Answer to Question #9669 Submitted to "Ask the Experts"

Category: Nuclear Power, Devices, and Accidents — Nuclear Accidents

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

Q

The media is reporting many uranium dose levels from Japan's damaged nuclear plants, but no plutonium levels have been reported.

Can Geiger-Mueller counters measure plutonium as well as uranium? If not, what instrument should the general public purchase to measure plutonium?

At what plutonium dosage can small cell (algae, plankton, etc.) marine life be damaged by plutonium?

What groundwater tables and waterways are adjacent to Daiichi Nuclear Plant 2?

A

The radioactivity that has been released from the reactor site and identified so far has been the fission products that volatilize most easily at some of the higher temperatures that the fuel has endured. Thus, radioisotopes of iodine, with 131I being the most important from a potential dose impact standpoint, have received considerable publicity. Radioactive isotopes of cesium, namely 137Cs and 134Cs have also been identified. I have not seen any information as to releases of uranium. The uranium oxide that fuels the reactors provided the 235U atoms that fissioned as the reactors operated. The fissioning of uranium produced lighter mass fission fragments, and these are the source of the radionuclides that have been noted in reports of releases.

Plutonium, of which 239Pu is the most notable isotope, is produced during reactor operation by various nuclear reactions and decays events. The 239Pu comes from the capture of a neutron by 238U in the fuel; the product that is produced undergoes one decay event and so transforms to 239Pu. The plutonium is not at all volatile, although it could be entrained in particulate form in a fire involving damaged fuel. It is not very soluble in water. It might appear in very small quantities in water effluents from the site if the fuel has been severely damaged and potentially contaminates water that has been in contact with the fuel and  released over long periods.

A Geiger-Mueller (GM) detector is not a very effective instrument for assessing possible plutonium, or uranium for that matter, in the environment, since the detector’s response would be overwhelmed by natural background radiation and possibly by radiation from released fission products that might be present. The plutonium, primarily an alpha particle emitter, would expectedly be present at very low levels, if at all, in environmental samples, and it would not be feasible to measure it with a GM detector.

There have been some specially designed field instruments that have been used for determining the presence of plutonium in surface soil and some other environments. These have relied on measurement of some low-yield, low-energy x rays that are emitted during decay of the plutonium, and have had relatively limited applications.

Depending on the type of media that might be contaminated and the extent of contamination it is often necessary to perform time-consuming and expensive laboratory analyses to evaluate plutonium content. Such analyses involve chemical separation procedures to isolate plutonium, subsequent electrodeposition of the plutonium, and alpha counting using sophisticated laboratory counting equipment.

I do not have specific information as to the concentrations of plutonium in sea water that would be deleterious to small cell marine life. The current recommended water concentration limit (in the United States) for long-term exposure of human beings to drinking water polluted by 239Pu is 0.74 bequerels per liter. Plutonium tends to accumulate in marine sediments, and this limits its bioavailability. However, some organisms may concentrate some radionuclides, and this bioaccumulation may impact permissible levels. A 1974 paper by Hodge et al. (Rapid Accumulation of Plutonium and Polonium on Giant Brown Algae, Health Physics, v. 27, pp. 29-35, 1974) showed the bioaccumulation factors up to a few hundred for plutonium by these large marine algae, and it seemed to increase with available surface area. There have been some studies that show that zooplankton retain very little ingested plutonium; rather, they very efficiently pass the plutonium though their digestive systems to be excreted in fecal matter.

I am not able to give you specific information about the groundwater aquifers and surface waters in the Daiichi vicinity. It is likely, however, that, since the nuclear plants are on the ocean coast, the general groundwater flow will be away from the land mass.

George Chabot, PhD, CHP

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