Cosmic-ray muon radiography for reactor core observation

Annals of Nuclear Energy 78 (2015) 166–175

Contents lists available at ScienceDirect

Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene

Cosmic-ray muon radiography for reactor core observation Kuniyoshi Takamatsu a,⇑, Hiroaki Takegami a, Chikara Ito b, Keiichi Suzuki c, Hiroshi Ohnuma c, Ryutaro Hino a, Tadahiko Okumura d,e a

Nuclear Hydrogen and Heat Application Research Center, Japan Atomic Energy Agency, 4002 Narita-cho, Oarai-machi, Higashiibarakigun, Ibaraki 311-1393, Japan O-arai Research and Development Center, Japan Atomic Energy Agency, 4002 Narita-cho, Oarai-machi, Higashiibarakigun, Ibaraki 311-1393, Japan c Kawasaki Geological Engineering Co., Ltd., 2-11-15, Mita, Minato-ku, Tokyo 108-8337, Japan d Engineering Advancement Association of Japan, 3-8-19, Tranomon, Minato-ku, Tokyo 105-0001, Japan e Energy Society of Japan, 2-3-7, Shimbashi, Minato-ku, Tokyo 105-0004, Japan b

a r t i c l e

i n f o

Article history: Received 29 August 2014 Received in revised form 8 November 2014 Accepted 17 December 2014 Available online 3 January 2015 Keywords: Muon Radiography Nuclear reactor HTTR Fukushima Daiichi Nondestructive inspection

a b s t r a c t One of the critical problems that have arisen from the accident at TEPCO’s Fukushima Daiichi nuclear power plant is the removal of fuel debris. For solving this problem, an examination of the internal reactors has been planned to identify the fuel debris. However, the high radiation dose around the reactors has necessitated the development of a remote sensing method that would enable observation of the internal reactors from the outside. In our study, we focused on a nondestructive inspection method by which cosmic-ray muons could be used to observe the internal reactor from outside the reactor pressure vessel (RPV) and containment vessel (CV). We conducted an observation test on the high-temperature engineering test reactor (HTTR) at the Japan Atomic Energy Agency to evaluate the applicability of the method to the internal visualization of a reactor. We also analytically evaluated the resolution of existing muon telescopes to assess their suitability for the HTTR observation, and were able to detect the major structures of the HTTR based on the distribution of the surface densities calculated from the coincidences measured by the telescopes. Our findings suggested that existing muon telescopes could be used for muon observation of the internal reactor from outside the RPV and CV. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction We focused on the use of cosmic-ray muons as a nondestructive method for observing the internal reactor of a nuclear power plant (NPP) from outside the RPV and CV. Because muons can travel long distances through materials (Nagamine, 2003), they have been used in a nondestructive method for inspecting large structures (Minato, 1986). Although this method has been applied in the investigation of underground structures (Suzuki et al., 2011), volcanoes (Tanaka et al., 2010), and underground rocks (Shiratori et al., 2010), its application to in-reactor inspection is at a very early stage (Morris et al., 2014). The use of the method to distinguish among Pb, Fe, and C in a mass-conserved system has also been demonstrated (Shoji et al., 2011). In the present study, we conducted an observation test on a high-temperature engineering test reactor (HTTR) at the Japan Atomic Energy Agency (JAEA) to evaluate the applicability of the method to the internal visualiza-

⇑ Corresponding author. Tel.: +81 29 266 7718. E-mail address: [email protected] (K. Takamatsu). http://dx.doi.org/10.1016/j.anucene.2014.12.017 0306-4549/Ó 2014 Elsevier Ltd. All rights reserved.

tion of a reactor. We also analytically evaluated the resolution of the measurement apparatus used for the HTTR observation. 2. Experiments 2.1. Overview of the HTTR The HTTR was built at the Oarai Research and Development Center of the JAEA, and is the first HTGR in Japan (Saito et al., 1994). The main specifications of the HTTR are listed in Table 1. The reactor consists of the core and internal components, which are contained in an RPV of height 13,200 mm and diameter 5500 mm (Fig. 1). The fuel assemblies are of the pin-in-block type, as shown in Fig. 2. Fig. 3 shows a horizontal cross section of the HTTR. The core comprises fuel assemblies, CR guide blocks, and replaceable and permanent reflector blocks, as shown in Figs. 1 and 2. A perpendicularly stacked row is referred to as a column, and the core consists of 61 columns. The core is divided into a fuel region, which includes fuel assemblies (Figs. 1 and 2) and a replaceable reflector region that surrounds the fuel region. Control rod guide blocks are installed in both regions. The active core,

167

K. Takamatsu et al. / Annals of Nuclear Energy 78 (2015) 166–175

which is 2900 mm high and has an effective diameter of 2300 mm, consists of 30 fuel columns and seven control rod guide columns.

2.2. Overview of the experiments The applicability of the use of muons for in-reactor inspection was evaluated by an inner reactor observation test conducted on the HTTR using existing muon telescopes that were developed for subsurface exploration.

2.2.1. Structures of the muon telescopes The muon detector of the apparatus used for the test consisted of two plastic scintillators, two photo-multipliers, two discriminators, a coincidence circuit, and a counter. The major specifications of the three telescopes are listed in Table 2. The entry of a muon into the scintillator causes light emission, and the light is then converted into an electrical signal, which is multiplied by the photomultiplier. The discriminator is used to remove the environmental gamma rays. The employed telescope uses the coincidence method to selectively detect a muon coming from the observation direction (Fig. 4) (Akiyama et al., 1991). The method counts the muons passing the two scintillators simultaneously, and the extension of the line that connects the two scintillators constitutes the measurement direction.

CR standpipe RPV Upper shield block Permanent reflector block Replaceable reflector block Core restraint mechanism Fuel element Hot plenum block Support post Lower plenum block Core support plate Core support grid

2.2.2. Multichannel-type telescope Fig. 5 shows photographs of the two types of muon telescopes used for the observation. The first is a multichannel-type telescope, shown in Fig. 5(a), which is capable of simultaneous measurements in five directions using one main spherical detector and five sub-detectors. The sub-detectors were arranged at intervals of 15°.

2.2.3. Single-channel-type telescope A single-channel-type telescope, shown in Figs. 5(b) and 5(c), has the basic configuration of a muon telescope and uses the coincidence method. The area of the scintillator of single-channel telescope A was approximately half of those of the scintillators of the other telescopes.

Item

Specification

Thermal power Coolant Reactor outlet coolant temperature

30 MW Helium 850 °Ca 950 °Cb 395 °C 4.0 MPa 12.4 kg/sa 10.2 kg/sb Graphite 2,900 mm 2,300 mm 2.5 MW/m3 Low-enriched UO2 3–10 wt% (Avg. 6 wt%) Prismatic block Steel (2 1/4Cr-1Mo) 1

Core structures Core height Core effective diameter Power density Fuel Enrichment Fuel element type RPV Number of main cooling loops

Primary helium gas tube Side shielding block RPV Fuel element Replaceable Reflector block Permanent reflector block

Table 1 Main specifications of the HTTR.

Reactor inlet coolant temperature Primary coolant pressure Primary coolant flow rate

Auxiliary helium gas tube

a Rated operation mode: operation at reactor outlet coolant temperature of 850 °C. b High-temperature test operation mode: operation at reactor outlet coolant temperature of 950 °C.

CR guide block CR : Control rod RPV : Reactor pressure vessel Fig. 1. Core and internal components of the reactor.

2.2.4. Densities of the major structures of the HTTR The muon coincidence was measured at the periphery of the CV of the HTTR. Table 3 lists the major structures of the HTTR and their material densities. The structures are variously made from graphite, concrete, and steel. Although the densities of graphite and concrete are considerably less than that of steel, the total volume of the structures made from graphite and concrete is comparatively large.

2.2.5. Measurement point The muons were measured at five different points on the same floor of the HTTR, as shown in Figs. 6(a) and 6(b). Measurement

168

K. Takamatsu et al. / Annals of Nuclear Energy 78 (2015) 166–175

Fig. 2. Block-type fuel of the HTTR.

North CR guide block Active core

1 F4 F3

F3

F3

F3

F4

F2

F3

F1

F2

F4

F1

F3

F1

F1

F1

F3

F4

2

6

6

12

6

3

Irradiation block

F2

Fuel assembly

F3 6 F4

F2

F3

Replaceable reflector block

7

F3 F3

4 5

F1 F4

F2

F2

F2 F3

F1

Top replaceable reflector block

F3

8

F4 CR RSS

9

Bottom replaceable reflector block

Column

BP insertion hole Neutron source

CR : Control rod RSS : Reserved shutdown system BP : Burnable poison The underlined numbers (e.g., 6, 6, 12, and 6) under the respective fuel elements denote the total number of blocks. Fig. 3. Horizontal cross sections of the HTTR.

points located approximately 22 m below the ground were used to measure the major underground structures of the HTTR. The azimuth and zenith angles were varied at each point, and the measurement direction was determined by a combination of the two angles.

2.2.6. Azimuth angle Figs. 6(a) and 6(b) show the employed apparatus and azimuth angle for each measurement point. The multichannel telescope was used for the measurements at points A, B, and C. Singlechannel telescope A was used for the measurements at points A

169

K. Takamatsu et al. / Annals of Nuclear Energy 78 (2015) 166–175 Table 2 Major specifications of the muon telescopes. Type

Multichannel

Single-channel A

Single-channel B

Scintillator size (mm)

Main Sub(5)

/245 (spherical plastic scintillator) /245  t50 (cylindrical plastic scintillator)

/175  t50 (cylindrical plastic scintillator)

/245  t50 (cylindrical plastic scintillator)

1.4 491 10 3–3.5

1.0 240 10 3–3.5

1.4 491 10 3–3.5

Distance between scintillators (m) Cross-section (cm2) Solid angle (°) Discrimination levela (MeV) a

Cuts off environmental c-rays.

and D, and single-channel telescope B was used for the measurements at point E. The direction from each measurement point to the center of the reactor core was defined as the zero azimuth angle. 2.2.7. Zenith angles Fig. 7 shows an example of the zenith angle, which was varied between 0° and 70° in steps of 5°, resulting in 15 directions for each azimuth angle. 2.2.8. Measurement time The measurement time for one measurement direction was determined by considering the relationship coincidence number and measurement accuracy. In muon-based observation, an adequate coincidence number is required to maintain accuracy. Regarding the accuracy,pffiffithe coefficient variation of the muon meaffi surement is defined as NN  100%, where N is the coincidence number in consideration of Poisson distribution (Knoll, 2010). Although the accuracy in improved by a lower coefficient variation, a longer measurement time is required for such, and this ultimately decreases the measurement efficiency. Based on measurements for subsurface exploration, a coincidence of over 1000 should be observed within a practical measurement time. The coefficient variation for a coincidence of 1000 is approximately 3%. An adequate measurement time for obtaining a coincidence of 1000 in the observation of the HTTR was estimated. The representative surface density was calculated based on the densities and sizes of the different structures of the HTTR, and was determined to be approximately 20–30 hg/cm2, where h in hg/cm2 indicates hector (102). Approximately one day is required to observe 1000 coincidences for this surface density. The measurement time for each measurement direction was therefore set at 23 h, and the total measurement cycle was set at 24 h to allow one hour for preparation for the next measurement.

Fig. 4. Schematic illustration of the coincidence method.

3. Analyses 3.1. Simplified analytical model of the HTTR Fig. 5(a). Multichannel muon telescope for inner structure observation.

The resolution of the muon telescope used for this study was evaluated by simple analysis. Fig. 8 shows the analytical model of the HTTR, comprising a reactor core, a RPV, a CV, and space and piping. The model has been simplified to improve the calculation efficiency. 3.2. Measurement lines for the surface density analysis Cubic voxels measuring 0.5  0.5  0.5 m was used to develop the model. The density of each voxel was assigned based on the densities of the materials of the major components of the NTTR. The effective density of the space and piping was determined by

their volume ratios. The densities assigned to the components are listed in Table 3. Fig. 9 shows the measurement lines used for the analysis. The measurement lines were arranged parallelly and spaced at 1 m. The zenith angle was varied between 0° and 70° in steps of 5° at each measurement point. The results of the surface density analysis were used to evaluate the resolution of the muon telescope. When the entire core was replaced by water, the maximum change in the surface density was approximately 17 hg/cm2, which is approximately equal to the surface density calculated for a core length of 10 m, and also

170

K. Takamatsu et al. / Annals of Nuclear Energy 78 (2015) 166–175 Table 3 Material densities of the structures of the HTTR model. Structure

Density (g/cm3)

Reactor core Reactor pressure vessel (RPV) Containment vessel (CV) Biological shield Reactor building Ground Piping and space Heat exchangers

2–3 8 8 2.35–2.5 2 1.8 0.5 6

Fig. 5(b). Single-channel muon telescope A for inner structure observation.

Fig. 6(a). Measurement points and azimuth angles for multichannel telescope.

Fig. 5(c). Single-channel muon telescope B for inner-structure observation.

approximately equal to the difference between the density of the core and that of water (2 g/cm3). Conversely, when the core was partially replaced, the center of the bottom of the reactor core was replaced by cubic water. The maximum changes in the surface density that resulted from replacement by 2, 1, and 0.5 m3 of water were 2.5, 0.5, and 0.06 hg/cm2, respectively. These changes indicate the tendency of the surface density to decrease with decreasing water volume.

Fig. 6(b). Measurement points and azimuth angles for single-channel telescope.

3.3. Detailed analytical model of the HTTR The detailed analytical model of the structures of the HTTR is shown in Fig. 10. Parts with densities less than 0.5 g/cm3 are transparent. In the future, we will make more detailed and complicated analytical models by means of three dimensional CAD data.

171

K. Takamatsu et al. / Annals of Nuclear Energy 78 (2015) 166–175

55 m

Fig. 7. Zenith angles of measurement at point B.

52 m

50 m

Density (g/cm3) Fig. 10. Detailed analytical model of the HTTR.

Fig. 8. Simplified analytical model of the HTTR.

Fig. 11. Solid angle in the coincidence method.

3.4. Muon attenuation by the structures In this analysis, the time dependence of the muon incident was ignored. The muon number, which was determined by the solid angle of the muon telescope (Fig. 11), was considered as a constant. The muon attenuation by the structures was evaluated using Miyake’s experimental equation (Miyake, 1979), which is as follows:

Il ðh; hÞ ¼

Fig. 9. Measurement lines for the surface density analysis.

AM a ðh sec h þ aÞ  expðbh sec hÞ; hþH

ð1Þ

where Il ðh; hÞ is the muon flux per unit time (s1), per unit area (cm2), and per unit solid angle (sr1); h is the surface density; h is the zenith angle; and the coefficients AM ¼ 174, H ¼ 400, a ¼ 11, a ¼ 1:53, and b ¼ 8  104 . This experimental equation was valid for the Kolar Gold Field in India by using the constant parameters which were adjusted to fit the experimental data in consideration of standard rocks, as shown in Fig. 12. The different terms of this relation represent the pion decay into muons, the

172

K. Takamatsu et al. / Annals of Nuclear Energy 78 (2015) 166–175

Fig. 12. Miyake’s experimental equation (Miyake, 1979) evaluated in the Koral Gold Field in India.

production energy spectrum, and the depth-energy relation. The experimental equation is useful for the whole depth range until 10,000 m water equivalent (m.w.e.). The surface density in a particular direction was calculated using Eq. (1). 4. Results 4.1. Coincidence at each measurement point Fig. 13 shows the relationship between the total coincidence over 23 h and the zenith angle for each measurement point. In the figure, the coincidence measured by single-channel telescope A are doubled because the area of the scintillator of the telescope was half that of the other scintillators. As can be observed, the coincidence varied with the measurement point, zenith angle, and azimuth angle. The range of the coincidences measured in this test was approximately 300–5000. In particular, over 1000 coincidences were observed for a zenith angle smaller than 50°. Consequently, an adequate number of coincidences to confirm the applicability of this method was obtained within the range of the main components of the HTTR. The coincidences may be a reflection of the respective HTTR structure because the relationship between the coincidence and the zenith angle was different for each measurement point. As noted above, the surface density was calculated from the measured coincidence using Miyake’s equation (Miyake, 1979) (Eq. (1)). The coincidence C coin was determined using

C coin ¼ Il ðh; hÞ  tAX;

ð2Þ

where t is the measurement time, A is the cross section of the scintillator, and X is the solid angle formed by two detectors. 4.2. Surface density at each measurement point Fig. 14 shows the distribution of the surface densities obtained from each measurement point. As can be observed, the surface

density varied with the measurement direction, and were higher in the directions of the reactor structures made of concrete, such as the reactor core and biological shield. For example, the surface densities for zenith angles smaller than 10° at point B are relatively higher. These results suggest that a concrete wall exists only directly above point B and is approximately 12 m high. 4.3. Surface densities in the directions of azimuth angles of 15° and 30° at point C To confirm the measurement results, the surface densities determined from the measured coincidences were compared with those calculated from the densities of the HTTR structures and the pass-through distances of the muons in the structures. The employed densities of the structures were the same as those used for the analysis (listed in Table 3), and the pass-through distances were obtained from the design drawings of the HTTR. Fig. 15 shows the surface densities in two directions from point C, namely, 15° and 30° azimuth angles. The direction of an azimuth angle of 15° passes through the reactor core, and that of an azimuth angle of 30° passes through only the CV. Regardless of the existence of the reactor, the variations of the surface densities determined from the coincidence and the densities of the structures were similar and dependent on the zenith angle. The surface densities determined from the measured coincidence apparently reflected the structures of the inner HTTR. 4.4. Discussion The above results confirm the applicability of muon observation using existing muon telescopes to in-reactor inspection. However, the correspondence between the surface density and the type of structure was unclear for some measurement directions because the size and density properties of some of the components of the HTTR used in the present study were too complicated for the

K. Takamatsu et al. / Annals of Nuclear Energy 78 (2015) 166–175

173

Fig. 13. Coincidence at each measurement point.

Fig. 14. Results of surface density at each measurement point.

resolution of the muon telescopes. In this method, the maximum change in the surface density measured from each measurement point was determined for the volume of each replaced object using existing muon telescopes. A small change in the surface density is difficult to detect because of the time variation of the muon flux. A very long measurement time is therefore required to measure a

small change in the density of a small object. For example, it was estimated that more than 10,000 coincidences were required to detect a change in surface density of 0.5 hg/cm2 in the HTTR, and this implies a measurement time for one direction of approximately 10 days. In actual measurements, the available measurement time is determined by considering the total measurement

174

K. Takamatsu et al. / Annals of Nuclear Energy 78 (2015) 166–175

Fig. 15. Comparison of the surface densities determined from the measured coincidences and calculated from the structure densities.

Fig. 16. Fukushima Daiichi NPP.

period. Consequently, the minimum detectable volume in the HTTR observation using existing muon telescopes was estimated to be approximately 1–2 m3 for a density difference of 2 g/cm3 in the HTTR. The measurement conditions such as the measurement time are also determined by the resolution of the telescope, which is the interval between the measurement lines.

4.5. Limitations of the application of the method to the Fukushima Daiichi NPP A simulation of the Fukushima Daiichi NPP has given an indication of the enormous time and effort that would be required for a practical muon CT scan of the plant (Borozdin et al., 2012). The obtained surface densities were representative of those that would be used for the CT scan and, in principle, could be used to create a 3D map of the scanned volume, although much more measurements and azimuth angles would be required using a larger detector.

However, an analysis of the pre-accident surface densities using the design charts of the Fukushima Daiichi NPS reveals some obvious differences compared to the post-accident condition. This can be attributed to the presences of fuel debris, which are expected to be larger than 1 m in the RPV and PCV, as shown in Fig. 16. We are presently working on improvements for the Fukushima Daiichi NPP, to make the space resolution smaller than 20 cm and the measurement time shorter than two weeks. A more detailed analysis may, however, be necessary for the observation of the plant because the structure and measurement distance from the reactor are different from those of the HTTR structure used in the present study. It would thus be very useful to improve the muon telescope that would be used for the in-reactor inspection.

5. Conclusion The applicability of the nondestructive observation method in which muons are used for in-reactor inspection was evaluated by

K. Takamatsu et al. / Annals of Nuclear Energy 78 (2015) 166–175

an observation test conducted on an HTTR using existing muon telescopes that utilize the coincidence method. The measured coincidences of the muons were found to vary with the structure of the HTTR. The main structures were detected based on the distribution of the surface densities calculated from the measured coincidences. This suggests that the nondestructive observation method using muons can be applied to the observation of the inner reactor of an NPP from outside the RPV and CV. It is, however, necessary to improve the analysis method and apparatus resolution to enable the detection of smaller objects in the reactor, such as fragments of fuel debris. Acknowledgments This study used some results obtained by the Mechanical Social Systems Foundation and was supported by the JKA Foundation. The authors would also like to thank the chairperson of ‘‘Preliminary Test for Visualization of Inner HTTR Committee’’, Prof. T. Tokunaga, as well as the committee members for their helpful comments. References Akiyama, N., Suzuki, K., Tonouchi, S., Minato, S., 1991. An equipment for measuring cosmic ray angular distribution: development for non-destructive inspection for large scale structures. Radioisotope 40 (2), 71–74.

175

Borozdin, K., Greene, S., Lukic, Z., Milner, E., Miyadera, H., Morris, C., Perry, J., 2012. Cosmic ray radiography of the damaged cores of the Fukushima reactors. Phys. Rev. Lett. 109, 152501. Knoll, G.F., 2010. Radiation Detection and Measurement, fourth ed. Wiley (2010). Minato, S., 1986. Bulk density estimates of buildings using cosmic rays. Appl. Radiat. Isot. 37, 941–946. Miyake, S., 1979. Cosmic ray observations in deep underground. J. Phys. Soc. Jpn. 34 (4), 292–301. Morris, C.L., Bacon, J., Ban, Y., Borozdin, K., Fabritius, J.M., Izumi, M., Miyadera, H., Mizokami, S., Otsuka, Y., Perry, J., Ramsey, J., Sano, Y., Sugita, T., Yamada, D., Yoshida, N., Yoshioka, K., 2014. Analysis of muon radiography of the Toshiba nuclear critical assembly reactor. Appl. Phys. Lett. 104, 024110. Nagamine, K., 2003. Introductory Muon Science. Cambridge University Press, ISBN 1139439219. Saito, S. et al., 1994. Design of high temperature engineering test reactor (HTTR). JAERI, 1332. Shiratori, Y., Motoshima, A., Suzuki, K., 2010. Possibility of application to rock around underground opening of measuring technique that uses cosmic-ray muons. In: EIT-ISCE Symp. Eng. Geo-hazards, pp. 31–36. Shoji, D., Tanaka, H.K.M., Takamatsu, K., 2011. Development of a simple-material discrimination method with three plastic scintillator strips: for easy inspection of mass-conserved system. Nucl. Instrum. Methods Phys. A 654 (1). Suzuki, K., Ohnuma, H., Kubota, R., Asanuma, H., 2011. Feasibility study of multichannel cosmic ray muon telescope and 3D tomography. In: Proc. 10th SEGJ Int. Symp.; Nov. 20–23; Kyoto, Japan. Tanaka, H., Uchida, T., Tanaka, M., Shinohara, H., Taira, H., 2010. Development of a portable assembly-type cosmic-ray muon module for measuring the density structure of a column of magma. Earth Planets Space 62 (2), 119–129.