Thinning behavior of solid boron carbide immersed in molten stainless steel for core disruptive accident of sodium-cooled fast reactor

ABSTRACT

Boron carbide (B₄C)–stainless steel (SS) eutectic reaction behavior is one of the most important issues in the core disruptive accidents (CDAs) of sodium-cooled fast reactors (SFRs). In this study, the immersion experiments using B₄C pellets with molten SS were conducted to evaluate the CDA sequences such as contact event of solid B₄C with degraded core materials including SS at very high temperature. The immersion experiment aims at understanding the kinetic behavior of solid B₄C–liquid SS reaction based on the reduced thickness of B₄C pellet after the experiment in the temperature ranges from 1763 to 1943 K, which is higher than the temperature of solid B₄C–solid SS reaction. Based on the kinetic consideration of the reaction rate constants for solid B₄C–liquid SS reaction, it was found that similar temperature dependency was identified between solid B₄C–liquid SS and solid B₄C–solid SS. Besides, the reaction rate constants of solid B₄C–liquid SS were smaller than those of solid B₄C–solid SS extrapolated in higher temperature region by two or more orders of magnitude due to two different evaluation method for B₄C side/SS side. It was confirmed that this difference was reasonable through the consideration of previous reaction tests in solid–solid contact for B₄C side/SS side.

KEYWORDS:

• Sodium-cooled fast reactor
• core disruptive accident
• boron carbide
• stainless steel
• eutectic reaction

Nomenclature

k	=	reaction rate constant for SS in solid B4C–solid SS reaction [m2/s]
kIM	=	reaction rate constant for B4C in solid B4C–liquid SS reaction [m2/s]
x	=	reacting amount of SS in solid B4C–solid SS reaction [m]
xIM	=	reacting amount of B4C in solid B4C–liquid SS reaction [m]
t	=	reaction time in solid B4C–solid SS reaction [s]
tIM	=	immersion time at the 20 mm from the bottom end of B4C pellet [s]
R	=	gas constant [J/K/mol]
T	=	temperature [K]

Acknowledgments

This study is the results of “Technical development program on a fast reactor international cooperation, etc.” and “Technical development program on a fast reactor common basis.” entrusted to Japan Atomic Energy Agency (JAEA) by the Ministry of Economy, Trade and Industry (METI). The authors are deeply grateful to Mr. Onose H., Mr. Iinuma R., and Mr. Toida K. (Tokokikai Co.) for their assistance in many experiments and data processing.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Ministry of Economy, Trade and Industry.