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
The average number is for thermal fission depends on nuclide and neutron energy.
Delayed Neutrons:
Small fraction (1% Thermal) of neutrons are emitted as delayed neutrons.
The come from the Decay of Fission Products.
The time is from some seconds to minutes.
The average number depends strongly on:
Fissioning nucleus.
Energy of the inducing neutron.
DELAYED Neutrons are ESSENTIAL to control the Nuclear Reaction Average Number of PROMPT Neutrons DELAYED Neutron Fraction Average TOTAL Number of Neutrons
9.
Delayed Neutron Emission Source: J.K. Shultis and R.E. Faw, “Fundamentals of Nuclear Engineering”, Marcel Dekker, 2002
The probability of resonace absorption is very high
p , the probability of escaping the resonance is very low.
k ∞ << 1.0.
Reactor design solutions:
Increase p : Use an EFFECTIVE MODERATOR.
Increase f : Use more fissile material.
Increase : use a fuel with more neutrons per fission and low
25.
Core Design Estimates Source: J.K. Shultis and R.E. Faw, “Fundamentals of Nuclear Engineering”, Marcel Dekker, 2002 Increasing But, there is no U-233 in Nature, one must “make” it.
There is too little moderator , is small, p is very small and
There is too much moderator, is large, f is small and
There is an optimal that gives a maximum for
Source: J.K. Shultis and R.E. Faw, “Fundamentals of Nuclear Engineering”, Marcel Dekker, 2002 Only Heavy Water as a Moderator can be used for an homogeneous natural uranium reactor
27.
Quantification of the Thermal Cycle Source: J.K. Shultis and R.E. Faw, “Fundamentals of Nuclear Engineering”, Marcel Dekker, 2002
So, how can we build a nuclear reactor with Uranium without Heavy Water as Moderator ?.
Increase f
Increasing 235 U content from 0.72 % to > ~ 2.5% : ENRICHMENT
More fissile fuel will increase more chance of absorption by fuel.
AND/OR
Increase p
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).
29.
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
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