nonreturn valve

"We had no time to fit a nonreturn valve--the pipe's open to vacuum at this end.

Dammit, why doesn't it have some method of protecting itself against gross physical injury, a nonreturn valve arrangement or some such?

"The nonreturn valve of the connector is jammed in the closed position," the Nidian said in its staccato, barking speech.

At Pickering, the vacuum duct is closed by nonreturn valves, which would prevent flow of the steam/air mixture from the duct to a non-accident reactor unit following a LOCA.