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
A129 A129 = 6.476 (8) =1.248 (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. The sampling technique and method for sample preparation are detailed in Ref 6 [6]. Beside the determination of the main components by quadrupole mass spectrometer, the noble gas concentrations were measured as well. The xenon content of the sampled gas was determined by a noble gas mass spectrometer [6]. Table 2: Xenon isotope ratios dissolved in the primary water of the block-3 (values during the shut-down are bold-typed)
Date of 129Xe 130Xe 131Xe 134Xe 136Xe Xenon (ppb) sampling 132Xe 132Xe 132Xe 132Xe 132Xe 2000 03 11 0.762 0.117 0.798 0.704 0.809 0.5 2000 03 12 0.724 0.127 0.779 0.705 0.81 1.1 2000 05 19 0.588 0.084 0.771 0.88 1.11 3.2 2000 05 20 0.505 0.095 0.728 0.89 1.13 4.8 2000 06 17 0.559 0.0639 0.739 1.08 1.42 2.2 2000 06 18 0.349 0.0553 0.604 1.1 1.47 7.7 2000 07 28 0.222 0.0409 0.565 1.49 2.19 4.6 2000 07 29 0.229 0.0327 0.592 1.41 2.13 6 2000 07 29 0.237 0.0285 0.627 1.35 2.06 6.9 2000 07 29 0.528 0.0519 0.671 1 1.4 9.3 2000 07 29 0.724 0.105 0.741 0.696 0.857 55.1 2000 07 29 0.726 0.109 0.742 0.717 0.885 43.4 2001 03 15 0.324 0.069 0.63 1.303 1.725 20.2 2001 03 15 0.299 0.045 0.604 1.27 1.876 27.1 2001 04 27 0.14 0.026 0.498 1.276 1.961 33.52 2001 04 28 0.194 0.032 0.517 1.235 1.866 106.5 2001 06 01 0.052 0.015 0.537 1.433 2.259 4.3 2001 06 01 0.06 0.023 0.498 1.36 2.163 5.3 2001 07 20 0.095 0.091 0.295 1.275 2.005 2.5 2001 07 21 0.006 0.071 0.301 1.291 2 4.4 2001 07 21 0.0005 0.008 0.316 1.321 2.017 146 2001 07 21 0.121 0.021 0.381 1.257 1.846 188.2 2001 07 21 0.226 0.04 0.435 1.133 1.673 208.5 2002 03 14 0.193 0.0298 0.587 1.489 2.23 0.6 2002 03 14 0.0218 0.00173 0.489 1.392 2.029 0.5 2002 04 26 0.59 0.165 0.738 0.809 1.089 0.2 2002 04 26 0.322 0.0588 0.671 0.97 1.965 0.2 2002 04 26 0.301 0.0834 0.699 1.02 2.055 0.2
Proceedings of the International Conference Nuclear Energy for New Europe, Kranjska Gora, Slovenia, Sept. 9-12, 2002 0702.5 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 (Table 2.), that probably means the dissolved gas contained fission produced xenon. Usually xenon occurs just in very low concentration (expressed in parts per billion, ppb) in the dissolved gas, but the isotope ratios, mostly the 134Xe/132Xe and the 136Xe/132Xe, shift to the direction of the fission yields. The 134Xe/132Xe ratio is 0.3883 in the air, contrarily, the measured values vary between 0.7 and 1.5 depending on the air and the fission origin. The presence of the fission xenon can be well demonstrated through the 136Xe/132Xe isotope ratio. The measured values of 136Xe/132Xe ratio change between 0.8 and 2.3 despite the value of 0.3298 in air. Analysing these data, we can conclude that the reactor in the block-3 contains inhermetic fuel rod(s), from which mobile fission products are released to the primary water. In the other reactors, there is not such an effect like this. How long is the damaged inhermetic fuel rod in the reactor? If we want to give an estimation of the period of the damaged inhermetic fuel rod spent in the reactor the three-isotope or the six-isotope calculation has to be done. The Table 3 shows the time results of the different calculations, expressed in months. It is conspicuous that the values are very diverse. In the ideal case, the values have to be between 0 and 36, because the fuel rod spends three years in the active zone of the reactor. A possible reason of the high deviation can be that more than one fuel rod might be damaged and the ages are different. In our opinion, this assumption is quite improbable. We suggest that there should be just one or coeval inhermetic fuel rods, but these assumptions cannot be confirmed yet. Table 3: The time (age) values calculated from the xenon isotope ratios expressed in months (values during the shut-down are bold-typed) Date of Three-isotope calculation Six-isotope sampling 132,134,136Xe 131,132,136Xe 129,132,136Xe 130,134,136Xe 134Xe 136Xe 2000 03 11 -45 -36 4.3 57 -9.5 4.3 2000 03 12 -46 -20 32.9 66 11 32.9 2000 05 19 -20 -19 21.8 62 8.8 21.8 2000 05 20 -18 -2 49.2 83 28.2 49.2 2000 06 18 -16 21 35.2 67 20.9 37.9 2000 07 28 6.1 10.7 -21 27.9 -11.4 -20.8 2000 07 29 20 7.8 -16 33.9 -2.2 -15.8 2000 07 29 26 3.4 -10 35.1 4.9 -9.4 2000 07 29 18 7.9 -13 10.1 -1.2 -12.5 2000 07 29 11.2 2.1 16.4 -16 14.5 16.3 2000 07 29 6.2 0.3 5 3.2 5.4 5 2001 03 15 -42 8.9 8.9 63 -5.5 8.8 2001 03 15 18.6 10.5 -4.2 32.3 4.4 -4.2 2001 04 27 31.5 24.8 25.4 35.8 27.9 25.4 2001 04 28 28.2 24.5 24.3 24.5 25.9 24.3 2001 06 01 32.7 13.4 12.9 37.2 21 12.9 2001 06 01 37.8 20.3 20.8 38.3 27.9 20.8 2001 07 20 38.1 47.7 29.8 40.5 33.3 29.9 2001 07 21 33.2 47.2 46 39.5 40.7 29.9 2001 07 21 27.4 45.2 45.2 32 38.3 45.2 2001 07 21 17.2 43.1 41.4 19.8 32.4 41.4 2001 07 21 26.7 41.1 40 32.2 34.8 40 2002 03 14 13.7 6.9 -17.4 -31.2 -6 -17.4 2002 03 14 7 24.4 40.6 -145 28.7 40.6 2002 04 26 27 -4.7 25.8 42 26.2 25.8 2002 04 26 89 -2.9 -23.6 43.1 45.2 -23.2 2002 04 26 87 -9.1 -29 43.2 40.5 -28
The negative values and those, which exceed 36 months, are invalid. The main reasons are the imprecise measurement in case of low concentration of xenon and the indefinite input parameters of the calculations. However, the values can provide some valuable information about the age of the inhermetic fuel rod. Figure 3 shows the couples of age and sampling date and the
Proceedings of the International Conference Nuclear Energy for New Europe, Kranjska Gora, Slovenia, Sept. 9-12, 2002 0702.6 lines fitted to the values in case of a few calculations. The imperfect fit of the lines has to be taken into account in the evaluation (R2 is varying between 0.002 and 0.47!). The sampling was made four times a year whenever a block was shut down. The block-3 usually is shut down in August (the last shut-down each year), and the first sampling of the block- 3 is scheduled the next Spring, about 8 months later. Therefore, the age of the inhermetic fuel rod may be 8, 20 or 32 months in March. In Figure 3 there are three lines with negative slopes (No.3,4,6),giving false results: according to these lines the fuel rods will get younger in time, which is obviously impossible. Even so, these fitted lines with negative slope give an average age during the three years: in Figure 3, the third and the sixth line provide an average age of 15 months, the fourth line 25 months, which are quite acceptable. The trend of the other lines is in good agreement with the expectations. The lines refer to an advanced age during the time in the reactor. In Figure 3, the first and second line reaches the zero value in September and in March 2000, respectively, but the failed rod might not be installed later than August 1999, because we have found fission produced xenon in the whole year in 2000.
100 50 1., with 132Xe, 134Xe and 136Xe 80 40 2 131 132 136 2 60 R =0.47 30 2., with Xe, Xe and Xe R =0.15 40 20 20 10 0 Months 0 Months -10 -20 -20 -40 -30 -60 1999.08.28 2000.03.15 2000.10.01 2001.04.19 2001.11.05 2002.05.24 2002.12.10 -40 1999.08.2 2000.03.1 2000.10.0 2001.04.1 2001.11.0 2002.05.2 2002.12.1 8 5 1 9 5 4 0
50 100 3., with 129Xe, 132Xe and 136Xe 40 2 50 2 30 R =0.002 R =0.15
20 0
10 -50 Months 4., with 130Xe, 134Xe and 136Xe Months 0 -100 -10
-20 -150 1999.08.2 2000.03.1 2000.10.0 2001.04.1 2001.11.0 2002.05.2 2002.12.1 -30 8 5 1 9 5 4 0 1999.08.28 2000.03.15 2000.10.01 2001.04.19 2001.11.05 2002.05.24 2002.12.10 50 50 5., Six-isotope calculatuion and with 134 Xe 40 2 40 2 R =0.34 30 R =0.003 30 20 20 10 10 Months Months 0 6., Six-isotope calculation 136 0 -10 and with Xe
-10 -20
-20 -30 1999.08.28 2000.03.15 2000.10.01 2001.04.19 2001.11.05 2002.05.24 2002.12.10 1999.08.2 2000.03.1 2000.10.0 2001.04.1 2001.11.0 2002.05.2 2002.12.1 8 5 1 9 5 4 0
Figure 3: The time values and the fitted lines in function of the sampling Furthermore, the slope of the first line of part 1 in Figure 3 is too high, hence the age reaches the 40 months during only a period of one year. The most correct result is given by the line observed in the fifth graph. This line provides an appropriate age of the damaged rod: 12- month age is estimated to October of 2000, the 24-month age to September of 2001 and the 36- month age to July of 2002. This line shows that the failed fuel rod spent three years in the reactor till August 2002. Taking into account the data and the calculations it can be seen that the damaged fuel rod was installed into the reactor in 1999. Presumably, it was removed from the reactor during the refuelling procedure in Summer 2002. In order to determine more accurately the age of the
Proceedings of the International Conference Nuclear Energy for New Europe, Kranjska Gora, Slovenia, Sept. 9-12, 2002 0702.7 damaged fuel rods special attention has to be turned to the improvement of xenon measurements, and reliable input parameters necessary for the calculations have to be determined.
ACKNOWLEDGEMENTS
We thank Ms. Magdolna Mogyorósi for her assistance in the sampling.
REFERENCES
[1] K. Balogh, I. Berecz, D. Bódizs, Method for Detection of Fuel Elements for Checking of Leakage by Mass Spectrometric measurements of Fission Noble Gases, Patent 191 119, 1984. (in Hungarian)
[2] C. Leuthrot, A. Brissaud, A. Harrer, IAEA Tecdoc-709. IAEA, Vienna, 1992, pp.67
[3] I.I. Lohtev, S.A. Bezdenezhnik, Estimation of defect fuel rods detection under elevate temperature, Technical report NZHK, No. 951-39-85, 1985.
[4] P. de Bièvre, Y. Aregbe, K. Mayer, S. Valkiers, Release of anthropogenic xenon to the atmosphere: a large-scale isotope dilution, International Journal of Mass Spectrometry and Ion Processes 154, 1996, pp.89-87.
[5] M. Ozima, F. Podosek, Noble Gas Geochemistry, Cambridge University Press, Cambridge, 1983, pp.15-40.
[6] L. Palcsu, M. Molnár, Zs. Szántó, I. Futó: Dissolved stable noble gas measurements from primary water of Paks NPP, Proceedings of International Conference on Nuclear Energy in Central Europe, Portorož, Slovenia, 10-13 September 2001.
Proceedings of the International Conference Nuclear Energy for New Europe, Kranjska Gora, Slovenia, Sept. 9-12, 2002