Experimental study on eutectic reaction between fuel debris and reactor structure using simulant materials

Highlights

• Simulated experiments on eutectic reaction between fuel debris and structure conducted.

• Positive effect of temperature and contact pressure on reaction rate confirmed.

• Ratio of Sn particle size to Pb pellet diameter has non-monotonous effect.

• Effect of Sn particle shape confirmed according to the change of porosity.

• Reaction rate in block-block samples is generally larger than particle-pellet ones.

Abstract

It is important to ascertain the mechanisms underlying the eutectic reaction between different reactor materials that might be encountered during a Core Disruptive Accident (CDA) of Sodium-cooled Fast Reactors (SFR) since such reactions will affect the accurate progression of a severe accident. In this study, motivated to understand the characteristics of the probable eutectic reaction between fuel debris and the lower head of reactor vessel, a series of simulated experiments has been conducted at the Sun Yat-sen University using a couple of rather lower-eutectic-point (456 K) materials (namely Sn particles and Pb pellet). The experiments were carried out in a self-designed experimental system, which mainly consists of a sample holder and a visible resistance furnace. To acquire a relatively comprehensive understanding, a variety of experimental parameters such as the reaction temperature (463–483 K), contact pressure (0.4–1.2 MPa), Pb pellet diameter (10–25 mm) along with the diameter (0.3–3 mm) and geometry (spherical, cylindrical and droplet-shaped) of the Sn particles have been taken. Through detailed analyses, it is found that the reaction temperature and contact pressure can have noticeable positive impact on the reaction rate. As for the size of Pb pellet and Sn particles, with increasing the diameter ratio of Sn particles to Pb pellet, a non-monotonous effect is observed due to the competing role between the contact area and contact pressure. An evident influence of Sn particle geometry on reaction rate has been verified in accordance with the variation of particle-bed porosity. The analyses in this work also suggest that the reaction rate in previous experiments using block-block samples is generally larger than present experiments using particle-pellet ones, especially at a higher temperature. Knowledge and evidence obtained from this work will be utilized for the design of future high-temperature experiments using actual reactor materials as well as for the improved validations of eutectic-reaction-related models incorporated in fast reactor severe accident codes.

Introduction

The evaluation of Severe Accident (SA) is of critical importance for the research and development of nuclear reactor systems (esp. the advanced Sodium-cooled Fast Reactors (SFR)) considering its potential risk associated with the radioactive releases from nuclear power plant (Tentner et al., 2010). Over the past decades, plenty of knowledge and evidence regarding the Core Disruptive Accidents (CDAs) of SFR have been gained, with the accumulation in experimental work, theoretical modeling as well as the improvement of computer codes (Cheng et al., 2015, Yamano et al., 2009). In some countries (e.g. Japan and France), a systematic identification of key phenomena during SFR severe accidents has been made according to their specific reactor designs (Suzuki et al., 2014, Tobita et al., 2016, Bertrand et al., 2018). For example, it becomes gradually understood that during the material-relocation phase of a postulated CDA of SFR, because of the gravity-driven force, the molten core materials might be discharged out of the core region and then relocate through some potential paths (e.g. the control rod guide tubes) into the lower plenum (Tentner, et al., 2010, Cheng et al., 2019a). Such core materials, after being rapidly quenched and fragmented into small solid particles (as a consequence of the melt-sodium interaction), are expectable to sediment and form debris beds over the core-support structure and/or in the lower inlet plenum of the reactor vessel (Cheng et al., 2013, Cheng et al., 2014, Cheng et al., 2019a, Cheng et al., 2019b, Tentner, et al., 2010).

Since the adequate cooling of the formed debris beds along with their neutronically subcritical configuration is an essential premise to achieve the In-Vessel Retention (IVR) (Cheng et al., 2019a, Cheng et al., 2019b), in the past in the field of fast reactor safety lots of investigations on the so-called debris bed formation and relocation phenomena have been carried out with an aim to protect the reactor vessel from being melt through (Cheng et al., 2013, Cheng et al., 2014, Cheng et al., 2019a, Cheng et al., 2019b, Morita et al., 2016). For instance, recognizing the significance of the debris-bed geometry (esp. height) on its heat-removal capability, by assuming an initially conically-shaped debris bed, several series of investigations (including experimental analyses, empirical modeling along with numerical calculations) on the debris bed self-leveling behavior have been conducted by Cheng et al., 2013, Cheng et al., 2014, Morita et al., 2016, Tagami et al., 2018. It should be noted that through those studies, much of valuable knowledge related to IVR from the thermal aspect has been successfully accumulated (Cheng et al., 2013, Cheng et al., 2014, Cheng et al., 2019a, Cheng et al., 2019b), thereby greatly stimulating us to investigate more comprehensively the failure mechanism of the reactor vessel (due to the interaction with accumulated debris beds) from other relevant aspects (esp. the material side).

The failure of reactor vessel in the lower head is a complex process that is not simply a matter of melting point (Mustari et al., 2014, Mustari et al., 2017, Hofmann, 1999). It is theoretically probable that during the CDA relocation phase, when the temperature in the lower part of reactor vessel (e.g. due to the decay heat of fuel) exceeds the eutectic point of fuel and stainless steel, the direct contact between the particulate debris with the reactor vessel may result in the occurrence of eutectic reaction that melts the structure materials far below their melting point (Mustari et al., 2014, Mustari et al., 2015, Mustari et al., 2017, Hofmann, 1999). Thus, great concern must be given to the mechanisms of this reaction in order to hinder the underestimation of melt-through of the reactor vessel (given only melting point is taken into account). Unfortunately, over the past decades, although some pioneering studies regarding eutectic reaction for both LWRs and fast reactors during a severe accident have been carried out, most of them were focused on the interaction between different materials at the core region (e.g. the boron carbide (B₄C) control rod and stainless steel cladding). It is believed that for SFR, though at normal operation B₄C is stable thermodynamically and compatible with the stainless steel cladding due to its very high melting point of 2584 K, in a SA the core temperature may exceed 1473 K (the eutectic point), in which the eutectic reaction between B₄C and stainless steel could be reasonably expected (Veshchunov and Hofmann, 1995, Nagase et al., 1997, Sasaki et al., 2015, Sasaki et al., 2016, Shibata et al., 2015, Xiong et al., 2019), thereby making the melting point of cladding drop significantly and resulting in rapid failure of control rod tube. In addition to boron carbide (B₄C) and stainless steel, in field of reactor safety additional studies using other material couples such as B₄C and Zircaloy (Veshchunov and Hofmann, 1994, Sasaki et al., 2016), Zircaloy and Inconel (García et al., 1992), Zircaloy and (Ag, In, Cd) (Veshchunov and Hofmann, 1996), Alumina and Zircaloy (Hofmann et al., 1989) along with UO₂ and Zircaloy (Hofmann and Kerwin-Peck, 1984) have been also carried out in the past. However, it should be noted that the above studies were generally conducted within the microscopic level, i.e. with a focus on the reaction kinetics (Hofmann et al., 1989, Veshchunov and Hofmann, 1995, Nagase et al., 1997, Sasaki et al., 2015, Ueda et al., 2016), spatial distribution of elements during reaction (Veshchunov and Hofmann, 1995, Veshchunov and Hofmann, 1996, Steinbruck, 2004, Jr et al., 2012, Sasaki et al., 2015, Sasaki et al., 2016, Zheng et al., 2018) as well as the chemical composition of the products formed (Veshchunov and Hofmann, 1995, Nagase et al., 1997, Steinbruck, 2004, Jr et al., 2012, Sasaki et al., 2015, Sasaki et al., 2016, Ueda et al., 2016, Zheng et al., 2018). Aimed at acquiring more evidence and database for understanding the macroscopic characteristics (esp. the reaction rate), recently a series of simulated experiments using block-shaped low-eutectic-point materials was performed by Xiong et al.(2019) at the Sun Yat-sen University (SYSU). In the experiments, different experimental parameters such as reaction temperature, contact pressure (used to simulate the swelling intensity) along with the contact geometry (square, upright and inverted isosceles trapezoids) have been utilized (Xiong et al., 2019). Nevertheless, since the background of all the above studies is restricted to the assembly of control rod or fuel rod, it is no wonder that the samples used in their work are mainly in a shape of block or rod.

Regarding the eutectic reaction within the lower head of reactor vessel, up until now knowledge and experimental data are extremely rare. Mustari et al., 2014, Mustari et al., 2015, Mustari et al., 2017 are the numbered representative researchers, even if their work is actually oriented to the LWR meltdown accident. In their work, the eutectic reaction phenomenon (e.g. in a solid–liquid system) was numerically analyzed using the Moving Particle Method (MPS) based on some pioneering experiments such as the TREAT experiments conducted at Argonne National Laboratory in which Armco Iron is immersed in a molten uranium pool (Mustari et al., 2014, Mustari et al., 2017). As indicated above, different from LWR severe accident, due to the efficient quenching and fragmentation of molten core materials in sodium environment, particulate debris beds, instead of liquid corium, are more likely to accumulate within the lower head. In order to fill the gap of knowledge and database (thereby enhancing the evaluation of fast reactor safety), it is therefore easily understood that a deeper and comprehensive experimental investigation on the eutectic reaction between fuel debris and reactor vessel is pressingly needed.

Focusing on the above aspects, in recent years a systematic experimental study on the eutectic reaction between fuel debris and the reactor vessel has been launched at SYSU. Our research is divided into two steps. In Step I, we investigate the mechanism of eutectic reaction using low-melting-point metals, aiming at checking the validity of our experimental system and accumulating useful knowledge for the followed investigations at higher-temperature conditions. In Step II, efforts will be made to carry out the investigations using actual reactor materials under a range of rather high temperatures. The current study belongs to the Step-I investigations, in which Sn particles and Pb pellet, due to their rather lower eutectic point (456 K) and economical cost, are selected respectively as the simulant materials for fuel debris and structure wall. The experiments are conducted in a self-designed experimental system, which mainly consists of a sample holder and a visible resistance furnace. To acquire relatively comprehensive understanding, a variety of experimental parameters such as the reaction temperature (463–483 K), contact pressure (0.4–1.2 MPa), Pb pellet diameter (10–25 mm) along with the diameter (0.3–3 mm) and geometry (e.g. spherical, cylindrical and droplet-shaped) of the Sn particles have been taken. The remaining of this article is organized as following. In Section 2, conditions of our present experiments are briefly described, while the obtained results as well as their explanations are discussed in detail in Section 3. Knowledge and experimental data from this work will be utilized for the design of Step-2 high-temperature experiments as well as for the improved validations of eutectic-reaction-related models incorporated in fast reactor severe accident codes in China.