Main Process of Neutron Interaction With Nuclei of Medium

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From the course by National Research Nuclear University MEPhI

Nuclear Reactor Physics Basics

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National Research Nuclear University MEPhI

Nuclear Reactor Physics Basics

13 ratings

This engineering course is designed to introduce students to a range of concepts, ideas and models used in nuclear reactor physics. This course will focus on the physical theory of reactors and methods of experimental studies of the neutron field. This course is based on the course “Neutron transport theory” which has been taught at the National Research Nuclear University “MEPhI” for the past 20 years. What you'll learn: • Define basic processes that may occur in the reactor core, laws, equations, and the limits of applicability of models describing the neutron field in the reactor; • Demonstrate practical experience of calculating the distribution of neutrons in media; • Demonstrate the ability to analyze the process of slowing down neutrons in various media (typical for nuclear fission reactors) from the standpoint of understanding the physics of the process; • Evaluate important reactor parameters including performance and safety.

From the lesson

Introductions to Nuclear Reactor Physics

In this module students will learn about the neutron field and main functions to describe it. Students will study the concepts of neutron flux, net current, one-way currents and vector of net current.

Main Process of Neutron Interaction With Nuclei of Medium4:34

Microscopic Cross-Sections6:14

Mean Free Path2:52

Meet the Instructors

Yury Volkov
Senior lecturer, PhD in Nuclear Engineering
Department of Theoretical and Experimental Physics of Nuclear Reactors

After studding this lecture, the student should be able to: define

main processes of neutron interaction with nuclei of a medium; define

the microscopic and macroscopic cross sections and mean free path.

There are two main mechanisms of the neutron interaction with nuclei

of a medium: the first is scattring.

Scattering — it is an interaction of a neutron with a nucleus

which results only in a redistribution of the kinetic energy and the angular

momentum between neutron and nucleus.

The second absorption. Absorption – it includes all interactions of a neutron with

a nucleus resulting in the appearance of new nucleus and new particles

(including neutrons). Further, the term collision of a neutron

with a nucleus will be understood as the scattering and absorption.

For a better understanding of the nature of nuclear reactions, it is very

useful to implement representation of nucleus as a finite potential well.

There are energy levels in the potential well which characterize

the exited states of the nucleus. The bottom of the potential

well characterizes the stable state. When the neutron

is colliding to collide with a nucleus with the velocity v,

it has the kinetic energy T. When the neutron’s energy is close

to the energy of an excited state the neutron can penetrate into the nucleus and

create the compound state. All nuclear reaction occurs though

the mechanism of compound nucleus except one – the potential

elastic scattering (scattering on a nucleus potential without penetration).

The potential elastic scattering — occurs when neutron is deflected

by a nucleus without being absorbed. Potential elastic

scattering conserves kinetic energy and is often visualized as

a "Billiard Ball" type of collision. In this case, some kinetic energy

of the neutron is transferred to nucleus of the target atom. The momentum

is conserved. An important role in explaining the mechanism of many

nuclear reactions played the assumption of N. Bohr (1936) about

two stages of nuclear reactions. The first stage is capturing by the nucleus

of a neutron (at the characteristic distance for nuclear forces ~ 10 in the power minus 15 m),

and the formation of the intermediate nucleus which is called

compound (or compound-nucleus). When the neutron penetrates

into the nucleus, it adds the excitation energy (E_ex), which is the sum of

its kinetic energy and binding energy. The compound nucleus

lives as usual 10 in the power minus 17 seconds and subsequently the excitation energy

has to be removed. The second stage — a decay of the compound nucleus.

There are different ways to remove the excitation energy — the one which

we called nuclear reactions undergoing by the mechanism of compound

nucleus. What happeneds next depends on the probability of

nuclear reactions. The first case — neutron can escape with the

same energy (not direction), this is the resonance elastic scattering.

The second – the neutron can drop down to a lower energy level

(not lower than the binding energy) and escape with a lower

kinetic energy. The residue part of the excitation energy is removed by

the γ-emission. It names the inelastic scattering.

The others the neutron absorption reaction

is the most important type of reactions that take place

in a nuclear reactor. The third option, the neutron can drop down

to the bottom of the potential well and create a new stable state.

All excitation energy is removed by the γ-emission.

It's name the radioactive capture, (n, γ) reaction.

The neutron can drop down to the bottom of

the potential well, but the excitation energy goes to the split the nucleus — fission,

(n, f) reaction; the creation of new particles — reactions of nucleus spallation.

In summary all nuclear reaction occurs with the mechanism of compound

nucleus generation. Except of the only one — the potential elastic scattering.

The next step is to learn how to calculate the probability of the reactions.

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