Inspectors for the International Atomic Energy Agency have been to some of the world's most sensitive nuclear facilities — from North Korean reactors to Iranian uranium plants. But it all seems straightforward compared to what awaits them at the Zaporizhzhia Nuclear Power Station in southern Ukraine.

Since March, the plant has been occupied by Russian forces, and run by a skeleton crew of Ukrainian workers. When they arrive, inspectors will walk past the boarded-up hulk of the main administrative building, which was pummeled by rocket-propelled grenades during the initial invasion. A nearby courtyard holds the charred remains of military tents, razed by a retaliatory Ukrainian drone strike in late July. In recent weeks, shells have punched through the roofs of vital support buildings, and wildfires have threatened the plant's power lines.It's all happening at a nuclear facility — Europe's largest — that even in peacetime can be daunting, says Lars van Dassen, the executive director of the World Institute for Nuclear Security, a nonprofit in Vienna. Van Dassen has visited Zaporizhzhia, and says its six massive reactors and sprawling auxiliary buildings make the site a challenge to 

The world's nuclear watchdog has its work cut out

The International Atomic Energy Agency is the world's nuclear watchdog. In the past, it has been charged with making sure that nations do not illicitly pursue nuclear weapons. Inspectors have caught inconsistencies in North Korea's plutonium inventories, and checked that Iran's uranium enriching centrifuges are not producing bomb-grade material.But the agency also conducts more run-of-the-mill inspections at nuclear power stations all over the world, according to Shirley Johnson, a former nuclear inspector with the agency who now runs a U.S.-based private consultancy. Though important, "the most boring inspection you can do is a power plant," Johnson says. Inspectors typically check the books and make sure the reactor's inventory of nuclear fuel and nuclear waste matches what's on paper. They'd also make direct measurements to ensure that the nuclear material is really what it's reported to be. Normally "you can do a power reactor in half a day," Johnson says. 

The IAEA has been to Zaporizhzhia many times before on these routine missions, says Kevin Veal, the head of international nuclear safeguards at the U.S. National Nuclear Security Administration. "The agency has had a really good handle on activity at the facility for well over two decades," he says, referring to the IAEA.

This inspection is far from routine, however. The inspectors' visit was delayed for months, until the latest fighting around the plant put enormous diplomatic pressure on Russia, according to Patricia Lewis, the director for international security at Chatham House in London.Nuclear plant inspections,