How does deuterium interacts with neutrons?

[Physicsissuef]

How does deuterium interacts with neutrons? I sow in nuclear power plants is used heavy water, and it says because the heavy water interacts with neutrons? Is it mechanically stopping them, or there is some other kind of interaction?
btw- how neutrons in deuterium stick together, how they are attracted, since they don't have any electrical charge?

 

[shelanachium]

Deuterium in nuclear reactors acts as a 'moderator', slowing down fast neutrons to low speeds where they are more easily captured by uranium or plutonium to induce fission.

Deuterium makes a good moderator because it is only twice as massive as a neutron, so a neutron colliding with it gives up a high proportion of its kinetic energy on each collision and is rapidly slowed to 'thermal' energy, i.e. that of the atoms in its environment.

Ordinary hydrogen, having almost the same mass as the neutron, would seem to be an even better moderator, and indeed in some reactors ordinary water acts as moderator. But ordinary hydrogen captures more neutrons than does deuterium so more are lost before being able to induce U or Pu fission, and enriched fuels (containing more than the 0.7% natural abundance of the fissionable isotope U-235) must be used. D2O-moderated reactors can use natural, unenriched uranium.

Even deuterium captures some neutrons, forming small amounts of H-3 (tritium) which is radioactive (half-life 12 years) though the radiation is of very low energy.

In deuterium there is only one neutron (and a proton), so no neutrons 'stick together' in it. However indeed most nuclei contain many neutrons, and also many protons - these latter having a positive charge might be expected to repel each other. Look up 'strong force' on the web for explanations of why complex nuclei can be stable. It is a force between nucleons (protons and neutrons) which is much stronger than the electrostatic forces causing protons to repel each other, but is effective only at very short distances.

 

[Physicsissuef]

And why some atoms absorb more neutrons then others? Why some radioactive elements need fast neutrons to make nuclear reactions, and some elements not?

 

[shelanachium]

Neutron reactions with nuclei

You will find a discussion of this in this same forum -(Neutron Capture) cross-section.

Slow neutrons tend to react with most nuclei, though with varying ease. Being uncharged, they are not repelled by the nuclei, and just as it easier to catch a slow-moving ball than a fast one, a slow neuton is more easily captured than a fast one.

Fast neutrons are needed when the reaction is one which absorbs energy, which must therefore be provided by the fast neutron, e.g the breakup of a light nucleus. For example Be-9 + n ---> 2He-4 + 2n absorbs energy, so fast neutrons must be used.

Other reactions needing fast neutrons are those with high activation energies, such as the fission of common uranium (U-238). This merely absorbs slow neutrons and does not break up - it produces U-239 which then beta-decays in two steps tp Pu-239. Note that U-238 has an even number of neutrons, and addition of a neutron when there is already an even number releases relatively little energy, so the resulting nucleus is not 'shaken up' much. (Again see post on neutron cross-sections.) However fast neutrons will fission U-238, the energy they bring with them disturbs the nucleus enough to provoke fission.

Pu-239, and also U-233 and U-235, do fission with slow neutrons. Note all these nuclei have odd numbers of neutrons. The addition of another releases much energy, enough to 'shake up' the nucleus so that it fissions.

Hope this helps!

 

[Physicsissuef]

Why fast neutrons of U-238 "shake up" the nucleus, and why fast neutrons does not "shake up" the U-235?

 

[shelanachium]

Fast neutrons can indeed cause U-235 to fission, but much less efficiently than slow ones. Slow neutrons are more easily captured by most nuclei, and the energy released on capture by U235 (which has an odd number of neutrons, 143), is sufficient to provoke fission. That released on slow neutron capture by U-238 (even number of neutrons, 146) is not sufficient to provoke fission, so ONLY fast neutrons can do so.