The Nuclear Regulatory Commission has ordered nearly a third of the nation's nuclear reactors to upgrade their venting systems.

The order, which applies to 31 boiling-water reactors, is part of the commission's review measures following Japan's 2011 Fukushima nuclear power disaster.

"Strengthened vents will help these plants continue to protect the public and the environment even if emergency systems can't immediately stop an accident," NRC Chairman Allison M. Macfarlane said in a statement Thursday.

"By safely releasing built-up pressure and hydrogen, the plants will preserve the buildings that contain radioactive material," she said.

Depending on their refueling schedules, plants will have different deadlines to comply with the requirements, but the plants need to have completed some improvements by June 2014.

Thursday's directive supersedes the commission's March 2012 order for the 31 reactors.

"The new order is in line with the with the industry's ideas on the most effective means to address the venting issue, and we consider the timing of the phased approach to be achievable," Steve Kerekes, a spokesman for the Nuclear Energy Institute, a Washington industry group that had raised concerns about the 2012 decision, said in a statement to Bloomberg.

"We look forward to working with the NRC and stakeholders to develop the guidance for implementation of the order," Kerekes said.

Separately, a survey by Platts released Thursday revealed nuclear power plant operators in the United States might have to spend nearly $3.6 billion in the next three to five years on modifications to the nation's 102 nuclear units in response to Japan's 2011 Fukushima nuclear disaster.

The cost estimate does not include NRC's new orders for the 31 nuclear reactors but includes capital expenditures for previous requirements ordered by the commission, post Fukushima, such as new equipment to handle threats such as floods and earthquakes, Platts reports.

Platts said its survey revealed a wide range of estimates for the cost of complying with NRC's post-Fukushima requirements. That's because the nation's fleet of reactors is not standardized, said David Lochbaum, director of the nuclear safety project of the Union of Concerned Scientists.

"It's not going to be the same fix for each plant," Lochbaum told Platts, adding older reactors might require more upgrades than newer ones.

For example, Duke Energy of Charlotte, N.C, which operates 11 reactors and owns the Crystal River-3 plant in Florida, which has been offline since September 2009, said it would spend $600 million on the upgrades. And for its 17 units, Chicago-based Exelon expects to spend $400 million for upgrades in the next five years.