Radiation levels near the site of a failed missile test on Russia’s White Sea reached as high as 16 times normal immediately following the Aug. 8 incident, according to the state meteorological service.

The radiation levels at the six stations declined steeply within half an hour and were close to normal levels by 2:30 p.m., according to Roshydromet. Severodvinsk, a city of 180,000, is about 40 kilometers from the offshore platform where the explosion occurred.

The blast killed five atomic scientists during a test of a missile engine that used “isotope power sources” on an offshore platform in the Arkhangelsk region, close to the Arctic Circle, the state nuclear monopoly Rosatom said over the weekend. The Defense Ministry initially reported two were killed in the accident, which it said involved testing of a liquid-fueled missile engine. The ministry didn’t mention the nuclear element.

Gamma radiation measured at six of eight testing stations in Severodvinsk at 12 p.m. ranged from 4 to 16 times the port city’s normal rate of 0.11 microsieverts per hour, with one observation point showing 1.78 microsieverts per hour, according to a Roshydromet statement. The World Nuclear Association estimates the hourly dose from flying at 30,000 feet in North America is 3 to 4 microsieverts.

Norway said it had stepped up radiation monitoring after the incident but hadn’t detected anything abnormal. Finland’s Radiation and Nuclear Safety Authority said it didn’t expect to find an increase in radiation after the incident.