LIMERICK - The Exelon nuclear plant has been given two more years to make a crucial upgrade to the reactor buildings intended to prevent explosions like those that occurred during the Fukushima nuclear disaster in 2011.

In March 2012, the Nuclear Regulatory Commission ordered the Limerick Generating Station to take a number of steps designed to prevent the build-up of explosive gas in the reactor containment building.

Hydrogen built up inside three reactor buildings at the Fukushima Daiichi when the cooling water pumps failed as a result of the generators having been made inoperative by the tsunami that swamped the seaside plant in the wake of an underwater earthquake off-shore.

After an analysis of the events that unfolded in Japan, the NRC made a number of recommendations to plants across the country, particularly to those like Limerick, which use the same reactor design as the one that failed across the Pacific in 2011.

Among the changes the NRC required of Limerick was the installation of a 'hardened wetwell vent' system, the design of which would be 'used to remove combustible gases from the containment building before they could trigger an explosion following a severe accident,' NRC spokesman Neil Sheehan explained in an email to The Mercury.

'The installation of such vents is one of our post-Fukushima requirements,' Sheehan wrote.

On Feb. 26, Exelon wrote to the NRC and asked for a two-year extension on the deadlines for installing the new safeguards in each of its two reactors.

On April 14, the NRC informed Exelon that its request for an extension had been granted.

'Exelon will now have until the end of a refueling outage in spring 2018 to complete the work at Unit 1 and until the end of a refueling outage in spring 2017 to complete the work at Unit 2,' Sheehan wrote.

'The company had requested one additional refueling cycle to wrap up the vent design and installation work,' wrote Sheehan.

According to NRC's letter to Exelon, the staff agreed with the company's rationale for the delay.

The letter, from Eric J. Leeds, director of the office of nuclear reactor regulation, notes that other changes Exelon can make to the Limerick plant prior to the new vent system being installed will 'provide a portion of the desired increase in capability' to deal with the circumstances the vents are needed to prevent.

Leeds further noted that the NRC staff 'concluded that a sequence of events such as the Fukushima Daiichi accident is unlikely to occur in the United States based on the current regulatory requirements and existing plant capabilities.'

Sheehan confirmed that Limerick is not the only nuclear plant to seek, or to receive, an extension of this type.

Seven other reactors, including three others in Pennsylvania - Susquehanna 1 and 2 and Peach Bottom 3 - have been granted similar extensions. All but the two Susquehanna plants are owned and operated by Exelon.

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