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Early Detection of Micro-Cracks in NPP Piping

#A-1741


Development of a System of Early Detection of Micro-Cracks in Primary Piping of Nuclear Power Plants

Tech Area / Field

  • ENV-EHS/Environmental Health and Safety/Environment
  • ENV-MRA/Modelling and Risk Assessment/Environment
  • FIR-MAT/Materials/Fission Reactors
  • FIR-NSS/Nuclear Safety and Safeguarding/Fission Reactors

Status
3 Approved without Funding

Registration date
16.04.2009

Leading Institute
Armenian NPP, Armenia, Metsamor

Supporting institutes

  • Institute of Mechanics, Armenia, Yerevan

Collaborators

  • Skoda JS a.s., Czechia, Plzen\nThe University of Western Ontario / Department of Electrical and Computer Engineering, Canada, ON, London\nNuclear Research and Consultancy Group, The Netherlands, Petten\nCalifornia State Polytechnic University Pomona, USA, CA, Pomona\nMcMaster University, Canada, ON, Hamilton\nUniversity of Florida / Laboratory for Development of Advanced Nuclear Fuels and Materials, USA, FL, Gainesville

Project summary

After all, safety of nuclear power industry facilities is reduced to the guaranteed integrity of physical barriers and reliable localization of nuclear and radioactive materials.

An opportunity for early detection of through micro-cracks and control of their development should prevent critical damage of the primary cooling circuit pipeline elements, resulting in essential increase of the NPP safe operation level.

At present, the following systems of continuous control of leak in the primary cooling circuit pipelines are used:

  1. System of leak monitoring from measurement of humidity or the leak volume activity, which actually is an indicating instrument and has the following disadvantages:
    • leak location is not determined;
    • sensitivity does not exceed 40-50 l/hour;
    • operational speed, that is response time to the leak occurrence is usually not less than 1 hour.
  2. System of leak monitoring from measurement of acoustic noise (ALUS type and its modifications) with the following disadvantages:
    • leak location is determined as a pipeline section between a pair of transducers and is usually about 6 m;
    • a continuous set of the industrial noise database is required;
    • interpretation of information is rather complicated and not always unambiguous;
    • sensitivity does not exceed 20-30 l/hour;
    • operational speed, that is response time to the leak occurrence is usually 5-8 min, taking into account the operator final decision making time.
  3. Periodic inspection of the primary cooling circuit pipeline metal condition is carried out during scheduled preventive maintenance and has the following known design disadvantages:
    • configuration of the primary cooling circuit pipeline is such that considerable part of welded seams is technologically inaccessible to mechanical ultrasonic examination. Manual diagnostics is less informative and subject to impact of human factor;
    • the availability of asymmetric welded seams connecting pipes with non-uniform wall thickness complicates operational control and introduces an uncertainty element in calculation algorithms.
Besides, occasionally it is necessary to control additional pipeline sections, because the structure of the pressure compensator manifold (surge lines 200 mm in diameter) uses spring suspensions without stiff supports, which can result in excessive thermal displacement of elements and additional concentration of stresses in closing welded seams, especially in transitional regimes of reactor unit.

Hence, the existing monitoring system of integrity of the primary piping elements has a series of above mentioned shortcomings, which can be removed as follows:

  • To develop a system of early detection of through micro-cracks in primary cooling circuit pipelines aimed at control of their development and localization of leak section;
  • To carry out theoretical estimation of the most probable areas of leak formation and experimental verification of calculation results aimed at increasing the control efficiency and reducing cost of the offered equipment.

When forming a part of Soviet atomic energetic system with well-defined functional and information limits, Armenian nuclear power plant received a strictly dosed and hierarchically authorized access to some scientific achievements in this branch of industry.

The work scope of the offered Project is especially important for the Armenian NPP, because it could partially compensate the known lack of scientific and research support of its operation after collapse of the Soviet Union.

The basic objective of the proposed Project is increase of safety and reliability of NPP operation due to development of a method and apparatus for early identification of through micro-cracks in primary cooling circuit pipeline, control of their development and localization of leak section.

The offered system will provide decrease in probability of both critical damage of the primary cooling circuit pipeline elements and equipment failure during operation.

Effectiveness of use of such system is determined not only by its proposed high sensitivity (10-3 l/hour), but also by a correct choice of the pipeline sections for leak monitoring. Therefore, identification of pipeline sections with the highest probability of leak formation would be an important component of the problem solution. Development of a system of leak detection in primary cooling circuit pipeline (when the leak rate is yet much less than the critical one) and determination of its location should allow the NPP operational staff observing crack development and making better decisions.

Optimization of location and number of controlled pipeline sections will be carried out by means of identification of the most dangerous sections in terms of through micro-cracks formation, for which purpose:

  • Calculations of mechanical loads on the main circulating line and surge line will be performed within the framework of linear theory of elasticity;
  • Conformity of these calculations with real situation will be checked by measurements of deformations and displacements of the pipeline elements at normal operation mode of NPP.

Development of topology of a system of strain-gauge transducers for remote detection of deformations and displacements as well as corresponding software for data gathering and processing will allow obtaining experimental information for correlation of the calculation results.

Also, displacements of equipment elements of primary cooling circuit pipeline will be measured at transient processes of reactor unit and changes in spatial orientation of the primary cooling circuit pipeline equipment (due to above mentioned displacements) resulting in possible failures of the primary cooling circuit pipeline equipment and/or formation of through micro-cracks, will be estimated.

The method of leak identification consists of a correct choice of the physical phenomenon, which forms a basis for the measurement and provides insignificant dependence on environmental conditions, uniqueness of the measurement results and high sensitivity. It is supposed to use the dependence of leak through micro-cracks on the volume activity of a radionuclide used as a reference mark for the NPP primary coolant piping. Sampling and transportation of the leak-related steam-air mixture to the activity measuring module is performed by carrier gas through a set of controlled valves, system of collectors and sampling boxes.

The apparatus for determination of leak location and the radioactive substance leakage rate for the primary coolant circuit pipeline section will contain an aspiration pump, controlled valves and collector connected to the measuring chamber, in which an instrument for measurement of reference radionuclide volume activity in steam-air mixture should be placed. Controlled areas of the primary coolant circuit pipelines, selected as the most dangerous in terms of leak formation, will be tightly encircled with boxes equipped with the nozzles for input and output of carrier gas at two diametrically opposite points; the output nozzles will be connected to the collector through the controlled valves, and the aspiration pump will be placed at the output of measuring chamber.

The offered system will use a gamma spectrometer, common to the whole system, to measure activity of leak from any controlled section of the NPP primary coolant circuit pipeline.

The sampling device and transportation system will provide accuracy of leak localization within ±10 cm and delivery time to the measuring unit not more than 20 seconds.

During the Project implementation, obtaining of the following results is expected:

  1. Development of a high-sensitivity system of early identification of through micro-cracks in the primary coolant circuit pipelines and control of their development, which allows:
    • preventing guillotine rupture of the primary coolant circuit pipelines as well as loss of coolant;
    • reducing safety shortage for the primary coolant piping less than 200 mm in diameter;
    • excluding the necessity for visual search of leak spots and, accordingly, direct exposure of operating personal to radioactive elements;
    • determining - in a unique and simple manner - co-ordinates of the pipeline sections, in which through cracks are formed;
    • an unambiguous assigning of the measured activity to a concrete leak with known co-ordinates;
    • reducing radiation exposure of maintenance personnel and nondestructive testing operators;
    • reducing the nuclear power plant emission activity.
  2. Theoretically justified and experimentally confirmed data on maximum stresses in the primary piping elements arising at transient processes, and a list dangerous zones with the highest probability of through micro crack formation.
    • development and realization of a monitoring system and registration of displacements of the primary coolant circuit pipelines.
The Project realization will enable solution of several problems serving the ISTC purposes:
  • Reorientation of activity and maintenance of long-term professional work of scientists and experts earlier specialized in development and production of weapons of mass destruction, to the solution of the civilian sector problems.
  • The Project will allow the scientists involved in weapon production, to reorient their activity and acquire skills in the area of NPP safety. This problem is important for Armenia, where nuclear power plant is in use.
  • The Project collaborators are recognized leaders in maintenance of safety of nuclear power plants, protection of the environment and population against the NPP radioactive emissions. Their participation in the Project will guarantee continuity of methodological and technical approaches to the Problem tasks.
  • The experience gained in the course of the Project implementation could be used by the countries which build their energy policy upon the operation of existing and construction of new nuclear power plants.


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