Thanks to the Wigner effect moderator graphite rods get "charged up" by atoms displaced by neutrons to the wrong place in the lattice. This can store up to 2.7 kJ/g, a pretty decent amount of energy (lithium batteries have 0.46-0.72). It can be released by heating the graphite to 250 degrees C. In current reactors this is just a problem, but in EP this might be used for high performance energy storage. With nanotechnology the graphite can likely be turned into something even more energetic (and a bigger fire hazard).
TNT has an energy density of 4.61 kJ/g. TNT can be replaced with octanitrocubane with energy 8.5 kJ/g or octaazacubane at 22 kJ/g - these are likely molecules that requires nanotech to be made. A lot of the design would likely be in stabilizing nasty compounds like these.
To power plasma rifles I would go for nuclear isomer batteries. Tantalum 180m has 41,340 kJ/g that can (assuming EP tech!) be released as gamma rays. Or maybe it can be used as a "gamma grenade" irradiating everything within a few hundred meters. To get it you need to either separate the isomer from natural tantalum, or charge it up using a nuclear reactor.
http://en.wikipedia.org/wiki/Energy_density
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