Treeing initiation time

Although the rRNA tree of life is able to place organisms broadly into their correct relative positions, it does not have a reliable inbuilt clock (Graur. Martin, 2004), and so, until recently the timing of events on the tree of life has been poorly known. In a recent review Shen and Buick (2004) used fossil, rather than molecular evidence to place some initial time constraints on the evolutionary appearance of the Archaea, Bacteria and Eucarya (Fig. 6.6). 

It is necessary to check units to be sure they make sense. Fault trees are often associated with event trees in which only the initiator has the units of frequency and the fault trees are dimensionless probability. This dimensionless is achieved by failure rates being paired with a mis.sion time. 

Function event trees are developed to represent the plant s response to each initiator. The function event tree is not an end product it is an intermediate step that provides a baseline of information and permits a stepwise approach to sorting out the complex relationships between potential initiating events and the response of the mitigating features. They structure plant respoases to accident conditions - possibly as time sequences. The transition labels of function event trees (usually along the top of the event tree) are analyzed to provide the probability of that function occurring or not occurring. 

The OWR protective systems were modeled with event tree diagrams for the time sequence following an initiating event to fuel damage or safe shutdown. Fault trees were used to find the probability of failure of each protective system in a particular event tree. 

As a general rule, boiler section deposits initially are often soft and porous but are prone to become harder and more adherent as they age. Thus, the presence of hard, dense scales and deposits in a boiler indicates that deposition has been occurring for a considerable time. The deposition of scales often produces a layered effect similar to the rings of a tree, indicating periodic changes in the water chemistry and perhaps providing some clues as to the history of the deposits. 

Contemporary forest declines were initiated about 1950-1960, virtually simultaneously throughout the industrial world at the same time as damage to aquatic systems and structures became apparent. A broad array of natural and anthropogenic stresses have been identified as components of a complex web of primary causal factors that vary in time and space, interact among each other, affect various plant growth and development systems and may result in the death of trees in mountainous ecosystems. As these ecosystems decline, the alterations in forest ecology, independent of the initial causal complex, become themselves additional stress factor complexes leading to further alterations. 

Closely related to the annuity is the perpetuity. This is an amount of money set aside at time zero that will provide something indefinitely. Care for a cemetery lot is frequently purchased as a perpetuity. The owners of the cemetery agree for a given initial fee to cut the grass, rake the leaves, trim the trees, and do whatever else is necessary to preserve the lot s appearance forever. 

The event tree can be used quantitatively if data are available on the failure rates of the safety functions and the occurrence rate of the initiation event. For this example assume that a loss-of-cooling event occurs once a year. Let us also assume that the hardware safety functions fail 1% of the time they are placed in demand. This is a failure rate of 0.01 failure/demand. Also assume that the operator will notice the high reactor temperature 3 out of 4 times and that 3 out of 4 times the operator will be successful at reestablishing the coolant flow. Both of these cases represent a failure rate of 1 time out of 4, or 0.25 failure/demand. Finally, it is estimated that the operator successfully shuts down the system 9 out of 10 times. This is a failure rate of 0.10 failure/demand. 

Next the team develops a chronology of events based on the available known times and sequences. This document is usually referred to as a timeline. (See the detailed description of timeline development in Section 9.3.) Unconfirmed assumptions regarding chronology should be clearly identified as unconfirmed, and action should be initiated to verify assumptions. Many investigators use relatively simple timelines (instead of sequence diagrams) with the logic tree methods because the logic tree itself shows the interactions of events and conditions. 

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