# Development of Offshore or Onshore Disposal Facility for Disaster Wastes and Decontamination Wastes Polluted by Radiation

Representative Researcher : Tsuchida, Takashi (Hiroshima University),

Associate Researcher : Endo, Satoru, Shizuma, Kiyoshi (Hiroshima University), Watabe, Yoichi. (Port and Airport Research Institute), Ueno, Kazuhiko, (Pentad Ocean Co. Ltd)

## Summary

For recovery from Fukushima No. 1 nuclear power plant accident, the management and disposal of huge amount of wastes and soils contaminated by radioactive cesium is urgent requirement. The wastes consists of the disaster wastes and waste soils accompanied by the decontamination works, incinerated ash of contaminated plants and trees and bottom mud of discharged water, river and sewage. In three years after the event, the wastes in heavily contaminated areas still remains without any treatments. The predicted volume of waste soils, which were placed in various places in the area, also ranging from 16 million m $ ^{3} $ to 41 million m $ ^{3} $ . The lack of disposal facilities for these soils is hindering the decontamination works. In historical earthquake disasters in Japan, the disaster wastes had been disposed in a large scale disposal facility located in offshore or onshore areas. The purpose of this study is to show the technical requirements of the facility for wastes contaminated by radiation, and the feasibility is estimated.

Based on the properties of wastes and soils contaminated by radioactive cesium, the technical conditions and requirements for bottom clay layer with low permeability are determined as following table;

Technical Item	Condition	Background and references
Radioactivity concentration disposed in the facility	100,000 Bq/kg	The maximum radioactivity concentration of wastes reserved in the interim storage facility. The average concentration investigated in this study was 24,800 Bq/kg.
Average dissolution ratio of wastes	10%	The average concentration investigated in this study was 6.7%.
Average density of wastes	1.5t/m³	The average values investigated in this study
Average void ratio of wastes	0.80	The average values investigated in this study
Thickness of bottom clay layer with low permeability, D	D=4.0m	In the case of disposal facility for normal wastes, D is 2.0m.
Water head difference h in and out the facility	h=4.0m	According to the investigation report of disposal facilities for normal wastes, h is about 2m.
Radioactivity of seepage ground water	90Bq/kg (Cesium137)	Acceptable value for discharged water from nuclear power plant
Treatment of effluent water by wastes reclamation	No effluent water	Effluent water is discharged to ocean after treatment in disposal facilities for normal wastes.
Permeability of bottom clay layer	less than 5×10-10m/s	To reduce the radioactive concentration, the travel time of seepage water is made to be longer than 233 years. The standard of facility for normal wastes are 1×10-10m/s (5m thickness), or 1×10-8m/s (0.5m thickness).
Rare of cesium absorption of bottom clay layer	90%	No description in the standard for normal wastes

Artificial deformable geomaterials for seepage water control structure was developed by mixing the marine clays with bentonite. According to a series of laboratory tests, it was shown that the developed material satisfied the technical requirements shown in above table.

The seepage water control structure must be safe against the great earthquake and Tsunami. The centrifuge tests were carried out to study the behavior of

Fig.1 Cross section of seawall structure

seawall and the seepage control structure at the back of seawall. The results show that the seepage water control structure is safe by taking larger distance from the seawall. From the result of the under-water shaking table model tests, the safety of seawall structure and the seepage water control structure was confirmed. The numerical simulation was carried out to estimate the safety against L2 Tsunami. The result showed that the all the facility will be submerged, and that by the installation of counter works and the embankments, the flow velocity and the flow volume would be reduced drastically as to keep the safety of the surface cutoff layer and the wastes inside.

Based on the results of study, the structure of disposal facility was proposed as Fig.1. The construction cost of 5 million m $ ^{3} $ facility was estimated as 26,600 yes per 1m $ ^{3} $ volume of wastes and that of 10 million m $ ^{3} $ facility was 22,200 yen per 1 m $ ^{3} $ , which were considered to be feasible not only technically but also from cost-benefit considerations.

Key words: waste disposal facility, radio active cesium, Fukushima-Daiichi nuclear power plant, radiation disaster, contaminated soil, marine clay, bentonite, permeability