Safety Analysis Approaches – ISA vs. DSA – One Safety Analyst’s

Opinion

John Farquharson

jfarquharson@absconsulting.com

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Introduction

For commercial nuclear fuel cycle facilities (e.g., enrichment, fuel fabrication), the NRC requires compliance with 10 CFR 70.61 through an Integrated Safety Analysis (ISA)

For DOE nonreactor nuclear facilities, the DOE requires compliance with 10 CFR 830 through a Documented Safety Analysis (DSA)

This paper looks at similarities and differences between the ISA and DSA approach

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Similarities

Both regulations have been in existence for approximately a decade (since ~2000)

The processes analyzed are both nonreactor, nuclear facilities with similar potential accidents of interest (i.e., loss of confinement, fires, nuclear criticality accidents)

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Similarities (cont.)

Both regulations reference a standard for the structure of the safety basis documents• DOE-STD-3009 for DSAs• NUREG-1513 (ISA guidance)

Both regulations address multiple receptors• “Facility workers”• Co-located workers• Public

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Similarities (cont.)

Consequence thresholds and categories for radiation and toxic exposures are similar

Likelihood categories are generally similar (order of magnitude bins)

Both standards reference the Center for Chemical Process Safety (CCPS) “red book” for hazard analysis methodology

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Differences

ISA promotes a layer of protection analysis (LOPA) approach with an approved scenario risk matrix used to:• Judge acceptability of credited controls

– Items relied on for safety (IROFS)

• Provide guidance for probability of failure values for controls

• Screen out low likelihood initiating events

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Differences (cont.)

DSA is more consequence-driven• Qualitative guidance on acceptable controls• No allowances to screen out initiating events• No approved risk matrix• Some DOE facilities (e.g., Pu) may have potentially

higher consequences as compared to NRC-regulated ISA facilities

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General Hazard Procedure (either approach)

Perform hazard identification Perform hazard evaluation

• List all available controls Select safety controls

• IROFS for ISA• Safety class or safety significant structures, systems,

and components (SSCs) for DSA Detailed accident analysis Derive agreement for operations of controls

• Management measures for ISA• Technical safety requirements for DSA

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Main Differences Between DSA and ISA Approach

Method for acceptance of risk due to postulated operational accident

DSA – pick controls based on qualitative guidance• Engineered over administrative controls• Passive over active, etc.

ISA – guidance in risk matrix approach that factors:• Likelihood of postulated initiating event• Probability of failure on demand of IROFS

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LOPA

More quantitative than a hazard and operability (HAZOP) analysis

Less quantitative than fault tree/event tree analyses

Focuses on one scenario at a time Looks at Independent Layers of Protection (IPLs) Is another tool for judging risk

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Layers of Defense Against a Possible Accident

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LOPA is limited to evaluating a single cause-consequence pair

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IPL1 IPL2 IPL3

Initiating Eventsuccess

Consequencesexceeding criteria

Undesired, buttolerable outcome

Safe outcomesuccess

success

ConsequenceOccurs

ImpactEvent

Frequency

failure

failure

failure

Undesired, buttolerable outcome

Key:Thickness of arrowrepresents frequency ofthe consequence if laterIPLs are not successful

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DSA Guidance for Choosing Safety Controls

From DOE-STD-3009, choose controls that: 1.Are preventive over mitigative2.Reduce source term3.Are passive over active4.Are engineered over administrative5.Are nearest source 6.Have the fewest active features7.Reduce risk the most8.Are effective for other accidents…

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ISA Guidance for Choosing Safety Controls

10 CFR 70.61 – Performance Requirements

(b) The risk of high consequence events must be limited. Engineering and administrative controls shall be used to keep events highly unlikely (guidance in NUREG-1520 as <1E-5/yr) or their consequences less than high

• High consequence event– acute worker dose 100 rem – person outside controlled area dose 25 rem

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ISA Guidance for Choosing Safety Controls (cont.)

10 CFR 70.61 – Performance Requirements (c) The risk of intermediate consequence

events must be limited. Engineering and administrative controls shall be used to keep events unlikely (guidance in NUREG-1520 as <1E-4/yr) or their consequences low

• Intermediate consequence event– not a high consequence event– acute worker dose 25 rem – person outside controlled area dose 5 rem

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NUREG 1520 — Risk Matrix

Consequence Category 3

High

Likelihood Category 1:

highly unlikely

2 acceptable

1 acceptable 2 acceptable

Consequence Category 2

Intermediate

Consequence Category 1

Low

Standard Review Plan Risk Matrix

6 unacceptable 9 unacceptable

6 unacceptable

Likelihood Category 2:

unlikely

Likelihood Category 3:

not unlikely

3 acceptable

3 acceptable

4 acceptable

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Likelihood

10 CFR 70.65 requires the applicant to define the likelihood terms “unlikely,” “highly unlikely,” and “credible.” All credible high-consequence events must be highly unlikely, and credible intermediate-consequence events must be unlikely for the risk to be acceptable. Events that are not credible may be exempt from the use of controls

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Likelihood of Occurrence

Composed of the following two elements:1. The frequency of the initial event occurring despite

prevention measures

2. The reliability or effectiveness of protection measures that protect against the event progressing to the accident

a. IROFSs

i. Active engineered controls (AECs)

ii. Passive engineered controls (PECs)

iii. Administrative IROFSs

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Not Credible Events

External events < 1.0E-6/y Process deviations requiring many unlikely

human actions/errors for which there is no motive or reason

Process deviations for which a convincing argument, based on physical laws, shows that they are not possible or unquestionably extremely unlikely

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Highly Unlikely Events

Double contingency protection Likelihood index < -5 Estimated likelihood below 1.0E-5/y

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Unlikely Events

Engineered, hardware controls with high grade of management measures

Enhanced administrative controls Likelihood index > -5 and < -4 Estimated likelihood below 1.0E-4/y

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Likelihood Category

1

2

3

NUREG 1520 — Table A-8: Determination of Likelihood Category

Likelihood Index T (= sum of index numbers)

T - 5

- 5 T - 4

- 4 T

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NUREG 1520 — Table A-9: Failure Frequency Index Numbers

Frequency Index

Number

-6*

-4*

-3*

Based on Evidence

External event with frequency <10-6/yr

No failures in 30 years for hundreds of similar IROFS in industry

No failures in 30 years for tens of similar IROFS in industry

Based on Type of IROFS**

Exceptionally robust passive engineered IROFS (PEC), or an inherently safe process, or 2 independent active engineered IROFS, PEC, or enhanced administrative IROFS

A single IROFS with redundant parts, each a PEC or AEC

Comments

If initiating event, no IROFS needed

Rarely can be justified by evidence. Further, most types of single IROFS have been observed to fail.

-2*

-1

0

No failure of this type in this plant in 30 years

A few failures may occur during plant lifetime

Failures occur every 1-3 years

A single PEC

A single AEC, an enhanced administrative IROFS, an administrative IROFS with large margin, or a redundant administrative IROFS

A single administrative IROFS

1 Several occurrences per year

Frequent event, inadequate IROFS

2 Occurs every week or more often

Very frequent event, an inadequate IROFS

Not for IROFS, just initiating events

Not for IROFS, just initiating events

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NUREG 1520 — Table A-10: Failure Probability Index Numbers

Probability Index

Number

-6*

-4 or -5*

Probability of Failure on Demand

10-6

10-4 - 10-5

Based on Type of IROFS

Exceptionally robust passive engineered IROFS (PEC), or an inherently safe process, or 2 redundant IROFS more robust than simple administrative IROFS(AEC, PEC, or enhanced administrative)

Comments

If initiating event, no IROFS needed

Rarely can be justified by evidence. Most types of single IROFS have been observed to fail.

-3 or -4* 10-3 - 10-4 A single passive engineered IROFS (PEC) or an active engineered IROFS (AEC) with high availability

-2 or -3*

-1 or -2

10-2 - 10-3

10-1 - 10-2

A single active engineered IROFS (AEC), or an enhanced administrative IROFS, or an administrative IROFS for routine planned operations

An administrative IROFS that must be performed in response to a rare unplanned demand

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Footnotes for Tables A-9 and A-10

* Indices less than (more negative than) -1 should not be assigned to IROFS unless the configuration management, auditing, and other management measures are of high quality, because without these measures, the IROFS may be changed or not maintained.

** Failure frequencies based on experience for a particular type of IROFS, as described in this column, may differ from values in column 1; in this case, data from experience take precedence.

Severity of Consequences

The severity of consequences of an accident is measured in terms of resulting health effects, including fatalities or exceeding personnel exposure limits

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10 CFR 70.61 – Performance Requirements

High consequence event– Acute worker dose 100 rem – Person outside controlled area dose 25 rem– Person outside controlled area intake 30 mg

soluble U– Acute chemical exposure (from or produced by

licensed material) that could endanger a worker’s life or could cause irreversible or serious, long-lasting health effects to persons outside the controlled area

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10 CFR 70.61 – Performance Requirements (cont.)

Immediate consequence event– Not a high consequence event– Acute worker dose 25 rem – Person outside controlled area dose 5 rem– 24-hour average release of radioactive material

outside restricted area concentration > 5,000 times Table 2, App B, Part 20

– Acute chemical exposure (from or produced by licensed material) that could cause irreversible or serious, long-lasting worker health effects or mild, transient health effects to persons outside the controlled area

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Comparisons – DSA vs. ISA

DSA – qualitative guidance on picking controls

ISA – agency-wide accepted risk matrix approach

ISA – justification for operational events being “noncredible”

Same controls selected?

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