These poor workers at the plant don't even need to go inside the Reactor 1 reactor building to get exposed to high radiation.
1000 millisieverts is 1 sievert.
Mainichi Shinbun (5/21/2011), citing information from TEPCO's press conference on May 11:
TEPCO announced on May 21 that the debris that emits 1000 millisieverts/hour radiation was found outside the reactor building of the Reactor 3, on the south side. According to TEPCO, it is a pile of concrete bits and paper-like materials. It is the highest radiation ever found on the debris outside the buildings.
And here's the latest contamination map ("survey map") from TEPCO, released on May 11. The debris is located between the Reactor 3 building and the concrete pump (that they call "elephant"). The radiation of 1000 millisieverts/hr was measured on the surface of the debris:
You can view all the past maps here.