.. _section-9.1:

Introduction
------------

In Version 5.7, a new metallic fuel performance model for pre-transient and
transient analysis has been introduced.
The new fuel model is described in the remaining sections of this chapter, the description
of DEFORM-5, which is considered deprecated, has been moved to :numref:`section-A9.8`.


The integrated metallic fuel performance model accounts for the coupled
nature of the uranium plutonium zirconium metallic fuel phenomena and
its impact/evolution on/during safety analysis. Given the user-provided
operating history, initial geometry, fresh fuel composition, mass,
cladding type, and the calculated fuel pin and coolant temperature
distribution, MFUEL, the metallic fuel performance model, simulates the
fuel and cladding behavior as a function of time. The main capabilities
of MFUEL are prediction of: 1) Fuel pin mechanics, and compositional and
dimensional changes; 2) Clad failure; and 3) Fuel pin thermal
resistance. Achieving these high-level goals accurately is strongly
related to the model performance of individual physical processes taking
place within the fuel pin during operation. Metal fuels typically
operate above half of their solidus temperatures during normal as well
as off-normal conditions. Availability of thermal activation provides
the driving force for various diffusional processes leading to complex
phase transformations, micro-structure evolution, significant amount of
fuel swelling, interconnected porosity formation, excessive amounts of
fission gas release, and fuel clad chemical interactions. Clad failure
in fast reactors primarily occurs as a result of creep rupture augmented
by clad wastage formation. The reaction is driven by thermal creep
induced dislocation motion, grain boundary cavity nucleation, and growth
and breakup of grain boundaries. The high-level complexity and limited
available data requires introducing physics-based modeling approaches to
gain extrapolation ability and sensitivity with respect to various
conditions.

As shown in :numref:`figure-9.1-1`, the MFUEL driver evaluates the different physical
phenomena, described in :numref:`section-9.2` at each time step. At the end of
each time step, the MFUEL driver updates the fuel pin geometry, fuel pin
composition, plenum pressure, fuel porosity, bond sodium infiltration,
and clad failure margin. Each of these quantities has an impact on the
fuel pin thermal-hydraulics, reactivity feedback, and margin to fuel
melting.

.. _figure-9.1-1:
.. figure:: media/Fig1.png
   :align: center
   :figclass: align-center
   :width: 6.30425in
   :height: 4.26239in

   MFUEL - Integrated Metal Fuel Models. Processes shown in red
   are handled by other SAS models and processes shown in green are handled
   by the MFUEL driver.