In analogy with combustion,
"Heat generated by molecule burning
to more stable molecules.
and with Einstein's insight E = mc^{2} 
Energy generated by
less stable nucleus "burning" into more stable ones.
What then is "burning" and
In chemistry, O_{2} sets zero for stability.
In nuclear physics,
the atomic mass unit u is set by ^{16}_{8}O = 16 u,
Definitions  what is
Stable nucleons more neutrons than protons.  
Last stable isotope is Bi209 (see ^{208}Pb).  
U half lives.^{234}U: ~½ 10^{6} y; ^{235}U: ~10^{9} y; ^{238}U: 4 10^{9} y.  
Decay routes:  
Above stability line:  Z > → Z1 + β^{ +} + &gamma 
Below stability line:  Z > → Z+1 + β^{ } + &gamma 
Large Z elements  Z > → Z2 + α + γ 
Difference between nucleus mass & constituent nucleons  
^{56}_{26}Fe: most bound  
⇒ merge: fusion  
⇐ split: fission  
Leftmost dip: ^{4}He = α + 2 e^{} 
Units: Standard: ^{16}_{8}O ≡ 16 u.
u (amu or atomic mass unit) = 1.6606 10^{27} kg
Convert: u = 930
MeV = 0.149 nJ; neutron = 1.0087 u, proton = 1.0073 u. Ref:
Nuclei Mass Table
Element  Energy  unit  
U235  235.0439  u  

Ba141  140.9144  u  
Kr92  91.9262  u  
2 n (1.0078)  2.0156  u  
excess  0.1877  u  
174.4  MeV  
28  pJ  
72  TJ/kg 
On a mass basis (235 1.66 1027 = 0.39 1024 kg); ^{235}U fission produces 71.7 TJ/kg.
Methane: 50 MJ/kg. (TNT: 4 MJ/kg).
Fission is million times more efficient!
^{235}_{92}U plus slow neutron has other fission
reactions; see text.
Average number of neutron released is 2.4. That there is more than one neutron permits a chair reaction .

1. Stable energy requires neutrons cooled & most absorbed. 2. Reaction products in excited states that decay by emitting α (He4 nucleus), β (both + &  electrons) and γ (MeV xrays). 
Most neutrons are prompt; 0.7% delayed ms→min. This will be important for controlling nuclear fission reactors for safety.
⇑ Relative penetration of α, β & γ.
⇐ Distribution ^{235}U fission fragments. Most radioactive; some long lifetimes. 