Pressure and Composition of Gas Mixtures in Fuel Rods for Pressurised Water Reactors by an Ultrasonic Sensor F.AL Gabr 1, JY. Ferrandis 1, D. Baron 2, P. Chantoin 3 1 Laboratoire d Analyse des Interfaces et de Nanophysique, LAIN, France 2 Electricité De France, France 3 Independent consultant, France 1. Introduction Around 400 power reactors are operated around the world and the fuel cost on a 30 years period is close to the reactor cost (around 1.5 billions) therefore the fuel discharge burnup increase is an essential economical challenge for utilities. Fuel pellets made of Uranium Dioxide (UO 2 ) or mixed oxides (UO 2 and PuO 2 ) are stacked into a zirconium alloy pipe called the fuel rod. There are between 45 000 to 51 000 rods in a reactor core, depending on the core size. They are presently discharged by 1/3 or 1/4 every year. Core management is foreseen for the near future on an 18-month period instead of 12. The clad is sealed under a Helium atmosphere at a pressure equal or superior to 2.5 MPa before the irradiation starts, which partly compensates the effect of the external coolant pressure (water at 15.5 MPa) and prevents the clad from collapsing. During the nuclear reaction in water reactors two phenomena are contributing to the internal pressure increase; on one hand the production of fission gases, mainly Xenon and Krypton, released in the free volume of the rod; on the other hand, the total free volume (initially about 18 cm 3 in the standard design) decreases down to, or less than, 10 cm 3 at the end of life, mainly due to the combination of fuel swelling and cladding creep down. (Figure 1). Moreover, the fuel pellets are submitted to various stresses during the nuclear reaction mainly due to the steep radial temperature gradient and as a consequence of the transmutation inducing local physical-chemical property changes. The global result is a fuel pellet expansion prompting a strain on the zirconium alloy cladding. When a strong contact is established, a chemical interaction layer of zirconium oxide (containing fission products) of several microns appears on the inside. Another thicker oxide layer is induced on the outer cladding surface due to the interaction with the cooling water. All of these phenomena put the pipe under stress. Thermal-mechanical codes are used to simulate all these phenomena and predict the fuel rod behaviour throughout its life. One of the limiting parameters for the burnup increase is the gas pressure in fuel rods, which Figure 1. Evolution of the free volume: a) rod internal volumes for MOX fuel; b) rod internal volumes for UO 2 fuel a) b) 278
Z 1 =ρ 1 v 1 Z 2 =ρ 2 v 2 Z 3 =ρ 3 v 3 Figure 2. Impedance matching for three layers e could lead to a gap reopening if this pressure increases considerably beyond the coolant pressure. This criterion was previously defined as follows: The rod internal pressure must be less than the nominal Reactor Coolant System pressure Now, it has been relaxed in this way: The instantaneous cladding creep-out rate shall not exceed the instantaneous fuel swelling rate (i.e. the fuel-to-clad gap does not open): this is the so-called "no lift-off" criterion Up to now the determination of fission gas pressure in fuel assemblies was determined, at most, on a few rods extracted from pre-selected assemblies to obtain one of the data point on the curve FGR = f (burnup). Several methods were developed to measure the pressure and avoid as much as possible the puncturing of the rods, i.e. measure of 85 Kr directly or after accumulation in the plenum by freezing with liquid nitrogen. Companies have been working without great success on a thermal method developed in Russia (problem of conductibility when oxide layer develops) and the one method currently used is the rod puncturing which is actually performed in hot cells after shipping from the reactor pool. The whole process is relatively expansive and therefore limits the number of measurements, which, in turn, is detrimental to the statistics. The consequence is that the curve of available data FGR = f (burnup) shows a substantial dispersion mainly due to the lack of data. The dispersion reflects also the dispersion of fabrication data, of irradiation data and stochastic errors. Apart from the statistics, a second problem has been the examination cost. Nowadays the nondestructive examination on one rod i.e. rod extraction from the assembly, transportation and nondestructive examinations (dimensional changes and FGR) costs around 76 000. However a good knowledge of the fission gas pressure in different operating conditions is essential for good fuel modelling which, in turn would reduce the safety margins and improve the fuel economics. To measure this pressure and the composition of the gas by a non destructive, noninvasive method we propose here a technology based on acoustics. The operability of the method has already been demonstrated [1,2,3]; however several improvements are proposed to make the method industrially operational within the three next years. Two major interacting improvements are under consideration: An improved acquisition and data processing method to clean the signal and then enhance the measurement accuracy and performance; The development of adapted sensors to this type of measurement (up to now the sensors were bought over the counter and not specifically developed for this measurement). In this paper are given: The state of the method development: We propose a method based on acoustics to determine, via the speed of ultrasound, the composition and the pressure of a gas mixture from the outside of the pipe and thus provide an answer to this non-destructive evaluation application. The experimental set-up used to achieve those measurements will be described and the principle of the specific sensor which was especially designed to transfer energy through the pipe wall (prime condition to perform time of flight measurements) will be recalled; Future plans; Conclusions. 2. State of the Method Development 2.1. Experimental Setup and Method Figure 3. Adaptation of the piezoelectric sensor frequency to the 570 µm wall thickness of the rods and to the water thickness layer Some authors have already described a technique using echography to measure ultrasonic velocity in gases in a vessel [4]. However, this method requires the introduction of sensors inside the pipe, which is not possible in the case of nuclear fuel rods. An experimental methodology needs then to be defined. The main problem, before measuring precisely the time of flight of an 279
acoustic wave in a medium, is to make the wave propagate in the medium. The gas is contained in a 1 cm Zircaloy pipe. The thickness of the rod is 570 µm. Small acoustic impedance of gas compared to solid one induces a strong insertion loss, which could prevent any propagation of ultrasonic waves inside pipes. Indeed, in the case of a Zircaloy/Helium-Xenon mixture interface, the reflection coefficient (defined as below) is almost equal to 1, Equation (1): ZZirc Zgaz R = (1) Z + Z Zirc gaz Z1 (water) Delay line Z3 (gas) Piezo Z2 (Zircaloy) Figure 4. Scheme of the sensor on the Zircaloy pipe Figure 5. A specific mechanical holder to fix the transducer to the rod where: Z i = ρ i.v i, Z i is the acoustic impedance of medium i, ρi the density and v i acoustic waves velocity; α T is the coefficient of transmission of the acoustic wave through a layer of thickness e, Equation (2): T 4Z1Z 3 α = (2) 2 2 2πe Z Z 2 2 2πe ( Z1+ Z3 ) cos + ( Z2 + ) sin λ λ 2 1 3 Z2 where: λ 2 is the wavelength of the acoustic waves propagating in the layer. When e = λ/2, the layer is transparent. When e = λ/4, then α T = 1 for Z 2 = (Z 1 Z 3 ) 1/2. The adequate choice of material allows higher impedance matching. For this reason, we propose to adapt the piezoelectric sensor frequency to the 570 µm wall thickness of these rods and to the water thickness layer (Figure 3) to reduce the different acoustic impedance mismatches to ensure sufficient wave propagation inside the gas. We chose a frequency of 4 MHz in order to let the ultrasound go through the pipe wall. Indeed the pipe wall being 570 µm thick is a half wave for this frequency. The Zircaloy layer is then transparent. Thus impedance matching concerns piezoelectric material and gas. A quarter wavelength water layer allows an increase in transmission: Z water is nearer than (Z gas.z Pz ) 1/2. This interferometric structure guarantees an optimal transmission of the ultrasonic energy inside the pipe free volume. With our device, we reduce the reflection coefficient at the Zircaloy/gas interface so that performing time of flight measurements is possible without the use of signal processing developments. We realised a half-cylindrical sensor for this purpose, which is described in Figure 4. The pipe is immersed in water. A specific mechanical holder has been designed to fix this transducer to the external surface rod (Figure 5). The relative position of the echoes is used to perform the time of flight measurements. Any complementary information that could be carried by the amplitude of the reflected echoes such as the reflection coefficient or the attenuation, both of which are dependent on the pressure, is not accurate enough. In an ideal case, the reflection coefficient is measurable and close to the models for normal incidence of the ultrasonic beams. But the system here is a pipe, which is resonant and there are many defects in the thickness of the layers, which lead to a significant imprecision, for instance, the lack of knowledge in the dimensions of tubing walls (570 µm ±5%) or the uncertainty on the echoes amplitude introduced by the material mechanical evolution under irradiation. 2 280
2.2. Experimental Results and Discussion In normal PWR operation, the range of pressure and temperature experienced by the Zircaloy fuel rod cladding is 60 to 155 MPa and T = 298-450 K, with an inner gas composition of Helium, Xenon and Krypton. Proportions range from 1 to 30% of Xenon and from 0.1 to 2% of Krypton. The maximum released is observed obviously after 4 or 5- year cycle. 2.2.1. Pressure Determination We are going to give the experimental results obtained for one gas (Nitrogen, N2) and for the Helium-Xenon gas mixture. Given in Figure 6 is an echogram representative of the type of signal we are dealing with. The Fourier transform of the signal amplitude makes analysis and then the screening of interferences possible; then the signal is treated with an inverse Fourier transform enabling the recovery of a signal with less interference and a better accuracy of the signal amplitude, which is used to determine the gas pressure. The amplitude of picks corresponding to echoes obtained on a time scale (Figure 6) or according to frequencies is a function of the gas pressure and of the nature of the gas. A sensor calibration with a piece of pipe full of a gas mixture at a given pressure and in a given proportion is sufficient to perform measurements on an unknown mixture at an unknown pressure. Figure 7 plots the signal amplitude (experimental) according to the pressure for two experiments using the same sensor but with two different mechanical settings. It is observed that according to the setting, the pressure red is 60 bars with the first setting and 70 bars with the second and represents a relative deviation of around 20%. This fairly important difference is due to the quality of the sensor and the generation of interferences in the cavity. It shows that much progress can be made by improving the sensor and the data acquisition treatment, which is precisely the subject we propose for future development. With the improvements foreseen an accuracy of the same order of magnitude on the pressure as that ob- Figure 6. Echogram for a pressure of 130 bars Figure 7. Signal amplitude (experimental) 281
tained with rod puncturing measurements is expected. Figure 6 represents the observation of 14 echoes in the tube filled with nitrogen at 130 bars. The times of flight are 45.6 µs, 88.8 µs and 132 µs. The method lies in the measurement of the times of flight between several return journeys (up to ten) of the acoustic wave inside the gas. From this time value one can get the velocity of the ultrasonic waves for one return journey from the relation (3): 2d ν = (3) t In this equation d is the inner diameter of the pipe (d = 8.36 mm ±28.5 µm) and t the time of flight for one return journey across the pipe. Therefore, the propagation speed of the ultrasonic waves in gas versus the pressure for nitrogen (less expensive for method calibration) can be represented in Figure 8. Using a thermodynamic model and virial expansion which is not detailed in this paper, a veloc- 395 Velocity (m/s) in Nitrogen Vs Pressure 390 385 380 375 370 365 360 355 Experimental values Model 350 345 0 20 40 60 80 100 120 140 160 Pressure (10 5 Pa) Figure 8. Theoretical and experimental velocities for Nitrogen, T = 300 K Figure 9. Ultrasounds velocity in the Helium-Xenon mixture at various pressures versus the proportion of Xenon at 4 MHz, T = 300 K 282
ity correlation versus pressure has been obtained and validated experimentally from 40 to 140 bars with our sensor for a fuel rod filled with nitrogen gas. For this pressure range, velocity deduced from time of flight measurement varies from 360 to 390 m/s. 2.2.2. Composition Determination The range of internal pressure and temperature for the Zircaloy tubing which is used is 60-100 MPa and T = 298-450 K, with a proportion of Xenon up to 5%. In these conditions the sound velocity variations versus the composition of the mixture are obtained for different pressures. The composition can be deduced from velocity measurements with Equations (4) and (5): R T ν = γ (4) M = x M + x M (5) He He Xe Xe where: γ ratio of the specific heat of the gas at constant pressure c p to the specific heat at constant volume c v ; R perfect gas constant; T temperature; M molecular weight of the gas and x i volume fraction of the gas i. A virial expansion enables pressure at second order to be taken into account. Figure 9 shows that in the case of fuel rods initially filled with pure Helium gas, 10% volume fraction of Xenon decreases velocity from 1050 to 520 m/s whereas pressure variation from 50 to 100 bars induces only a 20 m/s increase which means that the ultrasound velocity is very little modified by the gas pressure but very sensitive to gas composition. These predictions have been found to agree with measurement on Helium-Argon gas mixtures. In this case Argon has been used only for cost considerations and does not change the principle of the method validation. For a pressure between 50 bars and 100 bars and for a Xenon composition between 0 and 10% (strait part of the curve on), determining the ultrasound velocity obtained with our method gives the composition of the gas mixture. If an uncertainty of 4-5% is made on the velocity determination, the composition of the gas can be ascertained to a 99% of accuracy when the pressure is known. If the pressure is not known the uncertainty on the Xe proportion would slightly vary according to the pressure. The error made on the composition if a pressure of 150 bars instead of 50 bars is considered (extreme case), would be 10%. Figure 9 shows also that beyond 10% Xe the curve is becoming flatter (Xe is becoming dominant) which would introduce an additional error, however this uncertainty would be more than compensated by the pressure increase and by the higher proportion of xenon in the mixture. It is interesting to note that if the composition is known through the experimental knowledge acquired during the past 35 years, an accurate enough velocity measurement could lead to the pressure. As for one gas, the speed of ultrasound measurements will directly lead to the determination of the pressure via specific thermodynamic models, which are not detailed here. The krypton has not been taken into account since it represents only 1.1% of the total weight of the fission products (in comparison, Xenon represents 12.7%). Rapid evaluation showed it would not affect the measured ultrasound velocity. The determination of the pressure and composition would have to be made by successive approach in order to converge toward the true value: 1. Calculate the composition with an estimated pressure value; 2. Calculate the pressure with the composition found in 1; 3. Calculate again the composition with the pressure found in 2; 4. etc Such an approach would lead with the method as it is to an evaluation better than 10% of the pressure and of the same order of uncertainty than the rod puncturing method after the improvement foreseen in our programme for future development are carried out. 3. Future Plan As seen before, the principle and the modelling of these measurements are well under control, however the accuracy and the repeatability of the pressure measurement can be improved. A new programme is proposed here following the observation that an appropriate data acquisition and processing method was able to considerably improve the amplitude and time of flight measurements and that many interferences observed were due to the quality of the sensors used up to now. The detailed analysis of the signal shows that it is necessary to develop three major aspects: The piezo-electric ceramic; The generator/receiver which should be able to provide the adequate electric signal; Acquisition and data processing. To accomplish this task a three-year work plan has been established based on the set-up of an international group of committed and interested organisations belonging to research safety, industry utilities and international organisations. Application to WWER Fuel Rods The WWER fuel rods design does not include a 283
spring and the fuel stack is blocked with a spring circlip [5]. This configuration should permit the pressure measurement without modification of the rod design. In this case our method would be directly operational. With this method the oxide layer formed on the outside of the cladding at the plenum level is not a problem as the wave velocity in the oxide is close to the velocity in the zirconium alloy. 4. Conclusions Rod internal pressure is among the safety criteria applied in nuclear power fuel safety analysis, which is a life-limiting factor for nuclear fuel. A good knowledge of its evolution in steady state, abnormal or accident conditions is therefore essential for good fuel modelling which, in turn would reduce the safety margins and improve the fuel economics. Up to now the determination of fission gas pressure in fuel assemblies was determined, mostly, on a few rods per assembly to reveal one of the data point on the curve FGR = f (burnup). To avoid as much as possible the rod piercing, several methods were developed but without great success. The method currently used today is the rod piercing which does not allow for good measurement statistics and has been found to be quite expensive (around 76 000 per rod) and the curve FGR = f (burnup) shows an important dispersion mainly due to the lack of data. The dispersion reflects the dispersion of fabrication data, of irradiation data and stochastic errors. To measure this pressure and the composition of the gas by a non destructive, non-invasive method we propose here a technology based on acoustics. It has been demonstrated that the measurements are accurate enough to reach, via the ultrasound velocity, the pressure and the composition of a gas mixture. However several improvements are proposed to make the method industrially operational within the three next years. This new work is proposed following the observation that an appropriate data acquisition and processing method was able to considerably improve the amplitude and time of flight measurements and that much interference was due to the quality of the piezo-electric and the generator/receiver system. An accuracy of the same order as that experienced with rod puncturing is expected with the optimized method. The system could be used nearly as it is on already irradiated WWER fuel. Two major interacting improvements are under consideration: An improved acquisition and data processing method to clean the signal and then enhance the measurement accuracy; The development of adapted sensors to this type of measurement (up to now the sensors were bought over the counter and not specifically developed for this measurement). To support this work we propose a 3-year work plan based on postgraduate and PhD work to reach the industrial level. Finally it is proposed to carry out the development inside an international structure of interested and committed organizations including Research, Safety, Industry, Utilities and possibly international organizations. 5. References [1] Thèse de spécialité: J.P. Marco. Etude et réalisation de capteurs micro-acoustiques spécifiques pour la mesure de pression à travers une paroi Université de Montpellier II juillet 1996. [2] Thèse de spécialité: M.F. Narbay. Mesure de pression et de composition des gaz de fission dans les crayons combustibles des centrales à réacteurs à eau pressurisé par méthode acoustique Université de Montpellier II Janvier 2000. [3] Determination of the Composition of a Gas Mixture in a Nuclear Fuel Rod by an Acoustic Method. INSIGHT, 42, 603-605, 2000. [4] Compressibility and Sound Velocity Measurements on N 2 up to 1 GPa. P. Korbeek, N. Trappeniers, S.N. Biswas, International Journal of Thermophysics, 9(1), 103-116, 1988. [5] Design and Performance of WWER Fuel. Technical Report Series, 379, International Atomic Energy Agency, Vienna, 22, 1996. 284