How is phosphorus-32 produced?
The most important nuclear reactions for the generation of anthropogenic P-32 are
Cl-35 (n, α) P-32,
S-32 (n, p) P-32, and
P-31 (n, γ) P-32.
Since especially chloride, sulfate, and phosphate can occur as impurities in water, typical P-32 activity concentrations in the reactor coolant of light water reactors are in the range of several kBq per litre. (Sources for impurities in the reactor water of BWR’s can be leaking condensers; in the primary coolant of PWR’s, phosphorous compounds in the added boric acid. Sulfate might also be released from cation exchange resins.)

In the laboratory, P-32 can be produced (even as an experiment with students) using the S-32 (n, p) P-32 reaction with fast neutrons. A sample of pressed elemental sulfur is placed close to a neutron source for several days. After that, the sulfur is washed with boiling water. The conversion of the α-sulfur allotrope to β-sulfur at 95.3 °C helps to release the phosphorous compounds from the solid sulfur matrix to the water. Thus, the resulting P-32 solution can be made carrier-free.