Add support for CSAS-Shift sequence (csas5-shift, csas6-shift)

[Copilot]

Overview

This PR adds support for the CSAS-Shift sequence as described in the SCALE CSAS-Shift manual. CSAS-Shift integrates the Shift Monte Carlo solver into the Criticality Safety Analysis Sequence (CSAS) framework, providing improved performance and scalability for criticality safety calculations.

What is CSAS-Shift?

CSAS-Shift uses the Shift Monte Carlo radiation transport solver instead of KENO for criticality calculations. It offers:

• Improved performance - Faster solutions with better parallel scaling
• Full KENO compatibility - Supports both KENO V.a and KENO-VI geometries
• Simple transition - Users just change =csas5 or =csas6 to =csas5-shift or =csas6-shift

Changes Made

New Model: Scale-shift

• Created .fz/models/Scale-shift.json with output variables matching Scale-keno
• Parses k-eff, energy of lethargy, nubar, and mean free path from CSAS-Shift output
• Uses same variable syntax (&(variable)) and comment character (') as other models

New Calculator

• Created .fz/calculators/Scale-shift.sh to execute SCALE with CSAS-Shift sequences
• Validates successful completion by checking for "Congratulations" message
• Supports both =csas5-shift and =csas6-shift input formats
• Created .fz/calculators/Localhost_Scale-shift.json for local execution

Example and Documentation

• Added examples/Scale-shift/godiva.inp showing csas6-shift usage
• Updated README.md with:
  • Scale-shift in supported calculation types
  • Output variables documentation
  • Usage examples with the fz Python API
  • Input file format example highlighting the difference from regular CSAS
• Updated CHANGELOG.md with new features

Testing

• Extended test suite to validate Scale-shift model, calculator, and example files
• All tests passing ✓
• CodeQL security scan: 0 vulnerabilities ✓

Usage Example

import fz

# Run CSAS-Shift calculation with varying sphere radius
results = fz.fzr(
    input_path="examples/Scale-shift/godiva.inp",
    input_variables={"r": [8.5, 8.6, 8.7, 8.741, 8.8, 8.9, 9.0]},
    model="Scale-shift",
    calculators="Localhost_Scale-shift",
    results_dir="godiva_shift_results"
)

print(results[['r', 'mean_keff', 'sigma_keff']])

Input File Format

Simply replace =csas6 with =csas6-shift in existing KENO input files:

'Input generated by GeeWiz SCALE 6.0.2
=csas6-shift
godiva
ce_v7_endf
read composition
 uranium     1 den=18.742 1 293
                                 92234 1.0202
                                 92235 93.7112
                                 92238 5.2686   end
end composition
read geometry
global unit 1
 sphere 10    &(r)
 media 1 1 10
 boundary 10
end geometry
end data
end

Compatibility

• Compatible with SCALE 6.2+
• Follows the same patterns as existing Scale-keno, Scale-tsunami, and Scale-xsdrnpm models
• No breaking changes to existing functionality

Closes #[issue_number]

Original prompt

add support for CASS-shift sequence, described in https://scale-manual.ornl.gov/CSAS-Shift.html

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