## Abstract

Modeling the propagation of radiative heat waves in optically thick material using a diffusive approximation is a well-known problem. In optically thin material, classic methods, such as classic diffusion or classic P_{1}, yield the wrong heat wave propagation behavior, and higher-order approximation might be required, making the solution more difficult to obtain. The asymptotic P_{1} approximation [Heizler, Nucl. Sci. Eng., Vol. 166, p. 17 (2010)] yields the correct particle velocity but fails to model the correct behavior in highly anisotropic media, such as problems that involve a sharp boundary between media or strong sources. However, the solution for the two-region Milne problem of two adjacent half-spaces divided by a sharp boundary yields a discontinuity in the asymptotic solutions that makes it possible to solve steady-state problems, especially in neutronics. In this work we expand the time-dependent asymptotic P_{1} approximation to a highly anisotropic medium using the discontinuity jump conditions of the energy density, yielding a modified discontinuous P_{1} equation in general geometry. We introduce numerical solutions for two fundamental benchmarks in plane symmetry. The results thus obtained are more accurate than those attained by other methods, such as Flux Limiters or Variable Eddington Factors.

Original language | American English |
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Pages (from-to) | 189-207 |

Number of pages | 19 |

Journal | Nuclear Science and Engineering |

Volume | 192 |

Issue number | 2 |

DOIs | |

State | Published - 2 Nov 2018 |

Externally published | Yes |

### Bibliographical note

Funding Information:We acknowledge the support of the PAZY Foundation under grant number 61139927. The authors thank Roee Kirschenzweig for the use of the IMC code for radiative problems, Stanislav Burov, and the anonymous referees for their valuable comments.

Publisher Copyright:

©, © 2018 American Nuclear Society.

## Keywords

- Radiative transfer
- Su-Olson benchmark
- asymptotic P approximation

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