hazardous material) is proper control of the material. The desired outcome is that handling of radioactive materials never results in what is called “radioactive contamination” or an excessive dose. Radioactive contamination is simply the presence of radioactivity where it is not meant to be. Whether this is in the environment, the city or even worse, in or on people, radioactive contamination is not good.
Radioactive contamination is very different from radiation in the same way that a light bulb is different than light. Radiation is generated and emitted by radioactive materials just as light is generated and emitted by a light bulb. Radioactive materials are the source of radiation but they are not the radiation themselves.
When radioactive contamination finds its way onto people (i.e, their skin), the best removal means is simple mild soap and water (without scrubbing). As a general rule, whatever will remove common environmental dirt and dust from a surface will also remove radioactive contamination. This is because generally, radioactive contamination is just some kind of dust, dirt or material which also happens to be radioactive. When radioactive contamination finds its way inside a person (from eating contaminated foods or breathing contaminated air), removal of the material is much more difficult and much more important. There are special medical treatments for different kinds of radioactive material intakes but the best approach is to insure they are never needed. Preventing an intake comes through good engineering design, proper training and disciplined operations.
The critical difference between contamination on the surface of a person and inside that person is the amount of biological damage which will occur. The most extreme example is that of alpha radiation (this is a rather heavy particle emitted with a lot of energy) such as that which comes from uranium, thorium and many of their radioactive decay products. If alpha emitting radioactivity is contaminated only on the surface of your skin, the large majority of the dose will strictly occur only in the layer of dead skin cells which make up the surface of your skin. Giving a large dose of radiation to dead cells has no health consequence to those cells whatsoever (they are already dead). If the contamination gets inside your body, such as into your lungs or blood stream, then the alpha particles will cause damage to bone surfaces, lung tissue and so on, resulting in known health consequences at sufficiently high doses. Generally, if the radioactivity taken into the body is small (here small typically meaning similar to the amount normally taken in from natural sources), regulatory agencies tend to not play much if any role depending on the circumstances.
The first approach to safely handling radioactive materials is always to use engineered controls as much as possible. Examples of this include encapsulating the radioactive material in very strong welded metal vessels or chemically fixing the radioactive material in a solid single piece. This approach can result in radioactive sources the size of grains of rice (such as used in medicine for some cancer treatments) all the way up to radioactive sources the size of an engine block or bigger for powering interplanetary space vessels. Proper training requires that handlers understand all the correct procedural steps along with the technical basis for those steps. This required knowledge includes all the basic science behind the possible upset conditions and any follow on recovery actions. Having this knowledge will substantially discourage if not eliminate skipping or ignoring any steps in the procedures. Disciplined operations simply means that radioactive material handlers are indoctrinated with following all procedure steps word for word and never trying to find short cuts or quick fixes. This would include all requirements for work approval and control which can mean a lengthy process of revision and review of work instructions prior to conducting any work.
How to safely handle radioactive materials
There is a common saying in the nuclear field and it goes like this, “time, distance and shielding”. What this means is that the primary methods to reduce radiation exposure involve those three items, and here’s why. If you cut your exposure time in half for a fixed radiation field, you cut your dose in half. If you double the distance between you and a radiation source, you cut the dose rate down by a factor of 4. If you double the shielding between you and a source, the dose rate goes down exponentially (which is a fancy way of saying a whole lot). These 3 tools then are on the front line of reducing radiation dose to those who handle radioactive materials. Another item of basically equivalent importance is known as the source term. The source term is the actual radioactive material itself which if cut in half will typically cut dose rates in half. Often this does not help because the source is chosen to give just the needed dose rate and making it less will not serve its purpose, but in some instances this is not the case.
After making a concerted effort at optimizing the factors of time, distance and shielding when using radioactive materials, the focus on handling becomes more detailed. This includes things like personal protective clothing. You could dress out in the most layers of clothing and the highest engineered protection factor clothing allowable but this would typically make the job take a lot longer. Spending longer periods of time exposed to radioactive materials will typically defeat the purpose of the protective clothing. If the job takes twice as long because workers have to wear respirators or bubble suits means their external exposure to any gamma radiation has just doubled. This is just the opposite of the goal in radiation safety to maintain all doses as low as reasonably achievable.
Even after all that, an equally important factor to consider is the chemical form of the radioactive material. If the material is in the form of a respirable powder (a powder so fine it would be like floating dust and you could breathe it in) then very substantial controls would be required. In this case, concern would be on preventing the radioactive material from spreading outside those spaces where it is wanted. The worst case would be allowing it to get inside of people and after this, allowing it to be released into the environment. If the radioactive material is say a piece of metal, control is typically much simpler. In this case, an engineered shield along with appropriate training of users is often fully adequate to control the source.
Stay tuned for next weeks subject which will be contamination with radioactivity.
Understanding radioactive material shipments
When you think of shipping radioactive materials, do you think of shipping kitty litter? How about special medicines for hospitals, smoke detectors or exit signs? Well all of these things (and many more) could constitute actual transportation of radioactive materials. Some may or may not be exempt depending on the amount or concentration of radioactivity in the materials (kitty litter for example often contains only naturally occurring radioactive potassium and so is not controlled). It has been estimated that roughly 1% of all hazardous materials shipped in the United States is radioactive material.
There is a large array of different types of radioactive materials which are shipped around the country (and the world) every day. This is not considering the natural radioactivity present in all things but rather those concentrated amounts of radioactivity requiring differing levels of containment and control. These materials include nuclear medicine for hospitals, materials for industry and of course fuel for nuclear reactors to make that wonderful thing we call electricity. Pretty much anything whose radioactive content has been increased due to artificial means, when moved from one place to another over public areas, constitutes federally regulated transport of radioactive materials. Depending on the amount and concentration, various shipping means are allowed by federal regulations for moving these materials.
The transportation of radioactive materials is regulated by various federal organizations. The Department of Transportation limits things like what kind of placards (signs stuck on the back and sides of a tractor trailer) and radiation levels are allowed on the exterior of shipping containers driving over public roads. The Nuclear Regulatory Agency (NRC) has to approve all type B shipping containers which are used to move materials of higher risk. These containers are required to go through some pretty impressive testing before the NRC will approve their use in public transportation. This testing includes, a 30 foot free fall onto a very hard surface, 40 inch drop onto a steel girder, almost 1500 F burn test for half an hour and immersion under 50 feet of water. With all of these tests, the vessel cannot be found to leak at all in order for that type of vessel to be approved. It is actually pretty impressive so that a container could be caught in a tunnel fire, dumped in a lake or even roll off an overpass and not be expected to rupture, it is very robust.
Some radioactive materials are shipped in simple sturdy boxes if the radioactivity is low enough. This can be due to a low concentration or a small total amount (such as smoke detectors) or only containing naturally occurring radioactive materials (such as lantern mantles, potash and welding rods). The type of shipping container can vary substantially depending on the content. Some shipments of radioactive materials require shielding such as nuclear medicines and spent fuel from nuclear reactors. Typically the shielding material is simply lead but in some rare instances, high density plastic might be used for shielding neutron sources. The legal limits for external dose rates are never zero. Such an insistence would imply that even shipment of foods like potatoes, avocadoes and bananas would have to give “zero” external dose rates because these foods, being full of potassium, generate measurable radiation fields due to potassium being naturally radioactive. Rather, the legal external dose rate limits are deemed adequate by the regulators for protecting the public and the environment when all risks are taken into consideration.
Stay tuned for next weeks subject which will be handling radioactive materials.
For the love of neutrons
Neutrons are lovely little things, even with all of their silly oddities. Typically found in the nucleus of an atom, neutrons have no charge yet they generate their own magnetic field. They hold the nucleus of all complex atoms together through a force that is neither gravitational nor electrical in nature. They do not naturally exist on their own for long as they are radioactive all by themselves and will undergo radioactive decay. It is interesting that if an atom does not have enough neutrons, it will also be radioactive in and of itself. Likewise, if an atom has too many neutrons, it will be radioactive. Kind of like table salt in your diet, too much or too little can be bad.
Neutrons have no charge and so generally are neither attracted to nor repelled from anything. If they get very, very close to some atoms, they can be pulled into that atoms nucleus, which often will leave the resulting atom radioactive. An atom needs to have just the right amount of neutrons in its nucleus to be stable (the nucleus is at the very center of an atom which also contains the positively charged protons and almost all the atoms mass).
Some atoms which are very large have so many protons and neutrons in their nucleus that if you were to split up that atom, both energy and extra neutrons are released among the remaining bits. These extra neutrons can then be utilized to split up additional atoms and continue this process which is known as nuclear fission. A critical state in a nuclear reactor occurs when the reactor has a self sustaining reaction taking place. So long as fuel is adequate, the reaction will continue, and generally only enriched uranium is used for the fuel.
The underlying physical science behind all this is fairly well understood and explained. Neutrons often simply scatter off the nucleus of other atoms. This scattering is similar to different balls running into each other with the neutron typically being the smaller ball. When neutrons interact with hydrogen, the scattering is like that of billiard balls running into each other. It is this very interaction which makes materials with lots of hydrogen preferable for neutron shielding. As with billiard balls, the average velocity of the balls after scattering is approximately half of what the incoming balls velocity was. Examples of materials with large amounts of hydrogen include water, plastic, rubber and to some extent even concrete. By repeating this scattering process many times, the neutron can be slowed down and absorbed by various poisoning atoms in the material. These scattering materials are known as moderators because they slow down the neutrons. In this way, shielding of neutrons is accomplished through moderating the neutrons and then absorbing them into the shielding material itself.
Silly little neutrons, moderation in all things applies even to them and yet they are critical to holding our world together.
Stay tuned for next weeks subject which will be Transportation of Nuclear Materials.
Radiation Shielding Made Simple
If you can’t see, taste, smell or feel radiation, why then do we need shielding from it, and for that matter how much and what kind? Different types of radiation require different kinds of shielding. Some kinds like alpha radiation can be stopped with a piece of paper whereas neutrons are not even stopped by many inches of pure lead! Tricky as this may sound, the key (as with all engineering and technology) is found back in the very basics of simple science.
The fundamental makeup of an atom is that of the tiny little positively charged nucleus being orbited by a cloud of negatively charged electrons. The electrons are those same little bits of charged material that have to travel through electrical wiring to your indoor lighting and through your fancy electronics. The nucleus of the atom on the other hand, is much more subtle and elusive. Unlike the electrons, the nucleus is not only difficult to see but also requires more effort to be readily controlled. The nucleus does not take part in chemical reactions, it does not give off or reflect visible light and it contains almost all of an atom’s mass. When it comes to shielding ionizing radiation, the nucleus is usually only important for neutrons and even then, only a select few nuclei are very good at it. When shielding most kinds of ionizing radiation, it is pretty much just the electrons which get involved with stopping the radiation. Suffice it to say that electrons are pretty important in oh so many ways. They are even that part of the ozone layer which protects us from many of the suns harmful rays. Besides the possibility they can pose of electrocuting someone, if they are accelerated to go really, really fast they can become ionizing radiation and possibly even a safety hazard.
Beta radiation is simply electrons traveling so fast, that when they interact with the electrons of material into which they are penetrating, they are able to knock those electrons away from their host atoms. When the electrons of a material are ripped off in this way, we call the material ionized and the radiation which caused it is referred to as ionizing radiation.
When ionizing photon radiation is being shielded by a simple wall or shell, the effectiveness of the shielding is not so difficult to calculate. If the radiation comes from scattering, the situation can be much more difficult and requires more sophisticated techniques to handle correctly. When a gamma ray photon interacts with material, the most likely interaction will simply be it scattering off of electrons in that material. When the photon scatters, it gives up some of its energy to the electron and bounces off with lower energy in a different direction. This scattered photon may still have enough energy to be ionizing and so must be considered when evaluating the safety factor associated with any given shield design. In some cases, the electron which was scattered away may also have enough energy to be ionizing.
Because an alpha particle is really just the nucleus of a helium atom (i.e., a helium atom with no orbiting electrons), it is slowed down very quickly and effectively in materials. This is mainly due to its higher charge of +2 allowing it to interact with the electrons (having a charge of -1) in a material very easily and effectively. It is for these reasons that a piece of paper has the ability to completely stop most alpha radiation.
The nucleus is made up of positively charged protons and neutrally charged (no charge) neutrons. The attractive force from having opposite charges on the electrons and the protons is what keeps them together making up an atom (with the electrons orbiting the nucleus). The nucleus has a little more to it with the neutrons and protons mixed in there. The neutrons provide the “glue” which holds the positively charged protons together in the nucleus even though identical charges on protons would otherwise repel them and push them away. Neutrons are also a form of ionizing radiation but that will have to wait for the next column.
Radon basics for the layman
Radon is a naturally occurring radioactive gas. It is colorless, tasteless and has no smell. It is heavier than air and is basically in every breath of air we take. Radon is the only decay product from naturally occurring radium, radium itself is a naturally occurring decay product (after many previous decay processes) of uranium. Sound confusing? It gets better, radon has decay products which are also radioactive which decay into other radioactive elements and so on with each decay generating photon and either beta or alpha radiation! Eventually they all decay into different isotopes of lead. If that were not enough, radon is also a noble gas! This means it does not bind to other atoms to form molecules. As such it is able to squeeze through the smallest of cracks and find its way through air, soil and sometimes even rock (if there are sufficient cracks in the rock).
Radon is created every time a naturally occurring atom of radium undergoes radioactive decay. As such, radon is being created at effectively a constant rate at all times in the earth. Radon tends to have higher concentrations in the mornings due to the natural temperature inversions that are usually present then. A temperature inversion often occurs in the morning due to the heat being given off by the earth overnight such that when the conditions are right, the air increases in temperature as you go higher. Because temperatures usually become lower with increasing altitude, when this is reversed, it is called temperature inversion. When a temperature inversion is in place, there is no mixing of the air on the earth’s surface with air at higher altitudes. When radon leaks into the air in these conditions, it tends to stay near the surface because it is heavier than air. Without wind to dilute the radon by mixing surface air with that higher in the atmosphere, the highest radon concentrations tend to occur in the mornings. Likewise, radon concentrations tend to be larger in the winter months when there is less wind than in summer months. Another way of looking at this is that when the earth is heated by the sun, the air next to the ground is heated and rises. Cooler surrounding air then rushes in (sometimes from as far asCanada) to replace the space of that heated rising air. The motion of this cooler air replacing the heated rising air is simply wind, even the very origin of wind (hence there tends to be higher surface radon concentrations in the winter and mornings).
Radium is in pretty much present in all dirt all over the planet, but it is concentrated in some kinds more than others. When a home has a basement which is not sealed airtight against the surrounding soil, radon generated in that soil can leak into the house. Typically, homes do not have problems with elevated radon, particularly if they do not have a basement. Those homes which do have radon problems can substantially reduce the radon with simple ventilation. Opening a window can reduce levels by a great deal in just a few minutes if a cross breeze is present (and blowing through the house). Similarly, filtered ventilation will reduce the airborne radioactivity.
Although radon is a noble gas (and so does not bind to other things to form molecules), all of the decay products of radon are heavy metals. The radioactive decay products of radon are able to bind to other molecules including much larger items such as dust and smaller particles. As such, large fractions of the radon decay products can be bound to breathable dust. These radioactive atoms decay rather quickly. Even though they build up on all kinds of air filters, the vast majority of the radioactivity will have decayed away within a few hours on used air filters after the airflow has stopped. The radioactivity is actually of a form that you can readily measure it with common handheld radiation detectors.
Stay tuned for next weeks subject which will be Radiation Shielding.
Eating and drinking radioactive materials
The intake of too much radioactivity is not only dangerous, it can also be deadly. In fact, in order to realize this, one needs only remember the recent cloak and dagger story of the former Russian spy, Alexander Litvinenko. This man was intentionally poisoned by too much internal radioactivity (possibly through drinking contaminated tea). This only underscores the importance of placing safety controls on storage and handling of radioactive materials. Still, this does not mean radioactivity cannot be safely handled. Other lethal items such as drain cleaner, rat poison and ammonia can be relatively safely handled in your own home. Radioactive material handling does take much more training and equipment to safely handle, but when properly engineered and administered, radiation safety controls and programs have been shown to be effective throughout industry. As in any human endeavor, accidents have occurred and people have been hurt which is only a reminder to never lose our vigilance in demanding excellence from ourselves.
Radioactivity in and of itself is not something to be feared just because it is radioactive. Radioactivity is really just about as natural and normal as anything one can imagine in nature. It should however, be respected and handled in accordance with the level of risk it poses. A unit which places different amounts of radioactivity into measures of risk is the annual limit of intake (ALI). An ALI basically is how much radioactivity you can literally swallow to obtain a lifetime dose of 5 rem (a rem is a unit of radiation dose). The reason 5 rem of dose is chosen is that 5 rem is the regulatory limit of radiation dose a person can get when employed as a radiation worker here in the United States. The value of 5 rem is approximately 6 times larger than the average dose Americans receive each year from natural and medical sources. An ALI is more simply characterized as the legal limit instituted by federal regulators for radiation safety purposes. Most nuclear facilities have administrative control limits much lower than the maximum of 5 rem and often don’t have any workers receiving more than around 0.1 rem from work activities.
There are other ways of comparing different values of internal radioactivity besides legal limits of intake. Intake limits are typically measured in Bq which is equivalent to 1 radioactive atom disintegration per second. One of these comparisons to consider is the naturally occurring radioactivity which we get inside our bodies just as a result of living here on earth. Some examples of approximate averages include 4500 Bq of naturally occurring potassium or 650 Bq of naturally occurring Rubidium. This gets into our bodies from eating regular foods and breathing regular air. Typical water might have as high as 200 Bq per cubic meter of radium alone and air might typically have around 40 Bq per cubic meter of radon and its radioactive decay products (floating around you and being breathed in and out). Lots of numbers I know but the point is, radioactivity is in fact all around us and even a part of us, pretty much of the whole universe as we know it. Other common radioactive materials found in nature include uranium, thorium, and radium. Perhaps one of the more famous radionuclides in our bodies is carbon-14 because it is used in radioactive dating and is also incorporated into all other living things on earth. Make no mistake, this does not mean that unlimited exposures to ever increasing values of these radioactive materials is safe or without risk. Radiation generated from radioactive materials can be deadly and measurably carcinogenic (cancer causing) at sufficiently high levels.
For the isotope Pu239 (this is the plutonium isotope used in nuclear weapons), the legal limit for air concentration is 2 Bq per cubic meter (where the radionuclide would be attached to airborne dust particles). This is based on an assumed year long exposure at this level which would result in a total yearly intake of almost 5000 Bq. At these levels, for this isotope, the yearly dose limit of 5 rem can be expected to an individual receiving this exposure. The specifics of converting intakes of radioactivity to dose are not very simple and depend on chemical form of the radioactivity and details on its intake (whether it was swallowed or breathed in). A trained health physicist can carry out these kinds of calculations for all known types of radioactivity in pretty much any chemical form. The goal of course always comes back to protecting the public, workers and the environment.
Stay tuned for next weeks subject which will be radon.
Can radioactive waste ever be safe?
When you look around you at the city, pretty much everything you see was either grown or mined. As such, almost all these things will certainly be thrown away someday (unless recycled) and so returned back to the earth. This does not just apply to the trash which gets taken out to the curb regularly but also those materials flushed down the toilette or thrown to the wayside. This can be buildings which just got so old that they had to be torn down and thrown away or the road which had to be scraped up and replaced. Even when materials are recycled, some waste is generated but it all sticks around in some form or another. In the end, what makes up a city either ends up in a dump or stays on the surface. This of course also applies to all forms of hazardous waste including radioactive materials.
Hazardous waste can loosely be defined as anything that cannot be safely eaten or drank. More specifically, anything which when put in soil would make that soil unsafe for use in growing food or obtaining drinking water. Such materials typically need special disposal controls. Common household hazardous wastes are bug killers, motor oil and antifreeze along with anything else labeled toxic, poison or corrosive (to give a short list). Typically, the controls for hazardous waste disposal include safe transportation to the disposal facility where the disposal facility would provide sufficiently long term isolation of the waste to control the hazard.
The basic concept behind radioactive waste disposal follows the pattern for hazardous waste disposal in many ways. This starts with carefully documenting everything in the waste containers with sufficient detail that measurement evidence can be provided to determine whether the waste meets the acceptance criteria for the disposal site. Radioactive waste then has to be transported with adequate safety controls (to include such things as posting, shielding and containment) to protect the public and environment in case of an accident. The disposal site itself then has to meet and follow a typically very long list of federal and state regulations to again demonstrate adequate protection of workers, the public and the environment.
One of the unique characteristics of nuclear waste is that when the radioactive decay process has ended, the material is effectively decomposed into just “dirt” again. It would not be the same type of dirt that it came from necessarily. As an example, when radium (which is a naturally occurring radioactive element) is mined from the earth, the other elements present would be chemically separated out to isolate the radium but when the radioactive decay process of the radium has ended, the final product would not have the same elements which were originally removed. Similarly if radioactive cobalt from a medical irradiator were disposed of, after the radioactive elements had completed their decay process, only the element nickel would remain.
The decay of the radioactive waste into stable elements depends on the particular waste itself. Different isotopes decay at different rates. The decay rates depend on certain constants unique to each isotope which can be described in terms of a half life. The radioactive isotope of cobalt used in medical irradiators has a half life of 1925 days. The radioactive isotope of Thorium naturally found throughout all of the known soil on earth has a half life of 14 billion years. The half life describes how long it takes for half of the radioactivity present to undergo radioactive decay. This means if you had a given number of radioactive cobalt atoms today, in 1925 days (just over 5 years), half of those atoms would have decayed into stable nickel and the other half would have still been radioactive cobalt. After another 1925 days, the remaining cobalt would have lost half of its atoms to decay meaning only ¼ or 25% of the original number of radioactive cobalt atoms would still exist (the rest would become stable nickel). Some radioactive materials (most of the naturally occurring ones) decay into another radioactive element which also decays to another radioactive element and so on until they finally decay into stable lead.
Whatever type of radioactivity is being disposed of at whatever concentration, there are federal and state regulations limiting transportation and disposal options to maintain environmental and public safety.
Stay tuned for next weeks subject which will be inhalation and ingestion of radioactivity.
Nuclear Reactor Accidents
The most famous nuclear reactor accidents have been those of 3 mile island, Chernobyl and now Fukushima. The 3 mile island event did not hurt anyone in a measurable way (other than to scare a lot of people). Chernobyl on the other hand did kill many dozens of first responders due to acute radiation sickness because they tried to put out the fire. With the government of the former Soviet Union not dispensing iodine pills to block the uptake of radio-iodine, there was also a measurable increase in thyroid cancers, particularly to children. Likewise, childhood leukemia’s also had a measurable increase over the natural rates prior to the Chernobyl event. Fukushima is almost a mix of the prior two events in a very particular way. Fukushima had a very large release of radioactivity. However, past and future radiation doses from Fukushima have not and are not expected to hurt the public in any measurable way. Both the American Nuclear Society and Health Physics Society have evaluated the radiological health consequences and concluded they are below measurement capability (see http://hps.org/fukushima/#ANSreport).
In order for radiological effects to become measurable, two general categories of radiation exposure are expected. The first is acute radiation syndrome which requires very large doses of radiation. By taking a natural background dose of radiation that you would get in a day, and increasing this by about a million, then giving this to a person in a matter of hours, this will bring about this effect. The other general category is the promotion of latent cancers. If a person is exposed to radiation levels around ten thousand times larger than daily background, a statistically significant increase in the likelihood of obtaining cancer can be expected (just large enough to be measured). The reason for this is that cancer is basically quite natural in that if you live long enough, you naturally have a 25 to 35% chance of getting cancer with no elevated radiation exposure history. In order to increase the likelihood of obtaining cancer from a radiation exposure, the dose has to be pretty high in order to distinguish between natural variations in occurrences for large populations. Kind of like asking which straw broke the camels back when you put them all on at once.
The 3 mile island incidence was a combined effect of not having the greatest design and perhaps more importantly, inadequate training. Chernobyl was the result of multiple disciplinary failures of the operators conducting an unapproved test on a poor design (they were actually trying to prove with the test that the design was better than it was). The largest two factors making Chernobyl so long lasting were the lack of a containment structure (to hold any radioactive releases in) and its use of a graphite core. The graphite core (think charcoal briquettes) caught on fire after the nuclear reaction had stopped causing more of the radioactivity to be released with the smoke. The initial release should have largely been stopped were a containment structure present and the release would have largely stopped after the initial explosion if the core had not caught on fire.
Fukushima was the result of a reactor facility only designed to withstand approximately a 20 foot tsunami yet subsequently subjected to a tsunami more than 40 feet in height. Had the barriers stood, then maybe only half the water would have come in preventing the accident but the excess water pouring over the barriers broke the barriers down exposing the facility to the full force of the wave. Without the ability to continually cool the reactor core, heat slowly built up requiring sporadic release of steam. Even when the nuclear reaction is turned off, the radioactivity in the fuel continues to generate small amounts of heat which if not removed can slowly build up. If the heat continues to grow indefinitely (even if only slowly), then the materials which are solid will eventually want to melt (such as the fuel) and the materials which are liquid (such as water coolant) will want to boil and vaporize. At such extreme temperatures, a process (hydrolysis of the water) can take place where the heat can separate the hydrogen and oxygen in water resulting not in just water vapor but hydrogen gas and oxygen gas. If these gases are allowed to build up in confined areas, a spark can ignite them (hydrogen gas is flammable in the presence of oxygen) resulting in an explosion.
Stay tuned for next weeks subject which will be nuclear waste.
