Answer to Question #9678 Submitted to "Ask the Experts"

Category: Radiation Basics

The following question was answered by an expert in the appropriate field:

We know that we are getting radiation from Japan now in small doses. What is to prevent it from growing as Japan's emissions grow in the future? Since we are getting it now at the level Japan is at now, why would we not get more later as Japan gets more later?

You are not alone in your concern about radioactivity released in Japan reaching the United States and the potential impact it might have. We all occupy a globe of finite dimensions, and we share some common environmental transport paths through which materials generated on part of our planet may be carried by prevailing forces to locations very remote from the source.

As we discuss this issue, I should first note that one great advantage we have with respect to our assessment of radioactive materials, as opposed to many nonradioactive toxic species, is that we often have much greater sensitivity for detecting the radioactivity by way of the radiations that the material emits than we do for most nonradioactive materials. Thus, even when the concentrations of radioactive atoms are so low as to represent no health concerns, the material may still be detectable using the instrumentation that has been developed and is currently available and operational in many laboratories throughout our country and the world. That is the situation we presently have regarding detection of radioactive fission products that have been transported from Japan on the winds across the Pacific Ocean.

The diluting power of the atmosphere is extremely large, and the airborne concentrations of some fission products that have reached our shores from the Fukushima Daiichi site are very low. As the radionuclides reach our country they may remain airborne, some may deposit from the air directly on to soil, vegetation, and bodies of water, and some may be washed out of the atmosphere to the earth with natural precipitation. In all cases, resulting levels have been extremely low, not adding any meaningful increment to the radiation doses that we already receive from natural background radiation sources.

If the situation in Japan should worsen such that higher levels of radioactivity were to be released to the atmosphere, and the prevailing wind patterns remained as they have been, then we might expect to see the concentrations of some radionuclides increase in our country, but I cannot imagine a scenario in which such levels would reach a point where they presented any significant health concern.

We must keep in mind that the reactors have been shut down and some fission products of initial concern, especially the radioactive iodines, of which 131I is the most important, are decaying with rather short half-lives—e.g., 131I has an 8-day half-life, which means that every eight days any previously existing 131I would have been reduced in amount by a factor of two. Unless something changed drastically for the worse in Fukushima—in particular, a new fire that affected fuel in the reactors or greater losses of cooling water that allowed the fuel temperature to increase markedly—I would not expect levels in this country to reach markedly higher levels than we are now measuring.

We should keep in mind that there may be local occurrences at the Japanese reactor sites that may produce temporary increases in releases of some radioactive species—e.g., the releases of radioactive materials into the ground and sea water associated with leakage of contaminated water through the recently discovered crack in a concrete pit. Such events may produce notable localized increases of radioactivity and associated radiation levels in some environmental media in the Fukushima area, but may result in no increases in radioactivity in our country. In order for us to experience increased radioactivity levels, not only must there be increases in levels released from the Japanese site, but there must also be an effective transport mechanism to get the material to our country. Atmospheric transport is the most likely means for transporting material here, but this is only effective if the reactor products are released to the air. Material that is released to the ground or ground water in Japan and ultimately may reach the ocean will be partly sequestered by silts and other possible biota and/or be diluted to such a great extent that we would not expect to see measurable amounts in sea water near our shores.

What we see in the way of radioactive contaminants reaching our country is clearly related to what is happening in Japan, but there is not a one-to-one correlation between amounts released there and what we measure here because of the intervening factors that affect the distribution and migration of material in the environment. The physical form and properties of the released material—gas or solid or liquid—and radioactive half-life, chemical properties, affinity of inert and bio-materials for uptake of the material, and available transport pathways affect the amounts that will reach us. The radioactivity that has been able to escape from the reactors into the air environment and be transported to remote locations has been volatile components that were volatilized at elevated temperatures when some water coolant was lost, thus allowing the fuel to heat up to the point of being partially damaged with an attendant loss of some of the cladding that provides a barrier against leakage of fission products from the fuel matrix. This radioactivity includes primarily some radioactive noble gases, of which 133Xe is an example (5.2-day half-life), radioactive iodines, of which 131I (8-day half-life) is the most notable, and radioactive isotopes of cesium, primarily 134Cs (2.1-year half-life) and 137Cs (30-year half-life).

Naturally, there is a greater concern on our part with certain food products produced in sections of Japan, where levels of contamination in soil, vegetation, and water might be relatively high, that might be imported into the United States. Our country has banned imports of fresh vegetables, fruits, and milk from Japan. It is also requiring radiation monitoring, upon arrival, of other food products from Japan.

In summary, your inferences about the relationships between levels of radioactivity being released from the Fukushima site and levels being measured here have some validity, but the present situation does not seem to portend ever-increasing releases from Japan and, in the event of continuing and/or increasing releases from the debilitated reactors, the levels that we will experience in this country may vary significantly with the characteristics of the radioactivity released and any changes in environmental conditions that influence transport of the material from Japan to the United States. There is no reason to believe that future levels will present any notable health impact to citizens in this country.

Naturally, we are all hoping for the safest and most expeditious resolution and minimization of the threats caused by the extreme events that have transpired in Japan. The direct catastrophic impacts of the earthquake and the tsunami have wrought terrible consequences that have been a cause of great grief for Japanese survivors as well as the world community and will require massive commitments of manpower and resources to remediate. The nuclear events involving the reactors at the Fukushima Daiichi site present an added burden that will require continuing efforts and resources to minimize the effects on safety and health of the Japanese workers and general population. The likely permanent losses of several of the reactors as electrical generators present another significant loss that might negatively affect Japan’s overall well-being and recovery. We must trust that the United States, along with much of the developed world, will continue to provide assistance in the forms of financial aid, materials, technical resources, and expert manpower to assist Japan in its recovery.

George Chabot, PhD, CHP

Answer posted on 4 April 2011. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.