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Strength monitoring of a prestressed concrete containment with grouted tendons
Nuclear Engineering and Design 216 (2002) 213– 220 www.elsevier.com/locate/nucengdes
Strength monitoring of a prestressed concrete containment with grouted tendons Zaozhan Sun a,*, Sujuan Liu a, Songtao Lin b, Yongjin Xie b a
Nuclear Safety Center, State En6ironmental Protection Administration, 54, Hong Lian Nan Cun, Haidian District, Beijing 100088, China b Engineering Structure Laboratory, Central Research Institute of Building and Construction, M.M.I., Beijing, China Received 15 March 2001; received in revised form 25 January 2002; accepted 28 January 2002
Abstract Existing containments are all required to be checked for their strength periodically through inservice inspection programs. In China, all the containment prestress tendons are protected by cement grouting, except the sampling tendons for testing. To improve the methodology and accuracy of the present inservice inspections for grouted tendon containments, a new scheme of containment strength verification is proposed. It applies to all prestressed containments, and it can be used as a continuous monitoring tool. The greatest advantage of the new inspection scheme is that it suggests another possible way of monitoring the prestress levels in concrete when tendon force measurements become impossible in a case where all the tendons are grouted with cement. The measured quantities are the displacements of critical points in the containment cylinder and the dome. The apparatus is installed permanently outside the containment, and the data readings can be done any time. The improved accuracy of the apparatus contributes to make these measurements a meaningful source of continuous monitoring data. The application of the new scheme in Qinshan Nuclear Power Plant has verified its practicability. At the same time, it reveals that proper application of such a monitoring system requires careful beforehand arrangements. © 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The containment plays a significant role in ensuring nuclear safety. China’s currently effective regulatory document, HAF 0200(91), ‘Code on the Nuclear Power Plant Design’ (China NNSA,
* Corresponding author. Tel.: + 86-10-622-58197; fax: + 86-10-622-57804. E-mail address: [email protected], sunzaozhan@sohu. com (Z. Sun).
1991), regards the concept of defense in depth as an important part of the safety principles. It requires that a series of echelons of equipment and procedures be provided in order to prevent accidents and to ensure appropriate protection in the event that accident prevention fails. HAF 0200(91) states that ‘To keep the release of radioactivity to the environment below acceptable limits in accident conditions, a system of containment shall be provided unless it can be demonstrated that the release of radioactivity can be limited by other means’. The number one require-
0029-5493/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 0 2 ) 0 0 0 6 1 - 4
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ment for a qualified containment that can realize the goal of the third level defense is that it must have sufficient strength. This is a prerequisite for the containment to carry out the confinement function. Up to now, all the containments of nuclear power plants built in China have adopted cement grouted prestress systems. To ensure that prestress losses over a plant lifetime of decades remains below prescribed values, almost all the nuclear safety regulatory bodies in the world require inservice inspections of the containment prestress tendons. Qinshan nuclear power plant (QNPP) was originally committed to follow US RG 1.90 (USNRC, 1977) to do the tendon inservice inspections, even though the required liftoff forces of tendons can not be measured because the reserved ungrouted tendons had been grouted with cement. Because the pressurized containment strength test must be done during plant outage periods and the process of instrumentation and pressurization costs much time, the pressurized tests are usually not performed very frequently. To facilitate the instrumentation and observations and to shorten the time period of the test, the Central Research Institute of Building and Construction, who took the responsibility of QNPP’s prestressed tendon inservice inspection, developed a method that allows all the instrumentation and observations be done outside the containment. The method is similar to French practices. More accurate apparatus and reading tools were developed so that the monitoring of prestress status under plant operational state was also made possible. This makes up, more or less, for the deficiencies in real time monitoring of the containment prestress level because of the grouting of the originally reserved ungrouted tendons. The scheme of QNPP’s prestress level inservice inspection was changed, after the review and endorsement of National Nuclear Safety Administration (NNSA), to the following: pressurized tests once every 10 years plus annual unpressurized observations. In fact unpressurized observations can be done whenever needed because the apparatus is installed permanently outside the containment. With the improvement of test skills and measuring accuracy through practice, a more
reasonable and reliable inservice inspection scheme is expected to be established in the near future. Furthermore, the new scheme helps a lot in creating a database, which can be used in checking the design theories and in understanding the changing properties of prestressed concrete containments.
2. The necessity of real time monitoring for prestress levels For inservice inspections of containment strength with grouted tendons, US RG 1.90 prescribes three activities. They are the liftoff measurement of ungrouted tendons, prestress level measurement that is also known as Alternative A or deformation measurement that is also known as Alternative B during periodic pressure tests and visual inspections during pressure tests. The Alternative A is comparatively difficult to implement and the instruments are also difficult to maintain, so it is not popular in practice. For the Alternative B, RG 1.90 requires a test pressure at containment design pressure or so, and at a frequency of once in about 7 years. The French RCC-G (France EDF, 1980) also requires liftoff measurements for ungrouted tendons. For the pressure tests RCC-G only requires deformation measurements for a test pressure at containment design pressure and at a frequency of once in a decade. The Chinese EJ/T 926-1995 (China CNPC, 1996) presents similar requirements to those of RCC-G. The Russian PNAE G-10-021-90 (Russia ENTEK, 1993) requires, beside the ungrouted tendon observations, only one containment strength test for the entire lifetime of the plant. This test is done at the plant commissioning stage and repeated strength testing are only necessary when components affecting the containment strength are repaired or replaced. There are different requirements for checking containment strength by inservice inspections in different countries, except that the ungrouted tendons are required everywhere. The inspections can provide prestress level information whenever needed. RG 1.90 states that ‘Some tendons (otherwise identical) are left ungrouted and are pro-
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tected from corrosion with grease. The changes observed in these tendons are not intended to represent the changes due to environmental or physical effects (with respect to corrosion) in the grouted tendons. Instead, these test tendons will be used as reference tendons to evaluate the extent of concrete creep and shrinkage and relaxation of the tendon steel’. RCC-G assumes that although it is difficult to measure performance changes for the prestressed tendons, the load cells installed in ungrouted tendons can measure the force reductions caused by containment strength degradation, and a limit value can be determined so that the tendon forces are identified to be less than those used in design calculations. It can be seen that the ungrouted test tendons are not unnecessary but play an unique role especially if they can be used as a real time monitoring tool. However, checking forces in test tendons does not reveal the exact strength conditions of the grouted tendons because of the different environmental conditions for the two types of tendons. This paper stresses the necessity of some kind of real time monitoring because of the special importance of a qualified containment. Periodic pressure tests may also be necessary at present. RCC-G states that there is still no fully satisfactory method of ensuring containment integrity. Among the various protection measures used, cement grouting is a good and effective method. But, it is still not impossible for some tendons to fail completely during the containment lifetime. Considering that such failures are a local phenomenon and are of a slow process, some local cracks may be found during periodic tests and before complete failure occurs, provided that the test pressure is adequately large. This brings about a dilemma. To make sure the containment strength degradation is detected sufficiently early before it reaches an unacceptable condition, the time interval between two pressure tests should be short. On the other hand, to reduce as much as possible the testing induced structural damage, pressure tests should not be very frequent. Implementing some kind of real time monitoring can deal with this difficult situation by providing timely information.
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3. The philosophy of the Qinshan new inservice inspection scheme The original design of containment prestress system of QNPP follows the requirements of RG 1.90. There were three tendons, respectively in hoop, vertical and dome part, which remained ungrouted but greased to protect the tendons from becoming rusted. The purpose of doing so was to permit a possible real time monitoring of the prestress level through installing load cells in these tendons. All the rest of the tendons were cement-grouted for rust prevention. After the initial structural integrity test (ISIT), all the ungrouted tendons were grouted with cement mainly because of grease leaking, and this made impossible the real time monitoring of the prestress level. But, on the other hand, the above discussion explains the necessity of some kind of real time monitoring, as well as pressure tests with proper time intervals. Real time monitoring means that it must be possible to be carried out during plant operation. Therefore, all the measurement and reading apparatus must be installed permanently outside of the containment, and they must be available whenever needed. On the other hand, the implementation of this plan will bring the following advantages. First, it may reduce the required plant outage period for pressure tests. Because of the limited space, installation of instruments required by RG 1.90 Alternative B in the containment presents great difficulty. The installation and uninstallation may require a long time, which in turn requires a long outage period. Second, the permanent installation of instrumentation outside the containment and the any-time reading of observed data provide the possibility of the gradual substitution of special strength test personnel with regular plant workers. Third, the radioactivity, if any, is lower outside the containment than inside it and therefore outside containment measurements are beneficial to the test personnel both physically and psychologically. How can a plant implement the outside of containment measurements? Making reference to the French practice, QNPP chose to adopt an indirect method. The overall deformation of the
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containment is measured and is used to judge the strength status. The deformation of the cylindrical wall is measured by recording the displacements of selected points relative to a fixed point, and the deformation of the dome is measured by recording the displacements of selected points relative to the girder. The measurements for pressure tests can be realized by recording the position changes of selected points before, between, and after the establishment of the pressure, and real time monitoring requires recording of the time dependant position changes of selected points under plant operational conditions. The deformation of a prestressed containment is usually very small even under the pressure test condition, not to mention under the operational conditions. Therefore, the measurements require very high accuracy apparatus. Besides, the very small expected change of prestress level during the plant lifetime requires the real time monitoring to be based on very accurate predictions of time dependent prestress losses. Central Research Institute of Building and Construction, which took the work of QNPP’s containment strength inservice inspection and succeeded in developing the needed high accuracy instruments and apparatus. This is a large step forward and makes possible a new way of real time monitoring of prestressed containment strength.
4. The main technical issues of the new scheme With the permission of NNSA, QNPP modified the containment strength inservice inspection scheme. The new scheme adopts 10-year-period pressure tests together with annual normal condition readings. The acceptance criteria for the nuclear safety evaluation can be determined based on the following considerations. First, the pressure test should verify that the prestress system provides the containment with so high a strength that it remains within elastic conditions under design basis accident pressures. Second, the normal condition monitoring activities should provide the Utility and the Regulatory Body with sufficient confidence that the prestress system, since the last pressure test, has not experienced changes beyond expected acceptable values.
The 10-year interval is based on the RCC-G requirements. The periodic resistance tests are not planned. However, during periodic overall leaktightness tests (every 10 years) a complete reading of instrument meters is made. The acceptance criteria for the pressure test puts stress on the elastic condition of the containment. In other words, what the test examines is the strength of the containment at the time when it is tested, and it can be obtained by finding the relationship between the measured containment deformation and the test pressure. The requirements of ACI-359 (US ACI, 1986) include (1) the deformation in the expected maximum deformation location should recover more than 80% within 24 h after the complete removal of the pressure, and (2) the maximum deformation measured at the expected maximum deformation location should not be 30% or more than the expected value. The second requirement can be eliminated if the deformation recovers more than 90% within 24 h after the complete removal of the pressure. Because the measured value is a relative quantity reflecting the difference between the state in the test and the state before the test starts, and this value is comparative larger, the pressure test does not impose very high accuracy requirements on the instruments and apparatus. It is also easier to implement and the results are easier to use for strength evaluations. The annual normal condition monitoring stresses the time-dependent containment strength changes. It is intended to make sure that, except for the expected strength changes caused by factors such as time dependent prestress losses, the containment bearing capacity does not experience unacceptable change caused by unexpected factors. For the acceptable prestress losses, RG 1.90 gives the limit values shown in Fig. 1. RG 1.35.1 (USNRC, 1990) also gives similar limit value requirements. It can be seen from the figure that for the lifetime of a nuclear power plant, usually 40 years, the allowed maximum prestress losses are generally less than 10% of the initial prestress forces. Such losses usually produce only very small containment deformations. Because the quantity that needs to be measured is very small, the requirements for the instruments and appara-
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tus are very high. Furthermore, to monitor the time dependent changes of the containment strength during the whole lifetime, the base point must be immovable and unchangeable over the whole lifetime if the deformation is selected as the representative quantity.
5. The implementation of the new inspection scheme QNPP has adopted a new inservice inspection scheme in order to evaluate more accurately the containment strength condition. Making reference to RG 1.90 and RCC-G, the new scheme includes the following inspection items: 1. Visual inspection. 2. Strain measurements on the concrete surface. 3. Temperature measurements on the concrete surface. 4. Overall deformation measurements. 5. Dynamic characteristics measurements. The visual inspections are conducted following the requirements of US ACI 359 (ASME Code, Volume III, Section 2). Visual inspections include examining the protection condition of the anchorages. The surface conditions, such as carbonization and cracks, of the stress concentration areas are also examined. The width and length of cracks are measured. The cracks with a width larger than 0.2 mm and a length larger than 150 mm are
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recorded. The examinations are performed before pressurization, at the peak test pressure, and after the depressurization. The liner of the containment is also examined visually. The changes of the liner’s condition 24 h after the depressurization compared to the initial condition are recorded. Strain measurements are realized by installing permanent strain transducers on the containment concrete surface. The transducers are covered with protective casks. Strain measurements, together with the deformation measurements, provide basic data for the evaluation of the containment strength condition during pressure testing. The strain measurements are mainly used to evaluate the condition of the concrete. During pressure tests, they are used to check if local stresses in concrete goes beyond the prescribed limits. Sequential readings over a long time period under constant pressure provides information which helps in determining the prestress level and helps to evaluate the concrete creep and shrinkage. No special strain meters are used that respond only to volumetric and temperature changes in concrete. But, temperature measurements are done simultaneously with the strain measurements. Strains due to volumetric and temperature changes in concrete can be numerically calculated from the data of readings. The strain gauges and thermocouples are installed at representative locations of the containment. There are altogether 13 groups of strain gauges. They are
Fig. 1. Typical band of acceptable prestress level.
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Fig. 2. Measurement of dome deflection.
distributed along a generatrix of the cylinder, along an azimuth line of the dome and along a circumferential line of the cylinder. There are three kinds of temperature measurements. The first kind is used to get an overall temperature distribution picture. The second kind is to measure the temperatures at assumed key points so as to calculate the temperature field. The third kind is to measure the temperatures where strain gauges are installed so the temperature effects on the strain readings can be evaluated. The overall deformation measurements consist of the dome deflection, the radial deformation of the cylindrical wall, and the vertical deformation of the wall. The dome deflection measurement is relative to the girder, taking the girder as the reference point. High accuracy levels are installed at midpoints between the dome vertex and the girder as seen in Fig. 2. The radial deformation measurement of the wall is carried out using a plumb line method outside the containment. The system consists of a sweeping line, upper bracket, protective cask, sleeve tube, reading pan, plumb, damper and other components as seen in Fig. 3. Optical microscopes are used for making the readings from the reading pan. The vertical deformation measurement of the wall also uses the plumb line method as seen in Fig. 4. The influence of the temperature on the sweeping line must be taken into account. A material with a low expansion coefficient is selected. The new inspection scheme was put into practice on December 30, 2000. There are 13 strain measurement points and 17 deformation measur-
Fig. 3. Schematic of the plumb line system of radial deformation.
ing points, with 5 being vertical and 12 radial. The base point was determined as the slab of the containment. The final accuracy of the measuring system is 9 0.25 mm. The preliminary results of the pressure test show that, ‘24 h after the complete removal of the pressure, the residual deformation in the area of the expected maximum deformation area did not exceed 12% of the deformation under peak pressure. The maximum deformation measured at the expected maximum deformation area did not exceed 90% of the calculated value and did not increase more than 10% compared with the measured results of the ISIT’. This met the acceptance criteria requirements. A detailed data analysis report will be compiled later.
Fig. 4. Vertical deformation measurement.
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What should be mentioned here is that great difficulties were encountered during the installation of the instrumentation because the original containment design was not intended for such inspection activities. The plumb line system should best go straight down to the base slab, but this is very difficult because there are already existing piping and equipment installed in the buildings surrounding the containment. Therefore, the proper implementation of such a scheme needs early consideration during reactor design. What is more important is that the monitoring of time dependent prestress losses can only be against the state after the absolute reference point has been established. This means that the prestress level monitoring of QNPP’s containment can only make judgements compared to the state 10 years after the completion of the tendon tensioning. However, for new containments, the authors believe that early implementation will overcome the difficulties and provide much more information.
6. The application prospects of the new inspection scheme The greatest advantage of the new inspection scheme is that it suggests another possible way of monitoring the prestress levels in concrete when tendon force measurements become impossible in cases such as when all the tendons are grouted with cement. Besides, it is clearly a simpler and more reliable method. First, the installation of the instruments and apparatus for deformation measurements can be a one-time effort and they can be used permanently once the installation has been finished. The subsquent work does not necessitate specialists or additional tools, but rather requires only routine observations and data recordings. Second, if the instruments and apparatus are installed before the tendon tensioning, the whole deformation process caused by the prestress forces can be recorded, including that during the tensioning period. Third, the base slab of the containment can be considered as a rigid body because of its very high stiffness. Therefore, the continuous monitoring
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referenced to the base slab can provide real time information reflecting the containment strength status, which should be more complete than that provided by tendon force measurements. Fourth, the accuracy can meet the current requirements and it is therefore a practicable method. The accuracy of the instrumentation is improved through carefully selecting the locations, the installation conditions, the operation conditions and protection methods. Long time beforehand readings are also done to improve the accuracy. The instruments for strain and deformation measurements are so installed and protected that they are assumed to be used till the decommissioning of the plant. This means that the measurements can be done repeatedly relative to the same baseline. The QNPP application provides an example of the measurement performance. According to the theoretical analysis results (Qinshan Nuclear Power Corporation, 1996), the full designed prestress forces cause the cylindrical shell to experience a radial displacement of 4.71 mm at the expected maximum deformation point, and the dome experiences a vertical displacement of 12.93 mm at the vertex. If the expected degradation of the containment strength is to be guaranteed within the acceptable limits for its entire life time, the safety criteria of the deformation measurements should make sure that the strength related deformation during the entire life time, referenced to the state when the ISIT is just completed, does not exceed about 10% of the deformation caused by the initial prestress forces. This value should fall in the interval between 0.5 and 1.5 mm, depending on different designs. The current deformation measurements do not give errors exceeding 0.25 mm, and so the measurements should be regarded as meeting the requirements. The authors envision many application prospects for the new scheme. The successful implementation will change the exclusive use of tendon force measurements for prestress level monitoring. The construction of the prestress system can also be simpler and more reliable. The successful application of real time deformation monitoring suggests the possibility of eliminating the inservice inspection pressure tests just from
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the viewpoint of strength. This is because acceptance of the real time deformation monitoring, the normal condition monitoring will be considered as able to provide sufficient confidence in the containment strength. A potential improvement of this method is the automatization of the monitoring. If the strength status of the containment can be demonstrated as real time signals on the control room panels, the safety characteristics of the plant will be improved. Also, this may affect the contents of Operating Procedures, Emergency Operating Procedures and Emergency Plans.
7. Conclusions and summary The following conclusions summarize the findings presented in this paper: 1. Because of the special importance of the containment, some kind of real time monitoring of its strength is desirable in addition to the periodic pressure tests. 2. The real time monitoring of the containment strength, which is mainly determined by the prestress level, can also be realized through overall deformation measurements as an alternative to tendon force measurements. 3. The deformation measurements used in real time monitoring of containment strength must
select a reference point that is immovable and unchangeable over the whole plant lifetime and must have a very high accuracy. 4. A new deformation measurement method has been adopted by QNPP and it has demonstrated to work well. 5. The new inservice inspection scheme has other promising application prospects.
References China NNSA, 1991. Code on the Safety of Nuclear Power Plant Design. HAF0200(91). USNRC, 1977. Inservice Inspection of Prestressed Concrete Containment Structures with Grouted Tendons. RG 1.90 (Rev. 1). France EDF, 1980. Design and Construction Rules for Civil Works of PWR Nuclear Islands. RCC-G. China CNPC, 1996. Code for Prestresed Concrete Containment Design of Nuclear Power Plant with Pressurized Water Reactors. EJ/T 926-1995 (in Chinese). Russia ENTEK, 1993. Regulations for the Arrangement and Operation of Localizing Safety Systems at Nuclear Power Plant. PNAE G-10-021-90 (English Version). US ACI, 1986. Code for Concrete Reactor Vessels and Containments. ACI359 – 86. USNRC, 1990. Determining Prestressing Forces for Inspection of Prestressed Concrete Containments. RG 1.35.1. Qinshan Nuclear Power Corporation, 1996. Final Safety Analysis Report of Qinshan 1 Nuclear Power Plant (Revised), (in Chinese).
Strength monitoring of a prestressed concrete containment with grouted tendons Zaozhan Sun a,*, Sujuan Liu a, Songtao Lin b, Yongjin Xie b a
Nuclear Safety Center, State En6ironmental Protection Administration, 54, Hong Lian Nan Cun, Haidian District, Beijing 100088, China b Engineering Structure Laboratory, Central Research Institute of Building and Construction, M.M.I., Beijing, China Received 15 March 2001; received in revised form 25 January 2002; accepted 28 January 2002
Abstract Existing containments are all required to be checked for their strength periodically through inservice inspection programs. In China, all the containment prestress tendons are protected by cement grouting, except the sampling tendons for testing. To improve the methodology and accuracy of the present inservice inspections for grouted tendon containments, a new scheme of containment strength verification is proposed. It applies to all prestressed containments, and it can be used as a continuous monitoring tool. The greatest advantage of the new inspection scheme is that it suggests another possible way of monitoring the prestress levels in concrete when tendon force measurements become impossible in a case where all the tendons are grouted with cement. The measured quantities are the displacements of critical points in the containment cylinder and the dome. The apparatus is installed permanently outside the containment, and the data readings can be done any time. The improved accuracy of the apparatus contributes to make these measurements a meaningful source of continuous monitoring data. The application of the new scheme in Qinshan Nuclear Power Plant has verified its practicability. At the same time, it reveals that proper application of such a monitoring system requires careful beforehand arrangements. © 2002 Elsevier Science B.V. All rights reserved.
1. Introduction The containment plays a significant role in ensuring nuclear safety. China’s currently effective regulatory document, HAF 0200(91), ‘Code on the Nuclear Power Plant Design’ (China NNSA,
* Corresponding author. Tel.: + 86-10-622-58197; fax: + 86-10-622-57804. E-mail address: [email protected], sunzaozhan@sohu. com (Z. Sun).
1991), regards the concept of defense in depth as an important part of the safety principles. It requires that a series of echelons of equipment and procedures be provided in order to prevent accidents and to ensure appropriate protection in the event that accident prevention fails. HAF 0200(91) states that ‘To keep the release of radioactivity to the environment below acceptable limits in accident conditions, a system of containment shall be provided unless it can be demonstrated that the release of radioactivity can be limited by other means’. The number one require-
0029-5493/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 0 2 ) 0 0 0 6 1 - 4
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ment for a qualified containment that can realize the goal of the third level defense is that it must have sufficient strength. This is a prerequisite for the containment to carry out the confinement function. Up to now, all the containments of nuclear power plants built in China have adopted cement grouted prestress systems. To ensure that prestress losses over a plant lifetime of decades remains below prescribed values, almost all the nuclear safety regulatory bodies in the world require inservice inspections of the containment prestress tendons. Qinshan nuclear power plant (QNPP) was originally committed to follow US RG 1.90 (USNRC, 1977) to do the tendon inservice inspections, even though the required liftoff forces of tendons can not be measured because the reserved ungrouted tendons had been grouted with cement. Because the pressurized containment strength test must be done during plant outage periods and the process of instrumentation and pressurization costs much time, the pressurized tests are usually not performed very frequently. To facilitate the instrumentation and observations and to shorten the time period of the test, the Central Research Institute of Building and Construction, who took the responsibility of QNPP’s prestressed tendon inservice inspection, developed a method that allows all the instrumentation and observations be done outside the containment. The method is similar to French practices. More accurate apparatus and reading tools were developed so that the monitoring of prestress status under plant operational state was also made possible. This makes up, more or less, for the deficiencies in real time monitoring of the containment prestress level because of the grouting of the originally reserved ungrouted tendons. The scheme of QNPP’s prestress level inservice inspection was changed, after the review and endorsement of National Nuclear Safety Administration (NNSA), to the following: pressurized tests once every 10 years plus annual unpressurized observations. In fact unpressurized observations can be done whenever needed because the apparatus is installed permanently outside the containment. With the improvement of test skills and measuring accuracy through practice, a more
reasonable and reliable inservice inspection scheme is expected to be established in the near future. Furthermore, the new scheme helps a lot in creating a database, which can be used in checking the design theories and in understanding the changing properties of prestressed concrete containments.
2. The necessity of real time monitoring for prestress levels For inservice inspections of containment strength with grouted tendons, US RG 1.90 prescribes three activities. They are the liftoff measurement of ungrouted tendons, prestress level measurement that is also known as Alternative A or deformation measurement that is also known as Alternative B during periodic pressure tests and visual inspections during pressure tests. The Alternative A is comparatively difficult to implement and the instruments are also difficult to maintain, so it is not popular in practice. For the Alternative B, RG 1.90 requires a test pressure at containment design pressure or so, and at a frequency of once in about 7 years. The French RCC-G (France EDF, 1980) also requires liftoff measurements for ungrouted tendons. For the pressure tests RCC-G only requires deformation measurements for a test pressure at containment design pressure and at a frequency of once in a decade. The Chinese EJ/T 926-1995 (China CNPC, 1996) presents similar requirements to those of RCC-G. The Russian PNAE G-10-021-90 (Russia ENTEK, 1993) requires, beside the ungrouted tendon observations, only one containment strength test for the entire lifetime of the plant. This test is done at the plant commissioning stage and repeated strength testing are only necessary when components affecting the containment strength are repaired or replaced. There are different requirements for checking containment strength by inservice inspections in different countries, except that the ungrouted tendons are required everywhere. The inspections can provide prestress level information whenever needed. RG 1.90 states that ‘Some tendons (otherwise identical) are left ungrouted and are pro-
Z. Sun et al. / Nuclear Engineering and Design 216 (2002) 213–220
tected from corrosion with grease. The changes observed in these tendons are not intended to represent the changes due to environmental or physical effects (with respect to corrosion) in the grouted tendons. Instead, these test tendons will be used as reference tendons to evaluate the extent of concrete creep and shrinkage and relaxation of the tendon steel’. RCC-G assumes that although it is difficult to measure performance changes for the prestressed tendons, the load cells installed in ungrouted tendons can measure the force reductions caused by containment strength degradation, and a limit value can be determined so that the tendon forces are identified to be less than those used in design calculations. It can be seen that the ungrouted test tendons are not unnecessary but play an unique role especially if they can be used as a real time monitoring tool. However, checking forces in test tendons does not reveal the exact strength conditions of the grouted tendons because of the different environmental conditions for the two types of tendons. This paper stresses the necessity of some kind of real time monitoring because of the special importance of a qualified containment. Periodic pressure tests may also be necessary at present. RCC-G states that there is still no fully satisfactory method of ensuring containment integrity. Among the various protection measures used, cement grouting is a good and effective method. But, it is still not impossible for some tendons to fail completely during the containment lifetime. Considering that such failures are a local phenomenon and are of a slow process, some local cracks may be found during periodic tests and before complete failure occurs, provided that the test pressure is adequately large. This brings about a dilemma. To make sure the containment strength degradation is detected sufficiently early before it reaches an unacceptable condition, the time interval between two pressure tests should be short. On the other hand, to reduce as much as possible the testing induced structural damage, pressure tests should not be very frequent. Implementing some kind of real time monitoring can deal with this difficult situation by providing timely information.
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3. The philosophy of the Qinshan new inservice inspection scheme The original design of containment prestress system of QNPP follows the requirements of RG 1.90. There were three tendons, respectively in hoop, vertical and dome part, which remained ungrouted but greased to protect the tendons from becoming rusted. The purpose of doing so was to permit a possible real time monitoring of the prestress level through installing load cells in these tendons. All the rest of the tendons were cement-grouted for rust prevention. After the initial structural integrity test (ISIT), all the ungrouted tendons were grouted with cement mainly because of grease leaking, and this made impossible the real time monitoring of the prestress level. But, on the other hand, the above discussion explains the necessity of some kind of real time monitoring, as well as pressure tests with proper time intervals. Real time monitoring means that it must be possible to be carried out during plant operation. Therefore, all the measurement and reading apparatus must be installed permanently outside of the containment, and they must be available whenever needed. On the other hand, the implementation of this plan will bring the following advantages. First, it may reduce the required plant outage period for pressure tests. Because of the limited space, installation of instruments required by RG 1.90 Alternative B in the containment presents great difficulty. The installation and uninstallation may require a long time, which in turn requires a long outage period. Second, the permanent installation of instrumentation outside the containment and the any-time reading of observed data provide the possibility of the gradual substitution of special strength test personnel with regular plant workers. Third, the radioactivity, if any, is lower outside the containment than inside it and therefore outside containment measurements are beneficial to the test personnel both physically and psychologically. How can a plant implement the outside of containment measurements? Making reference to the French practice, QNPP chose to adopt an indirect method. The overall deformation of the
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containment is measured and is used to judge the strength status. The deformation of the cylindrical wall is measured by recording the displacements of selected points relative to a fixed point, and the deformation of the dome is measured by recording the displacements of selected points relative to the girder. The measurements for pressure tests can be realized by recording the position changes of selected points before, between, and after the establishment of the pressure, and real time monitoring requires recording of the time dependant position changes of selected points under plant operational conditions. The deformation of a prestressed containment is usually very small even under the pressure test condition, not to mention under the operational conditions. Therefore, the measurements require very high accuracy apparatus. Besides, the very small expected change of prestress level during the plant lifetime requires the real time monitoring to be based on very accurate predictions of time dependent prestress losses. Central Research Institute of Building and Construction, which took the work of QNPP’s containment strength inservice inspection and succeeded in developing the needed high accuracy instruments and apparatus. This is a large step forward and makes possible a new way of real time monitoring of prestressed containment strength.
4. The main technical issues of the new scheme With the permission of NNSA, QNPP modified the containment strength inservice inspection scheme. The new scheme adopts 10-year-period pressure tests together with annual normal condition readings. The acceptance criteria for the nuclear safety evaluation can be determined based on the following considerations. First, the pressure test should verify that the prestress system provides the containment with so high a strength that it remains within elastic conditions under design basis accident pressures. Second, the normal condition monitoring activities should provide the Utility and the Regulatory Body with sufficient confidence that the prestress system, since the last pressure test, has not experienced changes beyond expected acceptable values.
The 10-year interval is based on the RCC-G requirements. The periodic resistance tests are not planned. However, during periodic overall leaktightness tests (every 10 years) a complete reading of instrument meters is made. The acceptance criteria for the pressure test puts stress on the elastic condition of the containment. In other words, what the test examines is the strength of the containment at the time when it is tested, and it can be obtained by finding the relationship between the measured containment deformation and the test pressure. The requirements of ACI-359 (US ACI, 1986) include (1) the deformation in the expected maximum deformation location should recover more than 80% within 24 h after the complete removal of the pressure, and (2) the maximum deformation measured at the expected maximum deformation location should not be 30% or more than the expected value. The second requirement can be eliminated if the deformation recovers more than 90% within 24 h after the complete removal of the pressure. Because the measured value is a relative quantity reflecting the difference between the state in the test and the state before the test starts, and this value is comparative larger, the pressure test does not impose very high accuracy requirements on the instruments and apparatus. It is also easier to implement and the results are easier to use for strength evaluations. The annual normal condition monitoring stresses the time-dependent containment strength changes. It is intended to make sure that, except for the expected strength changes caused by factors such as time dependent prestress losses, the containment bearing capacity does not experience unacceptable change caused by unexpected factors. For the acceptable prestress losses, RG 1.90 gives the limit values shown in Fig. 1. RG 1.35.1 (USNRC, 1990) also gives similar limit value requirements. It can be seen from the figure that for the lifetime of a nuclear power plant, usually 40 years, the allowed maximum prestress losses are generally less than 10% of the initial prestress forces. Such losses usually produce only very small containment deformations. Because the quantity that needs to be measured is very small, the requirements for the instruments and appara-
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tus are very high. Furthermore, to monitor the time dependent changes of the containment strength during the whole lifetime, the base point must be immovable and unchangeable over the whole lifetime if the deformation is selected as the representative quantity.
5. The implementation of the new inspection scheme QNPP has adopted a new inservice inspection scheme in order to evaluate more accurately the containment strength condition. Making reference to RG 1.90 and RCC-G, the new scheme includes the following inspection items: 1. Visual inspection. 2. Strain measurements on the concrete surface. 3. Temperature measurements on the concrete surface. 4. Overall deformation measurements. 5. Dynamic characteristics measurements. The visual inspections are conducted following the requirements of US ACI 359 (ASME Code, Volume III, Section 2). Visual inspections include examining the protection condition of the anchorages. The surface conditions, such as carbonization and cracks, of the stress concentration areas are also examined. The width and length of cracks are measured. The cracks with a width larger than 0.2 mm and a length larger than 150 mm are
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recorded. The examinations are performed before pressurization, at the peak test pressure, and after the depressurization. The liner of the containment is also examined visually. The changes of the liner’s condition 24 h after the depressurization compared to the initial condition are recorded. Strain measurements are realized by installing permanent strain transducers on the containment concrete surface. The transducers are covered with protective casks. Strain measurements, together with the deformation measurements, provide basic data for the evaluation of the containment strength condition during pressure testing. The strain measurements are mainly used to evaluate the condition of the concrete. During pressure tests, they are used to check if local stresses in concrete goes beyond the prescribed limits. Sequential readings over a long time period under constant pressure provides information which helps in determining the prestress level and helps to evaluate the concrete creep and shrinkage. No special strain meters are used that respond only to volumetric and temperature changes in concrete. But, temperature measurements are done simultaneously with the strain measurements. Strains due to volumetric and temperature changes in concrete can be numerically calculated from the data of readings. The strain gauges and thermocouples are installed at representative locations of the containment. There are altogether 13 groups of strain gauges. They are
Fig. 1. Typical band of acceptable prestress level.
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Fig. 2. Measurement of dome deflection.
distributed along a generatrix of the cylinder, along an azimuth line of the dome and along a circumferential line of the cylinder. There are three kinds of temperature measurements. The first kind is used to get an overall temperature distribution picture. The second kind is to measure the temperatures at assumed key points so as to calculate the temperature field. The third kind is to measure the temperatures where strain gauges are installed so the temperature effects on the strain readings can be evaluated. The overall deformation measurements consist of the dome deflection, the radial deformation of the cylindrical wall, and the vertical deformation of the wall. The dome deflection measurement is relative to the girder, taking the girder as the reference point. High accuracy levels are installed at midpoints between the dome vertex and the girder as seen in Fig. 2. The radial deformation measurement of the wall is carried out using a plumb line method outside the containment. The system consists of a sweeping line, upper bracket, protective cask, sleeve tube, reading pan, plumb, damper and other components as seen in Fig. 3. Optical microscopes are used for making the readings from the reading pan. The vertical deformation measurement of the wall also uses the plumb line method as seen in Fig. 4. The influence of the temperature on the sweeping line must be taken into account. A material with a low expansion coefficient is selected. The new inspection scheme was put into practice on December 30, 2000. There are 13 strain measurement points and 17 deformation measur-
Fig. 3. Schematic of the plumb line system of radial deformation.
ing points, with 5 being vertical and 12 radial. The base point was determined as the slab of the containment. The final accuracy of the measuring system is 9 0.25 mm. The preliminary results of the pressure test show that, ‘24 h after the complete removal of the pressure, the residual deformation in the area of the expected maximum deformation area did not exceed 12% of the deformation under peak pressure. The maximum deformation measured at the expected maximum deformation area did not exceed 90% of the calculated value and did not increase more than 10% compared with the measured results of the ISIT’. This met the acceptance criteria requirements. A detailed data analysis report will be compiled later.
Fig. 4. Vertical deformation measurement.
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What should be mentioned here is that great difficulties were encountered during the installation of the instrumentation because the original containment design was not intended for such inspection activities. The plumb line system should best go straight down to the base slab, but this is very difficult because there are already existing piping and equipment installed in the buildings surrounding the containment. Therefore, the proper implementation of such a scheme needs early consideration during reactor design. What is more important is that the monitoring of time dependent prestress losses can only be against the state after the absolute reference point has been established. This means that the prestress level monitoring of QNPP’s containment can only make judgements compared to the state 10 years after the completion of the tendon tensioning. However, for new containments, the authors believe that early implementation will overcome the difficulties and provide much more information.
6. The application prospects of the new inspection scheme The greatest advantage of the new inspection scheme is that it suggests another possible way of monitoring the prestress levels in concrete when tendon force measurements become impossible in cases such as when all the tendons are grouted with cement. Besides, it is clearly a simpler and more reliable method. First, the installation of the instruments and apparatus for deformation measurements can be a one-time effort and they can be used permanently once the installation has been finished. The subsquent work does not necessitate specialists or additional tools, but rather requires only routine observations and data recordings. Second, if the instruments and apparatus are installed before the tendon tensioning, the whole deformation process caused by the prestress forces can be recorded, including that during the tensioning period. Third, the base slab of the containment can be considered as a rigid body because of its very high stiffness. Therefore, the continuous monitoring
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referenced to the base slab can provide real time information reflecting the containment strength status, which should be more complete than that provided by tendon force measurements. Fourth, the accuracy can meet the current requirements and it is therefore a practicable method. The accuracy of the instrumentation is improved through carefully selecting the locations, the installation conditions, the operation conditions and protection methods. Long time beforehand readings are also done to improve the accuracy. The instruments for strain and deformation measurements are so installed and protected that they are assumed to be used till the decommissioning of the plant. This means that the measurements can be done repeatedly relative to the same baseline. The QNPP application provides an example of the measurement performance. According to the theoretical analysis results (Qinshan Nuclear Power Corporation, 1996), the full designed prestress forces cause the cylindrical shell to experience a radial displacement of 4.71 mm at the expected maximum deformation point, and the dome experiences a vertical displacement of 12.93 mm at the vertex. If the expected degradation of the containment strength is to be guaranteed within the acceptable limits for its entire life time, the safety criteria of the deformation measurements should make sure that the strength related deformation during the entire life time, referenced to the state when the ISIT is just completed, does not exceed about 10% of the deformation caused by the initial prestress forces. This value should fall in the interval between 0.5 and 1.5 mm, depending on different designs. The current deformation measurements do not give errors exceeding 0.25 mm, and so the measurements should be regarded as meeting the requirements. The authors envision many application prospects for the new scheme. The successful implementation will change the exclusive use of tendon force measurements for prestress level monitoring. The construction of the prestress system can also be simpler and more reliable. The successful application of real time deformation monitoring suggests the possibility of eliminating the inservice inspection pressure tests just from
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the viewpoint of strength. This is because acceptance of the real time deformation monitoring, the normal condition monitoring will be considered as able to provide sufficient confidence in the containment strength. A potential improvement of this method is the automatization of the monitoring. If the strength status of the containment can be demonstrated as real time signals on the control room panels, the safety characteristics of the plant will be improved. Also, this may affect the contents of Operating Procedures, Emergency Operating Procedures and Emergency Plans.
7. Conclusions and summary The following conclusions summarize the findings presented in this paper: 1. Because of the special importance of the containment, some kind of real time monitoring of its strength is desirable in addition to the periodic pressure tests. 2. The real time monitoring of the containment strength, which is mainly determined by the prestress level, can also be realized through overall deformation measurements as an alternative to tendon force measurements. 3. The deformation measurements used in real time monitoring of containment strength must
select a reference point that is immovable and unchangeable over the whole plant lifetime and must have a very high accuracy. 4. A new deformation measurement method has been adopted by QNPP and it has demonstrated to work well. 5. The new inservice inspection scheme has other promising application prospects.
References China NNSA, 1991. Code on the Safety of Nuclear Power Plant Design. HAF0200(91). USNRC, 1977. Inservice Inspection of Prestressed Concrete Containment Structures with Grouted Tendons. RG 1.90 (Rev. 1). France EDF, 1980. Design and Construction Rules for Civil Works of PWR Nuclear Islands. RCC-G. China CNPC, 1996. Code for Prestresed Concrete Containment Design of Nuclear Power Plant with Pressurized Water Reactors. EJ/T 926-1995 (in Chinese). Russia ENTEK, 1993. Regulations for the Arrangement and Operation of Localizing Safety Systems at Nuclear Power Plant. PNAE G-10-021-90 (English Version). US ACI, 1986. Code for Concrete Reactor Vessels and Containments. ACI359 – 86. USNRC, 1990. Determining Prestressing Forces for Inspection of Prestressed Concrete Containments. RG 1.35.1. Qinshan Nuclear Power Corporation, 1996. Final Safety Analysis Report of Qinshan 1 Nuclear Power Plant (Revised), (in Chinese).