Internal dose calculations

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Project log

Andor Andrasi
added 2 research items
The idea of the IDEA project aimed to improve assessment of incorporated radionuclides through developments of more reliable and possibly faster in vivo and bioassay monitoring techniques and making use of such enhancements for improvements in routine monitoring. In direct in vivo monitoring technique the optimum choice of the detectors to be applied for different monitoring tasks has been investigated in terms of material, size and background in order to improve conditions namely to increase counting efficiency and reduce background. Detailed studies have been performed to investigate the manifold advantageous applications and capabilities of numerical simulation method for the calibration and optimisation of in vivo counting systems. This calibration method can be advantageously applied especially in the measurement of low-energy photon emitting radionuclides, where individual variability is a significant source of uncertainty. In bioassay measurements the use of inductively coupled plasma mass spectrometry (ICP-MS) can improve considerably both the measurement speed and the lower limit of detection currently achievable with alpha spectrometry for long-lived radionuclides. The work carried out in this project provided detailed guidelines for optimum performance of the technique of ICP-MS applied mainly for the determination of uranium and thorium nuclides in the urine including sampling procedure, operational parameters of the instruments and interpretation of the measured data. The paper demonstrates the main advantages of investigated techniques in comparison with the performances of methods commonly applied in routine monitoring practice.
In several countries guidelines exist concerning the requirements of whole-body counter measurements where the performance criteria for the quantities of bias, precision and minimum detectable amount are specified. One or more of these quantities are mostly the subject of intercomparison programmes. The whole-body counter laboratory of the KFKI Atomic Energy Research Institute has paid great attention to the good quality of measurements carried out for the determination of radionuclides incorporated in the human body. Participation in intercomparison programmes proved to be a very important and useful tool to check the capability and reliability of methods used. This laboratory participated in nine international intercomparison exercises organised in recent years, by means of which the counting efficiency has been tested for different detectors, monitoring geometries, body shapes and sizes, calibration and evaluation methods used. It can be concluded that in the majority of cases the results obtained were well within the range of performance criteria generally accepted in whole-body counting.
P. Zagyvai
added a research item
An occupational incorporation event occurred at the Radioactive Waste Treatment and Disposal Facility in December 2013 at Püspökszilágy, Hungary. Internal contamination due to 241Am was discovered by a regular routine whole body counting measurement at the Centre for Energy Research, Hungarian Academy of Sciences. After that, a whole body counter was calibrated for an organ counting geometry. For preliminary calibration, a home-made MIX-D chest phantom was applied simulating uniform lung activity distribution by 241Am point sources located in different positions within the lung volume of the phantom. In order to carry out a more precise calibration, a Lawrence Livermore National Laboratory (LLNL) chest phantom was provided by the International Atomic Energy Agency. For counting efficiency over the lungs, values of 0.46±0.19 and 0.55±0.07 cps kBq−1 were obtained for the MIX-D and the LLNL phantom, respectively; thus, the results are in good agreement.
P. Zagyvai
added 3 research items
DESCRIPTION This report gives technical guidelines for radio-iodine monitoring following a nuclear incident. Monitoring aspects addressed include the choice of detectors, the calibration and measurement process, factors affecting measurements, measurement uncertainties, the preparation of equipment and measurement locations, the measurement time, the measurement of very young children, management of results and measurements performed by members of the public. This latter point is addressed by making recommendations to professionals to enable them to train and inform citizens. Interpretation of measurements is addressed by providing data on doses per unit measurement, enabling the direct conversion of a measurement into thyroid doses and committed effective doses. The information provided can be used to calculate doses for different age groups, for the embryo or foetus, and for different radio-iodine isotopes. Interpretation of measurements following iodine prophylaxis is also addressed, as well as the case of doses from intakes of short-lived radio-iodine isotopes that cannot usually be measured in the thyroid.
DESCRIPTION This report presents the result of the work carried out by the Work Package 6 of the CAThyMARA project. This work focused on the direct assessment of the thyroid dose or committed effective dose from measurements. As a result dose per content functions are derived for different ages (new born, 1,5,10,15-years old children and adult) and for five radio-iodine isotopes (131-135 I) and 132 Te. Thyroid dose to the foetus is also obtained from measurement of the mother. If short-lived radioiodine isotopes cannot be measured correction factors to 131 I measurement are given. The interpretation of measurements in case of stable iodine prophylaxis is also addressed. This report also discusses the parameters influencing thyroid dose assessment. Finally a review of existing software for internal dose assessment is given and a demonstration software for dose assessment in case of nuclear emergency is presented. Approval WP5 leader Lara Struelens: 30/05/2017 OPERRA coordinator Jean-René Jourdain: 31/05/2017 2 1. PREFACE This report presents the methods and the results regarding the direct dose assessment from measurement of radio-iodine isotopes in the thyroid. Several cases are considered: from the newborn to the adult, the foetus and public members of the public who took stable iodine. This report also discusses the sources of uncertainties related to dose assessment, presents a review of existing software for dose estimation in case of emergency and finally presents a demonstration software dedicated to both professionals and citizens carrying out their own measurements. It is necessary, prior to any other text, to mention that materials presented here are based on some assumptions and a priori set boundaries. The considered scenarios cover serious accidents on light water nuclear reactors (PWR/BWR) in which population is subject of single acute intake. Dose assessment is performed during early phase after the accident, thus no intake by (chronical) ingestion is covered. Intake by ingestion was not covered in this project for two reasons: firstly because of the time limitation to carry-out the work, secondly because it can be reasonably assumed that food ban will be correctly implemented and followed after a reactor accident, at least for a vast majority of the population. One have to keep in mind, that the main purpose of the dose assessment system described in the CAThyMARA guidelines and here is the classification of subjects to those who need medical attention or follow-up and to those who not. Dose reconstruction for epidemiology is not considered as a primary goal, since is usually done when more accurate data are available. Thus robustness and easy use of the dose evaluation method, particularly in situations without detail information, was prioritised over high precision assessment. The system stands on old, however well-established, models related to ICRP 60 general recommendation, i.e. it does not consider dosimetric models and feature from newer ICRP documents (e.g. ICRP 103, 100, 130). The main reason for such an approach is that the updated ICRP dosimetric models were not yet fully available and implemented at the time of this project. However the use of the older system does not necessarily limit applicability since it is in general conservative. Moreover introduced systems may be easily adopted to the new one when needed. Simplistic approaches, such as one dose per measurement function for rather large spectrum of intake parameters, is used when inaccuracy due to aggregation is smaller than uncertainty of the input parameters. Other arguments for this simple approach is that it is less prone to evaluation error (mismatching) and may be used by a less skilled personnel. While guidelines is written in prescriptive form, this report is more of scientific nature. It shows particular issues in detailed way and tries to indicate limitation of the used approach in order to explain the methodology used in the guidelines. It may be read independently however it is recommended to read the guidelines prior to this report.
Tritium may exist in several chemical and physical forms in workplaces, common occurrences are in vapor or liquid form (as tritiated water) and in organic form (e.g. thymidine) which can get into the body by inhalation or by ingestion. For internal dose assessment it is usually assumed that urine samples for tritium analysis are obtained after the tritium concentration inside the body has reached equilibrium following intake. Comparison was carried out for two types of vials, two efficiency calculation methods and two available liquid scintillation devices to highlight the errors of the measurements. The results were used for dose estimation with MONDAL-3 software. It has been shown that concerning the accuracy of the final internal dose assessment, the uncertainties of the assumptions used in the dose assessment (for example the date and route of intake, the physical and chemical form) can be more influential than the errors of the measured data. Therefore, the improvement of the experimental accuracy alone is not the proper way to improve the accuracy of the internal dose estimation.