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  control rod was manually withdrawn.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.

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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.

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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.

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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.

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Chernobyl. Results of the Accident
Configuration of the Reactor after the Accident
Source ANNEX J EXPOSURES AND EFFECTS OF THE
CHERNOBYL ACCIDENT, UNSCREAR, 2000
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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.

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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.

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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.