It's not very viable yet because the only form of it we reliably achieve is a bomb. This makes the only useful spacecraft to use it something like Project Orion.

Such a device, while possible, is very fuel-inefficient: you need several kg of plutonium just to ignite the tritium. Peak mechanical and radiation loads are very high.

A thruster which could produce much lower thrust for a much longer time seems much safer and cheaper to implement, even if detonating nuclear bombs in space were not an issue. And a issue it is: you need to do it rather far away from all other spacecraft, which means some weird orbital plane.