Radionuclides to be considered in dose estimates following accidental airborne releases

Transcription

1 Radionuclides to be considered in dose estimates following accidental airborne releases K.G. Andersson, S.P. DTU Nutech, Technical University of Denmark ESS Target Station Design Update Name Affiliation Authors K.G. Andersson, S.P. DTU Nutech, Risø, Denmark Reviewers P. Jacobsson, T. Hansson ESS-AB, Sweden ESS-AB, Sweden

2 Document history Version Release date Comment 1 27/08/2012 Creation of version #1 2 30/08/2012 Comments by D. Ene implemented

3 Radionuclides to be considered in dose estimates following accidental airborne releases Introduction Assessment of radiological risk to man and environment from a hypothetical accident involving release of radioactive substances to air requires input data and a range of assumptions. Input data needed include the source term, i.e. identification of the radionuclides involved and the amounts released and the assumptions include specifications of the circumstances under which atmospheric transport and subsequent exposure of man occurs. Input data on accidents was made available at a meeting at ESS in Lund on 13 th of August 2012 identifying radionuclide inventory after 5 y operation (Ene, 2011) and specification of accident scenarios ( 2012). The meeting decided to focus on Design Basis Accident conditions and use release factors specified for elements representing volatiles and aerosols. This approach required further identification of radionuclides by combining the list of elements of volatiles and aerosols with the list of radionuclide inventory. For practical purposes a ranking of radionuclides according to radiological significance was needed in order to identify the radionuclides that contribute most to radiation dose to man from the accident. This report describes the ranking procedure applied and the radionuclides identified. Methodology Obviously, tritium needs to be considered in the dose calculations, due to the high concentrations in several parts of the ESS facility, wherefrom releases could occur, as well as the long physical half-life compared with most other radionuclides of potential relevance, and the high expected release fraction. Other contaminants of potential concern are defined with respect to element in the ESS document Specification for revised dose assessment ( 2012), and with respect to radionuclides of these elements in the ESS nuclide inventory report (Ene, 2011). Assessments to be made relate to accidents involving a monolith temperature of 500 C, and the elements of concern here would be Na, Ga, K, Rb, Cs,

4 Se, Sn, In, Zn, Te and Cd (assumed to be aerosols), and P, Cl, O, N, Br, F, S and I (assumed to be volatiles) ( 2012). Various isotopes of each of these elements are present in the inventory. The relative importance of each radionuclide needs to be identified to create a manageable list of only the radionuclides of importance for detailed consequence calculations. For this purpose, we calculated a risk index related to the internal dose (by ingestion of contaminated food) in an accident situation by multiplying the inventory content (after 5 y operation) of each radionuclide by the ICRP ingestion dose factor (ICRP, 1996) and the release fraction from the tungsten material (0.1 for volatiles and 0.01 for aerosols). Also external dose needs to be represented in this ranking, and another risk factor was calculated by multiplying the inventory content by a dose rate factor for exposure from a large, homogeneously contaminated plane ground surface (unit: Sv/s per Bq/m 2 on the ground) which was taken from Eckerman & Ryman, 1993), and a time integration factor (unit: s), which takes into account a subsequent exposure period (1y), and the physical half-life of each radionuclide. From experience it is expected that the magnitude of ingestion doses and external exposure from the ground would for 137 Cs contamination be of approximately the same magnitude (see, e.g., Roed et al., 2006). The relationship between ingestion and external dose risk index was for all other radionuclides scaled by the same factor that made the two ranking factors for 137 Cs balance. At this point the external and internal dose risk indices were combined by simple addition. The following ten radionuclides represent more than 96 % of the total of the dose ranking factors for the potentially release radionuclides (in this order): I-125, I-126, I-120, I-123, Te-121, Cd-109, Rb-83, I-121, Sn-113 and Se-75 (see Table 1). Table 1. Ranking of the most important radionuclides, based on estimated risk indices from ingestion dose and external exposure. Radionuclide T½, s Q (target), Bq* Ranking parameter I E E E+07 I E E E+06 I E E E+05 I E E E+05 Te E E E+05 Cd E E E+05 Rb E E E+05 I E E E+05 Sn E E E+05 Se E E E+05 *after 5 y operation

5 Also noble gases need to be considered. These are in the present context radionuclides of Ar, Kr and Xe. Naturally, none of these deposit, and contribute mainly to dose through radiation from the contaminated plume while dose from inhalation is insignificant. With the default assumption (Swedish Radiation Safety Authority, 2009) for accident calculations of a wind speed of 3 m/s and a distance to the representative person of only a few hundred metres, the physical halflife is of very little importance (provided that it exceeds a few minutes). It should also be noted that very little progeny of these released noble gas would be produced over the very limited time that it takes for the noble gases to spread over a very large area, thus resulting in very small individual doses. A risk factor related to the dose contribution from each noble gas contaminant can thus be calculated by multiplying the inventory content by the dose factor from Eckerman & Ryman for submersion in contaminated air of semi-infinite dimensions. The following ten radionuclides represent more than 99 % of the total of the thus calculated noble gas dose ranking factors (in this order): Xe-121, Xe-123, Xe-125, Xe-120, Xe-127, Xe-122, Kr-77, Kr-79, Kr-76 and Kr-88. Conclusions A short list of radionuclides of importance in connection with accidents has been created on the basis of information of source strengths in target following 5 years operation. On this basis it will be possible, given assumptions agreed on at the meeting in Lund on the 13 th of August 2012 regarding effective release height of 45 m to calculate the resulting dose per Bq emitted during the accident.

6 References: Archive number Eckerman K F and Ryman J C (1993). External exposure to radionuclides in air, water, and soil. Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA, Federal Guidance Report No. 12. Ene, D. (2011). Evaluation of ESS safety relevant concerns assuming two basic concepts for the target station, EDMS archive number , , ESS, Lund, Sweden. T. (2012). Specification for revised dose assessment, ESS /1, , ESS, Lund, Sweden. ICRP Publication 72, Age-dependent doses to members of the public from intake of radionuclides: Part 5, Compilation of Ingestion and Inhalation Dose Coefficients, ICRP, Elsevier Science Ltd., Oxford, Roed, J., Andersson, K.G., Barkovsky, A.N., Fogh, C.L., Mishine, A.S., Ponamarjov, A.V. & Ramzaev, V.P. (2006). Reduction of external dose in a wet-contaminated housing area in the Bryansk Region, Russia, J. Environmental Radioactivity, vol. 85 (2-3), pp Swedish Radiation Safety Authority (2009), Decision, 2nd April Reference: SSM 2008/1945, Order with regard to analysis of the radiological environmental impacts of nuclear reactors Oskarshamn 1, 2 and Oskarshamn 3.