Boron, Sand, and Chernobyl
Author: A Millisecond of Eternity
I recently came across HBO’s highly rated miniseries Chernobyl by chance. Curious to learn more, I sought it out and watched it. The series is almost textbook-quality in every aspect of its production, from its plotting and cinematography to its sound design. Even though viewers already know how the disaster ended, the oppressive, tragic atmosphere remains deeply affecting. It recreates the historical setting of the 1986 Chernobyl nuclear disaster to a remarkable degree, and I strongly recommend it to interested readers.
Returning to the subject: most readers will know something about the 1986 Chernobyl disaster, the worst nuclear catastrophe in human history. Estimates suggest that it released more than 400 times as much radioactive material as the atomic bomb dropped on Hiroshima during the Second World War. A report issued by relevant United Nations agencies in 2005 estimated that nuclear radiation might eventually cause 4,000 deaths. Greenpeace International’s 2006 estimate was far more alarming: the potential death toll in Ukraine, Russia, and Belarus could reach 93,000, and radiation had induced cancer in roughly 270,000 people across those countries.
In the second episode, scientist Valery Legasov proposes dropping boron and sand into the reactor. His reasoning is that boron can absorb neutrons and slow the chain reaction, while sand can smother the fire by cutting it off from the air. Why, then, were sand and boron the scientists’ first response? If a similar nuclear disaster occurred today, would they do the same thing?
What happens when a burning reactor core lies exposed? According to Kathryn Huff, a nuclear reactor engineer and professor at the University of Illinois Urbana-Champaign, an exposed core burning in open air creates at least two problems.
The first is the continuing nuclear fission reaction. When uranium undergoes fission, it releases neutrons. Those neutrons strike other uranium atoms and split them, releasing still more neutrons, while the uranium atoms release increasing amounts of energy. Because the reaction is no longer controlled, it produces an almost unimaginable level of radiation, posing a lethal danger to anyone who tries to approach the reactor.
The second—and more serious—problem is that the fire sends vast quantities of smoke, dust, and debris into the air. Most of the material produced by the explosion came from the nuclear reactor, and some came directly from the core. This included radioactive elements and isotopes formed when uranium atoms split.
This is the most dangerous part of the accident. Some of these isotopes can harm the human body, and they are far more radioactive than the materials encountered in everyday life. Other substances are not only highly radioactive but also highly mobile in the environment. Here, mobility means that the isotopes can enter living organisms and cause severe problems. Take iodine-131, a radioactive isotope of iodine. Just as they absorb ordinary iodine, living cells also absorb iodine-131.
Chernobyl’s smoke and dust contained large quantities of iodine-131. It could drift for thousands of kilometers before entering rivers, falling onto soil, or being absorbed directly by plants, animals, and people. Our thyroid glands depend on iodine and absorb iodine-131 just as they absorb ordinary iodine, creating a serious, long-lasting source of radiation inside the body. This is why people in affected areas should take iodine tablets after a nuclear disaster. Flooding the thyroid with nonradioactive iodine reduces its uptake of radioactive iodine isotopes from outside the body. Iodine tablets should not be taken casually in ordinary circumstances, of course, because doing so may cause problems such as hyperthyroidism.
Sand and Boron
The scientists’ immediate decision to drop sand and boron now makes sense. In fact, the mixture dropped at Chernobyl also included clay and lead. They were trying to address the first and second problems as far as possible. Sand could smother the fire by excluding air, while boron could, in theory, slow or suppress the nuclear reaction.
In a nuclear reactor, some isotopes accelerate the reaction while others slow it down. Sustaining a nuclear chain reaction requires enough radioactive isotopes to be packed closely together for the many neutrons they release to strike and split other nuclei. The chain-reaction diagram illustrates the process: ① a uranium-235 atom absorbs a neutron, splits into two smaller new atoms, and releases three neutrons and binding energy; ② one neutron is absorbed by uranium-238 and does not continue the chain reaction, another does not collide with any atom, and the final neutron strikes another uranium-235 nucleus, releasing two neutrons and continuing the chain reaction. In this “Sand and Boron” section, the accompanying image shows graphite moderator blocks ejected from the core; the largest block contains a complete control-rod channel. When certain isotopes interact with neutrons, however, the structure of their nuclei gives them some probability of absorbing those neutrons. Uranium—especially uranium-235—tends to capture a neutron and then undergo fission immediately. Boron has both a high neutron-absorption cross section and the ability to absorb neutrons across a broad range of energies, so it is particularly likely to capture them. In theory, therefore, dropping enough boron—or boron compounds—into the exposed core of Reactor No. 4 would absorb large numbers of fast-moving free neutrons, slowing or stopping the reaction.
Despite the enormous sacrifices involved, the plan had no obvious effect, in part because the way the helicopters dropped the material prevented the neutron absorber from reaching the core. A 1997 BBC report noted: “The intense radiation killed several pilots. Despite these sacrifices, very little neutron absorber reached the core.”
The outcome fell short of expectations, but the principle behind using sand and boron—stopping the reaction with a neutron absorber and reducing radioactive particles in the air—was sound. If a similar disaster happened today, the response would still rely on the same underlying theory. The greatest difference is that modern nuclear power stations are designed from the outset to handle much of the emergency response themselves.
Modern reactors are equipped with chemical sprays that can cover the entire reactor building and remove radioactive isotopes from the air before they escape.
Unlike the Chernobyl Nuclear Power Plant, nuclear facilities in the United States are entirely enclosed within sealed reinforced-concrete structures. These containment structures are designed to be extremely robust; in theory, even a major explosion would not breach them. A small jet could crash into a building’s protective shell without exposing the reactor core. In 1988, the U.S. government wanted to know how strong the reinforced concrete used in nuclear reactor facilities really was, so it designed an experiment. An F-4 Phantom jet was launched at approximately 770 km/h and sent head-on into a concrete slab. One can imagine the consequences of a 9/11-style attack on a nuclear reactor, but fortunately the test anticipated the threat. The 3.66-meter-thick reinforced concrete emerged almost unscathed. Although the image of the collision is shocking, the aircraft inflicted surprisingly little damage on the concrete block. The deepest scar measured only 60 mm, and the structural damage amounted to little more than a scrape.
Any small mistake can be fatal, however, so the U.S. Nuclear Regulatory Commission (NRC) drafted emergency manuals hundreds of pages long, and those manuals include the use of boron. Each of the 98 nuclear reactors operating in the United States has such a manual. These documents tell emergency personnel what to do in a range of emergencies that seem plausible but are, in fact, extremely unlikely.
Although the NRC has developed guidance for more extreme disasters since the 9/11 attacks, the manuals do not discuss a Chernobyl-style event in detail. “Even so,” Huff said, “when it came to extinguishing a fire over an exposed core, sand and boron ultimately deserve at least some of the credit.”
Finally, I would like to remind everyone not to venture there blindly just because “exploring Chernobyl” sounds cool. Some parts of the Chernobyl Exclusion Zone still have high radiation levels or uncertain conditions, and entering them without due care may be dangerous. Anyone who genuinely plans to visit should follow all local rules and the instructions of professional guides and site administrators.
References
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