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 SAS4A/SASSYS‑1 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 SAS4A/SASSYS‑1 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 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.

10.4.1. Fuel constituent Radial Migration

As discuss in 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 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 Internal Arrays Required for Implementing IFR Handbook U-Pu-Zr Fuel Properties Data and Multiple Radial Fuel Zones Option

Definition/Comments

Array Name

Arrays by Radial Node and Axial Segment in Block COMC

  1. Fuel mass

FUELMS(11,24)

  1. Pu mass in fuel

FUPUMS(11,24)

  1. Zr mass in fuel

FUZRMS(11,24)

  1. Fe mass in fuel

FUFEMS(11,24)

  1. Ni mass in fuel

FUNIMS(11,24)

  1. Total porosity in fuel

PRSTY2(11,24)

  1. Logged sodium mass in fuel

FUNAMS(11,24)

Arrays by Fuel Type in Block FPTVAR

  1. First solid-state transition temperature of fuel

TEMALF(8)

  1. Last solid-state transition temperature of fuel

TEMGAM(8)

  1. Pre-calculated coefficients in the quadratic equation for fuel thermal conductivity

ACPF(8,7)

  1. Pre-calculated coefficient in the quadratic equation for fuel thermal conductivity

AKFU(8,5)

  1. Pre-calculated parameters in fuel theoretical density equation

ADFU(8,2)

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].

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.