ML17262A007
| ML17262A007 | |
| Person / Time | |
|---|---|
| Issue date: | 08/25/1976 |
| From: | Levine S Office of Nuclear Regulatory Research |
| To: | Rusche B Office of Nuclear Reactor Regulation |
| References | |
| RIL-0007 | |
| Download: ML17262A007 (4) | |
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Saw Levin~
San.1. r..vine, Jcti.nq Dinetor Office of !hclaa..r Requla.tory aasearch DIST:
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- Xelber Curtis
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- Murley Norry Levine
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AUG 2 5 1976
'1'HE SIMMER CODE Analyses of hypothetical core disruptive accidents (BCDA's) for liquid-
"metal cooled fast breeder re~ctors (IMFBR's) have predicted the evolution of highly distorted molten core qeometries, and this has led to concern about recriticality during such accidents.
In mid-1974 the Los Alamos Scientific Laboratory (LA.SL) began development of a code called SIMMER (for ~n, Implicit, M_ultifield, M.ulticomponent, Eulerian, Recriticality) to follow the motion and neutronics of Ui!FBR cores during BCDA's.
This manual describes SIMMER-I, which is the first in a series of SIMMER computer codes designed to examine the dynamics of disrupted cores in LMFBR's.
Overview of SIMMER-T Models SIMMER-T couples space-dependent neutronics with multifield, multicompon-ent fluid dynamics in an (r,z) representation of an LMFBR core.
'l'he code includes different neutronics models for accuracy and efficiency comparisons.
A time-dependent neutron transport option is included to accurately calculate the neutron leakage and streaming if large voided regions develop during an accident calculation.
The diffusion approx.iJnation breaks down in situations where material densities approach zero, but when such situations are not expected, a diffusion option is provided for including space-dependent effects. Either the quasistati*c method or the extrapolation method is available for calculating the time-dependent behaYior of the flux in both the transport and diffusion options.
Cross sections used in the space-dependent neutronics calculation are determined at each neutronic time step using the Bondarenko formalism.
The code includes additional options for performing point kinetics neutronics or using input values of power vs. time for scoping studies of whole core accidents or small scale studies of single subassembly phenomena.
The SIMMER~T fluid dynamics model is based on that developed in KACHINA program in which the relative motion of two fields, liquid and vapor, is calculated using the Implicit Multifield (IMF) method.
The KACHINA model has been extended by the addition of a third component to each field to
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AUG 2 5 1976 follow the motion of the three LMFBR materials: fuel, steel, and sodimn.
In addition, a structure field has been added to model solid fuel, cladding, and subassembly can walls.
Within each field, the materials, or components, move with the velocity of the field.
'!'his means, for example, that liquid fuel and liquid steel in the liquid field have the same velocity at a point in space.
On the other hand, separate energy equations are solved for each component except for those in the vapor field where the mixture of vapors is treated with a single energy equation.
The structure field is fixed in space and acts as an infinite momentum sink for drag forces and mass transfer. Separate continuity and energy equations for the three structure components predict the the:nnal state of the structure field.
If a can wall is in a mesh cell, radial motion in that.cell is inhibited until the can wall is predicted to fail. Failure of structure in the present model depends only on melting.
Melted materials enter the liquid field, while freezing materials leave the liquid.field and enter the structure field.
Within the structure and fluid dynamics equations, terms are included to account for coupling between the fields and the components.
Monentum coupling is accounted for by drag force terms; energy coupling is accounted for by heat transfer terms, and mass coupling is accounted for by phase transition terms.
The drag correlations are simple in form.
The heat transfer coefficients are simple constants multiplying the temperature differences between fields and components to determine heat transfer rates.
Phase transitions are modeled with greater complexity.
Melting and freezing are calculated by comparison of material energies with solidus and liquidus energies, and vaporization and condensation are calculated using nonequili-brium conduction-limited phase transition models in which the phase transition rates are proportional to the difference between saturation and component temperatures.
SIMMER-T and the SIMMER Program In SIMMER-T development has concentrated on the numerical methods of two-phase fluid dynamics and time-dependent neutronics.
The physics of exchange
PUG 2 5 1975 coefficients for the two-phase fluid dynamics equations has received much less attention.
These exchange coefficients model the interaction phenomena of heat transfer, momentum exchange, and phase transition and are primarily based on flow of dispersed droplets in a vapor field, al though flow of bubbles in a liquid fie*ld is treated in some cases.
Other two-phase flow regimes are not modeled, and it is likely that the user will find situations in which the S:IMMER*I exchange coefficients are not valid.
An important effort prior to the release of SIMMER-II will be the development of better models for these exchange coefficients.
The tabular equations of state that are part of the SIMMER program are not included in SIMMER-I.
The equa_tions of state for sodium, iron, and uranium-dioxide have undergone extensive investigation using techniques Which consistently describe the material equation of state for the density and energy range from normal atmospheric conditions to low densities and/
or high energies.
In place of these more sophisticated equations of state, which require tabular data and interpolation routines for use in SIMMER, analytic representations of the equations of state have been included in SIMMER-I.
A later modification of SIMM~-I will include tabular equations of state.
The original scope of the SIMMER code was to examine the possibility of secondary criticality following disasseJr!bly of an LMFBR core.
This included tracking the large scale motion*of materials prior to, during, and after*
disassembly.
Despite the elementary nature of the physics modeling, it has become apparent during the development Qf SIMMER-I that the eventual capabilities of SIMMER will allow the examination of other LMFBR accident Situations such as gradual core meltdown, fluidization, and development of a disrupted core, i.e., the transition phase.
The techniques also appear to be useful in the analysis of single subass~mbly phenomena involving fuel mel~
down, extended material motion, freezing, plugging, clad relocation, and fluidization.
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