From the Net: 'Emergency shutdown at Illinois reactor — Smoke was actually steam containing radioactive material — Workers evacuated' (Energy News)

Title: Byron Station Declares Unusual Event
Source: Exelon Press Release
Date: Jan 30, 2012
Emphasis Added

[...] Operators at Byron Generating Station declared an Unusual Event at 10:18 a.m.CT, due to the loss of offsite power and Unit 2 coming offline.

The nuclear facility’s diesel generators activated as designed to provide power to the facility when there is a loss of offsite power to the facility. The facility remains in a safe condition. Station engineering experts are looking into the cause of the loss of offsite power.

Byron Station is designed to depressurize to reduce steam pressure as part of the many redundant safety systems built into the facility. Steam from the unit is released through safety relief valves that are specifically designed for this purpose.

The steam, which will evaporate quickly, contained expected levels of tritium.

Local residents may see or hear the steam release in progress, which will continue throughout the day until the unit cools down. These types of station releases are regulated by the Nuclear Regulatory Commission.

There is no health or safety impact to workers or to the public from the release, and Exelon Nuclear has notified allappropriate local, state and federal officials of the Unusual Event. [...]

From EnergyNews.