NEUTRON CYCLEAND
FOUR FACTOR FORMULAPRESENTED BY,
TEENA THANKACHENREG NO: 630-14126012
NUCLEAR FISSIONChief method of producing energy.Tremendous amount of energy is
released.When heavy nuclei are bombarded with
protons, deuterons, neutrons,α particles etc , then the nucleus is caused to breakdown into two roughily equal parts , known as fission fragments. This process is called NUCLEAR FISSION.
Frisch and Meitner in 1939 used the word FISSION .
NUCLEAR CHAIN REACTIONWhen a neutron produces fission
in uranium nucleus, besides the fission fragments a few fast neutrons are also emitted. If one or more of the emitted neutrons are used to fission of other nuclei, further neutrons are produced and the process is repeated.
The reaction thus becomes self-sustained and is known as CHAIN REACTION.
NUCLEAR CHAIN REACTION
NUCLEAR CHAIN REACTIONThe reaction is controlled in such
a way that only one of the neutrons emitted in a fission causes another fission, then the fission rate remains constant and the energy released steadily.
Such a reaction is called CONTROLLED CHAIN REACTION.
It is used in Nuclear Reactors.
NEUTRON LIFE CYCLE
NEUTRON LIFE CYCLEIn thermal reactors,neutrons that
cause fission are born at a much higher energy level than required
To make fission more probable,these neutrons must be “slowed” to thermal energy
PWRs use water as a moderatorWhen moderating “fast”
neutrons,gains and losses occur
This process is referred to as NEUTRON LIFE CYCLE .
Explains factors involved in controlling nuclear fission rate
Proper management of the neutron life cycle makes control of a nuclear reactor possible
Some of the fast neutrons born by fission in one generation will cause fission in the next generation
But…Fission neutrons “travel” through
a series of events as they slow to thermal energies, leak, or are absorbed in the reactor
Referred to as the neutron life cycle
Simplified neutron life cycle: All neutrons are born as fast neutronsSome fast neutrons are absorbed by
fuel and cause fast fissionSome fast neutrons leak out of reactor
coreSome fast neutrons undergo
resonance capture while slowing downAll remaining fast neutrons become
thermalized
Some thermal neutrons leak out of core
Some thermal neutrons absorbed by non-fuel material
Some thermal neutrons absorbed by fuel and not cause fission
Remaining thermal neutrons absorbed by fuel and cause thermal fission
K Neutron production from fission in one generationNeutron absorption in the preceding generation
pfK
EFFECTIVE MULTIPLICATION FACTOR Keff
Describes neutron life cycle in a real, finite reactor
A reactor of finite size will have neutrons leak out of it
Defined as ratio of neutrons produced by fission in one generation to number of neutrons lost through absorption and leakage in preceding generation
◦Like K∞, by its value, tells whether a new generation of neutrons is larger, smaller, or same size as preceding generation
◦ Also known as six-factor formula
fpLLK thfeff
INFINITE VS. EFFECTIVE MULTIPLICATION FACTOR
If leakage is small enough to be neglected, multiplication factor depends only on balance between production and absorption called Infinite multiplication factor
Also called four-factor formula , considers factors shown below:
pfK
With leakage included,considers six factors
fpLLK thfeff
FOUR-FACTOR FORMULAAlso known as Infinite
Multiplication Factor Used to consider a reactor of
infinitely large size where no neutron leakage can occur
Defined at ratio of neutrons produced by fission in one generation to number of neutrons lost through absorption in preceding generation
K
EFFECTIVE MULTIPLICATION FACTOR (KEFF) & CRITICALITYWhen value of keff is 1, a self-sustaining
chain reaction of fissions is occurring◦Neutron population is neither increasing nor
decreasing◦Called “critical” or critical reactor keff = 1
When neutron production is greater than the losses due to absorption and leakage◦Reactor is supercritical◦keff > 1◦Neutron flux is increasing each generation
EFFECTIVE MULTIPLICATION FACTOR (KEFF) & CRITICALITYWhen neutron production is less
than losses due to absorption and leakage◦Reactor is subcritical◦Keff < 1◦Neutron flux is decreasing each
generationWhen keff is not equal to exactly
1, neutron flux and therefore reactor power will be changing
INFINITE MULTIPLICATION FACTOR
Four factors independent of size and shape of reactor and do not consider any neutron leakage from the reactor.
Where : pfK
= fast fission factor
p = resonance escape probability
= reproduction factor
f = thermal utilization factor
FAST FISSION FACTOR (ε) = No of fast neutrons produced by all
fissions No of fast neutrons produced by thermal fissions•First event neutrons
incur after birth•Caused by neutrons that are in fast energy range•Results in a net increase in fast neutron population
Neutrons must pass close to a fuel nucleus while still fast
Value affected by fuel concentration and physical arrangement proximity to moderator
Essentially 1.00 for a homogenous reactor, fuel atoms surrounded by moderator atoms (rapid moderation)
Cross-section for fast fission in uranium-235 or uranium-238 is small
Still an appreciable number of fast neutrons cause fission in uranium-235, uranium-238, and plutomium-239
A large fraction of fast fissions occur with uranium-235 because of its wider fission energy spectrum
In a heterogeneous reactor (PWR/BWR), fuel atoms packed closely together in fuel pellets within fuel rods and assemblies ◦Neutrons emitted from fission of one fuel atom have a good chance of passing near another fuel atom before slowing down
◦Results in some fast fissionFor PWRs, 1.02 is a good value for ε,
with a range of 1.02 to 1.05
RESONANCE ESCAPE PROBABILITY (ρ)
ρ = No: of neutrons that reach thermal energy
No: of fast neutrons that starts to slow down
After fast fissions occur, neutrons continue to diffuse throughout reactor
Collide with nuclei of fuel, non-fuel material, and moderator◦ Lose energy in each collision and slow down
All nuclei within reactor core have some probability of absorbing neutrons ◦ Microscopic cross-section for absorption (σa) for
each materialσa is not a constant value,
dependent on energy level of incident neutron
Absorption cross-sections increase as neutron energy level decreases
THERMAL UTILIZATION FACTOR (f)
f=No:of thermal neutrons absorbed in the fuel
No: of thermal neutrons absorbed in all reactor materials
After thermal non-leakage, thermalized neutrons still dispersed throughout the core where they are subject to absorption by either fuel or non-fuel material
Thermal utilization factor describes how effectively thermal neutrons are being absorbed by fuel or underutilized by non-fuel materials
Thermal utilization factor is always less than one◦Not all thermal neutrons are absorbed in fuel◦These neutrons are lost to the fission process
A value range for thermal utilization factor is 0.70-0.80
REPRODUCTION FACTOR (η) η= No: of fast neutrons neutrons produced by
thermal fission
No: of thermal neutrons absorbed in the fuel
Most neutrons absorbed in fuel cause fission, but some do not
Reproduction factor represents net gain in neutron population
Value range of 1.65-2.0