Why reactor-circuit water was no place to make tea

Was reactor water used to make tea during a Royal Navy submarine refit?

In the recollection given, it was water from the associated heat-exchange system rather than water circulating through the reactor itself.

In the refit anecdote as remembered here, the water came from the heat-exchange side of the cooling system. It was not drawn directly from the circuit circulating through the reactor. The exact incident and plumbing are recalled with uncertainty, so the account should not be treated as a verified technical description of a particular submarine. The practical point is the same: personnel were directed to obtain drinks in the galley instead of improvising with unfamiliar systems elsewhere in the boat.