1.3. Documentation Overview¶
The rest of this manual contains details of the modeling capabilities of SAS4A/SASSYS‑1. The chapter organization shown in Table 1.3.1 reflects the major model delineations. Each chapter provides in-depth descriptions of the models, including model formulations, solution techniques, and input descriptions. It is critical that users understand the relationships between their model input and the model formulations given in this manual. Failure to understand these relationships can result in broken models and misleading results.
SAS4A/SASSYS‑1 provides a detailed, multiple-channel thermal/hydraulic treatment of the reactor core. Each channel represents a fuel pin, its cladding, the associated coolant, and a fraction of the subassembly duct wall. Other positioning hardware, such as wire wraps or grid spacers, is usually lumped into the structure field with the duct wall. Within a channel, the flow is assumed to be one-dimensional in the axial direction, and the temperature field in the fuel, cladding, coolant, and structure is assumed to be two-dimensional in the radial and axial directions. Usually, a channel represents an average fuel element in a subassembly or a group of subassemblies. A channel may also represent pins in blanket or control subassemblies. Alternately, a single channel may also be used to represent the hottest pin in an assembly, or any other subset of a subassembly. The axial extent of a channel covers the entire length of a subassembly, including the core, the axial blankets, the fission gas plenum and the spaces above and below the pin/cladding geometry. Different channels may be used to account for radial and azimuthal design geometry, power, coolant flow, and burnup variations within the reactor core.
Chapter |
Subject |
|---|---|
1 |
Introduction (this chapter) |
2 |
SAS4A/SASSYS‑1 User’s Guide |
3 |
Pin Heat Transfer and Single‑Phase Coolant Thermal/Hydraulics Model |
4 |
Reactor Point Kinetics and Reactivity Feedback Models |
5 |
PRIMAR‑4: Primary and Intermediate Loop Thermal/Hydraulics Model |
6 |
Plant Control and Protection Systems Model |
7 |
Balance‑of‑Plant Thermal Hydraulics Model |
8 |
DEFORM‑4 Oxide Fuel and Cladding Mechanics Model |
9 |
DEFORM‑5 Metal Fuel Cladding Mechanics Model |
10 |
SSCOMP Metal Fuel Characterization Model |
11 |
FPIN2 Metal Fuel and Cladding Mechanics Model |
12 |
TSBOIL Two‑Phase Coolant Thermal/Hydraulics Model |
13 |
CLAP Molten Cladding Dynamics Model |
14 |
PLUTO2 Fuel‑Coolant Interaction Model |
15 |
PINACLE In‑Pin Fuel Relocation Model |
16 |
LEVITATE Fuel Relocation Model |
Chapter 2 contains a general user’s guide for SAS4A/SASSYS‑1, including a complete description of the standard input file. Although Chapter 2 includes a summary description of every input parameter, it is essential that users consult the relevant chapters to understand the relationship between the input and the model formulations.
Chapter 3 contains the description of the formulation for the SAS4A/SASSYS‑1 pin heat transfer and single-phase coolant thermal/hydraulics model. The subassembly-to-subassembly heat transfer model has been improved, and axial conduction in the coolant has been added. A sub-channel model has been introduced to provide accurate predictions of intra-assembly temperature and flow distributions.[1‑16] This modeling addition is being validated with results from the EBR-II Shutdown Heat Removal Tests [1‑17] as part of an International Atomic Energy Agency Coordinated Research Project.[1‑18]
Chapter 4 contains the description of the formulation for the SAS4A/SASSYS‑1 reactor point kinetics, decay heat, and reactivity feedback models. A new addition to this module is the ability to represent more detailed decay heat characteristics in multiple regions of the core. This module provides the reactor power level to the core thermal/hydraulics models for determination of the heating rate in the fuel, and receives core materials temperature and geometry information to calculate the reactivity feedbacks employed in the solution of the point kinetics equations.
Chapter 5 presents a full description of the formulation for the PRIMAR‑4 sodium loops thermal/hydraulic model. This model provides boundary coolant pressure and flow conditions for the core channel models, including transient heat losses through normal and emergency heat removal systems and the transient performance of pumps. PRIMAR‑4 includes the option for multiple core inlet and outlet coolant plena, permitting exact representation of the actual EBR-II coolant systems geometry. Compressible volumes in PRIMAR‑4 may also be coupled with external computational fluid dynamics simulations to better represent flow and temperature distributions during transients.
The plant control and protection system model described in Chapter 6 is mostly unchanged from prior versions of SASSYS‑1, except for the addition a sinusoidal function to represent oscillations in control-system signals.
The balance-of-plant (BOP) model described in Chapter 7 was implemented to permit 1) improved simulation of EBR-II design basis transients, 2) whole-plant analysis of IFR designs for optimization of advanced reactor control system strategies, and 3) core temperature margin assessments in unprotected accident sequences (i.e. beyond design basis accidents (BDBA) and anticipated transients without scram (ATWS)). In these latter sequences, core response depends strongly upon the performance of the balance-of-plant, because the core neutronic and thermal/ hydraulic behavior is determined by the availability of heat sinks outside the core. The BOP model couples to PRIMAR‑4 at the steam generator.
Chapter 8 provides a description of the DEFORM‑4 fuel element behavior model for stainless steel-clad oxide fuel, which is unchanged from prior versions of SAS4A/SASSYS‑1.
Chapter 9 contains the description of the DEFORM‑5 model, which treats the transient behavior of stainless steel and advanced (HT‑9) cladding for metal fuel elements. This model is aimed at predicting margin to cladding failure, and timing and location of failure in limiting transients. It includes physical phenomena unique to metallic fuel, such as fuel/cladding chemical interactions.
The SSCOMP model described in Chapter 10 reflects available metal fuel material properties evaluations recorded in the IFR Material Properties Handbook [1‑19]. An efficient correlation technique has been implemented in all SAS4A/SASSYS‑1 material properties routines that accurately generates the data from the IFR Handbook for use in all the modules of the code. It is planned to revise the material migration capability in SSCOMP for ternary fuel, to add models for fission gas generation and release, swelling, and all other phenomena needed to describe the transition from cold, clean, unirradiated conditions to hot irradiated conditions.
Chapter 11 contains the description of the FPIN2 metal fuel pin mechanics model [1‑20]. FPIN2 is a validated model for metal fuel pin transient behavior. Unlike DEFORM‑5, which treats only the cladding response, FPIN2 provides a finite-element solution of the fuel and cladding mechanics equations for the elastic/plastic response, including fission gas pressurization and migration, molten cavity formation and growth, and fuel/cladding chemical interaction and cladding thinning. The interface between SAS4A/SASSYS‑1 and FPIN2 has been designed to permit stand-alone execution of FPIN2 for direct verification or to replace the FPIN2 thermal/hydraulics calculation with the SAS4A/SASSYS‑1 counterparts for coupled calculations. The application for this model is design basis analysis of driver and experimental fuel elements in EBR-II for the purpose of margin-to-failure assessments.
The TSBOIL module for liquid metal coolant boiling and two-phase thermal/hydraulics calculations has been retained intact from previous versions of SAS4A/SASSYS‑1. The current model includes a set of modifications to describe the sudden release of non-condensable fission gas from a cladding rupture in the upper fission gas plenum of metal fuel elements and the subsequent plenum blow-down and liquid coolant expulsion. This option has been used to assess the safety implications of long-term fuel element irradiations in EBR-II [1‑21].
The CLAP and PLUTO2 models described in Chapter 13 and Chapter 14 are relevant only to oxide fuel, and have remained unchanged since the previous documentation.
The PINACLE model described in Chapter 15 and the LEVITATE model described in Chapter 16 have been upgraded for applications to metallic fuel [1‑22]. The model enhancements added to PINACLE and LEVITATE for metal fuel include fuel/cladding and fuel/structure chemical interactions and fission gas generation and migration with fuel swelling. Preliminary analyses of TREAT M-Series in-pile metal fuel tests have been completed [1‑23], and applications to severe accident sequences in metal-fueled IFR cores have been completed and documented [1‑24].