http://www.nea.fr/html/rp/chernobyl/c01.html<snip>
The accident
The accident occurred at 01:23 hr on Saturday, 26 April 1986, when the two explosions destroyed the core of Unit 4 and the roof of the reactor building.
In the IAEA Post-Accident Assessment Meeting in August 1986 (IA86), much was made of the operators' responsibility for the accident, and not much emphasis was placed on the design faults of the reactor. Later assessments (IA86a, UN00) suggest that the event was due to a combination of the two, with a little more emphasis on the design deficiencies and a little less on the operator actions.
The two explosions sent fuel, core components and structural items and produced a shower of hot and highly radioactive debris, including fuel, core components, structural items and graphite into the air and exposed the destroyed core to the atmosphere. The plume of smoke, radioactive fission products and debris from the core and the building rose up to about 1 km into the air. The heavier debris in the plume was deposited close to the site, but lighter components, including fission products and virtually all of the noble gas inventory were blown by the prevailing wind to the North-west of the plant.
Fires started in what remained of the Unit 4 building, giving rise to clouds of steam and dust, and fires also broke out on the adjacent turbine hall roof and in various stores of diesel fuel and inflammable materials. Over 100 fire-fighters from the site and called in from Pripyat were needed, and it was this group that received the highest radiation exposures and suffered the greatest losses in personnel. A first group of 14 firemen arrived on the scene of the accident at 1.28 a.m. Reinforcements were brought in until about 4 a.m., when 250 firemen were available and 69 firemen participated in fire control activities. By 2.10 a.m., the largest fires on the roof of the machine hall had been put out, while by 2.30 a.m., the largest fires on the roof of the reactor hall were under control. These fires were put out by 05:00 hr of the same day, but by then the graphite fire had started. Many firemen added to their considerable doses by staying on call on site. The intense graphite fire was responsible for the dispersion of radionuclides and fission fragments high into the atmosphere. The emissions continued for about twenty days, but were much lower after the tenth day when the graphite fire was finally extinguished.
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Thermal reactions between fuel cladding and water in spent fuel from US reactors can produce hydrogen =- lots of it - which is highly flammable...
http://books.nap.edu/openbook.php?record_id=11263&page=38The first two of these objectives could be compromised by a terrorist attack that partially or completely drains the spent fuel pool.2 The committee will refer to such scenarios as “loss-of-pool-coolant” events. Such events could have several deleterious consequences; Most immediately, ionizing radiation levels in the spent fuel building rise as the water level in the pool falls. Once the water level drops to within a few feet (a meter or so) of the tops of the fuel racks, elevated radiation fields could prevent direct access to the immediate areas around the lip of the spent fuel pool building by workers. This might hamper but would not necessarily prevent the application of mitigative measures, such as deployment of fire hoses to replenish the water in the pool.
The ability to remove decay heat from the spent fuel also would be reduced as the water level drops, especially when it drops below the tops of the fuel assemblies. This would cause temperatures in the fuel assemblies to rise, accelerating the oxidation of the zirconium alloy (zircaloy) cladding that encases the uranium oxide pellets. This oxidation reaction can occur in the presence of both air and steam and is strongly exothermic—that is, the reaction releases large quantities of heat, which can further raise cladding temperatures. The steam reaction also generates large quantities of hydrogen:
Reaction in air:
Zr+O2?ZrO2
heat released=1.2×107 joules/kilogram
Reaction in steam:
Zr+2H2O?ZrO2+2H2
heat released=5.8×106 joules/kilogram
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