FAQ ABOUT RADIOACTIVITY

  

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( HomeScience Radiation → FAQ )

For the past 14 years I have worked in radiation protection and finding safe solutions to the disposal of radioactive waste.  I know that radiation and radioactivity especially the man-made applications such as nuclear power reactors and atom bombs is an emotive subject for many people, and so in this section I try to answer some frequently asked questions about radiation and radioactivity.  Note that this is not intended to be a comprehensive account of all aspects of radiation - a search on the web for any of the terms used in here will give rise to much more detailed accounts, for example how power plants work.

 

 

General

What is radiation?
Where does it come from?
Are there different types of radiation?
What is a radionuclide?
What does "half life" mean?

Natural Radiation

What is natural background radiation?
What are cosmic rays?
Where does natural uranium come from?

Health Effects

What will radiation do to me?
What are the symptoms of radiation?
Is there a safe level of radiation?
Can radiation be good for me?
What is a radiation dose?
How do you calculate it?
Are all types of radiation equally dangerous?
What about medical uses of radiation?
What's the most dangerous type of radiation?
How much radiation will kill me?
What do "stochastic" and "deterministic" mean?
Are some radionuclides more dangerous than others?

Power Generation

What is nuclear fission?
What is nuclear fusion?
How do nuclear power plants work?
Do power plants produce much waste?
What happened at Chernobyl?

Radioactive Waste Disposal

How do we dispose of nuclear waste?
How do we know the disposal method is safe?
Can you prove that?
Why is deep disposal the best way?
But how can you be so sure?
Are there any "rules" for radioactive waste disposal?
What other options are there?
What are the issues facing waste disposal?
What happens to the waste before being disposed of?

 

What is radiation?

Radiation is one of the by products of radioactive decay of unstable atomic nuclei.

All atoms consist of a nucleus surrounded by a number of electrons.  The nucleus consists of protons and neutrons, with the number of neutrons being slightly larger than the number of protons for most nuclei.  The number of protons in the nucleus defines the element to which the atom belongs.  Atoms for which the nuclei have equal numbers of protons but different numbers of neutrons are called isotopes.  Thus, strictly speaking, all isotopes belong to the same atom, since the number of protons remains the same.  In neutral atoms, the number of protons and the number of electrons are equal.

Only certain nuclei are stable, in the sense that they retain there structure indefinitely.  For example, all carbon atoms have nuclei with 6 protons.  Carbon nuclei with 6 or 7 neutrons (plus the 6 protons) are stable (i.e. are stable isotopes of carbon).  Carbon nuclei with 8 neutrons are unstable and undergo radioactive decay.  This unstable isotope, carbon-14 (so-called because the nucleus has 6+8=14 particles in it), occurs in large quantities in the upper atmosphere.  Unstable species such as carbon-14 are described as being radioactive.

When such unstable nuclei undergo radioactive decay, radiation is emitted and the result is a new nucleus, and for many types of radiation a different atom (because the number of protons changes during the decay).  There are various types of radiation, and these are described below.

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Where does it come from?

Radiation and radioactive decay occur wherever radioactive materials are present, including natural situations.  Man-made sources of radiation include power stations (discharge of radioactive materials into the atmosphere and water bodies), medical applications of radiation (x-rays, treatment of certain cancers) and of course nuclear weapons.  Natural sources of radiation include the presence of radioactive elements (in particular uranium and thorium) in soils and rocks, and cosmic rays.

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Are there different types of radiation?

There are three principle types of radiation that occur naturally:

1.  Alpha radiation;

2.  Beta radiation;

3.  Gamma radiation.

Alpha radiation is usually only produced when heavy nuclei decay (by heavy we mean atoms with more than about 82 protons in their nuclei).  An alpha particle is in fact a helium nucleus, and is a particle that consists of two protons and two neutrons.  The result of an alpha decay is a nucleus with two fewer protons and two fewer neutrons.  Alpha radiation is the least penetrating type of radiation, and in fact is unable to penetrate the upper dead layer of skin on the human body.

Beta radiation occurs when either:  a proton in a nucleus becomes a neutron, or a neutron becomes a proton.  Beta particles are electrons or positrons.  In the former case, the decaying nucleus emits a positron (an electron with a positive charge) and is left with one less proton, and in the latter case the nucleus emits an electron and is left with one more proton.  Beta radiation is moderately penetrating.  It can penetrate skin, but has trouble passing through clothing.

Gamma radiation occurs when the nucleus undergoes an internal re-arrangement of the neutrons and protons.  Gamma radiation is simply very high frequency electromagnetic radiation (i.e. is similar in form to radio waves or visible light, but with a much higher frequency), and usually occurs in conjunction with alpha or beta decay.  Gamma radiation is the most penetrating of the three types of radiation, and can pass straight through the human body.

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What is a radionuclide?

The term radionuclide is the technical term for an isotope with an unstable nucleus.  Carbon-14 is an example of a radionuclide.

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What does "half life" mean?

The term half life refers to the time required for half of a given number of radioactive atoms to decay.  The half life of carbon-1400 is about 5730 years.  Therefore, if we start with 1,000,000 carbon-14 atoms, after 5,730 years we will be left with 500,000 atoms.  After 11,460 years we will be left with 250,000 atoms, and so on.

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What is natural background radiation?

Natural background radiation refers to radiation that occurs naturally, as opposed to through the activities of man.  The two principle sources of natural background radiation are the presence of naturally occurring radioactive materials (in particular isotopes of uranium and thorium) in soils and rocks, and cosmic rays, which originate from outer space.  Radon is a radioactive gas that causes serious problems in many homes, and it originates from the presence of natural uranium and thorium in the ground.

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What are cosmic rays?

Cosmic rays are nuclear particles that originate from outer space.  Some originate from the sun, whereas the remainder come from further a field.  Cosmic rays are mainly high energy protons, and when they reach of the top of the atmosphere they interact with the materials and gases there, in the process producing additional nuclear particles.  Some of these particles pass through the atmosphere and reach the ground surface.  The atmosphere attenuates cosmic rays so that their intensity at ground level is not as high as it is in the upper atmosphere.  Interestingly, frequent flyers, such as pilots, need to have their radiation dose due to cosmic rays monitored.

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Where does natural uranium come from?

Natural uranium (and other naturally occurring radioactive species such as isotopes of thorium) have been present in the ground ever since the earth was created, and were themselves created during the Big Bang!  Isotopes such as uranium-238 and thorium-232 are still present in the ground because they have very long half lives, and simply have not decayed away through the life of the universe (about 15 billion years).

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What will radiation do to me?

The effects of radiation on humans depends on the magnitude of the dose received.  Low doses of radiation result primarily in an increased risk of cancer or of passing on hereditary defects to offspring.  These effects are often called delayed effects, as they don't happen at the time of exposure.  Higher doses produce more immediate effects (called early effects), such as nausea, vomiting, skin burns and disruption to the internal body functions.

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What are the symptoms of radiation?

For high doses of radiation, the symptoms often include nausea, vomiting and diarrhoea.  If a powerful source of radiation is placed in the hand or a pocket, then a radiation burn is very likely to occur on the skin, especially if the time of exposure is long.  Further symptoms of high levels of radiation exposure include failure of the haematopoietic system (responsible for the production of blood cells and hence the body's ability to fight infection), failure of the gastrointestinal system and failure of the central nervous system.  Such failures are of course likely to result in death.

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Is there a safe level of radiation?

A difficult question, and one that may never be answered, because for low radiation doses it is difficult to determine if a cancer actually resulted from receiving low levels of radiation, or whether some other cause was responsible.  The current hypothesis is that even the smallest levels of radiation result in an increased risk of cancer.  Radiation experts call this the LNT (Linear no Threshold) hypothesis.  It is unlikely that this view will change in the foreseeable future.

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Can radiation be good for me?

Some scientists believe that low levels of radiation can be good for you, though this view is not shared by everyone.  The idea is that radiation can cause otherwise "sluggish" cells in the human body to respond and reproduce in a more normal manner.

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What is a radiation dose?

Radiation dose is a numerical way of estimating the amount of radiation absorbed by the body, sometimes referred to as radiation exposure.  The simplest estimates of dose simply calculate the amount of radiation energy deposited in the body.  The usual type of dose estimated by radiation experts is called effective dose and takes into account the varying sensitivities of the different parts of the body to radiation, and also the fact that different types of radiation have varying levels of danger.

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How do you calculate it?

Radiation dose is computed by estimating how much radioactive material is taken into the body (for example by eating contaminated food or inhaling contaminated air) and then multiplying that by the effective dose resulting from inhaling or ingesting unit amount of the radioactive material.  The effective dose per unit intake is computed from mathematical models that calculate the transport and radiation properties of radionuclides in the body.  Such models are called biokinetic models.

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Are all types of radiation equally dangerous?

No.  Alpha radiation is about 20 times more dangerous than beta radiation and gamma radiation.  That is, a given amount of alpha radiation causes about 20 times as much damage to cells in the body, compared with the same amount of beta and gamma radiation.  This is because alpha particles are heavy and carry a bigger electrical charge than beta particles and gamma rays.  This in part explains why isotopes of plutonium are so dangerous, as most of then ones in common use are alpha emitters.

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What about medical uses of radiation?

Various radioactive isotopes are used in medical applications, for example thin needles that are used to irradiate cancerous cells in the body ( a procedure called brachytherapy).  During the course of such treatment, the patient will unquestionably receive a radiation dose to healthy cells in the body, and hence possibly be subject to adverse effects of radiation.  However, in deciding whether such medical treatment is appropriate, it must be decided if the benefits of the treatment (ridding the body of an existing cancerous growth) exceed the costs (receiving extra radiation dose and possible subsequent effects).

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What's the most dangerous type of radiation?

Alpha radiation is the most dangerous type of radiation, and give rise to substantially higher effective doses than beta particles or gamma rays.

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How much radiation will kill me?

Before answering this question, we need to briefly discuss how radiation dose is measured.  Effective dose, as described above, is measured in a unit called a Sievert, which is usually abbreviated to Sv.  The demarcation between "low and "high" dose depends on a number of factors, not least the time period over which the dose is received, but as a rule a total dose of 0.5 Sv is considered to be the lower limit of what might be considered a "high" dose.

Now, the answer to the original question depends on whether we are talking about "low" or "high" levels of radiation.  For "high" levels of radiation, a dose of around 10 Sv will be sufficient to kill most people, usually by failure of the major internal bodily functions (nervous system, immune system, gastrointestinal system).  At "low" levels of radiation, the question is to consider the magnitude of the risks of getting a cancer (or serious hereditary effect).  This is because, in contrast to "high" levels of radiation, it is not certain that any effect will result.  We therefore have to deal in probabilities that a cancer will be induced (risks in this context can be considered as probabilities).  In the UK, the natural background radiation dose (about 2 mSv per year) results in a probability of about 0.0001 per year of getting a cancer.  In simpler language, this means that one year's worth of exposure to natural background radiation will result in a  probability of 1 in 10000 of cancer induction.

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What do "stochastic" and "deterministic" mean?

In this context, stochastic and deterministic refer to the nature of the response of the body to low and high levels of radiation respectively.

The term "stochastic" implies that an increase in radiation exposure results in an increase in the PROBABILITY of an effect occurring, i.e. the induction of a cancer.  There are two other features of stochastic processes that apply in the context of radiation.  First, there is no threshold with stochastic processes.  That is, even an infinitesimal amount of radiation exposure carries with it an infinitesimal probability (risk) of cancer induction at a later stage.  Secondly, doubling the radiation dose results in a doubling of the probability.

The term "deterministic" implies that an increase in radiation dose results in an increase in the CONSEQUENCE, for example increased severity of skin burns after contact with a powerful radiation source.  This should be contrasted with stochastic effects, where it is the probability of occurrence that increases with increasing dose.  Deterministic effects also have some characteristic features.  In particular, there is usually a threshold level, below which deterministic effects are not observed.  For example, deterministic radiation effects are not usually observed below doses of about 0.5 Sv.

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Are some radionuclides more dangerous than others?

Yes.  As a rule, alpha emitting radionuclides are more dangerous than beta and gamma emitting radionuclides, especially if they are taken internally into the body (e.g. through eating contaminated food or inhaling contaminated air).  External irradiation by beta particles and gamma rays can also be a serious problem for some radionuclides, for example through standing on contaminated ground or holding a powerful radiation source.  Dangerous alpha emitters include plutonium-239, and cobalt-60 is a strong gamma emitter for which external irradiation can be a problem.

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What is nuclear fission?

Nuclear fission is the process on which the operation of most nuclear power plants is based.  When certain nuclei (for example that of uranium-235) are bombarded with neutrons, the nucleus splits into two smaller nuclei of roughly (though not exactly) equal sizes.  Because of the binding characteristics of the neutrons and protons in the original and resultant nuclei (and also the famous mass-energy relation that Einstein discovered), the result of this split is the liberation of a large amount of energy, manifested in the kinetic energy of the resultant nuclei.  That is, when a uranium-235 nucleus undergoes a fission, the fission products fly apart from the original position at great speed.

Extra neutrons are also produced during this fission process, and these in turn can interact with further fissionable nuclei to induce further nuclear fission reactions.  In turn, these fission reactions produce yet more neutrons, and these can then induce further fission reactions.  This process of inducing successive fission reactions is termed a chain reaction, and the energy produced during these fissions is the basic energy that is utilised to produce electrical energy in the power station.

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What is nuclear fusion?

Nuclear fusion can, in some senses, be considered to be the opposite of fusion.  Instead of splitting heavy nuclei, in the fusion process light nuclei are forced to combine to form a larger nucleus.  Again, the binding characteristics of the original nuclei and the resultant nucleus (and of course the mass-energy relation) ensure that energy is released during the fusion process.

The fusion process can only occur at very high temperatures, because of the need to force nuclei with like charges together.  The fusion process occurs in many forms in the sun (where the temperatures are high enough for fusion to occur), and is the basic principle on which the H-bomb operates.

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How do nuclear power plants work?

Most nuclear power plants are based on the fission process, the fissionable material being uranium-235.  Uranium-235 occurs in uranium that occurs naturally in the ground, but not at a sufficient concentration for it to be possible to maintain a fission chain reaction in the reactor.  The first part of the "nuclear cycle" is therefore to enrich naturally occurring uranium, so that the concentration of uranium-235 is sufficient.  This is in fact a rather difficult procedure, as the predominant isotope in naturally occurring uranium is uranium-238, which is chemically identical to uranium-235.  These two isotopes also have virtually identical physical properties too.

This enriched uranium can then be used to establish a fission chain reaction in a nuclear reactor.  The energy generated through fission is used to heat water and generate steam, which in turn is used to drive electrical generators and produce electricity.

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Do power plants produce much waste?

Nuclear power plants generate radioactive waste at all stages of electricity generation.  During the operation of the plant, waste is generated and liberated into the environment.  This includes releases of gaseous products into the atmosphere, and liquid effluents into water bodies (e.g. rivers or the sea).  Further waste is generated when a batch of nuclear fuel (the enriched uranium) has been used up and needs to be replaced with a fresh batch.  The waste includes the fission products, and also heavy atoms generated through the bombardment of uranium with neutrons.  This waste needs to be processed (often referred to as reprocessing), stored, and eventually disposed of.

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What happened at Chernobyl?

The accident at Chernobyl occurred when a fission chain reaction was allowed to spiral out of control.  Fission processes in nuclear reactors need to be very carefully controlled - if the fission becomes too vigorous, then it needs to be slowed down, usually by inserting rods made of carbon that absorb the excess neutrons and reduce the rate at which nuclear fission occurs.

The reactor at Chernobyl was of a design that could, under certain circumstances, promote unstable fission chain reactions.  The explosion at Chernobyl occurred during testing in which the control of the fission process was undertaken manually by power plant workers (rather than by computer).  This is an inexcusable breach of safety regulations that simply would not be allowed to happen today, and one cannot blame the plant workers for what happened, who were merely acting on orders from their superiors.

This mode of operation, along with the design flaws in the reactor design, resulted in a fission chain reaction that could not be prevented from spiralling out of control by the power plant workers.  The consequences of this loss of control are painfully well known.

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How do we dispose of nuclear waste?

The disposal of radioactive wastes, such as those produced during nuclear power production, depends on the nature of the wastes.  Wastes are usually classified according to the danger they pose, and the classifications include Low Level, Intermediate Level and High Level radioactive waste.  These are usually abbreviated as LLW, ILW and HLW, respectively.  Some LLW is sufficiently inert that it can be disposed in ordinary household land-fill sites.  Most LLW and some ILW can be disposed in "shallow" waste repositories, built a few metres or more under the ground surface.  At the present time, it is believed that the remaining ILW and HLW is best disposed of in deep waste repositories, built many hundreds of metres under the ground.

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How do we know the disposal method is safe?

One can never say with 100% certainty that any radioactive waste disposal is completely safe, in much the same way that no other activity (e.g. crossing the road) can be considered totally safe.  However, before a disposal facility is built and operated, it undergoes extremely rigorous safety analyses to ensure that it will perform to the required safety levels (I do this type of work for a living).  In the UK and most other countries, this analysis is subject to public enquiry and can only be authorised by the government.  Perhaps because of this, very few deep disposal facilities have been authorised around the world.

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Can you prove that?

As noted above, one cannot demonstrate that a disposal facility is totally safe, in the sense that no radioactive wastes will ever return to the environment accessible by humans (and animals and plants).  Most safety assessments attempt to predict the quantities that will return and what the radiological consequences will be.  In the UK and most other countries (except the USA ...) it must be demonstrated that the radiological effects are negligible over a time period of at least 1,000,000 years.  The safety assessments consider the various mechanisms that will return radionuclides to the accessible environment, the most important of which is usually the dissolving of wastes in groundwater and their subsequent migration through rocks back to the ground surface.

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Why is deep disposal the best way?

Deep disposal is considered by many people to be the most effective means for disposing of ILW and HLW.  The reason is that deep disposal provides the greatest degree of isolation of the wastes, and hence would require the greatest length of time before the wastes could migrate back to the accessible environment (and hence give the radioactive materials the greatest amount of time to decay).  Although deep disposal might not completely prevent some wastes from re-appearing (especially those with long radioactive half lives), the time required for this to happen would result in considerable dilution of the wastes, with a consequent reduction in radiological consequences.

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But how can you be so sure?

Because the other options simply don't provide the same advantages of deep disposal.  For example, shallow disposal (for LLW and some ILW) provides a greater opportunity for future human activities to disturb the repository contents.  Indefinite storage at the ground surface (another often-cited alternative) is susceptible to terrorist and other deliberate attacks, for example 9/11 style sabotage.

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Are there any "rules" for radioactive waste disposal?

Yes, and in most countries the rules are very strict.  In the UK, the performance of any waste disposal facility is subject to radiological risk limits and targets.  Radiological risk is a measure of the probability of cancer induction from repository derived wastes, and in the UK, the risk limit is this:  in one year, there should be nor more than one fatal cancer induction per million people from repository derived wastes.  To provide some context, this risk limit is about 100 times smaller than the radiological risk that results from natural background radiation.

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What other options are there?

Apart from burial underground, the (serious) options include burial under the sea bed and firing the wastes off into space.  However, burial under the sea bed is banned by international law, and firing into space can easily be shown to be a hopeless option.  The problem is this.  While putting the wastes into space would provide the ultimate isolation of wastes from humans on earth, the consequences of a rocket crash during lift-off are unimaginable - imagine all that HLW being falling fro the sky and being scattered over the ground surface.  The safety record of rockets is not good enough to rule out the possibility of a crash, and it would be very easy to show that the radiological risks from this option are unacceptably high.

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What are the issues facing waste disposal?

There are many issues facing radioactive waste disposal at the present time.  One of the principal concerns at the moment is that there are large quantities of waste around the world currently in storage, waiting for someone to take a decision about what to do with them.  This is certainly true in the UK, where the refusal of planning permission for an underground laboratory at Sellafield in Cumbria threw the UK's deep disposal plans into chaos.  From a personal perspective, I think that a big issue is the communication of the issues to the general public.  To non-experts, the issues must seem very confusing, and unfortunately the safety analyses of disposal facilities and the results of those analyses are technically complex and challenging to understand - even for experts!  Many people, perhaps understandably, equate all talk of radioactivity with what happened at Chernobyl and atom bombs.  Convincing folk that this is not the case is a major challenge.

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What happens to the waste before being disposed of?

Most HLW and some ILW is extremely radioactive, and when it is removed from the nuclear power reactor where it is generated, is extremely hot (because of the high levels of radioactivity) and will remain so for many years.  Such wastes are stored in cooling ponds until the excess heat and radioactivity has been dissipated.  Once this has happened, the wastes go into surface storage until a satisfactory and permanent means of disposal can be found ...

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