QUESTION:
Why you don't use nuclear fuel?

ANSWER from Jim Frautnick on September 10, 1999:
Actually, Cassini does use nuclear fuel but it is used for creating heat
which in turn generates electricity for the spacecraft and science
instruments.  Radioactive pellets create heat as they decay and this is
converted to electricity by a generator.  This use of nuclear fuel is
required for Cassini because when it arrives at Saturn it is so far away
from the Sun that solar panels would not generate enough power.  Batteries
are too heavy and have trouble lasting for the 11-year Cassini.

Nuclear fuel for propulsion is not used for several reasons:  Safety,
expense, performance, and suitability.

The main propulsion needed for Cassini is to escape the Earth's gravity
field to begin the interplanetary journey.  Solid-fuel booster rockets are
better suited to this task and currently there are no nuclear rockets
available for this task.   Solid-fuel rockets cannot be turned off and on.
They are a one-shot burn event but very efficient for generating the energy
for getting into orbit or escaping Earth.  Once Cassini has escaped Earth's
gravity, additional energy boosts are gained from 1) planetary flybys, and
2) short bursts of propulsion from gas-fueled rockets.  The planetary
flybys take advantage of geometric opportunities to gain additional energy
from rotating planets.  Cassini used this technique to gain energy from
Venus, Earth, and Jupiter.  Venus was used twice because more energy was
needed to get to Jupiter and Saturn but the cost was extra time in the
inner solar system.  The on-board propulsion systems can generate short
bursts of energy by burning and exhausting gas through nozzles.  These
systems can be turned on and off for short trajectory corrections.