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Figure A.1. Variation of the detection efficiency with the distance for different age voxel phantoms and four detection systems (NOTE: Some measurement distances cannot be simulated because of the geometry of the detector and the phantom).
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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, t...
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Following international guidelines by the IEEE and a well-established calibration procedure developed at the Hungarian Institute of Isotopes in Budapest, the experimental absolute efficiency calibration of an Ortec n-type coaxial high-purity germanium (HPGe) detector (GMX Profile series) has been performed, in view of direct application to quantita...
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... From the figure, the apparent to true thyroid dose ratio on the 7th (14th) day after intake was 0.81 (0.76) in the case of exposure 24 h after the shutdown and 0.91 (0.90) in the case of exposure 72 h after the shutdown. These values would be reasonable for exposure in the postvessel melt-through phase when the release fractions of iodine and tellurium are similar to each other (21) . The radioactivity ratios of iodine isotopes and 132 Te for the Fukushima Daiichi nuclear power plant (Table 1) were also similar to those for a typical inventory in the core of a light water reactor (22) . ...
In a nuclear emergency, one of the actions taken for the sake of public is to monitor thyroid exposure to radioiodines. Japan’s Nuclear Regulation Authority recently published a report on such monitoring and proposed direct thyroid measurements with conventional NaI(Tl) survey meters (e.g. Hitachi model TCS-172) as a primary screening method. A previous study proposed screening levels (SLs) used in these simplified measurements as the net reading values of the TCS-172 device. Age-specific SLs were derived from a thyroid equivalent dose of 100 mSv due to the inhalation intake of 131I. This study addressed the possible influence of short-lived iodine isotopes other than 131I on the simplified measurements. In preparation for such measurements, the responses of the device for 132I as an ingrowth component from 132Te, 133I, 134I and 135I in the thyroid were evaluated by numerical simulations using age-specific stylized phantoms in addition to those obtained for 131I in the previous study. The radioactivity ratios of the relevant isotopes were taken from the inventory data of the Fukushima Daiichi Nuclear Power Plant. The results were used to predict the net readings of the device when 132Te-132I and 133I as well as 131I were inhaled at 24 or 72 h after the shutdown of a nuclear power plant. In these cases, the signals from 132Te-132I and 133I become undetectable a couple of days after intake, which could lead to underestimations of the thyroid dose. To estimate the thyroid dose accurately from the simplified measurements, it is necessary to identify the exact time of intake after the shutdown and the actual physiochemical property of 132Te that affects the thyroid uptake of 132I.
... The system photopeak efficiency calibration for thyroid measurements was performed using a neck phantom proposed by the Belgian Nuclear Research Centre (SCK·CEN) in the CAThyMARA intercomparison programme [11]. The neck phantom, described in detail in [3], consists of a PMMA cylinder (neck) which can host one pair of vials which simulate the thyroid lobes. ...
... After an acute inhalation, the radionuclides 132 Te, 137 Cs, 134 Cs and 103 Ru are typically distributed in the human body with no specific organ accumulation. On the contrary, all radioisotopes of iodine concentrate almost exclusively in the thyroid gland-which is positioned at the base of the front of the neck typically with an overlying tissue thickness of between 0.4 and 1.5 cm [5]-reaching a maximum about 24 h post intake. Therefore, in the event of a nuclear accident, specific whole-body and thyroid measurements are required for a correct dose assessment to members of the public. ...
... Concerning thyroid measurements, that are crucial in the case of a 131 I release from a facility producing radioisotopes for medical purposes, the use of simple portable non-spectrometric equipment, such as rate meters, offers an alternative to the preferred gamma-ray spectrometry technique [5]. In fact, since only radioiodine isotopes are retained in the thyroid gland, it is not strictly necessary to perform spectrometric measurements. ...
... Therefore the nonspectrometric equipment could be enough sensitive to be used to scan thyroids of internally exposed individuals. These instruments have the advantage of being much cheaper than more sophisticated (spectrometric) ones, readily portable and simple to operate (requiring a basic training) [5,6]. ...
In order to properly respond to an emergency caused by an accident in a nuclear power plant with a spread of radionuclides in the atmosphere, we propose a field procedure to perform a large-scale individual thyroid monitoring of internal contamination due to inhalation of 131I, by means of non-spectrometric equipment, in particular dose rate meters. Specific attention is paid to the individual monitoring of children, because of the very high radiosensitivity of the child's thyroid to the carcinogenic effects of ionising radiation. The device performance was evaluated by measuring mock iodine sources provided in the Child and Adult Thyroid Monitoring After Reactor Accident (CAThyMARA) intercomparison and, just for a scintillator dose rate meter, by means of 60 s acquisitions of healthy volunteers' thyroids. All the devices showed a remarkable accuracy in quantification of equivalent 131I activity in the thyroids of persons of all ages. The selected scintillator dose rate meter showed detection limit values resulting in a maximum committed equivalent dose to thyroid HT, assuming an acute 131I inhalation occurred five days before the measurement, equal to 10 mSv (related to five-year-old children). Considering the level of HT values associated with the calculated detection limit activities, the proposed procedure has a significant sensitivity to be used for fast internally thyroid monitoring in nuclear or radiological emergencies, allowing daily monitoring a large amount of individuals.
Internal monitoring of a large group of contaminated people is mandatory during nuclear and radiological emergency situations. The emergency preparedness and response will be implemented during such off-normal situations. The action plans will be based on the generic and operational criteria. Rapid monitoring results play a significant role in the decision-making. Most of the time the specialized internal contamination monitoring facilities may be far from the incident site. In such scenarios, independent internal contamination monitoring units as mobile units may be required to be set up near the affected site for primary screening of the contaminated individuals. In-vivo measurement techniques can be used for rapid screening of the public by using, field deployable in-vivo monitoring systems like Portable Whole-Body Monitor (PWBM) and Portable Thyroid Monitors (PTM), survey meters, laboratory-based Quick Scan Whole Body Monitor, etc. The present chapter briefly explains the preparedness and response strategies during nuclear or radiological emergency situations and the internal contamination monitoring protocols followed during such off-normal situations.
It has been long known that external radiation exposure in the area of the thyroid is closely associated with an increased risk for thyroid cancer, especially in children. Initially it was not clear if the increased risk also applied to internal radiation from radioactive iodine, especially iodine-131 (¹³¹I). Any doubt about this was erased in the wake of the Chernobyl accident that led to many thyroid cancers. This cemented the universal acceptance of the need to be prepared for the possibility of future accidents and, specifically, for the use of potassium iodide to mitigate the risk. The subsequent Fukushima accident reinforced this need, but also highlighted some of the complexities. Among them was the difficulty of rapidly evaluating the magnitude of the thyroid radiation doses in order to guide officials and the population about using potassium iodide. In retrospect, the Fukushima-related doses were not high enough to advise taking potassium iodide, but it was given in some areas and not others. Across the Pacific, along the West Coast of the United States, supplies of potassium iodide were needlessly sold out, although it is not known how much was actually ingested. The goals of this chapter are to inform health care workers about potassium iodide, how to promote potassium iodide distribution in advance of a possible release of radioactive iodine into the atmosphere, and how to inform the public in the event of such a release.
This study aims to numerically perform individualized calibration of a whole-body monitoring system for measuring the activity of ¹³¹I internal contamination. A set of ten computational phantoms inside the whole-body counter (WBC) unit were simulated using Monte Carlo code MCNPX 2.6 to investigate the effect of various morphometric parameters on the calibration factors. The counting efficiencies for separated source organs (εS) were calculated to convert the response of the detectors to the activity of ¹³¹I in the thyroid and other organs. The biokinetic model of iodine was combined with human respiratory tract model (HRTM), and distribution of iodine in body organs was calculated for three hypothetical scenarios (acute) and a typical real scenario (chronic). The organ-specific counting efficiencies (CEOrgan) and the total counting efficiencies (CETotal) were obtained as a function of time after the end of ¹³¹I radionuclide intake. The results showed that the thyroid was the major contributor to the CETotal (>90%) 2 days after the end of intake. Therefore, the individualized calibration process were performed using CEThyroid which is strongly dependent on the detectors solid angle seen by the thyroid more than other influencing factors. As a summary, a practical and sufficiently fast method was introduced to estimate the individual-related CEThyroid (within ±13%) for measurement times of >2d after the end of intake, based on estimation of detectors solid angle seen by a point in front of the neck.
A portable thyroid dose monitoring system has been developed at the Japan Atomic Energy Agency (JAEA) to assess the thyroid equivalent dose for workers and members of the public in a high dose rate environment. The background (B.G.) photon correction is required for an accurate measurement in a high dose rate environment at an early stage after a nuclear accident. The responses of the system to the B.G. photons were calculated with the simplified cylindrical phantoms made of PMMA or the voxel phantoms using the PHITS code. Then they were compared each other among the respective age groups. It was found that the cylindrical phantom with an appropriate size and properly arrangement can mimic the human body in the high dose rate caused by ¹³¹I sources. As such we successfully developed the B.G. photon correction method using the cylindrical phantoms; one of those is used as an effective way to calibrate the monitoring system by inserting a¹³³Ba point source into it. As an additional phantom is not required, this method has a great advantage in the practical operation of the B.G. photon correction.
The results obtained in a measurement campaign concerning the internal contamination by gamma emitting radionuclides of a large number of individuals are presented in this work. The aim is to assess the goodness of a spectrometric method in an emergency response following a nuclear power plant accident or a spread of radionuclides in the atmosphere due to a terroristic act. A HPGe portable spectrometer, deployed in a collective protection apparatus, was used for both whole-body and thyroid measurements. An adult Bottle Mannikin Absorption (BOMAB) and thyroid phantoms were used to evaluate the detector performance. The BOMAB phantom was provided by the Italian Institute of Ionizing Radiation Metrology (INMRI) for the ENEA intercomparison exercise. Thyroid phantoms were provided by the Belgian Nuclear Research Centre for the "Child and Adult Thyroid Monitoring After Reactor Accident" European intercomparison exercise. The instrument performance was further evaluated by collecting spectral data from healthy volunteers, using acquisition times of 180 s and 100 s, respectively, for whole-body and thyroid measurements. The detector showed a good accuracy in quantifying radionuclide activities in the adult BOMAB and in the thyroids of persons of all ages. The proposed method allows to detect in vivo activities leading to a committed effective dose E(50) and committed thyroid equivalent doses HT greater than 2 mSv due to all gamma emitting fission products, if the scan is performed within five days after intake. Assuming, for instance, an acute inhalation of Cs-137 and I-131, the obtained detection limit values for adult lead to a E(50) value equal to 0.08 mSv and a HT value of 0.27 mSv. The E(50) and HT values showed that the proposed method can be successfully used when the dose assessment must be rapidly performed for a large number of individuals in the eventuality of the scenarios previously mentioned.
The main aim of internal dosimetry in the frame of operational radiation protection is the evaluation of committed doses to verify the compliance of internal exposures with regulatory dose limits. To better understand the biological effects of internal exposures (cancer and non-cancer diseases), epidemiological studies can be conducted for estimating radiation-induced risks associated with intakes of radionuclides. In case of high levels of exposure when radiological events occur and for risk assessment, appropriate calculations of absorbed doses in Grays (Gy) to organs and tissues of exposed persons are required, but no reference methodology is currently available for internal dose assessments using data collected for epidemiology studies. Epidemiological studies and radiological emergency response in case of exposure to internal emitters may require different approaches, tools and methods for dose assessment comparing with operational internal dosimetry. This publication presents an overview of specific procedures associated with internal dosimetry for emergency response and epidemiology studies.
