.. _section-10.1: Introduction and Overview ------------------------- The Integral Fast Reactor (IFR) Program [10-1] at Argonne National Laboratory is developing an advanced liquid-metal-cooled nuclear reactor (LMR) that uses U-Pu-Zr metallic alloy fuel due to a number of safety and economic benefits. The post-irradiation examination of U-Pu-Zr fuel pins irradiated in EBR-II shows the formation of three annular zones (central, middle and outer) of considerably different alloy compositions, fuel porosities and densities [10-2, 10-3]. The uranium in the fuel migrates from the central and outer zones to the middle zone, and the zirconium and fission products migrate in the opposite directions, i.e., from the middle zone to the central and outer zones [10-3]. Due to zirconium depletion the middle zone has a solidus temperature significantly lower than that of the central or outer zone. The zonal fuel densities have been measured to vary from about 8000 kg/m\ :sup:`3` in the central zone to about 16000 kg/m\ :sup:`3` in the middle zone [10-3]. The average porosity of each zone is also found to be significantly different from each other [10-4]. This radial redistribution of fuel constituents and annular zone formation has a considerable effect on the thermal behavior of U-Pu-Zr fuel pins because the thermal conductivity, specific heat and density of the fuel all vary with composition and porosity. The radial power shape within the fuel pin is also changed by fissile (i.e., U and Pu) redistribution. During steady-state irradiation, the buildup of fission gas within the metal fuel produces considerably more fuel swelling than is observed in oxide fuel. As the steady-state irradiation proceeds in metal fuel, the fission gas is initially all retained in the fuel matrix. Grain boundary bubbles gradually form, producing fuel swelling. As the porosity continues to increase, the grain boundary bubbles can become interlinked, producing a long-range interlinked porosity that offers a path for fission gas release to pin plenum [10-4]. When metallic fuel was first used in EBR-II, a peak burnup of only 1 at. % was considered a reasonable objective because of the extensive fuel swelling caused by grain boundary bubbles. Once it was determined that this "breakaway" swelling appeared to be self-limiting, and if enough space (~25 to 30% of fabricated fuel volume) was provided in the fuel-cladding gap, the result was very little stress on the cladding and high burnups could be achieved [10-5]. In order to calculate the steady-state behavior of metal fuel, it is therefore necessary to model the fuel swelling due to grain boundary bubbles and the associated fission gas release in a consistent manner. Listed below are the important in-pin phenomena (including the two described above) 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 these phenomena in the SSCOMP module will provide SAS4A and SASSYS-1 codes with the capability to describe the transition of the fuel pin form cold clean as-manufactured conditions to hot irradiated swollen initial conditions for the transient fuel mechanics module FPIN2 [10-6]. The modeling of the following aspects of fission gas behavior (phenomenon 3 listed above) is required for calculating the initial conditions of FPIN2 transient calculation: (a) fission gas generation, (b) fission gas retained within 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 unlinked grain boundary bubbles, and in interlinked grain boundary bubbles) has also been expressed by the pre-failure in-pin fuel motion module PINACLE development [10-7]. Regarding irradiation induced fuel swelling (phenomenon 4 listed above), axial swelling will contribute to pre-transient fuel column length, a needed initial condition for 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 for 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. The purpose of this chapter is to present the current status and future modeling needs of the SSCOMP module. As discussed in :numref:`section-10.2`, the zone formation calculation method of the earlier SSCOMP model [10-4] is not incorporated into SAS4A/SASSYS‑1 Version 3.0 code due to its limitations. 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. At present, the data needed to account for the effects of zone formation on pin transient temperature calculation are specified through input data controlled by the input parameter IFUELC. As a starting point for SSCOMP development, :numref:`section-10.3` describes thermal properties of U-Pu-Zr metallic alloy fuel as a function of composition and temperature. These properties are needed in the analysis of the Mark-V fuel (U-20Pu-10Zr), and other metallic alloy fuels of different compositions, especially the different fuel compositions across the pin radius resulting from fuel constituent migration during steady-state irradiation. The implementation of the thermal properties data of the IFR Metallic Fuels Handbook [10-8], using a method of regionwise interpolation, is an important part of :numref:`section-10.3`. Thermal properties of mixed-oxide fuel are described in :numref:`section-3.15`, and not here. Future directions for modeling efforts are discussed in :numref:`section-10.4`.