Estimation Method for Residual Sodium Amount on Unloaded Dummy Fuel Assembly

Abstract

This study has researched an estimation method for the amounts of residual sodium film and sodium lumps on dummy fuel pins in the Japanese prototype fast breeder reactor Monju by fundamental experiments and demonstration experiments. The residual sodium amounts on the pin surface were measured using three types of test specimens: (a) single pin, (b) 7-pin assembly, and (c) 169-pin assembly. The single pin and 7-pin assembly experiments revealed that the withdrawal speed of the pins and improvement of the sodium wetting drastically increased the residual sodium amounts. Furthermore, the 169-pin assembly experiments measured the practical amounts of the residual sodium in the Monju dummy fuel assembly and demonstrated sodium draining behavior through small gaps between the pins. The estimation method includes four models such as a viscosity flow model, Landau-Levich-Derjaguin (LLD) model, an empirical equation related to the Bretherton model, and a capillary force model in a tube. These calculation results were comparable to the residual sodium amounts obtained by the experiments. In the tests of improving sodium wetting, the amounts of residual sodium on the test specimen were close to 1.4 times larger than those of the thin sodium film estimated by the LLD model. The increased amount of residual sodium by improving the sodium wetting was explained by the ratio of the adhesion energy (${\gamma }_{SO}-{\gamma }_{SL}$).

Keywords:

• Sodium cooled fast reactor
• dummy fuel assembly
• residual sodium
• wet cleaning process, capillary

Nomenclature

$A$	=	= circumference of pin
$b$	=	= equivalent radius of gap surrounded by three pins
Ca	=	= capillary number of liquid sodium at 200°C
$C_1$	=	= constant number
$C_2$	=	= constant number
$g$	=	= gravitational constant
$h_1^I$	=	= thickness of liquid sodium film by empirical equation on Bretherton model
$h_1^O$	=	= thickness of liquid sodium film by viscosity flow model
$h_2^O$	=	= thickness of liquid sodium film by LLD model
$L$	=	= height of sodium lump
$L_0$	=	= height of pin
$M_i$	=	= residual sodium amount on the assembly structure ($i$: driver fuel assembly and dummy fuel assembly)
$m_i$	=	= residual sodium amount on the pin surface ($i$: single fuel pin, driver fuel pin, and dummy fuel pin)
$n$	=	= number of the tests conducted
$S$	=	= area of gap surrounded by three pins
U0	=	= withdrawal speed of test piece
Greek	=
$\gamma$	=	= surface tension of liquid sodium
${\gamma }_{SL}$	=	= solid-liquid interface energy
${\gamma }_{SO}$	=	= solid-gas interface energy
$\eta$	=	= viscosity of liquid sodium
${\theta }_a$	=	= advancing contact angle
${\theta }_E$	=	= static contact angle
${\theta }_r$	=	= receding contact angle
$\rho$	=	= density of liquid sodium

Acknowledgments

One of the authors (M. K.) expresses gratitude to T. Yamaguchi, leader of the Sodium Technology Development Group, and colleagues at Japan Atomic Energy Agency’s Tsuruga Comprehensive Research and Development Center for various discussions.

Disclosure Statement

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