Nuclear weapons commonly lose huge portions of their potential energy when the core begins to fission/fusion, because the heat expansion caused from the nuclear reaction prevents the core from staying densely packed for length of time needed for all the fuel to react.
Staged thermonuclear weapons use explosive force from smaller nuclear bombs to crush the core to such a density that it fuses much better and has exponentially greater energy efficiency. Cost and size also increases exponentially as well, usually limiting their effective size to that of their delivery system. In theory there is no limit on how many stages you can add, though each additional stage will grow increasingly expensive and inefficient.
Salted nuclear weapons
These are designed with fallout in mind – in most nuclear scenarios, the majority of the casualties die from residual radiation rather than the fire and heat itself. The radioactive decay of Cobalt-60, for example, is short enough to produce intense levels of radiation, but long enough to render an area uninhabitable for years.