Average Logarithmic Energy Loss

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Do curso por National Research Nuclear University MEPhI

Nuclear Reactor Physics Basics

2 classificações

National Research Nuclear University MEPhI

Nuclear Reactor Physics Basics

2 classificações

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.

Na lição

Main Principals of Slowing down of Neutrons

In this module students will get knowledge concerning Main Principals of Slowing down of Neutrons. Students will learn about Neutron Spectrum in Absorbing and Non-Absorbing Medium.

Main Principals of Slowing down of Neutrons7:36

Kinematic of Slowing Down8:08

Average Logarithmic Energy Loss3:32

Conheça os instrutores

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

The nex quantity is the average logarithmic energy loss

or average logarithmic energy decriment.

In practice the following quantity is very useful for

the slowing down process description.

The average logarithmic energy loss

is denoted as the lowercase ξ.

This quantity can be obtained from

the formula for an average function

by using the elastic scattering law.

A benefit of the function is the independency

on the energy of incident neutrons.

The function depends only on the atomic mass of the nucleus.

For hydrogen the average logarithmic energy loss

equals 1, for heavy nucleus the function tends to zero.

The next important quantity is the lethargy.

Similarly to the energy, the lethargy is

the variable for a description of the slowing down process.

Lethargy is denoted as U and it equals

to the logarithm of the ratio of reference energy

and the current value of energy

corresponding to the value of u.

The reference energy — the maximal possible neutron energy

in a system (source energy) ~10 MeV.

Through the slowing down, the lethargy increases

in contrast to the energy.

The benefit of the variable

is that in terms of the lethargy

the average increment of lethargy doesn’t depend

on the current value of incident neutron energy

in contrast to the energy variable.

By using thisquantity it is very easy to calculate

the average number of collisions

for a neutron to nucleus to slow down.

From for example E1 = 1 MeV to E2 = 1 eV.

The result depicted on this slide.

One can see that the fastest slowing down

occurs from the scattering on the hydrogen.

Can we say that hydrogen is the best moderator?

So, first of all, we have to introduce three new quantities.

The first us the average logarithmic loss of energy

for mixture of nuclei.

The mixture means molecules

or a homogeneous medium which is composed of different nuclei.

The second, slowing down power

determines the moderator which better moderates.

The third, the moderation ratio or quality factor

defines the best moderator and emphasizes the fact,

that the best moderator has to do both, to better slow down

the neutron and worse to absorb the neutrons.

By the last criterion,

as it is presented in the table,

the best moderator is the heavy water.

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