Fusion Neutronics

Nuclear Engineering

Dr Jonathan Shimwell

This work was funded by the RCUK Energy Programme
[Grant number EP/P012450/1]

Image by Volker Steger

Objectives

cross sections

Reaction rate

mean free path

Fusion cross section

The DT reaction has a higher probability of occurring

The DT reaction has a lower threshold temperature

The DT reaction releases large amounts of energy 17.6MeV

What is fusion

A fusion reaction is the joining of two smaller atoms to form heavier ones.
A large amount of energy is released during this process .

Deuterium (D)

Tritium (T)

Helium 4

3.5MeV

1/5 of the energy

Neutron

14.1MeV

4/5 of the energy

Helium 5

17.6MeV

Energy emitted from nuclear reactions

Calculation of fusion energy from binding energy

H3 + H2 = He4 + n

Binding energy before = 3 x 2.83 + 2 x 1.11

Binding energy after = 4 x 7.07
Difference in binding energy is 17.57 MeV

Calculation of fission energy from mass difference

U235 + n= Ba145 + Kr87 + 4n

Mass before = 235.04393u + 1.00866u

Mass after = 144.92752u + 86.91335u + 4 x 1.00866u

Mass difference = 0.17708u = 164.95MeV

proton mass = 1.672623E-27 kg

neutron mass = 1.674929E-27 kg

1 atomic mass unit = 1.660540E-27 kg

binding_energy curve

plot of energy distribution (Muir) of neutron for different DD, DT and 150Kev, 20Kev

Typical fusion reactions

H2 + H2 = H3 + H1 Q=4MeV

H2 + H2 = He3 + n Q=3.3MeV

H2 + He3 = He4 + H1 Q=18.3MeV

N14 + H1 = O15 Q=7.35MeV

H2 + H1 = He3 Q=5.49MeV

C12 + H1 = N13 Q=12.86MeV

He3 + He3 = He4 + 2 H1 Q=1.95MeV

C13 + H1 = N14 Q=7.55MeV

H1 + H1 = H2 Q=0.42MeV

N15 + H1 = C12 + He4 Q=4.96MeV

H2 + H3 = He4 +n Q=17.6MeV

Maximum energy emitted from fission and fusion per gram

17.6 MeV emitted per DT fusion

Assuming every D reacts with a T.
1 gram of material.

~200MeV emitted per U235 fission

Assuming 100% U235 enrichment .
Assuming ever U235 fissions.
Ignoring decay heat.
1 gram of material.

number\ of\ moles =\frac{mass}{molar \ mass}

0.2 =\frac{1}{5}

number \ of \ atoms =

0.0042 =\frac{1}{235}

0.0042 \times 6.02 \times 10^{23} = 2.56\times 10^{21}

0.2 \times 6.02 \times 10^{23} = 1.204\times 10^{23}

1g\ gram\ of\ DT\ fuel\ 1.1\times 10^{24}\ J

1g\ gram\ of\ U^{235}fuel\ 0.5\times 10^{24}\ J

number\ of\ moles \times

Avogadro's\ constant

Gravitation confinement

The repulsive coulomb force is overcome by gravitational forces and tunneling

Extremely large masses are required as gravity is a relatively weak force.

The CNO cycle

The Proton-Proton reaction

Breeder blankets

To generate sufficient tritium fuel to sustain the reactor.
To convert kinetic energy of the neutrons to thermal energy.
To shield the exterior components from neutrons.

Fuel Cycle

Key reactions in breeder blankets

Questions

Which isotope is depleted?
Why at the front?
What is the best isotope for the front?

Li^6 + n = He^4 + H^3

Li^7 + n = He^4 + H^3 + n

Be^9 + n = 2He^4 + 2n

1\times 10^{21}

neutrons

per second

Neutron moderation

Neutron spectra at different depths in the blanket.
Neutrons are moderated by the material and lose energy.
Neutrons are also captured and produced by some reactions.

Fuel Cycle

Neutron spectra at different depths in the blanket.

Neutrons are moderated by the material and lose energy.

Neutrons are also captured by some reactions.

Selecting suitable materials

Fuel Cycle - tritium breeding

Fuel Cycle - neutron multiplication

Neutron interaction

Material 1

Material 2

Neutron birth

(n,n')

(n,f)

(n,n')

(γ,γ')

(n,nγ')

(n,pn')

(n,f)

(n,2n)

(n,α)

(n,γ)

Neutron

Electron

Gamma

Alpha

Proton

Radioactivity

Target

Nuclide

n,2n

n,g

n,p

n,pn

n,d

n,t

n,nd

n,a

n,He3

n,pd

n = neutron

g = gamma

t = tritium

p = proton

d = deuterium

He = helium

Common neutron induced reactions

Neutron number

Proton number

Irradiation

Time

Number of atoms

Shut down

New isotopes build up during irradiation
Radioactive isotopes decay and will eventually reach a point where decay rate is equal to activation rate.
Decay is more noticeable once the plasma is shutdown.
The activity is related to the irradiation time and the nuclide half life.
The decay process emits gamma rays leading to dose.
The dose is dependent on the activity, the energy of the gamma and biological response.

Radioactivity - activation

Radioactivity - fission products

Atomic number

Percentage of fission products

Radioactivity - fission products

EU Roadmap for fusion energy

3 main reactors (JET, ITER, DEMO) demonstrating large steps in understanding and performance.

Image source www.ccfe.ac.uk/mast_upgrade.aspx

Many smaller reactors testing, validating and experimenting on different plasma physics regimes, component designs, materials and feeding into the design of larger ground breaking reactors