Geoff Brumfiel

Experts have been left scratching their heads over one of the most concrete concessions President Trump says he received from North Korean leader Kim Jong Un during Tuesday's summit in Singapore.

At a press conference after their meeting, Trump said Kim had agreed to destroy "a major missile engine testing site."

Updated: 10:45 a.m. ET

North Korea closed its nuclear test site in spectacular fashion Thursday, blasting the site in what one observer described as a "huge explosion."

NASA and the U.S. Department of Energy say they have successfully tested a new type of nuclear reactor that could one day provide juice to colonies on other worlds. The reactor can power several homes and appears able to operate in harsh environments.

Former weapons inspectors say an apparent trove of information on Iran's nuclear weapons program will increase pressure for more intrusive inspections of its atomic sites.

On Monday, the Israeli government disclosed it had obtained thousands of pages of documents and nearly 200 CDs' worth of data on Iran's nuclear program.

As chemical weapons inspectors wait to investigate an alleged strike near the Syrian capital of Damascus, former inspectors say the challenges the current team faces are daunting.

The inspectors arrived in Syria on April 14, on a mission to investigate a suspected chemical attack in the Damascus suburb of Douma seven days earlier. Unconfirmed reports from the scene suggest that dozens may have died.

The type of nerve agent used to poison a former Russian spy and his daughter in the U.K. was developed in a top-secret laboratory in Moscow and was once a closely held secret of the Russian government.

Sergei Skripal and his daughter, Yulia, were found slumped on a bench in the city of Salisbury on March 4. Experts quickly assessed that Skripal — a former Russian intelligence official accused of spying for the British — had been poisoned with a nerve agent.

On Monday, British Prime Minister Theresa May named the agent in a speech before Parliament.

As the news broke of a school shooting in Parkland, Fla., on Wednesday, hundreds of Twitter accounts believed to be under Russian sway pivoted.

Many had been tweeting about places like Syria and Ukraine — countries where Russia is seeking to strengthen its influence. Suddenly the accounts shifted to hashtags like #guncontrol, #guncontrolnow and #gunreformnow. Tweets mentioning Nikolas Cruz, the name of the shooting suspect, spiked.

The Trump administration released a report on the state of America's nuclear weaponry on Friday. The assessment, known as a Nuclear Posture Review, mainly concerns U.S. nukes and missiles.

This year, deep inside a mountain, North Korea detonated a giant nuclear bomb.

The intercontinental ballistic missile that North Korea launched on Wednesday appears to be significantly larger and more powerful than previous versions, according to independent analysts.

North Korea says a new intercontinental ballistic missile tested on Wednesday proves it has a nuclear deterrent that can reach any target in the United States.

According to a statement from the Korean Central News Agency, the ICBM is capable of carrying a "super-large heavy warhead, which is capable of striking the whole mainland of the U.S."

Copyright 2018 NPR. To see more, visit http://www.npr.org/.

DAVID GREENE, HOST:

The tiny nation of Denmark has just three stations for monitoring atmospheric radiation. Each week, scientists change out air filters in the detectors and take the used ones to a technical university near Copenhagen.

There, Sven Poul Nielsen and other researchers analyze the filters. They often snag small amounts of naturally occurring radioactivity, radon for example.

European authorities are providing new details about a cloud of mysterious radioactive material that appeared over the continent last month.

Monitors in Italy were among first to detect the radioactive isotope ruthenium-106 on Oct. 3, according to a fresh report by France's Radioprotection and Nuclear Safety Institute, known as IRSN. In total, 28 European countries saw the radioactive cloud, the report says.

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