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Reaction between B4C and austenitic stainless steel in oxidizing atmosphere at temperatures below 1673 K
Journal of Nuclear Materials 466 (2015) 334e342
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Reaction between B4C and austenitic stainless steel in oxidizing atmosphere at temperatures below 1673 K Ryosuke Sasaki, Shigeru Ueda*, Sun-Joong Kim, Xu Gao, Shin-ya Kitamura Institute of Multidisciplinary Research Advanced Material, Tohoku University, 1-1 Katahira 2 Aoba-ku, Sendai, Japan
a r t i c l e i n f o
s y n o p s i s
Article history: Received 8 May 2015 Received in revised form 6 July 2015 Accepted 8 July 2015 Available online 5 August 2015
The control rod of a light water nuclear reactor is constructed of a pole comprising stainless steel filled with a boron carbide (B4C) core. To appraise the stability of this control rod in the event of a severe accident, the reactions of the system of B4C and grade 304 austenitic stainless steel (SS) were observed at 1473 K in Ar, air, and a mixture of both. To clarify the reaction mechanism and the influence of the oxygen partial pressure, the weight change ratio was monitored and differential thermal analysis was performed at the temperature range from room temperature to 1673 K to monitor the reaction under controlled oxygen partial pressure. The results showed that there was no direct reaction between B4C and SS. When the temperature was higher than the melting point of B2O3 (743 K), the molten B2O3 formed by oxidation of B4C covered the surface of SS by spreading wetting. This B2O3 layer functioned to transport oxygen from the atmosphere to SS, leading to accelerated oxidation of SS. As a result, a FeeCreNieBeO oxide phase covered the surface of SS. Oxygen continuously entered the oxide phase with prolonged reaction time, and oxides such as Fe2O3, Fe3O4, and FeOxeCr2O3 were found on the outer layer. Therefore, in the presence of B2O3 formed by oxidation of B4C, the oxidation of SS was accelerated below the eutectic temperature of the FeeC system. © 2015 Elsevier B.V. All rights reserved.
Keywords: Control rod B4C 304 grade stainless steel Steam Oxidation B2O3 Iron oxide
1. Introduction B4C and stainless steel are commonly used as building materials for constructing the control rod of boiling water reactors (BWR). The function of the control rod is to adsorb neutrons released from the fuel rod so as to limit the fission of fuels. The currently used structure of the control rod comprises a tube made of 304 grade stainless steel (SS) that is filled with B4C pellets loaded into a case made of 316 grade stainless steel. In the event of a severe accident, collapse of the control rod might happen above the eutectic temperature of the FeeC system, and consequently unreacted part of the control rod and the channel box containing the nuclear material may be exposed to high temperature steam. These phenomena could have an impact on the fuel rod degradation and contribute and/or accelerate the core degradation of the reactor core [1,2]. Therefore, the stability of this control rod under various conditions, especially under the circumstances that may occur in a severe accident, could have an influence on the management of a severe accident of BWRs.
* Corresponding author. E-mail address: [email protected] (S. Ueda). http://dx.doi.org/10.1016/j.jnucmat.2015.07.015 0022-3115/© 2015 Elsevier B.V. All rights reserved.
In order to appraise the stability of the control rod, the reactions that occur when B4C coexists with austenitic stainless steel (grade 304; hereafter called SS) under various circumstances should be clarified. B4C itself is stable at high temperature [3]. In previous studies, the reaction between B4C and SS was studied under nonoxidative conditions, and it was found that the reaction did not proceed rapidly at temperatures below 1473 K [4]. The oxidation of B4CeSS melts in steam is investigated and forming CreFe oxide on the surface of SSeB4C is observed at 1562 K [5]. A kinetic model of oxidation of B4C pellet above 1473 K is reported [6]. Moreover, degradation and oxidation of B4CeSSeZry control rods in steam [7,8] were investigated. However, in the presence of steam, the chemical reaction of B4C occurred at temperatures below 1473 K [9e11] with consequent generation of B2O3. The influence of B4C on atmosphere in severe accident [9] and the reaction rate [10] were also investigated. The rate of oxidation of B4C is limited by the oxygen supply through the liquid B2O3 layer on the surface of the B4C phase [11]. In such cases, the oxidation of B4C proceeded at temperatures lower than the melting points of both B4C and SS. During collapse of the fuel and control rods, B4C in the clad tube is oxidized by steam in the reactor [1,2]; molten B2O3 would then be formed even at temperatures below the eutectic temperature of SS
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and B4C. The melting point of B2O3 is 743 K [12], and the eutectic temperatures between B2O3 and FeO, B2O3 and Fe2O3 are 621 K and 673 K, respectively. Therefore, the solidus temperature of the system would be significantly decreased with the formation of B2O3, and the presence of molten B2O3 may accelerate the oxidation reaction of SS in steam in the event of a severe accident. To adequately evaluate the reactions during a severe accident, the formation of a liquid phase or oxidation at temperatures below the eutectic temperature of the FeeC system in the reaction of B2O3 and SS has not been sufficiently studied. In this study, considering the oxidation of B4C in atmosphere of steam or air, acceleration of oxidation of SS by B2O3 formed from B4C was studied. The reactions that occur when B4C and SS coexist are investigated at 1473 K in controlled atmosphere. In order to control atmosphere dry air and Ar were employed as atmosphere. Thus, to clarify the reaction mechanism and the influence of the partial pressure of oxygen on the reactions of the system, measurement of the weight change and differential thermal analysis are conducted during the reaction under controlled oxygen partial pressure.
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thermocouple was employed for controlling temperature of the hot zone. Dry air or high-purity Ar (6 N) was introduced into the reaction tube at a flow rate of 100 mL/min. The hot zone of the furnace was controlled by preheating to 1473 K, and the sample was subsequently inserted and held at that temperature for 1800 or 3600 s. After heating, the sample was withdrawn from the furnace and quenched in a helium gas stream. After cooling, the sample was encased in resin in a crucible. The sample was cut and polished for observation of the cross-section of the SS plate using field emission electron probe microanalysis (FE-EPMA). 2.3. Evaluation of reaction of B4C and SS in atmosphere with controlled oxygen partial pressure
Reagent grade B4C powder (50 mm) and reagent grade SS powder (304 type-L grade stainless steel, 100 mesh, 70 wt% Fe, 19 wt% Cr, and 11 wt% Ni, Alfa Aesar) and SS plate (304 grade stainless steel: 18e20% Cr, 8e11% Ni, Mn < 2%, Si < 1%, Fe balance, Nilaco) with dimensions of 20 10 1 mm3 were used as specimens. The surface of the SS plate was washed with ethanol to remove oil.
The B4C and SS reagent powders were mixed in a weight ratio of 1:1; this mixture is termed ‘mixed sample’. Approximately 10 mg of SS powder, B4C, or the mixed sample was placed in an alumina crucible (f5 mmID 2.5 mm). The sample was heated using a thermal gravimetry-differential thermal analysis (TG-DTA) instrument (SII, TG/DTA6300N), and the temperature was increased up to 1673 K at a heating rate of 10 K/min. In measurement of the weight change and differential thermal analysis, 10 mg of Al2O3 was employed as reference sample. Dry air, or a mixture of dry air and high purity Ar was introduced into the reaction chamber at a flow rate of about 50 mL/min. The oxygen partial pressure of dry air, and the air/Ar mixtures was controlled at 0.21, 0.015, or 0.008 atm. The experimental conditions for the sample and atmosphere are shown in Table 1. Relationship between the partial pressure of oxygen of the gas mixture and high temperature steam is discussed in Section 5.2.
2.2. Observation of interface between B4C and SS heated at 1473 K
3. Reaction between B4C and SS
Approximately 2.5 g of B4C powder was placed into an alumina crucible (f19 mm ID 40 mm) and the lower half of the SS plate was buried in the B4C layer without making contact with the crucible. For comparison, a crucible containing only the SS plate was also prepared. As shown in Fig. 1, a vertical electric resistance furnace with a reaction tube (f42 mm ID 1000 mm) was used to heat the sample. A PID controller connected to a B type
The cross-sections of the B4C layer and SS plate heated at 1473 K in dry air or Ar for 3600 s are shown in Fig. 2. The U-shaped area, dark colored area, and vertical line correspond to the Al2O3 crucible, B4C layer, and SS plate, respectively. The upper surface of the B4C powder bed in contact with the gas phase was oxidized to B2O3 and became darker; however, the lower part of the B4C layer changed only marginally. The surface of the upper side of the SS plate turned dark. The dark layer on the surface of the SS plate was thicker for the sample exposed to air than that exposed to Ar. Although Ar is an inert gas, the sample was oxidized by impurity of gas and air introduced when loading the sample. The maximum amount of oxygen introduced during the sample loading is estimated to be 0.006 mol, and the amount is much greater than impurity of Ar (6N) gas. However, the oxygen was purged from vicinity of the sample by the Ar stream. The dark phase on the SS plate was not observed at the interface between SS and B4C in the B4C layer. The concentration of elements on the surface of SS at the position indicated by the square in Fig. 2 is shown in Fig. 3. Similar data is presented in Fig. 4 for the sample heated for 1800 s. In the compositional image acquired in back scattered electron mode (COMP), the reacted SS phase (phase I), an oxide phase in contact with phase I (phase II), and an outer oxide phase (phase III) were observed on the surface of sample. Phases II and III should consist of a mixture of solid and liquid oxides, and a mixture of solid oxides, respectively. The thickness of the reacted phase formed in air and that formed in Ar were different. However, phases I and II and a thin oxide layer of the order of tens of mm (phase III) were observed with the use of Ar atmosphere. The black area between phases II and III observed in the sample exposed to air (a1) corresponds to the resin. The respective thicknesses of phase II for the samples heated for 1800 s and 3600 s in air as well as Ar atmosphere were about 200
2. Experimental procedure 2.1. Samples
Gas exhaust Mass flow controller
Silicon plug Gas inlet tube Reaction tube
Al2O3 crucible Heating unit Air or Ar gas
Al2O3 block
Fig. 1. Schematic of the experimental apparatus.
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Table 1 Experimental condition for reaction between B4C and SS in controlled atmosphere. No.
Sample
Partial pressure of oxygen (PO2) [atm]
Flow ratio (air: Ar)
Total flow rate [ml/min]
1 2 3 4 5 6
B4C SS B4C/SS B4C/SS B4C/SS B4C/SS
0.21 0.21 0.21 0.015 0.008 e
Air Air Air 1: 12 1: 23 Ar
50.0 50.0 50.0 55.4 53.0 50.0
Fig. 2. Cross section of sample heated at 1473 K for 3600 s (a: in Air, b: in Ar).
and 300 mm. The respective thicknesses of phase III for the samples heated in air for 1800 and 3600 s were about 250 and 500 mm. The thickness of phase II and III increased with an increase in the heating time. Based on a5, a6, b5 and b6 in Fig. 3, which shows the concentration map of B and O, the concentrations of O and B in phase I
were low, and diffusion of O and B into phase I was not observed. Based on a2, a3, a4, a6, b2, b3, b4, and b6 in Fig. 3, which shows the concentration map of Fe, Ni, and Cr, these species were uniformly distributed in phase I. On the other hand, the O content of phase II and III was high, and phase II is mainly composed of oxides of Fe, Cr, and Ni. Fe, Cr, and Ni are simultaneously oxidized. The concentration of Cr and Ni in phase II is higher than the concentration of these species in phase I. Phase III was subsequently formed from the oxide phase of FeeCreNieBeO (phase II). Phase III is mainly composed of iron oxides. The results of analysis of the surface of the SS plate in the B4C layer are shown in Fig. 5. The sample was heated in dry air for 3600 s. A thin oxide phase containing Cr was observed on the surface of SS; however, no reaction between B4C and SS was observed. If the oxidation of B4C by the gas phase is much faster than the diffusion of oxygen in the B4C layer, the partial pressure of oxygen in the B4C layer would be low. B2O3 was observed only on the surface layer; therefore, the partial pressure of oxygen at the interface of B4C and SS would be low enough. The reaction between B4C and SS in the B4C layer did not proceed without a supply of oxygen from the gas phase. In order to evaluate the oxidation of SS by oxygen in the gas phase, the SS plate only was placed in the Al2O3 crucible and heated in dry air or Ar atmosphere at 1473 K for 3600 s. Cross-sections of the sample plate after heating are shown in Fig. 6. When the surface of the sample was oxidized in either atmosphere, a 50e80 mm thick oxide layer was formed. The thickness of the oxide layer is much less than that of the layer formed on the sample with B4C in Fig. 3. This indicates that the presence of B4C enhanced the oxidation of SS by oxygen in the atmosphere. The results of line analysis of Cr, O, B, Ni, and Fe in the sample with B4C heated in dry air and Ar are shown in Figs. 7 and 8, respectively. B was detected in oxide of phases II and III. Phase II is composed of Fe, Cr, Ni, O, and B. Phase II and III contained B provided from B4C. Given that the observed part of the SS plate was separated from the surface of the B4C layer by 5 mm and did not make direct contact with B4C, B moved upward on the
Fig. 3. Cross section of SS plate heated at 1473 K for 3600 s (a): in Air, b): in Ar).
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Fig. 4. Cross section of surface of SS plate heated at 1473 K for 1800 s (a): in Air, b): in Ar).
oxidation of SS proceeded even at temperatures much lower than the eutectic temperature of the SS and B4C system. It can be seen from Figs. 3 and 6 that the reaction is much faster than oxidation of SS without B4C. Phase III is composed of Fe, Cr, O, and B. Cr was observed to be non-uniformly distributed in phase III (see Fig. 7). Moreover, the texture of phase III varied based on the atmosphere; the oxygen partial pressure affects the formation of the oxide phases. 4. Reaction between SS and B4C under atmosphere with various partial pressures of oxygen The weight changes and the results of DTA of B4C and SS powder in dry air are shown in Figs. 9 and 10, respectively. The weight of B4C increased significantly during the exothermic reaction at temperatures in the range of 857e1093 K; the weight further increased up to 1555 K, and decreased at temperatures above 1555 K. The reactions resulting in the weight increase can be represented as follows: B4C(s) þ 4O2(g) / 2B2O3(l) þ CO2(g)
DG1 ¼ 2790 þ 0:424T Fig. 5. Cross section of interface of B4C/SS heated at 1473 K in air for 3600 s.
B4C(s) þ 7/2O2(g) / 2B2O3(l) þ CO(g)
surface of the plate. Since B was detected at this position, some boron compounds may have moved from the B4C layer by condensation of gaseous species containing B or by spreading wetting of molten B2O3 on the plate. The melting point of B4C is over 2700 K [1,2], therefore liquid or gaseous B4C would not move to the surface of the plate. On the other hand, because the melting point of B2O3 is 753 K [13], under the current experimental conditions, B2O3 is stable as a liquid phase, and B2O3 easily reacts with the oxide phase on the surface of SS. Therefore, it is proposed that B2O3 diffused on the SS surface by spreading or the wetting reaction. Consequently, when SS and molten B2O3 were coexistent,
½kJ=mol
DG3 ¼ 2510 þ 0:338T
½kJ=mol
(1)
(2) (3)
(4)
Here, DG1 and T are the standard free energy change of the reaction represented by Eq. (1) and the temperature [K], respectively. Since the standard free energy change for the reaction in Eq. (1) at 973 K is 2390 kJ, oxidation of B4C could progress readily. If all of the B4C was oxidized to B2O3, the weight would increase 2.52-fold. The weight of the sample treated at 1073 K was 1.5 times larger than the initial weight. If it is assumed that the weight change was due to the oxidation reaction, the observed weight change indicates that 60% of the B4C reacted with oxygen. DTA shows a positive peak at 1058 K, and indicates that the oxidation reaction proceeded
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Fig. 6. Cross section of stainless steel heated at 1473 K for 3600 s (a) in Air, b) in Ar).
Fig. 7. Distribution of components near the surface of sample heated for 3600 s in Air.
rapidly at the given temperature. The DTA curve shows negative values at temperatures above 1058 K. Since the weight gain rate of the sample decreased at 1058 K, it can be deduced that the endothermic volatilization reaction of B2O3 proceeded at temperatures above 1058 K. The weight of SS did not increase at temperatures below 1287 K, and increased by 1.38 times at temperatures from 1287 K to 1601 K. When all of the Fe, Cr, and Ni in SS was oxidized and Fe2O3, Cr2O3, and NiO were respectively formed, the weight increased 1.42-fold relative to that of the initial sample. Here, the weight was 1.38 times that of the initial sample when a temperature of 1473 K was used; therefore, 90% of the sample was oxidized at this temperature. Since the specific surface area of the powder sample used in the present measurement is much larger than that of the plate-like sample, the observed reactivity of the former was much higher. However, the results show that SS will be oxidized at temperatures above 1287 K. The weight change and DTA of the mixture of B4C and SS heated
in dry air, or air/Ar mixtures are shown in Figs. 11e13. The partial pressure of O2 in the gas phase was 0.21, 0.015, and 0.008 atm for samples No. 3, No. 4, and No. 5 as shown in Figs. 11e13, respectively. From these analyses, the following observations were made: 1) The weight of the sample increased at temperatures above 878 K in all atmospheres. The weight increased significantly in atmospheres with a higher oxygen partial pressure. In addition, the weight increased slightly at temperatures above 1023 K. 2) A further weight increase was observed in the temperature range of 1415e1466 K under dry air and air/Ar atmosphere (Figs. 11e13). A peak corresponding to an exothermic reaction and a significant increase in the weight were observed in the temperature range of 1430e1494 K. 3) Another peak of an exothermic reaction was observed at about 1564 K in Figs. 11 and 12. 4) Oxidation reaction of SS rapidly progress in a wide temperature range, a trapezoid shaped peak corresponding to an exothermic
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Fig. 8. Distribution of components near the surface of sample heated for 3600 s in Ar.
Fig. 9. Weight change ratio and DTA of B4C in air.
Fig. 10. Weight change ratio and DTA of SS in air.
reaction was observed in the temperature range of 1430e1578 K in Fig. 13. 5) Weight loss and endothermic reaction were simultaneously observed at temperatures above 1560 K in all atmospheres.
1) The weight increase at temperatures above 878 K is caused by the formation of B2O3 from B4C. 2) The additional weight increase at 1415e1466 K is derived from the formation of the oxide phase (phase II) by oxidation of SS. The oxidation observed at 1287 K corresponds to oxidation of SS by air, as shown in Fig. 10. However, the weight increased at
These observations can be explained as follows:
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R. Sasaki et al. / Journal of Nuclear Materials 466 (2015) 334e342
Fig. 11. Weight change ratio and DTA of the mixture of B4C and SS in air (PO2 ¼ 0.21).
Fig. 13. Weight change ratio and DTA of the mixture of B4C and SS at PO2 ¼ 0.0083.
1 FeðsÞ þ O2 ðgÞ/FeOðsÞ 2
(5)
DG5 ¼ 269 þ 0:0628T
½kJ=mol
(6)
1 3FeOðsÞ þ O2 ðgÞ/Fe3 O4 ðsÞ 2
(7)
½kJ=mol
(8)
1 2Fe3 O4 ðsÞ þ O2 ðgÞ/3Fe2 O3 ðsÞ 2
(9)
DG7 ¼ 282 þ 0:111T
DG9 ¼ 242 þ 0:144T
½kJ=mol
(10)
The equilibrium constant of Eq. (5) (K5) is represented as:
. K5 ¼ exp DG5 RT ¼
Fig. 12. Weight change ratio and DTA of the mixture of B4C and SS at PO2 ¼ 0.015.
temperatures above 1415 K in the case of the mixture of SS and B4C as shown in Figs. 11e13. In this case, SS is oxidized by O2 through phase II including B2O3. This would mean that the ratelimiting step for the oxidation reaction of the SS is mass transport of oxygen in phase II when SS is mixed with B4C, whereas the oxidation reaction of SS is rate determining when SS is oxidized by the gas phase. 3) The additional peak at about 1564 K and the trapezoid peak observed in the range of 1430e1578 K represent formation of phase III by oxidation of phase II. As shown in Figs. 7 and 8, phase III is mainly composed of iron oxide. The oxidation reactions of Fe and the standard free energy changes of the reactions can be represented as follows:
aFeO 1=2
aFe $PO2
:
(11)
When FeO and Fe coexist, the activity of FeO and Fe are unity. Therefore, the partial pressure of oxygen in equilibrium with coexistent FeO and Fe can be derived from Eq. (11). The partial pressure of oxygen in equilibrium with Fe3O4 and FeO, and Fe2O3 and Fe3O4 can be derived in a similar manner. According to thermodynamic data [14], when Po2 was less than or over 0.0089 atm, Fe3O4 and Fe2O3 existed as stable iron oxide phases at 1473 K. In Figs. 11 and 12 (where the partial pressure of oxygen was over 0.015 atm), the stable oxide form of iron is Fe2O3. Fe3O4 is the stable phase in Fig. 13. The difference in the shape of the DTA peak might be due to the difference in the partial pressure of oxygen in the gas phase. 4) The weight loss at temperatures above 1560 K is due to volatilization of B2O3 given that the weight of the sample does not decrease as a result of the reaction between the condensed and gas phases.
R. Sasaki et al. / Journal of Nuclear Materials 466 (2015) 334e342
5. Discussion 5.1. Acceleration of SS oxidation by B2O3 formed from B4C In the present study, the oxidation reaction of SS was accelerated by coexisting B4C at temperatures below the eutectic temperature of SS and B4C. The mechanism of the acceleration is explained as follows: when B4C and SS are heated in oxidizing atmosphere, B2O3 is initially formed by oxidation of B4C, and is distributed on the surface of SS. Oxygen in the gas phase is supplied to SS through B2O3, and a solideliquid phase of the FeeCreNieBeO system is formed on the surface of SS. Furthermore, oxygen is supplied through the solideliquid phase. The solideliquid phase mediates oxygen transport from the gas phase to SS, and enhances the oxidation reaction. The solideliquid phase of the FeeCreNieBeO system is oxidized by the gas phase and forms a mixture of the FeOxeB2O3 phase and the Cr2O3eFeOxeB2O3 phase, where FeOx denotes iron oxides. 5.2. Reaction behavior of B4C and SS in steam atmosphere In the present experiments, the effect of B2O3 on oxidation reaction of SS was studied, and the influence of the partial pressure of oxygen on the reaction was investigated. When B4C and SS are exposed to a steam atmosphere, the oxidation reaction of B4C and SS might proceed. Here, the influence of oxygen formed by the decomposition reaction of water in BWR on oxidation of SS will be estimated thermodynamically. The decomposition reaction of H2O can be represented by the following equations:
1 H2 OðgÞ/H2 ðgÞ þ O2 ðgÞ 2
DG ¼ 248 0:0558T
(12)
½kJ=mol
(13)
The relationship between the partial pressures can be derived from the equilibrium constant (K) of Eq. (12). 1=2
K¼
PH2 $PO2
(14)
PH2 O
The ratio of H2 and O2 generated from H2O is H2/O2 ¼ 2/1. If the initial gas phase in the reaction chamber is assumed to be saturated with H2O, the ratio of H2/O2 is maintained at 2/1. The partial pressures of O2 and H2 are derived from the given partial pressure of H2O at a specific temperature. The relationship between the pressure of O2 and the temperature at a given partial pressure of H2O are shown in Fig. 14. The
341
initial gas phase was assumed to be composed of H2O. The solid, broken, and chain lines correspond to the initial pressures of H2O in the reaction field of 1, 10, and 100 atm, respectively. The partial pressure of O2 increases with an increase in the total pressure in the reaction chamber. For initial pressures of 10 and 100 atm, the partial pressures of O2 were 0.00032 and 0.0015 atm at 1473 K, respectively. These are similar to the values found under the present experimental conditions. When B4C is exposed to the steam atmosphere of a nuclear reactor in a severe accident, the oxidation reaction described in Section 5.1 is expected to proceed. In the present study, it was shown that the oxidation reaction of SS mediated by B2O3 progresses faster than the formation of the liquid phase by the reaction between B4C and SS at 1473 K. It was shown that the liquid phase is formed above the eutectic temperature of B4C and SS. During severe accident scenarios, the formation of liquid B2O3 could trigger the begin of the core degradation process at low temperature and progress of the SS oxidation reaction at temperatures below the eutectic temperatures of B4C and SS must be assumed. 6. Conclusions In the present study, the reaction between B4C and 304 grade SS was studied at temperatures below 1673 K. The influence of oxygen in the gas phase on the reaction between B4C and SS was investigated, leading to the following conclusions: The formation of B2O3 by the oxidation of B4C under air/Ar atmosphere is activated at temperatures above 873 K where no influence of variation of the oxygen partial pressure on the activation temperature was observed. The oxidation reaction of SS is activated at temperatures above 1287 K when a small amount of liquid B2O3 or the liquid phase of the FeeCreNieBeO system are simultaneously present. A phase comprising a solid and liquid mixture of the FeeCreNieBeO system and a solid phase mixture of FeOx and FeOxeCr2O3 are formed by oxidation of SS with B4C in steam. For analyzing the core degradation in a severe accident, progress of the SS oxidation reaction at temperatures below the eutectic temperatures of B4C and SS must be assumed. Acknowledgment This study had been carried out under a collaborative research with JAEA. Authors thanks to Dr. Kurata, JAEA and Professor Ohtani, IMRAM Tohoku University for his helpful advice, and Mr. Tashiro, IMRAM Tohoku University for preparing samples. References
0
logPo2 [ atm ]
-2 -4 -6 H2O
-8 -10
1000
1200
1400
1 atm 10 atm 100 atm
1600
Temp. [ K ] Fig. 14. Partial pressure of oxygen in H2O atmosphere.
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Reaction between B4C and austenitic stainless steel in oxidizing atmosphere at temperatures below 1673 K Ryosuke Sasaki, Shigeru Ueda*, Sun-Joong Kim, Xu Gao, Shin-ya Kitamura Institute of Multidisciplinary Research Advanced Material, Tohoku University, 1-1 Katahira 2 Aoba-ku, Sendai, Japan
a r t i c l e i n f o
s y n o p s i s
Article history: Received 8 May 2015 Received in revised form 6 July 2015 Accepted 8 July 2015 Available online 5 August 2015
The control rod of a light water nuclear reactor is constructed of a pole comprising stainless steel filled with a boron carbide (B4C) core. To appraise the stability of this control rod in the event of a severe accident, the reactions of the system of B4C and grade 304 austenitic stainless steel (SS) were observed at 1473 K in Ar, air, and a mixture of both. To clarify the reaction mechanism and the influence of the oxygen partial pressure, the weight change ratio was monitored and differential thermal analysis was performed at the temperature range from room temperature to 1673 K to monitor the reaction under controlled oxygen partial pressure. The results showed that there was no direct reaction between B4C and SS. When the temperature was higher than the melting point of B2O3 (743 K), the molten B2O3 formed by oxidation of B4C covered the surface of SS by spreading wetting. This B2O3 layer functioned to transport oxygen from the atmosphere to SS, leading to accelerated oxidation of SS. As a result, a FeeCreNieBeO oxide phase covered the surface of SS. Oxygen continuously entered the oxide phase with prolonged reaction time, and oxides such as Fe2O3, Fe3O4, and FeOxeCr2O3 were found on the outer layer. Therefore, in the presence of B2O3 formed by oxidation of B4C, the oxidation of SS was accelerated below the eutectic temperature of the FeeC system. © 2015 Elsevier B.V. All rights reserved.
Keywords: Control rod B4C 304 grade stainless steel Steam Oxidation B2O3 Iron oxide
1. Introduction B4C and stainless steel are commonly used as building materials for constructing the control rod of boiling water reactors (BWR). The function of the control rod is to adsorb neutrons released from the fuel rod so as to limit the fission of fuels. The currently used structure of the control rod comprises a tube made of 304 grade stainless steel (SS) that is filled with B4C pellets loaded into a case made of 316 grade stainless steel. In the event of a severe accident, collapse of the control rod might happen above the eutectic temperature of the FeeC system, and consequently unreacted part of the control rod and the channel box containing the nuclear material may be exposed to high temperature steam. These phenomena could have an impact on the fuel rod degradation and contribute and/or accelerate the core degradation of the reactor core [1,2]. Therefore, the stability of this control rod under various conditions, especially under the circumstances that may occur in a severe accident, could have an influence on the management of a severe accident of BWRs.
* Corresponding author. E-mail address: [email protected] (S. Ueda). http://dx.doi.org/10.1016/j.jnucmat.2015.07.015 0022-3115/© 2015 Elsevier B.V. All rights reserved.
In order to appraise the stability of the control rod, the reactions that occur when B4C coexists with austenitic stainless steel (grade 304; hereafter called SS) under various circumstances should be clarified. B4C itself is stable at high temperature [3]. In previous studies, the reaction between B4C and SS was studied under nonoxidative conditions, and it was found that the reaction did not proceed rapidly at temperatures below 1473 K [4]. The oxidation of B4CeSS melts in steam is investigated and forming CreFe oxide on the surface of SSeB4C is observed at 1562 K [5]. A kinetic model of oxidation of B4C pellet above 1473 K is reported [6]. Moreover, degradation and oxidation of B4CeSSeZry control rods in steam [7,8] were investigated. However, in the presence of steam, the chemical reaction of B4C occurred at temperatures below 1473 K [9e11] with consequent generation of B2O3. The influence of B4C on atmosphere in severe accident [9] and the reaction rate [10] were also investigated. The rate of oxidation of B4C is limited by the oxygen supply through the liquid B2O3 layer on the surface of the B4C phase [11]. In such cases, the oxidation of B4C proceeded at temperatures lower than the melting points of both B4C and SS. During collapse of the fuel and control rods, B4C in the clad tube is oxidized by steam in the reactor [1,2]; molten B2O3 would then be formed even at temperatures below the eutectic temperature of SS
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and B4C. The melting point of B2O3 is 743 K [12], and the eutectic temperatures between B2O3 and FeO, B2O3 and Fe2O3 are 621 K and 673 K, respectively. Therefore, the solidus temperature of the system would be significantly decreased with the formation of B2O3, and the presence of molten B2O3 may accelerate the oxidation reaction of SS in steam in the event of a severe accident. To adequately evaluate the reactions during a severe accident, the formation of a liquid phase or oxidation at temperatures below the eutectic temperature of the FeeC system in the reaction of B2O3 and SS has not been sufficiently studied. In this study, considering the oxidation of B4C in atmosphere of steam or air, acceleration of oxidation of SS by B2O3 formed from B4C was studied. The reactions that occur when B4C and SS coexist are investigated at 1473 K in controlled atmosphere. In order to control atmosphere dry air and Ar were employed as atmosphere. Thus, to clarify the reaction mechanism and the influence of the partial pressure of oxygen on the reactions of the system, measurement of the weight change and differential thermal analysis are conducted during the reaction under controlled oxygen partial pressure.
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thermocouple was employed for controlling temperature of the hot zone. Dry air or high-purity Ar (6 N) was introduced into the reaction tube at a flow rate of 100 mL/min. The hot zone of the furnace was controlled by preheating to 1473 K, and the sample was subsequently inserted and held at that temperature for 1800 or 3600 s. After heating, the sample was withdrawn from the furnace and quenched in a helium gas stream. After cooling, the sample was encased in resin in a crucible. The sample was cut and polished for observation of the cross-section of the SS plate using field emission electron probe microanalysis (FE-EPMA). 2.3. Evaluation of reaction of B4C and SS in atmosphere with controlled oxygen partial pressure
Reagent grade B4C powder (50 mm) and reagent grade SS powder (304 type-L grade stainless steel, 100 mesh, 70 wt% Fe, 19 wt% Cr, and 11 wt% Ni, Alfa Aesar) and SS plate (304 grade stainless steel: 18e20% Cr, 8e11% Ni, Mn < 2%, Si < 1%, Fe balance, Nilaco) with dimensions of 20 10 1 mm3 were used as specimens. The surface of the SS plate was washed with ethanol to remove oil.
The B4C and SS reagent powders were mixed in a weight ratio of 1:1; this mixture is termed ‘mixed sample’. Approximately 10 mg of SS powder, B4C, or the mixed sample was placed in an alumina crucible (f5 mmID 2.5 mm). The sample was heated using a thermal gravimetry-differential thermal analysis (TG-DTA) instrument (SII, TG/DTA6300N), and the temperature was increased up to 1673 K at a heating rate of 10 K/min. In measurement of the weight change and differential thermal analysis, 10 mg of Al2O3 was employed as reference sample. Dry air, or a mixture of dry air and high purity Ar was introduced into the reaction chamber at a flow rate of about 50 mL/min. The oxygen partial pressure of dry air, and the air/Ar mixtures was controlled at 0.21, 0.015, or 0.008 atm. The experimental conditions for the sample and atmosphere are shown in Table 1. Relationship between the partial pressure of oxygen of the gas mixture and high temperature steam is discussed in Section 5.2.
2.2. Observation of interface between B4C and SS heated at 1473 K
3. Reaction between B4C and SS
Approximately 2.5 g of B4C powder was placed into an alumina crucible (f19 mm ID 40 mm) and the lower half of the SS plate was buried in the B4C layer without making contact with the crucible. For comparison, a crucible containing only the SS plate was also prepared. As shown in Fig. 1, a vertical electric resistance furnace with a reaction tube (f42 mm ID 1000 mm) was used to heat the sample. A PID controller connected to a B type
The cross-sections of the B4C layer and SS plate heated at 1473 K in dry air or Ar for 3600 s are shown in Fig. 2. The U-shaped area, dark colored area, and vertical line correspond to the Al2O3 crucible, B4C layer, and SS plate, respectively. The upper surface of the B4C powder bed in contact with the gas phase was oxidized to B2O3 and became darker; however, the lower part of the B4C layer changed only marginally. The surface of the upper side of the SS plate turned dark. The dark layer on the surface of the SS plate was thicker for the sample exposed to air than that exposed to Ar. Although Ar is an inert gas, the sample was oxidized by impurity of gas and air introduced when loading the sample. The maximum amount of oxygen introduced during the sample loading is estimated to be 0.006 mol, and the amount is much greater than impurity of Ar (6N) gas. However, the oxygen was purged from vicinity of the sample by the Ar stream. The dark phase on the SS plate was not observed at the interface between SS and B4C in the B4C layer. The concentration of elements on the surface of SS at the position indicated by the square in Fig. 2 is shown in Fig. 3. Similar data is presented in Fig. 4 for the sample heated for 1800 s. In the compositional image acquired in back scattered electron mode (COMP), the reacted SS phase (phase I), an oxide phase in contact with phase I (phase II), and an outer oxide phase (phase III) were observed on the surface of sample. Phases II and III should consist of a mixture of solid and liquid oxides, and a mixture of solid oxides, respectively. The thickness of the reacted phase formed in air and that formed in Ar were different. However, phases I and II and a thin oxide layer of the order of tens of mm (phase III) were observed with the use of Ar atmosphere. The black area between phases II and III observed in the sample exposed to air (a1) corresponds to the resin. The respective thicknesses of phase II for the samples heated for 1800 s and 3600 s in air as well as Ar atmosphere were about 200
2. Experimental procedure 2.1. Samples
Gas exhaust Mass flow controller
Silicon plug Gas inlet tube Reaction tube
Al2O3 crucible Heating unit Air or Ar gas
Al2O3 block
Fig. 1. Schematic of the experimental apparatus.
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Table 1 Experimental condition for reaction between B4C and SS in controlled atmosphere. No.
Sample
Partial pressure of oxygen (PO2) [atm]
Flow ratio (air: Ar)
Total flow rate [ml/min]
1 2 3 4 5 6
B4C SS B4C/SS B4C/SS B4C/SS B4C/SS
0.21 0.21 0.21 0.015 0.008 e
Air Air Air 1: 12 1: 23 Ar
50.0 50.0 50.0 55.4 53.0 50.0
Fig. 2. Cross section of sample heated at 1473 K for 3600 s (a: in Air, b: in Ar).
and 300 mm. The respective thicknesses of phase III for the samples heated in air for 1800 and 3600 s were about 250 and 500 mm. The thickness of phase II and III increased with an increase in the heating time. Based on a5, a6, b5 and b6 in Fig. 3, which shows the concentration map of B and O, the concentrations of O and B in phase I
were low, and diffusion of O and B into phase I was not observed. Based on a2, a3, a4, a6, b2, b3, b4, and b6 in Fig. 3, which shows the concentration map of Fe, Ni, and Cr, these species were uniformly distributed in phase I. On the other hand, the O content of phase II and III was high, and phase II is mainly composed of oxides of Fe, Cr, and Ni. Fe, Cr, and Ni are simultaneously oxidized. The concentration of Cr and Ni in phase II is higher than the concentration of these species in phase I. Phase III was subsequently formed from the oxide phase of FeeCreNieBeO (phase II). Phase III is mainly composed of iron oxides. The results of analysis of the surface of the SS plate in the B4C layer are shown in Fig. 5. The sample was heated in dry air for 3600 s. A thin oxide phase containing Cr was observed on the surface of SS; however, no reaction between B4C and SS was observed. If the oxidation of B4C by the gas phase is much faster than the diffusion of oxygen in the B4C layer, the partial pressure of oxygen in the B4C layer would be low. B2O3 was observed only on the surface layer; therefore, the partial pressure of oxygen at the interface of B4C and SS would be low enough. The reaction between B4C and SS in the B4C layer did not proceed without a supply of oxygen from the gas phase. In order to evaluate the oxidation of SS by oxygen in the gas phase, the SS plate only was placed in the Al2O3 crucible and heated in dry air or Ar atmosphere at 1473 K for 3600 s. Cross-sections of the sample plate after heating are shown in Fig. 6. When the surface of the sample was oxidized in either atmosphere, a 50e80 mm thick oxide layer was formed. The thickness of the oxide layer is much less than that of the layer formed on the sample with B4C in Fig. 3. This indicates that the presence of B4C enhanced the oxidation of SS by oxygen in the atmosphere. The results of line analysis of Cr, O, B, Ni, and Fe in the sample with B4C heated in dry air and Ar are shown in Figs. 7 and 8, respectively. B was detected in oxide of phases II and III. Phase II is composed of Fe, Cr, Ni, O, and B. Phase II and III contained B provided from B4C. Given that the observed part of the SS plate was separated from the surface of the B4C layer by 5 mm and did not make direct contact with B4C, B moved upward on the
Fig. 3. Cross section of SS plate heated at 1473 K for 3600 s (a): in Air, b): in Ar).
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Fig. 4. Cross section of surface of SS plate heated at 1473 K for 1800 s (a): in Air, b): in Ar).
oxidation of SS proceeded even at temperatures much lower than the eutectic temperature of the SS and B4C system. It can be seen from Figs. 3 and 6 that the reaction is much faster than oxidation of SS without B4C. Phase III is composed of Fe, Cr, O, and B. Cr was observed to be non-uniformly distributed in phase III (see Fig. 7). Moreover, the texture of phase III varied based on the atmosphere; the oxygen partial pressure affects the formation of the oxide phases. 4. Reaction between SS and B4C under atmosphere with various partial pressures of oxygen The weight changes and the results of DTA of B4C and SS powder in dry air are shown in Figs. 9 and 10, respectively. The weight of B4C increased significantly during the exothermic reaction at temperatures in the range of 857e1093 K; the weight further increased up to 1555 K, and decreased at temperatures above 1555 K. The reactions resulting in the weight increase can be represented as follows: B4C(s) þ 4O2(g) / 2B2O3(l) þ CO2(g)
DG1 ¼ 2790 þ 0:424T Fig. 5. Cross section of interface of B4C/SS heated at 1473 K in air for 3600 s.
B4C(s) þ 7/2O2(g) / 2B2O3(l) þ CO(g)
surface of the plate. Since B was detected at this position, some boron compounds may have moved from the B4C layer by condensation of gaseous species containing B or by spreading wetting of molten B2O3 on the plate. The melting point of B4C is over 2700 K [1,2], therefore liquid or gaseous B4C would not move to the surface of the plate. On the other hand, because the melting point of B2O3 is 753 K [13], under the current experimental conditions, B2O3 is stable as a liquid phase, and B2O3 easily reacts with the oxide phase on the surface of SS. Therefore, it is proposed that B2O3 diffused on the SS surface by spreading or the wetting reaction. Consequently, when SS and molten B2O3 were coexistent,
½kJ=mol
DG3 ¼ 2510 þ 0:338T
½kJ=mol
(1)
(2) (3)
(4)
Here, DG1 and T are the standard free energy change of the reaction represented by Eq. (1) and the temperature [K], respectively. Since the standard free energy change for the reaction in Eq. (1) at 973 K is 2390 kJ, oxidation of B4C could progress readily. If all of the B4C was oxidized to B2O3, the weight would increase 2.52-fold. The weight of the sample treated at 1073 K was 1.5 times larger than the initial weight. If it is assumed that the weight change was due to the oxidation reaction, the observed weight change indicates that 60% of the B4C reacted with oxygen. DTA shows a positive peak at 1058 K, and indicates that the oxidation reaction proceeded
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Fig. 6. Cross section of stainless steel heated at 1473 K for 3600 s (a) in Air, b) in Ar).
Fig. 7. Distribution of components near the surface of sample heated for 3600 s in Air.
rapidly at the given temperature. The DTA curve shows negative values at temperatures above 1058 K. Since the weight gain rate of the sample decreased at 1058 K, it can be deduced that the endothermic volatilization reaction of B2O3 proceeded at temperatures above 1058 K. The weight of SS did not increase at temperatures below 1287 K, and increased by 1.38 times at temperatures from 1287 K to 1601 K. When all of the Fe, Cr, and Ni in SS was oxidized and Fe2O3, Cr2O3, and NiO were respectively formed, the weight increased 1.42-fold relative to that of the initial sample. Here, the weight was 1.38 times that of the initial sample when a temperature of 1473 K was used; therefore, 90% of the sample was oxidized at this temperature. Since the specific surface area of the powder sample used in the present measurement is much larger than that of the plate-like sample, the observed reactivity of the former was much higher. However, the results show that SS will be oxidized at temperatures above 1287 K. The weight change and DTA of the mixture of B4C and SS heated
in dry air, or air/Ar mixtures are shown in Figs. 11e13. The partial pressure of O2 in the gas phase was 0.21, 0.015, and 0.008 atm for samples No. 3, No. 4, and No. 5 as shown in Figs. 11e13, respectively. From these analyses, the following observations were made: 1) The weight of the sample increased at temperatures above 878 K in all atmospheres. The weight increased significantly in atmospheres with a higher oxygen partial pressure. In addition, the weight increased slightly at temperatures above 1023 K. 2) A further weight increase was observed in the temperature range of 1415e1466 K under dry air and air/Ar atmosphere (Figs. 11e13). A peak corresponding to an exothermic reaction and a significant increase in the weight were observed in the temperature range of 1430e1494 K. 3) Another peak of an exothermic reaction was observed at about 1564 K in Figs. 11 and 12. 4) Oxidation reaction of SS rapidly progress in a wide temperature range, a trapezoid shaped peak corresponding to an exothermic
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Fig. 8. Distribution of components near the surface of sample heated for 3600 s in Ar.
Fig. 9. Weight change ratio and DTA of B4C in air.
Fig. 10. Weight change ratio and DTA of SS in air.
reaction was observed in the temperature range of 1430e1578 K in Fig. 13. 5) Weight loss and endothermic reaction were simultaneously observed at temperatures above 1560 K in all atmospheres.
1) The weight increase at temperatures above 878 K is caused by the formation of B2O3 from B4C. 2) The additional weight increase at 1415e1466 K is derived from the formation of the oxide phase (phase II) by oxidation of SS. The oxidation observed at 1287 K corresponds to oxidation of SS by air, as shown in Fig. 10. However, the weight increased at
These observations can be explained as follows:
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Fig. 11. Weight change ratio and DTA of the mixture of B4C and SS in air (PO2 ¼ 0.21).
Fig. 13. Weight change ratio and DTA of the mixture of B4C and SS at PO2 ¼ 0.0083.
1 FeðsÞ þ O2 ðgÞ/FeOðsÞ 2
(5)
DG5 ¼ 269 þ 0:0628T
½kJ=mol
(6)
1 3FeOðsÞ þ O2 ðgÞ/Fe3 O4 ðsÞ 2
(7)
½kJ=mol
(8)
1 2Fe3 O4 ðsÞ þ O2 ðgÞ/3Fe2 O3 ðsÞ 2
(9)
DG7 ¼ 282 þ 0:111T
DG9 ¼ 242 þ 0:144T
½kJ=mol
(10)
The equilibrium constant of Eq. (5) (K5) is represented as:
. K5 ¼ exp DG5 RT ¼
Fig. 12. Weight change ratio and DTA of the mixture of B4C and SS at PO2 ¼ 0.015.
temperatures above 1415 K in the case of the mixture of SS and B4C as shown in Figs. 11e13. In this case, SS is oxidized by O2 through phase II including B2O3. This would mean that the ratelimiting step for the oxidation reaction of the SS is mass transport of oxygen in phase II when SS is mixed with B4C, whereas the oxidation reaction of SS is rate determining when SS is oxidized by the gas phase. 3) The additional peak at about 1564 K and the trapezoid peak observed in the range of 1430e1578 K represent formation of phase III by oxidation of phase II. As shown in Figs. 7 and 8, phase III is mainly composed of iron oxide. The oxidation reactions of Fe and the standard free energy changes of the reactions can be represented as follows:
aFeO 1=2
aFe $PO2
:
(11)
When FeO and Fe coexist, the activity of FeO and Fe are unity. Therefore, the partial pressure of oxygen in equilibrium with coexistent FeO and Fe can be derived from Eq. (11). The partial pressure of oxygen in equilibrium with Fe3O4 and FeO, and Fe2O3 and Fe3O4 can be derived in a similar manner. According to thermodynamic data [14], when Po2 was less than or over 0.0089 atm, Fe3O4 and Fe2O3 existed as stable iron oxide phases at 1473 K. In Figs. 11 and 12 (where the partial pressure of oxygen was over 0.015 atm), the stable oxide form of iron is Fe2O3. Fe3O4 is the stable phase in Fig. 13. The difference in the shape of the DTA peak might be due to the difference in the partial pressure of oxygen in the gas phase. 4) The weight loss at temperatures above 1560 K is due to volatilization of B2O3 given that the weight of the sample does not decrease as a result of the reaction between the condensed and gas phases.
R. Sasaki et al. / Journal of Nuclear Materials 466 (2015) 334e342
5. Discussion 5.1. Acceleration of SS oxidation by B2O3 formed from B4C In the present study, the oxidation reaction of SS was accelerated by coexisting B4C at temperatures below the eutectic temperature of SS and B4C. The mechanism of the acceleration is explained as follows: when B4C and SS are heated in oxidizing atmosphere, B2O3 is initially formed by oxidation of B4C, and is distributed on the surface of SS. Oxygen in the gas phase is supplied to SS through B2O3, and a solideliquid phase of the FeeCreNieBeO system is formed on the surface of SS. Furthermore, oxygen is supplied through the solideliquid phase. The solideliquid phase mediates oxygen transport from the gas phase to SS, and enhances the oxidation reaction. The solideliquid phase of the FeeCreNieBeO system is oxidized by the gas phase and forms a mixture of the FeOxeB2O3 phase and the Cr2O3eFeOxeB2O3 phase, where FeOx denotes iron oxides. 5.2. Reaction behavior of B4C and SS in steam atmosphere In the present experiments, the effect of B2O3 on oxidation reaction of SS was studied, and the influence of the partial pressure of oxygen on the reaction was investigated. When B4C and SS are exposed to a steam atmosphere, the oxidation reaction of B4C and SS might proceed. Here, the influence of oxygen formed by the decomposition reaction of water in BWR on oxidation of SS will be estimated thermodynamically. The decomposition reaction of H2O can be represented by the following equations:
1 H2 OðgÞ/H2 ðgÞ þ O2 ðgÞ 2
DG ¼ 248 0:0558T
(12)
½kJ=mol
(13)
The relationship between the partial pressures can be derived from the equilibrium constant (K) of Eq. (12). 1=2
K¼
PH2 $PO2
(14)
PH2 O
The ratio of H2 and O2 generated from H2O is H2/O2 ¼ 2/1. If the initial gas phase in the reaction chamber is assumed to be saturated with H2O, the ratio of H2/O2 is maintained at 2/1. The partial pressures of O2 and H2 are derived from the given partial pressure of H2O at a specific temperature. The relationship between the pressure of O2 and the temperature at a given partial pressure of H2O are shown in Fig. 14. The
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initial gas phase was assumed to be composed of H2O. The solid, broken, and chain lines correspond to the initial pressures of H2O in the reaction field of 1, 10, and 100 atm, respectively. The partial pressure of O2 increases with an increase in the total pressure in the reaction chamber. For initial pressures of 10 and 100 atm, the partial pressures of O2 were 0.00032 and 0.0015 atm at 1473 K, respectively. These are similar to the values found under the present experimental conditions. When B4C is exposed to the steam atmosphere of a nuclear reactor in a severe accident, the oxidation reaction described in Section 5.1 is expected to proceed. In the present study, it was shown that the oxidation reaction of SS mediated by B2O3 progresses faster than the formation of the liquid phase by the reaction between B4C and SS at 1473 K. It was shown that the liquid phase is formed above the eutectic temperature of B4C and SS. During severe accident scenarios, the formation of liquid B2O3 could trigger the begin of the core degradation process at low temperature and progress of the SS oxidation reaction at temperatures below the eutectic temperatures of B4C and SS must be assumed. 6. Conclusions In the present study, the reaction between B4C and 304 grade SS was studied at temperatures below 1673 K. The influence of oxygen in the gas phase on the reaction between B4C and SS was investigated, leading to the following conclusions: The formation of B2O3 by the oxidation of B4C under air/Ar atmosphere is activated at temperatures above 873 K where no influence of variation of the oxygen partial pressure on the activation temperature was observed. The oxidation reaction of SS is activated at temperatures above 1287 K when a small amount of liquid B2O3 or the liquid phase of the FeeCreNieBeO system are simultaneously present. A phase comprising a solid and liquid mixture of the FeeCreNieBeO system and a solid phase mixture of FeOx and FeOxeCr2O3 are formed by oxidation of SS with B4C in steam. For analyzing the core degradation in a severe accident, progress of the SS oxidation reaction at temperatures below the eutectic temperatures of B4C and SS must be assumed. Acknowledgment This study had been carried out under a collaborative research with JAEA. Authors thanks to Dr. Kurata, JAEA and Professor Ohtani, IMRAM Tohoku University for his helpful advice, and Mr. Tashiro, IMRAM Tohoku University for preparing samples. References
0
logPo2 [ atm ]
-2 -4 -6 H2O
-8 -10
1000
1200
1400
1 atm 10 atm 100 atm
1600
Temp. [ K ] Fig. 14. Partial pressure of oxygen in H2O atmosphere.
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