SUMMARY of NUCLEAR ENERGY CONCEPTS and TERMS
Fission:
Unstable (radioactive) elements spontaneously split (radioactive decay), emitting high energy particles. Collision of particles with other atomic nuclei can trigger further nuclear decompositions. A small amount of mass is converted into a large amount of energy, when atomic nuclei are split.Einstein equation: E = m c2
Conversion of mass to energy. E= energy, m = mass converted, c= speed of lightCritical Mass :
There is a threshold mass of a radioactive isotope at which the flux density of radioactive particles will sustain a chain reaction. If this reaction is uncontrolled the result is an atomic bomb explosion. If the radiation fluxes are controlled and limited, we call it a nuclear reactor, which can be the basis of an electric power plant.Types of Radiation:
Atomic Weight Charge Alpha radiation (Helium nucleus) 4 +2 Beta radiation (Electron) ~0 -1 Neutron 1 0 Gamma ray ~0 0
Alpha radiation:
Alpha is quickly absorbed by matter because the particles have a large probability of collision with nuclei. Sources external to the human body cause radiation absorption within the thickness of the skin. Radiation from airborne particles in the lung are absorbed by surface membranes lining the lung. Alpha emitters ingested with food cause radiation absorbtion by the lining of the gut. The risk of genetic damage to adult organisms is very small because absorption takes place in surface cells.Beta particles:
Beta particles penetrate to the deepest parts of the body and can cause genetic damage and disrupt the function of cells anywhere in the body. Building walls and earthwork provide substantial shielding.Gamma particles:
Gamma has the greatest penetration due to their small cross-section. Gamma particles can pass through ordinary materials. Effective shielding requires blankets of lead. Gamma radiation is a danger to all cells in the body.Uranium fission:
92U235 + 0n1 => 92U236 => Fission Products
92U238 + 0n1 => 92U239 + Gamma => Fission Products
92U239 =>
93Np239 =>
94Pu239 ..
Neptunium Plutonium
After many steps, (and a long time) the ultimate product is non-radioactive
Lead atoms. The neutrons, whose absorbtion is indicated above, come from
splitting of later fission products in reactions not shown here. Note that U-235
fission in the presence of U-238 causes the conversion of part of the U-238 into
Plutonium-239 which can be concentrated to make an H-Bomb. * BAD CONTROL SYSTEM DESIGN * POOR INSTRUMENTATION * OPERATING IN VIOLATION OF SAFETY RULES * OPERATORS UNABLE TO HANDLE EMERGENCY * BAD MANAGEMENT ETHICAL PROBLEMS IN NUCLEAR The Atomic Energy Commission (AEC), was formed to create a civilian nuclear
energy industry, and had conflicting responsibilities:
* PROMOTING NUCLEAR POWER * REGULATING PLANT SAFETY As a result of these conflicting interests
* NO LONG TERM WASTE DISPOSAL PLAN WAS COMPLETED * Future TERMINATION \ CLEANUP COSTS ARE NOT FACTORED INTO CURRENT ELECTRIC
RATES.
* POWER COMPANIES ARE LARGELY SELF-REGULATED Internal conflicts of the AEC were supposed to be resolved by splitting the
promotional and regulatory duties between the new agencies: MAJOR NUCLEAR POWER DISASTERS
Chernobyl
- is near Kiev, Ukraine, in the former Soviet Union. Destroyed by steam and
hydrogen explosions followed by fire, it caused many deaths on site, increased
cancer rates in the thousands of square miles it contaminated.
April 26, 1986
Three Mile Island
- Located 10 miles southeast of Harrisburg PA on the Susquahanna River. The
accident,and radiation release, caused no immediate deaths. The cleanup cost
more than $1.5 Billion.
March 28, 1979 September 12, 1978 T.M.I. Unit #2 dedicated.
January 1979 TMI #2 began commercial operation.
March 26, 1979 Emergency core cooling pumps tested, with diverter valves
switched to disconnect ECCS from reactor. Valves not switched back.
March 28, 1979, 4 a.m. Three Mile Island Incident began. Cleanup/termination cost $1.5+ BILLION.
BIBLIOGRAPHIC REFERENCE: After the Accident it was necesary to dispose of the radioactive gases,
water, and contaminated debris from radioactive plumbing etc. The water had to
be filtered to separate and concentrate radioactive contaminants for
disposal. After these were removed it was possible to begin dismantling the
pressure vessel and extract the fuel rods. It was not until then that the inside
of the core could be inspected. As the damaged reactor was brought under control,
it was known from radiation monitoring that there was a significant amount of
radioactive material in the bottom of the pressure vessel. In spite of this, the
power company still maintained that the damage to the core had been minimal. When
a robot with a video camera was lowered into the presure vessel, four years after
the accident, this is what it saw:
As outside experts had speculated, there had been a partial meltdown. Many
fuel rods melted causing the tubes to break, spilling their load of uranium fuel
pellets into the bottom of the pressure vessel. The pile of uranium pellets
supported a continuing fission which was not subject to shutdown by control rods.
That explained the continuing radiation and heating that plagued the operators
trying to regain control of the plant.
There was no possibility of repairing and restarting the reactor. It was
entombed in concrete and left in place. The other reactors at TMI were not
affected and are still in service. 1. MANUAL OPERATION WITHOUT 2. OPERATION WITH EMERGENCY COOLING SYSTEM DISABLED 3. LACK OF OPERATOR TRAINING FOR 4. FLAWS IN SYSTEM DESIGN and 5. MANAGEMENT DECISIONS TO 6. ABSENCE OF DISASTER PLANS
7. MANAGEMENT & REGULATORY * FLAMMABLE GRAPHITE NEUTRON MODERATOR
* THERMAL RUNAWAY PROCESS FASTER * SPREAD LARGE FRACTION OF RADIOACTIVE TMI CLEANUP EXPOSED THOUSANDS OF WORKERS The plant had 4 nuclear reactors, each with associated steam turbines and
electric generators. Two additional units were under construction at the time of
the accident, April 26, 1986. Each of these units was of the same Soviet design,
designated RBMK-1000.
Chernobyl was the location of the world's worst nuclear power plant disaster.
Massive amounts of radioactivity were released, a thousand square mile area will
be uninhabitable for many decades.
(Topical humor after the accident: What is finger-licking good and glows in
the dark? answer: Chicken Kiev.) Electric generating capacity 1000 Megawatt.
Thermal output of core about 2000 Megawatt.
1661 zirconium fuel rods, holding mix of U-238 and U-235; Plutonium-239 is a
byproduct, which can be extracted by reprocessing the fuel rod material. Each
fuel rod is enclosed in a heat transfer water channel
211 Boron control rods with 8 fuel rods / control rod
Graphite core 1700 tons, made up of graphite bricks 2. Boron control rods to reduce neutron flux for shutdown
3. Thermal transfer control - closed circuit water/steam loop, multiple water
pumps Nitrogen/Helium gas within containment - low thermal conductivity and
oxygen exclusion - pressure and gas mixture are controlled. emergency core
cooling water system (ECCS) Computer for fine control, operator controls set points of feedback
controllers Plant engineers found the plant unstable at low power levels, While a Western plant has 10-15 operators/shift, Chernobyl had 70/shift. Most
did not graduate from nuclear engineer training schools, other
engineer/technical training; took correspondence courses, and took qualifying
tests for promotions. Published odds million to 1 against an accident. A problem with RMBK-1000 reactor design is that the time constants for
changes in thermal output are short. Control depends on computer regulated
feedback control systems. The human operator could not react fast enough to
manually control it without the automatic controls.
Neutron absorption and heat transfer coefficients are very different for
water and steam, so neutron flux and thermal output changes rapidly as water in
the tubes through the core makes a transition from hot water to steam.
The normal operating temperature of core tubes is greater than the ignition temperature
of the graphite blocks of the core (carbon) in an O2 atmosphere. Its
normal environment is an atmosphere with no oxygen.
One closed loop through reactor core and steam turbines
Turbine building roof; specification said it should be fireproof. Materials for 1 km
x 50 m fireproof roof was not available. Control cable conduits supposed to be
fireproof. Material not available. Exception granted. Cement and tiles, etc.
Quality control problems. Director had to prioritize uses, discard defective
materials. Fittings often required remanufacture to meet specifications. C + H20 => CO + H2 2 CO + O2 => 2 C O2
Chernobyl Unit #4 was commissioned on December 21, 1983. after 3 months of
testing. Commercial operation began on March 27, 1984 with an untested emergency
shutdown power system to drive control rods and pumps between generator outage
and diesel power startup.
April 26, 1986 Plan was to test low power shutdown systems on weekend before
May Day. Considered routine. No analysis by safety officer at Chernobyl. No
analysis by Electric Power Ministry which approved it. Plan included
disconnecting ECCS.
Test began at 1 a.m. under manual control. Power dropped from 25% to 5%.
Control rods pulled. Power slowly increased. Water pumps slowed. Power output
surged. Re-insertion of control rods failed. Water channels ruptured.
The steam explosion cracked the concrete shell around the reactor core
releasing the protective atmosphere. Steam on hot graphite generates
H2 gas. Explosion at 1:30 a.m. Core disrupted. Graphite ignites.
Water on graphite fire was ineffective. The fire fighters were fatally
exposed to radiation from the rubble left of the core.
Next day, they began bombing the core with sand, boron and lead, dropped by
military helicopters. Several crews were fatally exposed to radiation. May 9, 1986 after 5000 tons of sand etc., the fire was smothered. Following which a
heat exchanger was built by tunneling under reactor.
By fall, Unit 4 was entombed. Units 1, 2, were back in service in middle of a
30 km diameter uninhabited zone, 6 months later.
4/25/86 14:00 Automatic Core Cooling switched off for shutdown experiment, but
experiment postponed to midnight. Power level reduced to 50%.
4/26/86 00:28 Control rods: switch from local to global control. Power plummets
to 30 Mw. Control rods raised.
4/26/86 01:03 Cooling pump 4 switched in
4/26/86 01:19 Shutdown signal from steam separators, blocked.
4/26/86 01:21 Caps on fuel rod channels seen bouncing
4/26/86 01:22 Pressure falls in steam drums
4/26/86 01:23 Local overheating in one sector of core. Control rod emergency
reinsertion, failed.
4/26/86 01:24 Explosion in graphite pile: ruptured steam tubes release 65
atmosphere pressurized water; water flashed to steam, steam ruptures
containment. Steam on hot carbon generates H2, mixture with
O2 gas which explodes.
4/26/86 01:24 Upper shield disintegrates. Graphite core ignites.
01:28 Call received at fire station.
01:35 Fire brigade fights fire on turbine building roof.
02:30 Plant Director arrives at control bunker.
04:00 Three more fire brigades arrive with fire hoses.
05:00 General of militia arrives from Kiev. 4/26/86 As reactor 4 burns, 286 men continue work on Units 5 & 6
construction. Emissions 12 million Curies/day
4/27/86 Begin helicopter drops of sand, boron, lead on burning reactor 4.
They dropped 150 tons on day 1.
4/28/86 Radiation alarms set off at Swedish nuclear plants. TASS denies any problem all day but at 9 pm admits
accident
4/29/86 Pripyat evacuated, 10 km exclusion zone population of 50,000; only 21,000
were left to evacuate.
5/1/86 After bombing reactor with 1000 tons sand, etc., emissions down to 2 million
Curies/day. Radiation surveys show 40 Curie/km2 beyond 30 km.
Affected population more than 120,000. 5/2/86 Miners begin tunneling from under reactor 3 to under reactor 4 to build
heat exchanger.
5/6/86 Official report of accident: 2 dead, 204 radiation burn cases
hospitalized. Plant under control.
5/7/86 Transport system in Kiev jammed by people fleeing.
5/8/86 Emissions down to 150,000 Curies/day. The Ukraine Politburo in Kiev
demands Soviets give recommendations on fallout countermeasures. Soviets
proclaim no danger. Kiev media asks people to wash produce, etc. School year
suspended May 15, 1986; children under 7th grade to be sent to summer camps.
5/9/86 Fire out
6/86 KGB seizes dosimeters and medical records in controlled zones.
Health consequences of radiation are a state secret. Milk seized, made into
butter, held until I-131 decayed. Meat stored, shipped elsewhere for 10:1
dilution.
8/27-28/86 Report to IAEA in Vienna, claimed 'full disclosure'. Soviets
downplayed reactor faults, blame staff, discussion was heated, and pointed out
discrepancies in numbers on radiation release. Six pages of report, detailing
contamination of land north of Chernobyl had been suppressed.
9/23/86 Concrete sarcophagus complete over Unit 4
9/29/86 Unit 1 recertified
10/1/86 Unit 1 back on line
11/5/86 Unit 2 back on line
7/7/87 Plant Directors, Engineers, Operators tried for violating safety
rules, dereliction of duty. Trial was in Chernobyl. Contaminated evidence
required lawyers to wear radiation protection suits.
7/27/87 After excluding evidence of design faults, all were convicted.
Sentences 3 to 10 years. 5/88 IAEA conference on Chernobyl at Kiev. Fallout affected 17.5 million people in
USSR, 2.5 million children under 7. 135,000 evacuated. 350,000 children sent to summer
camp. Still claiming only 31 dead, and 200 injured. (over 500,000 worked on
cleanup) September 1982 - Chernobyl Unit 1, after 5 years service, was shut down for
maintenance. Restarted with some valves closed. Result: no water flow in a few
channels. Explosion in core, a few fuel rods melted. Some radioactivity escaped
plant. No radiation survey was done outside plant. Streets of Pripyat were hosed
down. No announcement to population. Emergency core cooling system saved plant.
Chief Engineer, his deputy, and chief operator of the shift were all demoted and
transferred. 1980 Kursh power station. RBMK-1000 plant had a power outage,
Reactor damaged because control rods and circulation driven by electric
motors/pumps failed. Time delay to start diesel generators was 40 seconds During
which, power surge damaged some fuel rods. Solution : design a special generator
to tap turbine power as it spun down during shutdown, to power emergency
equipment.. Oct. 1982. Armyansk nuclear power station. Explosion. Subsequent fire
destroyed turbine building. Fall 1983 Chernobyl Unit 4 startup. Certification team saw anomalous
power surge when control rod insertion starts. Considered minor, had been
seen in another reactor. No explanation, not documented. June 1985 Balakovsky PWR power station, Valve burst, release of 300
degree C. steam, cooked 14 workmen. Safety regulations viewed as guidelines,
chief engineer regularly made exceptions.
Half life, T:
Time for half the atomic nuclei to spontaneously split.
The amount decays exponentially
N = No exp (-t/T)
N = Amount
of radioactive material, No = Initial amount, t = elapsed
time
THREE MILE ISLAND
WHAT WENT WRONG?
unreliable valves
unreliable indicators
bad panel design (no standard)
lack of
indicators to monitor fluid balances
disabled emergency core cooling
system
manual override of automatic shutdown
no one on duty was a nuclear
engineer
operators untrained for emergency manual control
unwilling to investigate anomalies
coverup of
problems
POWER REGULATION
- funded research in plant design
-
subsidized production of nuclear fuel
- defined safety procedures, poor enforcement
- inspecting, certifying plants
- certifying operators, poor training
- wastes are still
accumulating in temporary storage
- radioactive waste? NIMBY
- avoid reporting radiation
release or do not monitor releases.
- avoid safety regulations to save
money.
Nuclear
Regulatory Commission (NRC)- safety and standards
Dept. of Energy (DOE) -
research, promotion, waste disposal, and fuel rod production
T.M.I. ACCIDENT CHRONOLOGY in BRIEF
1970's AEC LOFT (Loss Of Fluid Test)
research canceled as economy measure.
- Filter in inner loop
switched offline to clean
- Pressure transient triggers shutdown sequence
- Core overheats, pressure relief valve sticks open, in manual override
- Water in core begins leaking out open relief valve
- Emergency
cooling pumps don't work !
- After more errors, 1/3 of core exposed,
partial meltdown of fuel rods results.
- 2nd day someone closes relief valve
(unrecorded).. situation stabilizes
- hydrogen gas bubble forms.
-
Governor / NRC, order partial evacuation
Daniel F. Ford, THREE MILE ISLAND: 30
MINUTES TO MELTDOWN,
Penguin Books, NY 1982
CLEANUP AFTER THE THREE MILE ISLAND ACCIDENT
Three Mile Island Unit 2 Reactor Core After Accident
AP photo Copyright 1999 The Washington Post Company
For more information on Three Mile
Island:
Websites , Bibliography
SIMILARITIES: CHERNOBYL and TMI
AUTOMATIC SHUTDOWN SYSTEM
- TMI, override
of automatic shutdown sequence
- Chernobyl , disabled to perform system
tests
- TMI, left switched
off after a prior test
- Chernobyl , disabled as part of system tests
EMERGENCY MANUAL OPERATION
-
Contrast: airline pilots must periodically 'fly' a
simulator where disaster
recovery is practiced.
UNRELIABLE COMPONENTS
CONCEAL PROBLEMS
- Delay and avoidance in
reporting problems
- Evasion of regulatory requirements/standards
DECISIONS THAT CANCER RISK
OF RELEASED
RADIATION IS
ACCEPTABLE.
DIFFERENCES: CHERNOBYL and TMI
CHERNOBYL
* HAD NO REINFORCED CONTAINMENT STRUCTURE
THAN HUMAN REACTION TIME
CONTENTS TO THOUSANDS OF SQUARE
MILES
OF ENVIRONMENT.
CHERNOBYL EXPOSED ABOUT 800,000
WORKERS
Chernobyl Nuclear Power Plant
Chernobyl is a town of 30,000 people, 70
miles north of Kiev, in the Ukraine. The V. I. Lenin nuclear power plant is
located 10 miles from the town of Chernobyl. Adjacent to the plant is the town of
Pripyat, which houses and services plant workers. The plant is on the Pripyat
River, near its mouth into the Kiev reservoir.
Reactor design: RBMK-1000
Boiling Water Reactor
Control of the reactor
1. Graphite Core, moderates neutron flux from
fuel rods
Chernobyl Reactor operations:
Central power authority dictated operating levels in managing
power grid
Unnecessary shutdown meant 600,000 ruble revenue loss, firing of
person responsible.
Local
practice was to manually pull control rods if downward fluctuation threatened
spontaneous shutdown. Response time to scram: 18 seconds (theoretically it was
claimed to be 3 seconds)
Regulations against manual control routinely excepted.
Staff:
Accident\safety plans:
Authoritarian control ..
staff & engineers do not question safety.
Accident planning was around a
scenario of 1 or 2 fuel rod/water channels bursting.
No plan included a
graphite fire.
Administration building had emergency bunker under it.
Reactor building was a water tight containment building.
Evacuation:
Plant director had authority in principle to order
evacuation of Pripyat. However a standing order made any nuclear accident a
state secret.
SAFETY PROBLEMS IN CHERNOBYL REACTOR DESIGN
System Dynamics:
Another safety problem with the design:
Heat exchange system:
Secondary loop to
condense steam to water after turbine
Construction problems:
Hazard Potential of Water on Hot Graphite -
Water Gas Reaction:
Often used as a
H2 generator in freshman chemistry labs, it has a similar
hazard if not carefully controlled:
Chernobyl Accident in Brief
Fallout:
Estimated total release was 30 million Curies. Thousands of
km2 became uninhabitable for many years. 17.5 million people exposed
to fallout. Perhaps thousands died within a few years after exposure. Official
death toll to radiation sickness: 31. Birth defect rate and cancer rate
increased. Abortion was recommended for all pregnant women in heavy fallout
areas.
CHERNOBYL ACCIDENT CHRONOLOGY
Ministry of Energy & Electrification demands to know when Unit 4 will be back on line ..
Chernobyl Director still thinks reactor is intact.
Evacuation ordered for 30 km zone.
Other, Earlier, Soviet Nuclear Accidents:
For more information : Chernobyl
websites, Bibliography
Problem set:
1. How much mass is converted to energy per day by
a 1000 megawatt nuclear power station? Assume the efficiency of thermal to
electric power conversion is 50%. Speed of light is 3 x 108 m/sec,
1 Joule = 1 kg m2/ssc2 = 2.78 x
10-7 kilowatt hour
2. If a 1000 Megawatt (electrical output) power station transfers its waste
heat to a river which provides a stream of cooling water continuously passing
through its heat exchanger. How much water (in m3/sec) has to flow through if the output water is 2 degrees
C. warmer than the input water? [Should you be surprised that power plants are
located on large rivers?]
1 Watt = 1 Joule/sec
Specific heat of water 4.18 kJ /( kg deg.)
( 4.18 kJ. raises the
temperature of 1 kg of water by 1 degree C. ) Density of water: 1
gm/cm3 , or 1 gm of water has a volume of 1 cm3
3. Why is the United States so worried about the RMBK reactor in North Korea that
we are willing to trade them a new PWR and new fuel in exchange for
dismntling their RMBK reactor?
4. Why might this country consider
building a few dozen new nuclear power plants, considering that there
have been previous serious accidents witth nuclear power plants?
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Last Updated on January 24, 2001 by