[1] K-431, completed around 1965 as unit K-31, was a Project 675 or Echo II-class submarine with two pressurized water reactors, each of 70 MWt capacity and using 20%-enriched uranium as fuel.

The contaminated forest area was later surveyed as 2 km2 (490 acres) in a swath 3.5 km (2.2 mi) long and 200 to 650 m (660 to 2,130 ft) wide.

Initial estimates of the radioactive release were about 74 PBq (2 MCi) of noble gases and 185 PBq (5 MCi) of other fission products,[4] but most of this was short-lived isotopes; the estimated release inventory one hour after the accident was about 37 TBq (1000 Ci) of non-noble fission products.

In part because the reactor contained no accumulated spent nuclear fuel, the fraction of biologically active isotopes was far smaller than in the case of the Chernobyl disaster.

[5] The same source suggests that the total release was about 259 PBq but due to radioactive decay this decreased to 43 TBq after 24 hours.