The 7th International Symposium on Supercritical Water-Cooled Reactors ISSCWR-7 15-18 March 2015, Helsinki, Finland ISSCWR7-2036 Irradiation capabilities at the Halden reactor and testing possibilities under supercritical water conditions Rudi Van Nieuwenhove Institute for Energy Technology (IFE), Halden Reactor Project P.B. 173, NO-1751, Halden, Norway Email: rudivn@hrp.no Abstract Different types of instruments have been developed both for in-pile fuel and materials studies at the Halden Reactor Project. Over the last years, several of the standard instruments have been upgraded to be able to tolerate much higher temperatures. In particular, several instruments are now able to operate up to 650 ºC and 250 bar, thus in the range suitable for supercritical water (SCW) studies. In addition, a feasibility study for an in-pile supercritical water loop has been carried out which shows that such a loop can be realized in the Halden reactor, allowing for all the instrumentation possibilities which are presently carried out in PWR and BWR conditions. Another, and cheaper alternative, is to perform corrosion experiments inside a small capsule in which supercritical water is maintained by means of gamma heating and external pressure lines. The conceptual designs of the SCW loop and SCW capsule will be highlighted. Keywords: Supercritical water, corrosion, fuel, instrumentation 1. Introduction The Halden Boiling Water Reactor (HBWR) started operation in 1958, being originally built in order to demonstrate the usefulness of nuclear power as an energy source for the process industry. The Halden Project is a joint undertaking of national organizations in 18 countries sponsoring a jointly financed program under the auspices of the OECD - Nuclear Energy Agency. An important part of this project is related to the study of nuclear fuel and the behavior of materials under nuclear radiation. Over the past 15 years, increasing emphasis has been placed on testing fuel and materials in the HBWR under thermal-hydraulic and water chemistry conditions representative of commercial nuclear power plants. These conditions are achieved by housing test rigs in pressure flasks connected to dedicated water loops. Eleven water loops are currently in operation in which boiling water reactor (BWR), pressurized water reactor (PWR) and CANDU reactor conditions are being simulated. Different types of instruments have been developed both for fuel and materials studies [1]. Many of these were already developed in the period 1966-1974, but have been continuously improved and refined since then. For materials studies, the following types of on-line in-pile measurements are presently possible; crack growth measurements, creep, stress relaxation and crack initiation. In addition, in-core reference electrodes have been developed for measuring the electrochemical corrosion potential (ECP). For fuel studies, various in-pile measurements are possible (see Table 1). 1
Table 1: Possible measurements on fuel rods Quantity Fuel center temperature Fuel elongation Cladding elongation Fuel rod internal pressure Fuel rod diameter Related to the following properties Fuel thermal conductivity, fuel cladding gap conductance Fuel densification and swelling Fuel pellet cladding interaction, oxide and crud deposition on cladding Fission gas release, fuel stack densification and swelling Cladding creep, pellet cladding interaction, fuel swelling The heart of many of our in-core instruments is the Linear Variable Displacement Transducer (LVDT). The LVDT (see Figure 1) is a versatile instrument used to transform a mechanical movement into an electrical signal. The primary coil is activated by a 400 Hz constant-current generator and the position of the magnetic core in relation to the coils affects the balance of the signal from the secondary coils. Thus any mechanical movement changes the position of the magnetic core, and the corresponding signal can be measured. The LVDTs were originally designed to operate under PWR conditions (350 C and 150 bar). b c d a Figure 1: Principle of the LVDT (a: primary coil, b: secondary coil, c: ferritic core, d: signal cables) The figure above shows the principle design of our LVDT. The LVDTs are made in many different sizes at customers request, but the most used size is the type 5 LVDT. This LVDT has a linear range of ± 2.5 mm, hence the type 5 designation referring to the linear range of the LVDT. The size of this LVDT is Ø 11.5 mm and a length of 55 mm. The signal cables used are 2-wire mineral insulated (Al 2 O 3 ) cables with Inconel 600 sheath. The outer diameter of the cables is 1.0 mm. Since the Halden Reactor Project started making in-core measurements, more than 2200 Linear Voltage Differential Transformers of different types have been installed in test rigs in the Halden Boiling Water Reactor. A failure rate of less than 10% after 5 year operation is expected for LVDTs. 2. Extension of instrumentation to SCW When designing a SCW reactor (SCWR), there is a need for studying physical properties of SCW (such as heat transfer), materials behavior as well as fuel behavior [2]. In fact, all the types of measurements which are presently performed on fuels and materials for Gen II and Gen III reactors will also be needed for Gen IV type of reactors, such as the SCWR. Therefore, there is a need to extend the present instrumentation capabilities to higher temperatures and more aggressive chemical conditions. Since the LVDT is the basic instrument for most of these 2
measurements, a high temperature version has been developed, capable of operation up to 700 C [3]. This was accomplished by using a different type of wire for the coils and a different way for connecting these wires to the signal cables. Stable operation for 2000 hours has been achieved at a temperature of 550 C. In addition, such LVDTs were also tested in supercritical water at 650 C and 250 bar. Such LVDTs were also tested in the Halden reactor, albeit at a lower temperature (300 C) for a period of almost three years. This means that all the measurements which were described in the previous paragraph are now also possible in SCW conditions (and in the core of a reactor). The crack growth measurement on Compact Tension (CT) specimens by means of the potential drop technique can already be applied in SCW, without major modifications. The set-up is shown in Figure 2. The CT is loaded by means of a miniature metallic bellows, coupled to an external pressure line and coaxial signal capable (radiation resistant) are coupled to extension arms to apply the DC current and measure the potential drops. Figure 2: Compact tension specimen for crack growth measurements. Reference electrodes, for measuring the ECP, have also been developed at the Halden Reactor Project [1,3]. A picture of such an electrode is shown in Figure 3. 3
Figure 3: Picture of a reference electrode for use in SCW. This electrode consists of a ceramic tube made of Yttrium Stabilized Zirconium oxide (YSZ), and filled with a mixture of an iron and iron oxide powder which surrounds a Fe conductor. The zirconium oxide body becomes an ionic conductor of oxygen ions at high temperature while being impermeable to other gases or water. These oxygen ions take part in the electrochemical reactions at the ceramic/water and at the ceramic/ iron oxide interface and thereby determine the potential of the inner Fe conductor. By measuring the potential difference between a working electrode (for instance a stainless steel sample) and this iron/iron oxide electrode, it thus becomes possible to measure its corrosion potential. Such an electrode has been successfully used in a SCW loop at JRC, in the Netherlands [4]. In case sufficient hydrogen is dissolved into the SCW, it is also possible to use a so-called Platinum electrode. This type of electrode can be used as reference electrode under so-called reducing conditions, which is normally defined as being when the molar ratio of hydrogen to oxygen is greater than 2. In this case, the platinum functions as a hydrogen electrode. The standard Halden Pt electrode (see Figure 4) is based on a mechanical seal between the ceramic (Mg stabilized Zirconium oxide or Mg PSZ) tube and the metal parts and is therefore relatively bulky. The outer diameter of the Pt tip is 13 mm. The diameter of the signal cable is 1 mm. Figure 4: Picture of the Pt-reference electrode Such an electrode has also been tested successfully at a SCW loop at VTT, Finland. Since the instruments will also be subject to corrosion, it is envisaged to apply a CrN coating (applied by PVD) to the most vulnerable parts. It has been shown [6] that such a coating provides perfect corrosion protection in supercritical water. It has also been tested on samples and fuel rod claddings in the Halden reactor in BWR and PWR conditions, showing excellent radiation and chemical resistance [5]. 3. Supercritical water loop possibility A feasibility study has shown that it is possible to install an instrumented supercritical water loop into the Halden reactor for materials and fuel studies. The external loop system will be similar to a PWR-loop system and with possibility for hydrogen addition. For materials studies, a flow rate of 0.1 kg/s is sufficient, while for fuel irradiations a flow rate of 0.35 kg/s is required. The 4
useable inner diameter of the in-pile section will be 35 mm (Fuel Flask Assembly) or 43 mm (Instrumented Loop System). The pressure flask will be made out of Inconel 718. Another tube surrounds the pressure flask in order to provide a thermally insulating gas gap (2-4 mm Ar or Xe). The heat exchanger represents the most challenging part of the whole SCW loop. The transition between subcritical and supercritical water occurs within the heat exchanger. The heat exchanger and the required electrical heaters and coolers will be located outside of the rig. A schematic drawing of the SCW loop system is shown in Figure 5. Figure 5: Schematic drawing of the SCW loop in the Halden reactor The heat exchanger considered is of the counter-flow type [7], consisting of a bundle of many parallel thin pipes, where the flow inside these thin pipes is opposite to the flow outside these pipes. For the case of a mass flow of 0.1 kg/s (materials testing loop), calculations show that one would need about 60 parallel tubes (outer diameter 3 mm, inner diameter 2 mm) with length of about 8.4 meter, while for the high mass flow rate of 0.35 kg/s, one would need about 90 tubes (outer diameter 4 mm, inner diameter 3 mm) with a length of about 8.4 meter. In these designs, the total pressure drop over the heat exchanger can be kept below 1 bar. Such a SCW loop would allow making all the in-pile measurements on fuels and materials which are presently carried out at the Halden reactor. Due to lack of financing, such a loop could however not be realized. 4. Supercritical water capsule A cheaper alternative to an SCW-loop is a capsule which can be loaded into an existing Pressurized Water Reactor (PWR) rig (in the Halden reactor), connected to a PWR loop. Inside the capsule, the temperature required to obtain SCW is provided by gamma heating of the material sample within. Thermal insulation is provided by means of a thin xenon gas gap. The advantage of locating such a capsule inside a PWR loop is two-fold: 1) One starts with a higher outer temperature (320 C) such that the required extra temperature increase can easily be achieved, and 2) The PWR pressure flask provides an extra barrier in case something goes wrong with the SCW capsule. The capsule can be equipped with thermocouples and external hydraulic pressure tubes (outer diameter 1 mm) allow continuous (but slow) refreshment of the water inside. For this purpose, it is envisaged to use a HPLC pump which can refresh the inner content several times a minute. 5
In addition, the pressure lines allow to regulate the pressure inside the capsule and to avoid the accumulation of hydrogen within the capsule. Dimensions of a possible design are shown in Table 2 and a drawing of the proposed capsule is shown in Figure 6. The assumed gamma heating is 0.8 W/g (typical for the core of the Halden reactor) and for simplicity, the material of the sample has been chosen to be Inconel 600. The sample should be placed near the middle (axially) of the reactor core region in order to minimize the neutron heat flux gradient. A separate capsule can be envisaged to pre-heat the water before it enters the main capsule. Table 2: Characteristics of a SCW capsule Item Outer (thermal insulation) tube Dimensions (mm) 19/17 (outer/inner diameter) Width of the xenon gas gap 0.5 Inner (pressure) tube 16/12 (outer/inner diameter) Sample diameter 6 Sample length 80 Total length of capsule 140 Figure 6: Drawing (cut-open view) of the proposed capsule for materials testing in supercritical water in the core of the Halden reactor. The temperature distribution within the capsule (finite element method) is shown in Figure 7. The temperature along the surface of the cylindrical sample is shown in Figure 8. 6
Figure 7: Temperature distribution within the SCW capsule. Figure 8: Temperature distribution along the surface of the cylindrical sample. Instead of one sample, one can of course also consider different samples (in different materials), stacked together. To remove the samples (for surface analysis and weight measurements), the capsule needs to be cut open in a hot cell. 7
5. Conclusions Various instruments have been developed at the Halden Reactor Project for the on-line and inpile study of fuels and materials under SCW conditions. It has been shown that it is technically feasible to realize an in-pile SCW-loop in the Halden reactor. For limited in-pile corrosion studies on materials in SCW, it has been shown that one could achieve this by means of a small capsule located within an existing PWR loop. References 1. R. Van Nieuwenhove, S. Solstad, IEEE Transactions on Nuclear Science, Vol. 57, Issue 5, (2010), 2683-2688. 2. T. Schulenberg, H. Matsui, L. Leung, A. Sedov, Super-Critical Water-cooled Reactors (SCWRs), GIF-INPRO Meeting, Vienna, Feb. 28 to March 1, 2013. 3. R. Van Nieuwenhove, Proceedings of a IAEA Technical Meeting held in Halden, Norway, 21-24 August 2012, In-pile Testing and Instrumentation for Development of Generation-IV Fuels and Materials, Session 1 (Instrumentation Development), Development and testing of instruments for Generation-IV materials research at the Halden reactor project, IAEA TECDOC-CD-1726, http://www-pub.iaea.org/mtcd/publications/pdf/te-cd- 1726/PDF/IAEA-TECDOC-CD-1726.pdf 4. K. Turba, R. Novotny, K.-F. Nilsson, P. Hähner, Progress in the qualification of candidate materials for Generation IV nuclear systems at the European Commission Joint Research Centre (JRC-IET), Nordic Forum for Generation IV Reactors, Status and activities in 2012, Nordic Nuclear Safety Research (NKS), 2012, (NKS-270). 5. R. Van Nieuwenhove, IFA-774: the first in-pile test with coated fuel rods, Enlarged Halden Program Group Meeting, Røros, HWR-1106, 7-12 September, 2014 6. R. Van Nieuwenhove, J. Balak, A. Toivonen, S. Pentiilä, U. Ehrnsten, Investigation of coatings, applied by PVD, for the corrosion protection of materials in supercritical water, The 6 th International Symposium on Supercritical Water-Cooled Reactors, ISSCWR-6, March 03-07, 2013, Shenzhen, Guangdon, China. 7. P. Vierstraete, R. Van Nieuwenhove, B. Lauritzen, NOMAGE4 activities 2011, Part II, Supercritical water loop, Nordic Nuclear Safety Research (NKS), 2012, (NKS-255). 8