Fukushima: the worst nuclear disaster since Chernobyl. A meltdown that made a part of the Japanese coast uninhabitable, leaving tens of thousands in shelters with no prospect of returning to their homes.

After the nuclear catastrophe in Japan, the U.S.’s Nuclear Regulatory Commission established a Japan Lessons-Learned task force to make recommendations about how to correct conditions that might lead to a similar disaster here.

The Fukushima reactors, like the reactors at 23 plants in the U.S.—including the Vermont Yankee plant near Brattleboro and the Pilgrim plant at Plymouth—were built to a design called Mark I. The Lessons-Learned task force recommended that operators of American Mark I plants install vents capable of withstanding high heat and steam pressure, fires, and minor explosions, and a year ago in March the NRC ordered them to do that.

But now the NRC is balking at another improvement recommended by its own professional staff: equipping the so-called “hardened” vents with high-capacity radiation filters.

Without the filters, the gases released through the vents to reduce pressure in a heating reactor would be highly radioactive; reactor managers would be faced with a grim choice between letting pressure build toward an explosion and “fire-hosing downwind communities with massive amounts of radiation,” in the words of Paul Gunter, director of the watchdog group Beyond Nuclear’s Reactor Oversight Project. The nuclear industry doesn’t want to pay to install the filters, which cost about $16 million each.

In January, Massachusetts state rep Tom Calter wrote the NRC on behalf of his constituents living in Plymouth, near the Pilgrim plant, to ask them to require Mark I plant operators to install the filters. On March 19 the five NRC commissioners voted on whether to implement the staff’s recommendation that the Mark I reactors be required to have the filters by December, 2017, a year after they are required to have the vents.

Commission chairman Allison Macfarlane voted in favor of ordering the reactor operators to install the filters, which she called “a prudent and appropriate safety enhancement.” Three commissioners, William C. Ostendorf, William D. Magwood and George Apostolakis, voted for the filters, but not by order—rather by a rulemaking process of the sort that would delay their installation for years. The other commissioner, Kristine Svinicki, voted against the filters.

The logic behind the votes, as laid out in the record of the commissioners’ comments, was sometimes surprising. Magwood, for example, conceded that reactor operators need “an array of tools and options” to use in dealing with “extreme events,” but added if that if they did not “take appropriate action in the event of a severe accident,” a loss of containment integrity would occur even if there were filters. In other words, they need an array of tools—but if they don’t use the tools properly, there will still be a catastrophe. The latter point is so obvious it hardly needs to be made, but why filters shouldn’t be added to that “array of tools” was not explained.

In any case, the vote has gone down, and the Mark I reactors won’t be getting filters—at least not soon. Said Gunter, “This is fundamentally a Fukushima lesson unlearned.”•