science a 52 lecture 22 may 1, 2006 nuclear power what is...

Spring 2006 © Harvard Science, A 52 FHA+MBMLecture 22, 1

Nuclear Power

What is it?What are its problems and

prospects?

Science A 52Lecture 22 May 1, 2006


Nuclear FissionOn of the most interesting accounts a fission and the discovery of the release of two or more neutrons duringfission is on the WEB along with the voices of the major participants. You can almost feel the excitement in their voices as they described their reactions as experimental results came in.Let us listed to a few.

http://www.aip.org/history/mod/fission/fission1/02.html


History of Nuclear Fissionhttp://www.aip.org/history/mod/fission/fission1/02.html

Here is a summary from the above site.It was only in 1938 tha t many scientists began to focustheir attention on uranium, the heaviest of known elements.Leading the pack were two German chemists, Otto Hahn andFritz Strassmann. For over thirty years, Hahn had beenworking with another talented scientist, Lise Meitner.However, Meitner was of Jewish ancestry, and had to fleeNazi Germany. Otto Hahn recalls...

Miss Meitner - Professor Meitner - had left our laboratoryon July 1938 on account of these Hitler regime things andshe had to go to Sweden. And Strassmann and myself,we had to work alone again and in the autumn of '38 wefound strange results.


By 1938, Hahn and Strassmann were among a number of scientists who were trying to find out what products are formed if you shoot neutrons into heavy elements. They hoped to find elements even heavier than uranium. Such altogether new elements would surely have scientific interest, and perhaps even practical uses. But the substances Hahn and Strassmann produced looked like radium or barium, two known and almost chemically identical elements...

HAHN: We made precipitations, Strassmann and myself,where we could be absolutely sure that there could be nothing else but either radium or barium.

Fission History - continued


Now Meitner and Frisch -her nephew -understood what had happened in Hahn andStrassmann's experiment. The neutrons which they had shot into uranium had indeedbeen captured by the uranium nucleus. But then the nucleus changed shape, vibrated,and came apart entirely. This was not the usual slight transformation of a nucleus.

The picture did however fit neatly with a recent theory of Niels Bohr's. He believedthat a nucleus behaves like a liquid drop, and a liquid drop when hit hard enough,might stretch until it broke in two. Now, if that happened to a nucleus, a lot ofenergy would be released—atom for atom, far more energy than any processseen till then.

This is what Meitner and Frisch thought happened to the nucleus of uranium after it had captured a neutron


http://www.atomicarchive.com/Fission/Images/fission.gif

A cartoon of U235 neutron capture

The target 92 U235 nucleus is often unstable with theadditional neutron,and in ~ 10-14 seconds it splits into twofragments of nearly equal mass that are also unstable andin turn eject neutrons and γ rays. Not all captures of aneutron of 92 U235 results in a fission. Some capturesproduce 92 U236+ γ ray energy.


Let us listen to comments onone who was there when the

first reactor went critical.http://www.aip.org/history/mod/fission/fission1/09.html


Energy release during fissionTo make the calculation of the energy released in fissionwe need to make a mass balance of all of the particlesbefore and after fission - we will fine that the mass after fission is less that that before. The “missing” mass has been converted to energy.

Some of the energy will be in the kinetic energy of the fission fragments, of the neutrons, and the gamma rays.

In particle physics energy is generally expressed in electronvolts.

1 eV = 1.6x10-19J 1eV is a very small unit of energy,but a proton has a very small mass. Let us see how to use this unit of energy.


Charge on the electron

http://www.recipeland.com/facts/Oil-drop_experiment - The_apparatus


Energy expressed in Electron Voltshttp://hyperphysics.phy-astr.gsu.edu/hbase/electric/ev.html#c2


http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ev.html#c1


FissionCalculation of mass conversion to energy

We can use Einstein’s mass-energy relation:

E = mc2

For nuclear calculations it is most useful to express theenergy in MeV ⇒million electron volts. An electron volt is the amount of energy given to anelectron accelerating it through a voltage difference ofone volt.

We need the famous equation of Einstein cast in massunits used in particle physics.



E(MeV) = m(amu) X 931One atomic mass units (amu) is 1/16ththe mass of O 16 .Using this scale the mass of theproton and the neutron are nearly 1.Mass of hydrogen atom = 1.00813 amuMass of the proton = 1.00758 amuMass of the neutron = 1.00897 amu



• We are now prepared to make a calculation of theenergy released in fission of 92U235. What we needto do is to determine the products of fission whenthis nucleus captures a neutron and fissions.

• The fission reaction is: U235 +neutron →2 fission fragments + ν neutrons +β- and γ-rays + energy

To proceed we need to know the fission fragmentsand then their weight in amu. The fission yield offission fragments from U 235 are known.


Glasstone, Samuel andEdlund,Milton;Nuclear Reactor Theory, vanNostrand,1952, pg.67



The isotopic weight of U235 = 235.124 amu, and the neutroninvolved in fission has an amu =1.00897. Therefore thereacting particles have a total weight of 236.133 amu and amass number of 235 + 1 = 236

The most probable type of fission, nearly 6.4% of the total,gives products with mass numbers of 95 and 139, addinggives a mass number of 234 - two mass numbers less that theinput, hence this fission must yield 2 neutrons. The fissionproducts are stable nuclides with masses of 94.945 and138.955, respectively. Doing the mass balances Before = After Fission236.133 = mass converted to energy +(94.945+138.955) +

2x1.00897∴mass converted to energy = 236.133 - 235.918 = 0.215 amu


Glasstone, Samuel, Nuclear Reactor Engineering,van Nostrand, 1955; pg.22


∴Energy released in this fission would be 931x0.215 = 198 MeV

The average values for all fissions of U235


Fission Neutrons per Capture•Fission of 92 U235 leads to the possibility of a chain reaction because an average of 2.5 neutrons are released in each fission of 92 U235. •Not every time a U235 atom captures a neutron does fission occur, sometimes just this reaction occurs: U235 + neutron →U236 +γ-rays•Making an allowance for non-fission capture, there are only 2.1 neutrons released for each time a neutron is captured in U235


Heat released by 1 pound ofFissionable Material

is 3.6 x 1010 BTU

orfission of 1 gram of materialgives roughly 1 Mw day of

power


Natural Uranium

• Isotopic Composition of Natural Uranium

99.282238

0.712235

.006234

%Massnumber


Making a Nuclear Reactor• It is impossible to make a nuclear power reactor

using natural uranium since the concentration ofU235 is so low. Only 2.1 neutron are available forthe chain reaction.

• Fission neutrons are released with high KE, andfission occurs most easily with thermal neutrons -low neutron absorbing material is needed to slowthe neutrons down without capture. Steel tubesand the like easily absorb thermal neutrons andcannot be used in a reactor using natural U.

• The first reactors built used natural U and werevery large - very pure graphite moderators.

• Now slightly enriched U is used in power reactors.


Making a Nuclear Reactor• The first reactors built at Oak Ridge and Hanford

were not for power but to create Plutonium239 a twostep decay product from neutron capture in U238.

• Pu239 can be used for bomb making and isseparated from U and fission products chemically.

• About 5% of the power of the reactor is in thedecay of the fission products (FP).

• This means that when the reactor is shut down,heat continues to be released by the FP andcooling must be provided.

• Disposal of the FP and the Pu produced remains aserious problem.

science a 52 lecture 22 may 1, 2006 nuclear power what is...

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