Page 33 - Neutrons for Sciences and Society

Horowitz, who had been at the Geneva Conference and had discussed the project with Maier-Leibnitz was satisfied to see that his ideas which had led to the presentation of this project now to be about to be realised, even if he would certainly have preferred construction to be sited at Saclay.
At the same time another group of scientists were studying the possibility of building a European pulsed pile. In a reactor, like those described above, fissile material and a moderator (light water, heavy water or graphite) are brought together to create
a critical assembly29 where the chain reaction takes place. This reaction is controlled by the introduction of matter to absorb neutrons (cadmium, boron)
to have a stable unit which
produces a constant flux of neutrons. One can also envisage a sub-critical assembly which only goes critical momentarily by the introduction either of additional fissile material, or an additional reflector/moderator. This gives rise to the pulsed reactor. The first, IBR-1 (Fig. 1.10), was built in 1960 in the USSR in the Joint Institute for Nuclear Research (JINR). The advantage of this technique
is that with a modest average
29 See section 4.1 for the definition of critical mass.
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Fig. 1.10: A segment of fissile U-235 on the rotating disk passes between two sub- critical plutonium assemblies creating a pulse of fission reactions.
Chapter 1 - Pre-history