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Chapter 1

Fusion and Fission Energy

In this chapter we will take a closer look at nuclear energy. Nuclear energy can be separated in nuclear fission(splitting of heavy atomic nuclei) and fusion of light nuclei as it can be seen in figure 1.2.

Figure 1.1: Fission and Fusion Reaction

Nuclear power delivers roughly 1/3 of the electricity produced in Europe. It is therefore an importantcontribution. However due to the sustainability Zeitgeist and the accident in Fukushima there has been arethinking in most of European countries. Other fast growing economies like China, India or South Koreahowever are building new Nuclear Power Plants.

Let us now look at the first branch from our nuclear tree, the fission.

1.1 Nuclear Fission

Nuclear fission for power production is based on the heavy element Uranium. The different isotopes are ra-dioactive, but because of their extremely long half-life (this is the time it takes before half of the initial amount

of a certain isotopes has undergone a spontaneous nuclear reaction, decaying to another isotope) these isotopesare still around on earth (that is 4.5 Gyear old). We will see that the nuclear decay that is important is theone that produces neutrons; however, most spontaneous decays are alpha decays: then a 4He nucleus is emitted(two protons + 2 neutrons).

Note that fissioning of 1 gram uranium yields as much energy as burning 2500 liters petrol or 3000 kilogramscoal.

1.1.1 Reactor Physics

Fission is splitting the heavy nucleus into two large fragments and a number of 3 neutrons. Much energy isreleased so the fragments and neutrons have high kinetic energy. Most important for power production is thatthe spontaneously produced neutrons can be absorbed by surrounding 235U nuclei. When that happens thisnucleus can directly split as well, producing new neutrons.

We can also take a look at the binding energy in atoms. For that we can take a look at figure 1.3. We cansee the difference in energy

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1.1. NUCLEAR FISSION CHAPTER 1. FUSION AND FISSION ENERGY

Figure 1.2: Fission Process

Figure 1.3: Binding Energy

The elements on the right side can be used for fission. The binding energy is then released. On the otherhand the elements on the right side can be used for fusion. The energy release for fission of 235U can bedetermined by:

235U + n→X + Y + vn + 207MeV (1.1)

Next we can take a look at the energy distribution and penetration depth:Energy (MeV) Range

Kinetic energy fission fragments X,Y 168 ¡¡1mmPrompt gamma-radiation 7 10-100 cm

Kinetic energy fission neutrons 5 10-100 cm

Gamma from fission products 7 10-100 cmBeta from decay of fission products 8 around 1 mm

Neutrinos 12 infiniteTotal 207

The energy from the neutrinos cannot be used since they dont have any mass.If we now want to reach 1W, how many fissions per second are needed? This can easily be solved by the

fact that 1 fission gives 200MeV of recoverable energy, so for 1 W we need:

1W

200 · 106 · 1.602 · 10−19(J/eV )= 3.1 · 1010fissionspersecond (1.2)

The next thing we would like to calculate is how much energy is released from 1 g of 235U ? To answer thatwe will first calculate the atoms in one gram which is:

6.022 · 1023(atoms/mol)

235(g/mol)= 2.56 · 1021atoms (1.3)

Now if all 235U atoms undergo the fission then this will give:

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1.1. NUCLEAR FISSION CHAPTER 1. FUSION AND FISSION ENERGY

2.56 · 1021 · 200 · 106(eV ) · 1.602 · 10−19(J/eV ) = 82GJ = 0.95MWd (1.4)

Now we can ask ourselves how long a household can get electricity from 1 g of 235U (if we take into account4000 kWh per year and assume a 35% thermal efficiency.

Well this can be calculated by:

82 · 109J · 0.35

4000 · 103 · 3600 = 2years (1.5)

The next point we will look at is the moderations. When looking at figure 1.4 we can see that the interactionprobability increases for lower energy levels. Thus we need to decrease the atoms to increase the interactionprobability.

Figure 1.4: Moderation

Neutrons produced by fission are very energetic (energies of MeV) and can be slowed down by collisionswith atoms. A moderator material is placed between the 235U containing pellets so that inside this moderatorneutrons can loose their energy. Most effective is water because this contains many protons: these are as light

as neutrons and therefore can take up more kinetic energy upon each collision (a neutron hitting a heavy atom just bounces off as hard as it came in). A room temperature neutron has an energy of 0.025 eV (25meV), somuch lower than the MeVs.

We further distinguish between moderator power and moderator quality. A good moderator should:

scatter a lot (

scat large)

have a large energy decrement per collision (ζlarge)

Absorb a little (

abs large)

Moderator power is then defined as ζ

scat and the moderator quality asζ

scatabs

.

We can further subdivide the ranges of materials used for fission in fissile isotopes which can be fissioned

by neutron capture, fissionable isotopes in which are the threshold in neutron energy and lastly fertileisotopes which can be turned into a fissile isotope.

1.1.2 Nuclear Reactors

A nuclear reactor consists of several components:

Moderator for slowing down the neutrons

Fuel where fission occurs and heat is produced

Control rods that absorb neutrons shutting down the reactor

Reflector to scatter back neutrons

System to transport heat (cold leg/hot leg)

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1.2. NUCLEAR FUSION CHAPTER 1. FUSION AND FISSION ENERGY

There are different kind of reactors. The first one is the Pressurized Water Reactor (PWR). In a PWR theprimary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energygenerated by the fission of atoms. The heated water then flows to a steam generator where it transfers itsthermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spins anelectric generator. Also there are Boiling Water Reactor (BWR). The boiling water reactor (BWR) is a typeof light water nuclear reactor used for the generation of electrical power. It is the second most common typeof electricity-generating nuclear reactor after the pressurized water reactor (PWR), also a type of light water

nuclear reactor. The main difference between a BWR and PWR is that in a BWR, the reactor core heats highpressure (1000 - 1100 psi) water, which turns to steam and then drives a steam turbine. In a PWR, the reactorcore heats very high pressure (2200 - 2500 psi) water, which does not boil. This hot water then exchanges heatwith a lower pressure (approx. 900 psi) water system, which turns to steam and drives the turbine.

1.1.3 Nuclear Fuel Cycle

A good image of the actual nuclear fuel cycle can be seen in figure 1.5

Figure 1.5: Nuclear Fuel Cycle

When looking at the material itself one starts with the raw uranium ore which is then converted to theyellow cake. At then end one receives the UO2 tablets. For the uranium resources it can be said that we stillhave enough uranium for the next 300 years. This is due to the fact that the earths crust contains 40 timesmore uranium than silver. This will cover for the next 100 years. Also there is still quite some undiscovereduranium and uranium as byproduct from phosphate deposits. This will yield an extra 200 years. Lastly wecould use the seawater which could bring us to an extra 6000000 years.

1.1.4 Spent Fuel Composition

There is always a waste coming from the fission energy. These are the actinides and the fission products.However only 4 % of the entire fuel is waste. The rest can be recovered. One of the main problems is the decayof radiotoxicity.

As a conclusion we can they the spent fuel consists of fission products, unused uranium and plutonium andAmericium. Fission products are already harmless after 300 years, Americium after 5000 years.

1.1.5 Safety of Nuclear Reactors

The safety of the nuclear reactor has ben discussed for quite some time. However the reactor is actually a stablesystem. A loss of coolant shuts down the reactor immediately and a loss of moderation shuts down the reactor.However core cooling is needed even after the reactor shut down.

1.2 Nuclear Fusion

Now let us look at the second part of our tree. If we go back to the binding energy that was introduced in 1.1.1,then we know that we can generate far more electricity from nuclear fusion. Currently scientists focus on the

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following reaction to create energy:

2H +3 H →4 H + n+ 17.6MeV (1.6)

Even though the D-T provides the lowest energy gain, it is the least difficult one to realize.If we look at the at figure 1.6 we can see how the process of fusion works.

Figure 1.6: Fusion Reaction

Now we can look at the individual materials that are needed for the reaction. Deuterium exists in nature,however tritium has a half life of only 12 years. Lithium is also naturally abundant.

1.2.1 Fusion Reactor Requirements

We will now take a closer look at what is needed for the fusion reaction in order to generate electricity. First of allhigh temperature is needed, secondly the collision rate of particles need to be sufficient in terms of high particledensity and sufficient long confinement time. There are different approaches for confinement mechanisms. Sinceordinary confinement does not work out due to the high temperature one can make use of gravitational power,intertial confinement or magnetic confinement (tokamak geometry)

For the fusion process a mixture of D and T is created, which is then heated up till both both elements will

start fusing and producing energy.Currently the research is done on the Tokamak. The Tokamak (see figure 1.7 creates toroidal magnetic fields

to confine particles in the 3rd dimension. Includes an induced toroidal plasma current to heat and confine theplasma

Figure 1.7: The Tokamak

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Within the Tokamak the magnetic field is confined in the following way:

Figure 1.8: Magnetic field of Tokamak

The limitations for the confinement are that scattering makes plasma particles difuse through the magneticconfinement, thus plasma can touch the wall of the torus. This leads to a cooling down of the plasma andfurthermore it leads to unwanted impurities which subsequently leads to a finite plasma lifetime.

1.2.2 R&D of Magnetic Fusion

The research of magnetic fusion started in the 1970s, from then onwards there was a steady learning curve till themid 1990s which is due to several experiments and research institutions like TFTR or JET. Since then furtherresearch has been slowed down but with the start of the ITER project it is assumed that the magnetic fusiontechnology will proceed. ITER stands for international thermonuclear experimental reactor. It is supported byseveral countries world-wide.

The fusion process can be divided in five steps:

1. The fusion reactor will heat a stream of deuterium and tritium fuel to form high-temperature plasma. Itwill squeeze the plasma so that fusion can take place. The power needed to start the fusion reaction willbe about 70 megawatts, but the power yield from the reaction will be about 500 megawatts. The fusionreaction will last from 300 to 500 seconds. (Eventually, there will be a sustained fusion reaction.)

2. The lithium blankets outside the plasma reaction chamber will absorb high-energy neutrons from thefusion reaction to make more tritium fuel. The blankets will also get heated by the neutrons.

3. The heat will be transferred by a water-cooling loop to a heat exchanger to make steam.

4. The steam will drive electrical turbines to produce electricity.

5. The steam will be condensed back into water to absorb more heat from the reactor in the heat exchanger.

The entire process is also depicted in figure 1.9

Figure 1.9: Magnetic-Confinement fusion process

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1.2.3 Challenges on the road towards fusion energy

Before fusion energy can actually be used for generating electricity there are still some challenges to overcome:

Plasma confinement works only for charged particles. The hot, 14MeV, neutrons and photons hit the wallsdirectly. Result: the walls will get an annual dose high enough to cause 20-30 dpa (20-30 displacementsper atom)

Hot He nuclei get trapped in the wall materials, leading to expanding gas bubbles.

Neutron induced transmutation will occur, causing radioactivity build up and the release of H and moreHe.

The energy of the plasma needs to be exported in a controlled way, without destroying the plasma or thewalls. Extreme materials demands.

Suitable materials do not exist yet. Candidate structural materials for plasma facing and breeding-blanketcomponents have a chemical composition that is based on low activation elements (Fe, Cr, V, Ti, W, Si,C, Ta).

High melting temperature materials are only W and C.

Superconducting magnets at T 4K are required close to extreme T regions and in intense gamma/neutronirradiation zones. The liquid He required to cool the wires comes with natural gas, but that may run outin the future.

The world stock of tritium is only around 27 kg (half-life 12.3 year), produced by (Canadian) heavy waterreactors . For future fusion energy generation the tritium will have to come from n capture in Li breederblankets around the Tokamak. NOTE: the dream of endless fusion using water as source of hydrogenisotopes is thus far from the goals in reality: It is the far more limited stock of 6Li that is used next to2H.

Proliferation issues: tritium as most easy fusion isotope is also relevant for building nuclear bombs.

And lastly there is the plasma quality factor Q. It is the amount of plasma heating from the fusion 4He

particles produced divided by the amount of energy put into the plasma heating. For jet the record was ataround 0.65 for 1 second.

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