Klingon B'Rel-class antimatter pod.
Antimatter pods were self-contained magnetized storage devices which contained antimatter fuel for starships.
History and specifications[]
An antimatter containment field was used to isolate the fuel inside from contact with normal matter, thus preventing a catastrophic reaction. (TNG reference: Star Trek: The Next Generation Technical Manual)
In 2370, Deep Space 9 prepared to receive a shipment of 20 antimatter pods destined for the Ambassador-class USS Hannibal. The Galaxy-class USS Enterprise had 30 antimatter pods. (DS9 novel: Antimatter)
In the Klingon B'Rel-class, antimatter was stored using magnetic isolation valves within duranium titanide tanks structurally reinforced to withstand battle damage. Storage racks in engineering held 20 pods and were rigged with self destruct technology. (ST reference: Klingon Bird of Prey Owners' Workshop Manual)