# Thermal neutron flux distribution in reflected core

oksuz_
Hi,

The figure given below was taken from "Nuclear Systems" by TODREAS and KAZIMI. it shows the effect of neutron reflector on the thermal neutron flux radial distribution.

Is this correct? Because it does not show the extrapolated distance. It seems to me that the reflector, somehow, produces neutrons and supplies the core.

thank you.

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• reflected core.png
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Staff Emeritus
This figure gives a bit more detail. You must think of both fast neutrons and thermal neutrons.
The heavy fuel load in the PULSTAR, combined with a relatively high fuel-to-moderator ratio, results in high fast neutron leakage at the core boundary. This causes a large thermal neutron “reflector hump” at the core periphery, (see Figure #1 below)yielding high thermal neutron fluxes at the sample irradiation facilities and beamports .

This source shows how to set up and solve the diffusion equations for the core and the reflector.
https://www.nuclear-power.net/nucle...s/neutron-diffusion-theory/reflected-reactor/
The two group case also shows the "reflector peak" in the reflector region. This reference offers the following explanation.
https://www.nuclear-power.net/nuclear-power/reactor-physics/neutron-diffusion-theory/reflected-reactor/ said:
One of the striking results of such solution is that the thermal flux reaches local maximum near the core-reflector interface. This behaviour cannot be derived using one-group diffusion method, because it is caused just by thermalisation of fast neutrons. The fast neutrons, which are produced in the core can enter the reflector at high energy, are not absorbed as quickly in the reflector as neutrons thermalizing in the core, because absorption cross-sections in the reflector are much smaller than in the core (due to the absence of fuel). The thermal neutrons accumulates then near the core-reflector interface, resulting in the local maximum, that is usually known as the reflector peak. This also reduces the non-uniformity of the power distribution in the peripheral fuel assemblies and also reduces neutron leakage, i.e. increases keff of the system (or reduces the critical size of the reactor). This effect can be seen in the following figure.

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• fluxdist.jpg
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• Two-Group-Method-Reflected-Reactor.png
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rpp
The plot is correct. Remember that this is a plot of the *thermal* flux. In two-group theory, there are fast neutrons and thermal neutrons.
Fast neutrons are born in the core (from fission) and travel into the reflector. In the reflector, the fast neutrons slow down to become thermal neutrons and can travel back into the core. Therefore you are correct, the reflector serves as a source of *thermal* neutrons to the core.

Refer to the plots in anorlunda's response to see the fast and the thermal flux.