# An improved target velocity sampling algorithm for free gas elastic scattering

## Abstract

We present an improved algorithm for sampling the target velocity when simulating elastic scattering in a Monte Carlo neutron transport code that correctly accounts for the energy dependence of the scattering cross section. The algorithm samples the relative velocity directly, thereby avoiding a potentially inefficient rejection step based on the ratio of cross sections. Here, we have shown that this algorithm requires only one rejection step, whereas other methods of similar accuracy require two rejection steps. The method was verified against stochastic and deterministic reference results for upscattering percentages in ^{238}U. Simulations of a light water reactor pin cell problem demonstrate that using this algorithm results in a 3% or less penalty in performance when compared with an approximate method that is used in most production Monte Carlo codes

- Authors:

- Argonne National Lab. (ANL), Argonne, IL (United States)
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

- Publication Date:

- Research Org.:
- Argonne National Lab. (ANL), Argonne, IL (United States); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

- Sponsoring Org.:
- USDOE National Nuclear Security Administration (NNSA); USDOE Office of Science (SC), Advanced Scientific Computing Research (ASCR) (SC-21)

- OSTI Identifier:
- 1425283

- Alternate Identifier(s):
- OSTI ID: 1426092

- Report Number(s):
- LLNL-JRNL-745886

Journal ID: ISSN 0306-4549; 138571; TRN: US1802077

- Grant/Contract Number:
- AC02-06CH11357; AC52-07NA27344

- Resource Type:
- Journal Article: Accepted Manuscript

- Journal Name:
- Annals of Nuclear Energy (Oxford)

- Additional Journal Information:
- Journal Name: Annals of Nuclear Energy (Oxford); Journal Volume: 114; Journal Issue: C; Journal ID: ISSN 0306-4549

- Publisher:
- Elsevier

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 97 MATHEMATICS AND COMPUTING; Monte Carlo; algorithm; rejection; target velocity sampling; 22 GENERAL STUDIES OF NUCLEAR REACTORS

### Citation Formats

```
Romano, Paul K., and Walsh, Jonathan A.
```*An improved target velocity sampling algorithm for free gas elastic scattering*. United States: N. p., 2018.
Web. doi:10.1016/j.anucene.2017.12.044.

```
Romano, Paul K., & Walsh, Jonathan A.
```*An improved target velocity sampling algorithm for free gas elastic scattering*. United States. doi:10.1016/j.anucene.2017.12.044.

```
Romano, Paul K., and Walsh, Jonathan A. Sat .
"An improved target velocity sampling algorithm for free gas elastic scattering". United States.
doi:10.1016/j.anucene.2017.12.044.
```

```
@article{osti_1425283,
```

title = {An improved target velocity sampling algorithm for free gas elastic scattering},

author = {Romano, Paul K. and Walsh, Jonathan A.},

abstractNote = {We present an improved algorithm for sampling the target velocity when simulating elastic scattering in a Monte Carlo neutron transport code that correctly accounts for the energy dependence of the scattering cross section. The algorithm samples the relative velocity directly, thereby avoiding a potentially inefficient rejection step based on the ratio of cross sections. Here, we have shown that this algorithm requires only one rejection step, whereas other methods of similar accuracy require two rejection steps. The method was verified against stochastic and deterministic reference results for upscattering percentages in 238U. Simulations of a light water reactor pin cell problem demonstrate that using this algorithm results in a 3% or less penalty in performance when compared with an approximate method that is used in most production Monte Carlo codes},

doi = {10.1016/j.anucene.2017.12.044},

journal = {Annals of Nuclear Energy (Oxford)},

number = C,

volume = 114,

place = {United States},

year = {Sat Feb 03 00:00:00 EST 2018},

month = {Sat Feb 03 00:00:00 EST 2018}

}