A bomb would just aid the spread of radioactive material in groundwater.
Moreover, the melted material would be at a very shallow depth, so little chance of confining radioactivity from the blast.

Latest in Mainichi Daily News:

Prime Minister Noda expressed confidence that the nuclear crisis at the Fukushima plant will be brought under control in the not-too-distant future.
Noda said Japan stands ready to continue to export its technology and expertise to emerging economies seeking to introduce nuclear facilities.....

Hoshono, minister for prevention of nuclear accidents, hopes that cold shutdown can be achieved at Fukushima by the end of the year, and that substantially lower emissions of radioactive substances can also be achieved by January.