Why is it that the water which constantly passes through the core of a nuclear reactor doesn't become radioactive? Despite passing so closely to the active nuclear fission reaction, as gamma particles are capable enough to penetrate thick surfaces. It ends up being converted to steam. What actually is going on in it?

  • $\begingroup$ physics.stackexchange.com/q/484026/23615 , this somewhat related question isn't here as an answer or duplicate but it can give you more background on how substances that aren't radioactive to begin with might encounter radioactive sources and become radioactive themselves. Of course that answer deals directly with the Chernobyl incident, not really your question per se. I think only water containing tritium as part of its molecule is radioactive to begin with. $\endgroup$
    – Triatticus
    Commented Sep 25, 2023 at 17:19
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    $\begingroup$ While O-16 can absorb a neutron to create O-17 (and a gamma, swamped out by all the fission gammas), O-17 is stable. Same thing with converting O-17 to O-18. Deuterium is stable, and the cross section for producing tritium from deuterium is about 10 microbarns, quite small. Your local ENDF mirror is your friend for nuclear cross section data. $\endgroup$
    – Jon Custer
    Commented Sep 25, 2023 at 18:05
  • $\begingroup$ Where did you read that? It does become radioactive, if not directly from radiation coming from uranium products activating water molecules, then because it has boric acid dissolved in it - enabling corrosion of the reactor parts and transmutation of boron. The water converted to steam is in secondary circuit, it is shielded from the radiation, so it does not become noticeably radioactive. $\endgroup$ Commented Sep 25, 2023 at 23:41
  • $\begingroup$ actually i was just studying about nuclear reactors for my exam and this question popped in my mind , the same water which gets heated due to the fission is sent in the form of steam and back $\endgroup$ Commented Sep 26, 2023 at 9:36

3 Answers 3


Hydrogen atoms can absorb a neutron and become deuterium (also releasing a 2.2 MeV gamma-ray in the process). Deuterium is not radioactive. Although if deuterium takes on another neutron it will become radioactive, the cross-section for that to happen is extremely small and neutrons are much more likely to find another hydrogen atom.

High energy charge particles will be slowed by cherenkov radiation (which is low energy) and various scattering processes.

Gamma-rays have a mean free path about 10-20 times longer in water than in lead, but since it's a lot easier to fill a large tank with water than with lead, it's a much better option. A 10 MeV gamma ray has a mean free path of about 30 cm in water vs. 2 cm in lead. The energy of the gamma ray will be converted into optical light via compton scattering and eventually photo-electric absorption. Nothing becomes radioactive.

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    $\begingroup$ And yet, tritium is a perennial concern. Have you read about what's going on at the Fukushima site lately? $\endgroup$ Commented Sep 26, 2023 at 2:45

At least in the PWR reactor, it is not converted to steam. There is a closed primary circuit, where the water is pumped between the Reactor Pressure Vessel (contacting the fuel fissile elements) and the lower chamber and U-pipes of the Steam Generator. In spite of being heated well above $100^{\circ}$C by the fuel elements, it stays liquid due to high pressure inside this circuit.

The liquid water of the secondary (also closed) circuit flows from the upper chamber of the Steam Generator, where it is heated by the hot U-pipes, turning to steam, passing through the turbines, losing heat (explained below), and returning liquid back in the Steam Generator.

Only that secondary closed circuit is cooled by external water (from the sea for example), that returns to nature. But it doesn't come in contact with the primary radioactive water (not even with the secondary one).


The answer is that the water does become radioactive, but it isn't very much, and the radioactive isotopes decay away fairly quickly. There are several things to consider.

  1. What is the probability of interaction? (given by the cross sections)
  2. What is the neutron flux? (high)
  3. How much time is the water in the core?
  4. What is the radioactivity of the induced nuclides?

For all of these reactions, the cross section of interaction is low, especially when compared to cross sections in fuel. The cross sections in natural water are on the order of milli-barns. Cross sections in other materials are on the order of barns, and in fuel, they can be hundreds of barns.

Hydrogen can absorb two neutrons to become tritium. Tritium remains in the water and is very difficult to separate, but isn't much of a biological harm. The tritium eventually decays with a 12.3 yr half-life. (Canadian CANDU reactors start with a lot deuterium, so they produce more tritium.)

A bigger issue in LWRs is that oxygen undergoes an (n,p) reaction to make radioactive N-16. However, it has a 7 second half-life and decays away quickly. You can mitigate the N-16 by holding the water in tanks for a short period of time until it decays.

I'm sure there are other reactions, but these are the big ones. Neither one is much of a problem for operating reactors. The water in the primary coolant is not discharged to the environment.


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