Many models have been created that have shown the correlation between the probability of an accident, with the state of preparedness in that area. In other words, the more likely an event, the more informed and prepared those in the effected area will be.
In this investigation I will use the probability of the occurrence of two events according to the AND logical gate, to demonstrate one way that the earthquake and tsunami on March 11th was likely underestimated in Japan. The two events may have been considered ‘independent events’ which would have caused their estimated probability of occurrence to be substantially less than a ‘common mode failure’ event.
Since it’s conception, the nuclear power industry has campaigned its willingness to make safety a top priority when identifying key plant vulnerabilities, and developing future plant designs.
The events at the Fukushima Daiichi Nuclear Power Station are the most recent example where the combination of considering a limited number of faults, and a conservative approach to the analysis of each fault has produced inappropriate at best, and at worst misleading insights about nuclear power plant risk.
What happens when those who could potential be affected are subjected to an information-processing constraint by various official and regulatory agents acting under state-contingent plans?
If the level of probability is relative to the level of preparedness, it could also mean the secrecy and classification of information prevents a system of checks and balances to be applied. This prevents independent analysis and verification to ensure the integrity, accuracy, and potential flaws in the methods used to make vital decisions.
The largest disasters in recorded history have generally started as a series of events that accumulates to form a seemingly impossible situation.
In the nuclear industry prior to the Fukushima disaster, it was thought that breach of containment was impossible, complete station black-out was defensed by safeguard after safeguard, and the dangers of nuclear power were well understood.
Why had virtually no thought been put into a multi-event disaster in one location, or the loss of back-up and off-site power?
Heat Readings of Reactor 1 and Reactor 2 at Fukushima Daiichi Plant
According to a report by the United States NRC, the situation at Fukushima Daiichi would have been substantially less severe if TEPCO had taken better actions following the earthquake and tsunami.
While the majority of the focus has been on the actions after the event, little discussion has developed any issues that could have been taken proactively to prevent the disaster. Being well-prepared is very costly, and the nuclear industry has built a reputation of avoiding spending profits on preventing potential or probable issues until after the fact.
After the events at Fukushima, the NRC inspected all of it’s nuclear power stations and checked the status of additional safeguards (B.5.b) mandated after the 9/11 terror attacks. The inspections showed that many of the safeguards were missing, inoperable, or never inspected and maintained. In many cases, proper training was not performed or maintained as well.
The NRC like many other regulatory agencies later decided that there was little to no additional safeguards that needed to be implemented to prevent future accidents that would require the suspension of operations or re-licensing of any existing nuclear power stations.
Estimating safety in the nuclear industry
Many skeptics around the world have asked whether the nuclear industry has done enough to be more prepared for rare events, but the public has failed to realize the lack of preparedness and its possible correlation to the identified weakness and bias of utilities and regulating agencies around the world today.
Probabilistic safety assessments (PSAs) are used by some nuclear power stations, mainly in developed nations, to identify and understand key plant vulnerabilities. PSAs are an effective tool for plant managers to target resources where the largest benefit for plant safety can be obtained.
The PSA analyzes the risk associated with operating the plant, as determined by a variety of metrics related to the different levels of damage to the plant, or it’s environment. It is preferred because of the logical and systematic approach that makes use of realistic assessments as a basis for the calculations.
In most operations where the design basis was premised from a limited evaluation of the information available, PSAs are a resource that can increase the level of understanding and safety.
Plant specific or Living PSAs (LPSA) are used to support testing methods, maintenance planning, and upgrading outdated technology at nuclear power stations. They are updated to reflect the current design and operational features and other existing plant information.
In theory, this provides the potential to increase the understanding of an inherent risk stemming from operational incidents or potential unforseen events. Traditional deterministic analysis are unable to properly calculate or take account for unknown or uncertain events as efficiently as PSAs.
There is no regulatory framework for the use of probabilistic estimates or PSAs in decision making. Not all countries adhere to the ‘risk-informed’ regulation, as there is no common understanding of the use of PSAs in relation to decision making.
The lack of regulation leads to a noticeable level of uncertainty of how to address certain elements of the PSA model. This also allows a variety of equations and methods to be used to gather information from probabilistic estimations.
There is a great deal to be learned and understood about regulating PSAs and how to best implement them viably in the nuclear industry. If assumptions or the adoption of a specific model for a certain element of a PSA are asserted, immediately the limits the usefulness and validity of the assessment plunge.
Examples of changes in logic models
Change in assumptions leading to changes in the success criteria; if an overly conservative approach is used to determine appropriate or relative issues and information, it can prevent accurate analysis.
Changes in level of detail required in the fault trees; can prevent full disclosure and limit understanding of the issue.
Changes in understanding of issues such as equipment qualification and their impact on component unavailabilities; can limit effectiveness and use of PSAs
Changes in system design or procedures; can invalidate PSAs integrity
As in any other probabilistic, or statistical method, in regards to the method or any component of the situation, the understanding, familiarity, integrity and bias of the operator has a direct effect on the results.
Any PSA which is used to support decision making at nuclear power stations should have a credible and defensible grounding firmly planted in reality, not for or against any operator or regulator motivated theory.
In its PRA Policy Statement, the US NRC stated “…PRA methods and data should be used in a manner that complements the US NRC’s deterministic approach and supports the US NRC’s traditional defence in depth philosophy.”
Any model created must accurately reflect the current status of the station selected. If the PSA is to increase any understanding of plant safety, it must be updated or modified to reflect changes in plant operations, methods, or known environmental and economical fluxes.
Common Cause Failure
A common-cause-failure (CCF) is when two or more components fail due to a common or related cause.
The probability of both an earthquake and a tsunami event as a “common mode failure” event was likely underestimated. As independent events, their estimated probability of occurrence would be substantially less than a common mode failure event.
Probabilistic Estimation
Identify the equation to determine the occurrence of two events.
The probability of the occurrence of two events according to the AND logical gate is:
P(A.AND.B) = P(A).P( A | B ) = P(B).P)+( B | A )
where:
P(A) is the probability of occurrence of event A
P(B | A) is the conditional probability of occurrence of event A, given that B occurs
P(A | B) is the conditional probability of occurrence of event B, given that A occurs
If the events A, B are independent the conditional probabilities become:
P(A | B) = P(A)
P(B | A) = P(B)
Therefore, when A, B are independent events:
P(A.AND.B) = P(A).P(B)
If the probabilities of occurrence of an earthquake and a tsunami are:
P(Earthquake) = P(Tsunami) = 10⁻⁴
If treated as dependent events, then:
P(Tsunami.AND.Earthquake) = P(Tsunami)P(Tsunami | Earthquake)
= 10⁻⁴×1
= 10⁻⁴
where: P(Tsunami | Earthquake) = 1
If considered independent, then:
P(Tsunami.AND.Earthquake) = P(Tsunami)P(Tsunami)
= 10⁻⁴× 10⁻⁴
= 10⁻⁸
where: P(Tsunami | Earthquake) = 1
As you can see there is a significant difference when you compare the probability of an earthquake and tsunami when considered as dependent or independent events.
Watch the TBS feed AND the TEPCO Webcam simulcast EXCLUSIVELY HERE http://lucaswhitefieldhixson.com/lucaswebcamwatch.html
By Lucas Whitefield Hixson who is Nuclear Researcher in Chicago, Illinois in USA.