Surprising myself with how much of this stuff I actually remember. Haven't worked in HP in a long time. :-)
First of all, this is a point source calculation. Your situation is more of a plane source or line source.
Secondly, if you have U235 in your channelhead, you have bigger problems than just the gamma dose rate. :-)
Thirdly, U235 has a much lower energy and less prevalent gamma than you're dealing with. Depending on the plant's chemistry your primary isotope will be Co-60 or Iron 59 (Fe-59).
Try the same calculation with one of those and you'll get a much different result.
One other thing with regard to SG work. (I did a LOT of it.) Hopefully you aren't using just lead shielding directly over the SG opening or to shield the internal side of the diaphragm. Depending on the contamination levels, doing that can actually INCREASE your apparent gamma dose rate (actually a Bremsstrahlung X-ray) caused by "braking" the high energy Beta particles in there.