Reactor Coolant is a quest that can be obtained from Proctor Ingram, on board the Prydwen, in Fallout 4..
In charge of maintenance regarding Liberty Prime now, as well as the Prydwen, Proctor Ingram is in need of a consistent supply of reactor coolant.
Speaking with Proctor Ingram after the Brotherhood of Steel have secured their victory across the Commonwealth, will reveal that she is in need of reactor coolant, for use in maintaining the Brotherhood's technology. For every container of coolant constructed for her, using gas canisters, antifreeze, nuclear material, and dirty water, she will reward the Sole Survivor with a small amount of bottlecaps. This quest remains active so long as the Sole Survivor remains in good graces with the Brotherhood of Steel, and can be completed an indefinite number of times for an indefinite number of bottlecaps.