TOR WOHN NO9600050 EXTERNAL RADIOACTIVE FALLOUT: DOSIMETRY AND LEVELS ::•••:•> .:\*--:.v.Sx ••'- JN^thS i'i i-t& - HZ* External doses from radioactive fallout: Dosimetry and Levels. Tor Wohni Norwegian radiation protection authority, Oslo University of Trondheim, NTH Submitted in partial fulfilment for the degree of Doktor ingenier Department of Physics Norwegian Institute of Technology University of Trondheim Tor Wehni: External doses from radioactive fallout Contents Preface and acknowledgement iii List of papers iv 1. Introduction and aims 1 2. Physical parameters influencing external dose levels 2.1 Introduction 3 2.2 Nuclide composition from different accidents 3 2.3 Activity depth distribution 6 2.4 Conversion factors from ground deposition to dose rate 9 2.5 Moisture, snow cover and deposition mechanisms 13 2.6 Conclusions 14 3. Technical and social factors influencing external dose levels 3.1 Introduction 15 3.2 Shielding factor 15 3.3 Decontamination 17 3.4 Social habits - Occupancy factors 19 3.5 Conclusions 19 4. Dosimetry and measurement techniques. 4.1 Introduction 21 4.2 Photon energy spectra 21 4.3 Measurement techniques 24 4.4 Appropriate dose quantities 27 4.5 Conclusions 32 5. Present work 5.1 Aims of present work 33 5.2 Nordic/Russian cooperation program - 33 Intercomparison test 5.3 Synopsis of papers 35 Tor W0hni: External doses from radioactive fallout 6. General discussion - Dosimetry 6.1 Introduction 39 6.2 Dosemeter systems 39 6.3 Dosimetry of penetrating radiation 42 6.4 Dosimetry of non-penetrating radiation 44 7. General discussion - Levels 7.1 Introduction 45 7.2 Corrections for background radiation a.o. 45 7.3 The influence of technical and social factors 47 7.4 Doses relative to the level of contamination 48 7.5 Annual dose reduction 50 7.6 Theoretical modelling of external doses 51 8. Conclusions 53 References 55 Papers I to V Tor W0hni: External doses from radioactive fallout iii Preface and acknowledgement The work described in this thesis was carried out at the Norwegian radiation protection authority (NRPA) in the years 1990 - 1994. The work started within the framework of a scientific and technical cooperation programme between the Leningrad (now St.Petersburgh) scientific research institute of radiation hygiene (SPIRHi and the radiation protection authorities in the Nordic countries. After the Chernobyl accident SPIRH established a local branch office in Novozybkov, situated in the heavily contaminated Brjansk region of southern Russia. During the cooperation programme I have attended several field investigations in this area, particularly in connection with individual external dose measurements in different villages around Novozybkov. As a spin-off from this project, similar external dose measurements were also performed in the most heavily contaminated municipality in Norway (0ystre Slidre), in close cooperation with an ongoing programme for whole body monitoring in this area. I am greatly indebted to the external dosimetry group at SPIRH for the scientific discussions and insight this has given into the external dosimetry problems connected with radioactive fallout, and especially to Victor Erkin and Vladislav Golikov. My Nordic coworker and fellow traveller in the SPIRH cooperation project has been Eva Wallstram at the Department of radiation physics at the University of Gateborg. Her methodical mind have been invaluable in the planning of the scientific content as well as the practical arrangement of the measurement programme. During the writing of the thesis, the assistance from the library staff at NRPA have been highly appreciated, and their readiness to help in the search for and the obtaining of relevant literature is greatly acknowledged. iv Tor Wehru: External doses from radioactive fallout List of papers The present work is based upon the following papers, which will be referred to in the text by their Roman numerals. I Wohni T. (1993). Dosemeterfor low level external radiation Radiat.Prot.Dosim. 4, 347-350. II Wohni T, Selnaes T and Strand P. (1994). External doses from Chernobyl fallout in Norway: Individual dose measurements in the municipality of Qystre Slidre. Radiat.Prot.Dosim. 2, 125-130. HI Erkin V, Wallstrum E and Wohni T. (1994). External doses from the Chernobyl fallout: Individual dose measurements in the Brjansk region of Russia. Radiat.Prot.Dosim.4, 265-273. IV Wohni T, Erkin V and Wallstom E. Dosimetry of external photon radiation. This report was presented at the symposium "Radiobiological consequences of Nuclear accidents: Radioecology and Health.", arranged in Moscow October 24th - 29th 1994. The report is accepted for publication in the forthcoming Proceedings volume from this symposium in Radiation Protection Dosimetry. The data in this paper is based on the measurement results from Paper II and Paper ffl. V Wohni T. (1995). Accident dosimetry of weakly penetrating radiation utilising carbon loaded LiFpellets in a Mylar phantom. Radiat.Prot.Dosim. In press. Chapter 1: Introduction and aims Chapter 1: Introduction and aims. External radiation represents a major contribution to the population doses from radioactive fallout. After the Chernobyl accident the dose from external radiation was the dominant contribution to the effective dose equivalent commitment in all but the countries in the southern latitudal region (< 40°). In the northern latitudal region (> 55 °) 60 % of the total dose was estimated as coming from external irradiation (UNSCEAR,1988). In chapters 2 and 3 of the present report, physical, technical and social factors influencing the external dose levels from radioactive fallout in general and radiocaesium in particular are discussed. In chapter 4 measurements techniques for assessing individual doses from penetrating and non-penetrating radiation are presented. In chapters 6 and 7 the results from this particular dosimetry project are discussed. As presented in chapter 5, the aim of the present work was twofold. Design, manufacture and calibrate suitable integrating dosemeters that can be worn by people in order to measure individual external doses from radioactive fallout. Apply the dosemeters in areas contaminated with radioactive fallout from the Chernobyl accident, in order to assess external doses; to the population. Analyze the results in the light of additional existing information concerning radioactive deposition, social habits, decontamination measures and other influencing technical and physical factors. The report deals with radioactive fallout in general, but experience and results from the Chernobyl accident will play a central role in the presentation. In this accident radiocesium very soon became the dominating nuclides. 137Cs is estimated to account for 85 % of the total external dose after the accident, while 134Cs contributed about 8 %. The rest was due to short lived radionuclides causing exposure only within the first year of the accident (UNSCEAR, 1988). Much of the assembled scientific data on external dose levels from fallout thus concern radiocaesium, which will play a dominating part in the discussion to follow. In this context the term "fallout" is synonymous with ground deposition. External doses from passing cloud or activity in air are not discussed, neither is contact exposure from radioactive Chapter 1: Introduction and aims material deposited on the body surface or clothes. Alpha radiation does not contribute to external irradiation, and are as such omitted from this discussion. Chapter 2: Physical parameters Chapter 2: Physical parameters influencing external dose levels. 2.1 Introduction. External dose levels are in general influenced by a number of physical parameters, like fallout levels, isotopic composition, activity depth distribution in the ground etc. The total amount of ground deposition is determined by the source term and by meteorological conditions, and is as such outside the scope of this report. The discussion in this chapter concerning migration of activity into the ground, mainly refers to rural environments. Some of the particular characteristics of urban environments are discussed in chapter 3. 2.2 Nuclide composition from different accidents. The isotopic composition of the fallout is a major factor for the formation of external doses. Large amount of radioactive material released into the environment will most likely be associated with accidents in nuclear reactors, nuclear fuel reprocessing plants or nuclear weapon detonation. The radioactive material released may be classified in three groups: - Fission products - Activation products - Uranium and transuranic elements. In general, fissions induced in ^U and 239Pu give rise to the main activity component from these sources. Fission products of nuclear weapons origin consist of more than 300 nuclides distributed over about 40 elements. The most important activation products are ^Mn and isotopes of Co and Fe. The relative contribution to the total fallout from each of the groups, and the relative importance of the different elements within each group will depend on the type of accident and other characteristics of the source term. In table 2.1 the isotopic composition of different well known nuclear accidents are shown, together with estimates of the global radioactive fallout from atmospheric bomb tests. Noble gases from the Chernobyl accident are not included, neither are 3H, 14C or activation products from the bomb tests. Published data concerning amounts of radionuclides released in the different accidents differ Chapter 2: Physical