A protective steel shell built to contain the remains of Chernobyl’s destroyed Reactor 4 can no longer perform its main safety function following a drone strike earlier this year, the UN’s nuclear watchdog has confirmed.

The International Atomic Energy Agency (IAEA) said that the New Safe Confinement (NSC) structure – the vast arch installed over the reactor and its ageing concrete “sarcophagus” – has “lost its primary safety functions, including the confinement capability” after being hit in February.

An expert mission from the agency completed a detailed safety assessment at the site last week. While the inspection found no permanent damage to the shield’s load-bearing elements or its radiation monitoring systems, the strike and subsequent fire seriously degraded parts of the multilayer protective envelope, including cladding and insulation.

According to Ukrainian authorities, the attack involved a drone carrying a high-explosive warhead that struck the northern section of the structure above a service area for the internal gantry crane, igniting a fire in the outer layers of the roof. Kyiv has accused Russia of carrying out the strike; Moscow has denied responsibility. The IAEA, consistent with its practice, has not attributed blame.

The February incident did not result in any measurable release of radioactive material, and radiation levels around the plant have remained within normal ranges for the exclusion zone. The innermost barrier of the NSC and the original concrete sarcophagus enclosing the reactor core were not breached.

Rafael Grossi, the IAEA’s director general, said that although immediate emergency repairs had been carried out on sections of the roof, more extensive work is required. He described “timely and comprehensive restoration” as essential to prevent further deterioration of the structure and to ensure long-term nuclear safety at the site.

Independent specialists have characterised the current risk to public health as low, largely because the most contaminated material remains encapsulated inside the original concrete shell built after the 1986 accident. The principal concern is that damage to the newer outer arch could, over time, allow water ingress, corrosion and weathering, making the older sarcophagus more difficult to manage and potentially increasing the chance that radioactive dust could be disturbed in future.

Chernobyl, located north of Kyiv near the border with Belarus, was the scene of the world’s most serious civil nuclear accident in April 1986, when a reactor test went wrong and triggered an explosion and fire. Large amounts of radioactive material were dispersed across Ukraine, Belarus and other parts of Europe. The Soviet authorities responded by erecting a hastily constructed concrete and steel enclosure over the destroyed reactor, designed with a projected lifespan of around 30 years.

Concerns over the long-term stability of that structure led to an international effort, coordinated by the European Bank for Reconstruction and Development, to design and fund the New Safe Confinement – a 36,000-tonne steel arch, more than 100 metres high and 257 metres across, which was assembled adjacent to the reactor and slid into place in 2016. It was engineered to operate for at least a century, allowing remotely operated cranes and machinery inside to dismantle the old sarcophagus and handle highly radioactive debris.

The drone strike and subsequent fire have now altered those assumptions. Ukrainian officials have indicated that the cost of fully repairing the NSC may exceed €100m, outstripping the remaining funds in the dedicated international Chernobyl account. The European Bank for Reconstruction and Development and several donor states have signalled a willingness to support restoration, but detailed plans and timelines have not yet been made public.

The incident has drawn renewed attention to the vulnerability of nuclear sites in active war zones. Since Russia’s full-scale invasion of Ukraine in 2022, the IAEA has maintained a continuous presence in the country, including at Chernobyl and at the Russian-occupied Zaporizhzhia nuclear power plant in the south. The agency has repeatedly warned that strikes on energy infrastructure and on or near nuclear facilities increase the risk of an accident, particularly if off-site power supplies used for cooling and safety systems are disrupted.

As part of its latest mission, the IAEA has also been inspecting electrical substations and grid nodes considered critical for nuclear safety. Grossi noted that these assets are “indispensable” both for supplying power to cooling and safety systems at nuclear plants and for transmitting electricity generated by those plants to households and industry across Ukraine.

For now, Chernobyl’s radiation levels remain stable and the site continues to be managed by Ukrainian personnel under IAEA supervision. The key question for Ukraine and its international partners is how quickly the damaged shield can be fully repaired, restoring the level of protection for which the New Safe Confinement was originally designed.