Chemical State Mapping of Degraded B4C Control Rod Investigated with Soft X-ray Emission Spectrometer in Electron Probe Micro-analysis

B4C is widely used as control rods in light water reactors, such as the Fukushima Daiichi nuclear power plant, because it shows excellent neutron absorption and has a high melting point. However, B4C can melt at lower temperatures owing to eutectic interactions with stainless steel and can even evaporate by reacting with high-temperature steam under severe accident conditions. To reduce the risk of recriticality, a precise understanding of the location and chemical state of B in the melt core is necessary. Here we show that a novel soft X-ray emission spectrometer in electron probe microanalysis can help to obtain a chemical state map of B in a modeled control rod after a high-temperature steam oxidation test.


Results and Discussion
, the B-Kα peak is observed at 183.5 eV for B. The B-Kα peak of B 4 C shifts to a slightly higher energy at 184 eV and shows a characteristic shape with decreased intensity at lower-energy. The change in the spectral shape is attributed to the bonding of B and C within B 4 C. The spectra of the iron borides FeB and Fe 2 B depict clear shoulders, designated as Sat.2 in the upper horizontal axis of Fig. 1, near 187 eV. These shoulders relate to the Fermi level of the material, as the electron behavior of B in iron borides is more metallic than that in the pure element 11 . The shoulders may also relate to the slightly higher energy of the peak of Fe 2 B than those of B and FeB. Notably, B 2 O 3 shows a distinctly different spectral shape compared to that of the other B-containing materials. The B-Kα peak of B 2 O 3 experiences a large shift to a lower energy at 180.5 eV and the formation of two additional satellite peaks: one higher (Sat.3: 192.5 eV) and one lower (Sat.1: 166 eV). Muramatsu et al. revealed that the higher-energy satellite peak originated from resonant X-ray Raman scattering and linked the lower-energy satellite peak to a transition from the B 2p and O 2s molecular orbitals to a core hole at the B 1s orbital 12 . Thus, we consider that EPMA-SXES has a sufficiently high energy resolution to determine the chemical state of B from the changes in the spectral shape of the B-Kα peak. Figure 2 shows (a-g) elemental mapping results obtained by conventional EPMA-WDS, (h) a secondary electron image, and (i) a back-scattering electron image on the cross-section of the B 4 C control rod model after exposure to high-temperature steam at 1250 °C for 30 min. Although B 4 C and stainless steel reacted significantly, B 4 C granules remain in the red-and pink-colored region in the (d) B map. Alloying elements of stainless steel such as Fe, Cr, and Ni are observed within the stainless tube where the B 4 C granules were placed before the oxidation test. Because these area also contains some amounts of B, liquefaction by eutectic interactions between B 4 C and stainless steel occurs under the high-temperature oxidation testing condition; the liquid solidifies as the test temperature is decreased. The electron images (h,i) show that the peripheral vicinity of the remaining B 4 C granule comprises a layer of reaction products from B 4 C with stainless steel and/or steam. The elemental maps show that the peripheral vicinity of the remaining B 4 C contains relatively dense O and C. The chemical state information of B will determine whether these are from B 2 O 3 formed by the oxidation of B 4 C or from other oxides.
In Fig. 3, we show the EPMA-SXES measurement result of (a-c) a B elemental map for the same position as Fig. 2 and (d) B-Kα spectra from the points A through K, as designated in the elemental map. The B-Kα spectra (solid lines) for the points of the remaining B 4 C granule present the characteristic peak shape of B 4 C, as shown in Fig. 1. However, the B-Kα spectra (dotted lines) for the points farther from the remaining B 4 C granule exhibit a unique shoulder at ~187 eV, similar to the shoulders in the spectra of the iron borides. Thus, EPMA-SXES supports that B in B 4 C reacts with the stainless steel to form borides. Although the peak of O-Kα (n = 3) is also observed near the B 4 C particle, the shape of the B-Kα peak differs from that of B 2 O 3 . Therefore, the O near the B 4 C granule probably results from other metal oxides.
To understand the reaction process of the B 4 C control rod model after high-temperature steam oxidation testing, it is beneficial to visualize the chemical distribution of B in the model. To clarify the chemical state, the elemental maps shown in Fig. 3(a-c) are re-calculated by the areal ratios of the peaks; P A is the total area of B-Kα in the range 178-188 eV, P B is the area from 178 to 186 eV, and P C is the area in 186-188 eV. Figure 3(e,f) show the chemical state maps obtained by calculating P B /P A and P C /P A , which display the existing areas of B 4 C and borides separately.
Thus, we succeeded in identifying the chemical state of B and in visualizing the spatial distribution of B with associated chemical states in the degraded B 4 C control rod model, based on EPMA-SXES with ultrahigh energy resolution. Our findings will aid the establishment of severe accident simulation codes and the prediction of the chemical state and physical location of B in the Fukushima Daiichi NPP. In addition, we emphasize that the present lab-scale SXES by EPMA has excellent energy-resolution and higher spatial resolution not only for light elements but also for higher atomic number elements compared with SXES by synchrotron radiation facilities. EPMA-SXES permits the visualization of the chemical state of elements in various materials at the micro-scale.

Methods
Reference materials. Five B-containing crystalline materials were prepared; B granules (3-7 mm) and B 2 O 3 powder were purchased from Kojundo Chemical Laboratory Co., Ltd. FeB and Fe 2 B (500 μ m in diameter) were purchased from Goodfellow Cambridge Ltd. B 4 C granules were supplied by a domestic BWR manufacturer in Japan. The chemical compositions were shown elsewhere 7 .
High-temperature steam oxidation test. We used a control rod model containing B 4 C granules 7 as neutron absorbers, SUS304L as cladding tube, SUS316L as control rod blade, and Zircaloy-4 as the channel box material as shown in Fig. 4. The B 4 C granules were placed in the cladding tubes, which are 0.6 mm in thickness, 4.8 mm in outer diameter and 20 mm in length. The vibration method was used for filling B 4 C granules with a packing density of approximately 70%. Then the ends of the tubes were sealed with setscrews of SUS304. Three sets of stainless steel tubes filled with B 4 C were sandwiched by the control rod blade material (SUS316L) and the channel box material (Zircaloy-4), which was coated with an oxide film of approximately 20 μ m in thickness on the surface, equivalent to that formed on the actual component by exposure to water vapor at 1000 °C for 10 min.
The high-temperature steam oxidation test was performed on the model for 30 min at 1250 °C, at which point the eutectic interaction of B 4 C and stainless steel is known to be rapid. The heating of the model was performed by a horizontal electric furnace using a KERAMAX (LaCrO 3 ) heater. Firstly, the model was set on a sample table located in a low-temperature part of the reaction tube. Secondly, the furnace was kept at the target temperature under flowing humidified Ar gas. Thirdly, the sample table was moved into the high-temperature part of the reaction tube using a heat-resistant push rod. After 30 min in this region, the test was completed by transferring the  sample stage, using the push bar, to the opposite low-temperature region. The Ar gas was humidified by bubbling the Ar at 800 cm 3 /min into a constant-temperature water bath kept at 70 °C.
After the high-temperature steam oxidation test, the model was cut at the axial direction center position and embedded into resin. The cross-section was polished with SiC papers and colloidal silica. The final surface of the cross-section was Pt-coated to prevent charge-up.

EPMA-WDS and SXES.
The polished sample was examined by a JXA-8500F field emission EPMA (JEOL) equipped with WDS and SXES. The acceleration voltage and beam current of the scanning electron beam were 15 keV and 5.5 × 10 −7 A, respectively. The energy range of SXES was 54 to 220 eV. The nominal energy resolution for SXES was 0.22 eV. Figure 5 shows the soft X-ray emission spectra of B-Kα from B granules obtained by a conventional WDS (type LDE2H) and the newly developed SXES (type JS200N). The advantages of SXES in energy resolution are evident. The broad asymmetrical peak is characteristic of covalent bonding 11 .