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fission
splitting of large atomic nuclei into 2 smaller nuclei
fission release energy
increase in binding energy loss in mass
fission types
spontaneous or induced
fission energy released when
binding energy per nucleon of products is more than binding energy per nucleon of reactants
nuclear fission in power stations
induced
indcued fission how
bombard heavy nuclei with neutron
nuclear fission enery amount release
150-200 Mev
energy released is (ek)
ek of fragments and neutrons
technical issues probability
of fission high enough to ensure enough fission events take place every second
technical neutrons
sufficient neutron to go on and create more fission events establishing a self sustaining chain reaction
technical free neutrons
controlled
technical heat
extracted efficiently
technical employee
shielded from radiation emitted from reactor
critical mass of fuel ratios
bigger mass = smaller sa:v ratio
smaller ratio means (critical mass of fuel)
smaller % neutrons likely to escape
critical mass shapes
sphere less escape, sheet more escape
critical mass of fuel def
min amount of fissile material in shape of sphere required to establish a self sustaining chain reaction
super critical
lead to explosion too many n
subcritical
reaction eventually dies out not enough n
when mass of fissile self sustaining, mass in ciritical state where
no increase or decrease in power, temp, neutron pop
critical mass of fuel depends on
concen of u235
geometry of core and fuel rods (affect SA)
uranium 238 why cants chain
neutron has lower energy than og neutron, below fiussion threshold
fast neturon
high speed neutron
thermal neutron
slow neutron
why need thermal
because has higher prob of being absorbed and causing fission
fuel rod purpose
contain fissionable u235 to produce heat
fuel rod made up of
uranium oxide
chernobyl fuel was
unenriched
control rod p
absorb excess neutrons
control rod comp
boron or cadium
control rod chernobyl
boron and light water
moderator p
slow down fast n to thermal speeds so that it can be absorbed by u235
moderator comp
graphite, water, heavy water
chernobyl moderator
graphite
coolant p
extract heat from reactor and stop meltdown
coolant comp
water, heavy water
coolan chernobyl
water
reactor shielding p
protect worker from radiation
reactor shielding comp
concreate and lead
chernobyl reactor shielding
none
why control boron and cadium
readily absorb n without fissioning
neutron flux
number of neutrons passing through at a time
half life and activity relationship
shorter the half life, higher the activity
half life
the time it takes for activity to halve
activity
number of nuclei that decay per second
unenrich coolant
heavy water
enrich coolant
light water
why unenrich heavy water
heavy water not as likely to absrob the scarce neutrons in fuel
why enrich light water
light water readily absorbs neutrons, controlling numb of neutrons from the enrich fuel
xenon 135 what
strong n absorber
xenon 135 where did it come from
buildup from daily activity
goal for safety test
see how quickly back up generator could start if power was cut, how quickly could pumps restart
safety test caused
decrease in power below the intended range for reactor4
cooling system
disabled
when power far below stable level what did they do
pull out all the control rods
what happened once control rods out
no power increase due to the xenon buildup as well as the water absorbing neutrons
power surge caused
emergency shutdown, control rods reinserted
What happens when the control rod comes back?
Graphite moderator displaces water.
Where is the only moderator located in this scenario?
At the bottom.
What is the effect of having no control of neutrons at the bottom?
Increased neutron flux at the bottom forms a power hotspot.
What is the result of increased neutron flux at the bottom?
Increased rate of fission and energy.
What issue arises with the control rod in this situation?
got stuck in this critical position
positive feedback loop
- normal water hot
- boils to steam
- less water to act as control
- decrease n absorbed, neutron flux increase
- rate of fission increase
- more water boils to steam
repeats until eventually no water left to act as control
negative feedback loop
- heavy water hot
- boils to steam
- loss of heavy water to slow down neutron
- decrease rate of fission
- energy output decrease
- hot water cools, steam content decrease
- more heavy water to moderate neutron
- rate of fission increase
repeats
how enriched u made
using gas centrifuges
enriched u what isotopes
96% U238
4% U 235
unenriched u what isotopes
99.7% u-238
0.3% u-235
what reactor chernobyl
RBMK
how xenon build up
as neutron flux decreases
low power pumps effect
water no pump
- heat not being removed efficiently
- areas of reactor got hotter than others
- build up of voids of steam
increase void content, increase steam pockets what do to fission
less water absorbing neutrons
- fission rate increased
- xenon absorbed many and decayed
- little control over rate of fission
bad design features of chernobyl
-control rods slow to reinsert
- positive feedback loop due to light water coolant
- no containment building
radiation health effects
- thyroid cancer
- acute radiation poisoning
chain reaction
self sustaining fission reaction spread by neutrons
heavy water
2 H2O
1
light water
1 H2O
1
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