Boiling and Drying Accident of High-Level Liquid Waste in a Reprocessing Plant: Examination of the Time-Dependent Temperature Increase of the Waste and of the Generation Rates of the Individual Components Released into the Gas Phase

Abstract

Using the amount, composition, and decay power density of high-level liquid waste in a storage tank, the temperature change of the waste up to 600°C and the corresponding vapor and gas release rates of H₂O, HNO₃, NO₂, NO, and O₂ as a function of time after the loss of cooling function were obtained by the following method. The heat balance equations in and around the tank were derived, and the solution of the waste temperature change was numerically obtained using the vaporization rates of H₂O and HNO₃ and the generation rate of NOx, which were both obtained from the experiments using the simulated liquid waste. Utilizing the temperature versus time curve obtained from the equation, the release rates of the components described above were obtained as a function of time. This information on the progress of the accident can be used to study the Leak Path Factor of radioactive materials, especially of volatilized Ru, and further, it becomes basic information when considering accident management and suppressing the impact of a disaster.

Keywords:

• Reprocessing
• high-level liquid waste
• severe accident
• boiling, drying

Acknowledgments

The analysis using the MCNP code was performed by Yuri Tajimi, Daisuke Takano, and Tami Mukohara of TEPCO Systems Corporation. The authors are very grateful for their contributions.

Disclosure Statement

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

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.

Notes

^a The liquid waste changes from a solution state to a dry state with increasing temperature. Hereinafter, “waste” covers both states.

^b By 600°C, the generation of vapors and gases that transports radioactive materials outside the building will cease. Further, since it takes more than 10 days to reach 600°C as shown in Sec. X, it is possible to take measures during this time such as introducing water into the cell and submerging the storage tank.

^c Sections III, VII, and VIII are applicable to the initial stage, and Sec. IX is applicable to the late stage. Sections IV, V, VI, X, XI, and the Appendixes are applicable to both stages.

^d Since H₂O and HNO₃ are vaporized from nitric acid solution, the interaction between H₂O and HNO₃ needs to be taken into account. The influence of this effect is explained in Sec. IV.C.3.

^e The tank is made of SUS with a thickness of 0.03 m. The thermal conductivity of SUS is 17 W∙m⁻¹∙K⁻¹, and the heat transfer coefficient through the tank wall is 17/0.03 = 570 W∙m⁻²∙K⁻¹, which is about 100 times larger than the natural convection heat transfer coefficient (2 to 5 W∙m⁻²∙K⁻¹), and therefore, the heat transfer resistance from the tank to the cell air is controlled by natural convection.

^f The meaning of this equal sign is explained in Sec. III.A, No. 5.

^g 2440 is obtained from the values in Table IV.

^h The shape of the top and bottom is slightly mortar-like rather than flat (see Fig. 1).

ⁱ A debris thickness becomes 0.32 m.