DASSH

Standard Code Description

1. Coding Language and Computing Platforms
   DASSH is written in Fortran 90+ and executables are available for Linux, Windows, and macOS architectures. DASSH was initially developed in Python but transitioned to Fortran due to performance requirements. DASSH was developed to couple with the Argonne Reactor Computation code suite and thus it directly links to DIF3D output to construct its power distribution. Alternatively, DASSH has a user definable geometry and power distribution, along with user definable fuel, structure, and coolant heat transfer properties, which allows it to obtain results independently of the Argonne Reactor Computation code suite.
2. Description of Purpose
   DASSH is a subchannel code developed at Argonne to perform full core steady-state thermal analysis on fast spectrum reactors (i.e. liquid metal coolants with hexagonal grids). It is primarily built to model forced convective flow and is not presently capable of dealing with natural circulation. Internally, it has mixing correlations for wire wrapped pins typical of fast reactors along with correlations for non-wire wrapped pins and grid spacers. DASSH allows the user to define an assembly as a standard pin lattice, a tube lattice (flow through tubes rather than around pins), and a porous body medium geometry. The last of these allows the user to adjust the heat transfer coefficients to match the behavior of a real system without dealing with the complex geometry features of that system in the subchannel code. 
   
   For a given reactor, DASSH allows the user to determine an orifice strategy across multiple (fuel cycle) time point evaluations. The built in flow search algorithm will adjust the assembly flow rates to meet user specified constraints on 1) peak 2-sigma clad temperature, 2) peak coolant temperature, and 3) desired bulk outlet temperature across all provided time points.
3. Typical Running Time
   A single time point evaluation with DASSH for a 3-meter tall reactor with ~300 driver and 546 total assemblies takes less than 1 minute obtain the coolant and pin temperature distributions in the entire domain on a modern compute workstation. The typical time required for an analyst to determine an optimal orifice design is ~1 hour but this strongly depends upon the number of time points being considered.
4. References
   1. M. A. Smith, ​“DASSH-F: Subchannel Based Thermal Analysis,” ANL/NSE-24/82 Rev. 1, January (2026).
   2. M. Atz, M. A. Smith, F. Heidet, ​“DASSH software for ducted assembly thermal hydraulics calculations – overview and benchmark,” Transactions of the American Nuclear Society 123 pp. 1673-1676 (2020).
5. Primary Authors
   M. A. Smith, Nuclear Science and Engineering Division, Argonne National Laboratory
6. Materials Available
   The source code and precompiled executables are available via a standard licensing process.
7. Sponsor
   U.S. Department of Energy, Office of Nuclear Energy