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On a different Stack, I encountered an answer that was making a comparison to nuclear powered naval ships. It was stated that the physical shielding being used was various kinds of metals. This seems... odd to me.

I assume that water is probably being used to cool the reactors in ships. But, water is also excellent at shielding. Considering how abundant water is for the environments that naval ships operate in, do the ships use water for shielding? And if not, why?

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The primary use/purpose of water in light water reactors is neutron flux moderation and cooling/steam generation. Water is only used for shielding during re-fueling. In land based PWR's (pressurized water reactors) the thickness of water to effectively protect workers from the radiation in the core when it is open for refueling is around 23 feet. Basically there's a 23 foot swimming pool (don't go swimming in one) between the fuel rods and the workers.

Shielding with various metals of different thicknesses is much more economical for a ship and a power plant. While there is a lot of water available at sea, there's no need to drag around enough water inside the ship to shield people from the radiation. You may as well sink the boat. Metal shielding is the way to go.

Also consider that nuclear powered naval ships do not often refuel, maybe once every 20 years or never. Since the core will not be opened, no need to provide a fluid barrier.

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    $\begingroup$ Considering that the extent of my knowledge of nuclear fuel and water shielding comes from a web comic, I find a 23 foot thick water shield somewhat mind-boggling. $\endgroup$
    – Ellesedil
    Commented Mar 31, 2016 at 19:54
  • $\begingroup$ I should mention that 23 feet is adequate to protect workers while they are removing fuel rods, which means the rods are lifted higher in the water than 23 feet, so true the 23 feet is not the minimum thickness to reduce the radiation, it is adequate for the task. Still, even if it only required 2 feet of water to reduce radiation there are other factors that make water shielding in a boat an issue: 1) added weight (required volume/weight of water compared to metal would be an interesting comparison) 2) what if the shielding water leaks and can't be replaced? $\endgroup$
    – willpower2727
    Commented Mar 31, 2016 at 20:08
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    $\begingroup$ *replaced without sinking the boat :) $\endgroup$
    – willpower2727
    Commented Mar 31, 2016 at 20:18
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To shield a reactor we have to take care of gamma rays and nuetrons. Neutrons are shielded easily if they are slow. We slow them through elastic collision with a light material and most economical one (water) is used. To shield fron gamma ray heavy material that is lead or iron is used. The answer to your question is that water is most probably used.

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