Geosciences Column: Following Fukushima: what happened to the iodine isotopes?

The March 2011 earthquake, 130 km off the coast of Japan, resulted in a 10-40 m high tsunami inundating Japan’s Pacific coast and caused the release of radionuclides from Fukushima Daiichi Nuclear Power Plant (NPP).  The demise of three of the reactors was widely covered in the media, with worldwide coverage of the potential effects of radiation release both close to the plant and further afield.

Following the accident, significant quantities of radionuclides were released into the environment. Those of particular concern included ¹³⁴Cs, ¹³⁷Cs and ¹³¹I as large amounts of these isotopes are produced during nuclear fission and they both have high gamma ray energy. Radioiodine can enter the human body through food and drinking water, where it accumulates in the thyroid, creating an irradiation risk. However, the time required for ¹³¹I to decay is on the order of 8 days, so the irradiation risk rapidly decreases with time.

The location and impact of the March 2011 earthquake in Japan (click for larger). (Credit: Wikimedia Commons users W. Rebel and Jon C.)

This iodine isotope, however, has a much longer lifetime (it has a half-life of 1.57 x 10⁷ years). This means that oceanographers can trace the path of the more harmful ¹³¹I retrospectively, based on where they find ¹²⁹I (its decay product). There’s one caveat: the level of ¹²⁹I in the environment is already elevated beyond natural concentrations. Release of ¹²⁹I during nuclear fuel reprocessing has caused ¹²⁹I to accumulate in both soils and seawater over time.

Consequently, knowing the background level of ¹²⁹I is crucial to knowing what effect the Fukushima NPP accident had on the local environment. Thanks to a little routine water sampling, water samples from the northwest Pacific were taken before the plant went into operation – phew! So, we know that the concentration of ¹²⁹I before the Fukushima NPP accident: it was on the order of 1-2 x 10 x 10⁷ atoms per litre – what happened after it?

Takashi Suzuki and his team from Japan’s Atomic Energy Agency took a closer look at the ¹²⁹I in water samples at the surface and at depth following the event and found elevated ¹²⁹I throughout the water column, particularly in the surface mixed layer.

Depth profiles of 129I before and after the Fukushima NPP accident. Green, brown and orange points were from late April, early June and late October 2011, respectively. (Credit: Suzuki et al. 2013)

¹²⁹I ranged from 1-90 x 10⁷ atoms per litre after the accident, the highest measurement being a whopping 73 times larger than pre-accident levels! That certainly sounds like a lot, but what does it mean for the consumers of local seafood?

Using the concentration of ¹²⁹I in the water column, the accumulation of radioactive iodine in marine animals and the amount of seafood consumed by average Japanese individual, Suzuki found that the risk to human health was negligible (6.7-550 x 10^-11 Sv per year), some 100 million times lower than the annual does limit (1.0 x 10^-3 Sv per year)! Good news for all concerned.

By Sara Mynott, EGU Communications Officer

These findings are published as part of a series on the impacts of Fukushima’s NPP discharges on the ocean. Published in Biogeosciences, this series brings together research on the dispersal pathways, transport processes and deposition of radionuclides following the accident, which you can access here.

References:

Aoyama, M., Uematsu, M., Tsumune, D., and Hamajima, Y.: Surface pathway of radioactive plume of TEPCO Fukushima NPP1 released ¹³⁴Cs and ¹³⁷Cs, Biogeosciences, 10, 3067-3078, 2013.

Suzuki, T., Otosaka, S., Kuwabara, J., Kawamura, H., and Kobayashi, T.: Iodine-129 concentration in seawater near Fukushima before and after the accident at the Fukushima Daiichi Nuclear Power Plant, Biogeosciences, 10, 3839-3847, 2013.