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Neutrons from spallation (file A08)

by Ludwik Kowalski, Sylvie Leray and David Whittal

A very large number of neutrons must be continuously available to transmute kilograms of radioactive wastes over reasonable time periods. We already know that a subcritical chain-reaction system can multiply initial bunches of neutrons by a large factor, for example 10 or 20. For convenience the term bunch refers to a number of neutrons injected into a subcritical system, from the outside, in each second. The injected bunches must obviously be as large as possible. One way to generate them is through the process called spallation.

It is well known that a cascade of nuclear reactions initiated by a cosmic ray proton in the upper atmosphere produces a large number of secondary particles at the sea level. Similar cascades can take place in solid and liquid materials. A block of material bombarded by high energy protons is called a target. A diagram of a cascade initiated in a target is shown in Figure 4. First all the energy is concentrated on just one particle, then it is shared among several secondary particles emitted along the direction of the initial proton. Secondary particles produce tertiary particles, and so on, as indicated by long arrows. Thus the total number of cascade particles increases while the average energy per particle decreases. The atomic nuclei participating in a cascade often become excited and evaporate particles, mostly neutrons. The spatial distribution of evaporated neutrons is more or less uniform, as indicated by short arrows. For the purpose of this article the term spallation is loosely used as a common reference to all nuclear reactions (cascade and evaporation) leading to production of neutrons.

click to see Figure 4 (use the back button to return later).

The actual number of neutrons per cascade fluctuates from event to event but the average number is a well defined and measurable quantity. That number depends on the kinetic energy of initializing protons and on the material in which cascades are taking place. For example, in lead each 1000 MeV proton is expected to generate 25 neutrons over a length which is close to 110 centimeters. A spallation target for protons of 1000 MeV can thus be visualized as a lead cylinder whose length exceeds 110 cm and whose diameter is 40 centimeters or less. Exposed to a beam of protons, the target acts as a source of neutrons, as illustrated in Figure 5 . The space surrounding the target is called a blanket; it is used for the storage of materials to be irradiated by neutrons. The actual target geometry must be chosen to maximize the number of neutrons in the blanket. Suppose that the beam from an accelerator is 10 mA (which amounts to 6*1016 protons per second) and that each proton produces, on the average, 25 neutrons. Then the number of neutrons per second is going to be 25*6*1016=1.5*1018.

click to see Figure 5 (use the back button to return later).

A subcritical chain-reaction system can be introduced into the blanket to further multiply the number of available neutrons by a large factor. Suppose that a bunch of 1.5*1018 neutrons is introduced into the blanket from a spallation target each second and that the multiplication factor of the self-extinguishing chain reaction is 20. Then the number of neutrons in the blanket, in each second, is going to be 20*1.5*1018 = 3*1019. The dependence of the neutron production rate on the beam current is an important safety feature, nuclear reactions taking place in the blanket, including the chain reaction, can be stopped suddenly by turning the accelerator off. No mechanical neutron-absorbing rods are necessary to control the rate at which heat is generated in a subcritical system sustained by an accelerator.

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