# Does water absorb neutrons?

Water is used as a shielding material in nuclear reactors. What's its function as a shield? Does it absorb neutrons and is there a reaction between water and neutrons?

• Water slows neutrons down. This is called "moderation" and it is important in reactors because some fission materials like Uranium 235 are preferentially split by slow (thermal) neutrons, but after the fission occurs they emit several fast neutrons themselves. So if we want to build a reactor with low concentration fission materials, then we need to make the process of neutron capture efficient. This is accomplished by moderator materials like graphite (carbon) and water which slow the fast neutrons down so that they are easily captured by the fission material. – CuriousOne Jun 4 '15 at 6:35
• I know the function of water as a coolant and a moderator.. I want to know about its shielding – user2974951 Jun 4 '15 at 6:56
• Pure water is not a particularly efficient shielding material for neutrons unless used in large quantities but one can add salts of elements like boron to make it more useful. I think this is also useful for fast reactor shutdown. – CuriousOne Jun 4 '15 at 7:03
• Hydrogen is the best material to 'slow' the fast (~2MeV) prompt fission neutrons, mainly because of the excellent mass match leading to maximum energy transfer from the neutron to a proton in a collision. So, the neutrons that escape the pool are generally slower, and boron has a huge capture cross section for the slow neutrons. At neutron production facilities they tend to use materials more like paraffin, with even more hydrogen per mass/volume (plus being solid helps move it around as necessary). – Jon Custer Jun 4 '15 at 12:33
• Paraffin and other waxes were popular in the early days, but the possibility of melting means they need containment which is an issue for some applications. (Certainly the safety guys at JLAB didn't like waxes.) Plastics have similar hydrogen fraction but aren't quite as dense, so they are an alternative in some applications---especially as PVC include chlorine or polyurethane can be boron doped. No idea if anyone has figured out an stable gadolinium doped plastic yet – dmckee Jun 4 '15 at 13:58

Neutrons don't see water, but instead see hydrogen and oxygen. The relevant cross-sections for "thermal" neutrons are $$\begin{array}{rccl} & \sigma_\text{scatter} & \sigma_\text{capture} & \text{(in barns)} \\ \text{hydrogen} & 82 & 0.33 \\ \text{oxygen} & 4.2 & 0.000\,19 \end{array}$$ So you can see that the primary effect of the water is for the neutrons to scatter off of the hydrogen. This exchange will tend to bring the neutrons into thermal equilibrium with the water (at which point we can actually refer to them as "thermal" neutrons, with typical kinetic energy $kT \approx 25\,\rm meV$).

From the neutron's perspective water is a gas of hydrogen atoms with number density $$n = \rm\frac{1\,gram}{cm^3} \cdot \frac{1\,mole\,H_2O}{18\,gram} \cdot \frac{2\,mole\,H}{1\,mole\,H_2O} =0.11\rm\,\frac{mole\,H}{cm^3}.$$ The oxygen is a 2% correction the scattering and completely irrelevant for capture. The probability of traversing a path length $\ell$ without an interaction is $\exp -n\sigma\ell$, so the mean free path between scatters for neutrons in water is roughly $$\ell_\text{scatter} = \frac{1}{n\sigma_\text{scatter}} \approx \rm2\,mm$$ and between captures is roughly $$\ell_\text{capture} = \frac{1}{n\sigma_\text{capture}} \approx 450\rm\,mm.$$ This suggests that a few decimeters of water is pretty efficient at moderating fast neutrons down to room temperature, but a few meters are required to completely convert thermal neutrons into gamma rays due to capture on hydrogen (energy 2 MeV). A few meters of water is pretty efficient at turning gamma rays into heat.

Water serves for water reactors in up to 3 roles: as a moderator, as a coolant and as shielding.

Water is excellent shield for alfa radiation of transuranic elements, -like any condensed matter, decreasing radiation to half in probably less then 1 mm thickness.

It is very good shield for beta electron based radiation, mainly from fission products and neutron decays, with half-thickness typically about 1-2 cm.

It is not good shield for gamma radiation, half-thickness more then 20 cm.

It is bad shield for neutrons in sense of low absorption cross-section, but it is very good shield - as all hydrogen rich matter - in sense of slowing neutrons down to thermal speed random movement. Those not finding their absorption fate in fission or other material decay to proton, electron and e-antineutrino with a halftime about 14 min.

Products of water neutron absorption is deuterium for light hydrogen, resp. tritium for deuterium ( heavy water or minor D content in natural water ).

1H has better moderating slowing down effect than deuterium, due better matching proton - neutron masses. But unfortunately, 1H has much larger absorption cross section for neutrons then deuterium. Therefore heavy water can be used for reactors using unriched natural uranium - see PHWR and CANDU rector, but light water reactors need to use enriched uranium.

Water isn't a particularly effective radiation shield, and as far as I know it is not used as a shield in the reactors themselves.

However it is used as a shield in the pools where fresh nuclear waste is stored. These are known as spent fuel pools, and they use large thicknesses of water (around 10 metres) to absorb the radiation emitted by the fuel.

• Can you please tell me what reactions are taking place in the absorption process between water and neutrons? – user2974951 Jun 4 '15 at 18:35
• @AmilaPasan: water doesn't react with neutrons to any significant extent. It shields against energetic particles by inelastically scattering them, and against gamma rays by Compton scattering. – John Rennie Jun 5 '15 at 5:21
• well, experimental reactors have water moderators en.wikipedia.org/wiki/Light-water_reactor . One could see cerenkov radiation from the rods – anna v Dec 20 '16 at 13:33
• @annav: that's just inelastic scattering. There is no significant reaction between the nuclei in the water molecules and the neutrons. – John Rennie Dec 20 '16 at 15:04
• well, if you see the crossections for capture by rob's answer, the reactions are with the nuclei – anna v Dec 20 '16 at 15:11

Water acts as a coolant in nuclear reactors. It is run through pipes in and around the reactor to absorb the immense heat generated.