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Why is standing by a pool of water used to store radioactive depleted rods safe whereas standing by a radioactive piece of metal is harmful? Why is the water in the second loop of a two-loop reactor not radioactive if it is so close (but not touching) to the contaminated/radioactive water of the first loop?

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I will assume that for the pool of water distilled water is used.

As we know, water molecules consist of Oxygen atoms and Hydrogen atoms.

In order for any substance to become a source of radioactivity some of its atoms must be transmuted to a radioactive material.

My understanding is that there is some probability of a Hydrogen nucleus capturing a neutron, turning the Hydrogen into Deuterium. However, Deuterium is stable; not a source of radioactivity.

There is some probability of Deuterium capturing a neutron, turning the Deuterium into Tritium. Tritium has a half life of 12 years.

But clearly the rate of production of Tritium will be very low.

My understanding is that the possible transmutations of Oxygen are into stable isotopes of Nitrogen and Fluorine


Hence in order for the water in the pool to become activated: metal from conduits and so on dissolves into the water at some non-zero rate; these atoms can become activated.

Source of this information:
Answer to question on Quora:
Does water become radioactive after being used in nuclear power plant?



[Later addition]

So we have that the water of a storage facility pool is activated to only a very small degree.

That raises the question: if the activity of the radioactivity is denied opportunity to activate another substance, where does the energy go?

The energy ends up as heat, and that's it.

The emitted radiation is energetic, so it will knock water molecules into separate atoms. Those loose atoms quickly encounter each other again, and form a bond. This bond formation generates heat.

Neutrons decay into a proton and an electron and a neutrino, the proton and electron soon encounter each other and combine, creating a Hydrogen atom, and some heat.

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This always depends on the specifics. You have three questions:

  • Why doesn’t radioactive water emit radiation? If it's radioactive, e.g. because it's tritiated, it does: by definition.
  • Why is standing by a pool of water used to store radioactive depleted rods safe? The water in this case is a shielding against the radioactivity emitted by the depleted rods. So it is safe because the water does what it's put there for: shield you from the radioactivity of the rods.
  • Why is the water in the second loop of a two-loop reactor not radioactive if it is so close to contaminated/radioactive stuff? For that it would need to get "activated". Activation is possible depending on the type of radioactivity (neutrons in particular do that) and the type of material (water doesn't get activated easily). So it will become a bit radioactive, just not nearly as much as the rods themselves.
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  • $\begingroup$ Whatever water does or does not do, I am certain its causes causes were not about definitions. $\endgroup$ Commented Jul 24, 2021 at 15:08
  • $\begingroup$ Thank you, “The water in this case is a shielding” Why is water so special? What are its physical/chemical properties that make water so special that it is used as a shielding? $\endgroup$ Commented Jul 24, 2021 at 18:29
  • $\begingroup$ it's cheap. it contains lots of hydrogen which has the same mass than neutrons so it moderates neutrons well, i.e. they loose their kinetic energy quickly. plus it's liquid and thus can be pumped around. $\endgroup$
    – rfl
    Commented Jul 24, 2021 at 19:06

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