Testing of a Steel Containment Vessel Model'

V. K. Luk', M. F. Hessheimef,

T. Matsurnotob, K. Komineb, and

J. F: Costello'

"Sandia National Laboratories, Albuquerque, New Mexico, USA

Nuclear Power Engineering Corporation, Tokyo, Japan b

'United States Nuclear Regulatory Commission, Washington, D.C., USA

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement., recorn- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

rs 794- 1

Abstract

A mixed-scale steel containment vessel model, with 1:lO in containment geometry and 1:4 in shell thicknesses, was fabricated to represent an improved, boding water reactor (BWR) Mark II containment vessel. A contact structure, installed over the model and separated at a nominally uniform distance from it, provided a simpWied representation of a reactor shield building in the actual plant. This paper describes the pretest preparations and the conduct of the high pressure test of the model performed on December 11-12, 1996. * 1. Introduction

The Nuclear Power Engineering Corporation (NUPEC) of Japan and the U.S. Nuclear Regulatory Commission (hXC), Office of Nuclear Regulatory Research, have been co- sponsoring and jointly funding a Cooperative Containment Research Program at Sandia National Laboratories. The purpose of the program is to investigate the response of representative models of nuclear containment structures to pressure loading beyond the design basis accident and to compare analytical predictions with measured behavior. This is accomplished by conducting static, pneumatic overpressurization tests of scale models at ambient temperature. At present, two tests are planned: a test of a model of a steel containment vessel and a test of a model of a prestressed concrete containment vessel.

This paper describes the pressure testing of a mixed-scale model of a Steel Containment Vessel (SCV) (see Fig. l), including the test sequence and the termination criteria. The model is representative of the steel containment vessel for an Improved Mark II Boiling Water Reactor plant. The geometric scale is 1:lO. Since the same materials were used for the model as for the actual plant, the scale on the wall thickness was set at 1:4. A steel contact structure (CS) was placed over the SCV model prior to testing to represent some features of the reactor shield building in the actual plant.

Instrumentation of the model consisted of over 800 channels of data, including strain gages, displacement transducers, temperature and pressure sensors as well as video monitoring. The high pressure test of the model was conducted on December 11-12, 1996. A detailed test plan

794- 1

'This work is jointly sponsored by the Nuclear Power Engineering Corporan'on and the US. Nuclear Regulatory Commission. The work of the Nuclear Power Engineering Corporation is performed under the auspices of the Ministry of International Trade and Indmry, Japan. Sandia National Laboratories is operated for the US . Departmemr of Energy under Contract Number DE-AC04-94AL.8SOOO.

governed the conduct of the tests. This paper contains a brief discussion of the post-test observations and the preliminary test results.

In addition to the tests, pretest analyses were performed to predict the struchiral behavior of the model using finite element models with the ABAQUS code [ 11. The analysis effort supported the design of the model and the development of an instrumentation plan. Separately, a set of pretest analytical predictions, euphemistically referred to as a Round Robin analysis, was conducted by analysts from eight organizations in Germany, India, Italy, Japan and the U.S. Their efforts will contribute to the goal of validating analytical methods for the evaluation of the structural behavior of actual containments under severe accident conditions. A pretest Round Robin meeting was held on October 1-2,1996 to compare pretest predictions of the behavior of the model at the same prescribed locations. A detailed report [2] is being prepared to document the analysis results from all participants.

2. Description of the SCV Model

The model was fabricated at the Hitachi, Lid., Japan. It was transported to Sandia and eventually installed in the ‘fragment barrier’ structure. The fragment barrier housed the SCV model during instrumentation and was designed to contain the fragments and safely vent the overpressure from a catastrophic failure of the model at a maximum pressure of 13.8 MPa (2000 psig). A schematic of the SCV/CS assembly is shown in Fig. 1. The SCV model is scaled 1:lO in overall geometry and 1:4 in wall thicknesses from an actual Japanese Improved Mark-I1 B WR containment. Whereas the design pressure of the prototype containment is 0.3 1 MPa (45 psig), the scaled design pressure, Pd, for the mixed scale model is 0.78 MPa (1 13 psig).

The model consists of sections with different wall thicknesses. Except at the knuckle region, all thickness variations occur on the exterior surface of the model so that the interior surface is smooth. A representation of an equipment hatch opening with a thickened reinforcement plate is included in the model and the hatch cover is welded shut. The flanges of the top head are represented by a single thick steel ring scaled to match the hoop stiffness of the scaled flanges. The model also includes several stiffeners representative of the actual plant and the material used in the consmction of the actual plant (JIS SGV 480 and SPV 490). A l l seams are full penetration welds.

The pomon of the model above the ring support girder approximates the major features of the actual containment, but the remaining lower portion does not and its presence serves merely to complete the pressure boundary. The wetwell and the wall-basemat junction of the actual containment is not included in the model. Likewise, other hatches, airlocks, penetrations, and internal constraints are omitted from the model.

A steel bell-shaped contact structure, made of 38 mm thick, AIS1 SA516 Grade 70 steel, was installed over the SCV model prior to the pressure tests. The model was expected to come into contact with the CS at about 4 to 5 times the design pressure, resulting in deformation and failure modes which are more representative of the actual plant. The CS was welded to the ring support girder with a partial penetration weld and its top end was open, positioned at the elevation of the knuckle region of the model (see Fig. 1). The purpose of the CS is to allow an investigation of the responses of the SCV model against an almost rigid surrounding structure when it expands under internal pressurization, so that the deformation behakor of the model is more representative of the expected responses of actual plants. The CS is not, however, intended to simulate the concrete shield building in physical plants. There is a nominal gap of 18 mm between the SCV model and the CS. This design allows the model to expand as a stand-alone structure during the early phase of the pressurization sequence before global contact between the two structures occurs.

794-2

I.

3. Pretest Preparations

In the 18 months following the anival of the model at Sandia, the CS was designed, fabricated and installed; instruments were installed on the model and the CS; the press~zation system was designed and fabricated; and the data acquisition and display systems were designed and implemented. The model was ready for the pressure test in the early part of October 1996.

3.1 Instrumentation

There were more than 800 instruments installed on the model and the CS. These instruments served to measure strains, displacements, pressure and temperature, to provide video coverage, to detect contact between the two structures, and to detect micro-cracking.

3.1 .I Strain Measurements Standard electrical-resistance strain gages were used to measure the strain responses of the model during the pressure test. The decision on the density, location and orientation of the strain gages was based on the accumulated experiences of the project staff and the pretest analytical predictions of the model responses. Multi-element strip gages were installed in areas with relatively large strain gradients and three-element rosette gages were used in areas where biaxial strain data were desired. Single element gages were installed for point strain measurements in low gradient areas and as complemenmy gages for evaluating bending moments when placed on the exterior of the model, complementing an inner-surface strip gage.

3.1.2 Displacement Measurements and Contact Detection Horizontal and vertical displacements of the model were monitored by installing variable-resistance linear displacement transducers (resistance potentiometers) inside and outside of the model. The interior potentiometers were anchored on a central support column installed inside the model, whose movements were monitored by potentiometers attached to the bottom head of the model. The exterior potentiometers were anchored on the interior wall of the fragment barrier. The relative displacements between the model and the CS were measured by using ten linear variable differential transformers (LVDTs) installed through holes in the CS. In addition, a total of 55 pin-type microswitches (contact detectors) were inserted through holes in the CS wall. These contact detectors were installed on the CS in meridionally aligned rows arranged every 90' to detect the onset of contact between the model and the CS. The switches were triggered when the gap was reduced to less than 1 mil.

3.1.3 Pressure and Temperature Measurements Two pressure transducers were installed inside the model to measure the internal nitrogen pressure at all times during the pressure test. Thermocouples were installed on the interior and exterior surfaces of the model to measure local surface temperatures which are used to provide temperam compensation for each installed strain gage. Two resistance temperature detectors were also installed to monitor the nitrogen gas temperature.

3.1.4 Video Coverage Three high pressure rated video cameras were used to monitor the interior of the model, two directed toward the top head and one directed toward the equipment hatch. In addition, seven standard video cameras with accompanying still cameras were installed to monitor the exterior of the model, three directed toward the top head, three directed toward the equipment hatch and one for an overall view.

3.1.5 Acoustic Emission System An acoustic emission system was used in the high pressure test. It consisted of twenty four sensors mounted on the model, eighteen on the inside and six on the outside. This system was controlled by a computer that calculated the approximate locations of the sources of all acoustic emissions detected by the sensors during the test. This system would provide information to signal the onset of a potential failure of the model.

794-3

3.2 Pressurization System

The pressurization system consists of two main components: the pressure soyrce and the valve gallery. For the high pressure test, the pressure source consisted of a truck with liquid nitrogen that was gasified and regulated to a constant pressure and temperature. The operation of the pressurization system was executed by a programmable logic controller that controls various valves, two flowmeters, and several sensors on the valve gallery.

3.3 Data Acquisition and Display Systems

The data acquisition system was designed and developed to store and record on a real time basis all data gathered on all instruments. The data acquisition system satisfied vigorous software validation and verification tests and configuration file accuracy checks to make sure that the system was capable of recording test data corresponding to properly designated instruments. The data display system received the pool of test data from the data acquisition system on a short time delay basis and displayed specific information on model behavior in response to some prescribed instructions. The data display system played a vital role in providing information on the stability of data on all instruments to allow the test to proceed in accordance with a prescribed test sequence.

4. Summary of Test Conduct

Three separate tests of the model were conducted:

leak and instrumentation test (0.2 Pd) (conducted on October 4, 1996) low pressure test (1.5 Pd) (conducted on November 4, 1996) high pressure test (model failure or 15.9 Pd) (conducted on December 11-12, 1996) 4.1 Leak and Instrumentation Test

The leak and instrumentation test was conducted to check the functionality of the pressurization system, the installed instruments and the data acquisition system. The model was subjected to three cycles of pressure loadings in the test: frst cycle to 0.1 Pd and the other two cycles to 0.2 Pd. In each cycle, the pressure was held for a long period of time to complete all necessary functionality checks.

4.2 Low Pressure Test

The low pressure test provided a performance check on all operating systems at a higher pressure level than the leak and instrumentation test while the model still behaved elastically. This test also served as a “dry-run” for the high pressure test, The pressure rise followed an incremental step of 1/6 Pd with a dwell time of at least 6 minutes. All data were collected during the dwell time and compared with the following criterion:

Qi - Qt - Qi - At

0.02

where Qt and Ql - N are the data at the current and the previous time interval, respectively. The next pressure increment was allowed only after a minimum 80 % - 90 % of the data had satisfied this criterion. The pressure was gradually increased to a maximum level of 1.5 Pd during which the pressure was held constant for 30 minutes. Gradual unloading in three steps followed this holding period.

4.3 High Pressure Test

The high pressure test was allowed to proceed only after the functionality checks of all operating systems were completed because this test was planned to undergo a monotonic

794-4

pressure rise and the cycle of unloading and reloading was not desirable. The pressure increment followed procedures similar to those used in the low pressure test.

depressurization. The pressurization sequence for this test, as shown in Fig. 2, was divided into three stages:

4.3.1 Test Sequence The high pressure test was a continuous test without

e

8

e

First stage (0 - -4.6 Pa) According to Sandia's pretest analysis [3], the conical section of the model expands 9 mm (equal to one scaled gap dimension in the actual plant) at an internal pressure of approximately 4.6 P d . During the test, the end of this stage actually occurred at an internal pressure of 4.2 Pd when the average displacement of four displacement transducers (O', 90", 180" and 270") at a given elevation reached 9 mm. The model behaved essentially in the elastic domain throughout this stage.

Second stage ( ~ 4 . 6 Pa) This pressure condition was held constant for 30 minutes to demonstrate the accomplishment of the original Phase I test objective, which was intended to make sure that the model behaved as a stand-alone smcture and no contact between the model and the contact structure occurred during this stage.

Third stage (-4.6 P, - model failure or 15.9 P,) The model behaved in the plastic domain throughout this stage. As the pressure continued to increase, so did the @ne required to arrive at a state of steady structural response. Accordingly, the incremental pressure rise for each step was reduced and the dwell time during each step was lengthened.

4.3.2 Termination Criteria The high pressure test would be terminated when either the model experienced a failure or the internal pressure reached 15.9 Pd (12.4 MPa or 1800 psig). The model failure could mean a structural failure, including a catastrophic failure or development of a major crack in the model, or a functional failure that occurred when the pressurization system could no longer maintain pressure at a given level inside the model. The functional failure of the model might be caused by the development of arrays of minute cracks on the model resulting in substantial leakage. If the bottom head penetrations leaked during the test resulting in the termination of the test, such leakage was not considered failure.

5. Preliminary Post-Test Observations

After sixteen and a half hours of continuous operation, the high pressure test was terminated when a substantial leak was detected and the pressurization system at its maximum flow capacity could not maintain pressure inside the model. The maximum internal pressure achieved during the test was 5.97 P d (4.66 MPa or 676 psig).

Preliminary post-test inspection of the model revealed a large tear, approximately 230 mm long, along the weld seam at the edge of the equipment hatch reinforcement plate. The tear was found on the left side of the equipment hatch (from an interior view) and preliminary inspections suggest that the tear may have initiated at a point of roughly 30 mm below the material change interface and propagated in both directions before it stopped. In addition, a smaller meridional tear, approximately 55 mrn long, was found next to a semi-circular opening (situated at about 2004 in the internal stiffening ring above the equipment hatch. The preliminary results for the high pressure test are discussed in detail in Reference 4.

6. Conclusion

After several years of preparations in designing and fabricating the SCV m@el and the contact structure, in instrumenting the two structures, and in developing the pressurization system and the data acquisition system, the SCV model test was successfully completed on December 11- 12, 1996. A special feature of this test project was the presence of the CS over the SCV model. Even though the CS was not intended to represent the major features of a surrounding shield building in an actual plant, its presence enabled the contact between the two structures to take place during the high pressure test. The data of model deformation behavior after contact will help validate analytical modeling related to the contact phenomenon.

An extensive post-test investigative effort is currently underway. All instrumentation data will be examined closely and compared with the pretest analytical predictions. There is a tentative plan to conduct a post-test failure analysis to simulate the failure mechanisms of the major tear at the equipment hatch and to develop a failure scenario. In addition, metallurgical evaluation of the two tears will also be conducted.

7. References

1. ABAQUS/Standard User’s Manual, Versions 5.2 - 5.5, Hibbitt, Karlsson and Sorensen, Inc., Pawtucker, RI, 1993 - 1995.

2. Luk, V.K. and Klamerus, E.W. 1996. Round Robin Pretest Analyses of a Steel Containment Vessel Model and Contact Structure Assembly Subject to Static Internal Pressurization. NUREG / CR-6517, SAND96-2899.

3. Porter, V.L., Carter, P.A. and Key, S.W. 1996. Pretest Analyses of the Steel Containment Vessel Model. NUREG / CR-6516, SAND96-2877.

4. Matsumoto, T., etc. 1997. Preliminary Results of Steel Containment Vessel Model Test. SMiRT 14, Lyon, France, August 17-22, 1997.

1 ,

c.

Fig. 1 A schematic description of the SCV/CS assembly

High Pressure Test 800

700

600

500

400

300

200

100

0 3 200 400 600 1000

Tim (mh)

Fig. 2 Detailed breakdown of the pressurization sequence for the high pressure test

794-7