PLENTIFUL ENERGY

Further cementing the decision was an event in the Fermi-1 reactor in 1966. Fuel
had melted during a startup when flow to one of the subassemblies had been
blocked by a structural component that had come loose. The subassembly partially
melted, affecting its immediate neighbors as well. The operators continued to
increase power without realizing that a subassembly wasn‘t being properly cooled.
The lesson drawn at the time was that the higher melting point of oxide would be
safer; it was thought it would have more margin-to-failure, and perhaps, lesser
implications if fuel did fail. And the ease with which the switch to oxide was
accepted was abetted by the fact that oxide was now the established choice in the
rapidly expanding commercialization of the LWR.
However, oxide has its disadvantages too. It has a limited ability to conduct heat.
It is an insulator, really, and it has limited specific heat as well, (a measure of the
ability to absorb heat with limited temperature rise), which means that fuel
temperature is quite sensitive to power. It will withstand very high temperatures,
however, and the temperatures at the centerline of the fuel pins reach 2,000 o C or
more. The wide range in temperature tolerance is a positive trait in normal
operation, but the high temperature can increase the energy releases in possible
accidents. And it is not entirely compatible with sodium. But it does withstand
radiation very well and it does not swell. Its breeding properties are only fair, as
will be explained below, but when it was chosen at this time it was felt that they
were good enough for a start, and better breeding fuels could be developed later. On
this basis, in the late 1960s and early 1970s a number of first-of-a-kind oxide-fueled
sodium-cooled fast reactor demonstration plants were constructed in several
countries around the world.
These reactors, and a few that came later, were fueled with mixed uraniumplutonium
oxide fuel. Mixed oxide fuel experience has been favorable. [1-2] A
comprehensive oxide fuel data base now exists that can support future commercial
reactor licensing efforts. High burnup potential, beyond 20% burnup, has been
demonstrated. The consensus worldwide is that oxide fuel is fully developed, and it
is certainly the de facto reference fuel for fast reactors. Its characteristics, good and
bad, are very well established.
Other ceramic fuel types have also been tried. Carbide and nitride in particular
received some attention in the mid-1970s. The U.S. fast reactor development
program began a significant program on these fuels, motivated by improvement in
breeding. These fuels, while still ceramic, do breed better and allow new core
loadings to be built up faster than in the reference oxide fuel. At this time, concerns
about breeding had arisen in the U.S. fast reactor demonstration reactor project,
CRBR, then underway. And in the early seventies, a rapidly expanding fast reactor
economy was still confidently planned that would follow light water reactors as the
next generation of nuclear power. So an ―advanced fuels program‖ to test the
performance of the carbide and nitride fuels in fast reactors was begun.
119