As the nuclear plant crisis in Japan reveals more vulnerabilities in facility operation and design, calls are being renewed to change the way nuclear plants are evaluated and regulated in the United States.

Just a week ago, officials at the US Nuclear Regulatory Commission reported that America's nuclear plants are safe, noting that some gaps had been addressed or were being addressed as a result of an initial safety review to apply lessons learned from the tsunami-stricken Fukushima Daiichi nuclear plant.

Up to that point, the Fukushima facility had experienced an alarming string of crises: partial core meltdowns, station blackout, exhausted backup batteries, hydrogen explosions, fires in spent-fuel pools, failed cooling systems, and radioactive water and air released to the environment. In the US, 23 reactors have the same design as those at Fukushima. But NRC regulators assured the public last week that those reactors had been updated with special safety equipment. "Hardened vents" on the reactors would prevent a hydrogen buildup and explosion, they noted.

Since then, however, the news from Fukushima has gotten worse. On Sunday, Tokyo Electric Power Co. said the uranium fuel core in its No.1 unit had melted completely – a worse condition than previously believed – and apparently seared a hole in the reactor vessel. Core meltdowns at the No. 2 and No. 3 reactors also are more serious than earlier believed.

Then, on Tuesday, The New York Times reported that the Fukushima reactors had the same hardened emergency vents that NRC officials had said made similar plants in the US safe from hydrogen explosions. The newspaper also reported, in a separate story, that the NRC had been warned years ago that the vents were subject to failure in a crisis.

On Thursday, critics pounced.

The antinuclear group Beyond Nuclear charged that, while some US Mark I reactors possess the same venting systems that failed in Japan's crippled plant, the NRC knows that other Mark I reactor operators may not have installed – or may even have uninstalled – those venting systems. (NRC officials insist all such reactors are equipped with the vents.) If the venting systems at Fukushima Daiichi had worked as designed, they would have prevented damage to containment from the hydrogen explosions, it said.

"The NRC left the retrofit of this experimental venting system to the voluntary discretion of the US reactor operators,” said Paul Gunter, director of reactor oversight at Beyond Nuclear, in a statement. “Now that this experimental containment vent is demonstrated to have failed at Fukushima, we need to know who installed it at US plants, who didn’t, and the justification for the continued operation of these deeply flawed and dangerous reactors."

Other nuclear watchdogs say tougher US scrutiny of nuclear power is needed at every level – from approval of reactor designs, to licensing of operators, to relicensing of older reactors, to letting nuclear reactors operate at higher outputs.

"We need to suspend licensing and relicensing, and suspend reactor certification decisions,” Arjun Makhijani, a nuclear physicist and president of the Institute for Energy and Environmental Research, said Thursday in a conference call with reporters. "Not the processes involved – but the decisionmaking itself – should stop because we don't know whether we're making decisions that are consistent with safety."

Beyond Nuclear wants the NRC to hold a public meeting in each emergency planning zone for reactors with a design like the one at Fukushima. The NRC should "revoke all prior approvals for the installation of the vent and instead require operators to submit a license amendment request with full public hearing rights,” said Mr. Gunter.

For its part, the NRC is conducting a 90-day review in which lessons learned from the Fukushima crisis will be factored into safety regulations for US plants. It is now more than 30 days into that review. In addition, the commission is conducting a longer review to incorporate more far-reaching changes. It has also reaffirmed the capability of the hardened vent system.

"Every US Mark I [boiling water reactor] has a hardened vent," writes Scott Burnell, an NRC spokesman in an e-mail interview. "The NRC continues to conclude the hardened vents at US BWRs can carry out their intended purpose of maintaining containment integrity, therefore the plants can continue operating safely with the hardened vents that exist today."

As to the overall safety of US nuclear plants, the NRC "continues to conclude that US plants are meeting regulations to ensure safe reactor operation and to protect the public if an accident were to occur at a reactor or a spent fuel pool," Mr. Burnell writes. "The NRC’s licensing processes carefully examine well-supported technical information to come to scientifically and legally defensible conclusions regarding both new and existing reactors."

Others, however, say the problem is not that the NRC is unable to identify problems, but that it often simply does not follow through on addressing known problems.

"They've identified the right issues for the most part, but it's not clear that they're planning to apply anything but band-aids," says Edwin Lyman, a nuclear physicist and expert on reactor safety with the Union of Concerned Scientists, a nuclear watchdog group. "In most cases they tend to underplay the potential for severe accidents – and as a result those regulations don't cover severe accidents in a serious way."

One example: NRC's approval of a buildup of spent-fuel rods in pools. The Fukushima crisis has now demonstrated the vulnerabilities of that practice, but US reactors of the same design have generally been found to have less, not more, emergency battery power to cool reactors during power outages. Most such reactors also have much larger loads of spent fuel in their cooling pools than did their counterparts at Fukushima.

"It's important that the NRC also be taking a harder look at its own decisionmaking over the last decade or so ... on the issue of spent-fuel pool [storage] and dry-cask storage," said Peter Bradford, a former NRC commissioner.