### What is Rattlesnake?

Rattlesnake is a radiation transport solver for the linearized Boltzmann radiation transport equation (RTE). It can model neutron, photon and thermal radiation interactions with background materials. Modeling radiation transport accurately is essential to the design and safety of nuclear facilities including nuclear reactors. Rattlesnake relies on the material properties describing how radiation particles interact with background media and field variables like the temperature and the material density to compute various quantities of interests, including the radiation flux distribution, variable reaction rates, radiation-induced power profile, etc. These can be provided to the analyst or to other physics for multiphysics simulations.

### How does Rattlesnake work?

Rattlesnake is a MOOSE-based application for solving the multigroup radiation transport equation. It provides various discretization schemes, including the self-adjoint angular flux (SAAF) and least squares (LS) formulation with continuous finite element method (FEM), the first-order formulation with discontinuous FEM. The angular discretization can be applied with the discrete ordinates method (SN), the spherical harmonics expansion method (PN) or using the diffusion approximation. All these schemes are leveraged with the multi-scheme capability within Rattlesnake, thus allowing the assignment of more suitable schemes on various subdomains with different levels of resolution in order to optimize the use of computing resources. Rattlesnake solves steady-state, transient and eigenvalue problems with arbitrary order of scattering anisotropy. It also owns all the features provided by the MOOSE framework, including unstructured higher-order meshes, massive parallelization, dimension agnosticism, etc. Rattlesnake is designed for multiphysics simulations. Its applications to the fully-coupled multiphysics simulation and the tightly-coupled multiphysics simulations with data transfer have been both successfully demonstrated. It interacts with YAKXS, a multigroup cross section library management toolkit, for incorporating the cross sections generated with lattice physics codes.

The detailed power distribution of all fuel pins for the C5G7-2D benchmark:

Transient of LRA benchmark:

Heterogeneous ATR calculation with total number of degrees of freedom 4,178,001,600 with 4,207,728 elements, 12 groups and 80 streaming directions:

It takes about 3.58 hours with 2048 cores on Falcon.