* NRC chairman endorses hardened vents after Fukushima visit
* Nuclear critic Markey said agency action too lenient
HOUSTON, March 19 (Reuters) - The U.S. Nuclear Regulatory Commission (NRC) on Tuesday delayed a decision to order operators of more than two dozen older nuclear plants to install filtered vents as part of the agency's post-Fukushima safety review, according to an NRC memo posted on its website.
The commission moved forward with an order requiring operators at 31 boiling-water reactors with Mark I and Mark II containments - similar to the Fukushima Daiichi design - to modify or install "hardened vents" to more effectively and safely release excessive containment pressure after a serious accident.
But the commission's action fell short of requiring operators to install so-called "filtered" vents able to retain radioactive material during severe accidents as recommended by NRC staff.
The NRC instead will hold a lengthy rule-making process to further study filter options and to develop a final rule by March 2017, six years after reactors at Fukushima were damaged by an earthquake and tsunami.
In January, the NRC staff recommended that filters be installed with the vents while the nuclear industry presented less costly proposals.
The inability of operators to open vents at the Fukushima plant led to damaging hydrogen explosions.
NRC Chairman Allison Macfarlane said her decision reflected a recent trip to the devastated Fukushima Daiichi plant and the surrounding area.
"I came away from the visit with a strong conviction that this must never happen again," Macfarlane said in a separate statement.
"Since we have now seen first hand the results of a severe accident at a Mark I facility, it is time to align the approach the NRC endorsed over 20 years ago to use containment vents to mitigate severe accidents with the actual physical capability of those vents to operate in severe accident conditions," Macfarlane said.
U.S. reactors with the Mark I containment design already have hardened vents that will likely only need modification to meet NRC requirements. But operators of the nation's eight Mark II reactors will have to install hardened vents.
None of the 103 U.S. reactors have filtered vents.
Macfarlane also seemed to favor the addition of filters. "Other than the additional incurred cost, there are no substantive downsides to implementation of filters and they are the international standard," she wrote.
Nuclear power accounts for about 20 percent of the nation's power, but the future of nuclear power is uncertain due to competition from low-cost natural gas-fired generation and lack of government action to limit greenhouse gas emissions.
The commission directed the staff to look at different filter options, including adding filters to vents or using other performance-based approaches.
Rep. Edward J. Markey, a Massachusetts Democrat and nuclear industry critic, said the NRC "abdicated its responsibility to ensure public health and safety" by delaying a decision on use of filtered vents for several years.
Markey has long called for the NRC to endorse its own technical staff's work and to quickly adopt all recommendations made by the agency's Near Term Task Force on Fukushima, including the use of filtered vents that would work in a severe nuclear accident situations.
"Instead of following its top experts' safety recommendations, it chose to grant the nuclear power industry's requests for more studies and more delays, and even after the study is completed there is still no guarantee that the NRC will ever make this common sense requirement mandatory," Markey said.