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MEDICAL PHYSICS IN <strong>THE</strong> BALTIC STATES 7 (2009) Proceedings of International Conference “Medical Physics 2009” 8 - 10 October 2009, Kaunas, Lithuania IGNALINA NUCLEAR POWER PLANT DOSE MONITORING SYSTEM *, Oleg Klepalov*, Kęstutis Gediminskas *, Romas VILKAS *, Mindaugas ZIKAS *, Gediminas ADLYS * * Kaunas University of Technology **Ignalina Nuclear Power Plant Abstract: The occupational exposure control is one of the important problem ensuring safety of personnel during operation of nuclear power plant. The TLD-system RADOS was set into operation in December of 1994. The software of this system ensured data interface between new system and existing database of INPP and provided a multi-user mode of data access. As a next step for the upgrading of the personal dosimetry system and preventing the overexposure of personnel in NPP a new electronic dosimetry system using RAD-80 and RAD-51dosimeters was implemented in 1997. Moreover the dynamic exchange of personal exposure from the electronic dosimetry data allowed planning the dose budget more exactly during implementation of different kinds of activities (especially during maintenance period). Analysis and comparison of data obtained using both dosimetry systems has shown that in general the parallel application of the TLD-system and electronic dosimetry system allowed to improve the quality of dose reduction measures, which represented the effectiveness of the implementation of ALARA program in NPP. Keywords: personal dosimetry system, TLD, dose monitoring, ALARA 1. Introduction The first unit of Ignalina NPP went into service at the end of 1983, the second unit was set into operation in 1987. Their design lifetime was projected out to 2010 - 2015. A total of four units were originally planned on this site. The Ignalina NPP contains two RBMK-1500 reactors. This is the most advanced version of the RBMK reactor design series. Compared to the Chernobyl NPP, the Ignalina NPP is more powerful (1500 MW versus 1000 MW), and is provided with an improved ACS (Accident Confinement System). Data base of Ignalina NPP consists of data on about 3500 persons, therefore the problem of duly and correct record-keeping of the personnel occupational exposure data and management of personal doses is one of the important problems ensuring safety of personnel during operation of the plant. During first years of the INPP operation film dosimeters of type IFKU were used for the monitoring of personnel occupational exposures. However the old system had several essential imperfections, such as: • low range of measurement (0.5 – 20 mSv); • special conditions for processing the dosimeters were required; • possibility to lose data if the film is spoilt by light or by heat; 83 • automatic registration and transmission of the data was not provided etc. It was clear that some measures to improve the existing personal dosimetry system have to be considered taking into account the growing strictness of requirements and international recommendations that occupational concerning personnel exposure. The first step for improving the personal dosimetry system at INPP was made in 1989. In this year INPP started to use new type of TL dosimeters with Al2O3 detectors. These dosimeters had a number of advantages as compared to film dosimeters: • wider range of measurement (0.1-10 4 mSv); • stable sensitivity; • mechanical durability. Besides after modernization of the dosimeter processing unit the possibility to transfer data from the measurement unit to a computer was provided, that made the creation of the exposure database for NPP workers possible. However this system had some weaknesses too: very high photosensitivity of detectors, manual processing were the main problems using this system. To overcome the above mentioned problems the project for the developing and improvement of the personal dosimetry system in INPP was initiated in 1994 within the framework of the "Bilateral Technical Assistance Program between Sweden and Lithuania in order to enhance the radiation protection safety at Ignalina NPP".
O. Klepalov et al. / Medical Physics in the Baltic States 7 (2009) 83 - 85 2. Personal dosimetry system The TLDs most commonly used in medical applications are LiF:Mg,Ti, LiF:Mg,Cu,P and Li2B4O7:Mn, because of their tissue equivalence. Other TLDs, which are used because of their high sensitivity, are CaSO4:Dy, Al2O3:C and CaF2:Mn. TLDs are available in various forms (e.g. powder, chips, rods and ribbons). A basic TLD reader system consists of a planchet for placing and heating the TLD, a PMT to detect the thermoluminescence light emission and convert it into an electrical signal linearly proportional to the detected photon fluence and an electrometer for recording the PMT signal as a charge or current. A basic schematic diagram of a TLD reader is shown in Fig. 1. TLDsystem which is used at Ignalina NPP is presented in Fig.2. Fig.1. TLD reader. Fig.2. Schematic diagram of the TLD system "RADOS" in Ignalina NPP. External exposure of personnel in the reference year was controlled once a month using thermoluminescent dosimeters. 84 Control of the externel exposure of personnel working under hazardous conditions was performed using both: thermoluminescent and electronic RAD dosimeters at the same time. The staff was allowed to work in a controlled area, presuming a dose of 0.2 mSv per day if, the difference between the collected individual dose and annual dose limit did not exceed 5.0 mSv after the work. If the difference between the collected individual dose and the annual dose level did not exceed 3.0 mSv, the staff was allowed to work under hazardous conditions, accounting 0.05 mSv per day. In 2008 individual dosimetric control was carried out for 3599 workers. 2320 workers of the NPP staff and 1279 workers from contractor's organization staff (CO). 3. Results The average individual dose of 0.91 mSv was estimated to CО + INPP personnel in 2008: 0.96 mSv for NPP personnel and 0.83 mSv for the personnel from contract organizations (Fig.3). Maximum individual dose for INPP staff was 18.09 mSv, maximum dose to sole worker from the contractor organization was 19.98 mSv. Data on the number of investigated workers and received collective doses are provided: for 2008 - in Table1; for the whole period of operation of Ignalina NPP –in Fig.4. 1. Table. 2008 Ignalina NPP and contractor's organization personnel irradiance indicators.
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