Overnight, NRC analysts continued their review of radiation data related to the damaged Japanese nuclear reactors. The analysts continue to conclude the steps recommended by Japanese authorities parallel those the United States would suggest in a similar situation.
The Japanese authorities yesterday recommended evacuation to 20 kilometers around the affected reactors and said that people who live out to 30 kilometers should shelter in place (stay inside).
Those recommendations parallel the protective actions this country would suggest should dose limits reach 1 rem to the entire body and 5 rem for the thyroid, an organ particularly susceptible to radiation uptake.
A rem is a measure of radiation dose. The average American is exposed to approximately 620 millirems, or 0.62 rem, of radiation each year from natural and manmade sources.
Eliot Brenner Public Affairs Director
U.S. NRC Blog 
JoAnne, those recommendations (1rem/5rem) are for total dose, not dose rate. 1 Sievert equals 100 rem. 1 mSv (milliSievert) equals 100 millirem (or mR, mrad). In the English system, the Roentgen, rad, and rem are essentially equivalent units (for most types of radiation). In the SI system, everything is a factor of 100 larger. Yes the dose will of course decrease with distance, as with any form of radiative energy (light, etc.). Direct radiation drops by the inverse square law (1/d^2). Radioactivity distributed by the atmosphere (fallout) also decreases with distance, but is a matter of dispersal. Here’s a simple analogy: the further you are from a fire, (a) the lower the temperature from the radiative energy (heat), and (b) the less ash and smoke you will get on you.
It is not realistic to ask about what will happen if you have a full or complete meltdown, especially if you are asking about impacts on the US West Coast. Consider that the fully damaged Chernobyl plant explosion in 1986 (not just a melt down) contaminated everything for a few hundred miles at most, resulting in lots of issues that you have in mind. In that case, there was really no containment at all, and the problem was driven by improper actions of the operators as well as a bad reactor design. In Japan, while you did get hydrogen explosions (I wonder about the hydrogen recombiner systems), the main issue driving things is simply removing what reactor operators call “decay heat” from the core as it continues to shut down. That will result in core melting if uncovered, releasing lots of curies of radioactive fission products into the atmosphere if we assume that the containment is now gone or otherwise ineffective.
The situation could get worse if the Japanese are not able to keep water over the core such that the fuel melts and causes a critical mass to form. This is why it is useful to put neutron absorbing material such as boron into the core area to keep criticality as unlikely as possible. Commercial nuclear plants use borated water to suppress criticality so that they can load in more fuel, so many in the industry understand borated water, and boron products are a favorite since each boron atom can absorb or capture multiple neutrons.
The key to arresting the issues here might be in the form of boron used. Boron silicate sand-like materials could help suppress reactivity but might not allow cooling if they are just dumped into the core area. I personally would prefer the use of a couple of tons (maybe ten tons) of the boron loaded Raschig rings used by the Department of Energy for controlling Plutonium residue reactivity, described at http://www.orau.org/ptp/collection/miscellaneous/raschigrings.htm but the only place I ever say those myself was at the DOE Rocky Flats Plant, which has been decommissioned. If we could find those at other DOE facilities, dumping a lot of those into the damaged Japanese reactors would help control reactivity as well as allow continued heat removal, even if by natural circulation.
The problem with such “bright ideas” is that they have no place to go. The people who have to deal with blog inputs may not have much background or knowledge on the issue, else they would be over there on the “expert” team. I was the team coordinator for DOE’s “expert” team at the Rocky Flats Plant back in 1989 when we had to do a Criticality Safety Assessment, so I learned about boron impregnated Raschig rings at that time. We resolved all the technical issues, but the “whistleblower” that we proved to be right (raised a safety issue) died penniless because DOE would not reimburse his court costs, much less award him for avoiding a criticality accident. The point of that is we really can not trust institutions in Government to be able to react properly outside their rice bowls. Regulators are able to regulate, not fix.
Nevertheless, this could all be an academic discussion if the Japanese are successful in getting electric power back to their reactors.
EFB
Virtually all fission products are heavier than air. The chances of a particle being transported for thousands of miles by normal winds is incredibly small. “Fallout” from atmospheric atomic tests were large because the clouds were sent into the atmosphere by a nuclear blast. Normal trade winds simply don’t have the power to deliver these particles for thousands of miles.
Ms. Nadell
Does the amount of radiation decrease as the distance froml the source increases? Yes. Protection from radiation depends on time, distance and shielding. Time (stay time) interacts with the millisieverts/hour. Distance from the source impacts the dose rate/hour, much as stepping away from a heat source reduces the heat effects. Shielding (lead, masonry, water) lowers dose rate. There is a discussion above that explains sieverts/ REM conversion.
Most of the world uses the international standard for units (SI) which is why Japanese and IAEA press releases use Seiverts to report the dose to the public. The US does not use the SI system and instead uses rem as the measure of dose to the public. To compare the dose units, 100 mrem is equal to 1 mSv (this is similar to 1 yard equaling .914 meter (SI unit)). Therefore, the average annual dose that a person in the US receives is 620 mrem which is equal to 6.2 mSv.
The NRC has said and continues to say that we do not expect unsafe levels of radiation to reach the U.S.
Here we are, full meltdown of multiple reactors looming, and we have YET To receive ANY information on what risks a full meltdown would pose to the Pacific Coast. Can we please have some information regarding the subject?
Re: “Those recommendations parallel the protective actions this country would suggest should dose limits reach 1 rem to the entire body and 5 rem for the thyroid, an organ particularly susceptible to radiation uptake.”
Your post should be clear about where dose rates are measured! At the reactor, at the plant boundary, or in the public domain near the site boundary. The media who does virtually no research of the facts has a habit of running with these numbers into the houshold of the public!
The values being reported in the media are in micro or millisieverts… Can you please describe average annual does in those values versus R and mR? And it would be helpful if future dose discussion could be in mSeiverts, too. Thanks!
Agreed … but it doesn’t look like they’re willing to do that – or give recommendations based upon what to do if a full meltdown were to occur. I don’t want to evacuate California, but I also don’t want to die here. We haven’t received an OUNCE of information as to what kind of threat a full meltdown would pose to the West Coast. All we keep hearing are “Currently, there are no threats to the Pacific Coast.” Or, “The Pacific Coast is under no danger, as long as there is no full meltdown.” Well what if there were a full meltdown? It is looking like a very, very real possibility. I think we have the right to know what potential dangers we face.
Thank you for the above post; it has both fact and value. I encourage you take the next step and publish the results of your dose projections based on complete fuel melt and containment failure at Fulushima-Daiichi Unit 2, for the curennt evacuation and sheltering boundaries and at the West Coast of the U.S. Please be clear in how you phrase your rersults. For example, if the entire fission product inventory of Unit 2 was released during the explosion, the dose to a person located at 30 kilometers from the reactor for the entire next year would be “x” total effective dose equivalent and “y” committed dose equivalent to a child’s thyroid. That same scenario would result in doses of “X” and “y” to the maximially exposed person living on the West Coast of the U.S.
You should also put the values in the context of the normal exposure of average perason in the US from naturally ocurring radioactive materials.
Thanks
As stated, the recommendations for max radiation exposure is 1rem for the whole body and 5rem for the thyroid for the general public annually. The news media keeps tossing around numbers in “millisieverts/hour?” How do these two numbers compare? There are lots of statistics and no real explanation of what these numbers mean and the general public is completely confused. Does the amount of radiation decrease as the distance froml the source increases?