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Defense in Depth Part I: A War for Safety

Thomas Wellock

One hundred years ago the French and German armies of World War I devised a new defensive strategy called “defense in depth.” Its aim was to prevent an enemy breakthrough of an army’s frontline with a deep system of interconnected trench lines and strong points.

Defense in depth circa WWI. Photo courtesy of the Library of Congress

Defense in depth circa WWI. Photo courtesy of the Library of Congress

Popularized in all its desperation and grisly effectiveness in films such as All Quiet on the Western Front, defense in depth has become the NRC’s official metaphor in the battle to protect the public from radiation hazards. It is the key concept governing nuclear safety in using multiple strategies in safety-system design, operations, and emergency procedures and planning.

The NRC’s use of the term has roots in the Manhattan Project of World War II. Military metaphors seemed particularly apt for those charged with ensuring the safety of the early plutonium production reactors at Hanford, Washington. They worried about the potential for a reactor “catastrophe” from a radiation release of “explosive violence.” Their solution was to erect multiple “lines of defense” of trained operators and emergency personnel, carefully sealed fuel rods, shielding walls, backup cooling and power systems, and even a backup to the backup shutdown system—a final solution so drastic that it would destroy the reactor to save the operators lives. Fittingly, its moniker derived from another military term — the “last ditch” safety device.

After the war, the “lines of defense” in reactor safety were categorized into functions by Atomic Energy Commission safety committees:

  1. Features that made a reactor inherently safe;
  2. “Static,” or physical, barriers, such as containment buildings, were to halt the escape of radiation; and
  3. Active systems were to shut down and cool the reactor in the case of unusual conditions.

While the AEC’s safety approach became more coherent, there was no consensus among experts over the relative importance of each category. Some experts focused mostly on a design’s physical barriers, while others gave weight to all three categories and included reactor operation too.

Over time, “defense in depth” replaced the scattered concept of “lines of defense.” Its first use appears to have been in 1958 to describe safety design in the plutonium extraction processes at Hanford. In a 1965 letter to Congress, AEC Chairman Glenn Seaborg applied the term to civilian reactor safety as an accident prevention and mitigating strategy.

It provided, he wrote, “multiple safeguards against the occurrence of a serious accident, and for containment of fission product release.” The term stuck.

But the story continues. The Office of Nuclear Regulatory Research has published a report on the history of defense in depth up to the present, which covers the term’s application to the whole nuclear fuel cycle. It’s a fascinating look at how this bedrock safety concept has evolved under the influence of events and new knowledge. We’ll have more on this report on Wednesday.



Tuesday’s Nuclear History Quiz

Chemist Lise Meitner with students (Sue Jones Swisher, Rosalie Hoyt and Danna Pearson McDonough) on the steps of the chemistry building at Bryn Mawr College. Courtesy of Bryn Mawr College. (April 1959)

An internationally renowned physicist, left, talks to students on the steps of the chemistry building at Bryn Mawr College circa 1959. Some consider her to be among the most significant women scientists of the 20th century. She was honored with the Enrico Fermi Award by the Atomic Energy Commission (the NRC’s predecessor agency) in 1966. In 1992, an element – the heaviest known in the universe – was named for her.

What is her name?

What is the name of the element?

Photo courtesy of Bryn Mawr College

Moments in NRC History: Regulating for Safety and Non-Proliferation, Part II

Thomas Wellock

RTR_2Part I of our Research and Test Reactor Series looked at the promise and unique safety challenges of research reactors, beginning with North Carolina State’s first civilian-owned reactor in 1953.

In Part II of our video series, we look at how the focus on safety of these reactors evolved into a concern about their security.

The Atomic Energy Commission (the NRC’s predecessor) had developed design requirements for research reactors with large safety margins that tolerated errors. Extensive training and supervision was required of licensed operators. Sabotage was foiled by making the reactors’ uranium fuel difficult to remove or destroy.

However, weapons proliferation became a persistent concern. Reactor designers favored fuel highly enriched in fissionable uranium-235. Uranium-235, however, was also the stuff of atomic bombs.

Initially, the AEC only permitted export of reactor technology with low enrichment, but in the 1960s, it granted international requests to U.S. manufacturers for high performance research reactors. The reactors needed only small quantities of enriched fuel, and it was believed bilateral agreements and regular inspections would assure the used fuel was returned to the U.S.

But events in the 1970s – including India’s detonation of a nuclear device made possible with fissionable material from a Canadian research reactor — demonstrated the limits of this approach.

Lowering the fuel enrichment was seen as a viable solution. In 1978, the Department of Energy launched a program to develop a low enriched fuel that met the performance needs of research reactors. In the U.S., operators of 20 research reactors opted to switch to low-enriched fuel.

MIHAfter the 9/11 attacks, the United States launched the Global Threat Reduction Initiative to accelerate the conversion to low-enriched fuel. Twenty-seven reactors around the world, including six in the United States made this conversion, taking out of circulation enough fissionable material to make 20 crude bombs.

The NRC also pursued enhancements against sabotage and theft with better staff background screening, access controls, security searches, and coordination with emergency responders.

The decline of the nuclear industry since the 1970s and the production of isotopes abroad have reduced the need for research reactors in the U.S. Their numbers have dwindled to about 30. This brought a new concern — the vulnerability of the nation’s isotope supply for medical uses, especially molybdenum-99.

The video explores how that vulnerability is being addressed and how the NRC continues to ensure research reactors operate safely in today’s threat environment. I hope you’ll take the time to watch the video.

NRC Celebrates A Milestone — 40 Years of Safety and Service

Tom Wellock
NRC Historian

It’s been 40 years since the Nuclear Regulatory Commission began operations on January 19, 1975. To be sure, the agency inherited a mixed legacy from its predecessor, the Atomic Energy Commission. The AEC had established an approach to reactor safety still used today, but critics claimed it worked too closely with the nuclear industry to promote nuclear power. As a new agency, the NRC had to demonstrate that it would be an unbiased, independent regulator.

40yearsOver the years, domestic and international events have challenged the NRC to define what independence meant. A new video on the NRC’s YouTube channel shows us how, in the early years, the essential elements of the NRC’s character were developed and remains today. For example, as the video shows, between 1975 and 1979, the NRC dealt with a major fire at the Brown’s Ferry nuclear power plant in Alabama,  a controversial request to export uranium to India, staff dissent over reactor safety, and tough questioning of its research conclusions regarding the probabilities of nuclear accidents.

From these experiences, the NRC learned that being an independent safety regulator took more than legislation. It meant cultivating a diverse staff, seeking out dissent and heeding critics. Safety research needed to be conducted free of perceived bias, and it learned the limits within which a regulatory agency may act under the United States’ constitutional separation of powers.

All these lessons have proven to be an asset for the NRC when it dealt with its greatest crisis — the 1979 accident at Three Mile Island; and in learning lessons from watershed nuclear events at both Chernobyl and Fukushima. We hope you’ll take a few minutes to watch the video.

Part II: Ensuring Safety in the First Temple of the Atom

Thomas Wellock
NRC Historian

https://www.lib.ncsu.edu/specialcollections/digital/text/engineAs noted in Part I of this story on the NC State research reactor, the Atomic Energy Commission (AEC) was very anxious to promote the world’s first civilian reactor. But its enthusiasm was tempered by the challenge of placing a reactor safely on a busy college campus and developing an approval process for non-AEC reactors.

The AEC turned to its Reactor Safeguard Committee, the forerunner of today’s Advisory Committee on Reactor Safeguards. The Committee was formed in 1947 to evaluate the safety of new reactors proposed by AEC laboratories and contractors.  “The committee was about as popular—and also necessary—as a traffic cop,” recalled Safeguard Committee Chairman Edward Teller.

The Committee’s most significant contribution was establishing a conservative approach to safety given the engineering uncertainty of that era. “We could not follow the usual method of trial and error,” Teller said. “The trials had to be on paper because the actual errors could be catastrophic.” The Committee developed a “simple procedure” of challenging a reactor designer to write a “hazard summary report” that imagined the worst “plausible mishap”—soon known as a “maximum credible accident”—and demonstrate the reactor design could prevent or mitigate it.

Five NC Stte physics professors designed the reactor. Here, in the reactor control room (left to right front row) are Clifford K. Beck and Arthur C. Menius, Jr. Standing is Newton Underwood, three unidentified students, Arthur Waltner and Raymond L. Murray.

Five NC State physics professors designed the reactor. Here, in the reactor control room, (left to right front row) are Clifford K. Beck and Arthur C. Menius, Jr. Standing is Newton Underwood, three unidentified students, Arthur Waltner and Raymond L. Murray.

The Committee focused on several hazards, including a surge in the chain reaction called a reactor “runaway,” a catastrophic release of radioactive material from fire, sabotage, or an earthquake, and hazards from routine operation that might result from leaks or inadvertent exposures. The Committee asked NC State to address these concerns in a “hazards summary report.”

To meet the Committee’s desire for inherent safety, NC State proposed a “water boiler” reactor, which was believed to have “student-proof” safety margin given its strongly “negative coefficient” of reactivity that limited greatly the possibility of a runaway. NC State also developed interlocks and an extremely dense concrete shielding to discouraged sabotage.

In order for NC State to commit the funds to such a long-term project, it needed an early approval. This created a dilemma since the college did not yet have a detailed, complete design.  The AEC used a two-step conditional approval that was similar to its later construction permit/operating license process. In step one, construction did not begin until NC State addressed the most important design safety issues. When it did, the AEC agreed by contract to supply enriched fuel. The fuel was not delivered, however, until NC State resolved all outstanding safety questions and a final inspection took place. With that, the first civilian reactor in history went critical in September 1953.

The AEC approach to safety at NC State foreshadowed many later regulatory practices. As important as the 1954 Atomic Energy Act is to current regulatory practice, it is interesting to see that many of the critical elements have even deeper roots back toward the beginning of the atomic era.


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