CHOPS: Module to Compute Pointwise Disadvantage Factors and Produce a Cell-Homogenized CENTRM Library

M. L. Williams and L. M. Petrie

Introduction

CHOPS (Compute HOmogenized Pointwise Stuff) computes pointwise (PW) disadvantage factors from the PW zone fluxes on a CENTRM output file, and then multiples the disadvantage factors by continuous-energy (CE) cross section data in a CENTRM library to generate a new cell-homogenized CENTRM CE library. The PW disadvantage factor for zone “Z”, as a function of energy E, is calculated from the expression,

(116)$D_{Z}(E)=\frac{\Phi_{Z}(E)}{\Phi_{C}(E)}$

where $$\Phi_{Z}(E)$$ is the CENTRM PW flux spectrum averaged over the volume of zone Z in the cell, and $$\Phi_{C}(E)$$ is the PW flux averaged over the entire cell volume. The cell-homogenized CE cross section for a nuclide “j” is equal to

(117)$\sigma_{C}^{(j)}(E)=\sum_{Z} F_{Z}^{(j)} D_{Z}(E) \sigma_{Z}^{(j)}(E)$

where $$F_{Z}^{(j)}$$ is the fraction of all nuclide–j atoms contained in zone Z, and $$\sigma_{Z}^{(j)}(E)$$ is the CE cross section for nuclide-j at the temperature of zone Z. When multiplied by the cell-homogenized number density of nuclide-j and by the cell-average flux, the cross section expression in (116) gives the correct average reaction rate at energy E.

CHOPS is used in the automated double heterogeneity sequence in SCALE, in which a low-level heterogeneity, such as microspheres in a granular fuel element, are smeared into a homogenized absorber region appearing in the second level heterogeneity, such as fuel pellet or pebble appearing in a lattice. The disadvantage factors provide for flux weighting of the PW XS data so that the spatial self-shielding is treated correctly in the homogenized geometry. A second CENTRM PW transport calculation is performed with the cell-averaged PW library output by CHOPS in order to account for the additional self-shielding of the absorber pellets/pebbles in the lattice. CHOPS is called automatically by the XSProc module for double-heterogeneous unit cells, or it can run as a standalone code.

CHOPS Input Data

DATA BLOCK 1

0$$LOGICAL UNIT ASSIGNMENTS (10 entries. Default values given in parentheses) 1. lold – logical unit number of input CENTRM XS library (1) 2. lnew – logical unit number of output CENTRM homogenized XS library (2) 1. lflx – logical unit number of input CENTRM PW flux library (3) 2. ldis – logical unit number for edit of PW disadvantage factors (0) 3. n15 – logical unit number for scratch (15) 4. n16 – logical unit number for scratch (16) 5. n17 – logical unit number for scratch (17) 6. n18 – logical unit number for scratch (18) 7. n19 – logical unit number for scratch (19) 8. nsq – sequence number used in filename on unit “lnew” (1) [Example: if lnew=11 and nsq=3: output filename of homogenized library= ft11f003] 1$$ INTEGER PARAMETERS (5 entries )

1. idtap – identifier for the new library (55555)

[for macro library, the value of idtap is made negative]

2. nprt – output print option: 0 = > min print; 1 = > normal; 2 = > max print (0)

3. iden – if=0 = > define homogenized XS id = id on CENTRM flux file (0)

if>0 = > define homogenized XS id to be, (iden*106 + ZA)

1. macr – type of XS output: 0 = > microscopic ; 1 = > macroscopic (0)

2. icorr – not used (0)

2** REAL PARAMETERS (3 entries )

1. tole – tolerance used to thin pointwise cross-sections (0.0025)

( 0.0 means no thinning is done )

1. cleth – maximum lethargy between thinned pointwise cross-sections

points that allow a point to be discarded (0.25)

3. vfrac – multiplier applied to all output XS’s [eg, grain fraction] (1.0)

T [ TERMINATE DATA BLOCK 1 ]

CHOPS I/O units

Table 135 shows default logical unit numbers used by CHOPS. These values may be changed in the 0 array of input.