26.08.2015   ACCELERATOR DRIVEN SUBCRITICAL REACTORS

Use of additional neutrons

We show in section 4.1.2 that the additional number of neutrons provided by an ADSR is the number of initial spallation neutrons N0 whatever the value of the multiplication coefficient ks. It is then possible to estimate the addi­tional transmutation capabilities of an accelerator. Typically an accelerator can provide 1.5 x 1025 neutrons/year/mA. Thus if all neutrons are captured in the nuclei to be transmuted, one gets an annual transmutation rate of 25 moles/year/mA. For example, the production rate of 233U would be around 5kg/year/mA. One sees that a 20 mA accelerator, as typically considered for ADSRs, would produce 100 kg/year of 233U, in addition to the production of the subcritical system, which in the best case amounts to 50kg/GWe/year. Thus ADSRs can increase quite significantly the 233U breeding capabilities. For 239Pu the gain is smaller since a fast breeder can produce up to 150kg/GWe/year.

Similarly around 2.5 kg of long-lived fission products could be trans­muted each year per mA proton beam. Since a typical 1 GWe reactor produces each year 4 kg of 129I, for example, a 20 mA accelerator could transmute annually the production of more than ten reactors.

3.1.1 Elementary processes

In a nuclear reactor neutrons are produced, slowed down and captured.

Furthermore, energy is produced by the fission process and, to a lesser

extent, by radioactive decay. The most important nuclear characteristics of

a nucleus present in a reactor are therefore:

• The fission cross section aF.

• The capture or (n, y) cross section ac.

• The number of neutrons q emitted following the capture of a neutron by a fissile nucleus. This quantity is crucial to the possibility of establishing a chain reaction. It can be split into two factors: the probability that an absorption* leads to fission, aF/(aF + ac) = 1/(1 + a) where a = ac/aF; and the mean number of neutrons v emitted per fission. Thus, q = v/(1 + a).

• The scattering cross sections, either elastic o, or inelastic o-in, which control the propagation of neutrons in the medium.

• The atomic mass of the nucleus A which controls the amount of slowing down of the neutron following an elastic scattering. After scattering at an angle в in the centre of mass, the final laboratory energy of a neutron, whose initial energy is E0, is given by

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