Assertion: heavy water's neutron | /r/HasProperty | directed at fissionable material
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text: It is used to slow the neutrons being directed at the fissionable material, by means of the molecules of the moderator physicaly impacting the incoming neutrons and absorbing some of the kenetic energy they posses, thus slowing them down, in the same way that two billiard balls impacting each other would slow down the incoming one (or both if they were both moving).