VERAIn User's Manual

Abstract

The VERA common input is an ASCII file and is designed to be modular. The input is divided into separate modules (or blocks) to describe the different geometric objects in the core and to define specific modeling options for each of the physics codes.Geometric objects are defined as the physical "parts" of the reactor core, which includes fuel assemblies, control rod assemblies, removable burnable poison assemblies, and detectors. By defining each geometric object as a separate block, the objects can be described independently of each other, relying on very little global information. The independent descriptions make quality assurance (QA) easier, they allow objects to be defined in one cycle, and they allow objects to be reused in subsequent cycles without concerns about input conflicts. Another advantage of the module approach is that it is easier to shuffle fuel assemblies and to insert and withdraw "inserts" such as control rods, detectors, and removable burnable poison assemblies into the fuel assemblies as the core configuration changes.Additional modules/blocks are used to define modeling options and parameters for each of the physics codes. Separating the geometry description from the modeling options allows all of the physics codes to share the same geometry description, and it also allows the same input to be used with multiple physics codes.The VERA input blocks are as follows:CASEID This block contains an input title card.CORE This block describes the core layout, including the core map, assembly locations, control rod locations, and assembly insert locations. The CORE block contains data that does not change during a cycle depletion.STATE These blocks describe reactor core operating parameters (statepoint values) at a particular point in time. Parameters include inlet temperature, pressure, power, control rod positions, and others. STATE values can, and usually do, change at each statepoint.ASSEMBLY These blocks contain the geometry and physical description of the nuclear fuel assemblies. The assembly descriptions do not include control rods, detectors, or inserts.INSERT These blocks contain the geometry and physical description of the assembly inserts. An insert is a generic term used to describe a removable burnable poison assembly or a thimble plug assembly.CONTROL These blocks contain the geometry and physical description of a control rod assembly. A control rod assembly is similar to an assembly insert, except that it can move during operations.DETECTOR This block contains the geometry and physical description of a detector string. EDITS This block contains information about what edits the code should produce.COUPLING This block contains parameters for coupling different physics codes together.In addition to the blocks listed above, additional code-specific blocks contain options specific to each physics code. Examples of code-specific blocks are COBRATF, MPACT, and SHIFT. Additional code-specific input blocks can be added as new physics codes are added to the core simulator.The following sectio...

The NILO-CMFD Method for Iteratively Solving Coupled Neutron Transport–Thermal Hydraulics Problems

“…We refer to this basic approach as Multiphysics CMFD (M-CMFD). As a concrete example, the M-CMFD strategy is implemented in the Michigan Parallel Characteristics-based Transport (MPACT) code [55,80] allowing it to couple to a variety of thermal hydraulics solvers, e.g., an internal simplified thermal hydraulics (simTH ) solver [32], the COBRA-TF (CTF) subchannel code [49,65,87], and the CFD code STAR-CCM+ [113]. MPACT's ability to couple to this diverse set of thermal hydraulics codes lends itself as evidence to the ease of code coupling that loosely-coupled methods facilitate.…”

“…We use the name Relaxed CMFD (R-CMFD) to refer to the Multiphysics CMFD (M-CMFD) method described above when relaxation is included. R-CMFD is the default multiphysics iterative method used in the MPACT code for steady-state eigenvalue problems [55,80]. To be clear, M-CMFD is just R-CMFD without relaxation.…”

“…The relaxation factor α affects the heat source distribution supplied to the thermal hydraulics problem in R-CMFD Step 2. α = 0 corresponds to full relaxation, with α = 1 indicating no relaxation. Typically, a value of 0.5 < α < 0.7 is used for reactor physics applications (e.g., the MPACT code uses α = 0.5 as its default [80]) -although, the optimal relaxation factor varies from problem to problem, and costly parametric studies are required to determine the optimal value for any given scenario. Also, for sufficiently difficult problems, R-CMFD will diverge, regardless of the relaxation factor chosen.…”

“…(Eq. (6.32a) is similar to the adaptive Wielandt shift expression used in MPACT [80]. In our numerical tests, we use r = 0.98, c 1 = 10, λ m = 0.)…”

“…In Tables (7.2)-( 7.3), we show both the root-mean-squared (rms) and maximum (max) relative difference between R-CMFD and NILO-A solutions of the assembly pin-powers, fluid temperature, and fuel temperature as a function of convergence tolerance ϵ. For a given simulation, ϵ was used in both the "k tolerance" and "flux tolerance" cards in the VERA common user input file [80]. As expected, both measures of solution difference decrease as the convergence tolerance ϵ is tightened.…”