Radiation levels inside the containment will be many times higher than usual, but that is okay because no one needs routine access inside containment buildings and no humans will be over exposed. The containment walls, reactor coolant piping, and other equipment inside the containment building will condense and capture much of the radioactive materials that are entrained in the water. Other than those vented noble gases mentioned above, essentially nothing will be released to the environment.
If you do not believe me, please read the compact, two page article that was published in the 20 September 2002 issue of Science Magazine titled Nuclear Power Plants and Their Fuel as Terrorist Targets. That article, jointly written by 11 of the most eminent nuclear specialists in the United States clearly lays out the possible consequences of the worst possible damage that can occur at a light water reactor with a containment dome licensed to "Western" standards.
Additional ReadingANS Nuclear Cafe Tsunami updates
Brave New Climate discussion thread Japanese nuclear reactors and the 11 March 2011 earthquake
World Nuclear News (March 12, 2011) Battle to stabilise earthquake reactors (This article is being updated as events unfold.)