Nuclear Research

Programs To Ensure

Public Health and

Safety

September 2001
Contents

Introduction.......................................................................1

Fuel Behavior and High Burnup Fuel........................................2

Plant Aging................................................................................3

Plant Material Conditions...........................................................4

Spent Fuel and Waste Storage.................................................4

Digital Instrumentation and Controls..........................................4

Thermal Hydraulics and Severe Accident Codes......................5

Probabilistic Risk Analysis.........................................................6

New Reactor Designs................................................................7

Radiation Protection...................................................................8

Decommissioning....................................................................8

Operational Data Assessment...................................................8

Human Performance..................................................................9

Use of Technical Standards and Codes....................................9
1

Introduction

Nuclear Safety

Research

NRC Headquarters

The Nuclear communities. Its mission includes: (1) 

Regulatory complementing the front-line regulatory 

Commission is activities of licensing, inspection and 

responsible for oversight, (2) independently examining 

assuring safety evolving technology and anticipated 

in the design, issues, (3) striving to be a center of 

construction, and excellence, and (4) having a major role in 

operation of every strategic arena in implementing 

commercial key strategies of NRC’s Strategic Plan.

nuclear facilities 

and in the other RES contributes to the agency’s 

uses of nuclear regulatory decision-making by providing 

materials, such technical advice, analytical tools and 

as in medicine information for staff to identify and 

and industrial resolve safety issues, make regulatory 

activities. As a decisions, develop regulations and 

key component guidance, conduct independent analyses 

of nuclear safety, to support decisions to grant or deny 

the NRC carries licensee proposed changes, renew plant 

out a research operating licenses, evaluate operating 

program to experience, and evaluate proposed 

provide designs and technologies, and enhance 

independent efficiency and effectiveness of NRC 

information and expertise needed to programs and processes. 

support the NRC’s decision-making 

process and to identify and characterize As part of its critical role at the agency, technical questions that may become the office coordinates research activities important safety issues in the future. within the NRC and leads the agency’s NRC’s regulatory research is designed to initiative for cooperative research with improve the agency’s knowledge in the U.S. Department of Energy, the areas where uncertainty in knowledge nuclear industry, universities, and exists, where safety margins are not well international partners. Research characterized, and where regulatory sponsored by the office is performed decisions need, or will need to be primarily by national laboratories of the confirmed. Thus, the development of Energy Department, but universities and sound technical bases allows proper private firms also are used. Further, the focus on safety issues and more RES staff possess broad knowledge in realistic decisions. many scientific and engineering fields 

and is frequently called upon for its 

The NRC’s research program is carried expertise by other offices of the NRC.

out by the Office of Nuclear Regulatory 

Research (RES). RES develops its RES’ efforts focus on a wide range of program with consideration of technical issues such as those discussed Commission direction and input from in this booklet. program offices and other stakeholders, 

including the academic and international 
Reactor accidents involving a large absorption of a neutron. The fissile 

release of radioactivity can only happen material is used up and converted to 

when fuel melts. There are only two energy and fission products. This 

ways to melt fuel: (1) too much power, process causes changes in the structure 

and (2) not enough coolant (water). Too of the fuel and results in a gradual 

much power can develop if there is a deformation of the fuel and a reduction 

loss of reactivity control and insufficient in the neutron population and thus a 

coolant, and insufficient coolant can less efficient nuclear reaction. The total 

result from a large water leak in the energy released in fission by a given 

reactor. The NRC amount of nuclear 

requires that fuel is called the fuel 

reactors be burnup and is 

designed such that measured in 

they can withstand megawatt days 

major reactivity (Mwd). The fission 

accidents and loss- energy released per 

of-coolant accidents unit mass of the fuel 

with very limited fuel is termed specific 

damage such that burnup of the fuel and 

melting could not is usually expressed 

occur. Postulated in megawatt days per 

accidents of these metric ton (Mwd/t).

types are studied 

using experiments Fuel damage criteria, 

and analyses to established in the 

demonstrate that 1970s, were 

fuel damage will be developed for normal 

adequately limited. operation and several 

These accidents are postulated accidents. 

being reexamined to The fuel damage 

confirm that recent criteria were defined 

increases in nuclear in terms of the 

power plant output amount of energy 

or newer alloys used released from the 

in metal rods fuel. The industry, for 

housing nuclear reactor fuel preserve economic reasons, has requested and, 

the original low limits on fuel damage. the NRC has approved, fuel burnup to 

higher levels than was originally 

Nuclear fuel for light water reactors anticipated. RES is focused on a 

release energy through fission (the reevaluation of NRC regulatory criteria 

splitting of a uranium atom). The fuel for fuel damage and modification of 

consists of uranium dioxide ceramic related analytical tools for higher burnup 

pellets that contain both fissile and fuels. It is essential to investigate the 

fertile materials. Fissile material is behavior of the fuel and fuel rods under 

material in which fission can be caused high burnup conditions to confirm fuel 

by neutrons with low energy. A fertile integrity and reliability under these 

material is a substance which can be conditions. 

converted into fissionable material by 

2

Fuel Behavior and

High Burnup Fuel

Examination of Fuel Rods

NRC
3

Plant Aging

A better of degraded 

understanding of tubes, causes 

the effect of age- of degradations, 

related and methods 

degradation on for evaluating 

structures and and maintaining 

passive integrity. This 

components (e.g., helps ensure 

buried piping, that the tubes 

masonry walls, are designed 

and flat bottom to maintain 

tanks) is needed structural 

to ensure that integrity under 

adequate margins operating and 

are maintained under all design load postulated accidental conditions. 

conditions for the current and any 

extended operating life of nuclear power Reactor Pressure Vessel Internals

plants. This research assembles the 

age-related degradation data base for The cylindrical, steel reactor vessel structures and passive components, and its internal equipment that identifies their age-related degradation houses the fuel rods provide the mechanisms, develops a technical heat source for the generation of evaluation of the effects of their steam to turn a turbine for the degradation and the significance to risk, production of electricity. Cracking of for more risk-informed licensing reactor pressure boundary decisions. components, such as piping, 

continues to be observed. 

Steam Generator Tube Integrity Degradation of these components 

could cause leaks or breaks that 

Steam generators are devices which could impair the ability to shut down 

use the heat of water circulating and cool the reactor core. Issues 

through the reactor core to generate being addressed by RES include: 

non-radioactive steam to drive (1) crack initiation and growth, (2) 

turbines. The hot radioactive water cracking due to stress corrosion, 

flows through steam generator tubes and the effects of irradiation on 

heating the non-radioactive water stainless steel.

which is outside the tubes. Thus, the 

tubes form a critical boundary, Structures

preventing the release of 

radioactivity and, therefore, are Aging structures can lead to designed to maintain their structural changes in structural properties, integrity under operating and which could lead to weakening and postulated accident conditions. failure. Degraded structures are of Various forms of degradations have particular concern when subjected occurred in steam generator tubes. to external loads such as 

earthquakes. Risk studies for 

With safety paramount, the steam degraded structures are being 

generator integrity research program conducted.

assesses the adequacy of inspection 

for identification and characterization 

Measuring a crack discovered in a weld in

a reactor coolant system pipe.

Nuclear Research Programs To Ensure Public Health and Safety
4

Plant Material

Conditions

This research area focuses on the Certification of Compliance for 

evaluation of structural integrity and intermediate burnup fuel. The second is 

material conditions of components and the licensing of dry cask storage for high 

systems in nuclear power plants and burnup fuel. RES is developing the 

other nuclear facilities under normal technical basis for ensuring the 

operating and accidental conditions. The continued safe performance of dry 

research addresses material exposure to storage systems for long-term storage 

reactor operating temperatures, stresses, of spent nuclear fuels and high-level 

irradiation environments, water coolant radioactive waste under extended 

chemistry, cyclic loading, and general service conditions, 20 to 100 years.

wear. Research also includes evaluation 

of non-destructive examination methods 

for performing in-service inspections of Digital Instrumentation

plant components and systems. and Controls

Spent Fuel and Waste The general direction of the nuclear 

industry is to replace their analog Storage instrumentation and control equipment 

with digital equipment because of the 

difficulty in replacing analog equipment There are two licensing issues associated modules and the advantages of newer with the long-term storage of spent equipment. It is expected that plants will 

retro-fit their protection systems, control 

systems, and eventually the majority of 

their control rooms. This will result in a 

mixture of analog and digital (hybrid) 

equipment that will require complex 

interfaces with an increased potential 

for errors. For example, industry is 

currently developing new modern control 

and diagnostic systems to improve 

reliability, and advanced neutron and 

flow sensors to reduce the uncertainty of 

power measurements. While digital 

technology has the capability to improve 

both operational performance and 

safety, there are challenges to the 

introduction of this technology into 

nuclear power plants. The everchanging digital technology with its 

complexity and unique failure modes 

requires research programs to provide 

a safety focus and guidance to the 

nuclear fuel. The first is the renewal of industry with sufficient lead time.

existing dry cask storage licenses and 

Storage casks at the Surry Nuclear

Power Plant in Virginia

NRC
5

Thermal-Hydraulics be balanced. The situation of either too 

Thermal-hydraulic research involves little coolant flow or too much energy 

development of computer codes and data production can result in the fuel 

that are used to model the reactor system overheating and potentially melting. 

to ensure that a balance is achieved and 

to assess the consequences of such an Additionally, RES maintains and 

imbalance and mitigating actions taken if continues to improve the usefulness of 

an imbalance occurs. computer codes that are used to analyze 

reactor operations and functions. These 

The energy source of nuclear power codes are used in analyzing many types 

reactors is the nuclear fission process that of transients and accidents. They also 

occurs within the fuel. Water is pumped are used to resolve fuel behavior 

into the reactor and passes over the fuel technical issues.

elements. 

In passing Severe 

over the fuel Accidents

elements Severe 

the coolant accident 

(water) is research 

heated, studies the 

which is detailed 

then used to behavior of 

generate reactors and 

steam. the radiation 

Various released 

forms of during such 

steam accidents and 

generators examines 

are used in means to 

nuclear prevent such 

reactors. accidents and 

The nuclear protect the 

reactor itself public. 

is the steam generator in the case of Severe accidents are that highly 

boiling water reactors, where the steam is improbable group of accidents that 

formed directly within the reactor core. involve serious, prolonged overheating of 

The steam generator is fed with water, most of the nuclear fuel which could then 

which is totally or partially evaporated to result in the release of large amounts of 

steam. The steam is then passed through radiation and radioactive materials to the 

the turbine, which drives the electrical containment and possibly the 

generator. The very low pressure exhaust environment. For some of these severe 

steam from the turbine is passed to a accidents, the radiation may escape from 

condenser, where it is converted back into the nuclear power plant and be carried 

water and recirculated to the steam by the wind exposing people in the 

generator. The removal of energy in the surrounding area.

fuel and the production of the energy must 

Thermal Hydraulics and

Severe Accident Codes

An illustration of a Boiling Water Reactor

Nuclear Research Programs To Ensure Public Health and Safety
6

Probabilistic Risk

Analysis (PRA)

PRA is an important element of the significance of operating experience on 

NRC’s regulatory processes. PRA is a a plant-specific basis. It also provides 

systematic method for incorporating trending information on industry and 

realistic information on the design and group-specific risk performance. Thus, 

operation of a facility into a logical and the ASP program provides a risk 

integrated model. Using this model, both perspective of nuclear power plant 

the overall reliability and risk of the events to inform agency response and 

facility, as well as the relative importance regulatory decisions. Analytical tools to 

of components, human actions and support these analyses are also 

errors, and potential accident initiators, developed and maintained.

can be estimated. Research is important 

to risk-informed decision-making, Risk-Informed Regulation

providing information on the likelihood, RES prepared guidance for making 

consequences, or mode of progression of risk-informed regulatory decisions. The 

postulated accidents and providing a risk guidance describes acceptable means 

context for regulatory decisions. for assessing the nature and impact of 

licensing changes when the change 

Plant Operating Data request is supported by risk information. 

Nuclear power plant operational data Another RES effort focuses on 

provide information on relevant operating providing a better balance to nuclear 

experience that is used to enhance plant plant technical requirements among 

inspections of risk-important systems, to those needed to provide defense-inperform technical reviews of proposed depth, to maintain appropriate safety 

license amendments, and to support margins, and to limit risk. For example, 

improvements in the reactor oversight RES is working to develop the technical 

process. Therefore, RES operates and basis to improve the realism of 

maintains the Sequence Coding and evaluations of reactor pressure vessel 

Search System (SCSS) that contains (RPV) integrity to support risk-informed 

information about events at nuclear modifications to the associated 

power plants in a computer-searchable regulations. The results of probabilistic 

framework. RES also operates and fracture calculations and PRA 

maintains the Common-Cause Failure considerations will be integrated to 

(CCF) database. Common-cause develop revised pressurized thermal 

failures have the potential to adversely shock screening criteria for incorporation 

impact the safety of nuclear power into the nuclear plant regulations. 

plants. Thus, review of operating events Risk-informing may involve relaxing 

at power reactors can disclose risk- requirements in some areas in 

significant interactions, phenomena, and combination with increasing 

behavior in the design and operating of requirements in other areas to 

power reactors that were not previously achieve a better balance.

recognized or analyzed.

PRA Standards

The Accident Sequence Precursor (ASP) The increased use of PRA in the 

program provides analyses of the risk regulatory decision-making process 

NRC
7

requires high-quality analyses There is a possibility that in the next 

(i.e., scope, methodology and data five to 10 years orders for new nuclear 

used in such analyses). This applies to power plants will become a reality. 

PRAs developed by industry to support However, for such orders to be realized, 

specific, risk-informed licensing actions future reactor designs must be attractive 

as well as to PRAs developed by NRC from an economic and safety 

staff to analyze specific technical issues perspective. Many of these designs 

or to support risk-informed Commission will propose features very different 

decisions. To this end, NRC has been than current reactor designs. Since 

working with the American Society of safety and licensing are major 

Mechanical Engineers (ASME) and the considerations for any future designs, 

American Nuclear Society (ANS) to it is most efficient, timely, and in the 

develop national consensus standards national interest for NRC to prepare 

on PRA. When developed, such for licensing future designs by having 

standards on PRA quality will help to early interactions with the designers 

ensure that PRAs developed in and developers as encouraged by the 

accordance with the standards are Nuclear Regulatory Commission’s 

robust, consistent, and defensible and Policy Statement on Advanced 

provide a suitable basis upon which Reactors.

regulatory decisions can confidently be 

made. While the ASME and ANS Early interaction would be to:

maintain overall responsibility for this 

effort, active NRC and industry 

participation has been, and will 

continue to be, essential to the 

development of such standards.

PRA Methods

RES is continuing to develop and 

improve methods for risk analysis to RES plays a key role in the agency 

better support realistic, risk-informed efforts to prepare for reviews of future 

decision making. Areas where work is reactors for licensing in the U.S. This 

currently underway to improve methods includes developing analytical tools, 

are plant aging, human reliability, fire, data, and guidance to support licensing 

and digital instrumentation and control reviews as well as evaluating new 

systems. Uncertainty exists concerning technologies, such as that associated 

with the proposed Pebble Bed Modular the nature and magnitude of the 

Reactor (PBMR). contributions of these aspects to plant 

risk. The new methods will complement 

Some key issues that need to be the methods developed to-date, further 

addressed through research, so as to reducing uncertainties and improving 

facilitate the licensing of future reactors realism and improving agency 

are: qualification of the fuel and its decision making. fabrication; development of the 

radiological source term; performance 

of high temperature materials;

Nuclear Research Programs To Ensure Public Health and Safety

– Understand reactor designs

– Identify safety issues and plans for 

their resolution

– Establish a framework for licensing 

that recognizes the unique features 

of these designs

New Reactor Designs
8

verification and validation of design and removing a facility from service followed 

safety analysis codes; probabilistic risk by reducing residual radioactivity to a 

assessment; use of commercial-grade level that permits the release of the 

components; and new digital systems, property for unrestricted use and, under 

hardware and software. certain conditions, restricted use. This 

includes removal of all intact radioactive 

Radiation Protection materials and the clean-up of any 

residual contamination to acceptable 

regulatory levels. Additionally, RES With regard to the NRC’s current examines environmental contamination regulations on radiation exposure based associated with a nuclear facility in on risk estimates, RES accumulates data terms of potential off-site transport of on the effects of radiation on the human radioactive material and compliance with body through several types of studies. any standards for termination of the RES uses this data to re-evaluate the operating license. current health effects of computer models 

and to evaluate health effects caused by 

long-term exposures to low levels 

of radiation.

Decommissioning Data from operating reactors is 

continually assessed to learn more 

For reactors and other facilities that are about operational safety. RES collects, 

nearing the end of their productivity, RES analyzes, and disseminates data 

has initiated studies on safe and effective accumulated during operations and 

means of decommissioning nuclear assesses trends in performance from 

these data. It evaluates operating 

experience to provide insights into and 

to improve the understanding of the risk 

significance of events that have occurred 

at licensed facilities. The evaluation 

may disclose risk-significant interactions, 

phenomena, and behaviors at power 

reactors that have not previously been

recognized or analyzed. For example, 

data accumulated from many operating 

reactors can show trends in aging of the 

reactors. Examinations of individual 

plants and events at nuclear plants is 

also used to develop guidance and 

standards on the use of risk 

assessments that are needed for the 

power plants and other nuclear facilities. agency’s reactor oversight program. 

Decommissioning is the process of safely 

Operational Data

Assessment

NRC

Dismantlement of the Trojan Nuclear

Power Plant in Oregon.
9

Human Performance research activities 

at the NRC focus on the interaction of 

people with the systems and the 

environments in which they work. The 

importance of these interactions is that 

they can strongly affect personnel 

performance and thus overall system 

safety at a nuclear facility. Indeed there 

is considerable operational evidence that 

poor human performance has been a 

factor in more than 50% of the incidents 

at nuclear power plants and an even 

greater percentage of incidents involving 

the use of nuclear materials for medical 

and industrial applications. 

At NRC we look at the performance 

of people in normal and abnormal 

conditions, and at the effects of the work 

environment, including physical and 

cognitive effects, on both individual and 

team performance. 

This includes humansystem interfaces, 

training, procedures, 

shift working hours, 

and a safetyconscious work 

environment. In 

addition, human performance research 

addresses human 

characteristics, such 

as knowledge, skills, 

and abilities, fatigue, 

and fitness for duty. 

It is important that 

human performance is supported by an 

organizational environment that promotes 

a culture of safety and provides its 

personnel with the programs, support, 

staffing, and tools necessary to perform 

the jobs safely and comfortably. 

NRC’s research program in human 

performance is anticipatory in that it

addresses emerging technology’s 

potential effect on human performance. 

In addition, human performance research 

develops and establishes the technical 

bases for NRC initiatives, such as, 

inspection guidance for evaluating 

emergency operating procedures, a 

systems approach to training, human 

system interface design for current and 

advanced control station design, human 

performance contributors in events, 

communications-related corrective action 

plans, shift working hours and fatigue 

management programs.

Use of Technical

Standards and Codes

RES coordinates the development of 

consensus and voluntary standards for 

agency use. 

Consensus 

standards are 

technical standards 

that support 

regulatory needs 

pertaining to 

materials, 

components, and 

processes. They 

are developed by 

experts from 

industry, 

universities, or 

government 

agencies. The 

agency serves on committees of the 

national and international organizations 

(i.e., American Society of Mechanical 

Engineers, the American National 

Standard Institute, the International 

Commission on Radiation Units and 

Measurements, and the National Council 

on Radiation Protection) that develop

Nuclear Research Programs To Ensure Public Health and Safety

An NRC inspector collects data at a

licensed facility.

Human Performance
FOR ADDITIONAL INFORMATION

CONTACT:

Office of Public Affairs

U.S. Nuclear Regulatory Commission

Washington, D.C. 20555

(301) 415-8200

E-mail: opa@nrc.gov

Visit our website at www.nrc.gov

10

these standards. The NRC frequently 

endorses these standards as guidance or 

adopts them into its regulations.

In short, the NRC’s Office of Regulatory 

Research goals are to maintain safety, 

increase public confidence, make NRC 

activities and decisions more effective, 

efficient, and realistic, and reduce

unnecessary regulatory burden on 

stakeholders. It carries out the agency’s 

research program to provide independent 

information and expertise needed to 

support the Commission’s decisionmaking process and to identify and 

characterize technical questions in 

anticipation of safety issues that may 

arise in the future. 
NUREG/BR-0282 Rev. 1

Office of Nuclear Regulatory

Research

U.S. Nuclear Regulatory

Commission