energy, power, and climate change 8.4 nuclear power recall the two types of nuclear reactions which...

Energy, Power, and Climate Change8.4 Nuclear Power

Recall the two types of nuclear reactions which yield energy: fission and fusion.

Fission breaks large nuclei into smaller ones, each of which is higher on the curve.

Each process results in the release of nuclear energy.

Fusion joins small nuclei into larger ones, each of which is higher on the curve.


NUCLEAR FISSIONFission occurs naturally in elements whose atomic numbers are greater than iron's.

236U 142Ba + 92Kr + 2(1n) 92

56

36 0

As an example, consider uranium-236 fissioning into barium-142 and krypton-92.

The atomic numbers Z balance on each side:

The atomic mass numbers A DO NOT balance:

We are short TWO, easily remedied by adding two neutrons to the reaction:

Question: Why didn't we use two protons instead of two neutrons?


NUCLEAR FISSION

Observing the table above calculate the mass deficit that occurs in the reaction 236U 142Ba + 92Kr + 2(1n).

92

56

36 0

Z Symbol A Mass (u)

92 U 236 236.045563

56 Ba 142 141.916361

36 Kr 92 91.926270

0 n 1 1.008664

The products have a total mass of

141.916361u + 91.926270u + 2(1.008664u) = 235.859959u

The reactant has a total mass of = 236.045563u

The mass deficit is just the difference:236.045563u - 235.859739u = 0.185604u


NUCLEAR FISSIONFind the energy, in electron volts and joules, released during the fission reaction 236U 142Ba + 92Kr + 2(1n).

92

56

36 0 The mass deficit was just calculated: 0.185604u

Recall that 1 u = 931.5 MeV:

E = 0.185604u 931.5 MeV1u

= 172.9 MeV

Recall that 1 eV = 1.610-19 J:

E = 172.9 MeV 1.610-19 J1 eV

106

1 M

= 2.810-11 J

FYI: This energy released per reaction seems rather low. The next sample problem will illustrate how much energy a kilogram of U-236 will release if it is all fissioned.


NUCLEAR FISSIONFind the energy density in J / kg for uranium being used in the reaction 236U 142Ba + 92Kr + 2(1n).

92

56

36 0 The mass of 1 mole of U-236 is 236 grams = 0.236 kg.

1 kg U 1 mol U0.236 kg U

6.021023 reactions1 mol U

2.810-11 Jreaction

Energy Density U = 7.11013 J/kg

Fuel Fuel Type Energy Density (MJ/kg)

Protons Nuclear 300,000,000

Uranium-235 Nuclear 90,000,000

Petrol Fossil 46.9

Diesel Fossil 45.8

Biodiesel Fossil 42.2

Crude Oil Fossil 41.9

Coal Fossil 32.5

= 71,000,000 MJ/kg

FYI: Compare fission energy density with fossil fuel energy density.


NUCLEAR FISSIONSo how does one create and control the fission process?Again, let's use the following reaction for illustration: 236U 142Ba + 92Kr + 2(1n).

92

56

36 0 The fission process can be triggered by striking the U-236 nucleus with a neutron of optimum energy:

As long as the two released neutrons have the right amount of kinetic energy, they will each trigger another fission, beginning a chain reaction.

FYI: If the neutrons have too much K, they will pass through the nuclei without triggering a fission event. If they have too little K, they will bounce off of the nucleus without splitting it.


NUCLEAR FISSION

Primary

Secondary

Tertiary

1

2

4

8

Exponential Growth

CHAIN REACTION

FYI: If each neutron in the reaction triggers another reaction, the result is an exponential reaction rate growth, and a nuclear explosion.


NUCLEAR FISSIONTo control the fission rate, a moderator is used.

The moderator absorbs or adds energy to the neutrons to adjust their kinetic energy to the "optimum value.""Optimum value" would depend on the use of the fission reaction:

For a nuclear detonation you would optimize the K value so that BOTH neutrons triggered subsequent fissions, creating EXPONENTIAL GROWTH.For nuclear power production you would optimize the K value so that A SINGLE neutron triggered subsequent fissions, creating SELF-SUSTAINMENT.


NUCLEAR FISSION

The reaction cross section is a measure of the probability that a particular reaction will occur at a particular incident kinetic energy.

Neutron K (eV)

Rea

ctio

n C

ross

S

ectio

n

10-1 100 101 102

resonances

The cross-section illustrated above shows that a particular reaction is most likely to occur when the incident neutron has a kinetic energy of between 1 and 10 eV.

To understand how fission reaction rates are controlled by the moderator, recall the reaction cross-sections we discussed earlier:

FYI: Nuclear power plants thus use a moderator to allow approximately one-half of the reaction-released neutrons to continue the fission process, while the other half are removed from the process by the moderator.

FYI: Besides the moderator as a control, critical mass is also a design factor. If the amount of fissile material is too small, most of the reaction-released neutrons escape the uranium and cannot cause further fissions. The minimum amount of fissile material needed to sustain a chain reaction is called the CRITICAL MASS.


THE NUCLEAR REACTOR

238U will release neutrons when split (similar to the reaction shown for 235U). However, rather than sustaining further fissions, most of the product neutrons are absorbed by the uranium and the chain reaction does not sustain itself.

Natural uranium is made up of uranium-238 (99.3%) and uranium-235 (0.7%).

238U, consequently, has a large critical mass.235U, on the other hand, will sustain fission reactions very readily, and thus has a much smaller critical mass.Enriched uranium is uranium which has had its 235U content increased through various (expensive and difficult) processes including diffusion and centrifuge.Reactor grade uranium has been enriched to about 3-5% uranium-235.Weapons grade uranium has been enriched to about 99% uranium-235.


THE NUCLEAR REACTORA fuel rod is made up of uranium oxide pellets: Many fuel rods are placed parallel to one another until a sustained fission begins.The whole shebang is under water, which is circulated to remove the heat released by fission.The reaction rate can be moderated by graphite control rods or other neutron-absorbing materials.Moving the rods in and out controls the reaction rate.

FYI: The water acts like the radiator of your car. It removes excess heat from the system. If it didn't, the reactor would soon burn its rods, damaging them, or perhaps melting itself down in a Chernobyl-like catastrophe.

FYI: The goal of controlling a reaction rate is to prevent the run-away exponential growth of reactions. Thus for every fission reaction, we want to produce ONE fission reaction. We want to somehow remove all but one of the released neutrons from the cross-section energy that produces fission.


THE NUCLEAR REACTORLight water reactors use ordinary H2O for the coolant.Heavy water reactors use D2O for the coolant.The difference is this: Light water absorbs the energy of the neutrons better than heavy water, through a process called neutron capture.

1n + 1H 2H +

Since the water absorbs neutrons that could otherwise be used for further reactions, light water reactors need to have a higher enriched uranium than heavy water reactors.

0 1 1

The tradeoff is in the difficulty of producing enriched uranium, as opposed to the difficulty in producing heavy water. Recall that deuterium only comprises 0.02% of naturally-occurring hydrogen (and thus 0.02% of water).

FYI: Canada uses heavy water reactors, whereas the US uses light water reactors. Why?


THE NUCLEAR REACTORSo how do we obtain useable energy from a nuclear reactor?The coolant in the core is circulated through a heat exchanger (under great pressure, so that it cannot begin to boil and it remains liquid).

Reactor Core Heat Exchanger

TurbineGenerator

Condenser

The steam generated in the heat exchanger turns the turbine.The heat exchanger is kept cooler by the condenser.

FYI: The condenser prevents the heat exchanger from overheating. And the heat exchanger prevents the core from overheating.

The generator is turned by the turbine, producing electricity.

Question: How many water reservoirs are there, and why are they kept separated.


THE NUCLEAR REACTOROf course we can create a Sankey diagram for a typical nuclear reactor: Because of the difficulty of enrichment, we include that energy in our diagram.

Energy in uranium ore

Energy used in processing

Wasted Heat

Energy remaining in spent fuel

Useable electricity


PROBLEMS WITH NUCLEAR ENERGY

Enrichment of uranium or separation of heavy water is very technically difficult and expensive.

Mining of uranium ore is dangerous and prone to causing environmental damage.

Possibility of Chernobyl-like meltdown if the reactor is not designed to prevent that type of failure.

FYI: Most reactors are designed so that in the event of a "loss of coolant accident" (LOCA) the neutron K will exceed the optimum value and fission will stop automatically. Chernobyl was one of those rare reactors that was not so-designed.

When uranium-238 absorbs a neutron it becomes U-239, then undergoes a -decay into neptunium-239, which then decays again into plutonium-239, which is even more fissionable than uranium. This can be used in another reactor (good) or for nuclear weapons (can be bad).The biggest problem is WASTE.

Low-level waste includes radioactive coolant, and other reactor parts that were in contact with the fissioning material.High-level waste is the material in the fuel rods.

FYI: HLW is dealt with in the following two ways at this time:

(1) It is stored under water on site for several years (to cool off), then it is sealed in steel cylinders.

(2) It is reprocessed to separate the plutonium and any remaining uranium-235. The waste from this process yields highly radioactive materials with shorter half-lives than uranium or plutonium.


BENEFITS OF NUCLEAR ENERGY

Potential of over 2000-years worth of fissionable fuels at our disposal.

The main benefit of nuclear fission is NO CO2 EMISSIONS.


FUSION

Fusion has the potential of producing more power than fission, but an economical self-sustaining fusion reactor has not yet been produced.

Fusion is the combining of light elements into heavy ones with the release of energy.

Hydrogen nuclei are trapped and accelerated by the magnetic field of a tokamak magnetic bottle.Fusion can at present be achieved only in bursts, and only at great energy "expense" - more than is gotten out of the reactor.

Fuel Fuel Type Energy Density (MJ/kg)

Protons Nuclear 300,000,000

Uranium-235 Nuclear 90,000,000

Petrol Fossil 46.9


FUSION - THE SUNFusion is sustained in the sun because of the great gravitational pressures which squish hydrogen nuclei together in the center of the sun.


FUSION - THE SUNThe fusion reaction on the sun looks like this:

Question: Where is the anti-electron (positron) in this reaction?

FYI: The temperature in the core is estimated to be 15 million K! Only fusion can produce this temperature. Without this outward radiative pressure to counter the inward gravitational force, the sun would collapse into a neutron star.


FUSION - THE HYDROGEN BOMBIf a fission bomb is properly designed, it can be used to trigger fusion. The combination fission-fusion device is called a hydrogen bomb.


FUSION - THE HYDROGEN BOMB

Question: Should this diagram be available on the internet to anyone in the world?

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