Sodium cooled nuclear reactors safer? Not necessarily so

While no sodium-cooled reactors currently operate in the United States, the U.S. Department of Energy (DOE) is working with industry on a number of “advanced” reactor designs, including the Sodium-Cooled Fast Reactor (SFR).  One of the SFR’s safety advantages, to quote the DOE, is that the design provides a “Long grace period for corrective action, if needed.” SRE’s meltdown transpired over a two-week period. Fermi Unit 1 had indications of inadequate core cooling in June that were repeated in August and dismissed until extensive damage occurred in October 1966. The “if needed” grace period is never long enough when warning sign after warning sign is dismissed or ignored.

DOE did acknowledge some “challenges” for the SFR: their higher speed and higher energy neutrons can embrittle and degrade nearby materials, liquid sodium coolant reactors with air and water and degrades concrete, and the opaqueness of the liquid sodium coolant complicates in-service inspections and maintenance.

Thank goodness for the “Long grace period for corrective actions, if needed.” That and the fact that SFRs only operate in cyberspace where the primary threat is carpal tunnel syndrome


Nuclear Plant Accidents: Fermi Unit 1, Union of Concerned Scientists
 , director, Nuclear Safety Project | July 12, 2016, Disaster by Design

Jorge Agustin Nicolás Ruiz de Santayana y Borrás, also known as George Santayana, wrote that “Those who cannot remember the past are condemned to repeat it.”
Disaster by Design/Safety by Intent #39 described the partial meltdown of the reactor core at the Sodium Reactor Experiment (SRE) in California. Workers at the Fermi Unit 1 reactor in Michigan must have remembered this accident pretty well, since they duplicated almost every key aspect of it just seven years later.

So, perhaps a companion to Santayana’s point is “Those who remember the past are condemned to repeat it, unless they take steps to prevent it.” Had SRE’s owners copyrighted their accident script, Fermi Unit 1’s owner would probably have had to mail them a royalty check.

Fermi Unit 1(Newport, MI) – October 1966 Unit 1 at the Enrico Fermi Atomic Power Plant had a fast breeder reactor cooled by liquid sodium. The operators achieved initial criticality of the reactor on August 23, 1963. An extensive testing program kept the reactor at very low power levels—too low to make electricity—until December 29, 1965. Completion of the low-power testing program enabled the operators to increase the reactor power level to the 10% and later 50% power testing plateaus.

The 50% testing plateau included a 60-hour steady state run that began on August 5, 1966, and ended on August 7. During this run, workers noticed abnormally high temperatures of the liquid sodium flowing out of some fuel elements. Outlet temperatures 20 to 25% higher for one fuel element than outlet temperatures for other fuel elements had been observed during June 1966. The outlet temperatures were 40 to 47% above other outlet temperatures during the August 1966 run at 50% power.

The reactor was shut down after the 60-hour run. Workers relocated four fuel elements that exhibited high outlet temperatures to other positions in the reactor core. They wanted to determine whether the high temperatures were caused by the fuel elements or were due to faulty thermocouples providing falsely high indications.

The operators restarted the reactor on October 4, 1966. They slowly and steadily increased the reactor power level. By the mid-afternoon of October 5, the reactor power level had reached about 15%. Plant parameters did not look right to the operators. The outlet temperatures of some fuel elements still indicated abnormally high. And now the operators noticed that the control rods were more withdrawn from the reactor core than expected for this power level.

At 3:09 pm, plant conditions deteriorated further. The radiation monitors in the ventilation exhaust ducts from the reactor building alarmed and automatically shut dampers to isolate flow to the environment from this pathway. This radiation monitor’s reading could not be easily dismissed as a faulty instrument—radiation monitors in four other areas of the plant were also indicating high readings. The operators shut down the reactor.

Debris in the reactor

Two fuel elements were found to have partially melted due to inadequate cooling. A crumpled piece of metal was recovered from the core sodium inlet plenum at the bottom of the reactor vessel below the reactor core region. Media reports at the time claimed that a beer can left inside the vessel or piping during construction blocked flow through the reactor core and caused the partial meltdown.

Examination of the metal debris determined that neither alcohol nor poor housekeeping caused the partial meltdown. Instead, a feature installed late in the reactor’s design intended to provide better protection in event of a meltdown triggered a meltdown………

Missed Opportunities = Pre-Existing Problems = Reactor Accident

Like the sinking of the Titanic leading to the capsizing of the Eastland three years later, the good intention of making the plant safer actually compromised its safety……….

While no sodium-cooled reactors currently operate in the United States, the U.S. Department of Energy (DOE) is working with industry on a number of “advanced” reactor designs, including the Sodium-Cooled Fast Reactor (SFR).  One of the SFR’s safety advantages, to quote the DOE, is that the design provides a “Long grace period for corrective action, if needed.” SRE’s meltdown transpired over a two-week period. Fermi Unit 1 had indications of inadequate core cooling in June that were repeated in August and dismissed until extensive damage occurred in October 1966. The “if needed” grace period is never long enough when warning sign after warning sign is dismissed or ignored.

DOE did acknowledge some “challenges” for the SFR: their higher speed and higher energy neutrons can embrittle and degrade nearby materials, liquid sodium coolant reactors with air and water and degrades concrete, and the opaqueness of the liquid sodium coolant complicates in-service inspections and maintenance.

Thank goodness for the “Long grace period for corrective actions, if needed.” That and the fact that SFRs only operate in cyberspace where the primary threat is carpal tunnel syndromehttp://allthingsnuclear.org/dlochbaum/nuclear-plant-accidents-fermi-unit-1

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