|Project:||„LBB (Leak Before Break)“ for Units 5 и 6 NPP – “Kozloduy””|
|Client:||NPP – „Kozloduy“|
Concept “Leak Before Break” (LBB) is based on following principle:
- On stage of design and material selection, or
- In the period of exploitation (i.e. ex-post) based on analyses from the information of the realized project,
by using certified programs and verified methods of fracture mechanics and also by performing experimental programs with aim to demonstrate the behavior according to the LBB scenario with sufficient safety factors under normal and accident loading conditions, demonstrates the impossibility of complete destruction of the component with through wall crack without pre-formation of stable leakage that can be detected in a timely manner by means of available control before the crack reaches a critical value under the conditions of instability.
For the high-energy pipelines of nuclear power plants, made from (elastic-)plastic materials which have high resistance to unstable crack growth, the probability of hypothetical double ended guillotine break (DEGB) is too low even in accident loadings. Instant catastrophic break is always preceded by a steady, sub-critical increase of the crack. This circumstance allows either the crack to be detected long before it becomes through wall (TW) or the leak to be detected with the help of Leak Detection System (LDS), before TW crack (with length ≥2cLD‑), through which the coolant leaks, reaches the critical length 2cc. In result it becomes possible in time discovery of defected cross sections, safely shut down of the reactor unit, perform a repair or replacement of the component with a defective section and thus excluding a sudden break of the pipeline. This is the essence of the “Leak Before Break” (LBB) approach concept.
The purpose of implementing the LBB concept is enhancement of the operational safety of NPPs. Object of analysis are high-energy pipelines with diameter above 150 mm (DN ³150 mm) from primary coolant system and their adjacent non-isolationable parts, as well as the main feedwater pipelines (the part located inside the containment), which are part of the second circuit.
The applied methodology for research of pipelines according the LBB concept is developed on the basis of the common safety analysis methodology (Standard Review Plan. Section 3.6.3. Rev. 1 “Leak-Before-Break Evaluation Procedures,” U.S. Nuclear Regulatory Commission Report, NUREG-0800 (Formerly NUREG-75/087), March 2007.) and relevant regulatory documents of Russia’s nuclear industry, including РД ЭО 1.1.2.05.0939-2013 Руководство по применению концепции безопасности «течь перед разрушением» к трубопроводам действующих АЭУ. – Москва: ОАО «Концерн Росэнергоатом», 2013.
Despite of the sufficient resistance to the various degradation mechanisms (e.g. Corrosion, corrosion/erosion wearing, deformation due to creep, thermal aging and fatigue) during its lifetime, as a dominant mechanism for possible damage of the pipelines of primary and secondary circuits, during extension the lifetime of units in NPPs with WWER, the fatigue of the material should be considered. Apart from this, especially for the lines of the main feedwater pipelines TX41-TX44, made of carbon steel Ст20 (without corrosion protection cladding)and consequently subjected to corrosion, furthermore during the analyses were considered and (determined by ultrasonic thickness gauge) significant thinning of the pipeline bend walls (in some places around and little bit over 4 mm) which significantly exceeds the normative corrosion correction coefficient с2, which is 1,2 mm per 30 years of exploitation.
For critical areas of analyzed pipelines are considered the welding joints and the most loaded pipe bends. As an initial defect is chosen postulated semi-elliptical surface defect of no greater size than 2c0=1.0 t and а0=0.2 t÷0.25 t (where t – wall thickness of the pipeline), for axis aspect ratio a0/c0=0.33÷0.4. Based on the analysis of the growth of the postulated initial defects (a0, c0) in the critical zones and taking into account the extension of the lifetime to 60 years it is possible to conclude the fact that cyclic growth of the defects due to fatigue of materials (so called fatigue crack growth, i.e. FCG) in the end of service life (of 60 years) is negligible small; its maximal predicted value by depth ∆a is less than 1 mm and is obtained for pipeline 5YA20.
For all analyzed pipelines the condition of stability of defects under maximum accident load (usually a SSE load is taken for the calculation) is fulfiled. For all of them is met the criterion of negligence of FCG required in subsequent analysis of the LBB concept, scilicet af < 0.5t, 2сf < 0.5·2cc, where af и 2сf – the depth and length of developed (after 60 years of service) initial defect, 2cc – critical length of the defect.
The comparison between the analytically calculated sizes of detectable TW cracks, through which is possible leak with the respective flow rate, on one hand, and on the other hand the possibility of predicting and detecting leakages with existing in units 5 and 6 of NPP “Kozloduy” LDS FLÜS (working on the principle of humidity control, with high degree of sensitivity of 1 l/h (0.017 liter per minute); manufactured by Siemens) shows that the lowest required value for sensitivity obtained for the system TQ40 in Unit 6 is 0.3 l/min (>>0.017 l/min). Hence, the eventual expansion of already in service LCS FLÜS for all considered pipelines in LBB concept can be justified in full volume.
The mentioned above dictates the requirement in the frame of conceptual design to be made a list of specific proposals for improvement and upgrading the existing LDS FLÜS. One of these improvements is by installing sensors for measuring the volume radio activity of the air in the measurement lines so the LDS FLÜS to be upgraded to analogue of LDS AKFLÜS. The design proposes, instead of measuring activity in “immediate” proximity around the monitored component (as it is in the original AKFLÜS), to carry out “remote” measurement (measurement at remote point from occurrence of the leakage) by means of the resources of the LDS FLÜS (already installed in units 5 and 6) of “portions” medium from monitored pipelines. Both AKFLÜS and the lines of upgraded FLÜS will continue to measure humidity. Moreover, by means of an additional connected to the standard LDS FLÜS appliance – for example УДГ-1Б system, will monitor also the level of volume radio activity and by its value will quantify the amount/rate of fluid discharged through occurred leakage.
In the frame of conceptual design are suggested two new extra LDS, working on different physical principles.
As an example of LDS based on monitoring of the heat distribution in the premises and on the surface of the pipelines by means of heat vision, a video surveillance system based on the use of infrared (IR) stationary video cameras is considered. The idea that stands behind this is pretty simple – what is monitoring for is the effect of unusual spot of increased temperature on the outside surface of the heat insulation due to leakage from TW crack. In order to significantly increase the stay time of the cameras in the containment, it is envisaged to develop a protective screen/cover of lead and lead glass to protect against ionizing radiation (which will reduce the intensity of radiation in the installed point with 70%) for the camera’s optics.
Also is developed conceptual design for LDS using as detector fiber optic net. Through high-sensitivity fiber optical cable (FOC) lines, permanent temperature monitoring of pipelines can be carried out. If FOC are located directly on the surface of the pipeline, leakage can be detected by monitoring local temperature decrease in accordance with Jaule-Thompson‘s effect; if the fiber lines are on the surface of the thermal insulation by expected appearance of hot spots with a local increase in temperature.
The logical continuation of the activities for this task is a further elaboration of a detailed technical design for supplementing and upgrading (improving) the existing systems, components and equipment of the LDS, as well as the subsequent delivery and installation of new ones, which will meet with full measure the requirements of the LBB concept and to be considered as fully implemented/applied in Units 5 and 6 of Kozloduy Nuclear Power Plant.
The presented results and conclusions can be used as additional arguments in the justification of extending the service life of units 5 and 6 up to 60 years.