INTERIM STAFF GUIDANCE FCSE-ISG-15: NATURAL PHENOMENA 

HAZARDS IN FUEL CYCLE FACILITIES 

Purpose

The staff is issuing this interim staff guidance (ISG) to provide additional guidance for evaluating 

events that may result from natural phenomena hazards (NPH). The staff is required to assure 

compliance with Title 10 of the Code of Federal Regulations (10 CFR) Part 70, “Domestic 

Licensing of Special Nuclear Material,” Subpart H, “Additional Requirements for Certain 

Licensees Authorized To Possess a Critical Mass of Special Nuclear Material.” This guidance 

provides criteria and methods that the staff can use to review the treatment of NPH at fuel cycle 

facilities as evaluated in the facility integrated safety analysis (ISA) and described in the license 

application. 

Regulatory Framework

For facilities regulated under 10 CFR Part 70, Subpart H, 10 CFR 70.62(c)(1) requires, in part, 

that each licensee conduct and maintain an ISA that is of appropriate detail for the complexity of 

the process and that identifies, among other things, “potential accident sequences caused by 

process deviations or other events internal to the facility and credible external events, including 

natural phenomena.” The regulations in 10 CFR 70.62(c)(1) also require, in part, that each 

licensee or applicant identify the consequences and the likelihood of occurrence of each 

potential accident sequence and the methods used to determine the consequences and 

likelihoods. 

The regulations in 10 CFR 70.61, “Performance requirements,” requires, in part, that individual 

accident sequences resulting in high consequences to workers and the public be “highly 

unlikely” and that sequences resulting in intermediate consequences to these receptors be 

“unlikely.” 

For new facilities or new processes at existing facilities, 10 CFR 70.64(a)(2), “Natural 

phenomena hazards,” requires that the design must provide for adequate protection against 

natural phenomena with consideration of the most severe documented historical events for the 

site. 

Discussion

The staff is issuing this ISG to provide additional guidance to the NRC staff for the review of fuel 

cycle facilities ISA evaluation of accident sequences that may result from NPH. . This ISG will 

be incorporated into future revisions of Appendix D of Chapter 3 of NUREG-1520, “Standard 

Review Plan for the Review of a License Application for a Fuel Cycle Facility.” Specific 

emphasis was provided on seismic hazards due to recent events such as the Fukushima Daiichi accident and recent updates to the U.S. Geological Survey hazard curves. 

The purposes of the review of a licensee or applicant’s treatment of NPH are to support findings 

that the licensee’s safety program and facility, equipment, and procedures are sufficient to 

provide reasonable assurance that the performance requirements of 10 CFR 70.61, and other 
2 

regulatory requirements can and will be met. 10 CFR 70.61 requires that credible high 

consequence events be “highly unlikely” and credible intermediate consequence events be 

“unlikely”. 10 CFR 70.62(a)(2) requires the licensee to develop and maintain the information 

that demonstrates that these requirements are met. This licensee evaluation involves 

identifying accident sequences, which start with an initiating event, intermediate failure events 

(usually of item relied on for safety (IROFS) or structures) leading to a particular accident, often 

a release of hazardous material, identifying the consequences of the accident, and the overall 

likelihood of the accident sequence. The next step is to determine that the performance 

requirements have been met. Typically for NPH this involves showing that either (a) the 

frequency of the NPH is less than 1x10-6 per year (NPH is not credible for location i.e., 

hurricanes in Washington state), or (b) that failures induced by the NPH that can cause high 

consequences are “highly unlikely” or intermediate consequences are “unlikely.” The 

acceptance criteria for unlikely and highly unlikely are addressed in the guidance provided in 

Chapter 3 of NUREG-1520, Revision 1, “Standard Review Plan for the Review of a License 

Application for a Fuel Cycle Facility.” For each natural hazard phenomenon a frequency versus 

magnitude typically can be obtained, from which the applicant’s ISA can relate likelihood to the 

magnitude of that phenomenon that can cause an event leading to high or intermediate 

consequences. 

Additionally, implementation of 10 CFR Part 70 requirements for existing facilities may vary 

because of different definitions of likelihood proposed by licensees to comply with 10 CFR 

70.65(b)(9). Chapter 3 of NUREG-1520 describes the acceptance criteria for licensee- and 

applicant-proposed definitions of the terms highly unlikely and unlikely as they apply to the 

development and maintenance of the ISA. The staff review should include an assessment of 

the implementation of the definitions of likelihood in the ISA for natural phenomena events. The 

staff’s review should focus on ensuring that the definitions are consistent with accepted 

standards associated with each natural phenomena event. The implementation of the 

definitions of highly unlikely and unlikely may differ for each natural phenomena event, as 

applicable. However, there are several factors that should be considered when evaluating how 

licensees both define and implement unlikely and highly unlikely for natural phenomena events 

within the ISA. This is because there is no common approach to determine how probabilistic 

hazards are determined for various NPH and because different natural phenomena present a 

unique set of hazards to each facility. Therefore, the resulting performance of the facility under 

the various NPH and the contribution to overall risk should be considered. 

For example, some structures at existing fuel cycle facilities were built to a building code with 

design-basis earthquake having exceedance probabilities of 2x10-3 per year to less than 1x10-3 

per year. The current building code uses ground motions with a 4x10-4 annual exceedance 

probability. In contrast, standard building codes and industry standards require seismic designs 

based on probabilistic ground motions with 10 percent probability of exceedance in 50 years or 

2 percent probability of exceedance in 50 years. 

Guidance For Evaluation Of NPH ISA Events For Fuel Cycle Facilities

The guidance in “Seismic Hazards” in Appendix D of NUREG-1520 provides factors for 

evaluating earthquake-related events. Another factor to consider is the likely rate of release 

based on the damage sustained. For example, some facilities may lose dynamic confinement 

but maintain building integrity. In some processes, radiologically and/or chemically hazardous 

material is held inside its primary containment at subatmospheric pressure. Furthermore, an 
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earthquake that results in limited subatmospheric containment losses may allow adequately 

trained workers to evacuate and/or take mitigative actions. In other cases, buildings may be 

equipped with a seismically activated interlock (an IROFS) that will shut off the building’s 

heating, ventilation, and air conditioning system during an event, thus limiting any leakage of 

UF6 to the outside. 

The majority of fuel cycle licensees completed their ISAs after Subpart H of 10 CFR Part 70 was 

promulgated1

 in September 2000. These ISAs, in general, postulated that structures, systems 

and components (SSCs) will remain intact during credible seismic events. In some cases, it 

was concluded that a high radiological or chemical consequence was highly unlikely based on 

the assumption that the SSCs will adequately perform their safety functions during postulated 

NPH events. The staff should review the basis for the assumptions used in the ISA to ensure 

that adequate documentation (e.g., design basis information) exists to support the expected 

performance of SSCs and/or potential consequences of failures of SSCs. 

The license application and ISA must address natural phenomena events (e.g., tornadoes, 

hurricanes, and earthquakes) and other external events with a sufficient level of detail to 

characterize and assess their impact on facility safety and compliance with the requirements of 

10 CFR 70.61. The licensee’s assessment should indicate which events are considered to be 

not credible or highly unlikely and the basis for that determination. This assessment should also 

indicate which events could occur without adversely impacting safety and the basis for that 

determination. In order to comply with the regulatory requirements to prevent or mitigate the 

consequences of NPHs, licensees may need to maintain access to or possess equipment that is 

capable of limiting the consequences to public and worker health and safety in the event of 

multiple credible challenges and degraded or disabled resources. These protective measures 

could include long-term loss of functions, such as offsite power, onsite emergency power, offsite 

water supply, other offsite services, and transportation to access offsite resources. The 

licensee’s assessment should also identify the assumptions such as the design bases for the 

SSCs credited for prevention or mitigation of the consequences to the facility for these types of 

events. 

In order to meet the requirements of 10 CFR 70.62(c)(1), licensees should also evaluate the 

completeness of events as discussed in the ISA if current information about NPHs indicates that 

unanalyzed events may be credible and result in accidents that exceed the performance 

requirements of 70.61. The staff’s review should assure that the description of the site and the 

facility, the design basis, the underlying data, and the assumptions are appropriate to the 

current condition of the facility as documented in the current ISA. 

Evaluation of Structures and Components 

Licensees’ safety analyses or licensing and design bases should consider credible NPHs. As 

previously discussed, licensees should also evaluate the completeness of events as discussed 

in the ISA if current information about NPHs indicates that unanalyzed events may be credible. 

 

1

 Refer to 10 CFR 70.62(c)(3) which required, in part, that existing licensees submit for NRC approval, by 

April 2001, a plan that described the ISA approach; and by October 2004, or in accordance with the 

approved plan, a completed ISA. It also required licensees to identify performance deficiencies and to 

correct them with adequate compensatory measures. 
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This evaluation could lead to potential changes to the ISA assumptions regarding the treatment 

of a particular NPH event. Reviewers should evaluate the following: 

1. SSCs are designed and constructed to perform the safety function needed to meet the 

performance requirements with sufficient design basis documentation. 

2. The ISA evaluation identifies those SSCs that are designated as IROFS. 

 

3. The likelihood information (e.g., hazard curves) or consequence attributed to/related to 

an NPH needs to be accurate and clearly defined. 

4. Configuration management is established to evaluate additions, modifications, or 

deterioration that may lead to significant physical change in SSCs that can affect the 

intended safety function. 

5. Occurrence of an NPH, especially an earthquake, may affect multiple SSCs in a facility. 

This could lead to multiple concurrent failures. 

6. The likelihood of failure and potential consequences are valid for the type of event. 

Additional Considerations for Existing Processes At Existing Facilities

The following supplements the guidance in “Considerations for Existing Processes at Existing 

Facilities” from Appendix D of NUREG-1520. Many existing fuel cycle facilities were designed 

and constructed using applicable building codes and standards adopted by State or local 

authorities at the time the facility was constructed. These building codes and standards 

established minimum requirements for providing safety to life and property from seismic 

hazards. This goal of providing safety of life and property is accomplished through the 

specification of prescriptive criteria to achieve adequate performance of a structure to ensure its 

capability to withstand a defined intensity of earthquake ground motions. The development of 

seismic design criteria is an ongoing process of improvement which is reflected in the evolution 

of seismic criteria in building codes and standards. For example, a facility may have been 

designed and constructed with building codes and standards that contained criteria for seismic 

loads resulting from earthquakes having accelerations associated to 10 percent probability of 

exceedance in 50 years. If adequate documentation exists, such as design calculations or 

reference material, these facilities may be able to demonstrate that their SSCs will adequately 

perform with a low likelihood of structural collapse under the specified accelerations. 

On the other hand, some fuel cycle facilities were designed and built to a building code that did 

not prescribe criteria for seismic hazards. For example, most facilities built in the eastern United 

States were designed without consideration of potential earthquake hazards. Given that these 

licensees still must consider seismic hazards as part of their ISA, the licensee may be able to 

make an assessment using the as-built condition of the facility. This assessment includes a 

review of existing documentation such as drawings and construction specifications to identify 

as-built characteristics of the SSCs. The assessment can identify information such as 

properties of materials used in construction, structural systems used, and elements that can 

affect seismic performance. Walk-downs of the facility can identify and confirm as-built data 

gathered as well as identify deviations from original drawings. From walk-downs, the licensee 

can identify and provide special emphasis on components that can affect operations with 
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hazardous materials. The NRC reviewer should assure that the assessment demonstrates that 

the performance requirements have been met. 

Performance-Based Evaluation of Seismic Events

Licensees may demonstrate how portions of their facility meet the performance requirements of 

10 CFR 70.61 by using analytical seismic evaluation methods to demonstrate the performance 

of the SSCs in the facility and how structural failures of those SSCs may lead to consequences 

of concern from seismic events. Performance-based design is an engineering approach, where 

the design process is structured to achieve performance requirements (structural limit states) 

specified by owners or licensees to meet a risk level to ensure adequate protection of public 

health and safety. Demonstration of performance can be done by selecting “performance 

objectives” that combine a desired performance level with a specified earthquake hazard. The 

concept of performance objectives is used widely in building codes and standards, such as the 

International Building Code, and standards for nuclear facilities, such as American Nuclear 

Society (ANS) 2.26, “Categorization of Nuclear Facility Structures, Systems, and Components 

for Seismic Design.” The performance objectives can be established by licensees and 

applicants to take into consideration the radiological and chemical consequences of failures of 

SSCs. If these radiological and chemical consequences align with the performance 

requirements in 10 CFR 70.61, the NRC reviewer can validate compliance. The goal of seismic 

design calculations from building codes and standards is to provide a low risk of 

earthquake-related death or life threating injuries from a collapse of structures. When 

evaluating performance-based structural analyses for NPH, the reviewer should also consider 

how the licensee addresses the following: 

• Concurrent failures; 

• Proper assignment of an SSC performance goal with adequate construction methods 

based on accepted standards that provide acceptably low risk; 

• Defined seismic design categories and limit states that are adequately supported by 

analyses (graded approach); 

• Documented qualitative or quantitative values of the critical design parameters at which 

the SSC fails to perform its safety function; 

• Consideration of functions such as emergency response, control rooms, etc.; and 

• Limited deformations of design equipment and subsystems in order to maintain safety 

function. 

Graded Approach to Consideration of NPH 

The use of a graded approach to demonstrate compliance with applicable regulatory 

requirements is outlined in Appendix B of Chapter 3 of NUREG-1520 as it relates to the ISA. A 

graded approach recognizes the diversity of regulated facilities and processes, potential 

hazards, and safety functions needed to prevent accident sequences and/or mitigate the 

consequences of events from NPH. A graded approach can be used by applicants and 

licensees to place SSCs into different categories such that the required level of analysis, the 
6 

complexity of the design or evaluation approach, documentation, and actions are 

commensurate with: 

• The relative importance to safety of the structure or component or the amount of risk 

reduction attributed to the structure or component to meet the performance 

requirements; 

• The particular characteristics of the structure or component, and the complexity and 

potential consequences that could result in the event of failures; and 

• The magnitude of the NPH. 

The objective of building codes and standards for NPH protection is to design structures to a 

“life safety” performance level to prevent structural collapse or failures that could endanger lives 

or prevent safe egress of personnel. The reviewer should verify that the licensee’s approach 

considers how failures of components can affect regulated material, affect mitigation measures, 

and produce releases that lead to consequences that exceed the performance requirements of 

10 CFR Part 70. In reviewing the graded approach, the reviewer should consider how the 

licensee: 

• Prevents loss of structural integrity that could lead to a release in excess of the 

performance requirements; 

• Prevents loss of capability to perform functions important to safety during and/or after 

the natural phenomena event that could lead to consequences to the public and/or 

worker; and 

• Confines hazardous material. 

A prioritization or graded approach that considers risk may also serve as a decision tool to 

identify how deficiencies will be corrected. For example, aspects of the load bearing elements 

of the building structure that house hazardous material may be evaluated further using more 

complex methodologies. Using the prioritization approach, the licensee may establish a plan to 

retrofit or perform detailed analyses of critical SSCs first and then provide a plan with a 

schedule to evaluate SSCs relied on for a safety function of a lower risk reduction. 

The reviewer should ensure that the licensee’s “evaluation basis seismic hazard,” to serve as 

the input to the seismic re-evaluation, provides for adequate protection to meet the performance 

requirements in 10 CFR 70. The reviewer should consider if the evaluation basis event 

adequately characterizes conditions at the site such as soil properties and local seismicity. If 

the licensee’s evaluation of an SSC identifies a deficiency in that it does not meet the criteria 

under the selected hazard, the reviewer should evaluate the retrofit plan which may include 

several options considered by the licensee to address the deficiency. Such options may include 

• Conduct a more rigorous evaluation of the deficiencies to account for potential 

conservatism not accounted for in the simplified evaluation; 

• Retrofit the SSC to improve resistance to the selected hazard; 
7 

• Modify usage of the SSC to eliminate the hazards; and 

• Incorporate a combination of mitigation/prevention strategies to reduce the materials at 

risk (seismic isolation valves, bunkers, etc.). 

Other Considerations 

The regulatory requirements in 10 CFR 70.62(c)(1) require licensees to conduct and maintain 

an ISA. Consistent with the regulations and guidance in NUREG-1520, licensees should 

establish a process to periodically re-evaluate changes to the natural phenomena data and data 

collection methods, modeling techniques (hazard curves), and assessment methods to ensure 

assumptions are still valid. If the assumptions used for determining credibility of NPH events 

change, such as updated building codes and standards, the staff should assess the licensee’s 

documentation of the processes used to evaluate any impacts to the current safety basis that 

could be affected by these changes. 

The effects from failures and impacts to the facility from natural phenomena events should be 

considered in radiological safety, chemical safety, nuclear criticality safety and fire safety 

assessments. Particular attention should be given to the potential for natural phenomena 

events to cause multiple failures. Licensee emergency response functions should consider the 

necessary actions to prevent or mitigate the potential consequences of a natural phenomena 

event (i.e., criticality, disabled water supply, loss of electrical power, delayed off-site response). 

For example, licensees can establish procedures for assessing damage to facilities after severe 

natural phenomena events as well as emergency response procedures for re-entry to the 

facilities considering the effects of potential failure to alarms and instrumentation. 

The operation of fuel cycle facilities inherently involves multiple significant hazards, such as: 

large inventories of UF6, uranium dioxide (UO2) and triuranium octoxide (U3O8) powder, fissile 

material that can go critical, process and byproduct chemicals such as hydrogen fluoride (HF) 

and nitric acid (HNO3), and similar process hazards. Because NPH events can affect large 

areas of a facility, emergency response functions should consider how they would respond to 

events with multiple consequences and activities at the same time, which would normally 

require different responses.
The reviewer should ensure that the licensee’s “evaluation basis seismic hazard,” to serve as 

the input to the seismic re-evaluation, provides for adequate protection to meet the performance 

requirements in 10 CFR 70. The reviewer should consider if the evaluation basis event 

adequately characterizes conditions at the site such as soil properties and local seismicity. If 

the licensee’s evaluation of an SSC identifies a deficiency in that it does not meet the criteria 

under the selected hazard, the reviewer should evaluate the retrofit plan which may include 

several options considered by the licensee to address the deficiency. Such options may include 

• Conduct a more rigorous evaluation of the deficiencies to account for potential 

conservatism not accounted for in the simplified evaluation; 

• Retrofit the SSC to improve resistance to the selected hazard; 

• Modify usage of the SSC to eliminate the hazards; and 

• Incorporate a combination of mitigation/prevention strategies to reduce the materials at 

risk (seismic isolation valves, bunkers, etc.). 

Other Considerations 

The regulatory requirements in 10 CFR 70.62(c)(1) require licensees to conduct and maintain 

an ISA. Consistent with the regulations and guidance in NUREG-1520, licensees should 

establish a process to periodically re-evaluate changes to the natural phenomena data and data 

collection methods, modeling techniques (hazard curves), and assessment methods to ensure 

assumptions are still valid. If the assumptions used for determining credibility of NPH events 

change, such as updated building codes and standards, the staff should assess the licensee’s 

documentation of the processes used to evaluate any impacts to the current safety basis that 

could be affected by these changes. 

The effects from failures and impacts to the facility from natural phenomena events should be 

considered in radiological safety, chemical safety, nuclear criticality safety and fire safety 

assessments. Particular attention should be given to the potential for natural phenomena 

events to cause multiple failures. Licensee emergency response functions should consider the 

necessary actions to prevent or mitigate the potential consequences of a natural phenomena 

event (i.e., criticality, disabled water supply, loss of electrical power, delayed off-site response). 

For example, licensees can establish procedures for assessing damage to facilities after severe 

natural phenomena events as well as emergency response procedures for re-entry to the 

facilities considering the effects of potential failure to alarms and instrumentation. 

The operation of fuel cycle facilities inherently involves multiple significant hazards, such as: 

large inventories of UF6, uranium dioxide (UO2) and triuranium octoxide (U3O8) powder, fissile 

material that can go critical, process and byproduct chemicals such as hydrogen fluoride (HF) 

and nitric acid (HNO3), and similar process hazards. Because NPH events can affect large 

areas of a facility, emergency response functions should consider how they would respond to 

events with multiple consequences and activities at the same time, which would normally 

require different responses. 

DISTRIBUTION: 

FCSE r/f 

 ML15121A044 *BY EMAIL

OFFICE FCSE/PORSB OGC* NMSS/PORB NMSS/FCSE NMSS/PORSB 

NAME JMarcano NStAmour MKotzalas MBailey MKotzalas 

DATE 5/21/ 2015 5/25/ 2015 5/21/ 2015 6/26/ 2015 6/26/2015 

OFFICIAL RECORD COPY