Deposition of fission and activation products after the Fukushima Dai-ichi nuclear power plant accident

Abstract

The Great Eastern Japan Earthquake on March 11, 2011, damaged reactor cooling systems at Fukushima Dai-ichi nuclear power plant. The subsequent venting operation and hydrogen explosion resulted in a large radioactive nuclide emission from reactor containers into the environment. Here, we collected environmental samples such as soil, plant species, and water on April 10, 2011, in front of the power plant main gate as well as 35 km away in Iitate village, and observed gamma-rays with a Ge(Li) semiconductor detector. We observed activation products (²³⁹Np and ⁵⁹Fe) and fission products (¹³¹I, ¹³⁴Cs (¹³³Cs), ¹³⁷Cs, ^110mAg (¹⁰⁹Ag), ¹³²Te, ¹³²I, ¹⁴⁰Ba, ¹⁴⁰La, ⁹¹Sr, ⁹¹Y, ⁹⁵Zr, and ⁹⁵Nb). ²³⁹Np is the parent nuclide of ²³⁹Pu; ⁵⁹Fe are presumably activation products of ⁵⁸Fe obtained by corrosion of cooling pipes. The results show that these activation and fission products, diffused within a month of the accident.

Introduction

The Great East Japan Earthquake and the subsequent Tsunami on March 11, 2011, caused the cooling systems of the Fukushima Dai-ichi nuclear power plant to break down. The cooling of some of the nuclear reactors thus became impossible, and the pressure in the nuclear reactor container rose because of hydrogen buildup. Despite the venting operation performed for reactor No. 1 on March 12, the building was damaged by a hydrogen explosion. Immediately thereafter, ¹³⁷Cs was detected from dust collected within the premises of the power plant. Although a venting operation was performed for reactor No. 3 on March 13, a large-scale hydrogen explosion occurred the following day. On the other hand, dry venting was performed for reactor No. 2 without a hydrogen explosion, and a hydrogen explosion occurred in reactor No. 4 on March 15. For these reasons, the nuclear-reactor fuel rods and container underwent damage between March 12 and 15, and a large amount of radioactive nuclides was emitted into the environment. As of April 12, it was confirmed that the cores for the No. 1 to No. 3 reactors were damaged and that the fuel pellet had melted. In light of this, the Fukushima Dai-ichi nuclear power plant accident was elevated to a level 7 event on the International Nuclear Event Scale (INES) (Releases, 2011). The radioactive contamination in the environment is become a great concern in Japan for our survival.

In this study, environmental samples of soil, plant species, and water were collected on April 10—about one month after the accident—in front of the main gate of the Fukushima Dai-ichi nuclear power plant and 35 km away at Iitate village in Fukushima prefecture. Doses of radioactive nuclides were then measured by a Ge(Li) semiconductor detector without any chemical modification of the samples in order to comprehensively identify the nuclides that diffused into the environment.