Three workers at a Nebraska nuclear plant were exposed to high levels of radiation on April 3 after failing to follow protocol when removing a highly radioactive steel rod from the plant’s reactor vessel.

Workers at the Brownville plant – just minutes from Iowa’s southwest border – were not exposed to levels that surpassed federal guidelines, according to a Nebraska Public Power District spokesman, but the U.S. Nuclear Regulatory Commission is now investigating the incident – a process that may take several days.

The Omaha World-Herald reports:

Inspectors’ concerns surround a 27-foot-long stainless steel rod known as a “shuttle tube” that’s placed inside a nuclear plant’s reactor vessel.

The flexible tube, about as wide as a human index finger, acts as a sheath to protect instruments and sensors that monitor conditions in the reactor.

NRC officials said the April incident occurred when workers removed the tube — highly contaminated with radioactive material — through the bottom of the station’s reactor vessel.

Standard procedure is to remove such tubes through the top of the vessel, the NRC said.

The agency said a two-person NRC team traveled to the plant to spend several days examining the circumstances behind the incident, review the Nebraska Public Power District’s response to the event and calculate how much radiation NPPD workers were exposed to.

The Brownville plant is just one of two nuclear generators in Nebraska. Iowa has one plant, but is still mulling the option of adding a second – an issue that has sparked statewide debate in the the wake of Japan’s nuclear disaster.