Several Hundred different nuclides can be produced
They are all neutron rich and decay by - emission until a stable nuclide is reached: DECAY CHAIN .
Important Decay Chains
Hahn and Strassman discovered fission Characterization of Promethium and production of Samarium. Very effective thermal neutron absorbers Discovery and Production of Technetium for medical applications Production of Xe-135, largest low energy neutron absorption X-section
So, how can we build a nuclear reactor with Uranium without Heavy Water as Moderator ?.
Increasing 235 U content from 0.72 % to > ~ 2.5% : ENRICHMENT
More fissile fuel will increase more chance of absorption by fuel.
Construct an HETEROGENEOUS core by separating fuel and moderator.
More Fast neutrons escape the fuel
They are thermalized away from 238 U resonances more probability of escaping the resonances.
Heterogeneous reactors also have a higher (more fast fissions in 238 U).
Core Design Estimates Source: J.K. Shultis and R.E. Faw, “Fundamentals of Nuclear Engineering”, Marcel Dekker, 2002 HETEROGENEOUS CORE Fuel Moderator Neutron moderation fast thermal More Heterogeneous: f decreases More heterogeneous: p increases There´s an optimum for k∞ max
We surround the core with a material with a HIGH scattering-to-absorption cross section: REFLECTOR.
Reflectors reduce Leakage Reflectors reduce Peak-to-average power Reflectors reduce Fast Neutron Flux outside the core neutrons/cm 2 s center Source: J.K. Shultis and R.E. Faw, “Fundamentals of Nuclear Engineering”, Marcel Dekker, 2002