According to various sources, that's exactly what happened. The plant operators wanted to see how long the turbines would keep spinning and producing electricity after the electrical power supply was shut off. In order to do that, they had to disable several of the control systems, including those that would shut the reactor down in the event of overheating and loss of control over the chain reaction in the reactor core.
There's a Russian journalist, Elena Filatova, who has documented numerous trips into and through the affected region on her Kawasaki Ninja. It makes for fascinating, but haunting, viewing:
http://www.angelfire.com/extreme4/kiddofspeed/chapter1.html