The Korea Herald explains why a hydrogen bomb is such an important advancement by North Korea if they did in fact test one. It all comes down to the explosive power of an H-bomb compared to a nuclear fission bomb. Could you imagine what one of these things would do if used against Seoul or Beijing?:
Nuclear weapons are classified into atomic, neutron and hydrogen bombs. Among them, hydrogen bombs, also known as thermonuclear bombs, are known to be the most powerful due to their unique structure that expresses explosive power.
Unlike atomic bombs that derive their energy from nuclear fission — the splitting of atoms — hydrogen bombs obtain their explosive power from both nuclear fission and fusion, the process of forming a heavier nucleus from two lighter ones, such as the nuclei of the hydrogen isotopes tritium or deuterium.
While fission bombs can have as small as 1 kiloton of explosive power — North Korea’s 2013 nuclear test was known to have 6 to 7 kilotons — hydrogen bombs’ explosive power ranges into the hundreds of kilotons. [The Korea Herald]
You can read the rest at the link.