Nuclear Fission 1 Einstein:
(1905) Albert Einstein: He comes up with a little formula you may
have heard of:
E = mc2
This equation changes everything.
1 g = 9 x 1013 J (equivalent to burning 1000 tons of coal!!!)
Binding energy of helium nucleus can be calculated:
Mass of 2 protons and 2 neutrons : Mass of a helium nucleus: 4 x 1.67x10-27 kg
6.644 x 10-27 kg
2 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:
Mass difference: 4 x 1.67x10-27 kg - 6.644 x 10-27 kg = 3.6 x 10-29 kg
Ebind = mc2 = (3.6x10-­‐29 kg) x (3x108 m/s)2 = 3.2x10-­‐12 J = 20.2 MeV Binding Energy 3 Fusion vs. Fission Fission is •  the breaking apart of a nucleus •  what occurs during radioacPve decay •  naturally occurring and happens in power plants 4 Fusion is Fusion vs. Fission •  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 5 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.
238U
⇒
234Th
+ 4He + (4.2 MeV)
(92P+146N) (90P+144N)
•  The 238U reaction is SLOW, taking 4.5 billion years!
6 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
(92P+143N)
+n ⇒
140Cs
+
93Rb
+ 3n (+ 200MeV)
(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.
7 The 235U Chain Reaction
Did you notice that a byproduct of 235U fission by
neutron collision is MORE neutrons?
n Need something to slow down reacPon and prevent it going “criPcal” The 235U chain reaction is self sustaining when
natural uranium is enriched to 5% of 235U.
8 Fission Power Plant 9 The 235U Reactor:
•  Nuclear generators work by starting a chain reaction in Uranium
that is enriched in 235U.
•  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.
The Down Side:
•  Failure to control the reaction leads to a runaway or meltdown.
(Chernobyl + 3 mile Island)
•  235U reactors produce toxic materials including both ‘depleted’
rods and the containment vessel.
10 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…. 11 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%) 5 1) 
2) 
3) 
4) 
5) 
Nuclear reactor zone Sandstone Uranium ore layer Granit Water 12 RTGs :
Radioisotope thermoelectric generator 13 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 14 Nuclear Power 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? Courtesy of Fox TV 15 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 16 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 17 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 18