.. _section-10.4:

Future Directions for Modeling Efforts
--------------------------------------

Multiple annular fuel zones of different compositions formed at the end
of steady-state irradiation in U-Pu-Zr alloy fuel can not be supplied to
the |SAS| codes through the input data. The codes are capable
of using the input zone formation data in the steady-state and transient
temperature calculations. U-Pu-Zr fuel thermal properties can now be
evaluated by the codes as a function of composition and temperature,
using the method of regionwise interpolation of IFR Handbook data.
Besides the U-Pu-Zr fuel specific heat, theoretical density and thermal
conductivity, the codes can now evaluate the zonal solidus and liquidus
temperatures and heat of fusion. The effect of radial migration of fuel
constituents on in-pin radial power shape is yet to be modeled.

The purpose of the SSCOMP module is to provide |SAS| codes with
the modeling of all those phenomena that describe the transition of a
U-Pu-Zr fuel pin from cold clean as-manufactured conditions to hot
irradiated swollen initial conditions for the transition fuel mechanics
module FPIN2 [10-6]. As described in :numref:`section-10.1`, listed below are the
important in-pin phenomena occurring during steady-state irradiation,
that need to be modeled in order to characterize a U-Pu-Zr fuel pin at
the beginning of a transient calculation:

1. Thermal expansion of fuel and cladding,

2. Fuel constituent radial migration,

3. Fission gas behavior, and porosity formation and distribution,

4. Irradiation-induced radial and axial swelling of fuel and cladding,

5. Bond sodium migration into fuel and pin plenum,

6. Cladding constituent migration into fuel.

The modeling of the above phenomena will provide all the needed initial
conditions for the transient fuel mechanics module FPIN2. Future
modeling efforts will be directed towards modeling the above phenomena.
Detailed comments are made below about some of the above six phenomena.

.. _section-10.4.1:

Fuel constituent Radial Migration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As discuss in :numref:`section-10.2`, the zone formation calculation method of the
earlier SSCOMP model [10-4] is not incorporated into |SAS|
Version 3.0 codes due to its limitation. The study of fuel constituent
radial migration leading to zone formation was not at that time
developed to a state where an appropriate dynamic model could be
developed. It is expected that a dynamic model for calculating fuel
constituent radial migration, available in the literature or developed
on the basis of available zonal composition and radial boundary data,
will be incorporated into the codes. Such a dynamic model will be based
on pin power history during steady-state irradiation. If neither a
dynamic model nor a zonal composition and radial boundary database is
available in the near future, the earlier SSCOMP model will be
implemented in the codes.

.. _table-10.4-1:

.. list-table:: Internal Arrays Required for Implementing IFR Handbook U-Pu-Zr Fuel Properties Data and Multiple Radial Fuel Zones Option
    :header-rows: 1
    :align: center
    :widths: auto

    * - Definition/Comments
      - Array Name
    * - Arrays by Radial Node and Axial Segment in Block COMC
      -
    * - 1. Fuel mass
      - FUELMS(11,24)
    * - 2. Pu mass in fuel
      - FUPUMS(11,24)
    * - 3. Zr mass in fuel
      - FUZRMS(11,24)
    * - 4. Fe mass in fuel
      - FUFEMS(11,24)
    * - 5. Ni mass in fuel
      - FUNIMS(11,24)
    * - 6. Total porosity in fuel
      - PRSTY2(11,24)
    * - 7. Logged sodium mass in fuel
      - FUNAMS(11,24)
    * - Arrays by Fuel Type in Block FPTVAR
      -
    * - 8. First solid-state transition temperature of fuel
      - TEMALF(8)
    * - 9. Last solid-state transition temperature of fuel
      - TEMGAM(8)
    * - 10. Pre-calculated coefficients in the quadratic equation for fuel thermal conductivity
      - ACPF(8,7)
    * - 11. Pre-calculated coefficient in the quadratic equation for fuel thermal conductivity
      - AKFU(8,5)
    * - 12. Pre-calculated parameters in fuel theoretical density equation
      - ADFU(8,2)

.. _section-10.4.2:

Fission Gas Behavior, and Porosity Formation and Distribution
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The modeling of the following aspects of fission gas behavior is
required for calculation the initial conditions of FPIN2 transient
calculation: (a) fission gas generation, (b) fission gas retained with
grains, (c) fission gas retained in grain boundary bubbles that are not
linked, (d) fission gas contained in grain boundary bubbles that are
interlinked, and (e) fission gas released to pin plenum. The need for
modeling these three types of fission gas contained in fuel (gas within
grains, in unliked grain boundary bubbles, and in interlined grain
boundary bubbles) has also been expressed by the pre-failure in-pin fuel
motion module PINACL development [10-7].

.. _section-10.4.3:

Irradiation-Induced Radial and Axial Swelling of Fuel and Cladding
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Regarding irradiation-induced fuel swelling, the axial swelling will
contribute to pre-transient fuel column length, a needed initial
condition of the FPIN2 module. Radial swelling of fuel and cladding will
determine fuel-cladding gap closure and bond sodium migration into pin
plenum, a needed initial condition of the FPIN2 module. The modeling of
fuel swelling phenomenon has two important aspects: swelling due to
solid fission products, and fission gas induced swelling. Swelling of a
cladding material is mainly determined by neutron fluence which is also
a parameter in determining cladding rupture life used in the FPIN2
transient fuel mechanics calculation.

The SSCOMP module is being developed to model the above listed in-pin
phenomena during steady-state irradiation of metallic fuel pins. After
the fuel pin has been characterized at the end of steady-state
irradiation, the calculation of the relevant ones of these phenomena
during the transient will be performed by the DEFORM-5 model.