UPDATE 3-Storms knock out TVA nuclear units, power lines
* Severe storms moving across TVA transmission area
* Rainfall boosts hydro output along Tennessee River
* Nuclear reactors shut after losing outside power (Updates power status at nuclear plant)
HOUSTON, April 27 (Reuters) - Severe storms and tornadoes moving through the U.S. Southeast dealt a severe blow to the Tennessee Valley Authority on Wednesday, causing three nuclear reactors in Alabama to shut and knocking out 11 high-voltage power lines, the utility and regulators said.
Governors in Alabama, Arkansas and Tennessee each declared a state of emergency as more than 20 deaths were blamed on a recurring round of severe storms this week moving eastward across the southern United States. [ID:nN27120833]
All three units at TVA's 3,274-megawatt Browns Ferry nuclear plant in Alabama tripped about 5:30 EDT (2230 GMT) after losing outside power to the plant, a spokesman for the U.S. Nuclear Regulatory Commission said.
A TVA spokeswoman said the station's backup power systems, including diesel generators, started and operated as designed. External power was restored quickly to the plant but diesel generators remained running Wednesday evening, she said.
The Browns Ferry units are among 23 U.S. reactors that are similar in design to the crippled Fukushima Daiichi nuclear plant in Japan where backup generators were swept away in the tsunami that followed the massive earthquake on March 11.
Browns Ferry output had been reduced earlier in the day due to transmission line damage from a line of severe storms that spawned tornadoes as it moved through Mississippi, Alabama, Kentucky and Tennessee.
Details of the transmission outages and co-op power outages were not immediately available.
The government owned corporation said crews were working to make repairs, but the severe weather was forecast to continue into the evening, TVA said in a release.
Most of the damage by Wednesday afternoon occurred in the western part of TVA's service territory in Mississippi, Alabama and western Tennessee and Kentucky.
Cullman Electric Cooperative in Cullman, Alabama, is the only power company directly affected by TVA's transmission outage, TVA said in a statement.
Rainfall amounts between four and seven inches (10-18 cm) have fallen since Tuesday in the area. Eight of the nine dams on the Tennessee River were generating at full power to move water through the river system to help control flooding, TVA said.