Agree with Richard. The HR, the Hotel Okura, the Fujita and the International or Kokusai Hotel are all quite nice. Hopefully, you will find something for your dates at one of them. If all else fails and you are willing to step down a star or so, try one of the offerings at: http://www.japanhotel.net/ . Most of their offering are business hotels, but some are pretty nice. They also have listings for Hiroshima.

JR