• Waste management in Japan
• Circular economy in Japan
• Waste management in Asia
• Disaster waste management

Introduction

The March 2011 Fukushima Daiichi Nuclear Power Plant Accident caused the discharge of various radionuclides into the environment. They fell and were deposited on the ground in wide areas of East Japan. Among them, the major nuclides of human health concern were radiocesium isotopes (i.e., Cs-134 and Cs-137) because they were emitted in large amounts and have relatively long half-lives. After the accident, the Act on Special Measures Concerning the Handling of Radioactive Pollution was enacted in Japan, and radiocesium has been treated according to this Act. At the time of writing this article, treatment of disaster wastes and excavated soils has been progressing in the areas closer to the power plant. Under such circumstances, in addition to radiocesium, we thought that there would be a need to know more about other radionuclides that also have long half-lives including their concentrations in the environment and behaviors during treatment of the waste containing them (e.g., tendencies of their concentrations in incineration ash during waste volume reduction processes).

As an example of such radionuclides with half-lives of more than a year, we focused on radiostrontium (Sr-90) with a half-life of 28.8 years, almost the same as that of Cs-137 (30.2 years). Sr-90 was discharged by the accident also in a large amount (1.4×10¹⁴ Bq) after Cs-137 (1.5×10¹⁶ Bq)¹⁾. Behavior of Sr-90 in the treatment of radioactive disaster waste was not well known while that of radiocesium was becoming increasingly clear. There were also some difficulties in the measurement of Sr-90. The commonly used method for measuring radiocesium by gamma-ray spectrometry could not be applied to Sr-90 because it does not emit gamma-rays when decaying. The measurement of Sr-90 requires isolating it by complicated sample preparation processes and measuring beta-rays of generated Yttrium-90 after about two weeks. There were not many cases of Sr-90 measurement in the waste management field because of these complicated processes and time required.

Development and evaluation of simplified methods for Sr-90 measurements

For the above-mentioned reasons, we have explored much simpler methods to measure Sr-90 in incineration ash samples. The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan has published a manual to provide a standard method to measure Sr-90²⁾, but this method isolates strontium using cation exchange resin and iron hydroxide treatment and needs to repeat precipitation, filtration and dissolution many times. It is not simple. In order to simplify these processes, we considered an alternative method using a strontium adsorption disk called “Empore^TM Strontium Rad Disk” (Sr Rad Disk). However, incineration ash contains various elements including barium and calcium that interfere with strontium adsorption on Sr Rad Disk, as well as radioisotopes of lead and bismuth that impede beta-ray measurements. Thus, we have proposed the following procedures for strontium recovery: removal of lead and bismuth as hydroxide from alkalized sample solution followed by removal of barium and calcium by chromic acid treatment; and filtration of the residue by Sr Rad Disk.

We collected 14 incineration ash (i.e., bottom ash and fly ash) samples from 8 incineration facilities in East Japan during the period from July 2011 to November 2013 and measured their Sr-90 concentrations using the above-mentioned simplified measurement method as well as a modified MEXT method. The modified MEXT method is a method with hydroxide treatment, fuming nitric acid treatment and chromic acid treatment that were added to the original MEXT method so that it can be used for incineration ash.

The results were as follows. In the measurements of Sr-90 concentrations of fly ash samples with the simplified method and the modified MEXT method respectively, the results obtained by the two methods agreed well. Conversely, in the measurements of bottom ash samples, Sr-90 concentration values were determined only with the modified MEXT method. Almost no Sr-90 was detected by the simplified method; it was below the detection limit of 2.1 Bq/kg. Based on these results, we have been improving the simplified Sr-90 measurement method for incineration ash.

Tendencies of Sr-90 concentrations in incineration ash

In this survey, we found that Sr-90 concentrations in incineration ash, measured with the simplified method and the modified MEXT method, ranged from 2.3 to 7.8 Bq/kg for fly ash and from 3.1 to 7.2 Bq/kg for bottom ash (all values were after decay correction on the day of sample collection). These values were similar to the results of another survey (11.0 Bq/kg for fly ash and 9.2 Bq/kg for bottom ash after decay correction on the day of sample collection) that we conducted in municipal waste incineration facilities in Fukushima Prefecture in December 2012. These results show that Sr-90 concentration values in incineration ash in East Japan were between 1 and 10 or slightly over 10 Bq/kg. The ratios of Sr-90 to Cs-137 ranged from 1/130 to 1/5200 that were almost equivalent to those of the two elements deposited in the soil.

It is known that radiocesium tends to concentrate in fly ash during waste incineration processes. In our survey, the ratios of radiocesium (Cs-134 and Cs-137) concentration in fly ash to that in bottom ash were found to be in a range of 5.16 to 11.3. These results were consistent with the above-mentioned general tendency of radiocesium. In comparison, the ratios of Sr-90 concentration in fly ash to that in bottom ash were in a range of 0.375 to 1.33. These results indicate that almost no Sr-90 concentrates in fly ash, compared with radiocesium.

In addition to Sr-90, we will study other radionuclides including their concentrations and behavior in incineration and various other waste treatment processes.