Fault Tree Analysis - AMOS Online

14 Pages·2014·0.92 MB·English

by  Peter Dunscomb

Fault Tree Analysis
Peter Dunscombe, PhD, FCCPM, FAAPM, FCOMP
Professor Emeritus
University of Calgary
1
Disclosure
Peter Dunscombe is a Member of
TreatSafely, LLC

treatsafely.org
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Purpose of a Fault Tree Analysis
To make the (radiotherapy) system safer through
using postulated failure modes, tracing the failure
pathways back and, on the basis of the FTA,
• Identifying key core structural safety features
• Designing the QA/QC Program.
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Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
1
Learning Objectives
• Identify several varieties of Fault Trees
• Point out the disastrous consequences of failing
to learn from a Fault Tree Analysis
• Use Fault Trees to help identify key core
components of a safe radiation treatment
program
• Position QA/QC activities in the Fault Tree
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Root Cause Analysis (RCA)

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Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Fault Tree Analysis (FTA)
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
2
RCA and FTA
Look similar?
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Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
RCA and FTA
A Fault Tree Analysis can be regarded as a
hypothetical Root Cause Analysis.
• An actual event starts an RCA
• Postulated failure modes are used to start and
FTA.
• However, in both, the failure pathway is traced
back.
• Postulated failure modes can be imported from
a Failure Modes and Effects Analysis.
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Fault Tree Analysis
FTAs are extensively used in high risk, high
reliability industries such as the chemical,
nuclear and aviation industries.
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
3
Varieties of Fault Trees
•A Fault Tree can be descriptive or
quantitative.
•A quantitative Fault Tree can be developed
from reported data (Thomadsen) or expert
elicitation (Ekaette).
•A Fault Tree can be extended to a Root
Cause Tree by including Basic or Root
Causes.
Thomadsen et al. IJROBP 2003 (57) 1496
Ekaette et al. Risk Analysis. 2007 (27) 1397
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Performing a Fault Tree Analysis
A Fault Tree Analysis is normally carried out
by a small team:
•Leader – knowledge of FTA and subject area
of review
•Facilitator – expertise in FTA
•Content experts – knowledge of subject area
of review and preferably multidisciplinary in
our environment.

Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Varieties of Fault Trees
1.Standard (Engineering) Fault Tree
2.Root Cause Tree
3.Probabilistic Fault Tree (data based)
4.Probabilistic Fault Tree (elicitation based)
5.TG 100’s FTA

Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
4
Engineering Fault Tree
O ring hardened
Pump fails
Motor burns out
Presvsaulvree fraeillesase
Nuclear Explosion
Fuel rods stick
Event
Maunnudael rr eretrpaacitrion
And
Or
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Ekaette’s Fault Tree
Ekaette et al. Risk Analysis. 2007 (27) 1397
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Ekaette’s Fault Tree
Ekaette et al. Risk Analysis. 2007 (27) 1397
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5
Varieties of Fault Trees
1.Standard Fault Tree
2.Root Cause Tree
3.Probabilistic Fault Tree (data based)
4.Probabilistic Fault Tree (elicitation based)
5.TG 100’s FTA

Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Root Cause Tree
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Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Root Cause Tree
Thomadsen et al. IJROBP 2003 (57) 1496
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
6
Varieties of Fault Trees
1.Standard Fault Tree
2.Root Cause Tree
3.Probabilistic Fault Tree (data based)
4.Probabilistic Fault Tree (elicitation based)
5.TG 100’s FTA

Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Probabilistic Fault Tree (Thomadsen)
•Focused on HDR and LDR
Brachytherapy.
•Based on 134 reports (1980-2001) in the
NRC and IAEA databases.
•Produced a conventional FTA, a process
map and an example of a root cause
analysis tree.
•Classified failures according to three
taxonomies.
Thomadsen et al. IJROBP 2003 (57) 1496
Peter Dunscombe. Fault ITJrReeO ABnPa ly2s0is0, 3J u(ly5 72)0 1–4 2948, 2014 Austin, Texas
Probabilistic Fault Tree (Thomadsen)
Thomadsen et al. IJROBP 2003 (57) 1496
Peter Dunscombe. Fault TIJreReO ABnaPly 2si0s,0 J3u l(y5 270) –1 2449,8 2 014 Austin, Texas
7
Probabilistic Fault Tree (Thomadsen)
Thomadsen et al. IJROBP 2003 (57) 1496
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
IJROBP 2003 (57) 1498
Interesting quote from Thomadsen’s paper
“In industries such as nuclear power, where
probabilistic risk assessment originated, most
failures occur only when several systems fail
concurrently, and the combination of
probabilities becomes important. Most medical
events, although they have several root causes
and concurrent unusual situations, fail along a
single branch of the fault tree”
Thomadsen et al. IJROBP 2003 (57) 1496
Peter Dunscombe. Fault ITJrReeO ABnPa ly2s0is0, 3J u(ly5 72)0 1–4 2948, 2014 Austin, Texas
Prescient observation by Thomadsen et al.
“Errors often follow violations in
protocols, particularly failures to
perform verification procedures, and
indicators that things are not correct
are often present yet ignored during
events.”
New York Incident?
Thomadsen et al. IJROBP 2003 (57) 1496
Peter Dunscombe. Fault ITJrReeO ABnPa ly2s0is0, 3J u(ly5 72)0 1–4 2948, 2014 Austin, Texas
8
Prescient observation by Thomadsen
2003 2006
Radiation Offers New Cures, and
“Errors often follow violations Ways to Do Harm
in protocols, particularly
By WALT BOGDANICH
failures to perform verification
procedures, and indicators
that things are not correct are
often present yet ignored
during events.”
Thomadsen et al. IJROBP 200N3 (e5w7) Y14o9r6k Incident?
Peter Dunscombe. Fault ITJrReeO ABnPa ly2s0is0, 3J u(ly5 72)0 1–4 2948, 2014 Austin, Texas
Varieties of Fault Trees
1.Standard Fault Tree
2.Root Cause Tree
3.Probabilistic Fault Tree (data based)
4.Probabilistic Fault Tree (elicitation based)
5.TG 100’s FTA

Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Probabilistic Fault Tree Analysis (Ekaette)
•Focused on Treatment Preparation for
External Beam Radiotherapy.
•Expert team of 3 medical physicists, 1
oncologist, 7 therapists/dosimetrists.
•Examined NRC, ROSIS and IAEA reports
to identify what could go wrong.
•Expert elicitation required some training in
understanding probabilities.
Ekaette et al. Risk Analysis. 2007 (27) 1397
Peter Dunscombe. Fault ITJrReeO ABnPa ly2s0is0, 3J u(ly5 72)0 1–4 2948, 2014 Austin, Texas
9
Probabilistic Fault Tree Analysis (Ekaette)
Ekaette et al. Risk Analysis. 2007 (27) 1397
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Risk Analysis. 2007 (27) 1397
Probabilistic Fault Tree Analysis (Ekaette)
Overall, however, the expert probability estimates used in conjunction with
the fault tree method produced an overall incident probability result for the
Preparation domain of 0.37%, which is comparable to the 0.14–0.68%
incident probability range experienced in 2002–2005.
Ekaette et al. Risk Analysis. 2007 (27) 1397
Risk Analysis. 2007 (27) 1397
Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
Varieties of Fault Trees
1.Standard Fault Tree
2.Root Cause Tree
3.Probabilistic Fault Tree (data based)
4.Probabilistic Fault Tree (elicitation based)
5.TG 100’s FTA

Peter Dunscombe. Fault Tree Analysis, July 20 – 24, 2014 Austin, Texas
10