Code Manual

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The manual for SAS4A/SASSYS-1 describes code capabilities, user input, and major code modules. It is divided into 16 chapters.


SAS4A/SASSYS-1 provide 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. From ten to thirty channels normally provide sufficient discretization, depending on the core design. Significantly more channels can be used if necessary.


User's Guide

Chapter 2 contains a user's guide for SAS4A/SASSYS-1 applications, including a complete description of the standard input file.


Steady-State and Transient Thermal Hydraulics in Core Assemblies

Chapter 3 contains the description of the formulation for the SAS4A/SASSYS-1 pin heat transfer and single-phase coolant thermal/hydraulics model. A major new addition to this model is the capability to treat channel‐to-channel heat transfer due to conduction and convection at all axial locations between the channel inlet and outlet, permitting a consistent multiple-pin subassembly treatment. Also, the subassembly-to-subassembly heat transfer model has been improved, and axial conduction in the coolant has been added. These modeling additions have been proven in validation analyses of EBR-II Shutdown Heat Removal Tests, and are required for accurate predictions of intra-subassembly flow and temperature variations in EBR-II during transients from normal to shutdown operating conditions.

Reactor Point Kinetics, Decay Heat, and Reactivity Feedback

Chapter 4 contains the description of the formulation for the SAS4A/SASSYS‐1 reactor points kinetics and reactivity feedback models. The new addition to this module is an option for an EBR-II-specific reactivity feedback model that is being validated with analysis of reactor operating data and used for predictive calculations of margins in design basis analyses. 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.


Primary and Intermediate Loop Thermal Hydraulics Module

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. The major new addition to PRIMAR-4 is the option for multiple core inlet and outlet coolant plena, permitting exact representation of the actual EBR-II coolant systems geometry.

Control System

The plant control and protection system model described in Chapter 6 is unchanged from prior versions of SASSYS-1, except for the addition of an option to allow dynamic allocation of model data storage at execution time.


Balance of Plant Thermal/Hydraulics Models

The balance-of-plant (BOP) model described in Chapter 7 is new for SAS4A/SASSYS-1. It 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.

DEFORM-4: Steady-State and Transient Pre-Failure Pin Behavior

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.

DEFORM-5: Metallic Fuel Cladding Transient Behavior Model

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 is a new model for SAS4A/SASSYS-1, and 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.


SSCOMP: Pre-Transient Characterization of Metallic Fuel Pins

The SSCOMP model described in Chapter 10 has been revised to reflect newly available metal fuel material properties evaluations recorded in the IFR Material Properties Handbook. 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.

FPIN2: Pre-Failure Metal Fuel Pin Behavior Model

Chapter 11 contains the description of a major new addition to the SAS4A/SASSYS-1 cods, the FPIN2 metal fuel pin mechanics model. 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, and 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.


Sodium Voiding Model

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, with the addition of a set of modifications to describe the sudden release of noncondensible 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.

Cladding Motion Model — CLAP

The CLAP model described in Chapter 13 is relevant only to oxide fuel, and have remained unchanged since the previous documentation.


Non-Voided Channel Fuel Motion Analysis — PLUTO2

The PLUTO2 model described in Chapter 14 is relevant only to oxide fuel, and have remained unchanged since the previous documentation.

PINACLE: In-Pin Pre-Failure Molten Fuel Relocation Module

The PINACLE model described in Chapter 15 and the LEVITATE model described in Chapter 16 have been upgraded for applications to metallic fuel. 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, and applications to severe accident sequences in metal-fueled IFR cores have been completed and documented.


LEVITATE: Voided Channel Fuel Motion Analysis

See #PINACLE: In-Pin Pre-Failure Molten Fuel Relocation Module