Objective Provision Tree

Objective Provision Tree (OPT). There is a namral interface between the OPT and the PIRT in that the PIRT identifies phenomena and issues that could potentially be important to safety, and the OPT focuses on identifying design provisions [in fact essential lines of protection (LOP)]. The purpose of the OPT is to document the implementation of essential LOPs to ensure successful prevention, control, or mitigation of phenomena that could potentially damage the nuclear system. 

My object in writing this book is to present a coherent set of operational rules for the analysis of reaction mechanism in terms of symmetry, and to explore their scope and reliability. It is written from the viewpoint of Orbital Correspondence Analysis in Maximum Symmetry - OCAMS for short, hence its subtitle, but not with the intention of touting a homemade prodct The procedural details of OCAMS are of secondary importance its advantage lies in the provision of a coherent overview of the relation between symmetry and mechanism it allows us to see the forest without losing sight of the trees. For reasons of self-consistency, the book remains within the framework of molecular orbital theory reformulation of the OCAMS approach in valence-bond terms has not been attempted. 

For easier and more user friendly applicability, the method that is reviewed in this publication, including the overview of all challenges, mechanisms and provisions for all levels of defence, is illustrated in the form of objective trees . ... 

Section 2 addresses the strategy of defence in depth and the importance of fulfilling the safety functions (SFs) to achieve the objectives for the different levels of defence. Section 3 provides a detailed description of the approach for making an inventory of the defence in depth capabilities of a plant. Section 4 discusses the applications of the approach and how to use the approach for assessing defence in depth. Section 5 presents conclusions. A discussion of the SFs is presented in Appendix I. In Appendix II, the objective trees graphically represent how, for each relevant safety principle , the safety objectives of the different levels of defence can be achieved by establishing defence in depth provisions at different stages of the lifetime of the plant. A test application of the approach is summarized in Appendix III. Definitions are provided at the end of the book. 

There might be an impact on the performance of FSFs/SFs by challenges which are placed on a lower level of the objective tree. On the next lower level of the tree there are several mechanisms listed that can give rise to the challenges. Under each of the mechanisms, there is a hst of possible provisions that should be implemented in order to prevent the mechanisms from occurring and to prevent challenges to the SFs from arising. 

The objective trees are presented in Appendix II for all the levels of defence based on the approach described. The trees themselves are self-explanatory, i.e. no additional text is provided to explain the challenges, mechanisms and provisions. Further guidance can be found in Refs [2,4,5]. 

The following remarks can be made on the formulation of provisions in the objective trees ... 

Users of the method presented in this pubhcation are expected to review and compare provisions for defence in depth identified in the objective trees with the existing defence in depth capabihties of their plant. The objective trees provide the rationale for the bottom up method, starting with the screening of individual provisions. Users should evaluate for each provision the level of its implementation. If the implementation of provisions is satisfactory, then the relevant mechanism can be considered as having been prevented from occurring. Deviations should be discussed and either justified by compensatory features specific to the plant or reconsidered for further strengthening of the defence in depth of the plant. 

Feedback is welcome from users to the developers of the method for improving it. In this connection, the appropriateness of the mechanisms and challenges set in the objective trees needs to be verified. Comments on the text in the boxes of the objective trees are also welcome in order to address more precisely the relevant safety aspects. Special attention needs to be given to the objective trees of those safety principles that are applicable to more than one level of defence and to the need to make further appropriate distinctions between provisions belonging to different levels of defence. 

FIG. 15. Objective tree for Levels 1-4 of defence in depth (RCS, reactor coolant system LOOP, loss of off-site power UHS, ultimate heatsink). Safety principle (142) ultimate heatsink provisions. 

As described in Sections 3 and 4 of the present publication, a bottom up approach was used for screening. Screening for each safety principle started from the bottom of the relevant objective tree. Implementation of provisions aimed at avoiding the occurrence of related mechanisms was evaluated as a first step. In a second step, conclusions were derived for all mechanisms based on the level of implementation of the provisions. Finally, during the third step, conclusions were formulated regarding sufficient prevention of challenges. 

When the review is being performed as a self-assessment, the level of satisfaction heavily depends on the plant safety culture and the desire for improvement. In any case the approach helps in identifying missing or weak provisions. The understanding of the importance of the provisions and of the interactions among the provisions or mechanisms is improved because of the complexity of the approach and its visualization in the form of objective trees. 

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