Three Mile Island

Accident of 1979

Knowledge 

Management Digest

NUREG/KM-0001

December 2012

Office of Nuclear Regulatory Research
Three Mile Island on April 10, 1979, 13 days after the accident.
NUREG/KM-0001 1

Table of Contents

Introduction............................................................................................................ 3

The Accident .......................................................................................................... 4

Investigations and Lessons Learned ...................................................................... 5

Regulatory Actions................................................................................................. 6

Research................................................................................................................. 8

Timelines...............................................................................................................11

Retrospectives...................................................................................................... 14

DVD Topics ......................................................................................................... 16

President Jimmy Carter observes the radiation monitors in the Three Mile Island, Unit 

2 control room on April 1, 1979, accompanied by Pennsylvania Governor Richard 

Thornburgh and the NRC’s Harold Denton (front to back).
2

Three Mile Island Unit 2 Timeline
NUREG/KM-0001 3

Introduction

Although it caused no deaths or injuries, the accident at the Three Mile Island, 

Unit 2 (TMI-2) nuclear power plant was the most serious incident in U.S. 

commercial nuclear power history. The accident began in the early morning 

hours of March 28, 1979, at the power plant near Middletown, PA, and its effects 

on nuclear safety and regulation continue to this day. Three Mile Island spurred 

the U.S. Nuclear Regulatory Commission (NRC) to tighten and heighten its 

regulatory oversight of the nuclear power industry to ensure safety for the public 

and the environment. Investigations and the implementation of lessons learned 

brought about sweeping changes in the U.S. nuclear industry. These included 

improvements in emergency response planning, reactor operator training, human 

factors engineering, radiation protection, and many other areas of nuclear power 

plant operations. The NRC has intensively studied and documented the TMI-2 

accident. 

This multimedia knowledge management digest is the first in the NRC’s NUREG 

knowledge management (NUREG/KM) series. The agency intends for NUREG/

KMs to preserve knowledge for future generations of the important historical 

events and research that have shaped the NRC’s regulatory programs.

This knowledge management digest and the supporting DVD contain the reports 

that the NRC and other government organizations issued following extensive 

investigations in the accident. Although a few key documents have become 

available electronically, this NUREG/KM marks the first time that the NRC has 

digitized these historically important reports. On the DVD, the table of contents 

on the main welcome screen lists these reports and provides access to them.

In addition to the large collection of reports, the DVD also provides a multimedia 

presentation of the special NRC event, “The Accident at Three Mile Island—A 

Look Back: Preserving the Institutional Memory after 30 Years.”
4

The Accident

The sequence of certain events—equipment malfunctions, design related 

problems, and operator errors—led to a partial meltdown of the TMI-2 reactor 

core, resulting in a very small offsite release of radioactivity. In an atmosphere 

of growing uncertainty about the condition of the plant, on the morning of March 

30, 1979, Pennsylvania Governor Richard Thornburgh advised people within a 

5-mile radius of the plant to stay indoors. He also advised pregnant women and 

preschool-age children to leave the area.

The NRC sponsored the Special Inquiry Group to investigate the accident and 

write NUREG/CR-1250. Volume 1 of that report contains an informative and 

nontechnical discussion of the accident, including onsite actions and offsite 

responses. Another good resource on the accident is the main report by the 

President’s Commission on the Accident at Three Mile Island. Additional 

volumes of both these reports provide technical details.

The NRC’s training manual on reactor safety (NUREG/CR-6042, Section 2.2) 

provides a technical summary of the accident. The training manual also provides 

an overview of the regulatory implications of the accident (Section 2.3). 

Shortly after the accident, NRC staff gathers in the NRC trailer office at Three Mile Island.
NUREG/KM-0001 5

The chronology of events can be found in all of the above documents. The 

“Backgrounder” written by the NRC’s Office of Public Affairs and in the NRC’s 

1979 annual report NUREG-0690, Chapter 2 contain additional nontechnical 

summaries of the accident. These and other reports on the accident are provided 

on the accompanying DVD (see the section Accident Overview).

Investigations and Lessons Learned

Two weeks after the accident, 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 “Kemeny Report” was issued in October 

1979. The NRC published its initial response to the Presidential Commission’s 

recommendations in November 1979 as NUREG-0632, “NRC Views and 

Analysis of the Recommendations of the President’s Commission on the 

Accident at Three Mile Island.” A subsequent report, NUREG-1355, “Status of 

Recommendations of the President’s Commission on the Accident at Three Mile 

Island—A Ten-Year Review,” updated that initial response to include each of the 

44 recommendations made by the Kemeny Commission over a 10-year period.

To help gain a comprehensive insight into the 

accident, the NRC sponsored both internal and 

external investigations. The NRC asked the 

independent Special Inquiry Group, known as the 

“Rogovin Committee” to perform an investigation. 

The Special Inquiry Group provided a thorough 

analysis and assessment of the causes and implications 

of the accident. As already noted, the NRC published 

this work in NUREG/CR-1250, “Three Mile Island, 

A Report to the Commissioners and to the Public.” 

Working internally, the NRC created a LessonsLearned Task Force. The NRC task force identified 

and evaluated safety concerns originating from the 

TMI-2 accident that required licensing actions at other 

nuclear power plants. The NRC published the task 

force’s conclusions in NUREG-0578, “TMI-2 Lessons 

Learned Task Force Status Report and Short-Term 

Recommendations,” and NUREG-0585, “TMI-2 

Lessons Learned Task Force Final Report.”
6

Many other 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 (see NUREG-0558); the NRC’s Advisory Committee 

on Reactor Safeguards; the Bulletins and Orders Task Force of the NRC’s Office 

of Nuclear Reactor Regulation (see NUREG-0645); the NRC’s former Office 

of Inspection and Enforcement’s Special Review Group (see NUREG-0616), 

the Task Force on Lessons Learned (see NUREG-0600), and investigation of 

information flow during the accident (see NUREG-0760); the NRC’s Siting Task 

Force (site location requirements for nuclear power plants); the NRC’s Emergency 

Preparedness Task Force; the Staff Panel on the Commission’s Determination 

of an Extraordinary Nuclear Occurrence (see NUREG-0637); the NRC’s Office 

of Nuclear Regulatory Research; and the NRC’s former Office of Standards 

Development.

The DVD provides many of the key investigation reports on TMI (see sections 

Government Investigations and NRC Investigations and Lessons Learned).

Regulatory Actions

The NRC implemented a number of regulatory actions resulting from 

investigations and lessons learned reviews, and completed many more “spin-off” 

actions in the decades following the accident. The first wave of actions that 

the NRC approved, included NRC orders to individual licensees and generic 

communications, such as bulletins and generic letters issued to nuclear power 

plant licensees. The NRC used these regulatory tools in the days to months 

following the accident.

Once the various investigative groups had documented their findings, the 

Commissioners considered the recommendations. The agency consolidated 

all of the recommendations that the Commission approved into NUREG-0737, 

“TMI Action Plan,” published in 1980. The plan included approximately 371 

individual requirements. The NRC found that of these, 13,863 action plan items 

were applicable when reviewed against each specific licensed nuclear power plant. 

Some of these requirements involved changes to the internal NRC organization, 

processes, and practices. A few requirements caused the Commission to issue 

policy statements and specific changes to the NRC’s regulations through the 

rulemaking process. Both of these long-term regulatory tools required extensive 

internal and external stakeholder involvement, and were completed during the 

1980s. NUREG-0933, “Resolution of Generic Safety Issues,” documents the 

prioritization and closeout of the TMI Action Plan requirements.
NUREG/KM-0001 7

Generic Communications—Within 3 days of the accident, the NRC’s Office 

of Inspection and Enforcement issued a series of bulletins instructing all 

operating power plant licensees to take a number of immediate actions to avoid 

repeating several of the events that occurred during the accident, and which 

contributed significantly to its severity (Bulletins 79-05, 05A, 05B, 05C, 06, 

06A, 06B, 06C, and 08). The bulletins and related evaluations also substantially 

informed other staff activities, such as those associated with the generic study 

efforts of the Bulletins and Orders Task Force (NUREG-0645) and the Lessons 

Learned Task Force (NUREG-0585). The NRC issued other types of generic 

communications including generic letters that transmitted information and 

usually required action or a response, and information notices related to issues 

in which the licensees considered action appropriate. The DVD provides most 

of the generic communications relevant to TMI-2 (see the section titled Generic 

Communications and Policy Statements).

TMI Action Plan—Each of the investigating groups organized their 

recommendations in a different way. These recommendations were collected and 

transformed into discrete, scheduled tasks in NUREG-0660, “NRC Action Plan 

Developed as a Result of the TMI-2 Accident.” NUREG-0737, “Clarification of 

TMI Action Plan Requirements,” documents the specific items from NUREG-0660 

that the Commission approved for implementation at nuclear power plants. In 

October 1980, the NRC issued NUREG-0737 for implementation in Generic Letter 

80-090. This report comprises specific items, and includes additional information 

about schedules, applicability, method of implementation review, submittal dates, 

and clarification of technical positions. The Generic Issues Program prioritized and 

tracked the TMI Action Plan items contained in NUREG-0660 and NUREG-0737. 

NUREG-0933, “Resolution of Generic Safety Issues,” documents the closeout of 

action items.

The DVD provides the above NUREGs and other select NUREGs that were 

associated with the implementation and closeout of TMI Action Items (see the 

section titled Industry Wide Regulatory Actions).

Rulemaking and Regulatory Guides—In most cases, the NRC implemented 

the TMI Action Plan issues through the review of new licensee applications 

and the imposition of confirmatory orders and specific license conditions (not 

through specific changes to the NRC’s regulations). In several instances, 

implementation of the TMI Action Plan resulted in new or modified regulations. 

These regulations, and a few others that could be considered relevant to the TMI 

accident, included the following: upgrading emergency planning regulations 

in 1980, requirements related to hydrogen control in Mark I and Mark II 

containments for boiling water reactors (BWRs) in 1981, upgrading operations 
8

personnel and staffing requirements at nuclear power plants in 1983, issuing a 

licensee event report rule in 1983, requirements related to hydrogen control in 

Mark III containments for BWRs and ice condenser containments for pressurized 

water reactors (PWRs) in 1985, improving the backfitting process for power 

reactors in 1985, improving personnel dosimetry processing in 1987, updating 

the operator licensing requirements in 1987, and mandating participation in the 

Emergency Response Data System program in 1991.

In general, a regulatory guide (RG) describes one acceptable method for 

implementing agency regulations. Regulatory guides are not substitutes 

for regulations, and the NRC does not require that licensees comply with 

them. Notable regulatory guides that the NRC revised or created during the 

implementation of the TMI Action Plan included RG 1.8, “Qualification and 

Training of Personnel for Nuclear Power Plants”; RG 1.97, “Criteria for Accident 

Monitoring Instrumentation for Nuclear Power Plants”; RG 1.101, “Emergency 

Planning and Preparedness for Nuclear Power Reactors”; and RG 1.149, “Nuclear 

Power Plant Simulation Facilities for Use in Operator Training and License 

Examinations.” NRC annual reports provided on the DVD summarize relevant 

rulemakings and regulatory guides (see the section titled NRC Annual Reports).

NRC Commission Policy Statements—Several policy statements that the 

Commission issued were directly or indirectly related to the TMI Accident. A 

policy statement is not a regulation and does not impose specific regulatory 

requirements, but rather provides the Commission’s rationale and motivation 

for future regulatory positions. Two notable policy statements that continue to 

have far-reaching regulatory applications include “Severe Reactor Accidents 

Regarding Future Designs and Existing Plants,” issued in 1985, and “Safety 

Goals for the Operations of Nuclear Power Plants,” issued in 1986. The former 

provided the basis for redirecting NRC research programs and other regulatory 

programs, while the latter provided the basis for backfitting and regulatory 

analyses, and for the use of probabilistic risk assessment in risk-informed 

decisions on plant-specific changes to the licensing basis. The DVD contains 

NRC Commission policy statements relevant to TMI-2 (see the section titled 

Generic Communications and Policy Statements).

Research

The NRC’s approach to safety changed dramatically after the TMI-2 accident. 

The agency’s emphasis shifted from providing safety by relying on the traditional 

design basis approach to relying on a multifaceted approach, which emphasized 

improved operations, human factors considerations in control rooms and 
NUREG/KM-0001 9

emergency procedures, realistic performance of systems and containments under 

severe accident conditions, and probabilistic risk assessments (PRAs) to identify 

generic and plant-specific vulnerabilities. NRC research and development 

(R&D) programs also reflected this change.

After the accident, the scope and diversity of the NRC’s efforts in severe 

accidents increased substantially. Some examples included allocating additional 

resources to R&D activities, such as the construction of new experimental 

facilities, the development of analytical tools, and, in general, the development 

of the information needed to gain greater insights into severe accident behavior. 

Since the accident, the NRC, and to some extent the industry, have developed 

a large body of information on severe accidents, including the probability of 

severe accidents, core-melt phenomenology, associated accident sequences, and 

the effects of severe accidents on plant systems, components, and structures, 

especially those that provide barriers to 

fission product release to the atmosphere, 

such as the reactor containment structure.

The U.S. Department of Energy (DOE) 

also supported an extensive research 

program, known as the “TMI-2 Accident 

Evaluation Program,” to develop a 

consistent understanding of the accident. 

The primary objective of the program 

was to develop an understanding of core 

damage progression in the upper core 

region, the heat up and the formation and 

growth of the molten central region of the 

core, the relocation of approximately 19 

metric tons of debris to the lower head, 

and the release of fission products to the 

reactor vessel and the containment.

Collaboration between the Government 

and nuclear industry organizations also 

contributed important post accident 

research. In 1980, four organizations—the 

General Public Utilities (GPU), the Electric Power Research Institute (EPRI), the 

NRC, and the DOE—formed a group known as “GEND,” signing a coordination 

agreement to implement the TMI-2 Information and Examination Program. This 

program executed R&D activities relating to the cleanup of TMI-2 and the study 

of the accident for the enhancement of nuclear power safety and reliability. The 

Closed-circuit television inspections 

of the reactor core revealed a rubble 

bed approximately 5 feet below the 

normal location of the top of the fuel 

assemblies.
10

group documented the results of this program in a series of about 200 GEND and 

GEND-INF (informal) reports, as well as many other technical reports issued by 

the NRC, the DOE and its laboratories, EPRI, and GPU.

The DVD provides many of the NRC reports, which document R&D activities 

relating to the TMI-2 accident and its unique recovery and cleanup (see the 

section titled Recovery and Research Activities). GEND reports are also 

provided on the DVD (see the section titled GEND Research Reports). NRC 

annual reports provide summaries of NRC research activities (see section titled 

NRC Annual Reports).

At left is a status page 

recorded at the NRC 

Operations Center on the 

second day of the accident 

showing temperature 

readings that were measured 

by plant technicians with 

a handheld voltage meter 

on the wires from reactor 

core exit thermocouples. 

These thermocouples 

were temporary installed 

to measure coolant 

temperatures above the 

reactor fuel in support of 

startup testing of the new 

reactor. We now know that 

many of these temperature 

readings were measured at 

newly formed false junctions 

located in the “liquefied 

fuel” regions of the core. 

Today, all pressurized water 

reactors are required to 

have redundant, qualified, 

Class 1E instrumentation 

for core exit thermocouples, 

sub-cooling margin monitors, 

and reactor coolant 

inventory monitors.
NUREG/KM-0001 11

Timelines

Plant Recovery and Cleanup Milestones—By the close of the weekend of 

March 30/April 1, 1979, TMI-2 was determined to be in a stable shutdown 

condition. On Sunday, April 1, 1979, President Jimmy Carter, Rosalynn 

Carter, and Governor Thornburgh visited the damaged unit. Within 2 weeks, 

the governor’s precautionary advisory to pregnant women and preschool-aged 

children to vacate the area within a 5-mile radius of TMI was lifted. Within 1 

month, the reactor was placed in natural circulation, with decay heat flowing 

to the condenser and out of the cooling towers. The purging of radioactive 

Krypton gas from the reactor building atmosphere, followed by the first reactor 

building entry by two utility staff, was accomplished after the first anniversary 

of the accident. Shortly before the second anniversary, the reactor was placed in 

loss-to-ambient cooling mode, where heat losses are transferred into the reactor 

building ambient and out through the reactor building fan coolers.

The reactor vessel head was removed in 1985, while the removal of loose fuel 

debris from the reactor vessel began later that year, and was completed in 5 years. 

Workers used numerous manual and hydraulically powered long-handled tools 

to perform a variety of functions, such as pulling, grappling, cutting, scooping, 

and breaking up the core debris. In 1986, 7 years after the accident, a special 

Workers perform defueling operations from a shielded defueling work platform (DWP), 

which is located 9 feet above the reactor vessel flange. The platform has a rotating 

surface with 6-inch steel shield plates, and is designed to provide access for defueling 

tools and equipment into the reactor vessel.
12

core sampling drilling rig was used to reveal the extent of the hard crust layer 

of the damaged core (see inside back cover). Between 1986 and 1990, the DOE 

shipped the fuel debris by rail to the DOE site at the Idaho National Laboratory 

and placed the debris in wet pool storage for study and safekeeping. The 

evaporation of 2 million gallons of slightly contaminated accident-generated 

water at TMI was completed in 1993 over a 2- to 3-year period.

In late 1993, TMI-2 was placed in NRC-approved post-defueling monitored 

storage, a passive monitored state in which the plant will remain until the 

licensee decommissions both units simultaneously. From 2000 to 2001, the 

fuel debris was moved into NRC-approved aboveground, dry storage at a DOE 

facility to await final disposal.

NRC Actions at TMI—The NRC played an active role as the regulator, first by 

responding to the accident, then by approving and overseeing recovery activities 

by the licensee and its contractors. A team began to form with the arrival of the 

NRC’s Region I inspectors during the first day of the accident, and continued 

to expand throughout the weekend with the arrival of engineers, scientists, and 

public affairs specialists from headquarters and other regional offices. Early 

in 1980, the NRC established the TMI Program Office (TMIPO) within the 

NRC’s Office of Nuclear Reactor Regulation to provide overall direction for the 

TMI-2 cleanup operations and inspections (see NUREG 0698). About 30 NRC 

employees staffed the onsite TMIPO office during the first few years.

Important NRC actions during TMI-2 recovery included:

• Issuance of the environmental assessment (NUREG-0591) and Commission 

order to use the Epicor-II system in the decontamination of intermediatelevel contaminated accident water in the auxiliary building in October 1979.

• Issuance of the Commission order to implement the TMI-2 recovery 

technical specifications in February 1980 (NUREG 0647).

• Issuance of the environmental assessment (NUREG-0662) and Commission 

order to purge radioactive krypton gas from the reactor building atmosphere 

in June 1980.

• Issuance of the NRC action plan for cleanup operations in July 1980 

(NUREG-0698).

• Issuance of the draft Programmatic Environmental Impact Statement 

(NUREG-0683) for public comment in August 1980 and implementation in 

February 1981.
NUREG/KM-0001 13

• Creation of the public Advisory Panel for the Decontamination of Three Mile 

Island in November 1980 (see NUREG/CR 6252).

• Issuance of the Commission Policy Statement endorsing the Programmatic 

Environmental Impact Statement in May 1981.

• Issuance of the safety evaluation report on the use of the Submerged 

Demineralizer System to process the highly contaminated accident-generated 

water in the reactor building sump and reactor coolant system in June 1981 

(NUREG 0796).

• Issuance of an amendment to the memorandum of understanding with the 

U.S. Department of Energy in March 1982 to accept the entire reactor core 

and highly radioactive solid waste for research and long-term storage (see 

NUREG-0698, Revision 2, Appendix A).

• Approval of the licensee’s safety evaluation for the refurbishment and use of 

the reactor building polar crane in November 1983.

• Issuance of Supplement 1 to the Final Programmatic Environmental 

Statement (NUREG-0683) on dealing with occupational radiation dose in 

October 1984.

• Issuance of certificates of compliance in April 1986 for two shipping casks to 

be used for shipment of fuel debris to DOE by rail.

• Issuance of Supplements 2 and 3 to the Final Programmatic Environmental 

Statement (NUREG-0683) on the final disposal of 2.1 million gallons of 

slightly contaminated accident generated water in June 1987, and on 

dealing with post-defueling monitored storage and subsequent cleanup in 

August 1989.

• Approval in April 1989 to use the evaporator for the final disposal of 2.1 

million gallons of accident-generated water.

The NRC wrapped up onsite NRC involvement at TMI 2 by the time that the 

agency terminated the TMI-2 Project Directorate in February 1988. The NRC’s 

TMI resident inspector office took over the inspection program for TMI-2 and a 

headquarters project directorate assumed responsibility for the technical review 

and project management functions. In September 1993, the NRC approved the 

“post-defueling monitored storage” at TMI-2 and associated license conditions 

and technical specifications. The last meeting (78th overall) of the 10-member 

public Advisory Panel for the Decontamination of Three Mile Island Unit 2 was 

held in September 1993 (see NUREG/CR-6252 for lessons learned).
14

The timeline feature on the DVD provides a summarized chronology of key 

recovery milestones and NRC actions. Users may read a short description of 

many of the actions by clicking on the text on the timeline from the interactive 

window on the DVD (see the section titled Timeline).

Retrospectives

For the 30th anniversary of the TMI-2 accident, the NRC hosted a special all-day 

event titled, “The Accident at Three Mile Island—30th Anniversary, A Look Back: 

Preserving the Institutional Memory.” The event featured several presentations, 

including an overview of the accident by Brian Sheron, Director of the NRC’s 

Office of Nuclear Regulatory Research; perspectives from TMI-2 Reactor 

Operator Edward Frederick; lessons learned from TMI-2 by Gary Holahan, 

Deputy Director of the NRC’s Office of New Reactors, and former member of 

the TMI-2 Lessons Learned Task Force; and a historical perspective by NRC 

Historian Sam Walker.

The NRC issued certificates of compliance to the Department of Energy in April 1986 for 

two shipping casks used to ship fuel debris by rail to the Department of Energy’s Idaho 

National Laboratory.
NUREG/KM-0001 15

Following the individual presentations, the NRC hosted a panel discussion titled 

“Remembering the Accident.” The panel included a distinguished group, all of 

whom made key decisions during the accident: Richard Thornburgh, former 

Governor, State of Pennsylvania; Harold Denton, former Director, NRC Office 

of Nuclear Reactor Regulation, and lead Federal onsite manager; and Jessica 

Mathews, former assistant to President Jimmy Carter.

The DVD provides a multimedia presentation of this event (see section titled 

Multimedia).

A press conference during the early days of the accident with Pennsylvania Governor 

Richard Thornburgh (right) and Director of the NRC’s Office of Nuclear Reactor 

Regulation, Harold Denton (left).
16

DVD Topics

To help navigate the DVD, the outline below provides topics and headings that 

the user can access directly from the interactive window. The user may also click 

on the text on the Timeline for a short description and related documents.

Important Note: Many of the documents on the DVD are historical in nature, 

and may contain information that is obsolete or superseded by today’s regulations 

and research results. Please refer to the NRC’s public Web site (www.nrc.gov) 

for current information on regulations, Commission policy statements, regulatory 

guidelines, regulatory processes, and research results.

Main Welcome Screen

– Introduction by Dr. Brian Sheron, Director of the Office of Nuclear 

Regulatory Research, NRC

– List of Documents Found on this DVD

– Knowledge Management Digest (NUREG/KM-0001)

• Accident Overview

• Timeline

• Historical Documents

○ Government Investigations

○ NRC Investigations and Lessons Learned

○ Generic Communications and Policy Statements

○ Industrywide Regulatory Actions

○ GEND Research Reports

○ Recovery and Research Activities

○ NRC Annual Reports

• Multimedia

○ NRC Special 30th Anniversary Event

○ Photo Gallery
NUREG/KM-0001 17

Three Mile Island Accident of 1979

Knowledge Management Digest

Prepared by:

Brian McGrattan

Don Marksberry

Felix Gonzalez

Carolyn Siu

Amy Bonaccorso

Mark Henry Salley

Contributions by:

Publications Branch in the Division of Administrative Services of the 

NRC Office of Administration

The Department of Energy’s Idaho National Laboratory 

Dickinson College Community Studies Center 

National Archives and Records Administration

Independent spent fuel storage installation for the dry storage of the fuel debris from 

TMI-2 located at the Department of Energy’s Idaho National Laboratory. 
18

TMI-2 Core End-State Configuration. In 1988, the NRC reported the final state of the 

damaged TMI-2 reactor core, shown above. The accident was terminated by reflooding 

of the core. This action did not immediately stop further core melting, but it did prevent 

the core from melting through the reactor vessel.

Notes: (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 

(NUREG/CR-6042).
NUREG/KM-0001 19

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. Later, a core boring machine (lower right) 

was used to break apart once liquefied materials. 
This DVD was developed using Visual Basic Professional 2008. Make sure you 

have Windows XP and Visual Basic framework 3.5 or above for the DVD to 

work correctly.

NUREG/KM-0001

December 2012