Nuclear fission is the splitting of a nucleus. The term is borrowed from the biological sciences.

Recall the table showing binding energy per nucleon vs. atomic mass number:

Topic 7.3 ExtendedB – Nuclear Fission

Fission breaks large nuclei into smaller ones, each of which is higher on the curve.This results in the release of nuclear energy.

If we strike a uranium-235 nucleus with a neutron we can have the following fission reaction occur:

Topic 7.3 ExtendedB – Nuclear Fission

235U + 1n (236U*) 140Xe + 94Sr + 2(1n) 92 0 92 54 38 0

7.6 MeV

8.8 MeV8.3 MeV

We can estimate the energy release by reading the table:

7.6(235) = 1786 MeV

8.3(140) = 11628.8(94) = 827.2

1989.2 MeV

energy release 1989.2 - 1786 = 203.2 MeV

FYI: The uranium-236 nucleus is in an excited state before it breaks apart.

Question: This is all well and good - but we had to supply an energetic neutron to initiate the reaction. So how do keep the reaction going without constantly supplying energetic neutrons? Each fission produces 2 neutrons.

FYI: These neutrons can be used to sustain further reactions.

If each neutron in the fission reaction triggers another reaction, we get a cascade of reactions:

Topic 7.3 ExtendedB – Nuclear Fission

235U + 1n (236U*) 140Xe + 94Sr + 2(1n) 92 0 92 54 38 0

Primary

Secondary

Tertiary

1

2

4

8

Exponential Growth

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

FYI: Clearly, since such runaway rates do not occur in our naturally-occurring uranium on Earth, there must be some MODERATING process which prevents such runaway growth rates to occur.

CHAIN REACTION

Topic 7.3 ExtendedB – Nuclear Fission

Since for any lump of fissile material, some of the neutrons will escape into the surrounding environment, not all of the neutrons will be available for participating in the chain reaction.If the lump is large enough, then there will be enough neutrons to sustain the fission reactions.We call the minimum mass of a fissile material necessary to sustain fission the critical mass.One method of triggering a fission chain reaction is to combine subcritical masses into a critical mass:

Obviously for generating useful energy for peaceful purposes we need to control the reaction rate.

Topic 7.3 ExtendedB – 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, recall the reaction cross-sections we discussed earlier:

FYI: The key to controlling the reaction rate of a fission process is to control the kinetic energy of the NEUTRONS released in the fission reaction.

NUCLEAR REACTORS

Topic 7.3 ExtendedB – Nuclear Fission

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.

NUCLEAR REACTORS

Topic 7.3 ExtendedB – Nuclear Fission

A fuel rod is made up of uranium oxide pellets: Many fuel rods are placed parallel to one another until a sustained fusion begins.The whole shebang is under water, which is circulated to remove the heat released by fission.

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.

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 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.

NUCLEAR REACTORS

Topic 7.3 ExtendedB – Nuclear Fission

Light 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?

NUCLEAR REACTORS

Topic 7.3 ExtendedB – Nuclear Fission

So 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.

NUCLEAR SAFETY ISSUES

Topic 7.3 ExtendedB – Nuclear Fission

Our first issue, and probably the foremost issue in the publics' mind is the meltdown.A meltdown refers to a core meltdown, a situation where the coolant fails to keep the core temperature low enough to prevent catastrophic destruction of the fuel rods, and possible subsequent pressure buildup and conventional explosion with release of radioactive materials into the environment.A loss-of-coolant-accident (LOCA) can lead to a meltdown in some reactors. American reactors are designed so that in the event of LOCA, the reactor stops sustaining itself. Light water is necessary for neutrons of the correct energy cross-section to be produced for sustained fission. Chernobyl was not so designed, and loss of coolant led to catastrophic meltdown, explosion, and graphite moderator fires that sent radioactive plumes into the atmosphere.

NUCLEAR SAFETY ISSUES

Topic 7.3 ExtendedB – Nuclear Fission

Our second issue is one of spent fuel storage.Fuel rods last about five years before they need replacement. But after that time, they need to be removed, and stored.Since the remaining nuclear materials in the rods have half lives of hundreds of years, storage is a real problem.Hand in hand with storage is transportation. Spent radioactive materials must be transported through high population areas to their final locations.Spent materials must also be safeguarded against theft: dirty bombs, and even primitive nuclear devices, may someday be improvised by terrorists.

FYI: Someday the oil will run out. Fusion has still not been suitably developed, and coal plants produce tons of pollutants yearly. Solar and wind simply cannot meet our needs. Much as people might be against nuclear power, perhaps it will become our primary source of energy in the future.