PLENTIFUL ENERGY

The EBR-I core was small, football-sized. During the post-accident disassembly
of the molten core, it was observed that considerable amounts of molten fuel had
penetrated into the upper blanket coolant passages, with a maximum upward travel
of approximately five inches. The lower blanket region also filled with solidified
material from the fuel section. In the core region, two distinct sponge-like zones of
porosity had formed. Based on observations and measurements made on the core
during its later disassembly, an artist‘s reconstruction of a vertical section through
the damaged core was made, as shown in Figure 7-9. Also shown in Figure 7-9 are
what are considered to be the most reliable values of densities of the various
areas.[9] Based on out-of-pile experiments, it was conjectured that the porous
structure in the center of the damaged core may have resulted from vaporization of
NaK (the liquid sodium-potassium coolant) entrained in the molten fuel alloy and
subsequent expansion of the vapor. Furthermore, radiochemical analyses of Ce-144
distribution and mass spectrographic analyses of U-235 depletion confirmed that
little mixing had occurred in the molten volume and the fuel that dispersed into
blanket regions originated from the outer part of the core. Fuel dispersal was
therefore was outward, reducing reactivity, which is the key point.
Figure 7-9. Schematic view of vertical section through damaged
EBR-I core assembly
The question of whether the EBR-I power excursion was terminated by the core
dispersal or by the manual scram was analyzed in detail. [10] The analysis
concluded That the manual scram was the likely cause of termination, but the
dispersed core would also have terminated the excursion in time to prevent prompt
criticality.
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