There would be no concussion because, in the vacuum of space, there are very few atoms around to transmit the physical force of the explosion. There would be an intense flash of radiation and the atoms of the bomb itself would be accelerated very rapidly away from the explosion. On earth, a nuclear bomb creates such an intense flash of radiation that anything flamable that is near by is heated to the ignition point and it bursts into flame (if there is oxygen around). Even without the oxygen, the air immediately around the explosion is heated to incredibly high temperatures and that creates a pressure wave (the concusion). The radiation from the bomb travels at the speed of light but the concussion travels more slowly.