wp:Underwater explosion#Properties of water

(my bold)

Effect of neutron exposure on salt water (nuclear explosions only) – most underwater blast scenarios happen in seawater, not fresh or pure water. The water itself is not much affected by neutrons but salt is strongly affected. When exposed to neutron radiation during the microsecond of active detonation of a nuclear pit, water itself does not typically "activate", or become radioactive. The two elements in water, hydrogen and oxygen, can absorb an extra neutron, becoming deuterium and oxygen-17 respectively, both of which are stable isotopes. Even oxygen-18 is stable. Radioactive atoms can result if a hydrogen atom absorbs two neutrons, an oxygen atom absorbs three neutrons, or oxygen-16 undergoes a high energy neutron (n-p) reaction to produce a short-lived nitrogen-16. In any typical scenario, the probability of such multiple captures in significant numbers in the short time of active nuclear reactions around a bomb is very low. Salt in seawater readily absorbs neutrons into both the sodium-23 and chlorine-35 atoms, which change to radioactive isotopes. Sodium-24 has a half-life of about 15 hours, while that of chlorine-36 (which has a lower activation cross-section) is 300,000 years. The sodium is the most dangerous contaminant after the explosion because it has a short half-life.[2][self-published source?] These are generally the main radioactive contaminants in an underwater blast; others are the usual blend of irradiated minerals, coral, unused nuclear fuel, and bomb case components present in a surface blast nuclear fallout, carried in suspension or dissolved in the water. Distillation or evaporating water (clouds, humidity, and precipitation) removes radiation contamination, leaving behind the radioactive salts.

15 hrs per half-life x 10 half-lives

= 150 hrs

= 6.25 days