Here’s the science behind nuclear weapons testing
President Trump wants the U.S. to resume nuclear weapons tests. See what that would mean
Nuclear weapons tests were once a regular occurrence, but most countries haven’t tested in decades, following the adoption of the Comprehensive Nuclear-Test-Ban Treaty in 1996. Now, that moratorium might be nearing an end. Politicians, including U.S. President Donald Trump, have called for tests to resume.
The Comprehensive Nuclear-Test-Ban Treaty was signed but not ratified by the U.S. and a few other holdouts, which means the treaty hasn’t come into force. Up to now, most countries have abided by its provisions anyway. The last nuclear test was performed in 2017 by North Korea, the only country to test a weapon this century.
U.S. scientists currently certify the capabilities of nuclear weapons using a variety of experiments and computer simulations. As part of a program called Stockpile Stewardship, these studies provide evidence that the weapons work — no nuclear blasts needed.
Watch the video for more on how nuclear weapons are understood scientifically and what a resumption of testing could entail.
Nuclear weapons, explained
Transcript
The United States hasn’t tested a nuclear weapon since 1992, but that era of restraint may be about to end. President Donald Trump has stated that he wants the United States to resume nuclear testing, and the country has an aging nuclear arsenal and an expansive effort to modernize it that is currently underway. The pressure to test is higher than it’s been in decades.
Nuclear weapons may be the most important thing that you think about the least. They remain largely out of sight and mind in everyday life. But if a major nuclear war broke out, these weapons could cause unprecedented destruction. Such a war could directly kill hundreds of millions of people and disrupt Earth’s climate, causing widespread famine and billions of additional deaths.
At the moment, the U.S. and Russia each have more than 5,000 nuclear weapons. A handful of other countries have tens or hundreds of weapons. In my reporting for Science News, I dug into the history of nuclear testing, the science of how these weapons are studied in the absence of tests and what makes the underlying physics of nuclear blasts so complex.
And how a return to testing could carry serious global consequences.
Between 1945 and the 1990s, countries around the world performed more than 2,000 nuclear tests. But by the mid ‘90s, nuclear testing had largely ceased, ushering in a testing taboo. The only country to test a nuclear weapon this century is North Korea. Nuclear weapons policies are based on deterrence. The idea is this: If your enemy knows you have nuclear weapons and knows that they work, they will be dissuaded from attacking you out of fear of devastating retaliation.
But if the U.S. or another country broke the testing taboo, others would likely follow. In short, one country testing could trigger a domino effect.
Nuclear weapons are based on two main types of nuclear reactions: fission and fusion. Fission is the splitting of large atomic nuclei. Fusion is when two small atomic nuclei merge into one. Nuclear blasts begin with fission. Each fission initiates other fissions in a self-sustaining chain reaction. In modern weapons, that fission helps to kick off fusion reactions, greatly amplifying the energy released. In the era of testing, scientists would explode a weapon to ensure that the fission and fusion reactions were proceeding as expected.
When tests were phased out, scientists had to develop other methods to certify the weapons’ capabilities.
The U.S. now has an extensive program called Stockpile Stewardship, which combines sophisticated experiments and computer simulations to determine whether weapons will function if needed. Some of the most secretive experiments take place deep underground at the Nevada National Security Site, the same site where explosive tests of nuclear weapons were once performed.
Those experiments are different from the explosive tests of the past. They are subcritical, meaning they don’t produce a self-sustaining chain reaction or trigger a nuclear blast. In a subcritical experiment, scientists study the material at the heart of these weapons: plutonium. Every nuclear weapon in the U.S. has a pit, a hollow sphere of plutonium. When a weapon is detonated, explosives implode the pit enough that it goes critical, meaning it can host a self-sustaining fission chain reaction.
In a subcritical experiment scientists similarly blast plutonium with explosives, but not enough to cross that criticality threshold. They then study what goes on inside the plutonium using X-ray imaging and other diagnostics. One of the biggest concerns scientists are investigating with these experiments is how the weapons age. The U.S. stockpile consists of decades old weapons. Scientists need to know if they will perform as expected, or if aging plutonium and other components could cause them to fail.
In the early days of nuclear weapons, tests were performed above ground in the open air. That spread radioactive fallout, contaminating the land and potentially sickening and displacing nearby populations.
To prevent such harmful effects, testing moved underground. In underground tests, fallout was largely contained within a cavity formed by the blast deep below the surface, but occasional accidents might still harm people or the environment.
Nuclear tests above or below ground have now largely ceased. Subcritical experiments don’t produce a nuclear explosion, so there’s no contaminated cavity formed or risk of a major release of radiation.
Many scientists argue that subcritical experiments have given us a better understanding of nuclear weapons than we had during the testing era, making full scale tests unnecessary. Others believe that while testing could improve our knowledge of the weapons’ inner workings, the risks far outweigh the benefits. A return to nuclear testing could spark a new arms race and encourage countries without weapons to develop their own arsenals. And that, those scientists say, could push the world closer to nuclear catastrophe.
