Nuclear Physics

Year 13 Option 2006

Part 3 – Nuclear Fission

Nuclear Fission

In the 1930s physicists tried to make transuranic elements by bombarding uranium with neutrons

Neutron rich nuclei decay by beta emmision Possible to produce Np and Pu Write the nuclear equations for these

processes

Induced and Spontaneous Fission

Other light nuclei were also produced eg Ba and Kr – fission had occurred

This is an example of neutron induced fission In terms of the liquid drop model, why is it

necessary to induce the reaction? http://

www.visionlearning.com/library/flash_viewer.php?oid=2391&mid=59

Other nuclei can undergo spontaneous fission

Induced Fission

Neutron sources

The chain reaction is started by inserting some beryllium mixed with polonium, radium or other alpha-emitter. Alpha particles from the decay cause a release of neutrons from the beryllium as it turns to carbon-12.

Can you write a nuclear equation for this process?

Fission Products

Energy from nuclear reactions

Energy released from fission reactions Typical reaction products are:

U-235 + n ===> Ba-144 + Kr-90 + 2n + energy U-235 + n ===> Ba-141 + Kr-92 + 3n + 170 MeV U-235 + n ===> Zr-94 + La-139 + 3n + 197 MeV The total energy released in fission varies with the precise break up,

but averages about 200 MeV for U-235 or 3.2 x 10-11 joule. That from Pu-239 is about 210 MeV per fission. (This contrasts with 4 eV or 6.5 x 10-19 J per molecule of carbon dioxide released in the combustion of carbon in fossil fuels.)

These are total available energy release figures, consisting of kinetic energy values (Ek) of the fission fragments plus neutron, gamma and delayed energy releases which add about 30 MeV.

 Consider the masses (in amu )of these particles

U-235  neutron  La139  Mo 95  2 neutrons  7 electrons

 235.0439  1.0087  138.8061  94.9057  2.0174  .0038

 mass before  mass after fission

 236.0526  235.7330

   Lost mass  .3196 amu  

   Matter is destroyed, converted to Energy by E = mc^2

   Binding Energy 4.8 x 10^-11 J  

     

Binding Energy

Probability of fission occurring

The probability that fission or any another neutron-induced reaction will occur is described by the cross-section for that reaction. The cross-section may be imagined as an area surrounding the target nucleus and within which the incoming neutron must pass if the reaction is to take place. The fission and other cross sections increase greatly as the neutron velocity reduces. Hence in nuclei with an odd-number of neutrons, such as U-235, the fission cross-section becomes very large at thermal energies.

Neutron cross sections

Controlling Fission When one nucleus falls apart it sends out a few

neutrons. These can trigger fission in other nuclei, if they hit them. In a small piece of U235, the neutrons are unlikely to hit another nucleus before they leave. In a bigger piece, they have much more chance of hitting another nucleus. Once the average number of new fissions made by the neutrons from each previous fission gets bigger than one, the chain reaction grows until it blows the material apart.

The critical mass for uranium is 15kg , radius 6cm About the size of a grapefruit

Chain Reaction

U-235 and U-238 One of the differences between U235 and its

common relative U238 is that U235 fissions very easily. Fission is the process of "splitting" an atom, releasing large amounts of energy, mostly in the form of heat.

Unfortunately, U235 is relatively rare (approx. 0.71% of natural Uranium ore) so the uranium ore is processed to provide a mixture that has more of the U235 isotope in it (around 4%). This is called "enriched uranium." The byproduct of this processing is U238 with almost no U235 in it at all, and that is "depleted uranium."

Plutonium production

U238 is not very fissile. When bombarded by neutrons released by U235 fission, it absorbs neutrons and decays to become Pu239-- Plutonium. The Pu239 isotope of plutonium is fissile, and works even better than U238. It occurs very rarely in nature, and is mostly produced in nuclear reactors as a byproduct (or in so-called breeder reactors designed specifically to produce plutonium) and is used almost exclusively in nuclear reactors and nuclear weapons.

The Fission Chain Reaction

If a chain reaction is to be maintained, the minimum condition is that for each nucleus capturing a neutron and undergoing fission, on the average there must be another neutron which causes fission in another nucleus.

Neutron Balance in a Chain Reaction

100 slow neutrons cause fission of U-235 On average 256 further neutrons produced 100 for new fissions 90 captured by U-238 to form Pu-239 + 2beta 30 absorbed by moderator 20 captured by U-235 to become U-236 9 escape from core 5 absorbed by structure 2 absorbed by control rods

Delayed neutrons

Situation can become supercritical in a matter of milliseconds

Delayed neutrons are produced from fission products with various half lives

These delayed neutrons allow reaction to be controlled

The flux of delayed neutrons holds the balance between runaway reaction and one that grinds to a halt

Nuclear Power Plant

Fast Breeder Reactors The Pu-239 produced as a by-product in thermal

reactors can be used It is fissionable with fast neutrons – Pu-240 So no need for a moderator to slow them On average 2.91 additional neutrons are

produced per fission (2.56 in U-235) Only 1 needed to sustain reaction Others can be captured by U-238 surrounding

core to breed more Pu-239 Lots of U-238 left in world Liquid sodium used as coolant Together with toxicity of Pu gives major design

problems