27 August, Session 2A (continued)
Release from the Reactor and the Consequences for the Immediate Vicinity
The release of radioactivity into the environment occurred in 4 phases.
1.
The initial explosion propelled particles of fuel, complete with fission products into the atmosphere. A great deal of this fell locally, around Unit 4 and on the site.
2.
The rate of release decreased due to the material dropped from helicopters on top of the reactor. The composition of this release indicates that it is rather similar to the fuel itself.
3. The material on top of reactor insulates the core and the debris increases in temperature. Release increases and is predominantly of the more volatile fission products - iodine, caesium and tellurium although there is still a significant fraction from fuel particles.
4. A sharp reduction in release rate. This is attributed to improved cooling of the core and to the formation of more refractory chemical components which means that the fission products were locked chemically by the materials dropped into
the reactor.
A combination of calculation and measurement was used to evaluate the actual releases of radioactive materials from the core. These calculations seem to be based upon detailed, but relatively local measurements. The results indicated that the release on the first day was about 12 million curies. The major release occurred over a period of about 10 days and up to 50 million curies of activity were released in all. A similar quantity of radioactivity associated with noble gases was also released, but the radiobiological consequences would have been much less and it is usual not to include those quantities in further calculations. It is important to note that these releases were calculated on 6 May taking into account radioactive decay.
Over all, it is claimed that about 3 percent of the total radioactivity inventory was released to the environment, with differentially large amounts of iodine, caesium and tellurium, the more volatile species. Some 15-20% of the total inventory of the reactor were released.
Close to the reactor, the radiation levels were very high indeed in the early days of the accident. These levels seriously impeded recovery work and made continued operation of the other reactors difficult. Nevertheless, Unit 3, the one immediately adjacent to the damaged reactor continued to operate until 5 am about 31⁄2 hours after the accident. Units
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