Innovative in-pile instrumentation developments for irradiation experiments in MTRs

In-pile measurement is a key parameter for current and future neutron irradiation experiments in Material Testing Reactors. An ambitious program is managed by French CEA (Commissariat à l’Energie Atomique) with the aim of developing innovative in-pile instrumentation for MTRs irradiation experiments. The scope of these studies includes: radiation measurements, for instance neutron flux and gamma heating; these parameters are crucial to improve MTR irradiation monitoring. measurements of physical parameters inside the irradiation rigs, for example temperature, sample dimensions, fission gas release, etc. An overview of this program is presented, illustrated by details about some of the recent developments, such as: sub-miniature fission chambers for on-line fast neutron flux measurements, high-temperature thermocouples for long-term in-pile experiments, acoustic measurements of fission gas release in PWR fuel rods. Introduction: current in-pile instrumentation For more than 50 years, neutron irradiation experiments have been using in-pile instrumentation to monitor and measure in-situ major physical parameters. The most typical in-pile instrumentation currently implemented in MTRs around the World is summarized in table 1. Measurements Examples of usual in-pile instrumentation Fuel or material samples temperature Thermocouples with metal sheath and mineral insulation : Below 1100°C : Ni-Cr / Ni-Al (K type) Ni-Cr-Si / Ni-Si (N type) Above 1100°C : W-Re alloys (C type) Fuel rod or material sample dimensions Elongation sensors and diameter gauges : LVDT based sensors Strain gauges based sensors Hyper frequency resonant cavities Fission gas release in fuel rods Fuel rod internal pressure sensors [1] + fuel temperature measurements (centreline thermocouple) Fission products laboratory analysis Neutron flux Self Powered Neutron Detectors Fission Chambers Activation foil dosimeters (post-irradiation analysis) Gamma heating Calorimeters Gamma thermometers Table 1 : Most typical in-pile instrumentation currently implemented in MTRs This instrumentation was mainly developed more than 20 years ago, and the majority of research and development programs about in-pile instrumentation had been clearly reduced since the beginning of the Eighties. Nevertheless some recent issues highlighted that the need for very fine and various in-pile measurements is becoming more and more acute. These new requirements for high-quality instrumentation are due on one hand to constant improvement in modeling and simulation, and on the other hand to the specific needs of emerging irradiation programs, relating for example to fuel or material characterization for HTR, VHTR, Gen IV systems, fusion, etc. As a consequence, innovative measurements are a strong necessity for upcoming in-pile experiments. Overview of recent CEA’s R&D studies for in-pile instrumentation For some years, the CEA (Commissariat à l’Energie Atomique) is managing an ambitious program called “INSNU” to develop and qualify innovative in-pile instrumentation for MTRs irradiation experiments. The global chart of this project is given figure 1. It summarizes all the technical items which are in the scope of these studies. This document has been established by joining all current and prospective needs pointed out not only by research reactors operators but also by CEA customers for irradiation experiments. Figure 1: global chart of in-pile instrumentation needs for future irradiation experiments The capability to operate this variety of in-pile measurements is a crucial challenge for the quality of upcoming neutron irradiation experiments in current and future MTRs. To improve the capabilities of this project, CEA and SCK·CEN (Belgian Nuclear Research Centre) intend to team up and join efforts for research on advanced MTR instrumentation systems. A joint laboratory dedicated to in-pile instrumentation will carry on some of the future developments, particularly in the following domains: neutron flux measurements, gamma flux measurements, samples dimension measurements, fission gas release measurements. As a matter of fact, each instrumentation developed in the INSNU project is mainly intended for existing Material Testing Reactors located in CEA or SCK·CEN Research Centers, such as OSIRIS in Saclay (France) or BR2 in Mol (Belgium), but they are also designed to be implemented in projected neutron irradiation experiments of the future Jules Horowitz Reactor, which is planned to be in operation in Cadarache (France) by 2014. Figure 2: external and internal views of CEA’s OSIRIS reactor (70MW MTR located in SACLAY, near PARIS, France) An overview of several on-going developments carried out by CEA for in-pile instrumentation is given below. Sub-miniature fission chambers for neutron flux measurements For decades, in-pile neutron flux measurements have been operated using Self Powered Neutron Detectors, which signals are generally correlated with post-irradiation analysis of activation foil dosimeters. An important improvement has been recently achieved in this domain, with the development of CEA’s sub-miniature fission chambers for in-pile measurements of high thermal neutron fluxes (up to 4×10 18 n/m2.s). These 1.5mm external diameter sensors, containing a 235 U fissile deposit, were qualified in the SCK•CEN reactor BR2 in Mol (Belgium) between 2001 and 2004. This product has now been industrialized and is manufactured by PHOTONIS Company under the name “CFUZ53”. More details about this program are given in [2]. Figure 3: 1.5 mm sub-miniature fission chamber In addition, a new model of sub-miniature fission chambers dedicated to the measurement of fast neutron fluxes (E > 1MeV) has been studied and developed by CEA. As there is at this time no other sensor available to give online measurement of the fast neutron flux, this innovative fission chamber, based on a 242 Pu fissile deposit, is very promising for all irradiation programs related to in-pile material radiation damage investigations. Advanced prototypes of these “fast neutron flux fission chambers” have been manufactured and optimized. They are about to be irradiated for in-pile qualification campaigns in the OSIRIS and BR2 reactors during 2005 and 2006. High temperature thermocouples for long-term irradiations The necessity to carry on reliable and long-term in-pile high-temperature measurements has became particularly relevant for scientific programs relating on one hand to fission reactors studies (HTR, VHTR, Gen IV systems) and on the other hand to characterization of materials for nuclear fusion (ITER). The expected samples temperature for these long-term experiments ranges from 1100 to 1600°C. Such measurements however encounter a strong limitation due to existing high-temperature thermocouples inability to withstand long term irradiations. Indeed in-pile high-temperature measurements (above 1100°C) are usually carried out using C type or S type thermocouples, respectively made of Tungsten-Rhenium and Platinum-Rhodium alloys. But when used under irradiation, these thermocouples are altered by an important drift of their electrical signal mainly due to the transmutation of some of their elements. For example, submitted to significant thermal neutron flux, Rhenium and Tungsten are rapidly transformed into Osmium. These transmutations change the initial chemical composition of these couples, causing a permanent alteration of their thermoelectric response. This important drift requires proper compensation for future high-temperature irradiation programs. Facing this problem, different scientific teams in the 1980’s have studied some improvements of high-temperature thermocouples under long-term irradiation [3] [4], but these developments had not been completed. In this scope, and following the results of previous studies, a new type of thermocouple for high-temperatures in-pile measurements has been developed and industrialized by CEA, in collaboration with THERMOCOAX Company [5]. This sensor is made of thermoelectric elements based on Molybdenum and Niobium, shielded by mineral insulator and appropriate metallic sheath. Both Mo and Nb have very low neutron absorption cross sections. As a consequence, this thermocouple -theoretically hardly affected by transmutationsshould not therefore exhibit significant drift due to irradiation, even for enduring experiments. The first phase of this development concerned the high-temperature compatibilities of different materials usable for sheath, insulators and wires, including tests of different diameters wires. The tests were managed in a high temperature furnace, in the range from 1000 to 1600°C, in pure helium atmosphere and in contact with graphite. The most significant materials tested are listed in Table 2. The tests show strong reactivity between the majority of these materials at 1600°C; the most suitable associations were Mo and Nb wires with HfO2 insulation and Nb or Ta sheath. Figure 4: detailed and general views of the 2000°C THERMOCOAX furnace used for hightemperature studies Figure 5: sheath materials samples Wires Mo Nb Insulators Al2O3 HfO2 Sheath Ta Mo-Re 50% alloy Ti Mo Re Nb Table 2: materials tested for high-temperature compatibilities The second phase of the development was the definition of the thermoelectric response of the new Mo-Nb couple. This was established by using a very accurate calibration process in the temperature range from ambient to 1600°C. The result is shown in Figure 6. The curve obtained during this calibration is very consistent with previous results, for example from CEA and INL. The electromotive force of the Mo-Nb couple in the intended temperature range is about 14 μV/°C, which is of the same order as the e.m.f. evolution of standard high-temperature thermocouples, such as C or S types. Interpolation polynomials have been calculated for low and high temperatures. Figure 6: Thermoelectric response of the Mo-Nb couple (last results in red),