|Three Mile Island. 900 MWe PWR. High-pressure & high-temperature water isolated in closed reactor loop produce, via heat exchanger, steam for turbine.|
In emergency shutdowns (SCRAM), the reactors built in US have an emergency core cooling system (ECCS) that pump large amounts of water into a reactor core heating too fast. Designs of that era had (1) lots of pumps, valves, and water pipes in nuclear reactor and (2) vast arrays of indicators and knobs in control room.
Earlier in routine maintenance
2 valves (on steam side of secondary heat exchanger)
were inadvertently left closed.
Incident. began with failure of primary feedwater pump to the
heat exchanger. Loss of feedwater decreased heat transfer from primary
system with subsequent overheating and pressure increase.
By safety design, backup systems didn't start at once.
SCRAM occurred ⇒ pressure relief valve released excess pressure.
The three backup feedwater pumps,
disabled by the closed valves, didn't start after built-in
The pressure relief valve stayed
open long enough to lower pressure; then relief
valve should have closed; the stuck-open valve not noticed
for 2 1/2 hours.
By safety design, emergency core cooling
system turned on as pressure dropped, but
operators thought ECCS turned on by mistake & manually turned it
off. The now uncovered core boiled water and steam escaped
through the stuck relief valve.
Operators, seeing core cooling (due to
escaping steam) mistakenly turned off water pumps to core,
hoping to raise temperature. Big mistake:
The core was uncovered for 13.5 hours.
Liquid released by stuck valve ruptured a holding tank seal, ultimately spilling 400,000 gallons of radioactive water onto containment vessel floor!
The final toll:|
15-30% of the core was uncovered
45% of the core melted
70% of the core was damaged
20 tons of debris fell to reactor bottom
Considerable radioactivity released [1012 Bq]
Summary: human error was the main
cause; design-induced mechanical failure also important.
Some important lessons about the design of control rooms were learned by
power plant managers.
In March 2009, Senate Subcommittee on Clean Airr and Nuclear Safety
heard testimony from two panels of experts. The central message from
is true about about all human endeavor.
Several important issues emerge from these and other events. The textbook spends considerable space on them. They are:
|Improve design: reduce complexity & increase reliability.|
|Risk-benefit analysis; see below.|
|Health effect of ionizing radiation:textbook 20.5.|
|Waste storage; see below.|
Risk Benefit Analysis
Repeat studies point out that
Individuals will tolerate voluntary risks
at a level 10,000 greater than involuntary risks.
In 2002, deaths in US from:
|50,000 auto accidents||30,000 firearms incidents||600 airtravel||100 hurricanes|
|Similar statistics exist for the danger of alcohol, smoking, no seat-belts.|
Any of these studies can be challenged, but the basis message remains: the involuntary risk seems worse than the voluntary one.
Nuclear reactor use radioactive material. Even if the plants can eliminate all emissions, still the unused reactor fuel must be faced.
A nuclear reactor does not consume all its U-235. Even if did, the neutrons and other radiation released transforms some of the elements in the moderator, cladding and reactor vessel to radioactive ones -- often with long-life times or health hazards.