New trends in physics teaching, v.4; The ... - unesdoc - Unesco

knowledge of electromagnetism to the discussion of plasma confinement); (b) understand some
general features of nuclear structure and nuclear forces (no preliminary knowledge is assumed);
(c) understand the basic features of chain reactions and the way they are controlled; (d) apply
basic physical concepts in order to understand the design of nuclear reactors; and (e) understand
and apply the basic techniques in the discussion of safety requirements (e.g. the ‘probability tree’
and, if space allows, the Harrisburg incident as a case study) and the waste-disposal problem.
We feel that, since the nuclear fission energy problem is so hotly debated, authors must state
clearly their personal position at the beginning of the unit, in order not to influence covertly
teachers and students. Our position is - on the whole, and with some disagreement in the authors’
group itself - a moderate pro-nuclear one. But we are, in any case, firmly convinced that two
attitudes must be avoided: one is the ‘don’t disturb the experts’ position. People must insist on
being informed and must try to understand as far as possible the technical and political issues
and the reasons on which decisions are taken. The second negative positionis the ‘atom is the
devil’ attitude: we intend to stress that atomic technology is quite well known and can be understood
by the layman as much - or as little - as any other technology. Risks can be evaluated,
just as in any other field, without absolute certainty and withoat total obscurity, and we must
choose whether we want to run them, or to pay the price for avoiding them. It is probably true
that the atomic choice is part of a general ‘hard’ technological choice and that this may in the
long run lead to disaster; it is also probably true that people’s habits and the social structure cannot
change in a few years, and that in countries like Italy, with little or no local resources, fission
reactors can usefully smooth out the transition to the next generation of energy technologies.
A long description of the two units would be out of place here, and moreover they are not
yet complete; but it is possible to give an idea of some features that have proved interesting
during discussions of the preliminary drafts and classroom trials.
The nuclear fission unit [7] requires an introduction on nuclear structure (whether or not it
must become a separate unit is under discussion). The problem is introduced by the question:
what can we measure about nuclei? The answer is that we can measure very little; only radius
and mass, if we don’t want to consider very sophisticated experiments. Rutherford’s experiment
(as a model for scattering experiments) and a mass spectrograph are sketched. Knowledge
of mass leads to knowledge about energy (E = mc2 wil probably remain a ‘black box’) and we
can draw a curve of binding energy per nucleon, using experimental data. From the curve, we
may derive the main characteristics of nuclear forces. The first feature we notice is saturation,
i.e. binding energy per nucleon grows to a maximum, then decreases slowly. This means that
nuclear forces are short range: each nucleon interacts only with its nearest neighbours, and is
not influenced by the addition of other nucleons outside the interaction range. Coulomb repulsion
then accounts for the decrease in binding energy at the top of the periodic table. The exclusion
principle (already known from atomic structure) accounts for the fact that the number of
neutrons cannot be too high for a given 2; this provides us with a relevant insight about elementary
particles, namely that neutrons and protons are identical so far as nuclear forces are concerned.
Then there are minor corrections, such as magic numbers, parity effects, surface effects, giving
supplementary information.
Another point that proved interesting during classroom trials was the general pattern of fission
reactor design. Once students have understood how fission is controlled, they can draw a list of
possible fissile materials and possible moderators and try to match them. They immediately see
that a poor fissile material, such as natural Uranium, requires a moderator which is a poor
absorber, such as heavy water; if we enrich the fuel, we can use a less sophisticated moderator,
such as natural water, and so on.
We were quite pessimistic about nuclear fusion, considering it a very complicated subject:
Italy
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