Nuclear  Fission  

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Einstein:

(1905) Albert Einstein: He comes up with a little formula you may have heard of:

This equation changes everything.

E = mc2

1 g = 9 x 1013 J (equivalent to burning 1000 tons of coal!!!)

Binding energy of helium nucleus can be calculated:

4 x 1.67x10-27 kg 6.644 x 10-27 kg

Mass  of  2  protons  and  2  neutrons  :      Mass  of  a  helium  nucleus:  

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Einstein:

(1905) Albert Einstein: He comes up with a little formula you may have heard of:

E = mc2

Binding energy of helium nucleus can be calculated:

4 x 1.67x10-27 kg - 6.644 x 10-27 kg = 3.6 x 10-29 kg

Binding  Energy  

Mass  difference:  

Ebind  =  mc2    =  (3.6x10-­‐29  kg)  x  (3x108  m/s)2                                    =  3.2x10-­‐12  J  =  20.2  MeV  

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Fusion  vs.  Fission  Fission  is    •  the  breaking  apart  of  a  nucleus    •  what  occurs  during  radioacPve  decay  •  naturally  occurring  and  happens  in  power  plants    

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Fusion  vs.  Fission  Fusion  is    •  the  combining  of  two  nuclei  to  form  a  heavier  nucleus    •  what  occurs  inside  the  core  of  the  Sun  •  the  magic  bullet  for  solving  human  energy  problems…  

or  maybe  not  

Fission vs. Fusion •  If Fusion is the process of obtaining energy by adding things together.

•  Then Fission is the process of obtaining energy by Breaking things apart.

•  A fissionable element can spontaneously decay into one or more different (lighter) elements, releasing energy as it decays.

•  The 238U reaction is SLOW, taking 4.5 billion years!

238U ⇒ 234Th + 4He + (4.2 MeV)

(92P+146N) (90P+144N)

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Fission Power Generation •  Stimulated Fission occurs when the interaction with another particle triggers element breakup.

•  Fission reactors operate on this principle using a stable isotope of Uranium, 235U. About 0.7% of natural U is 235U.

235U + n ⇒ 140Cs + 93Rb + 3n (+ 200MeV)

(92P+143N) (55P+85N) (37P+56N)

•  This reaction is easier to trigger than P-P fusion because both the neutron and 235U are neutral.

•  It produces a LOT of energy per reaction, but not as much per mass as 3He fusion.

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The 235U Chain Reaction Did you notice that a byproduct of 235U fission by

neutron collision is MORE neutrons?

The 235U chain reaction is self sustaining when natural uranium is enriched to 5% of 235U.

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n  

Need  something  to  slow  down  reacPon  and    prevent  it  going    “criPcal”  

Fission  Power  Plant  

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The 235U Reactor:

•  Nuclear generators work by starting a chain reaction in Uranium that is enriched in 235U.

•  235U reactors produce toxic materials including both ‘depleted’ rods and the containment vessel.

•  The rate of the chain reaction rate is managed by;

1.  By inserting control rods to block neutrons.

2.  By cooling the reactor core to slow neutrons.

•  Failure to control the reaction leads to a runaway or meltdown. (Chernobyl + 3 mile Island)

The Down Side:

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Some Basic Reactor Designs:

•  PWR/BWR:  Regular  water  (coolant),  carbon  control  rods      (moderator)  and  enriched  Uranium  (fuel)  

•  PHWR:  Heavy  water  (coolant/moderator)  and  natural      Uranium  (fuel)  

•  RBMK:  Regular  water  (coolant),  graphite  moderator,      and  natural  Uranium  (fuel)  –  VERY  unstable  

•  LMFBR:  Liquid  metal  (coolant/moderator),  various      natural  fuel  sources  (Uranium,  Thorium)  

•  IFR:  Like  an  LMFBR  but  recycles  the  fuel    •  And  many,  many  others….  

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Natural Fission Reactor : •  Oklo  –  Gabon,  Africa  

•  Like  HWR  without  the  heavy  water  

•  1.7  billion  years  ago,  the  abundance  of  U235  much    higher  (3.1%,  now  0.7%)  

1)  Nuclear  reactor  zone  2)  Sandstone  3)  Uranium  ore  layer  4)  Granit    5)  Water  

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RTGs :

Radioisotope  thermoelectric  generator  

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RTGs :

RTGs  used  on  Pioneer  10  &  11,  Voyager  1  &  2,  Galileo,  Ulysses,  Cassini,  New  Horizons,  MSL,  Viking  landers,  Apollo  12-­‐17    Typically  provide  a  few  100  Waks  of  power    for  50-­‐100  years.      Because  RTGs  contain  plutonium  and  other    highly  radioacPve  material,  they  can  be  more    dangerous  to  launch  that  a  fission  reactor  

Nuclear  Power  

Courtesy  of  Fox  TV  

7%  of  World’s  Power  Plants    

16%  of  World’s  Electricity  No  new  Nuclear  Power  Plants  in  the  US  since  the  1970s.    

Should  we  build  more  plants?  

What  about  coal?  

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Power  in  Space  

•  No  such  things  as  hydro-­‐power  or  wind  power  in  space  

•  Solar  power  is  possible  but  increasingly  limited  pasts  Mars’  orbit  

•  All  past  or  current  missions  going  to  Jupiter  and  beyond  used  a  form  of  power  based  on  radioacPve  decay  

•  Juno  mission  will  use  solar  panels:  >10  kW  at  Earth  but  only  480  W  (max)  at  Jupiter  

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Solar  Flux  in  the  Solar  System  

•  1365  W/m2  at  the  top  of  the  Earth’s  atmosphere  (about  1000  W/m2  at  noon  

       at  equator  at  the  surface)  •  Decreases  with  (distance)2  :  Flux  *  R2  is  const.  •  Flux  at  Mars    =  (1365  W/m2)*(1.0/1.5)2  

                   =  607  W/m2    •  Flux  at  Jupiter    =    55  W/m2  

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Solar  Flux  at  Mars    •  Flux  at  MarPan  surface    ~  590  W/m2  (max)  •  Solar  Panels  the  size  of  a  football  field  (ISS)  

 =  3200  kW  intercepted  •  Solar  Panels  10  m  x  10  m  

 =  59  kW  •  But  solar  panels  not  100%  efficient,  best  ones  

 today  30%  è  960  kW  /  18  kW    ISS:  14%è  448  kW  /  8  kW  

•  Average  person  uses  250  kW  a  month,  9  kW  day      

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