Answer to Question #9903 Submitted to "Ask the Experts"

Category: Alpha Emitters

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

Q

This request is about calculating internal dose from an alpha intake.

I am interested in finding references or general information on how alpha recoil energy can impact solubility of the source contamination. For example what kind of biokinetic behavior one can expect from an old (~15 years) source term containing transuranic radionuclides sitting in a passive system environment? Can alpha recoil energy disintegrate the particles to make them easily resuspendable and change the solubility of the particles and hence implicate the resulting dose when compared to a freshly released transuranic source term? The transuranic radionuclides we are interested in are 239Pu, 241Am, and 242/244Cm.

A

The so-called “solubility" of oxides of alpha-particle-emitting isotopes of plutonium, americium, and curium can be markedly affected by radiolytic mechanisms. In a chemical sense, solubility implies ionization in solution. On that basis alone plutonium would be rated as totally insoluble at normal physiological or environmental pH since the solubility product constant is 7 × 10-52 (Cleveland 1970). However, some inhaled plutonium dioxide enters the blood after inhalation. In contrast, both americium dioxide and curium dioxide display somewhat more soluble behavior in the lung after inhalation deposition. When the biological uptake in fish of 241Am fallout is compared to the environmental uptake of 239Pu, the more soluble americium uptake is larger and more important (Suchanek et al. 1996).

For practical purposes, besides chemical solubility there is also considerable mobility for very small particles especially in biological systems and in the environment. Insoluble particles deposited in the lung that are smaller than 10 nm in physical diameter can readily move through available pores directly into the blood where they can complex with citrate and transferrin and can be deposited in the liver and skeleton (Raabe 1982).

So-called nanoparticles of the actinides can be highly mobile and appear to be soluble without actually chemically dissolving. This apparent solubility is caused by the natural fragmentation effect of alpha particle recoil. Fleischer and Raabe (1977 and 1978) showed that the apparent partial dissolution of plutonium dioxide in water or body fluids is really the results of fragmentation of nanoparticles as the result of alpha emission recoil. The much higher apparent solubility of freshly prepared particles of 238Pu dioxide compared to freshly prepared particle of 239Pu dioxide of about the same physical size is directly proportional to the differences in radioactive decay rate of these two alpha-emitting isotopes. Raabe et al. 1973 and Raabe et al. 1975 have shown that fine particles of 238Pu dioxide exhibit about a two-hundred-times faster dissolution rate under neutral conditions than do fine particles of 239Pu dioxide because of radiolytic fragmentation effects. The specific activity of 238Pu is about 200 times the specific activity of 239Pu. When the biological uptake and environmental mobility of 238Pu fallout are compared to 239Pu, the more mobile 238Pu is clearly larger.

The answer to the question is yes, the age of the samples can markedly affect the apparent solubility and biokinetic behavior due to internal and surface fragmentation of the materials caused by alpha emission recoil. However, if particle size and granularity are about the same, the 239Pu will appear much more insoluble than the 241Am or the 242/244Cm.

Otto G. Raabe, CHP

References:

  • Cleveland JM. The chemistry of plutonium. Gordon & Breach Science Publishers: New York; 1970.
     
  • Fleischer RL, Raabe OG. Fragmentation of respirable PuO2 particles in water by alpha decay—a mode of "dissolution." Health Phys 32:253-257; 1977.
     
  • Fleischer RL, Raabe OG. On the mechanism of "dissolution" in liquids of PuO2 by alpha decay. Health Phys 35:545-548; 1978.
     
  • Hardy EP, Krey PW, Vochok HL. Global inventory and distribution of fallout plutonium,Nature 241:444-445; 1973.
     
  • Raabe OG, Kanapilly GM, Boyd HA. Studies of the in vitro solubility of respirable particles of 238Pu and 239Pu oxides and an accidentally released aerosol containing 239Pu. Inhalation Toxicology Research Institute Annual Report1972-1973, LF-46 (UC-48), pp. 24-30, Lovelace Foundation for Medical Education and Research, Albuquerque, New Mexico. (Available from NTIS); 1973.
     
  • Raabe OG, Boyd HA, Kanapilly GM, Wilkinson CJ, Newton GJ. Development and use of a system for the routine production of monodisperse particles of 238PuO2 and evaluation of gamma emitting labels. Health Phys 28:655-667; 1975.
     
  • Raabe OG. Deposition and clearance of inhaled aerosols. Chapter 2 in Mechanisms in Respiratory Toxicology (H.R. Witschi and P. Nettesheim, Eds.), CRC Press, Inc., West Palm Beach, Florida, pp 27-76; 1982.
     
  • Suchanek TH, Lagunas-Solar MC, Raabe OG, Helm RC, Gielow F, Peek N, Carvacho O. Radionuclides in fishes and mussels from the Farallon Islands nuclear waste dump site, California. Health Phys 71:167-178; 1996.
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