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Intuitively I'd expect a nuclear reactor to produce gamma and neutron radiation powerful enough to knock hydrogen atoms/nuclei (or electrons) off $\rm H_2O$ molecules, some of which could recombine into hydrogen/oxygen gas. The web is saying this doesn't happen. Why not?

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This is a very interesting question for which I had to do some research. Here is what I found in this article:

The water in Light Water Reactors does not split into hydrogen ($H_{2}$) and oxygen ($O_{2}$) primarily due to the deliberate control of the reactor's water chemistry and operational conditions.

The article highlights that adding hydrogen to the coolant system prevents the accumulation of oxygen and hydrogen peroxide, which could otherwise lead to corrosion. This process encourages recombination reactions, converting potentially corrosive oxygen radicals back into water.

Additionally, the concentration of boric acid, which is used in the coolant as a neutron absorber, plays a critical role in minimizing water decomposition through radiolysis.

To summarize, managing boric acid levels, the temperature of the reactor, and using chemical additives such as hydrogen effectively reduces the risk of splitting water into $H_{2}$ and $O_{2}$, ensuring the reactor's safety and operational efficiency.

I found many more articles and websites discussing this by searching light water reactor radiolysis on Google Scholar!

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    $\begingroup$ Well, it says "The stable products of water radiolysis are molecular hydrogen, oxygen and hydrogen peroxide. An excess of molecular hydrogen is dissolved in the primary coolant water of pressurized water reactors (PWR) in order to prevent the accumulation of oxidant products..." I think this means that neutrons do break water molecules apart, but that dissolved H₂ catalyzes recombination so well that a separate recombination device is not necessary during normal operation. $\endgroup$
    – Qwertie
    Commented Feb 22 at 8:06
  • $\begingroup$ Ah yes, neutrons do produce radiolysis! Sorry for not stating it explicitly. However, the hydrogen dissolved into the coolant prevents that from happening in the sense of inducing recombination. I hope this makes sense. $\endgroup$
    – cconsta1
    Commented Feb 22 at 11:09
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Gamma radiation initiates radiolysis in water, producing molecular hydrogen. This process is significant even in some old water-filled spent fuel casks, so to prevent that special catalysts were included in construction - burners of radiolitic hydrogen. But, neutron radiation usually does not make significant products of radiolysis.

The reason for that is as follows. Gamma of reactor spectrum (up to 3 - 10 MeV) can react just with outer electron shells of atoms, so it destroys molecular chemical bonds. Neutrons react with the nuclei of atoms and initiate nuclear transformations, including producing unstable radioactive nuclides. For the case of pure water, neutrons just makes heavy isotopes of hydrogen and oxygen without splitting molecules but rates for this process are relatively low.

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