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Einfhrung in die Nuklearenergie

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Operator error and failure of control rod system (positive void coefficient) ... This second, chemical explosion brushed off the roof of the building. ... – PowerPoint PPT presentation

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Title: Einfhrung in die Nuklearenergie


1
Einführung in die Nuklearenergie
  • Vorlesung 14
  • Nuclear Reactor Accidents
  • Rafael Macián-Juan
  • E.ON Energie-Lehrstuhl für Nukleartechnik
  • Technische Universität München
  • macian_at_ntech.mw.tum.de

2
IAEA Nuclear Accident Scale
3
Historical Overview
  • Chalk River, Canada (1952)
  • Partial Meltdown of a 30MWt experimental reactor.
  • Cooled by light water. Moderated by heavy water.
  • Operator error and failure of control rod system
    (positive void coefficient).
  • National Reactor Testing Laboratory, Idaho USA
    (1955)
  • Partial meltdown of a 1.4 MWt experimental
    breeder reactor (EBR-I).
  • Operator error failed to stop an intentional
    power rise fast enough.
  • Windscale, England (1957)
  • Overheat and fire in a graphite moderated gas
    reactor used for Pu production.
  • Due to unknown physical process in graphite under
    neutron irradiation.
  • Reactor fire with release of 131I (T1/28.02 d)
  • National Reactor Testing Laboratory, Idaho USA
    (1955)
  • Reactivity insertion accident in a 3 MWt Test
    Reactor.
  • Three US Army technicians were killed when a
    control rod was manually withdrawn.
  • Primitive design that allowed manual removal of
    control rods and operator failure.

4
Historical Overview
  • Fermi Reactor, Detroit, USA (1966)
  • Partial meltdown of a 100 MWt commercial breeder
    reactor (one-of-a-kind design).
  • Blockage in the flow path of the Na coolant.
  • The reactor could resume operation until 1973.
  • Lucens, Switzerland (1969)
  • Partial fuel melting in a 30 MWt experimental
    reactor.
  • Loss of CO2 coolant.
  • Bronws Ferry, Alabama, USA (1975)
  • Fire in the cable of the control system.
  • The reactor shut down automatically and the
    safety systems kept the reactor in a safe state.
  • First accident in a commercial plant which showed
    the need for redundancy.
  • Three Mile Island, Pennsylvania, USA (1979)
  • Chernobyl, USSR (1986)

5
Historical Overview
  • Consequences of the major nuclear accidents

Estimated delayed cancers are calculated on the
basis of the Linear No-Threshold
Hypothesis. Source D. Bodanski, Nuclear Energy,
Table 15.1
6
Three Mile Island
Scheme of the TMI-2 Reactor
Failed Valve
7
Three Mile Island
  • General Description
  • The Three Mile Island Unit 2 (TMI-2) nuclear
    power plant was near Middletown, Pennsylvania.
  • It happened on March 28, 1979 and was the most
    serious in U.S. commercial nuclear power plant
    operating history.
  • It produced NO deaths or injuries to plant
    workers or members of the nearby community. But
    it brought about sweeping changes involving
    emergency response planning
  • It had important consequences in
  • Reactor operator training CREATION OF WANO
    (World Association of Nuclear Operators),
  • Human factors engineering,
  • Radiation protection, and
  • Many other areas of nuclear power plant
    operations.
  • The U.S. Nuclear Regulatory Commission tightened
    and heightened its regulatory oversight.
    Resultant changes in the nuclear power industry
    and at the NRC had the effect of enhancing
    safety.

8
Three Mile Island
  • Description
  • The accident began about 400 a.m. on March 28,
    1979, when the plant experienced a failure in the
    secondary, non-nuclear section of the plant.
  • The main feedwater pumps stopped running, caused
    by either a mechanical or electrical failure,
    which prevented the steam generators from
    removing heat.
  • First the turbine, then the reactor automatically
    shut down.
  • Immediately, the pressure in the primary system
    (the nuclear portion of the plant) began to
    increase.
  • In order to prevent that pressure from becoming
    excessive, the pilot-operated relief valve (a
    valve located at the top of the pressurizer)
    opened.
  • The valve should have closed when the pressure
    decreased by a certain amount, but it did not.
    Signals available to the operator failed to show
    that the valve was still open.
  • As a result, cooling water poured out of the
    stuck-open valve and caused the core of the
    reactor to overheat.

9
Three Mile Island
  • Description
  • As coolant flowed from the core through the
    pressurizer, the instruments available to reactor
    operators provided confusing information.
  • There was no instrument that showed the level of
    coolant in the core.
  • The operators judged the level of water in the
    core by the level in the pressurizer, and since
    it was high, they assumed that the core was
    properly covered with coolant.
  • There was no clear signal that the pilot-operated
    relief valve was open.
  • Alarms rang and warning lights flashed, the
    operators did not realize that the plant was
    experiencing a loss-of-coolant accident (LOCA).
  • They took a series of actions that made
    conditions worse by simply reducing the flow of
    coolant through the core. !!!!
  • Because adequate cooling was not available, the
    nuclear fuel overheated to the point at which the
    zirconium cladding ruptured and the fuel pellets
    began to melt. It was later found that about
    one-half of the core melted during the early
    stages of the accident.

10
Three Mile Island
  • Consequences
  • The TMI-2 plant suffered a severe core meltdown,
    the most dangerous kind of nuclear power
    accident.
  • In a worst-case accident, the melting of nuclear
    fuel would lead to a breach of the walls of the
    containment building and release massive
    quantities of radiation to the environment. But
    this did not occur.
  • Health Impact
  • Estimates are that the average dose to about 2
    million people in the area was only about 1 mrem
    (full set of chest x-rays 6 mrem, natural
    radioactive background dose of about 100-125
    mrem/y)
  • The maximum dose to a person at the site boundary
    would have been less than 100 mrem.
  • Most of the radiation was contained the actual
    release had negligible effects on the physical
    health of individuals or the environment.

The molten Core remained in the vessel
11
Chernobyl
  • General Description
  • The April 1986 disaster at the Chernobyl nuclear
    power plant in the Ukraine was the product of
  • A flawed Soviet reactor design coupled with
  • Serious mistakes made by the plant operators
  • In the context of a system where training was
    minimal.
  • It was a direct consequence of
  • Cold War isolation and
  • The resulting lack of any safety culture.

12
Chernobyl
  • The design Flaw s of the Reactor
  • The reactor had a POSITIVE VOID REACTIVITY
    COEFFICIENT.
  • Chernobyl's control rod design had a number of
    flaws which made an emergency shutdown unsafe if
    there were fewer than thirty control rods in the
    reactor.
  • During the accident, there was an attempted
    emergency shutdown with only six to eight control
    rods in the reactor--and it helped cause the
    power spike.
  • The uranium-graphite-water reactor is inherently
    instable, especially at low power.

Scheme of the Control Rods
13
Chernobyl
  • Control Rod Design Flaw
  • Due to a design error of RBMK reactors, the upper
    and lower parts of the control rods contain
    graphite.
  • According to the regulations, in a shut down
    reactor the control rod should be at position D.
  • During operation it should be at position C
    graphite is located in the reactor core instead
    of neutron absorbing borated steel.
  • Before the accident, however, due to the
    accumulated reactor poisons the automatic control
    system pulled the rods out to level A, which is
    not allowed. Therefore, the space of control rods
    was occupied by water instead of graphite.
  • If one inserts a control rod into the reactor in
    order to decrease power, graphite takes the place
    of water. Since graphite practically does not
    absorb neutrons, while water does, there will be
    a temporary increase in power, as it was had been
    observed earlier in Ignalina.
  • The operators were not informed on this
    phenomenon and thus they decided not to take into
    account the regulations limiting the extent to
    which a control rod can be pulled out. The Soviet
    competent leaders said later in vain "Under such
    circumstances, even the prime minister does not
    have the right to give permission to operate the
    reactor."
  • The dynamic behaviour, in those minutes the
    reactor was different from what the operators
    thought it was like. The fact that the design of
    the control rod moving equipment made the
    excessive control rod pulling out possible is
    considered as a further construction fault.

14
Chernobyl. Chain of Events
  • Origin of the accident
  • An electrical engineering test to see whether
    the power form the turbine coast-down could power
    the coolant pumps until the diesel generators
    could start.
  • Chain of Events
  • At 1 o'clock in dawn Friday, April 25, 1986 they
    started to reduce the 3.2 GW thermal power.
  • By 1300, the power went down to 1.6 GW. One of
    the turbines was disconnected from the reactor.
  • At 1400, the electric distribution center
    informed the Chernobyl Lenin NPP that the energy
    need of the consumers is greater than expected.
    Therefore, they did not decrease the power
    further build-up of 135Xe !!
  • The young electrical engineers mainly kept an eye
    on the electric supply of the pumps. They did not
    take into account that the xenon-poisoning at low
    power operation makes the reactor instable, as it
    was discovered by John Archibald Wheeler and
    Eugene Wigner as early as in the 1940s in
    Hanford.
  • The experts, as well as the decision-makers
    travelled to their weekend houses for Easter.

15
Chernobyl. Chain of Events
  • Chain of Events
  • Due to the accumulated reactor poison most
    control rods were pulled out far more than
    allowed by the regulations !!.
  • The operators themselves wanted to control the
    reactor instead of the "unimaginative"
    automatics. The emergency core cooling system was
    switched off - of course against the regulations
    - at 2 PM on Friday !!!.
  • At dawn on 26th, the automatics responsible to
    control the evenness of the power density of the
    huge reactor was switched off !!!!.
  • 028 AM, April 26, 1986. To make sure, the
    operators increased the flow rate of cooling
    water above the authorized value less vapor in
    core.
  • When they started to decrease the power from 1.6
    GW to the planned 0.7 GW, it went down more than
    expected due to the positive void coefficient it
    dropped to 0.03 GW (too low !!!!). They should
    have waited a day for the decay of the
    accumulated 135I and 135Xe and thus the
    instability caused by xenon-poisoning could have
    disappeared.
  • 107 AM the two operators started to hesitate
    referring to the regulations but they were
    commanded to pull the control rods even further
    out. In this way they managed to stabilize the
    power at 0.2 GW. (The regulations prohibit
    operation under 0.7 GW.) Thinking of the low
    thermal power they decreased the flow rate of the
    cooling water.

16
Chernobyl. Chain of Events
  • Chain of Events
  • 122 AM. The last data printed by the computer
    0.2 GW.
  • 123 AM. Eventually, the real experiment started.
    The operators disabled the SCRAM too !!!!!, which
    would have stopped the reactor in case the number
    of neutrons was rising too quickly. (This action
    was very much against regulations. In the case of
    a modern plant, this is physically impossible.)
  • 12320 AM. Hardly 20 seconds elapsed when, due
    to the loss of steam consumption of the turbine,
    the coolant temperature started to rise and
    consequently the control rods began to move
    downwards to reduce power. However, this resulted
    in situation B, when the place of water was
    occupied by graphite and so the power increased
    by several percents.
  • 12340 AM. The power of the reactor with
    positive void and control rod positive feed-back
    jumped to 0.32 GW from 0.2 GW. As soon as the
    operator observed it, he pushed the scram
    (emergency shutdown) button.
  • 12343 AM. The thermal power reached 1.4 GW. At
    some positions the reactor became supercritical
    to prompt neutrons too and thus uncontrollable.
    Thermal expansion due to the sudden superheating
    distorted the metal channels of the control rods
    and the sinking rods got stuck halfway no SCRAM
    was now possible.

17
Chernobyl. Chain of Events
  • Chain of Events
  • 12345 AM. The thermal power was 3 GW now. More
    and more of the cooling water boiled away. What
    was foresaw in the 50s happened here because of
    the positive void coefficient the chain reaction
    ran away in the whole reactor.
  • 12347 AM. Due to the uneven thermal expansion
    the fuel cladding failed.
  • 12349 AM. Thermal deformation of the fuel rods
    broke the coolant pipes. The suddenly generated
    steam caused a steam explosion and burst the
    reactor cover open.
  • 12400 AM. Above 1100 C water reacts with the
    zirconium alloy of the rod cladding. The product
    of the reaction is hydrogen. Because of the
    cracks, steam contacted graphite as well and this
    reaction lead to the production of carbon
    monoxide and hydrogen
  • The flammable hydrogen and carbon monoxide mixed
    with the oxygen of air and exploded. This second,
    chemical explosion brushed off the roof of the
    building.
  • Graphite started to burn in air and the smoke
    contaminated the building and its growing
    vicinity with radioactivity. Two persons, a
    technician and an electrical engineer immediately
    died.
  • The temperature inside the reactor reached 3000
    C. The fission products diffused from the fuel
    to the burning graphite and to the air from
    there.

18
Chernobyl. Results of the Accident
Configuration of the Reactor after the Accident
Source ANNEX J EXPOSURES AND EFFECTS OF THE
CHERNOBYL ACCIDENT, UNSCREAR, 2000
19
Chernobyl. Radiological Emissions
  • The radionuclides released from the reactor that
    caused exposure of individuals were mainly
    iodine-131, caesium-134 and caesium-137.
  • Iodine-131 has a short radioactive half-life
    (eight days), but it can be transferred to humans
    relatively rapidly from the air and through
    consumption of contaminated milk and leafy
    vegetables.
  • Iodine becomes localized in the thyroid gland.
  • For reasons related to the intake of those foods
    by infants and children, as well as the size of
    their thyroid glands and their metabolism, the
    radiation doses are usually higher for them than
    for adults.
  • The isotopes of caesium have relatively longer
    half-lives (caesium-134 has a half-life of 2
    years while that of caesium-137 is 30 years).
  • These radionuclides cause longer-term exposures
    through the ingestion pathway and through
    external exposure from their deposition on the
    ground.
  • Many other radionuclides were associated with the
    accident, which were also considered in the
    exposure assessments.

20
Chernobyl
21
Chernobyl. Exposure Estimates
  • Average effective doses to those persons most
    affected by the accident were assessed to be
  • About 120 mSv for 530,000 recovery operation
    workers,
  • 30 mSv for 116,000 evacuated persons, and
  • 20 mSv during the first two decades after the
    accident to those who continued to reside in
    contaminated areas.
  • In other European Countries
  • Average doses there were at most 1 mSv in the
    first year after the accident with progressively
    decreasing doses in subsequent years.
  • The dose over a lifetime was estimated to be 2-5
    times the first-year dose. These doses are
    comparable to an annual dose from natural
    background radiation and are, therefore, of
    little radiological significance.

22
Chernobyl. Exposure Estimates
  • The Chernobyl accident caused many severe
    radiation effects almost immediately
  • Of 600 workers present on the site during the
    early morning of 26 April 1986
  • 134 received high doses (0.7-13.4 Gy) and
    suffered from radiation sickness.
  • Of these, 28 died in the first three months and
    another 19 died in 1987-2004 of various causes
    not necessarily associated with radiation
    exposure.
  • In addition, according to the UNSCEAR 2000
    Report, during 1986 and 1987 about 450,000
    recovery operation workers received doses of
    between 0.01 Gy and 1 Gy. That cohort is at
    potential risk of late consequences such as
    cancer and other diseases and their health will
    be followed closely.
  • Apart from the dramatic increase in thyroid
    cancer incidence among those exposed at a young
    age, and some indication of an increased
    leukaemia incidence among the workers, there is
    no clearly demonstrated increase in the incidence
    of solid cancers or leukaemia due to radiation in
    the most affected populations.
  • Neither is there any proof of other
    non-malignant disorders that are related to
    ionizing radiation. However, there were
    widespread psychological reactions to the
    accident, which were due to fear of the
    radiation, not to the actual radiation doses.

23
UNSCREAR Conclusion
  • The accident at the Chernobyl nuclear power plant
    in 1986 was a tragic event for its victims, and
    those most affected suffered major hardship.
  • Some of the people who dealt with the emergency
    lost their lives.
  • Those exposed as children and the emergency and
    recovery workers are at increased risk of
    radiation-induced effects.
  • The vast majority of the population need not live
    in fear of serious health consequences due to the
    radiation from the Chernobyl accident.
  • For the most part, they were exposed to radiation
    levels comparable to or a few times higher than
    the natural background levels, and
  • future exposures continue to slowly diminish as
    the radionuclides decay.
  • Lives have been seriously disrupted by the
    Chernobyl accident, but from the radiological
    point of view, generally positive prospects for
    the future health of most individuals should
    prevail.

24
Accident Analysis
  • CONSERVATIVE CODES
  • Based on models and methods with a high degree of
    conservatism.
  • Safety margins are usually very large.
  • Eg. LOCA App. K based codes.
  • BEST ESTIMATE (BE) CODES
  • Models and Methods based on the Best Available
    science
  • Physically realistic solutions.
  • Better performance to model complex systems
    interactions.
  • They produce a Best Estimate result.
  • Provide solutions that can be used to optimize
    safety and operation
  • The plants gain margin for operation without
    violating safety limits.
  • CHARACTERISTICS of BE Codes
  • Physical Models
  • Empirical Correlations.
  • Mechanistic Models.
  • First Principles Models.
  • Numerical Models
  • PDE Conservation Equations.
  • Neutronic Description (Diffusion or Transport).
  • Control System Theory.
  • Numerical Methods
  • Finite Differences or volumes
  • Nodal Methods (Neutronics).
  • Implicit, Semi-implicit or Explicit time
    discretization.
  • Iterative solution methods convergence.
  • Component Based Codes.

25
Accident Analysis
vv
vl
26
Accident Analysis
  • Modern Best Estimate System Codes Describe the
    Flow Field by
  • Set of Coupled PDEs which represent conservation
    laws for
  • Mass.
  • Energy.
  • Momentum.
  • The System is closed for solution by Closure
    Laws
  • Physical Models for Heat Transfer.
  • Interfacial (vapor-to-liquid).
  • Structures to fluids.
  • Physical Models for Momentum Transfer.
  • Interfacial drag, Wall to fluid drag.
  • Pumps and turbines.
  • Especial Models for important physical phenomena,
    eg.
  • Critical Heat Flux (CHF).
  • Tracking of interfaces.
  • Thermodynamic Properties of the fluid(s).

Mass
Convective Transport of Mass
Energy
Momentum
27
Accident Analysis
  • Core Neutronic Behaviour
  • Solution of Neutron balance equation
  • Static Calculation (criticality).
  • Dynamic Behaviour (transient).
  • Neutron balance defined by
  • Leakage.
  • Fissions (POWER).
  • Absorptions.
  • Solution Methods
  • Nodal Methods for Diffusion Approximation.
  • Fast Solution Procedures.
  • Accurate power distributions.
  • Most Used in System Codes.
  • Advanced Transport Theory Methods.
  • More detailed treatment of neutron transport.
  • Computation Intensive.

Time dependent Neutron distribution n(t,r)
Diffusion D(t,r)
Fission Sf (t,r)
Absorption Sa (t,r)
28
Accident Analysis
Coupled Solutions integrate the main descriptions
of physical processes that determine the behavior
of a nuclear system. Based on the transfer of
information between main code physical solution
procedures.
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