CHernobyl assesment

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