Lecture 6: Accidents Flashcards
What is the unit of radioactivity?
Becquerels (Bq)
Name the two isotopes that dominate environmental impact?
Iodine-131
Caesium-137
What does I-131 emit?
Alpha
What does Cs-137 emit?
Gamma
What is the half-life of I-131?
8 days
What is the half-life of Cs-137?
30 years
How is the effective dose calculated?
Radiation energy dose x Weighting factor
What are the weighting factors for gamma, beta, alpha, protons and neutrons?
Gamma: 1
Beta: 1
Alpha: 20
Protons: 2
Neutrons: 2 to 22
How is the total effective dose calculated?
Sum effective doses over all organs in the body (applying weighting factor for susceptibility)
How many Sieverts are fatal?
10 Sv
What is the effect of ~1 Sv?
Increase in long-term risk of fatal cancer
What is the % increase of cancer per Sievert?
5%
What two mitigating action are possible following an accident?
Population sheltering
Population evacuation
What is the criteria for sheltering and evacuation?
Sheltering > 10 mSv
Evacuation > 50 mSv
What is Wigner energy?
A build up of potential energy due to neutrons bombarding graphite.
What happened in the Windscale accident?
Cooling fans turned off for annealing process to release Wigner energy and prevent sudden spontaneous heat up/ignition.
Thermocouple indicated continuing temperature increase.
48-hours later, operators realise core is on fire.
Try to extinguish fire by setting blowers onto full, using CO2, and using water.
Why was the use of water to extinguish the Windscale fire risky?
Metal-water reaction could generate hydrogen (explosive)
What was the total release of radioactivity due to Windscale?
1620 TBq (I-131 equivalent)
What happened in the Three-Mile Island accident?
Condensate pump stopped
Main feedwater pump tripped
Turbine tripped
Backup pumps failed due to maintenance.
Hotter core inlet temp.
Pressure increases.
Reactor SCRAM (rods in).
PORV (relief valve) opens and pressure falls.
After 13 secs, valve is stuck open but indicated as closed to control room.
Coolant continues to leak.
Pressurizer indicates OK coolant level but insufficient coolant in core.
…alternative tried.
How does the consequence of Three-Mile Island compare with Windscale?
Only 0.03% of radioactive release
What were three lessons learnt from Three-Mile Island?
Improvements to reactor instrumentation and control room displays.
Implement safety measures to mitigate severe core melt events.
Human factors are very important.
What was the initial cause of Chernobyl?
Carrying out an experiment: How long would the inertia of a coasting turbine keep reactor pumps running following a loss of power?
How was the Chernobyl experiment affected?
Kyiv City request to pause due to power shortage.
Crew change
What happened when power reached 25% in the chernobyl experiment?
Power continued to reduce so control rods withdrawn beyond limits.
What was the result of the of the Chernobyl experiment?
diesel generator and coasting power insufficient so water flow rate decreased and steam increased.
What happened after an increase of power was noticed in Chernobyl?
SCRAM.
What was the critical design fault in Chernobyl?
Graphite tips of control rods:
Displaced water and increased moderation thus increasing reactivity.
Core overheats and control rods become jammed.
What was the release of radioactivity in Chernobyl?
5 x 10^6 TBq (I-131 equivalent)
What was the release of radioactivity in Fukushima?
1.5 x 10^6 TBq (I-131 equivalent)
What was the initiating event of Fukushima?
Magnitude 9 earthquake followed by Tsunami.
What was the result of the Fukushima Tsunami?
Loss of off-site power.
Flooded the emergency diesel generators.
Therefore, lost power and cooling ability.
What were some design adaptations to the EPR as a result of accidents?
No penetrations in reactor vessel below core height.
Core melt catcher.
Concrete impact protection shell.
