This Report
1.6 Our report is divided into four parts. The first
describes the present situation, giving some of the history and
background to the current wastes in the United Kingdom's inventory.
This part also outlines some of the ways of dealing with nuclear
waste that have been used in the past or advocated to us. In the
second part we analyse the options for waste management in the
UK from the technical perspective, reaching conclusions on the
preferred method. We then look more closely at the constraints
on implementing this waste management strategy, in particular
the question of public acceptability. We conclude this part with
a review of policy and make recommendations for the future. In
the third part we consider related but separate waste management
issues, particularly reprocessing of spent nuclear fuel and the
stock of plutonium. The report concludes with a summary of our
recommendations.
1.7 A general introduction to radioactivity, nuclear
fission and the problems they pose is given in Box 1. The membership
of the Sub-Committee which produced this report is listed in Appendix
1 and the Call for Evidence we issued is set out in Appendix 2.
The Enquiry was based on the assistance of a wide range of individuals
and organisations who responded to the Call for Evidence: these
are listed in Appendix 3. We are most grateful to them all for
their time and effort. We also wish to express our thanks to those
whom we visited, who made presentations or provided briefing:
in particular British Nuclear Fuels plc (BNFL) Sellafield; the
UKAEA, Dounreay; and the many people who were so generous with
their time and hospitality during our visits to the US, Canada,
Sweden and France (see Appendix 4). Thanks go also to the staff
of the High Commission in Canada and Embassies in the other countries
for their assistance with our overseas visits. Particular thanks
go to our Specialist Adviser, Ms Marion Hill of W S Atkins, without
whose experience, expertise, and all manner of assistance, the
production of this report would have been immeasurably more difficult.
We very much appreciate her help and that of everybody who has
contributed to this Enquiry.
1.8 A glossary of terms and acronyms is in Appendix
5.
Box 1: An introduction to radioactivity
The nature of radioactivity
The nucleus of an atom may be considered to contain
neutrons and protons, the number of which is called the atomic
number. Radioactivity originates from the nuclei of atoms that
are unstable because they contain too few or too many neutrons.
In order to attain stability, these 'radionuclides' spontaneously
eject nuclear matter (radiation) as either alpha-particles (nuclei
of helium atoms), beta-particles (electrons), gamma-rays (electromagnetic
radiation), or neutrons. The eventual result is that unstable
atoms transform themselves into more stable atoms of the same
or other elements. For example, when the naturally occurring radionuclide
samarium-147 (with atomic number 147) ejects an alpha-particle
(with atomic number 4), unstable samarium atoms are transformed
into stable atoms of neodymium-143.
The transformation illustrated above is the process
of radioactivity. In some cases (samarium-147 is an example) a
radioactive 'parent' nuclide decays to a stable 'daughter' product.
For the heavier radionuclides, however, a chain of daughter products
may be involved, only the last of which will be stable. For example,
in the naturally occurring uranium chain the parent is uranium-238,
the radioactive daughter products include uranium-234, thorium-230,
radium-226, lead-210 and polonium-210, and the end of the chain
is a stable form of lead. The decay pattern of a single radionuclide
is exponential with time, is characteristic of the particular
radionuclide, and is precisely known.
A useful measure of the decay rate of a radionuclide
is the 'half-life', which is the time taken for half the atoms
in a sample of that radionuclide to transform themselves. For
a given radionuclide, activity and half-life are inversely proportional.
The more active the radionuclide, the shorter the half-life, so
the faster the decay. After a sufficient time has passed, almost
all of a radioactive sample will have decayed to stable products
and be no longer radioactive. That time, however, may be very
long, perhaps millions of years or more if the half-life is large
enough, and in that case the radioactivity will be correspondingly
weak.
For example, the radionuclide krypton-85 has a half-life
of 3934.4 days and it decays to the stable nuclide rubidium-85
by ejecting a beta-particle (electron). After one period of 3934.4
days, 50% of the atoms in a sample of krypton-85 will have transformed
themselves into rubidium-85; after 10 half-lives (107.8 years)
the sample will contain just 0.1% krypton-85 and 99.9% rubidium-85,
and the radioactivity of the sample will have decreased in proportion
to the amount of krypton remaining.
The emission of alpha- and beta-particles is accompanied
by a release of energy, most of which manifests itself in the
rapid motion of the particles ejected. This radiation is so energetic
that it can strip electrons from surrounding atoms, and so 'ionise'
them. The ionisation eventually dissipates itself as heat and
as damage to the surrounding material. Further energy can be lost
as gamma radiation, similar to X-rays but more energetic, which
is also ionising.
Nuclear fission
The emission of free neutrons only occurs in a process
known as nuclear fission, Here the nuclei of certain very heavy
'fissile' atoms, such as those of uranium, which have a large
excess of neutrons over protons (143 neutrons and 92 protons in
uranium-235), when excited by the capture of a neutron, split
into two nuclear fragments, themselves highly radioactive, with
the emission of a few free neutrons. In the fission process large
amounts of energy are released, mainly in the form of energy of
motion of the heavily ionising fission fragments. Thus a mass
of fissile material exposed to neutron irradiation rapidly develops
an admixture of 'fission products', and is therefore highly radioactive,
at least initially.
The free neutrons emitted in the process of fission
in a suitable mass of fissile material may themselves excite further
atoms to undergo fission. This process is enhanced if the neutrons
are first slowed down in a 'moderator' of light materials such
as water or graphite. If the neutrons emitted per fission cause
on average one further nucleus to undergo fission, the result
is a condition known as 'criticality', a self-sustaining chain
reaction. This process, properly controlled, is the basis of energy
production in the nuclear power industry. In a nuclear reactor
heat is generated in the fuel elements and their cladding by the
ionisation caused by the fission fragments, and by the slowing
down of neutrons in the moderator.
Such fission reactions are known to have occurred
in nature about two billion years ago, when the natural abundance
of the fissile uranium-235 (which has a half-life of 703.8 million
years) was much higher than it is today.
The large fragments resulting from the fission process
are in fact the nuclei of medium weight atoms. They are unstable
and they absorb further neutrons, but do not undergo fission.
In a nuclear power reactor they are the principal waste product
of the power generation process. Eventually, due to their ability
to absorb
neutrons, they build up in the nuclear fuel to such
an extent that the fission process is no longer efficient. At
this point the fuel must be replaced. However it may still contain
substantial quantities of fissile material, which may be recovered
and re-used as fuel if it can be separated from the waste fission
products, and this is known as reprocessing.
Problems with radioactivity
The energy released during radioactive decay causes
ionisation in the matter through which the radiation passes. The
concern over radioactivity arises from the resulting damage that
this can cause to surrounding material, especially living tissue.
The term used to describe and measure this damage is 'radiotoxicity':
it depends upon the rate of decay, the type of radiation emitted
(alpha-particle or beta-particle, for example), its energy, and
the nature of the surrounding material. For example, some tissues
are more sensitive than others. In living organisms, radiation
can kill cells, can disrupt genetic material (leading to cancers)
and, if the dose and dose rate are very high indeed, such as in
a nuclear explosion, can kill outright. In medicine, on the other
hand, these properties are used in controlled conditions to treat
cancer, by preferentially killing off tumour cells.
As will be clear from the above, radionuclides can
continue to emit radiation for considerable periods of time. Thus,
when radionuclides are used, measures must be taken to isolate
them from the environment so as to limit their potential for causing
harm. This applies not only to nuclear material while in use,
but also after use, and to the waste products that result. For
some long-lived radionuclides the period of isolation required
may be thousands or even millions of years.
The measures that have to be taken to shield against
the effects of radiation depend on whether it mainly consists
of alpha-particles, beta-particles, gamma radiation or neutrons,
or some mixture of these. Alpha-particles can only penetrate a
few centimetres of air and they can be stopped by a sheet of paper
or an outer layer of skin. Because they lose all their energy
in a very short distance, alpha-particles can be very damaging
to soft tissue (eg in the lung or digestive system if inhaled
or ingested). Beta-particles can penetrate somewhat further, although
they can be stopped by a few millimetres of plastic or metal.
Again, they are dangerous inside the body. Gamma radiation, which
often accompanies beta-particles, will easily penetrate the human
body, and can do both internal and external damage. It can only
be sufficiently attenuated by thick or heavy shielding, such as
lead, concrete, or some metres of water.
Neutrons can also penetrate moderate thicknesses
of matter. In the course of doing so, they slow down by repeated
collisions with atomic nuclei in the material through which they
pass. When a neutron collides with a nucleus there will be a considerable
release of energy as the nucleus recoils. This in turn causes
ionisation and further damage similar to that caused by an alpha-particle.
The neutron is finally stopped only when it is absorbed by an
atomic nucleus, a process made more likely if it has first been
slowed down. As the neutron is absorbed it may cause fission (as
described above), or the release of intense gamma radiation, or
some other process. In their slowing down and eventual absorption,
neutrons cause damage to both living tissue and reactor components.
Water is often used as a shield for neutron radiation
because it can both slow down and absorb neutrons and, if it is
extensive enough, can attenuate the associated gamma radiation.
Natural background radiation
Natural background radiation includes cosmic rays,
gamma radiation from rocks and soils, radon emitted into the air
from rocks and soils, and radionuclides (eg potassium-40) in foods.
The background radiation doses which people receive depend on
where they live, their habits and their diet.
For most people in the United Kingdom natural background
doses are much higher than the dose they receive from all man-made
sources of radiation. The average natural background dose to an
individual in this country is 2.2 millisieverts per year*. The
range of background doses is from about 1 millisievert per year
to about 100 millisieverts per year; the highest doses are in
areas where radon levels are high, such as parts of Devon and
Cornwall. The average dose to a member of the public from nuclear
power is about 0.0004 millisieverts per year and the highest dose
is about 0.2 millisieverts per year**.
*The unit of radiation dose is the sievert, which
is defined in terms of the energy deposited per unit mass of body
tissue, with weightings for the potential of the type of radiation
to cause damage and for the sensitivity of tissues. A millisievert
is one thousandth of a sievert.
**Data taken from National Radiological Protection
Board report NRPB-R263 and MAFF/SEPA report RIFE-3.
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