Chapter 9 in Radiobiology for the Radiobiologist is the best source I could find for this topic

Radioprotectors are agents that, when given before or during radiation, reduce the likelihood of early and/or late effects from developing.

Most radioprotectors are sulfhydryl compounds. They contain a sulfhydrl group (S-H) attached to a short carbon chain with a reactive compound at the opposite end.
The example from Hall et all is cysteamine:

(1)
\begin{align} \text{SH}-\text{CH}_2-\text{CH}_2-\text{NH}_2 \end{align}

These agents are only effect on radiation types that require oxygen for 'fixing' - ie. that function through indirect action of radiation.

## Mechanisms and Examples

The sulfhydryl group may act by chemically reacting with free radicals generated by indirectly ionising radiation and preventing their interaction with DNA

### Improvement of DNA Repair

The sulhydryl group may donate a hydrogen atom to assist in DNA repair pathways.