Trees succession

For general advice, see Apples (p.294). Most European plums will set some fruit without cross-pollination, but nearly all will yield better when cross-pollinated by another European cultivar. Japanese plums must be cross-pollinated by either a Japanese or American type. American plums also need cross-pollination for best yields. Rootstock choice can further influence your plum trees success. Ask a specialty fruit nursery for help in selecting a suitable combination of trees on appropriate rootstocks. 

Event Trees. Event trees use an inductive logic approach to consider the effects of safety systems on an initiating event. The initiating event is propagated through the various safety functions. Branching is dependent upon the success or failure of the safety function. 

The bottom-up method uses the same substitution and expansion techniques, except that now, the operation begins at the bottom of the tree and proceeds up. Equations containing only basic failures are successively substituted for higher faults. The bottom-up approach can be more laborious and time-consuming however, the minimal cutsets are now, 1 obtained for every intermediate fault as well as the top event. 

Using de Morgan s theorem (Table 2.1-1), the fault tree in Figure 3.4.4-9 is converted into the success tree shown in Figure 3.4.4-10. Notice that the effect of the theorem is to reverse tlie logic "AND" gates become "OR" gates and vice versa failure becomes success and vice versa. 

Success trees, by definition, are success oriented. Some analysts claim that a success tree is a state of mind and that a true success tree cannot be adapted from a fault tree by de Morgan s theorem but must be built from "scratch" to involve the psychology of success rather than of failure. 

Fig. 3.4.4-10 Success Tree Transformation of Figure. S.4.4-9 Fault Tree...

The event" list, across the top of the event tree, specifies events for which the probability of failure (or success) must be specified to obtain the branching probabilities of the event tree. Events that are the failure of a complex system may require fault tree or equivalent methods to calculate the branching probability using component probabilities. In some cases, the branching probability may be obtained directly from failure rate data suitably conditioned for applicability, environment and system interactions. 

The success of some systems depends on other systems. An event tree is constructed by placing support systems before the supported systems. This may require iterating the event tree construction to get appropriate ordering. If the system dependency is in the fault trees, it is not reflected in the event tree and only becomes apparent when the PSA is calculated. 

LOCA, is presented in Table 3.4.5-1. In preparing the event tree, reference to the reactor s design determines the effect of the failure of the various systems. Following the pipe break, the system should scram (Figure 3.4.5-2, node 1). If scram is successful, the line following the node goes up. Successful initial steam condensation (node 2 up) protects the containment from initial overpressure. Continuing success in these events traverses the upper line of the event tree to state 1 core cooled. Any failures cause a traversal of other paths in the evL-nl tree. 

LESF (Figure 3.4.5-5), exemplified for the large LOCA, is compared with SELF. Event tree headings are the refueling water storage tank (RWST) a passive component, an engineered safety system (SA-1) and four elements of the containment system. Other examples of the LESF method show human error in the event tree while the criteria for system success is usually in the tan It tree analysis. 

In any given situation, there may be different levels of dependence between an operator s performance on one task and on another because of the characteristics of the tasks theraseb e.s. or because of the manner in which the operator was cued to perform the tasks. Dependence levels between the performances of two (or more) operators also may differ. The analyses should account for dependency in human-error probabilities. In addition, each sequence may have a set of human recovery actions that if successfully performed will terminate or reduce the consequences of the sequence. This information, coupled with a knowledge of the system success criteria leads to the development of human success and failure probabilities which are input to the quantification of the fault iices or event trees. With this last step, the HRA is integrated into the PSA, and Pl. ise 4 is complete. 

A probabilistic statement of the likelihood of human-error events presents each error in the task analysis as the right limb in a binary branch of the HRA event tree. These binary branches form the chronological limbs of the HRA event tree, with the first potential error siai ting from the highest point on the tree. (Figure 4.5-4). Any given [ask appears as a two-limb branch the left limb represents the probability of success the right limb represents the probability of failure. 

Each binary fork is attached to a branch of the preceding fork and is conditioned by the success or failure represented by that branch. Thus, evei7 fork, represents conditional probability. Each limb of the HRA event tree is described or labeled, in shorthand. Capital letters (A) represent I ailure lower case letters (a) represent success. The same convention applies to Greek letters, which represent non-human error events, such as equipment failures. The letters S and F are exceptions to this rule in that they represent system success and failure respectively, in practice, the limbs may be labeled with a short description of the error lo eliminate the need for a legend. The labeling format is unimportant the critical task in developing HRA event trees is the definition of the events themselves and their translation to the trees. 

The development of the HRA event tree is one of the most critical parts of the quantification of human error probabilities. If the task analysis lists the possible human error events in the order of ihcir potential occurrence, the transfer of this information to the HRA event tree is fadlitutcd. Each potential eiTor and success is represented as a binary branch on the HRA event tiec. with subsequent errors and successes following directly from the immediately preceding ones. Cure should be taken not to omit the errors that are not included in the task analysis table but might affect the probabilities listed in the table. For example, administrative control errors that affect a task being performed may not appear in the task analysis table but must be included in the HRA event tree. 

The ESDs were then translated into associated event trees. A generic event tree was developed for all initiators not involving LOCA. The generic transient event tree for each category of the transient initiators and loss of offsite power were specialized by the impact of the initiators on the safety and support systems, from the success criteria of the mitigating systems, and the initiator-specific human actions which were modeled in the fault trees. 

Answer End points 1, 3, and 6 are success paths which are not modeled by a fault tree. The fault tree for slow melt is Figure 15.3.1-1. 

Put a branch in the containment event tree for the success or failure of the off-gas syst id... 

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