3 Mile island.pdf

Three Mile Island Unit 2
Case Study Overview
IAEA International Project on Managing the Decommissioning and
Remediation of Damaged Nuclear Facilities (DAROD)
31 August – 4 September, 2015
John P. Clements
Health Physicist,
Division of Decommissioning, Uranium Recovery, and Waste Programs

Accident Overview
• A combination of equipment malfunctions, design-related
problems and worker errors led to TMI-2's partial meltdown
and very small off-site releases of radioactivity

Accident Progression
• At about 4 a.m. on Wednesday, March 28, 1979, TMI-2
experienced a failure in the secondary, non-nuclear section
of the plant
• A mechanical or electrical failure prevented the main
feedwater pumps from sending water to steam generators
that remove heat from the reactor core
• The plant's turbine-generator and then the reactor itself
automatically shut down
• Immediately, pressure in the primary system (the nuclear
portion of the plant) began to increase

• To control pressure, a pilot-operated relief valve (a valve
located at the top of the pressurizer) opened -The valve
should have closed when the pressure fell to proper levels,
but it became stuck open.
• Control room instruments incorrectly indicated that the valve
was closed leaving staff unaware that cooling water was
pouring out of the stuck-open valve
• As coolant flowed from the primary system through the
valve, other instruments available to reactor operators
provided inadequate information, and no instrument showed
how much water covered the core

• Plant staff assumed if the pressurizer water level was high,
the core was properly covered with water and did not realize
the plant was experiencing a loss-of-coolant accident
• Water escaping through the stuck valve reduced the primary
system pressure so much that reactor coolant pumps had to
be turned off to prevent dangerous vibrations
• To prevent the pressurizer from filling completely, staff
reduced emergency cooling water amounts into the primary
system – starving reactor core of coolant, causing it to
overheat
• Fuel overheated to the point at which the zirconium cladding
ruptured and the fuel pellets began to melt
• Later found that about half of the core melted during the
early stages of the accident

• TMI-2 reactor is permanently shut down and all its fuel has
been removed
• Reactor coolant system is fully drained and the radioactive
water was decontaminated and evaporated
• Radioactive waste from the accident was shipped off-site to
an appropriate disposal area, and the reactor fuel and core
debris was shipped to the Department of Energy's Idaho
National Laboratory
• Current plan is to keep the TMI-2 facility in long-term,
monitored storage until the operating license for the TMI-1
plant expires, at which time both plants will be
decommissioned
Current Conditions at TMI-2

• A specialized U.S. NRC team was established shortly after
the accident - including representatives of the NRC’s Office
of Inspection and Enforcement (IE), the 5 NRC Regions,
and the Office of Nuclear Reactor Regulation (NRR)
• A Public Affairs Office was established in Middletown, PA,
and staffed on a 24-hour basis
Regulatory Approaches/Issues

• The NRC site team initially supported emergency response
functions
• Within days of the accident, the site team performed on-site
recovery activities, which can be broken down into four
major areas:
1) Reviewed system modifications and system additions
2) Reviewed all procedures (emergency and normal
operation and maintenance) which were necessary to
post-accident activities
3) Provided close and continuous monitoring for the
operations
4) Provided consultation, review, and analysis of the
ongoing rad-waste, cleanup, and health physics
activities

• The NRC’s licensing process was generally utilized to
allow for remediation/decommissioning of TMI-2
• For example, NRC Orders were issued to:
• Suspend operating license and require maintenance in
a shutdown condition per NRC approved procedures
• Permit the usage of the Epicor-II filtration and ion
exchange system to decontaminate intermediate level
waste water
• Incorporate revised technical specifications to reflect
the damaged condition
• Permit the licensee to purge the reactor building of
krypton-85
• Public comments were sought and several Environmental
Assessments were prepared

• The overall NRC regulatory approach was impacted as a
result of the TMI accident, both in the short term and the
long term
• Short Term (days to months) – The NRC issued several
orders to individual licensees and issued generic
communications, such as bulletins and generic letters, to
all nuclear power plant licensees
• Long Term (months to years) - Investigations and the
implementation of lessons learned brought about
sweeping changes in the U.S. nuclear industry – including
improvements in emergency response planning, reactor
operator training, human factors engineering, radiation
protection, and many other areas of nuclear power plant
operations

• Fuel melting and reactor core damage
had occurred
• Contaminated coolant water remained
in the basement of the containment
building
• The containment building was
contaminated but remained in-tact and
functional
• First entry into the reactor building
containment was conducted by two
utility staff on July 23, 1980 (image to
the right)
Technical Issues – Post Emergency
State of the Facility

Technical Issues – Post Emergency
State of the Core
1. Cold leg Loop 2B inlet
2. Cold leg Loop 1A inlet
3. Cavity
4. Loose core debris
5. Crust
6. Previously molten material
7. Lower plenum debris
8. Hard layer debris
9. Damaged in-core instrument guide
10. Hole in baffle plate
11. Coating of previously molten
material on bypass region interior
surfaces
12. Upper grid damage

• Decontamination of intermediate-level contaminated water
(defined as less than 3.7 MBq/mL [100 µCi/mL]) in the
auxiliary building
• Environmental Assessment issued and approval
granted to use the Epicor-II filtration system
• Purging of the reactor building atmosphere
• Environmental Assessment and evaluation completed
• Controlled and filtered purge accomplished over a 14
day period, per NRC approved procedures
• The maximum cumulative radiation dose and the
maximum dose rate measured at off-site locations were
a fraction of the limits allowed under NRC regulations
Technical Issues/Challenges

EPICOR-II System

• Decontamination of highly contaminated wastewater
• The NRC Approved the use of the Submerged
Demineralizer System (SDS)
• The SDS operated underwater, in one of the spent fuel
pools of TMI Unit 2
• It consisted of a liquid waste treatment subsystem, a
gaseous waste treatment subsystem, and a solid waste
handling subsystem
• The approval to operate the SDS did not include water
disposal

• Removal of fuel debris from the reactor
• Operators removed damaged fuel and structural debris
from the reactor vessel by “pick and place” defueling of
loose core debris
• Workers performed defueling operations from a
shielded defueling work platform (DWP) located nine
feet above the reactor vessel flange
• The DWP had a rotating 17-foot diameter surface with
six-inch steel shield plates, and was designed to
provide access for defueling tools and equipment into
the reactor vessel

Shielded defueling work platform (DWP)

Numerous manual and hydraulically powered long-handled tools
were used to perform a variety of functions, such as pulling,
grappling, cutting, scooping, and breaking up the core debris. These
tools were used to load debris into defueling canisters positioned
underwater in the reactor vessel.

• Microorganisms inside the reactor vessel
• A large population of microorganisms developed in the
reactor coolant system (RCS), clogging cleanup system
filters and hindering the view of defueling activities
• Licensee conducted a multi-phase program to restore
water clarity consisting of high-pressure hydrolancing,
the addition of hydrogen peroxide, and the use of a
high-pressure positive displacement pump
• A diatomaceous earth (swimming pool-type) filter was
operated in conjunction with the letdown and makeup of
batches of reactor coolant, to remove the organic
material and improve the clarity of the RCS water

• NRC and DOE signed Memorandum of Understanding
(MOU) on waste disposal
• The MOU formalized the working relationship
between the two agencies with respect to the
removal and disposal of solid nuclear waste
generated during the cleanup of TMI-2
• Significant step toward ensuring the TMI site would
not become a long-term waste disposal facility
• The MOU covered only solid nuclear waste, and did
not cover liquid waste resulting from cleanup
activities
• DOE also agreed to accept fuel and highly
radioactive resins from the water purification system

• NRC and DOE signed Memorandum of Understanding
• MOU addressed three basic categories of TMI-2 waste:
1) waste determined by DOE to be of generic value in
terms of beneficial information to be obtained from
further research and development activities
2) waste determined to be unsuitable for commercial
land disposal because of high levels of
contamination, but which DOE may also undertake
to remove, store, and dispose of on a reimbursable
basis from the licensee
3) waste considered suitable for shallow land burial,
to be disposed of by the licensee in licensed,
commercial low-level waste burial facilities

• Epicor resin waste disposal
• The NRC approved the licensee’s request to dispose
of Epicor resin liners via shallow land disposal, as
they were similar to typical reactor resin wastes
• Several shipments were also made to various
laboratories for testing purposes
• Submerged Demineralizer System resin waste disposal
• Submerged Demineralizer System liners were sent
to DOE
• DOE conducted research on glass vitrification
(solidification) of this type of solid waste at Hanford

• Disposal of slightly contaminated water
• The licensee was approved for a treatment/disposal
method involving the forced evaporation of the
water contaminated during the accident and used in
subsequent cleanup operations at the TMI site - to
be completed over a 2.5 year period
• Residue from this operation, containing small
amounts of the radioactive isotopes cesium-137 and
strontium-90, and large volumes of boric acid and
sodium hydroxide, would require solidification and
disposal as low-level waste

Institutional Framework
and Strategic Planning

• Organizational arrangements/structures shifted from
operational NRC oversight prior to the accident, to an
augmented on and off-site emergency response
structure during the emergency phase, and to a new
structure enhanced by lessons learned after the
emergency
• Changes to the NRC’s organizational framework
occurred in the establishment of a TMI Program Office
and a TMI-2 Project Directorate, and an Office for
Analysis and Evaluation of Operational Data (AEOD)
• The NRC formed a 12 member Public Advisory Panel
including local citizens, local and state governmental
officials, and scientists, meeting regularly with both the
public and NRC Commissioners

• The pre-existing regulatory and political decision
making processes were generally adequate in that the
NRC’s licensing process was utilized for many
decisions
• The current post defueling monitored storage state of
TMI-2 represents a unique approach as it placed the
facility into monitored storage for an unspecified period
of time
• Funding provisions are provided by the licensee, who
provides annual funding status and cost analysis
reports

• Several investigation committees were formed shortly
after the accident, which influenced
organizational/structure changes
• The President of the United States, Jimmy Carter,
appointed a 12-member Presidential Commission to
investigate the accident at Three Mile Island. This
group, known as the “Kemeny Commission,”
conducted a comprehensive investigation of the
accident and made recommendations based upon
their findings
• The NRC sponsored both internal and external
investigations, and asked the independent Special
Inquiry Group, known as the “Rogovin Committee”
to perform an investigation
Strategic Planning

• Many groups, both internal and external to the NRC,
also performed separate investigations.
• These included:
• The U.S. Congress and its General Accounting
Office (GAO)
• The Ad Hoc Dose Assessment Group, which
comprised various Federal agencies
• The NRC’s Advisory Committee on Reactor
Safeguards
Strategic Planning

• The TMI accident permanently changed both the
nuclear industry and the NRC
• Public fear and distrust increased
• NRC's regulations and oversight became broader and
more robust
• Management of the plants was scrutinized more
carefully
• Careful analysis of the accident's events identified
problems and led to permanent and sweeping changes
in how NRC regulates its licensees – which, in turn, has
reduced the risk to public health and safety
Institutional Framework Changes

• Upgrading and strengthening of plant design and
equipment requirements
• Includes fire protection, piping systems, auxiliary
feedwater systems, containment building isolation,
reliability of individual components (pressure relief
valves and electrical circuit breakers), and the ability
of plants to shut down automatically
• Identifying the critical role of human performance in
plant safety led to revamping operator training and
staffing requirements, followed by improved
instrumentation and controls for operating the plant,
and establishment of fitness-for-duty programs for plant
workers to guard against alcohol or drug abuse

• Enhancing emergency preparedness, including
requirements for plants to immediately notify NRC of
significant events and an NRC Operations Center
staffed 24 hours a day
• Drills and response plans are now tested by licensees
several times a year, and state and local agencies
participate in drills with the Federal Emergency
Management Agency and NRC
• Requirement for plant specific simulators established
• Integrating NRC observations, findings, and
conclusions about licensee performance and
management effectiveness into a periodic, public report

• Having senior NRC managers regularly analyze plant
performance for those plants needing significant
additional regulatory attention
• Expanding NRC's resident inspector program
• Expanding performance-oriented as well as safety-
oriented inspections, and the use of risk assessment to
identify vulnerabilities of any plant to severe accidents
• Strengthening and reorganizing enforcement staff in a
separate office within the NRC
• Establishing the Institute of Nuclear Power Operations,
and formation of what is now the Nuclear Energy
Institute to provide a unified industry approach to
generic nuclear regulatory issues, and interaction with
NRC and other government agencies

• Installing additional equipment by licensees to mitigate
accident conditions, and monitor radiation levels and
plant status
• Enacting programs by licensees for early identification
of important safety-related problems, and for collecting
and assessing relevant data so operating experience
can be shared and quickly acted upon; and
• Expanding NRC's international activities to share
enhanced knowledge of nuclear safety with other
countries in a number of important technical areas

• Three Mile Island Accident of 1979 Knowledge Management Digest,
NUREG/KM-0001, December 2012, U.S. NRC Office of Nuclear
Regulatory Research
• U.S. NRC Backgrounder on the Three Mile Island Accident,
http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html,
Page Last Reviewed/Updated Friday, December 12, 2014
• U.S. NRC Regulatory Process for Decommissioning & Actions Related to
Damaged Facilities after Accident, Larry W. Camper, Director Division of
Waste Management and Environmental Protection, U.S. Nuclear
Regulatory Commission, USA, IAEA International Expert s’ Meeting on
Decommissioning and Remediation after Nuclear Accident, January 28 –
February 1, 2013, Vienna, Austria,
http://www-pub.iaea.org/iaeameetings/IEM4/29Jan/Camper.pdf
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