They may not have known the exact number (well, for that matter, no one knows because the survey meter went overscale at 10 sieverts/hr), but they've been running past the area when they have to enter Reactor 1 building to install the external heat exchanger to the Spent Fuel Pool, according to the tweets by a worker currently at Fukushima I Nuclear Power Plant.
So much for TEPCO's word that "there is no work planned near the area, and there will be no effect on the progress of the work at the plant", which is technically correct, at least the first half.
There may not be any work planned in the area there, BECAUSE RADIATION IS TOO HIGH. There may be no work planned there, but the workers have to go near it to enter the Reactor 1 building. The radiation is so high that the workers, with all their full protection gear and equipment and construction materials they carry, have been running past the area in order to minimize radiation exposure.
So it's been known.
More information from his tweets:
(Someone asked if it's true that TEPCO does not have the survey meter that can measure more than 10 sievert/hr radiation.) I think that's correct. We do have the instrument to measure the radiation of the nuclear fuel rods, but I wonder what happened to that. Broke?
(Referring to the area where over 10 sievert/hr radiation has been detected,) the radiation level around the exhaust stacks has been too high to even go near, ever since the accident started. Not just the area around the stack for Reactors 1 and 2, but also the stack for Reactors 3 and 4.
Debris are removed by shielded heavy equipment or remote-controlled equipment. That (10+ sievert/hr) spot was found during such operation.
We have to run when we go by the stack to enter Reactor 1. That's hard work. My legs hurt already from climbing up and down the stairs in the reactor building. Now I appreciate elevators!
All the elevators inside the reactor buildings at Fukushima I Nuke Plant are broken.
As to the worker who measured the radiation with the survey meter attached to a stick, Hiroaki Koide of Kyoto University says he must have spent only 1 or 2 seconds near the stack to get 4 millisieverts radiation.
Let's see. 2.78 millisieverts for one second, if the radiation was 10 sieverts/hr.
Koide thinks the radiation level is impossibly high to be coming from inside the duct or stack. He suspects there is a piece or two of the spent fuel blown out of the Spent Fuel Pool when Reactor 1 and Reactor 3 had hydrogen explosions.
He also says the protection suits that workers wear do nothing to protect them from gamma rays, unless they wear lead vests, but even then only marginally.
(For more of his comment, there's a transcript of the radio program that he appeared in, in Japanese.)