To consider special demands, two gaskets floating between the flange faces are used for example in flanged connections of pressure vessels, valves, pumps etc. Tightness control can be one of the reasons to choose this design. The inner (primary) gasket has to tighten the internal pressure; the outer (secondary) gasket tightens possible leakage of the primary gasket. By the use of two identical gaskets in the same plane the procedure for the tightness proof is the same as in connections with one gasket. In many cases there are used two gaskets with different characteristics (deformation and tightness characteristics) in two tightening planes. The behaviour of this connection depends not only on the gasket characteristics but also on the tolerances of axial dimensions. The distribution of prestress on both gaskets is determined by the elastic-plastic deformations of both gaskets and tolerances of axial dimensions of the connection. The decrease of gasket stress in operation depends on the plastic deformations of both gaskets in operation, too. Depending on the demanded tightness class, the possible gasket stress during assembly and the gasket stress in operation determine the selection of suitable gaskets (with necessary gasket characteristics). It is necessary to take into account the internal pressure and possible pressure between primary and secondary gasket, too. In the paper the design of gaskets for this type of connection is discussed. Originally, it was used: - nickel ring as primary gasket, - asbestos packing as secondary gasket. Both gaskets have suitable deformation characteristics for the existing type of connection. The secondary gasket has low stiffness for the adjustment of an optimal stress on the primary gasket after prestressing. The primary gasket has a good relaxation behaviour. However, the nickel ring needs a very high local stress to achieve suitable tightness behaviour. This stress leads to plastic deformation of the flange surface during assembly, too, and this can induce leakage after the next assembly (with new rings) as operation experience showed. The use of asbestos packing is not allowed any more due to health reasons. This invoked a redesign of this gasket. The paper deals with gasket proofs and gasket characteristics for selection of suitable gaskets. Calculations of gasket stress distributions of both gaskets (elastic-plastic deformations) are presented. The analysis of the stress distribution allows reviewing the tightness class in operation incl. relaxation behaviour of the connection. Other recommendations are listed like the increase of connection safety by a reduction of bolt stiffness (better relaxation behaviour of the connection and higher bolt elongation), more accurately bolt elongation measurement, new assembly procedure, etc. Main target of the re-design is life time extension of connection, too.

There are basic technical (protection) objectives determined for assurance of nuclear power plant safety and the following generally belong among them: - Reactor pressure vessel shut down, - Long term maintenance of sub-critical state, - Long term cooling, - Prevention of radioactivity leakage. To ensure these objectives multi-step concept of deep protection is used for the design of a nuclear power plant and it includes: - Prevention of failures during normal and abnormal operation, - Control of failures and their consequences, - Minimizing of risks during accidents. Failure of operating systems is conservatively postulated for determination of systems requirements using for failure ensure as piping breaks. Ensure of these postulated failures come under multilevel safety approach. Failure consequences should be mainly ensured by design measures as separation of high energy piping, whip piping restrains etc. Efficiency of design measures have to be demonstrated. This passive safety procedure during design of new NPP can be applied. Application of this passive procedure for operating NPP can lead to technical and economical problems. It can be done by non precise and non sufficient requirements, current standards and documents. Leak before break concept (LBB) is very often out due to break operation conditions for successful concept usage. Beak preclusion concept was defined in Germany thirty years ago. The concept is developed from this time. Required quality of SSC is basic of this concept. The quality has to be received during manufacturing and assembly of new components to system or the quality passport has to be documented for SSC in operation before enlistment to the concept. During next operation they are sufficient and redundant measures necessary to control and to manage ageing phenomena (conceptual, technological, and physical) for exclusion of premature ageing. This proactive approach is also basic of documents from the last year’s required ageing and lifetime management. In Czech NPPs postulated failures and their consequences in accord with producer knowledge state at that time were insured. Postulated failures and their consequences were insured partly design measures and partly design supposed quality too. It is very difficult to realize new requirements on needed design provision on NPP in operation. It is impracticable in any cases. Needed national law for approach application exists in Czech from 1997. Regulation on lifetime management and national nuclear standards with specific requirements exist in Czech too. There are backgrounds for application proactive approach as it is used in Germany NPPs. New safety approach was provided in Czech NPPs. SSC are separated into three groups on the base safety approach: - SSC must not fail (guarantee of quality), - SSC may fail in rare case (preventative maintenance), - SSC may fail (failure orientated maintenance). The contribution deals about new Czech safety concept aspects, boundary conditions, needed document and proactive measures.

There are important mechanical systems, structures and components (SSC) in industrial equipments and nuclear power plants. These SSC are decisive for safety and economical operation and they mustn’t fail. There are principles used for these components to ensure their integrity during operation. Their fundamentals are as follows: – achievement of required quality during design, manufacturing and assembly, – quality assurance in following operation, – appropriate quality certificate in operation. The design should include all degradation mechanisms but some of them are very difficult to specify exactly and analyze before operation. The hardly specified degradation mechanisms have to be excluded from this concept. Appropriate operation measures have to be determined. Typical examples are operation vibration, material corrosion phenomena, thermal stratification, striping and dynamic loading during valve operation, etc. Design specification can determine basic design temperature, pressure, sustained loads and time history loadings during normal, abnormal and emergency operation only. Efficiency of the measures during operation has to be verified. Necessary measures have to be determined during design for verification and control of the real reasons during the SSC service. Both certificates and design of structure properties (material properties, dimensions, shapes, etc.) shall allow exclusion of systematic mistakes during design, manufacturing and assembly. Control of specified and unspecified degradation reasons during operation is the first redundant measure for insurance of required quality during operation and also timely assessment of inspection results regarding their influence on components quality. Appropriate measures shall be determined in time to minimize this influence if necessary. Check of degradation consequences is the second important measure. Relevant consequences have to be discovered in time before SSC failure. These measures shall be periodically updated in accordance with latest knowledge and their evaluation regarding the equipment. If relevant consequences are detected necessary measures shall be undertaken to exclude these consequences during following operation. Certificate of sufficient quality in operation have to be made before SSC is put in operation or after long term operation. Basis for this certificate are actual design and relevant load, real geometry, performed attachments, piping hanging and relevant operation loadings including media and specified postulated rupture according to current knowledge. Potential degradation mechanisms in SSC operation are necessary to determine and to prove sufficient insurance of their reasons. There are three levels assigned in the concept regarding SSC quality in future operation: – quality has to be guaranteed (prevent failure - proactive approach) by monitoring of causes of damage; – quality has to be maintained (preventative maintenance - proactive approach) by monitoring of damage results; – no specific demands on quality (maintenance triggered by damage - reactive approach) statistical approach for damage. The paper deals with theoretical background of the integrity concept on Czech NPPs and example of practical application is presented.