w DEPARTMENT OF COMMUNITY SERVICES AND HEALTH Australian Radiation Laboratory Radiation Protection in the Mining and Milling of Radioactive Ores Volume 1 Lecture Notes from a course conducted at the Australian Radiation Laboratory October 2029,1987 Edited by John F. Boas \RL/TR095 • -••• .,,, IvA• ... .... LOWER PLENTY ROAD ISSN 0157-1400 YALLAMBIE VICTORIA 3085 JULY 1990 TELEPHONE: 433 2211 ••ISLT-'T *Z?m»"".-•- AUSTRALIAN RADIATION LAB0RA10RY Radiation Protection in the Mining and Milling ot Radioactive Ores. Volume 1 Lecture Notes t-rom a course conducted at the Australian Radiation Laboratory October 20-29, 1987 Edited by John F. Boas LOWER PLENTY ROAD YALLAMBIE VIC 308b TELEPHONE: 433 2211 (i) Preface Ihe development of mining and milling of radioactive ores in Australia has given rise to an increased possibility of the exposure of industry employees and members cf the public to ionizing radiation. As part of its responsibilities "to act as the focal point of national expert advice" in public and occupational health matters relating to radiation, the Australian Radiation Laboratory conducted a two week course entitled "Radiation Protection in the Mining and Milling of Radioactive Ores" in February 1981. Similar courses were conducted in February 1983 and in August I98S. During these courses, the participants' time was divided approximately equally between lectures and practical work. An A.R.L. Technical Report (ARL TR/043) was printed in January 1982 and contained the lecture material. The evolution of the field was one of a number of factors which affected the organization of the course held in October 1987. The stabilization of interest in uranium mining and milling and the growth of the mineral sands industry required a change of emphasis, as sand mining had been only briefly mentioned in the 1981 Course. Secondly, the scientific emphasis of the course changed, for example, increased emphasis was placed on radioactive dust as experience has shown that this is as significant, or more significant a contribution to radiation exposure than radon daughter inhalation or external gamma radiation. Finally, the revision of the Code of Practice for the Mining and Milling ot Radioactive Ores, completed late in 198/, provided the stimulus for a revision of much of the lecture material. As a result, the 198/ Course was divided into two blocks of four days, where the first part covered mainly background material for those participants new to the industry and the second covered some more advanced aspects, including the 198/ Code and the changes and developments since 1981. As was the case with previous Courses, the participants' time was divided between lectures and workshop/laboratory exercises. This Technical Report covers the lecture material presented during the course. As with all active fields of science, there have been further changes and developments since 1987. The development with the greatest impact has been the revision of the dose estimates from the atomic bombs at Hiroshima and Nagasaki. As a result of this and other new information on the effects of (li) radiation exposure, the International Commission on Radiological Protection (1CRP) is currently (July 1990) revising its recommendations for limits to radiation exposure. Ihere has also been additional material in the literature on a number of other issues. As a result, some of the chapters contain material not available at the time of the course. In editing the lecture notes, the main emphasis was on consistency of format, rather than on avoiding overlaps in content. Ihus some chapters repeat material covered elsewhere. However, it was felt that where appropriate each chapter should be self-contained. It should be noted that the opinions expressed here are those of the authors. Accordingly, these volumes are not intended to serve as the authoritative source for standards of practice in the mining and milling of radioactive ores. Ihe appropriate standards and guidelines are to be found in the relevant Codes of Practice, Guidelines and Handbooks. Whilst some of the material presented is well known and relatively easily accessible in the literature, other sections contain material which is less accessible or is the result of original work by the authors which will be published elsewhere. As coordinator of the course, 1 would like to thank all those who participated for their contributions. Special acknowledgement is due to the clerical and typing staff of A.R.L. for their assistance in the preparation of the notes for the course. In particular, Judy Evans and Rhonda Austin carried most of the burden of converting authors' manuscripts into the form enclosed between these covers. Although most of the lectures were given by staff members of the Australian Radiation Laboratory, lectures were also given by staff members of Roxby Management Services Pty Ltd., the Office of the Supervising Scientist for the Alligator Rivers Region and the A.C.T. Administration, Environment Protection Branch. I would like to particularly thank Neale Brabham, Mike Carter and Hugh Crawley and their respective organizations, for their contributions. John Boas July 1990. (iii) A Note on the Australian Radiation Protection Standards Australia sets its radiation exposure limits by adopting those Recommendations of the International Commission on Radiological Protection (1CRP) that apply to our conditions. Ihese cover exposures of the public in general and of those whose jobs may lead to radiation exposure in the course of their work. I he exposure limits, set by the National Health and Medical Research Council (NHMRC) in 1980 were:- • b mi Ilisievert per year tor members of the public; and • bO mil lisievert per year for radiation workers with the additional requirement that exposures be kept "as low as is reasonably achievable". These numbers exclude exposures from natural background radiation (between 1 and 2 millisievert per year) and exposures from medical procedures (less than 1 millisievert per year). Since 1985, a further condition has been introduced that the exposure of members of the public over a long time should not exceed an average rate of 1 millisievert per year. Since 198/, new data and a re-evaluation of earlier information indicates "with reasonable certainty" that the risks associated with ionising radiation are about three times higher than was assumed when the above standards were set. The ICRP is presently engaged in revising its recommendations and a new set is likely to be finalised in November 1990. f-oI lowing this, the NHMRC will develop and promulgate a new set of recommended Australian standards tor radiological protection. fhese will be progressively incorporated into the appropriate legislation, codes of practice and guidelines. I he main changes to be recommended by the ICRP are likely to be:- • a reduction of the dose limit for occupational exposure from 50 millisievert per year to 20 millisievert per year, with some provision for year-to-year flexibility. This flexibility could take the form of an average over several years with the additional restriction that not all the dose is received in a single year (e.g. not more than 2 years average in a single year). (iv) • The current limit for the exposure of members of the public of 1 millisievert per year averaged over a lifetime will be made more stringent, by limiting the averaging period to five years. Although exposure to radiation in Australia is unusually low (public exposures above background are negligible and average occupational exposures are about 0.5 millisievert per year), the changes proposed by ICRP are likely to have a significant impact on some areas of radiation protection. Whilst the notes enclosed in these volumes were prepared following the existing standards, the reader should bear in mind the changes which may occur in the near future. ... •t'-f-'n'*-" ' 'wr~ -afirm)^-^-" •rv (V) A Glossary of Terms Activity: Radioactive decay rate usually in units of disintegrations per second (8q). The traditional unit, the Curie (Ci) corresponds to 3.7 x 10 disintegrations per second. Alpha Particle: A positively charged particle of atomic mass 4. tmitted in the radioactive decay of some nuclei. I he particle, which is made up of two protons and two neutrons, is the same as the nucleus of the helium atom. Atomic Hasi (A): The sum of the number of protons and neutrons in a nucleus. 238 Thus, the dominant isotope of uranium ( U) has an atomic mass of 238, which is the sum of its 92 protons and 146 neutrons. Atomic Number (Z): The number of protons in the nucleus of an atom. Attenuation Coefficient: The parameter which represents the absorption characteristics of an absorbing medium. Usually applied to gamma radiation. The absorption of gamma rays, under conditions when source, absorber, and detector are well separated is governed by ai exponential law where 1 is the incident intensity, x the absorber thickness and v, in units o of (length) is the absorption coefficient. It is often convenient to express absorber thickness in units of mass/area, by multiplying the linear thickness by the density, (for example g/cm ) in which case y takes the units 2 -I of area/mass (cm g ). Attenuation Length: I he thickness of absorber required to reduce the intensity of gamma rays by a factor /e. Also known as mean free path. Becquerel: (Bq) The SI unit of activity. 1 Bq corresponds to a decay rate of 1 disintegration per second. Beta Particle: An energetic electron emitted in the radioactive decay of some nuclei. More penetrating, and with a lower rate of ionisation than alpha particles. The beta particle may be either positively or negatively charged. Bremsstrahlung: High energy X-rays emitted by electrons which are retarded in the Coulomb field of nuclei (German for "Braking radiation"). Buildup Factor: A multiplying factor, greater than 1, which is frequently based on experiment, and which is used 1n shielding estimates, to correct "ideal" geometry estimates of absorption for the effect of extended source, absorber and detector.