Transcript Historical Perspective Nuclear Reactors and the Regulatory

The Science of
Nuclear Reactors and
the Regulatory
Framework
A Presentation to Wake Forest University
School of Physics
September 12, 2002
M.T. Cash
Historical Perspective
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Science and Law are often best
understood in a historical perspective
The regulation of nuclear power reactors
involves science, law, politics and many
other influencing forces
A review of the development of science
along with the regulatory framework is
interesting and illuminating
Discovery of Nuclear Fission
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James Chadwick “discovery” of the neutron
(1932)
Hahn and Strassman “discovery” of barium
atoms resulting from neutron bombardment of
uranium (1937)
Frisch and Meitner using ideas from others
develop the liquid drop model of fission to
explain Hahn Strassman (January 1939)
Mass Defect
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Splitting atoms was all very exciting and
fascinating in 1939, but practical value?
The observed nuclear mass is always
less than the summation of the
constituent nucleon masses
This mass defect is embodied in the
binding energy of the nucleus pursuant
to E = m c2 (A. Einstein 1905)
Application to Uranium Fission
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It can be shown that the binding energy per
nucleon in Uranium is approximately 7.59
Mev/nucleon
U-235
FPa + FPb + Energy
The resultant energy from fission will be found
to be approximately 200 Mev/Fission
Combustion of Carbon atom results in an
energy release of approximately 4 ev
Chain Reacting System
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Even with the enormous energy release
the reaction still needed a supply of
neutrons
In early 1939 it became apparent that a
number of neutrons (2 to 3) were emitted
per fission.
A nuclear chain reaction was possible.
Neutron Slowing Down Theory
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Using computers of the day (??) Fermi and
others developed a neutron slowing down
model
Neutrons born at high energies
Fission likelihood (cross section) changes in
relation to neutron energy
Neutrons slow down through collisions as they
move through materials (loose energy)
Factors Affecting Chain
Reacting System
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Composition of Fuel
Composition of Surrounding Materials
Physical Arrangement of Fuel
Physical Arrangement of other Materials
Quantity of Fissionable Isotope in Fuel
Four Factor Formula
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Simplified Algebraic View of Chain Reaction
Material Potential for Self Sustaining Chain
Reaction (Infinite Media)
K∞ = ή έ p f
ή Average Number of Fast Neutrons
έ Neutrons from high to low energy
p Resonance Escape Probability
f Thermal Utilization
Six Factor Formula
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Simple algebraic representation estimating the chain reacting
state of a physically real system
Keff = K ∞ Pnlth Pnlfst
P represents the non-leakage probability for either fast or
thermal neutrons
P represented by a formula largely dependent on physical
geometry of system
The solution of the relative geometric size values given certain
materials will yield the “critical size” of the reactor
Fermi and colleagues would undertake a famous experiment in
this regard
An important point: The systems self sustaining capability is
not dependent on the magnitude of the neutron population
Moving from Subcritical to
Critical Conditions
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The previous discussion focused on a chain
reacting system in a a steady state condition
A physical reactor however must be
assembled, and started up, hopefully not at
the same time
The concept of neutron kinetics or reactor
kinetics is useful and fundamental
Basic Kinetics and Inhour
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It can be shown, that the time dependent
neutron population in a reactor is:
Φ(t) = Φ0 e (t/Tp)
Tp is known as the reactor period or time for
reactor power to increase by a factor of e.
Reactor period can be estimated from certain
changes in Keff
These relationships demonstrate that Tp will
establish a stable value shortly after a change
in Keff
Basic Kinetics and Inhour
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These relationships will show that for a certain range
of Keff the reactor will be supercritical on delayed
neutrons
For certain large values of Keff the reactor would be
supercritical on prompt neutrons (not controllable)
Delayed neutrons come from fission products, some
of which have neutrons in the decay scheme. These
“delayed neutrons” enter the chain reacting system.
This time delayed contribution to the chain reaction
acts a natural control mechanism.
Inhour Approximation for
Changes in Neutron Population
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Neutron population is important because
fission rate is proportional to neutron
population
Neutron population and fission rate starts low
(at an artificial source level)
By changing reactor composition (control
rods) the population is allowed to increase (by
several orders of magnitude) to increase power
Control rods are re-adjusted to restore
criticality and maintain constant power
Basic Kinetics and Inhour
Basic Kinetics and Inhour
(Startup of CP 1)
Affects of Fission on
Surrounding Materials
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Fission Products deposit kinetic energy very near the site
of fission. Potential for high localized heat production
Heat must be transported from the fuel to avoid melting
the fuel material.
Following reactor shutdown, subcritical state, decay of
fission products generate heat. Heat must be removed to
avoid melting the fuel
Fission Products may exist in gaseous state in fuel rods
and represent a potential hazard
Radiation from fission and decay products requires
shielding of immediate environment (Particularly high
energy neutrons and gamma)
Development of Modern
Methods and Tools
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Current nuclear analysis for neutron transport, radiation
dose analysis and heat transfer relies on sophisticated
computer models
Typical neutron transport is based on neutron group
theory or a modified version of group theory.
Heat Transfer and Fluid Mechanics use various numerical
methods solution techniques for Navier-Stokes differential
equations.
The four factor and six factor formula are largely only of
interest from a historical perspective
Reactor Technology
Development
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Chicago Pile Number One
First Man Made Chain Reacting System
Chicago Squash Court
Team led by Enrico Fermi
Graphite blocks with Uranium Slugs in the blocks
Taken Critical in December 1942
Operated a very low “neutron flux” (power levels)
Crude safety features, no physical containment structure
Early Production Reactors
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By 1943 Large Plutonium production reactors were under
construction in the middle of the Eastern Washington
Desert
Design was large 28 by 36 foot graphite cylinder on its
side
Approximately 200 tons of small uranium plugs in
aluminum tubes (1000)
Approximately 75000 gallons per minute of cooling water
flowing over the uranium inside the tubes for cooling
Irradiate each plug for about 100 days then push it out to
decay for about 60 days
Plugs would go to chemical separation plants to extract
Plutonium for first atomic bomb
Developmental Reactors
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The Atomic Energy Act of 1946 as amended in 1954
established the Atomic Energy Commission to oversee
weapons production and the development of commercial
nuclear power
A number of different reactor types were investigated in this
early phase of reactor development
The Nuclear Navy Under Admiral Rickover functioned under the
AEC umbrella
By 1950 the Navy had focused on a Pressurized Water Reactor
Design for a propulsion plant
Prototype PWR plant in 1953
Nautilus launches in 1954
The pressurized water reactors place as dominant reactor
design is set
Shippingport
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The first commercial nuclear power generating
station
Pressurized water reactor design based on
naval prototype
Ground broken in 1954 and generated
electricity to the grid in 1958
Part of President Eisenhower’s Atoms to
Peace Program
Concept of Fission Product Barriers as a
Primary Safety Concept
Light Water Reactors
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The United States 103 power reactors
are all light water reactors
The moderator/coolant is ordinary water
There are two designs, boiling water
reactor (BWR) and pressurized water
reactor (PWR)
The PWR is the dominant reactor design
Typical Pressurized Water
Reactor
Safety Considerations in Early
Reactor Designs
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Defense in Depth and Fission Product Barriers
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The fuel cladding
The reactor coolant boundary
Containment structure
An early AEC analysis of severe accidents raised concern
over large scale fission product release
Concerns in the public regarding atmospheric weapons
testing became more pronounced
Remote siting and engineered safety features as
alternatives were debated in early decisions
AEC at that time had a dual mandate, reactor technology
development and reactor safety regulation
Development of Reactor
Regulation
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In the 1940s and 1950s a form of governmental entity
was emerging, arguably as the dominant “branch of
government”
The administrative agency was becoming a major policy
setting, law making, law enforcing entity
Administrative Agencies typically have congressional
delegations of power that allow rulemaking, adjudication
and enforcement
These powers are parallel to legislative, judicial and
executive powers
The emergence of administrative agencies is coincidental
in time with the early growth of commercial atomic energy
The Atomic Energy Act
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The 1946 Act created the Atomic Energy Commission coming
out of World War II with exclusive control of nuclear weapons
and nuclear reactor technology vested in the federal
government. No civilian use authorized at that time.
The Atomic Energy Act of 1954 established the legal framework
allowing commercial operation of nuclear reactors
The 1954 Amendment to the Act established a dual
development and regulatory role for AEC regarding commercial
reactors.
The dual role of development and oversight became
increasingly problematic over a period of years
In 1974 the Energy Reorganization Act of 1974 abolished the
AEC gave the weapons production responsibility to ERDA and
created the Nuclear Regulatory Commission (NRC). The
promotional role for government was largely abolished.
Nuclear Regulatory Commission
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The NRC is a “typical” Commission based
administrative agency
There are five commissioners, nominated by
the President and approved by the Senate
There is a Chairman of the Commission
There is a professional staff of non-political
appointees (approximately 2000 now)
The professional staff act in response to
general policy direction of the Commission
Regulatory Framework
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The NRC has authority to:
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Promulgate rules
Issue Violations and Civil Penalties
Grant Licenses to Operate Reactors
Suspend or Revoke Licenses to Operate Reactors
Issue Orders
Conduct adjudicatory hearings
Federal Agency Regulations are in the Code
of Federal Regulations (CFR)
NRC regulations are in Title 10 Energy (10
CFR)
NRC Regulatory Framework
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NRC Staff and Commission Authority can be
placed in two broad categories
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Licensing Activities
• Centered in Washington DC in the Office of Nuclear
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Reactor Regulation.
Maintain and Issue Revisions to Operating Licenses
Enforcement and Inspection Activities
• Centered in Four Regional Offices with Resident
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Inspectors at Each Nuclear Power Reactor.
Southeast Region (Region IV) is Headquartered in
Atlanta
Key Regulations as Relate to
Underlying Science and External
Forces
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General Design Criteria
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The GDC were developed in the early phases of licensing initial
power reactors after the 1954 Act.
Give objective criteria which licensees were to include in the
design of facilities
Criterion 10 through 19 (Under the General Heading Protection
by Multiple Fission Product Barriers) examples
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10 CFR 50 Appendix A
14 Reactor Coolant Pressure Boundary integrity standards
16 Containment Design
These fission product barrier “regulations" reflect the scientific
principles associated with the cladding, coolant boundary and
containment vessel as engineered features
Siting Reactors 10 CFR 100.11
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As noted earlier scientific analysis had calculated potential
consequences of hypothetical accidents
In addition, public concern arising mainly from weapons fallout
emphasized the focus on public safety concerns
As a means of addressing the issue a regulation and review
process surrounding reactor siting was put into place
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Meant to weigh and consider, actual site relative to population
Engineered Safety Features
Current Regulation is 10 CFR 100.11 “Factors to be Considered
When Evaluating Site”
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Sets radiation dose limits following certain postulated accidents
Exclusion Area Two Hour Dose Limits
Low Population Zone with same Limits for the entire period of the
accident
Three Mile Island and Hydrogen
Generation (10 CFR 50.46)
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Approximately 4:00 am on March 28, 1979 an unexpected but nonemergency automatic reactor shutdown (trip or scram) occurred at TMI
Do to a number of human errors and design problem a relatively minor
operational event cascaded into a major core damage event
Late into the sequence of events concern arose regarding an
unanticipated buildup of hydrogen gas in containment
Experts debated at that time whether the hydrogen gas could reach
“explosive levels”
Combustible levels did exist however the explosive concentrations and
conditions likely did not exist (4% by volume in air)
The hydrogen was generated from a high temperature water zirconium
reaction. (Fuel clad is made of Zirconium)
Normal operating temperatures of the clad during operation are
approximately 700 to 800 F0
In the accident conditions at TMI the cladding exceeded the melting
point of Zirconium and the temperature for rapid hydrogen generation
Three Mile Island and Hydrogen
Generation (10 CFR 50.46)
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The Accident at Three Mile Island was followed
by extensive changes in regulations and
regulatory oversight
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Including the Promulgation of 10 CFR 50.46
(Acceptance Criteria for ECCS)
Includes limits for peak clad temperature, maximum
cladding oxidation and maximum hydrogen generation
Which might result from loss of cooling accidents
Concluding Remarks
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The nuclear power industry in the United States is extensively
regulated by the NRC
The Three Mile Island Accident had no documented health
effects but resulted in a loss of public trust and led to significant
improvements in operations, training, emergency planning and
regulation
The existing regulatory framework is largely a reflection of the
initial development of reactor technology and science, political
forces surrounding initial reactor licensing and the Three Mile
Island Accident
The technology is fundamentally sound from a public policy
perspective
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No greenhouse gases
Cost competitive with coal for electrical generation
Domestic Source of Energy with stable fuel cost