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TopChazhma Bay Accident
Nuclear submarine refueling at shipyard in Chazhma bay has been completed on August 2, 1985. Pressure test revealed that bow reactor top cover was mounted untight due to the fragment of welding rod that was clenched between reactor top cover and copper pad on the reactor vessel. This circumstance required reactor cover removal and copper pad substitution. During the reactor cover lift on August 10, accidental clench and withdrawal of reactivity compensation lattice occurred that was followed by spontaneous chain reaction (SCR). As a result of energy burst internal reactor structures were destroyed and partially thrown out together with overheated steam-air mix, fire started onboard. Accident occurred with “fresh” nuclear fuel in the active core and radionuclide composition of the release contained short lived fission products (krypton, xenon, iodine) and activation products (mainly 60Co).
Some SCR features were assessed in Bogatov, Kiselev, and Shvedov (2011). Indirect assessments on the base of observable consequences of explosion made it possible to assess energy release about 100 kg of trinitrotoluene equivalent, fuel amount that formed critical mass was evaluated as several dozen of kilograms, about 1019 nuclear fission (energy equivalent Q ≈ 3.108 J) occurred. This value of specific energy release (~ 1000 cal/g(U)) is enough for fuel melting and dispersion into fine particles, corresponding temperature of steam generation was taken as 1800°K.
Assuming steam thermal capacity at this temperature Cp = 2.8.103 J/kg/grad, specific steam generation heat r = 2.3,106 J/kg, mass of evaporated water M at the overheat temperature ΔТ=1500°K can be roughly assessed as:
(1)Energy burst was fast, and main part of the release was contained in overheated steam-air mix. At the time of accident there was southeastern wind with velocity of 5 m/s, periodical drizzling rain, atmospheric stability class D by Pasquill (Sivintsev et al., 2005).