Senior nuclear engineers and scientists from the United States and 10 other nations urge that safety standards governing the ability of nuclear reactors to survive severe accidents be strengthened in light of Japan's Fukushima disaster.

The group, assembled by Roger Mattson, former director of the Nuclear Regulatory Commission's reactor systems safety division, and Nikolai Steinberg, former chairman of the Ukrainian nuclear regulatory authority, sent their statement April 6 to the head of the International Atomic Energy Agency.

"We express here our deep concern about the future of nuclear power in view of the consequences of the earthquake and tsunami at the Fukushima-Daiichi NPP [nuclear power plant] in Japan," says the group's statement to IAEA Director-General Yukiya Amano. "We are confident that only nuclear power that avoids being a threat to the health and safety of the population and to the environment is acceptable to society," says the statement, headed "Never Again: An Essential Goal for Nuclear Safety."

The statement says that a complete analysis of the Japanese nuclear crisis is not possible until more information is available. But it faults the safety preparations put in place by Tokyo Electric Power Co. (TEPCO), owner of the stricken complex.

"It appears that, in the siting and design of the Fukushima-Daiichi nuclear plants, an unlikely combination of low-probability events (historic earthquake plus historic tsunami leading to loss of all electrical power) was not taken sufficiently into account. ... [H]indsight shows that relatively inexpensive improvements, detectable by more extensive analysis beforehand, may have avoided these accidents altogether."

The letter to Amano was signed on behalf of the group by Jukka Laaksonen, director-general of the Finnish Radiation and Nuclear Safety Authority and chairman of the Western European Nuclear Regulatory Association.

"I am acquainted with Japan's system well enough not to be surprised now," he said in an interview last month. "Their management system is complex and schematic, and it might not give as much weight to engineering reasoning and the best know-how as to formal rules and formal position." He did not repeat this criticism in the group's message to the IAEA.

Second aftershock rocks Japan

Current assumptions about the ability of nuclear plants to withstand the most severe disasters and emergencies must be reconsidered, the group said.

"This horror of four units going out, with multiple core damage, it has to open our eyes to being willing to talk more about that," said Mattson, who led the NRC's "Lessons Learned Task Force" following the Three Mile Island nuclear accident in 1979. "Some people say the reactors are safe enough and we don't have to spend the money" to do more. "I don't think they will hold the day," he said.

A month after the March 11 earthquake and tsunami, TEPCO is still battling to overcome damage to the reactor cores and put the three crippled reactors on a path toward a cold shutdown, while facing the threat of further quakes.

At 5:16 p.m. local time yesterday, northeast Japan was hit with another strong aftershock measuring 7.0 magnitude, but no increase in radiation at the Fukushima Daiichi site was reported.

Another quake measured at under 6.0 magnitude hit near the plant at 2 p.m. local time today, temporarily disrupting the moving of contaminated water away from the Unit 2 area. Workers continued to inject inert nitrogen gas into Unit 1, to lessen the risk of a hydrogen explosion, but pressure readings indicated the gas may be leaking, NHK reported.

Japan's science ministry said yesterday that the amount of radiation accumulated over about half a month in parts of Fukushima prefecture now exceeds permissible levels for a whole year, the NHK news service reported. The radiation readings at one place 30 kilometers northwest of the plant exceeds by more than 14 times the recommended long-term exposure, NHK said.

The latest condition report by Japan Atomic Industrial Forum Inc. continues the bleak assessment of conditions at the plant. It estimates that the fuel rods in Units 1, 2 and 3 are exposed partially or fully and that 70 percent of the reactor fuel in Unit 1, 30 percent in Unit 2 and 25 percent in Unit 3 has been damaged. The integrity of the reactor pressure vessels containing the fuel is not known, and core cooling systems are not working, forcing a continued reliance on emergency freshwater pumps to try to cool down the fuel units. The operability of the damaged reactors' control rooms is listed as "poor" because of the loss of normal electric power supply.

The heavy damage to the reactor cores was estimated up in an internal report March 26 prepared by an NRC reactor safety team, first described by The New York Times. It says damaged fuel in Unit 1 "may have slumped to the bottom of the core and fuel in the lower region of the core is likely encased in salt and core flow is severely restricted and likely blocked. ... Injecting fresh water through the feedwater system is cooling the vessel but limited if any flow past the fuel. ... It is difficult to determine how much cooling is getting to the fuel." Conditions in Units 2 and 3 were assumed to be somewhat less severe. The team's recommendations included injection of nitrogen into the reactor containment structures if possible, to limit hydrogen explosions within the containment.

Avoiding the 'natural tendency' of complacency

David Lochbaum, director of the reactor safety program for the Union of Concerned Scientists, said last week that the attempts to cool down the reactor cores are being gravely challenged on several fronts.

"Their instrumentation, their ability to monitor what's going on, they don't have many options," he said, noting that workers at the reactor have no margin for error as they handle two spent fuel pools and three reactor cores. "It's hard to say that things are going to get better or things are going to get worse, because they have so many challenges to face on so many places that it's going to be difficult to be 100 percent right all five times."

Many in the group that signed the statement are members or former members of the International Nuclear Safety Group, affiliated with the IAEA. In addition to the United States and Ukraine, the statement's signers come from Finland, France, Germany, India, Lithuania, Russia, South Korea, Spain and Sweden. They asked Amano to put their statement on the agenda of the IAEA meeting scheduled for June to review the Fukushima accident. The other signers from the United States were Harold Denton, former director of the NRC's nuclear reactor regulation, and Salomon Levy, former design and manufacturing manager for General Electric's nuclear division. The 16 signers did not include anyone from China.

The group said that the safety requirements for existing plants, built to earlier safety standards, should be rigorously reviewed, as many plants are being relicensed to operate for up to 60 years. Training of top reactor professionals must be upgraded to better equip them for dealing with reactor accidents and emergencies.

Nuclear regulators should improve reactor inspections "and guarantee openness and honesty in reporting the findings of such inspections to the public. Routine inspections are important; however, even more important is the capability to recognize early indications of low probability incidents or circumstances," the group said.

The group also addressed the safety standards for some 60 new reactors under construction or planned in 15 countries, led by China. Safety rules for new reactors should be strengthened to assure that backup power can maintain reactor and spent fuel cooling in an accident until normal power is restored. "New reactors should ensure safety even if operating personnel are not able to provide immediate response in an emergency," the statement says.

The statement does not directly address the question of how the safety of older plants could be made to match that of the new reactor designs.

The nuclear experts stressed the need for a strong safety culture shared by plant owners, operators and workers.

"We know that due to a natural tendency of human beings for complacency, the nuclear safety regime can erode; i.e., if we do not continuously pursue safety, we can [lose] safety," they said. "There are occasional signs that national and international safety assessments and peer review missions are becoming more focused on demonstrating that safety is satisfactory and in compliance with national and international standards than on finding and correcting deficiencies, be they in design, operation, or the standards themselves."