How did the Fukushima nuclear accident affect wildlife?

radiation hotspot

Radiation hotspot in Kashiwa, Japan, February 2012. (Source: Wikipedia, public domain)

On March 11, 2011, a tsunami, a giant wave set off by an earthquake, struck the Fukushima-Daiichi Nuclear Power Station in Japan. The tsunami caused a catastrophic failure of the power station and a release of radioactive material that has been rated second in magnitude only to the Chernobyl disaster. The extent of the radiological impact of this event on surrounding wildlife has been a contentious topic, but the results of a recent study are cautiously optimistic.

The United Nations Scientific Committee on the Effects of Atomic Radiation oversaw a study by an international team of scientists, who evaluated the results of a 2011 environmental assessment of the area near the power plant and published their results earlier this year (Environ. Sci. Technol. Lett., 2014, 1, 198–203).The UN committee compiled data for the year following the accident; relevant reports and scientific papers provided additional data. Radiation effects were inferred by comparing compiled dose–response relationships.

Radiation exposures were evaluated for the first 3 months after the accident, during which short-lived isotopes played a significant role, and for a later phase (3–12 months), in which exposure was dominated by longer-lived isotopes. Radionuclide concentrations were measured over time and geographic area. The researchers used kinetic models to calculate how concentrations varied over time in the whole collection of organisms for each area (biota). They determined cumulative doses using calculations.

Adult butterflies collected in September 2011 showed more severe abnormalities than those collected in May 2011, indicating that cumulative radiation exposure had caused deterioration in the overall butterfly population. However, dosimetry uncertainties and other confounding factors complicate the interpretation of these observations. The recorded dose rates were much too low to cause the kinds of abnormalities the researchers observed, and when they tried to replicate the conditions in the laboratory, they needed ten times the radiation exposure to reproduce the same effects that they saw in the field. Other scientific data do not support the appearance of the observed effects at the dose rates recorded.

The research team noted that localized effects might have contributed to the abnormalities they saw in the butterflies. They also observed declines in some bird populations and exposures of macroalgae that exceeded corresponding benchmarks, which supports this possibility.

The team concluded that because the highest exposure levels lasted only a short time, there was likely no damage to the integrity of plant and animal populations overall. However, individual organisms in relatively contaminated areas might have been damaged during the weeks immediately after the accident. Especially at risk were individual members of species that are especially sensitive to radiation, that stay in one place rather than moving around, or that live in areas that received high doses of radiation.

This is a lay-audience summary of my writeup on The American Chemical Society’s website.

The Art of the Possible

I’ve been intending to start a lay-person’s version of my weekly postings on the American Chemical Society website. Several of my Twitter followers have mentioned that they are impressed, but confused, by these tech-heavy synopses, written for an audience of professional chemists. I thought I might wait for a week with an “accessible” topic, like medieval bones or counterfeit currency, but I decided that I might as well just jump in and start today.

I really picked a doozy of a week to start. This week’s post deals with an energy minimization study of silica- and germania-based zeolites. Say what?

Sodalite

Sodalite, a naturally occurring zeolite. Source: Wikipedia, used under Creative Commons Attribution Share-Alike License (posted by Ra’ike)

OK, let’s start with “zeolites”. These are inorganic materials, some found in nature, some made in a lab or a factory. If you’re going to get really picky about it, you can call them “microporous solids”. At the atomic scale, these materials form frameworks with a lot of open space, so you can use them as “molecular sieves”, or you can make them into containers for other molecules. Zeolites are great for catalytic converters in cars, for making all kinds of chemicals from petroleum, or for absorbing huge amounts of water in disposable diapers, among other uses.
Carbon dioxide molecules in zeolite cages. Source: Berend Smit laboratory, UC Berkeley, via Department of Energy’s National Energy Research Scientific Computing Center

Carbon dioxide molecules in zeolite cages. Source: Berend Smit laboratory, UC Berkeley, via Department of Energy’s National Energy Research Scientific Computing Center

Scientists have spent decades assembling databases of zeolite framework structures. Some of these structures already exist in the real world. Others “ought to” exist, from geometrical considerations, but they’ve never been made before. Every now and then, someone runs across one of these hypothetical structures, and they want to know whether it’s worth their time and effort to try and make the actual material. Maybe it has cavities of just the right shape, or it has channels of the right size, for some application that this person has in mind.

In addition to structural databases, scientists have access to databases that contain information on how stable certain chemical compositions and arrangements are. These databases are the results of years of research gained from chemical reactions, melting, compressing, and otherwise poking and prodding various materials. As a result, scientists can examine a series of framework structures to see which ones are the most stable.

A research group from Arizona State University recently published the results of their structural stability calculations, using hypothetical zeolite structures made from silica (silicon dioxide, the same stuff that beach sand is made of) and germania (germanium dioxide, which is chemically very similar to silica). (Chemistry of Materials 2014, 26, 1523–1527). They looked at framework structures based on building blocks containing one silicon (or germanium) atom surrounded by four oxygen atoms, each oxygen atom acting as a bridge between one building block and the next. These frameworks are referred to as “tetrahedral networks”. The group was especially interested in the angles formed by two silicon (or germanium) atoms and the oxygen atom bridging them, or T–O–T angles.

Schematic of tetrahedral building blocks. A silicon or germanium atom is at the center of each tetrahedron, and oxygen atoms are at each point. Source: Wikipedia, used under Creative Commons Attribution Share-Alike License (posted by Trikly)

Schematic of tetrahedral building blocks. A silicon or germanium atom is at the center of each tetrahedron, and oxygen atoms are at each point. Source: Wikipedia, used under Creative Commons Attribution Share-Alike License (posted by Trikly)

The research group did calculations for a series of known structures, distorting the structures over a wide range of T–O–T angles and watching to see whether the stability increased or decreased. For all of the silica structures, the structures became more stable as the angles increased from 120º to 140º. From 140º to 180º, the stability stayed about the same. Germania structures, however, were most stable in a narrow angular range between 128º and 130º.

Several commercially important zeolites have structures with T–O–T angles in a range that would make silica very happy, but would be an uncomfortable contortion for germania. In fact, of the 5824 stable frameworks described in the Atlas of Prospective Zeolite Structures, 994 have all of their T–O–T angles in the optimal range for silica (135º–180º), but only 48 are in the most favorable range for germania (117º–145º). Mixing germanium and silicon in the framework might extend the range of stable bond angles and increase the number of stable structures available for synthesis.