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Detection of Leakage of Fuel Elements by Xenon Isotope Ratios in Primary Water of Paks Npp

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Detection of Leakage of Fuel Elements by Xenon Isotope Ratios in Primary Water of Paks Npp International Conference Nuclear Energy for New Europe 2002 Kranjska Gora, Slovenia, September 9-12, 2002 www.drustvo-js.si/gora2002 DETECTION OF LEAKAGE OF FUEL ELEMENTS BY XENON ISOTOPE RATIOS IN PRIMARY WATER OF PAKS NPP L. Palcsu, Zs. Szántó, É. Svingor, M. Molnár, I. Futó, Z. Major Institute of Nuclear Research of the Hungarian Academy of Sciences Bem tér 18/c., 4026 Debrecen, Hungary [email protected] T. Pintér Paks Nuclear Power Plant P.O. Box 71., 7031 Paks, Hungary [email protected] ABSTRACT By measuring the xenon isotope ratios in the primary circuit it is possible to detect the presence of inhermetic fuel rods in the reactor. Because high amounts of Xe and Kr isotopes are produced during the fission of uranium the existence of leakages can be detected by measuring these dissolved isotopes. During the operation beside the fission of 235U, plutonium-239 is also generated by fast neutron capture of 238U. Due to the 239Pu splitting the isotope ratios of fission produced xenon and krypton are changed. Supposing that during the fission period the isotope ratios follow a linear trend, and measuring two isotope ratios it is possible to determine the time that fuel elements spent in the reactor. We have been continuously sampling the dissolved gas in the primary water of the four circuits in the Paks NPP since 1999. The examination of the circuit-3 in the four blocks requires a special interest, because we have found an anomaly in the xenon isotope ratios. On the basis of the measurement data and the theoretical calculations it seems that the damaged fuel element was installed into the reactor in 1999, and it was removed from the reactor during the refuelling procedure in Summer 2002. 1 INTRODUCTION Knowledge of the quality and quantity of the dissolved noble gases in the primary water of nuclear power plants is important to get information about the condition of the processes generating energy and the state of the equipments. Among others, measuring the xenon isotope ratios in the primary circuit it is possible to detect the presence of inhermetic fuel rods in the reactor [1]. In the nuclear power plant of Paks (Paks NPP) a fuel element consists of zirconium rods with diameter of 9 mm filled by uranium-oxide pastilles. The primary water streams along the zirconium rods being moderator and heat carrier at the same time. If a rod is damaged, cracked or it is leaky then fission products can enter into the primary water. Understanding of particularities of leak-checking techniques is important conditions for reliability of quality control. Different methods for detection of leakages are mentioned in the literature: for example measurement of 134Cs/137Cs in the primary water [2], mass spectrometric leak testing under elevated temperature 0702.1 0702.2 with capsulation helium as a trace gas [3]. The fission noble gases activities provide information on the occurrence and number of defects, while iodine, cesium and neptunium activities provide information on the size and growth of the defect. The existence of the leak can be sensitively detected by the measurement of the dissolved xenon (and krypton) because high amount (about 22%) of the fission products of uranium are the isotopes of xenon and krypton. For instance, in a reactor, which uses uranium fuel enriched in 235U to 3.5 %, working with a neutron flux of 13 -2 -1 3.2·10 n·cm ·s and a specific power of 1375 MW/tonneU, during three years 5.4 kg of xenon and 0.36 kg of krypton are generated from a tonne of uranium found by de Bièvre [4]. During the operation beside the fission of 235U, plutonium-239 is also generated by fast neutron capture of 238U. Due to the 239Pu splitting the isotope ratios of fission produced xenon and krypton are changed (Table 1), because the fission of plutonium gives another composition of Xe and Kr isotopes. In this paper the measurements and the calculation are focused on xenon, because more information can be obtained from xenon isotopes than from krypton. Table 1: Xenon isotope ratios produced in uranium fuel during 1 and 36 months and the atmospheric values [4] 1 month 36 months atmospheric 128Xe/132Xe 4.13·10-5 2.81·10-3 7.13·10-2 129Xe/132Xe 0 4.7·10-6 0.9832 130Xe/132Xe 8.12·10-5 3.32·10-4 0.1518 131Xe/132Xe 0.6046 0.3756 0.7876 134Xe/132Xe 1.6887 1.3433 0.3883 136Xe/132Xe 2.4957 2.1176 0.3298 The longer the period the rod is in the reactor, the more plutonium is produced during fission and the bigger the differences among the isotope ratios of xenon are. If the xenon isotope ratios from both uranium fission and from fission of uranium enriched with plutonium are known, it is possible to determine since when the damaged fuel rod is in the reactor by measuring the isotope ratios of xenon dissolved in the primary water. 2 THEORETICAL BACKGROUND The isotope ratios in the Table 1 represent the ratios of xenon from fission after a one-month and a 36-month period in a reactor [4]. During a longer period (36 months) plutonium was generated in appreciable quantity, hence the isotope ratios of xenon were quite different from that of plutonium-free fuelling period. We suppose that during the fission period the isotope ratios follow a linear trend. Figure 1: Three-isotope calculation: the fission produced yields are in red, the isotopes from the air in green. Proceedings of the International Conference Nuclear Energy for New Europe, Kranjska Gora, Slovenia, Sept. 9-12, 2002 0702.3 Measuring two isotope ratios (for example 134Xe/132Xe and 136Xe/132Xe) in the dissolved gas of primary water it is possible to determine the time of fuel elements spent in the reactor. In the case of three isotopes (that equals to two isotope ratios) a linear equation system can be described, in which the seven unknowns are the air yield (A132, A134, A136) and the fission yield (F132, F134, F136) of the certain isotopes (Figure 1) and the time spent in the reactor (t). The parameters of the equation system are the following ones: the xenon isotope ratios in the air, the measured dissolved xenon isotope ratios in the primary water, and the slopes (s134, s136) and the intersections (i134, i136) of the linear change of the xenon isotope ratios supposed to be in the reactor during the fission period. The equation system is composed of the following equations: æ 136 Xeö (F + A ) æ 134 Xeö (F + A ) ç ÷ = 136 136 ç ÷ = 134 134 ç 132 ÷ (1) ç 132 ÷ (2) è Xeø measured (F132 + A132) è Xeø measured (F132 + A132) A136 A134 = 0.3298 (3) = 0.3883 (4) A132 A132 F F 136 = s ×t +i 134 = s ×t +i 136 136 (5) 134 134 (6) F132 F132 F + A + F + A + F + A =1 136 136 134 134 132 132 (7) Solving the system of equations the origin of the three isotopes and the time spent in the reactor can be determined. This method is called the three-isotope calculation. As we can see in Table 1 the literature data are comparable in case of xenon isotopes both originating from uranium fission (first column) and uranium-plutonium mixture (second column). Therefore, it is necessary to measure the isotope ratios very accurately, which is quite difficult in case of low concentration of xenon. From theoretical point of view the restrictive factor is the validity of the isotope ratios used for calculations. For example Ozima [5] has found that the 134Xe/132Xe ratio from neutron-induced fission of 235U is 1.84, the 136Xe/132Xe is 1.48, which values are quite different from the values found by de Bièvre (Table 1). Our calculations were made using the values from the Table1: more authentic values could be found only with "in situ" experiments in the nuclear power plant. Unfortunately, there weren't any possibilities for this task. Another method is to use six isotopes for calculations (Figure 2). The base of the calculation is that the xenon-129 is originated only from the air, or its fission yield is negligible (Table 1) [4][5], hereby the air-originated xenon of the dissolved xenon in the primary water can be determined on the base of xenon-129 measurement. The residual xenon arises from the fission. Examining the fission produced isotope ratios the time can be calculated similarly as it was mentioned above. In this "six-isotope calculation", a linear equation system with eleven unknowns provides the yields of the several isotopes. Figure 2: Six-isotope calculation: the fission produced yields are in red, the isotopes from the air in green. Proceedings of the International Conference Nuclear Energy for New Europe, Kranjska Gora, Slovenia, Sept. 9-12, 2002 0702.4 A A 129 = 6.476 129 =1.248 (8) (9) A130 A131 A129 A129 = 0.9832 (10) = 2.532 (11) A132 A134 A 129 129 = 2.981 æ Xe ö A129 (12) ç 132 ÷ = A136 ç ÷ (13) è Xe ømeasured A132 + F132 æ 130Xe ö A + F æ 131Xe ö A + F ç ÷ = 130 130 ç ÷ = 131 131 ç 132 ÷ (14) ç 132 ÷ (15) è Xe ømeasured A132 + F132 è Xe ømeasured A132 + F132 æ 134Xe ö A + F æ 136Xe ö A + F ç ÷ = 134 134 ç ÷ = 136 136 ç 132 ÷ (16) ç 132 ÷ (17) è Xe ømeasured A132 + F132 è Xe ømeasured A132 + F132 (A + F ) =1 å i i , where i concerns the six isotopes (18) i 3 RESULTS AND DISCUSSION We have been continuously sampling the dissolved gas in the primary water of the four circuits in the Paks NPP since 1999.
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