TNAG-1503-FCO40-2061-Guangdong-nuclear-power-station-project-at-Daya-Bay-safety-c-1986 — Page 67

r exploded in the air. Volatile and gaseous radionuclides Were freely evolved into the air by the overheated fuel.

None of this would have happened had the operators, (by switching off vital safety systems and ignoring the rule about the minimum number of inserted control rods) not allowed the reactor just prior to the experiment to be poised on the threshold of just such a reactivity-excursion.

Development and Consequences of the Accident

The attention of the IAEA meeting now turned to the immediate consequences of the accident. Legasov continued the presentation, following the written report once more.

The initial release of radioactivity missed the adjacent town of Pripyat. Evacuation was delayed, but Legasov defended this saying that they were initially safer where they were. Indeed those in stone houses were forbidden to leave since the

masonry sheltered them from radiation. However, the graphite fire, the increase in graphite temperature and the continued release of activity soon made evacuation of Pripyat vital. was accomplished in 2 hours.

It

The amount of radioactivity released was greatest on the first and ninth days following the accident. The second peak occurred when decay heat and fire had raised the core to its maximum temperature of 2000°C. The graphite fire resulted in the production of a radioactive aerosol which went up into the atmosphere. It was to stifle the graphite fire and stop the escape of the aerosol that 5000 tons of sand, boron carbide and lead were dropped onto the exposed face of the reactor from helicopters. The boron carbide was to prevent fission from restarting locally. The lead was to absorb heat, absorb radionuclides and shield the helicopters from gamma radiation. The sand and clay were to act as aerosol filters. By 6 May the release of activity had ceased, or virtually so, the rise in temperature having been reversed by natural convection of air and by creating a forced flow of cold nitrogen through the

core.

Three and a half percent of the radioactive core material were released.

Temperature measurements in and around the reactor core could not be made except with simple devices such as thermionic valve amplifiers or materials of known melting point. The radiation fields rendered semiconductor instruments unreliable. With the exception of the reactor vault itself, radiation fields have now fallen from thousands of Roentgens per hour (the maximum recorded) to no more than one or two R per hour.

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