# How to Declare a Spherical Neutron Source for Thermal Neutron Flux Calculation?

TL;DR Summary
how I can declare a spherical neutron source to calculate the thermal neutron flux in a cell
c Created on: Tuesday, September 05, 2023 at 11:26
1 1 -1.58 -1
2 0 -2 #1
3 0 -3 #1 #2
4 0 3

1 rpp -0.5 0.5 -0.5 0.5 0 0.1
2 so 10
3 so 15

mode n
m1 15000. 0.12951 $matrix 6000. 0.01731 12000. 0.00546 11000. 0.008 20000. 0.35857 13000. 0.002184 8000. 0.40146 16000. 0.0118 26000. 0.0026 14000. 0.0101 imp:n 1 1r 0 1$ 1, 4
sdef cell=2 rad=d1 erg= 0.025 par=1
si1 0 10
f4:n 1
e4 1.01e-10 121i 0.025
nps 10000

A few things to think about,

The void cell is 4, so your imp line should be 1 1 1 0 not 1 1 0 1, right?

You have a point source at 0 0 0 (default source location) is this on the edge of two cells? It needs to not be on the edge. If you want to make cell 2 a homogeneous source, a sphere with a box cut out of it, that is tricky. A surface source might be easier from surface 2, but it depends a lot on what the problem is.

erg= has a space before 0.025

If you rename your input file to add .txt you can attach it to a post, or you can paste into BB code tags, then we can see the version with the correct formatting.

Greg Bernhardt and berkeman
Cell 1 is immersed in a thermal neutron source which is the cell 2

#### Attachments

• input file flux.txt
1,019 bytes · Views: 44

## 1. How do I define the geometry of a spherical neutron source?

To define the geometry of a spherical neutron source, you need to specify the radius of the sphere and the position of its center within your simulation or experimental setup. This involves using the appropriate software or mathematical framework to describe the spatial boundaries and ensure the source is correctly centered within the cell.

## 2. What parameters are needed to describe the neutron source?

The key parameters to describe a neutron source include the source strength (neutrons per second), energy spectrum of the emitted neutrons, spatial distribution (uniform or otherwise), and angular distribution. For a spherical source, you also need to specify the radius of the sphere.

## 3. How do I set up the boundary conditions for the neutron flux calculation?

Boundary conditions are crucial for accurate neutron flux calculations. Common boundary conditions include reflective, absorptive, or vacuum boundaries. You need to define these conditions at the edges of your cell to simulate how neutrons interact with the cell boundaries. Reflective boundaries imply neutrons bounce back, while absorptive boundaries mean neutrons are absorbed and do not contribute to further interactions.

## 4. What simulation tools can I use to calculate the thermal neutron flux?

Several simulation tools are available for calculating thermal neutron flux, including MCNP (Monte Carlo N-Particle), FLUKA, and GEANT4. These tools allow you to model the neutron source, define the geometry of the cell, set boundary conditions, and run simulations to obtain the neutron flux distribution.

## 5. How do I convert the neutron flux results to thermal neutron flux?

To convert the neutron flux results to thermal neutron flux, you need to apply energy-dependent weighting to the neutron flux distribution. This involves using the energy spectrum of the neutrons and integrating over the thermal energy range (typically 0.025 eV for thermal neutrons) to isolate the thermal component of the flux. Most simulation tools provide built-in functions or post-processing scripts to facilitate this conversion.

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