ORNL-5388 - the Molten Salt Energy Technologies Web Site

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given materials, particularly for oxide LMFBR fuel and sodium. Although there is no con-
clusive theoretical and/or experimental evidence, the most widely accepted theory is that
for an energetic'vapor explosion to occur, there must be intimate liquid-liquid contact
of the fragmented molten fuel particles and the contact temperature at the fuel-sodium
surface must exceed the temperature required for homogeneous nucleation of the sodium.
A considerable amount of evidence exists to suggest that for oxide fuel in the reactor
environment, the potential for a large-scale vapor explosion is extremely remote. The
key factor is the relatively low thermal conductivity of the oxide fuel, which does not
permit rapid enough heat transfer from the fuel to cause the fuel-sodium contact tempera-
ture to exceed the sodi um homogeneous nucleation temperature,
The primary difference between carbide and/or metal fuels as opposed to oxide fuels
is their relatively higher thermal conductivity. Under typical assumed accident conditions,
it is possible to calculate coolant temperatures which exceed the sodium homogeneous nucleation
temperature. This does not mean, however, that a large-scale FCI will necessarily
occur for carbide-sodium or metal-sodium systems. As noted above, these theories as mechanisms
for vapor explosion have not been completely substantiated. However, insofar as
the homogeneous nucleation criterion is adequate, it is clear that the potential for largescale
vapor explosion, at least in clean laboratory systems, is greater for carbide or
metal in sodium than for oxide in sodium. Continued theoretical and experimental study is
necessary to gain a thorough understanding of the details of the mechanisms involved and to
estimate the likelihood for vapor explosion under reactor accident conditions for any
breeder system.
Breeding Performance of Alternate Fuel Schemes
Table 4.6-2 shows that in terms of fissile production, the reference Pu/U core
with U blankets gives the best breeding performance regardless of fuel type (oxide, carbide,
or metal). For the carbide systems considered, a heterogeneous core design using
Pu/U carbide fuel with a U carbide blanket gives a breeding ratio of 1.550. For the metal
systems considered, a nominal two-zone homogeneous. core design using U-Pu-Zr alloy fuel
gives a breeding ratio of 1.614.
The increased fissile production capability of the carbide and metal fuels is
especially advantageous for the denatured cycles. A breeding ratio as high as 1.4 has
been calculated for a metal denatured system, and the breeding ratio for a carbide de-
natured system is not expected to be substantially smaller. However, a good part of the
fissile production of any denatured system is plutonium. Thus the denatured system is
not a good producer of 233U. However, when used with the energy park concept, where the
plutonium produced by the denatured systems can be used as a fuel, the denatured carbide
and metal uranium systems are viable concepts. Metal and carbide concepts may also prove
to be valuable as transmuter systems for producing 233U from 232Th.
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