Security analysts say a nuclear test would almost certainly prompt other countries to follow suit.
Russia's parliament has passed the last stage of a law to withdraw Moscow's ratification of the Comprehensive Nuclear Test Ban Treaty (CTBT), which now goes to President Vladimir Putin for signing. Here is a look at some questions that raises.
What is the CTBT?
The CTBT, agreed in 1996, bans "any nuclear weapon test explosion or any other nuclear explosion" anywhere in the world, with the goal of reducing and ultimately eliminating nuclear weapons. Its preamble says it is aimed at supporting disarmament and non-proliferation by constraining the development of nuclear weapons and stopping countries producing more advanced ones.
Which countries have signed and ratified?
A total of 187 states have signed the treaty, and 178 have ratified it in their parliaments.
Of the nine countries that possess nuclear weapons:
- Britain and France have signed and ratified.
- The United States, Israel and China have signed but not ratified. Russia signed and ratified but is now withdrawing ratification, saying it is "mirroring" the U.S. position.
India, Pakistan and North Korea have yet to sign or ratify.
What is the legal status of the treaty?
The treaty is not legally valid until 44 named countries sign and ratify it - all nine that have nuclear weapons, and 35 others that possess nuclear power and research reactors.
Does it have any practical effect then?
Yes, in practice the treaty has created a taboo against explosive nuclear tests. No country has carried such a test since the 1990s except North Korea, which conducted the most recent of its six tests in 2017.
The treaty established a global network of observation posts that can detect the sound, shockwaves or radioactive fallout from a nuclear explosion. When complete, it will comprise 321 monitoring stations and 16 laboratories, hosted by 89 countries. Around 90% are already operational, including in Russia and the United States.
What Does the Russian Shift Mean?
Russia says it is not planning to conduct a test, and will not do so unless the U.S. does. But some analysts see a Russian test as more likely now. Putin has issued what the West sees as nuclear threats since the start of the Ukraine war and may be keeping testing in reserve in case Russia's fortunes decline sharply, when he could use it to warn the West to back off. Publicly, he has not said if he thinks a test is needed or not.
What Would a Nuclear Test Achieve?
Not much in scientific terms, experts say, because full-scale detonations can be accurately simulated using "subcritical" tests without a nuclear chain reaction.
Both Russia and the United States have advanced research programs to understand weapon performance and behaviours, said Dylan Spaulding, senior scientist at the Union of Concerned Scientists: "That kind of research doesn't require full-scale testing, it requires computer modelling."
But experts say a test would send a hefty political signal.
Andrey Baklitskiy, senior researcher at the U.N. Institute for Disarmament Research, said any nuclear weapons state would do this only in exceptional circumstances:
"The argument 'Maybe our nuclear warheads will be a little bit better' - that wouldn't be a very good argument why you'd do this."
Should We Worry About a Resumption of Nuclear Testing?
Security analysts say a nuclear test would almost certainly prompt other countries to follow suit, wrecking the CTBT and setting off a new arms race. Anti-nuclear campaigners say it could cause environmental damage, although testing - unlike in the early decades of the Cold War - would be carried out deep underground. But it would also point to wider instability.
"If we're in a world in which testing is going on, the first thing it shows us is that nuclear risks have risen," said James Acton, co-director of the nuclear policy program at the Carnegie Endowment for International Peace.
"I think testing is both a symptom of growing tensions, and would also further exacerbate those tensions."
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