Time Dependent Radio-Toxicology of Fission Products M

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Radiological Risk Assessment (Wednesday, February 12, 2014 14:00)

Time Dependent Radio-toxicology of Fission Products M. Stern , A. Baram , S. Pistinner Soreq NRC, Yavne 81800, Israel

INTRODUCTION Ionizing radiation, emitted by radiological materials, is known to cause damage to biological tissue. Prolonged exposure to radiation may cause a vast array of harmful medical effects, from enhancing future probability of cancer, up to Acute Radiation Syndrome resulting in multi-system failure. In complex radiologic release events involving fission products (nuclear fallout, reactor failures), the products' physical decay chains dictate a time dependent product inventory. As the ratios between different products vary, so does the toxicology of the radioactive inventory as a whole. The temporally varying toxicological factors should be taken into account when producing radiological risk assessments for populations. In this paper we study the time varying toxicology of fission products, using a specialized model named Koala, developed in Soreq NRC. A significant and monotonous rise in the aggregate toxicity of ingested fission products was noted. This result carries important implications for risk assessment, as it partially cancels out the fission product physical decay. A similar, albeit less pronounced rise was found for external exposure. Factoring activity and toxicity together allows computation of effective source terms for simple events involving fission products. We demonstrate one such source term, based on fallout from a nuclear explosion. This source term may be easily introduced into suitable atmospheric dispersion models.

THE RADIOACTIVE INVENTORY Several important classes of radiological risk assessments involve fission products. These materials are product of nuclear reactions in fissile material (such as Uranium or Plutonium), used to release energy in nuclear reactors or weapons. The fission products are usually radioactive, and may pose a threat to the population. Characterization of the fission products is thus an important step in preparing radiological risk assessments for events like nuclear accidents or fallout. The fission products are divided into decay chains, such that the quantity of a single isotope satisfies the equation (1)

Where is the quantity of the isotope, is its decay constant (reciprocal mean lifetime), and are the branching ratios of other isotopes into the considered isotope. This relation corresponds to a large number of connected first order differential equations, one for each isotope involved in the problem. To solve this set of equations, we have developed a model named Koala, which utilizes several independent techniques to obtain solutions of the time dependent inventory(1). The currently preferred method is based upon solving the eigenvalue problem of the decay matrix represented by (2)

Where is the unity matrix. Obtaining the time dependent solution of the fission products inventory allows the computation of a crucial function for risk assessment, the inventory activity (3)

Koala's results were validated by comparing them to both analytical solutions of simple isotope systems, and experimental data. The following figures present comparison of Koala to experimental and numerical results for aggregate fission product inventories (for U235 and Pu239). Millage(2) considered numerically the decay of Pu239 using numerical programs, while Petrov et al.(3) studied activity measurements the inventory of U235. The results are normalized by the Way-Wigner approximation(4) denoted by

Figure 13. Comparison of Koala results to decay data. a) Millage's[2] numerical data for decay of Pu239 (using DKPOWR and ORIGIN2), b) Petrov et al. [3] experimental results for decay of U235. Activities are divided by the Way-Wigner(4) approximation According to this approximation, the activity on these graphs should form a straight line. The deviation from the Way- Wigner Approximation is clearly demonstrated. The nontrivial temporal behavior of the decay products is well reconstructed by the Koala model. This test and others show that the model is compatible for calculation of decay inventory for fission products in instantaneous events. A modification of the model allows computation of the time dependent decay inventory in the more complex settings of a reactor core.

RADIO-TOXICITY OF FISSION PRODUCTS Performing risk assessments for events involving fission products require a further elaboration on the aforementioned computation. Each of the isotopes may be assigned numbers, relating its instantaneous activity to the biological damage it may cause. These factors depend on the pathway of exposure: 1. Inhalation(5) – each inhaled Becquerel causes aggregate damage over decades. Toxicity for inhalation is assigned the units of .

2. External exposure due to the radioactive cloud(6) (Immersion) – proportional to the time integral of the airborne activity concentration ( ).

3. External exposure due to ground shine[6] – proportional to the ground activity concentration ( ).

Computing the risk of each isotope at a certain time entails finding its activity, and multiplying it by the relevant toxicity factors. This process facilitates the construction of an effective source term, comprised of an "effective isotope", with activity equal to the total inventory activity, and a time dependent toxicity conversion factor. It is convenient to group the isotopes according to their reactivity: Ideal gasses are non- reactive and are not deposited on the surface. This property also dictates that their radio-toxicity due to inhalation is null. Therefore, we will consider two effective isotopes referred to as "Solids" and "Ideal Gasses". The following figures show the time dependence of the effective toxicity conversion factors for the two effective isotopes, due to an instantaneous fission of Pu239 by fast neutrons.

Figure 14. Time dependent radio-toxicity of the two effective isotopes ("Solids" and "Ideal Gasses"), due to a) inhalation of fine particles, b) immersion in the radioactive cloud. In figure (2a) we find an unexpected phenomenon: The effective radio-toxicity due to inhalation significantly grows over time, as the inventory gradually accommodates more toxic isotopes. In the first two years following fission it may be approximated by a power law with exponent 0.6. This growth indicates that the inventory decay rate provides a poor estimate for the radiological risk to population. In fact, the inventory physical decay is partially offset by the sharp increase in radio-toxicity. The effective radio- toxicity due to external exposure (2b) does not show growth of such magnitude, but still displays a nontrivial temporal dependence. It is noteworthy that during the first few hours after the fission (the time window most relevant for risk assessment), one may still detect an effective radio-toxicity increase of nearly two orders of magnitude compared to the initial value. Let us now discuss a simple source term, due to a nuclear explosion of 1kt, which will result in fission of Pu239 nuclei. Koala allows calculating the activity of each radioisotope, and multiplying it by the relevant radio-toxicity factors. Factoring in a typical human respiration rate facilitates placing the radiological exposures due to inhalation and external radiation on the same footing. The following figure illustrates the "potential doses" due to the effective isotopes in each path of exposure. It is important to stress that the potential dose does not represent a real exposure a person may contact, but a measure of the source term, representing the hazardous materials in the atmosphere following the explosion. To obtain risk assessments for the exposures of the population, the source term is inserted into a suitable atmospheric dispersion model.

Figure 15. The potential dose due to a 1kt nuclear explosion. In the first two hours, most of the radiological exposure is caused by external exposure to solids. In later times, the primary cause of radio-toxicity is inhalation of radioactive aerosols. The noble gasses seem to contribute very slightly to the source term at all times. Let us remark that the comparison between external and internal exposures has an important caveat. The external exposure is immediate, and the internal exposure will be accumulated over the following 50 years. It is thus recommended to initially treat the external exposure, which may cause more severe biological effects.

REFERENCES 1. J. Harr, Precise Calculation of Complex Radioactive Decay Chains, AFIT/GNE/ENP/07-03 (2007). 2. K. K. Millage, Fission Product Decay Characteristics, AFIT/GNE/ENP/86M-5 (1989). 3. R. V. Petrov el al, Radioactive Fallout, Appendix IV (1963). 4. K. Way, E. P. Wigner, The Rate of Decay of Fission Products, Physical Review, Vol 73, 11 (1948). 5. International Basic Safety Standards for Protection Against Ionizing Radiation and for the Safety of Radiation Sources, Safety Series No. 115, International Atomic Energy Agency, Vienna (1996). 6. K. F. Eckerman, J. C. Ryman, External Exposure to Radionuclides in Air, Water, and Soil, Federal Guidance Report No. 12, EPA-402-R-93-081 (1993).