April 26, 2020
Day 117 of 366

 

April 26th is the 117th day of the year. It is World Intellectual Property Day, established by the World Intellectual Property Organization (WIPO) to “raise awareness of how patents, copyright, trademarks and designs impact on daily life” and “to celebrate creativity, and the contribution made by creators and innovators to the development of societies across the globe.”

In the United States, today is “celebrated” as National Audubon Day, National Dissertation Day, National Help a Horse Day, National Kids and Pets Day, National Pretzel Day, National Richter Scale Day, National South Dakota Day, and National Pet Parents Day. That last one is typically observed on the last Sunday in April.

 

Historical items of note:

• In 1564, playwright William Shakespeare was baptized in Stratford-upon-Avon, Warwickshire, England. His actual date of birth is unknown.
• In 1803, thousands of meteor fragments fell from the skies of L’Aigle, France. The event convinced European scientists that meteors exist.
• In 1865, Union cavalry troopers cornered and killed assassin John Wilkes Booth in Virginia.
• In 1933, actress, singer, and producer Carol Burnett was born.
• In 1954, Akira Kurosawa’s Seven Samurai was released.
• In 1958, actor Giancarlo Esposito was born.
• In 1962, NASA’s Ranger 4 spacecraft crasheed into the Moon.
• In 1977, actor Tom Welling was born.
• In 1979, actress Stana Katic was born.
• In 1981, Dr. Michael R. Harrison of the University of California, San Francisco Medical Center performed the world’s first human open fetal surgery.

 

In 1986, a nuclear reactor accident occurred at the Chernobyl Nuclear Power Plant near Pripyat, Ukraine in the Soviet Union, creating the world’s worst nuclear disaster.

The accident started during a safety test on an RBMK-type nuclear reactor, which was commonly used throughout the Soviet Union. If you recall the discussion of the SL-1 accident on January 3rd, the common reactor type in the United States uses pressurized water as the moderator to help control neutron production, and as a result, power and heat generation. The RBMK-type reactor uses graphite as a moderator with moving water to cool the core, which was a contributing factor to the accident.

Other contributing factors included active removal of decay heat (the heat generated from the decay of fission products), positive void coefficient properties (an estimate of how much reactivity is added to the core when voids appear in the system), and instability at low power levels.

The test was a simulation of an electrical power outage. This test was being used to help develop a safety procedure to maintain reactor cooling water circulation until back-up electrical generators could provide power to the complex. The blackout duration was about one minute and had been identified as a potential safety problem that could cause the reactor core to overheat. The hope was that residual rotational energy in a turbine generator could provide enough power to last for that minute, but while three tests had been conducted, no solution had been found.

On the fourth attempt, there was an unexpected delay of ten hours. Unfortunately, that pushed the test to a later shift, and the oncoming operators were not prepared for the experiment.

During the planned decrease of reactor power in preparation for the electrical test, the power unexpectedly dropped to a near-zero level. In a pressurized water reactor like those operating in the United States, this would likely result in the reactor shutting itself down due to the inherent negative coefficient of reactivity. But the RBMK’s low-power imbalances limited the ability of the reactor to burn off xenon-135, a fission product that hinders the rise of reactor power.

To overcome the xenon, operators raised power by disconnecting most of the control rods from their automatic control systems. Even worse, they manually extracted most of the rods to their upper limits to maximize positive reactivity and overcome the xenon to increase power.

The operators were able to only partially restore the specified test power, but the instability risk was not evident in the operating instructions, so the operators proceeded with the test. The operation of the reactor at the low power level and high poisoning level was accompanied by unstable core temperatures and coolant flow. Several alarms were triggered, but they were ignored to preserve testing power levels.

The combined effect of actions to this point was an extremely unstable reactor configuration. Nearly all of the 211 control rods had been extracted manually, including all but 18 of the “fail-safe” manually operated rods of the minimum 28 that were supposed to remain fully inserted to control the reactor even in the event of a loss of coolant.

Upon test completion, the operators triggered a reactor shutdown, but circumstances caused an uncontrolled nuclear chain reaction. A large amount of energy was suddenly released, vaporizing superheated cooling water and rupturing the reactor core in a highly destructive steam explosion. The result was an open-air reactor core fire that released radioactive contamination in a large plume for about nine days. The fire gradually released about the same amount of contamination as the initial explosion, resulting in a 10-kilometer exclusion zone around Pripyat and the evacuation of 49,000 people. The exclusion zone was later expanded to 30 kilometers when an additional 68,000 people were evacuated.

The explosion killed two of the operating staff. During the emergency response that followed, 134 station staff and firemen were hospitalized for acute radiation exposure. Twenty-eight of them died in the days to months following the accident, and an estimated fourteen cancer deaths were related to the event.

To reduce the spread of radioactive contamination from the wreckage and protect it from weathering, a protective sarcophagus was built by December 1986. This also provided radiological protection for the remaining operational reactors at the site. The sarcophagus was further enclosed in 2017.

Site clean-up is scheduled for completion in 2065.

The Chernobyl disaster is considered the worst nuclear power plant accident in history, both in terms of cost and casualties. The combination of the initial emergency response and later environmental decontamination have involved half a million personnel and approximately $68 billion. The accident resulted in safety upgrades on all remaining Soviet-designed RBMK reactors – as of 2019, ten of them remained operational – and continues to be an in-depth training topic for operators in the United States to this day.

 

The Thing About Today is an effort to look at each day of 2020 with respect to its historical context.

For more creativity with a critical eye, visit Creative Criticality.