Class-D nuclear warhead
Class-D Nuclear Warheads are considerably more compact than any nuclear device currently known and qualify as a "suitcase nuke".
The exact technical details are not revealed in the series, in keeping with its philosophy of avoiding technobabble. Probably a Class-D Nuclear Warhead is an implosion-type fission bomb. Given the small size of the device that Baltar delivered to Gina, the fissile core is extremely small, and the the apparatus which compresses it to supercritical mass is equally compact. The yield of a class D is difficult, if not impossible, to calculate from information seen in the series. It should be noted that many nuclear bombs, including the American version of a "suitcase nuke", can be configured for different yields ranging from .375 to 1.5 megaton easily ("dial-a-yield").
One explanation for its small size is that a nucleide other than plutonium is used in the fission primary. Aside from the rather unscientific introduction of a "new element", the most logical candidate would be Californium. This synthetic element, produced in a cyclotron, exists only in miligram quantities in the real world, but the Colonies might produce much more using advanced -- but plausible -- technology. Cf-251 has a critical mass of around two kilograms, and this could be reduced even further using currently existing techniques. The Class D warhead is advanced technology, but it is in no way physically impossible, nor must it rely on scientific principles unknown in the present day.
The primary drawback of Californium is that the half life is 790 years, meaning that long term storage is problematic. Plutonium has a half life of 24,110 years, and functional 100 pound simple implosion devices have been made with plutonium using existing 20th century technology.
Notes
- This article is assuming that the shown nuclear warheads are Class D.
- In the United States military, Class-D is the designation of a relatively light-weight Thermonuclear weapon. The W-28 warhead in the B-28 TN bomb is an example of this type.