No fly zone over Nebraska power plant June 6, what is this about ?

http://tfr.faa.gov/save_pages/detail_1_6523.html

Issue Date : June 06, 2011 at 2231 UTC
Location : FORT CALHOUN NUCLEAR POWER PLANT BLAIRNE, Nebraska near OMAHA VORTAC (OVR)
Beginning Date and Time : Effective Immediately
Ending Date and Time : Until further notice
Reason for NOTAM : Temporary flight restrictions
Type : Hazards
Replaced NOTAM(s) : N/A
Pilots May Contact : MINNEAPOLIS (ZMP) Center, 651-463-5580

Jump To: Affected Areas
Operating Restrictions and Requirements
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the TFR only extends up to

the TFR only extends up to 3,500' above sea level, so i don't think it's because they don't want anyone peeking into what's going on. it sounds like it's because there's a legitimate "hazard" there. could it be potential for explosion? if it was related to gasses, perhaps that's a safer range, 2 miles and 3,500' vertically?