Development of a Ranging Inspection Technique in a Sodium-cooled Fast Reactor Using a Plate-type Ultrasonic Waveguide Sensor

판형 웨이브가이드 초음파 센서를 이용한 소듐냉각고속로 원격주사 검사기법 개발

Abstract

In a sodium-cooled fast reactor, which is a Generation-IV reactor, refueling is conducted by rotating, but not opening, the reactor head to prevent a reaction between the sodium, water and air. Therefore, an inspection technique that checks for the presence of any obstacles between the reactor core and the upper internal structure, which could disturb the rotation of the reactor head, is essential prior to the refueling of a sodium-cooled fast reactor. To this end, an ultrasound-based inspection technique should be employed because the opacity of the sodium prevents conventional optical inspection techniques from being applied to the monitoring of obstacles. In this study, a ranging inspection technique using a plate-type ultrasonic waveguide sensor was developed to monitor the presence of any obstacles between the reactor core and the upper internal structure in the opaque sodium. Because the waveguide sensor installs an ultrasonic transducer in a relatively cold region and transmits the ultrasonic waves into the hot radioactive liquid sodium through a long waveguide, it offers better reliability and is less susceptible to thermal or radiation damage. A 10 m horizontal beam waveguide sensor capable of radiating an ultrasonic wave horizontally was developed, and beam profile measurements and basic experiments were carried out to investigate the characteristics of the developed sensor. The beam width and propagation distance of the ultrasonic wave radiated from the sensor were assessed based on the experimental results. Finally, a feasibility test using cylindrical targets (corresponding to the shape of possible obstacles) was also conducted to evaluate the applicability of the developed ranging inspection technique to actual applications.

제 4세대 원자로로 개발되고 있는 소듐냉각고속로는 냉각재로 사용되는 소듐의 물과 공기에 의한 반응을 방지하기 위하여 원자로 헤드의 개방 없이 회전 구동만을 이용하여 핵연료 교환을 수행한다. 따라서 핵연료 교환을 위해서는 노심과 상부내부구조물 사이의 공간에 원자로 헤드의 회전 구동을 방해하는 장애물의 존재 여부를 확인하는 검사가 반드시 선행되어야 하는데, 소듐의 불투명성으로 인해 통상적인 광학 장비를 사용한 검사방법으로는 장애물의 확인이 어렵고, 초음파를 이용한 검사방법이 적용되어야 한다. 이 연구에서는 불투명한 소듐 중에서 노심과 상부내부구조물 사이의 장애물 존재 여부를 확인하기 위하여 판형 웨이브가이드 초음파 센서를 적용한 원격주사 검사기법을 개발하였다. 웨이브가이드 센서는 원자로헤드 상부의 저온 구간에 초음파 트랜스듀서를 설치하고 길이가 긴 웨이브가이드를 사용하여 노심 상부의 고온 고방사능 소듐 내부로 초음파를 전파시키기 때문에, 고온 고방사능에 의한 손상 없이 장시간 적용이 가능한 장점을 가지고 있다. 먼저, 수평 방향으로 초음파를 방사시킬 수 있는 10 m 길이의 수평 빔 웨이브가이드 센서를 설계 제작하였고, 제작된 센서의 특성 분석을 위한 빔 프로파일 측정 및 기초 실험을 수행하여 방사되는 초음파의 빔 폭과 전파 거리를 평가하였다. 또한, 실제 장애물로 작용할 수 있는 부품들의 형상인 원형 목표물에 대한 원격탐지 성능시험을 수행하여 개발된 원격주사 기술의 유용성을 평가하였다.

1. Introduction

Nuclear power plants (NPPs) are important sources of electrical power in Korea. Currently, almost 30 % of the country’s electricity is generated by NPPs(1). Although an NPP can provide high-quality electricity, it also incurs some inherent problems, notably nuclear waste. One possible solution to the problem of nuclear waste is the use of the sodium-cooled fast reactor (SFR). The SFR can reduce nuclear waste considerably by recycling the spent fuel from a pressurized- water reactor (PWR). So far, various SFRs have been developed and are currently being developed in several countries(2). In Korea, the prototype Generation-IV SFR (PGSFR) has been undergoing development at the Korea Atomic Energy Research Institute (KAERI) since 2012. The PGSFR has a plant capacity of 150 MWe and is a pool-type reactor in which all of the primary components and primary sodium are located within a reactor vessel, as shown in Fig. 1.

Fig. 1Vessel cutout view of the PGSFR

Although the SFR reuses spent fuel, it also requires refueling to enable its ongoing operation. However, this refueling process is much more difficult than that of a PWR because the SFR uses sodium as its coolant. The reactor head must not be opened, even during the refueling, so as to prevent a reaction between the sodium and the air. Therefore, a refueling strategy that relies on rotating the reactor head of the SFR is employed to load new fuel and unload the spent fuel. As part of this process, it is necessary to ensure that there are no obstacles between the upper internal structure (UIS) and the reactor core, as shown in Fig. 2. However, this is quite difficult because sodium is optically opaque, such that monitoring or surveying of the in-vessel structures cannot be done using optical devices. Therefore, there is a need for an inspection technique that can monitor these obstacles (i.e., a ranging inspection technique(3,4)) despite their being immersed in liquid sodium that is both hot and radioactive.

Fig. 2Concept of reactor system of SFR

Several research efforts have addressed the monitoring and surveying of the in-vessel structures immersed in the hot radioactive liquid sodium. Most of these techniques have relied on ultrasound, and two types of ultrasonic sensors have been developed: immersion and waveguid sensors. The immersion sensor(5~8), which is directly immersed in the liquid sodium, can provide high-resolution images using high-frequency ultrasound, but its reliability in such a high-temperature and highly radioactive environment has yet to be confirmed. The highly radioactive and hot liquid sodium could possibly cause thermal and radiation damage to the piezoelectric and bonding materials of an ultrasonic sensor. On the other hand, the waveguide sensor(9~13) uses an ultrasonic transducer that is mounted in a relatively cold region (e.g., above the reactor head) and transmits ultrasonic waves into the liquid sodium using a long waveguide. Therefore, the transducer can offer better reliability as it is less susceptible to thermal or radiation damage.

This paper describes a ranging inspection technique based on an ultrasonic waveguide sensor. A 10 m plate-type waveguide sensor that can radiate ultrasonic waves horizontally was designed and manufactured. A basic experiment was first performed to investigate the characteristics of the developed waveguide sensor. Next, a feasibility test using cylindrical targets that simulate the shapes of actual obstacles was conducted and the results were assessed.

 

2. Ranging Inspection Using a Waveguide Sensor

2.1 Horizontal Beam Waveguide Sensor

To determine the presence of any obstacles between the UIS and the reactor core, an ultrasonic wave should be propagated between them. To achieve this, a waveguide sensor that can radiate an ultrasonic wave horizontally was newly developed. Figure 3 shows the developed horizontal beam waveguide sensor. It consists of a 10 m stainless steel (SS304) strip, an ultrasonic transducer (Krautkramer Benchmark series transducer, 12.7 mm in diameter, with a 1 MHz center frequency), a solid wedge, and an acoustical shielding tube. The width and thickness of the strip are 15 mm and 1.5 mm, respectively. A thin shielding tube, with a 25.4 mm outer diameter and a 1 mm thickness, is employed to prevent energy leakage into the surrounding liquid during propagation along the strip, as well as to provide a single radiation face. This tube is welded to the strip.

Fig. 3Horizontal beam waveguide sensor

To radiate the ultrasonic waves into the surrounding liquid, the ultrasonic transducer first generates a longitudinal wave in the wedge. Next, the ultrasonic wave propagating through the wedge is converted into a guided wave in the waveguide. Here, the angle (α) of the wedge is important for generating the required guided wave mode. This angle can be calculated by using Snell’s law, as follows:

where vW is the longitudinal wave velocity in the wedge and cp is the phase velocity in the waveguide, having a thickness d at a frequency f. In the developed waveguide sensor, an A0-mode Lamb wave is selected as the target guided wave mode because of its large out-of-plane particle displacement, which ensures high-efficiency radiation into the surrounding liquid(14,15). Finally, the ultrasonic wave is radiated into the surrounding liquid through the radiation end section of the sensor. Here, the angle (β) of the radiation end section is also important to the horizontal emission of the ultrasonic wave. This angle can be calculated from the radiation angle (θ) of the leaky wave, which can also be obtained by Snell's law, as follows:

Here, vL is the longitudinal wave velocity in the surrounding liquid. Because the phase velocity in a 1.5 mm thick SS304 strip at 1 MHz is cp=2351 m/s and the longitudinal wave velocity in water is vL=1480 m/s, the radiation angle can be calculated as θ=39° from Eq. (2). Therefore, one can determine that the angle of the radiation end section should be β=39° to horizontally radiate the ultrasonic wave in water.

To confirm the radiation property of the developed horizontal beam waveguide sensor, a beam profile measurement of the sensor was carried out. Figure 4(a) shows the experimental setup for measuring the beam profile of the developed 10 m horizontal beam waveguide sensor in water. A four-cycle tone burst centered on 1 MHz, as generated by a function generator, was used as the input signal. This was amplified by a power amplifier (RITEC GA-2500A) before being sent to the transducer. Once the ultrasonic wave is generated by the transducer, it propagates into the waveguide through the wedge and is converted into the A0-mode Lamb wave. Then, the A0-mode Lamb wave propagates through the waveguide and is radiated from the radiation end section into the water as a leaky longitudinal wave. In addition, this leaky longitudinal wave is measured using a needle hydrophone (ONDA, HNR-0500, frequency range of 0.25 to 10 MHz). To measure the ultrasonic waves at predetermined positions, a needle hydrophone was installed in an XYZ scanner and moved in the XZ plane at intervals of 0.5 mm. Finally, the signal measured at each position is amplified by a broadband receiver (RITEC, BR-640) and processed by C-scan software (UTEX, Winspect). Figure 4(b) shows the beam profile (i.e., the maximum amplitudes of the measured signals at predetermined positions) of the developed 10 m horizontal beam waveguide sensor obtained in the XZ plane. From the result, one can clearly see that the ultrasonic beam radiated from the sensor propagates horizontally.

Fig. 4(a) Experimental setup for beam profile measurement of the developed horizontal beam waveguide sensor, (b) Obtained beam profile in water at 1 MHz

2.2 Basic Experiment and Results

To investigate the characteristics of the developed ranging inspection technique, such as the inspection range and resolution, a basic experiment was first performed in water. Figure 5 shows the setup for the basic experiment. Two plate-type steel targets were installed in a large water bath with an inner diameter and height of 1480 mm and 820 mm, respectively. The distances of the targets from the sensor were 840 mm and 1200 mm, and the angles of the targets relative to the center of the sensor were −8° and 7°, respectively. The width, thickness, and length of the target were 50 mm, 3.2 mm, and 400 mm, respectively.

Fig. 5Setup for basic experiment

In the experiment, the vertical distance (L) between the targets and sensor was varied from 5 mm to 50 mm with an increment of 5 mm. The sensor was then self-rotated from −90° to +90° with an interval of 0.5° at each L. To control the developed 10 m horizontal beam waveguide sensor and obtain the inspection results, a remote inspection module and a ranging inspection program(Under Sodium Ranging Ver. 7) were used(16). Figure 6 shows the remote inspection module, which consists of an upper driving device and a lower guiding structure, as well as the ranging inspection program. The program mainly consists of two parts, a motion control part that controls the vertical movement of the sensor and a scan control part that is responsible for the rotation of the sensor and the image processing. While the sensor is rotated at intervals of 0.5° at each L by the remote inspection module and ranging inspection program, an ultrasonic wave centered on 1 MHz is radiated horizontally and the reflected wave is measured for each angle. A nine-cycle tone burst was used as the input signal and was amplified by a power amplifier (RITEC, RAM-5000-3C) before being sent to the transducer. The measured signals were amplified and band-pass filtered from 0.25 to 5 MHz by the same equipment.

Fig. 6Remote inspection module and main screen of ranging inspection program

Figure 7 shows the measured signals at −8° and 7° at L=5 mm. The signals appearing at 7.73 ms in Fig. 7(a) and at 8.2 ms in Fig. 7(b) are reflected waves from targets 1 and 2, respectively. The two target-reflected signals are clearly distinguishable, even though large amounts of noise generated by the motor driver are incorporated into the measurements. The images resulting from the basic experiment are shown in Fig. 8. The amplitudes of the measured signals in a time gate from 7.6 ms to 8.25 ms (the threshold level was 0.1 V, as shown in Fig. 7(a)) at all angles for each L were used to obtain each resulting image. From the figure, one can see that two target images are well indicated up to L=35 mm, although they are not very clear over L=25 mm. In addition, one can also see that the image of target 2 becomes slightly more intense than that of target 1 as L increases. This is caused by the beam spread of the ultrasonic wave. Because the beam width of the ultrasonic wave widens as it propagates, the beam width at target 2 is wider than that at target 1, despite the decrease in the amplitude. The beam spread angle of the ultrasonic wave radiating from the developed sensor in the XZ plane can be estimated from the result to be about γ= 1.91°.

Fig. 7Measured signals at (a) −8° and (b) 7° at L= 5 mm

Fig. 8Images obtained in basic experiment

Meanwhile, it can be seen that the image of target 2 is wider than that of target 1. This is mainly caused by the cylindrical image mapping. Because the inspection result is mapped in cylindrical coordinates, the image of target 2 has a wider arc for the same angle owing to the greater distance from the sensor. In addition, the beam spread in the XY plane also affects these results. Although the spread angle is very small, the front of the main beam widens both vertically and laterally as the wave propagates. From the results shown in Fig. 8, one can confirm that a flat obstacle located more than 1 m from the sensor when L≤35 mm can be well detected by the developed ranging inspection technique.

 

3. Feasibility Tests and Results

The shape of the sub-structures of the UIS in the SFR, which could become obstacles between the UIS and the reactor core, will be cylindrical or hexagonal, but not flat. In addition, the reflections from these shapes will be much weaker than those from a flat obstacle. To apply the developed ranging inspection technique to a real application, a feasibility test for a real shape was therefore conducted.

Figure 9 shows the experimental setup for the feasibility test. Two stainless steel (SS304) hollow cylindrical targets that simulate the sub-structures of the UIS were installed in a large water bath. The distances of the targets from the sensor were 400 mm and 470 mm, respectively; these distances were determined by considering the amplitudes of the measured reflection signals from the targets. The angles of the targets relative to the center of the sensor were −3° and 17.62°, respectively. A 15 mm thick plate (SS304) was placed under the targets and the sensor to simulate the reactor core. The diameter, thickness, and length of the cylindrical target were 50 mm, 1 mm, and 400 mm, respectively.

Fig. 9Experimental setup for feasibility test

In the experiment, the vertical distance (D) between the targets and the lower plate was varied between 5 mm and 50 mm with an increment of 5 mm, while the vertical distance (S) between the sensor and the lower plate was increased with an increment of 5 mm, from 5 mm to 25 mm, for each D. The sensor was then self-rotated from −90° to +90° at intervals of 0.5° for each S. The ultrasonic wave, centered on 1 MHz, was radiated horizontally, and the reflected wave was measured for each angle using the same equipment (RITEC, RAM-5000-3C) as that used in the basic experiment. A remote inspection module and a ranging inspection program were also used to control the 10 m horizontal beam waveguide sensor and thus obtain the inspection results.

Figure 10 shows the inspection results obtained for S=5 mm. From these results, one can see that there are no target images when the distance D is larger than 30 mm; two targets are well identified under D=25 mm although only one target is indicated at D=25 mm, given the low amplitudes of the reflected signals. The inspection results obtained for S=15 mm are also shown in Fig. 11. In this case, the targets are detected when the distance D is less than 35 mm. From these results, one can conclude that a target can be well identified provided the vertical distance between the sensor and target is less than 20 mm. Although not shown here, the results for other values of S(S=10 mm, 20 mm, and 25 mm) also exhibit similar aspects. Therefore, it can be stated that it is possible to apply the developed 10 m horizontal beam waveguide sensor to ranging inspection in an SFR, although the inspection coverage should be increased.

Fig. 10Inspection results for S=5 mm

Fig. 11Inspection results for S=15 mm

 

4. Conclusions

The clearance between the UIS and the reactor core in an SFR is an important factor affecting the refueling process, which involves the rotation of the reactor head. To assure this clearance, a ranging inspection technique using a 10 m plate-type horizontal beam waveguide sensor was proposed, and its feasibility was experimentally studied in this work. From the beam profile measurement and basic experiment, it was clearly shown that the developed waveguide sensor can radiate an ultrasonic wave horizontally, and that flat obstacles located over 1 m from the sensor can be easily detected by the developed technique. A feasibility test using cylindrical targets that simulate the real shape of a possible obstacle was also carried out in water, and the results showed that the clearance can be assured provided the vertical distance between the sensor and the target is more than 20 mm. In hot liquid sodium, however, the sound velocity and the wetting properties, both of which have a major effect on the ultrasonic radiation, are different from those in water. To apply the developed ranging inspection technique to actual applications in an SFR, further studies should therefore be undertaken to improve the inspection coverage and signal sensitivity, especially in hot liquid sodium.