- Email: [email protected]
Performance test of radiation detectors developed for ITER-TBM
Fusion Engineering and Design xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Performance test of radiation detectors developed for ITER-TBM ⁎
M. Angelonea, , R. Pilottib, F. Stacchic, M. Pillona, A. Klixd, P. Rajd, S. Loretia, G. Paganoa a
ENEA, Dipartimento Fusione e Tecnologie per la Sicurezza Nucleare, C.R. Frascati via E. Fermi, 45, 00044 Frascati, Italy Università degli Studi “Tor Vergata”, Dipartimento Ingegneria Industriale, via del Politecnico 1, 00100 Rome, Italy1 Università degli Studi “La Sapienza”, Dipartimento Ingegneria Energetica, 00100 Rome, Italy1 d Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany b c
A R T I C LE I N FO
A B S T R A C T
Keywords: ITER-TBM Self-power detectors Diamond detectors Tritium breeding Nuclear detectors Neutron flux
The validation of calculation tools used for the nuclear design and analysis of future fusion machines requires the availability of nuclear instrumentation able to measure the nuclear quantities of interest in the harsh environments typical of tokamaks (e.g. ITER) characterized by high radiation level and high temperature. This instrumentation needs to be developed and properly tested under reactor-relevant working conditions. In the EU the activities to develop advanced nuclear detectors for the ITER-TBM are coordinated and supported by F4E and carried out under a collaborative effort between ENEA and KIT performed under the European Consortium on “Nuclear Data and Experimental Techniques”. In this paper the activities carried out at ENEA Frascati to develop nuclear sensors (diamond and self-powered detectors) able to operate “in-core” under the harsh working conditions of the ITER-TBM are discussed. Furthermore, the performance of diamond detectors operated at T > 300 °C, as well as the performance of a selfpower neutron detector (SPND) made with Cr emitter are presented. The open issues and the technological challenges to be faced to further develop the detectors are also addressed.
1. Introduction
accessibility is very limited and this requires the use of miniaturized and reliable detectors. Presently there are not nuclear detectors suited for use in TBM and must be studied, developed and properly tested under reactor-relevant working conditions. This calls for a dedicated effort. To this end, since many years in the EU a technological program is ongoing under F4E coordination and partial financial support. The activities are carried out under a collaborative effort between ENEA and KIT coordinated under the European Consortium on “Nuclear Data and Experimental Techniques” lead by ENEA [3,4]. In this paper the activities so far carried out at ENEA Frascati for developing and testing artificial diamond neutron detectors (SCD) and self-powered neutron detectors (SPND) specifically designed to operate “in-core” under the harsh working conditions of the two EU ITER-TBMs, are reported. The open issues, the technological challenges to be faced, the limits and the perspectives of the developed detectors are also addressed.
To validate the performance of the test blanket module (TBM) of ITER under representative conditions TBMs based upon different concepts will be located in selected equatorial ports of ITER. Two European TBMs, the Helium cooled liquid lead (HCLL) [1] and the Helium cooled pebble bed (HCPB) [2], will be located in ITER for studying their performances. The tritium production rate (TPR) will be the main parameter to be investigated. However, to complete and assess the knowledge of a complex nuclear system such as the TBM other nuclear parameters are of interest and need to be measured, the goal being the validation of the calculation tools used for design. To this end a direct comparison with selected experimental quantities is mandatory. The development of detectors able to measure neutrons while operated in harsh environments is thus a must for fusion tokamaks. From the fusion neutronics point of view, nuclear measurement experiments in TBMs will be of great interest. In different phases of ITER operation, responses like tritium production, nuclear heating, material activation etc. will be measured at different locations. Unfortunately, the environment inside the TBM is characterized by high neutron and gamma fluxes and high temperature. Furthermore, the ⁎
1
2. Functional requirements and specifications of candidate sensors for TBM Despite the many intrinsic differences in the two EU TBM proposals
Corresponding author. E-mail address: [email protected] (M. Angelone). ENEA Guest.
https://doi.org/10.1016/j.fusengdes.2018.05.018 Received 18 September 2017; Received in revised form 16 February 2018; Accepted 3 May 2018 0920-3796/ © 2018 Published by Elsevier B.V.
Please cite this article as: Angelone, M., Fusion Engineering and Design (2018), https://doi.org/10.1016/j.fusengdes.2018.05.018
Fusion Engineering and Design xxx (xxxx) xxx–xxx
M. Angelone et al.
proposed detectors can work either in pulse or current mode, however due to the intense neutron flux the current mode will be most suitable if not the preferential one. For measuring or subtracting and/or suppressing the noise and/or the background the most used method is to locate “back-to-back” two detectors one of the two being incomplete (dummy detector) and to subtract the signal of the dummy detector from that of the true one; d) Radiation Hardness: it will characterize the working life of the detector and the degree of accuracy for the measured quantities, this is a point of concern; e) Burn-up: a well-known problem for fission chambers operated in high neutron flux reactors. When the number of reacting atoms is small compared to the numbers of atoms involved in the reaction rate, the continuous reduction of the material alters the response of the detector which must be corrected since the burn-up affects the accuracy of the measurements; f) Neutron-Gamma discrimination: this is an essential issue for any detector located in a mixed neutron-gamma field. This is also common to passive detectors whose response can be affected by gammas; g) Capability to withstand EM fields and transients; h) Safety & Integration: All the proposed detectors and detection systems must ensure integration with the environment and the safety levels according to the prescription for the other part and components of the TBM. It is thus necessary to prepare a dedicated R&D program where these requirements & specifications will represent the input for the design of nuclear detectors. Furthermore, a step–by-step approach seems the more appropriate so to have a selection procedure which is based on feed-back from the lesson learned at each step of the developing program.
Table 1 Summary of the nuclear working conditions in HCLL & HCPB TBM for DT phase (data refer to ITER at 500 MW fusion power [5]). HCLL Parameter
Max. Value
Neutron flux (n cm−2 s−1) Gamma Flux (g cm−2s−1) Temp. (°C) Nuclear Heating (Wcm-3) Tritium Prod. Rate (T cm−3s−1)
HCPB Max. Value
2.0 × 1014
Max. Uncertainty ± 10%
2.0 × 1014
Max. Uncertainty ± 10%
4.5 × 1013
± 10%
4.5 × 1013
± 10%
500 5.0
± 1.0% ± 10%
650 8.0
± 1.0 ± 10%
1012
± 5%
1011 ± 1012
± 5%
(e.g. the HCLL uses liquid Li-Pb while in the HCPB breeder and multiplier are in solid form), since both TBMs must withstand very similar working conditions (Table 1) they share common issues, problems and some similar solutions. This is the case of nuclear sensors/detectors to be located inside these TBMs to monitor and measure fundamental nuclear parameters (e.g. neutron flux and tritium production). Due to the many constrains (e.g. high neutron and gamma flux intensity, high temperature, scarce accessibility, limited space to host the detectors, integration issues, etc.), the neutron detectors must face very challenging working conditions and their development must fulfil and solve many technological challenges. To select the most appropriate detectors a list of functional requirements, constrains, parameters and specifications to be fulfilled by the proposed sensors was prepared, the most important are: a) Invasiveness: the detectors must fit inside the available channels which are about 5 mm diameter; b) Cabling: cables extending for several meters are needed to connect the active detectors to their electronics. Owing to the intense radiation field and the high temperature, mineral insulated cables (MI) that can withstand temperatures as high as 800 °C and neutron fluence > 1020 n/cm2 will be used; c) Electrical Signals: The
2.1. Definition of detectors, measurable quantities and uncertainty margin A fundamental aspect to design any measuring system is to define the quantities of interest and the accuracy required for each parameter to be measured. The main nuclear parameters to be measured in a TBM (as a function of time), are: a) Neutron and gamma fluxes (and energy spectrum, whether possible)
Fig. 1. Measured response of the SPND_Cr detector to Co-60 source at various doses. 2
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Fig. 2. Comparison between time dependent 14 MeV neutron emission (curve A) and current measured by the SPND_Cr prototype (curve B) exposed to 14 MeV neutrons at FNG.
their fine time resolution. Passive detectors usually provide time-integrated responses or responses with very coarse time resolution. A typical example is a neutron activation system (NAS) which consists of activation probes, a transport system for the activation probes and a measurement system for the activities induced in the probes. A NAS provides absolute neutron flux values, does not need special calibration for the purpose of local flux measurements and it does not suffer from burn-up effects. However, it has low time resolution and the need for insertion/extraction systems (e.g. rabbit system) with the further related need for integration in the system and it is therefore very intrusive. On the other hand, active detectors require cabling (and perhaps dedicated cooled channels) so the integration with the system is not necessarily easier than for passive detectors. Another point is to propose a reliable range of uncertainty margin that can be accepted for the various quantities to be measured. As a guidance for the detector development and qualification (and a measure for the achievement of the goal) a preliminary acceptable measurement uncertainty range was agreed upon. In particular, TPRs should be measured with ¡¾5% uncertainty and neutron fluxes with < ± 10%. Among the various active detectors presently considered for the EUTBMs self-powered neutron detectors (SPND), for total neutron flux measurement, and diamond detectors covered with 6LiF, for tritium measurement, are under development at ENEA while KIT is developing the passive system NAS and SPND [6,7]. The reason to choose these detectors as well as their basic working principles are discussed in [3,4].
Fig. 3. CAD view of the prototype diamonddetector realized at ENEA-Frascati.
3. Development and test of SPND for TBM Fig. 4. Typical PHS recorded at various temperatures with a 500 μm thick diamond detector with two Cr contacts irradiated with 14 MeV neutrons. The main n-Carbon reaction is shown. PHS recorded in at least 10 min, each.
The self-powered detector (SPD) is a very simple detector formed by two electrodes (emitter and collector) with in between a mineral insulator (MgO or Al2O3). An SPD is usually designed in a coaxial cylindrical fashion, the rod of emitter is encapsulated in a metallic tube (collector) with a tube of mineral insulator packed between them. The detector is directly linked to a mineral-insulated (MI) coaxial cable for signal transmission. The emitter is chosen for its high interaction crosssection with neutrons (or gammas) which produce high energy electrons. Some electrons travel across the insulator and stop in the collector forming the direct current (DC.) signal of the SPND. It can be demonstrated that the current is proportional to the neutron flux. No
at several positions; b) On-line tritium production; c) Nuclear heating. One proposal is to use both passive and active detectors. Active detectors are necessary for on-line monitoring (e.g. neutron flux and dose rate level) since they provide time-dependent response thanks to 3
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definitively establish the properties and the functionality of the detector. These tests are ongoing. The sensitivity to gammas was first investigated by irradiating the SPND_Cr detector with an intense Co-60 gamma ray source available at ENEA (CALLIOPE facility [12]). The measurements were performed at three different dose-rate (100, 300 and 500 Gy/h corresponding to a γflux of 0.47, 1.4 and 2.34 × 1010γ/cm2s). The results are shown in Fig. 1. It can be noted that the detector is somewhat sensitive to gammas. Good linearity of the produced current versus the gamma dose-rate was observed (inset in Fig. 1). The sensitivity to Co-60 defined as the ratio of the measured current per unit length of the emitter to the impinging gamma flux was thus derived resulting to be 0.3 × 10-25 (Am−1)/(g/m2s) ± 15%. Further tests are scheduled to better study the performances of the SPND_Cr prototype under different working conditions (e.g. at high temperature). A preliminary test was performed also with 14 MeV neutrons at the Frascati neutron generator (FNG) [13]. The results are shown in Fig. 2 where the response of the SPND_Cr detector is compared to the 14 MeV neutron emission recorded with the reference FNG neutron monitor. A good correlation between the SPND signal and the FNG neutron production is observed. Good proportionality between the measured current and the 14 MeV neutron flux impinging on the SPND was observed too. The sensitivity to neutrons was also derived, resulting 4.90*10−29 Am−1/n/m2s ( ± 17%). Accounting for the gamma sensitivity already reported and after calculating the gamma-ray flux by mean of simulation using the MCNP6 code [10], we concluded that the measured signal is affected by a contribution lower than 10% due to gammas. Further tests are ongoing and new measurements of the SPND_Cr detector in more intense fast neutron sources (e.g. accelerators) are foreseen. The data obtained both using gamma and 14 MeV neutron sources seem indicate that the prototype SPND_Cr can be used in a TBM-like neutron spectrum. However, more conclusive tests will be the ones at high temperature.
Fig. 5. PHS recorded at 330 °C for the 100 μm thick diamond detector with Crboron contacts. Measure lasting 45 min.
external bias is necessary. Depending on which particle initiates most electron emission events, an SPD can detect neutrons (so-called SPND) or gammas (SPGD). Indeed, any SPD is sensitive to both neutron and gamma radiation and this fact is to be accounted for during the design and testing phases, as it will be discussed in the following. The SPND presently available on the market were developed for nuclear fission reactors. They are not a priori suitable for reliable operation in ITER-TBM neutron spectrum since the latter is much “harder” than the ones of a nuclear fission reactor. Indeed, the most common used emitter materials (e.g. V, Co, Rh etc.) present high activation cross-sections (tens of barn) at thermal neutron energy but these cross sections are small at neutron energies typical of a TBM. Furthermore, despite in TBM the gamma flux is about ten times lower than the neutron flux the effect of gammas cannot be neglected. Last, but not least, the operation in the temperature range 300–600 °C is another point of concern. Commercial SPND are usually designed to operate at 200–300 °C and thus tests at temperature > 300 °C are mandatory. The above discussion indicates that the use of SPND with fast neutrons in a TBM environment requires a deep analysis of all the aspects related to the physics, technology and operation of this detector [8]. This study is ongoing at ENEA Frascati (in collaboration with KIT). The first step was to calculate and compare the activation induced in a number of different metals irradiated by different neutron spectra and including the ones typical of the EU-TBMs. The output of this study (performed using FISPACT code) was a list of a few metals which can produce enough saturation activity to be used as emitters in SPND designed for TBM [9]. Cr and Be appear interesting for the TBM and a prototype of SPND with Cr emitter was realized (SPND_Cr). The SPND_Cr prototype is basically similar to the commercial ones so to reduce technological problems. It uses a mineral cable (MI) 10 m long and the insulator is made of Alumina (as for the commercial detectors). The sheath is in stainless steel. The Cr emitter is a rod of 100 mm length and 2 mm diameter for a total Cr mass of about 2.5 gr. The SPND_Cr was simulated with the MCNP6 code [10]. Its response was calculated under the (simulated) neutron flux of the HCPB-TBM. Here we mention that about 80% of the signal is prompt while 65% is due to neutron-induced gammas rendering the SPND_Cr a prompt-SPND for TBM [11].
4. Development of diamond detectors for operation at high temperature Conventional solid state Si-based detectors are not applicable in harsh environment, on the contrary, diamond based detectors are very promising. Previous studies performed at ENEA [14] concluded that to operate at HT the diamond detectors must be fabricated by using dedicated materials and technologies. At ENEA Frascati several prototypes of diamond detectors made with different types of electrical contacts and using mineral cable (MI) and metal-oxide connectors (MOC) were realized (Fig. 3) and tested under 14 MeV neutron irradiation at FNG up to T = 400 °C. The used diamond films were usually 500 μm thick (4 × 4 mm2 surface) but two films were 100 μm thick. The detectors were designed for withstanding high temperature and characterize for not using welding or glue to realize electrical contacts which are made just by mechanical pressure. Technical details are in [15]. In our experiments a number of different detectors were exposed to 14 MeV neutrons and the measurements at high temperature (HT) were performed by using the same electrical heather and thermocouple used in [15]. Here we limit to report the most promising results. As an example Fig. 4 shows a typical Pulse Height Spectrum (PHS) recorded with a 500 μm thick diamond film and Cr contacts irradiated with 14 MeV neutrons at various temperatures. The typical 12C(n,α)9Be peak produced by 14 MeV neutrons in carbon is well visible up to T ∼250 °C with good energy resolution (FWHM ∼5%) despite its shift toward lower energy that seems indicate a reduction of the charge collection efficiency. Different metallization (by sputtering or evaporation technique) were studied for realizing the metal contacts (e.g. W, Cr, Ag, Pt, Ti/Pt) and in some cases (e.g. Cr, Ti/Pt and Ag) the metal deposition was followed by annealing at 500 °C per one hour in vacuum. For all the
3.1. Test of prototype SPND_Cr detector with 60Co gamma-rays and 14 MeV neutrons The prototype SPND_Cr was tested with gammas and 14 MeV neutrons. Both tests were performed at room temperature this because it is necessary first to understand the operation capability of this new and unique prototype. The tests at high temperature are mandatory to 4
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covered with 6LiF and irradiated at the ISIS spallation source was reported. This study pointed out the capability of diamond detectors to operate for a long lasting period (one week) at T = 150 °C, and at high neutron fluence while operated in pulse mode. However, many important problems have to be yet faced and solved. Among them, a problem is with the lack of neutron sources which for intensity and energy spectrum can reproduce those expected in TBMs. This is limiting the tests with neutrons and also arising some concerns for a proper and detailed study of the prototype detectors for TBM. This problem is common to all detectors to be developed for TBM and tokamaks and affect the experimental validation of the detectors.
studied detectors, above 230 °C÷240 °C the performances degraded quickly. However, it was found that once cooled down below the maximum working temperature the detectors performances were always reproduced. The best performances with metal contacts were obtained for a 500 μm thick SCD film equipped with two Ag annealed contacts which was perfectly working up to 240 °C [15]. Since almost the same behavior was observed for all the tested detectors, regardless of the metal contact used, it is supposed that it is due to some physical characteristics of the used diamond films (all commercial from Element-6). One of the hypotheses is that some traps are formed at the metal-diamond interface and that these traps are activated at temperatures > 220–230 °C. The observed differences among the achieved maximum working temperatures (of the order of 10–20 °C) can be achieved to the different metal contacts as well as to the fact that the contacts were or not post-annealed after deposition. In the attempt to improve the performances of the diamond detectors and to understand whether the type of contact impacts on the performances, a different type of electrical contact, made of a heavily doped boron layer 2–3 μm thick and deposited on one side of a 100 μm thick Element-6 diamond film was produced. The Boron contact was deposited by chemical vapor deposition (CVD) technique radiofrequency (RF) assisted. The boron concentration was of the order of 1020at/cm3. This allows to get an ohmic contact while the metal-carbon contacts can be either of the Schottky or ohmic type depending upon the construction procedure (e.g. by performing post deposition annealing at T > 500 °C of the metal contact, ohmic contact can be obtained). Details of the boron deposition technique are in [16]. The new detector was thus heated and irradiated with 14 MeV neutrons at FNG. The detector performed very well at temperature > 300 °C. Example of the results are in Fig. 5. The reasons why this detector shown so improved performances is not yet understood. Indeed, a second twin detector, still 100 μm thick, was realized but its behavior was similar to that of the other detectors made with two metal contacts. Its maximum working temperature was around 250 °C. Further analysis to investigate the problem is ongoing.
Acknowledgments The work leading to this publication has been funded partially by Fusion for Energy (F4E) under the Framework Partnership Agreement F4E-FPA-395. This publication reflects the views only of the authors and F4E cannot be held responsible for any use which may be made of the information contained therein. References [1] G. Rampal, et al., HCLL TBM for ITER-design study, Fusion Eng. Des. 75–79 (2005) 917. [2] L.V. Boccaccini, R. Meyder, U. Fisher, Test strategy for the European HCPB test blanket module in ITER, Fusion Sci. Technol. 47 (2005) 2015. [3] L. Leichtle, et al., The F4E programme on nuclear data validation and nuclear instrumentation techniques for TBM in ITER, Fusion Eng. Des. 89 (2014) 2169. [4] M. Angelone, et al., Neutronics experiments, radiation detectors and nuclear techniques development in the EU in support of the TBM design for ITER, Fusion Eng. Des. 96–97 (2015) 2. [5] The ITER tokamak, https://www.iter.org. [6] A. Klix, et al., Preliminary experimental test of activation foil materials with short half-lives for neutron spectrum measurements in the ITER TBM, Fusion Sci. Technol. 64 (2013) 604–612. [7] K. Tian, et al., Feasibility study of a neutron activation system for EU test blanket systems, Fusion Eng. Des. 109–111 (2016) 1517. [8] P. Raj, et al., Self-powered detectors for test blanket modules in ITER, 2016 IEEE NS Symp. (NSS/MIC/RTSD) (2016) 1–4, http://dx.doi.org/10.1109/NSSMIC.2016. 8069908. [9] M. Angelone, et al., Development of self-power neutron detectors for neutron flux monitoring in HCLL and HCPB ITER-TBM, Fusion Eng. Des. 89 (2014) 2194. [10] A General Monte Carlo N-Particle (MCNP) Transport Code. https://mcnp.lanl.gov/. [11] P. Raj: private communication. [12] S. Baccaro, A. Cecilia, A. Pasquali, Gamma Irradiation Facility at ENEA-Casaccia Centre (Rome), ENEA Technical Report RT/2005/28/FIS, (2005) ISSN/0393-3016. [13] M. Martone, M. Angelone, M. Pillon, The 14 MeV frascati neutron generator, J. Nucl. Mater. 212–215 (1994) 1661. [14] M. Angelone, et al., Spectrometric performances of monocrystalline artificial diamond detectors operated at high temperature, IEEE Trans. Nucl. Sci. 59 (5) (2012) 2416. [15] R. Pilotti, et al., Development and high temperature testing by 14 MeV neutron irradiation of single crystal diamond detectors, J. Instrum. (2016), http://dx.doi. org/10.1088/1748-0221/11/06/C06008. [16] M. Marinelli, et al., High performances 6LiF-diamond thermal neutron detectors, Appl. Phys. Lett. 89 (2006) 143509. [17] R. Pilotti, et al., High temperature long-lasting stability assessment of a singlecrystal diamond detector under high-flux neutron irradiation, Eur. Phys. Lett. 116 (2016) 42991.
5. Discussion and conclusion The results presented above indicate that some interesting and encouraging results were already achieved for SPND and diamond detectors developed for harsh environments. Some important points seems established such as the capability of the prototype SPND_Cr detector to be sensitive both to gammas and neutrons, the sensitivity to gammas resulting smaller compared to that of neutrons. As far as diamond detectors are concerned, up to now the most reliable operation range seems up to about 240 °C, at higher temperature the operation in pulse mode (PHS) seems questionable. Study of their response in current mode are ongoing to see whether under this mode of operation higher temperature can be withstood. To note that in [17] a study about the long-lasting behavior of a diamond detector
5
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Performance test of radiation detectors developed for ITER-TBM ⁎
M. Angelonea, , R. Pilottib, F. Stacchic, M. Pillona, A. Klixd, P. Rajd, S. Loretia, G. Paganoa a
ENEA, Dipartimento Fusione e Tecnologie per la Sicurezza Nucleare, C.R. Frascati via E. Fermi, 45, 00044 Frascati, Italy Università degli Studi “Tor Vergata”, Dipartimento Ingegneria Industriale, via del Politecnico 1, 00100 Rome, Italy1 Università degli Studi “La Sapienza”, Dipartimento Ingegneria Energetica, 00100 Rome, Italy1 d Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany b c
A R T I C LE I N FO
A B S T R A C T
Keywords: ITER-TBM Self-power detectors Diamond detectors Tritium breeding Nuclear detectors Neutron flux
The validation of calculation tools used for the nuclear design and analysis of future fusion machines requires the availability of nuclear instrumentation able to measure the nuclear quantities of interest in the harsh environments typical of tokamaks (e.g. ITER) characterized by high radiation level and high temperature. This instrumentation needs to be developed and properly tested under reactor-relevant working conditions. In the EU the activities to develop advanced nuclear detectors for the ITER-TBM are coordinated and supported by F4E and carried out under a collaborative effort between ENEA and KIT performed under the European Consortium on “Nuclear Data and Experimental Techniques”. In this paper the activities carried out at ENEA Frascati to develop nuclear sensors (diamond and self-powered detectors) able to operate “in-core” under the harsh working conditions of the ITER-TBM are discussed. Furthermore, the performance of diamond detectors operated at T > 300 °C, as well as the performance of a selfpower neutron detector (SPND) made with Cr emitter are presented. The open issues and the technological challenges to be faced to further develop the detectors are also addressed.
1. Introduction
accessibility is very limited and this requires the use of miniaturized and reliable detectors. Presently there are not nuclear detectors suited for use in TBM and must be studied, developed and properly tested under reactor-relevant working conditions. This calls for a dedicated effort. To this end, since many years in the EU a technological program is ongoing under F4E coordination and partial financial support. The activities are carried out under a collaborative effort between ENEA and KIT coordinated under the European Consortium on “Nuclear Data and Experimental Techniques” lead by ENEA [3,4]. In this paper the activities so far carried out at ENEA Frascati for developing and testing artificial diamond neutron detectors (SCD) and self-powered neutron detectors (SPND) specifically designed to operate “in-core” under the harsh working conditions of the two EU ITER-TBMs, are reported. The open issues, the technological challenges to be faced, the limits and the perspectives of the developed detectors are also addressed.
To validate the performance of the test blanket module (TBM) of ITER under representative conditions TBMs based upon different concepts will be located in selected equatorial ports of ITER. Two European TBMs, the Helium cooled liquid lead (HCLL) [1] and the Helium cooled pebble bed (HCPB) [2], will be located in ITER for studying their performances. The tritium production rate (TPR) will be the main parameter to be investigated. However, to complete and assess the knowledge of a complex nuclear system such as the TBM other nuclear parameters are of interest and need to be measured, the goal being the validation of the calculation tools used for design. To this end a direct comparison with selected experimental quantities is mandatory. The development of detectors able to measure neutrons while operated in harsh environments is thus a must for fusion tokamaks. From the fusion neutronics point of view, nuclear measurement experiments in TBMs will be of great interest. In different phases of ITER operation, responses like tritium production, nuclear heating, material activation etc. will be measured at different locations. Unfortunately, the environment inside the TBM is characterized by high neutron and gamma fluxes and high temperature. Furthermore, the ⁎
1
2. Functional requirements and specifications of candidate sensors for TBM Despite the many intrinsic differences in the two EU TBM proposals
Corresponding author. E-mail address: [email protected] (M. Angelone). ENEA Guest.
https://doi.org/10.1016/j.fusengdes.2018.05.018 Received 18 September 2017; Received in revised form 16 February 2018; Accepted 3 May 2018 0920-3796/ © 2018 Published by Elsevier B.V.
Please cite this article as: Angelone, M., Fusion Engineering and Design (2018), https://doi.org/10.1016/j.fusengdes.2018.05.018
Fusion Engineering and Design xxx (xxxx) xxx–xxx
M. Angelone et al.
proposed detectors can work either in pulse or current mode, however due to the intense neutron flux the current mode will be most suitable if not the preferential one. For measuring or subtracting and/or suppressing the noise and/or the background the most used method is to locate “back-to-back” two detectors one of the two being incomplete (dummy detector) and to subtract the signal of the dummy detector from that of the true one; d) Radiation Hardness: it will characterize the working life of the detector and the degree of accuracy for the measured quantities, this is a point of concern; e) Burn-up: a well-known problem for fission chambers operated in high neutron flux reactors. When the number of reacting atoms is small compared to the numbers of atoms involved in the reaction rate, the continuous reduction of the material alters the response of the detector which must be corrected since the burn-up affects the accuracy of the measurements; f) Neutron-Gamma discrimination: this is an essential issue for any detector located in a mixed neutron-gamma field. This is also common to passive detectors whose response can be affected by gammas; g) Capability to withstand EM fields and transients; h) Safety & Integration: All the proposed detectors and detection systems must ensure integration with the environment and the safety levels according to the prescription for the other part and components of the TBM. It is thus necessary to prepare a dedicated R&D program where these requirements & specifications will represent the input for the design of nuclear detectors. Furthermore, a step–by-step approach seems the more appropriate so to have a selection procedure which is based on feed-back from the lesson learned at each step of the developing program.
Table 1 Summary of the nuclear working conditions in HCLL & HCPB TBM for DT phase (data refer to ITER at 500 MW fusion power [5]). HCLL Parameter
Max. Value
Neutron flux (n cm−2 s−1) Gamma Flux (g cm−2s−1) Temp. (°C) Nuclear Heating (Wcm-3) Tritium Prod. Rate (T cm−3s−1)
HCPB Max. Value
2.0 × 1014
Max. Uncertainty ± 10%
2.0 × 1014
Max. Uncertainty ± 10%
4.5 × 1013
± 10%
4.5 × 1013
± 10%
500 5.0
± 1.0% ± 10%
650 8.0
± 1.0 ± 10%
1012
± 5%
1011 ± 1012
± 5%
(e.g. the HCLL uses liquid Li-Pb while in the HCPB breeder and multiplier are in solid form), since both TBMs must withstand very similar working conditions (Table 1) they share common issues, problems and some similar solutions. This is the case of nuclear sensors/detectors to be located inside these TBMs to monitor and measure fundamental nuclear parameters (e.g. neutron flux and tritium production). Due to the many constrains (e.g. high neutron and gamma flux intensity, high temperature, scarce accessibility, limited space to host the detectors, integration issues, etc.), the neutron detectors must face very challenging working conditions and their development must fulfil and solve many technological challenges. To select the most appropriate detectors a list of functional requirements, constrains, parameters and specifications to be fulfilled by the proposed sensors was prepared, the most important are: a) Invasiveness: the detectors must fit inside the available channels which are about 5 mm diameter; b) Cabling: cables extending for several meters are needed to connect the active detectors to their electronics. Owing to the intense radiation field and the high temperature, mineral insulated cables (MI) that can withstand temperatures as high as 800 °C and neutron fluence > 1020 n/cm2 will be used; c) Electrical Signals: The
2.1. Definition of detectors, measurable quantities and uncertainty margin A fundamental aspect to design any measuring system is to define the quantities of interest and the accuracy required for each parameter to be measured. The main nuclear parameters to be measured in a TBM (as a function of time), are: a) Neutron and gamma fluxes (and energy spectrum, whether possible)
Fig. 1. Measured response of the SPND_Cr detector to Co-60 source at various doses. 2
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Fig. 2. Comparison between time dependent 14 MeV neutron emission (curve A) and current measured by the SPND_Cr prototype (curve B) exposed to 14 MeV neutrons at FNG.
their fine time resolution. Passive detectors usually provide time-integrated responses or responses with very coarse time resolution. A typical example is a neutron activation system (NAS) which consists of activation probes, a transport system for the activation probes and a measurement system for the activities induced in the probes. A NAS provides absolute neutron flux values, does not need special calibration for the purpose of local flux measurements and it does not suffer from burn-up effects. However, it has low time resolution and the need for insertion/extraction systems (e.g. rabbit system) with the further related need for integration in the system and it is therefore very intrusive. On the other hand, active detectors require cabling (and perhaps dedicated cooled channels) so the integration with the system is not necessarily easier than for passive detectors. Another point is to propose a reliable range of uncertainty margin that can be accepted for the various quantities to be measured. As a guidance for the detector development and qualification (and a measure for the achievement of the goal) a preliminary acceptable measurement uncertainty range was agreed upon. In particular, TPRs should be measured with ¡¾5% uncertainty and neutron fluxes with < ± 10%. Among the various active detectors presently considered for the EUTBMs self-powered neutron detectors (SPND), for total neutron flux measurement, and diamond detectors covered with 6LiF, for tritium measurement, are under development at ENEA while KIT is developing the passive system NAS and SPND [6,7]. The reason to choose these detectors as well as their basic working principles are discussed in [3,4].
Fig. 3. CAD view of the prototype diamonddetector realized at ENEA-Frascati.
3. Development and test of SPND for TBM Fig. 4. Typical PHS recorded at various temperatures with a 500 μm thick diamond detector with two Cr contacts irradiated with 14 MeV neutrons. The main n-Carbon reaction is shown. PHS recorded in at least 10 min, each.
The self-powered detector (SPD) is a very simple detector formed by two electrodes (emitter and collector) with in between a mineral insulator (MgO or Al2O3). An SPD is usually designed in a coaxial cylindrical fashion, the rod of emitter is encapsulated in a metallic tube (collector) with a tube of mineral insulator packed between them. The detector is directly linked to a mineral-insulated (MI) coaxial cable for signal transmission. The emitter is chosen for its high interaction crosssection with neutrons (or gammas) which produce high energy electrons. Some electrons travel across the insulator and stop in the collector forming the direct current (DC.) signal of the SPND. It can be demonstrated that the current is proportional to the neutron flux. No
at several positions; b) On-line tritium production; c) Nuclear heating. One proposal is to use both passive and active detectors. Active detectors are necessary for on-line monitoring (e.g. neutron flux and dose rate level) since they provide time-dependent response thanks to 3
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definitively establish the properties and the functionality of the detector. These tests are ongoing. The sensitivity to gammas was first investigated by irradiating the SPND_Cr detector with an intense Co-60 gamma ray source available at ENEA (CALLIOPE facility [12]). The measurements were performed at three different dose-rate (100, 300 and 500 Gy/h corresponding to a γflux of 0.47, 1.4 and 2.34 × 1010γ/cm2s). The results are shown in Fig. 1. It can be noted that the detector is somewhat sensitive to gammas. Good linearity of the produced current versus the gamma dose-rate was observed (inset in Fig. 1). The sensitivity to Co-60 defined as the ratio of the measured current per unit length of the emitter to the impinging gamma flux was thus derived resulting to be 0.3 × 10-25 (Am−1)/(g/m2s) ± 15%. Further tests are scheduled to better study the performances of the SPND_Cr prototype under different working conditions (e.g. at high temperature). A preliminary test was performed also with 14 MeV neutrons at the Frascati neutron generator (FNG) [13]. The results are shown in Fig. 2 where the response of the SPND_Cr detector is compared to the 14 MeV neutron emission recorded with the reference FNG neutron monitor. A good correlation between the SPND signal and the FNG neutron production is observed. Good proportionality between the measured current and the 14 MeV neutron flux impinging on the SPND was observed too. The sensitivity to neutrons was also derived, resulting 4.90*10−29 Am−1/n/m2s ( ± 17%). Accounting for the gamma sensitivity already reported and after calculating the gamma-ray flux by mean of simulation using the MCNP6 code [10], we concluded that the measured signal is affected by a contribution lower than 10% due to gammas. Further tests are ongoing and new measurements of the SPND_Cr detector in more intense fast neutron sources (e.g. accelerators) are foreseen. The data obtained both using gamma and 14 MeV neutron sources seem indicate that the prototype SPND_Cr can be used in a TBM-like neutron spectrum. However, more conclusive tests will be the ones at high temperature.
Fig. 5. PHS recorded at 330 °C for the 100 μm thick diamond detector with Crboron contacts. Measure lasting 45 min.
external bias is necessary. Depending on which particle initiates most electron emission events, an SPD can detect neutrons (so-called SPND) or gammas (SPGD). Indeed, any SPD is sensitive to both neutron and gamma radiation and this fact is to be accounted for during the design and testing phases, as it will be discussed in the following. The SPND presently available on the market were developed for nuclear fission reactors. They are not a priori suitable for reliable operation in ITER-TBM neutron spectrum since the latter is much “harder” than the ones of a nuclear fission reactor. Indeed, the most common used emitter materials (e.g. V, Co, Rh etc.) present high activation cross-sections (tens of barn) at thermal neutron energy but these cross sections are small at neutron energies typical of a TBM. Furthermore, despite in TBM the gamma flux is about ten times lower than the neutron flux the effect of gammas cannot be neglected. Last, but not least, the operation in the temperature range 300–600 °C is another point of concern. Commercial SPND are usually designed to operate at 200–300 °C and thus tests at temperature > 300 °C are mandatory. The above discussion indicates that the use of SPND with fast neutrons in a TBM environment requires a deep analysis of all the aspects related to the physics, technology and operation of this detector [8]. This study is ongoing at ENEA Frascati (in collaboration with KIT). The first step was to calculate and compare the activation induced in a number of different metals irradiated by different neutron spectra and including the ones typical of the EU-TBMs. The output of this study (performed using FISPACT code) was a list of a few metals which can produce enough saturation activity to be used as emitters in SPND designed for TBM [9]. Cr and Be appear interesting for the TBM and a prototype of SPND with Cr emitter was realized (SPND_Cr). The SPND_Cr prototype is basically similar to the commercial ones so to reduce technological problems. It uses a mineral cable (MI) 10 m long and the insulator is made of Alumina (as for the commercial detectors). The sheath is in stainless steel. The Cr emitter is a rod of 100 mm length and 2 mm diameter for a total Cr mass of about 2.5 gr. The SPND_Cr was simulated with the MCNP6 code [10]. Its response was calculated under the (simulated) neutron flux of the HCPB-TBM. Here we mention that about 80% of the signal is prompt while 65% is due to neutron-induced gammas rendering the SPND_Cr a prompt-SPND for TBM [11].
4. Development of diamond detectors for operation at high temperature Conventional solid state Si-based detectors are not applicable in harsh environment, on the contrary, diamond based detectors are very promising. Previous studies performed at ENEA [14] concluded that to operate at HT the diamond detectors must be fabricated by using dedicated materials and technologies. At ENEA Frascati several prototypes of diamond detectors made with different types of electrical contacts and using mineral cable (MI) and metal-oxide connectors (MOC) were realized (Fig. 3) and tested under 14 MeV neutron irradiation at FNG up to T = 400 °C. The used diamond films were usually 500 μm thick (4 × 4 mm2 surface) but two films were 100 μm thick. The detectors were designed for withstanding high temperature and characterize for not using welding or glue to realize electrical contacts which are made just by mechanical pressure. Technical details are in [15]. In our experiments a number of different detectors were exposed to 14 MeV neutrons and the measurements at high temperature (HT) were performed by using the same electrical heather and thermocouple used in [15]. Here we limit to report the most promising results. As an example Fig. 4 shows a typical Pulse Height Spectrum (PHS) recorded with a 500 μm thick diamond film and Cr contacts irradiated with 14 MeV neutrons at various temperatures. The typical 12C(n,α)9Be peak produced by 14 MeV neutrons in carbon is well visible up to T ∼250 °C with good energy resolution (FWHM ∼5%) despite its shift toward lower energy that seems indicate a reduction of the charge collection efficiency. Different metallization (by sputtering or evaporation technique) were studied for realizing the metal contacts (e.g. W, Cr, Ag, Pt, Ti/Pt) and in some cases (e.g. Cr, Ti/Pt and Ag) the metal deposition was followed by annealing at 500 °C per one hour in vacuum. For all the
3.1. Test of prototype SPND_Cr detector with 60Co gamma-rays and 14 MeV neutrons The prototype SPND_Cr was tested with gammas and 14 MeV neutrons. Both tests were performed at room temperature this because it is necessary first to understand the operation capability of this new and unique prototype. The tests at high temperature are mandatory to 4
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covered with 6LiF and irradiated at the ISIS spallation source was reported. This study pointed out the capability of diamond detectors to operate for a long lasting period (one week) at T = 150 °C, and at high neutron fluence while operated in pulse mode. However, many important problems have to be yet faced and solved. Among them, a problem is with the lack of neutron sources which for intensity and energy spectrum can reproduce those expected in TBMs. This is limiting the tests with neutrons and also arising some concerns for a proper and detailed study of the prototype detectors for TBM. This problem is common to all detectors to be developed for TBM and tokamaks and affect the experimental validation of the detectors.
studied detectors, above 230 °C÷240 °C the performances degraded quickly. However, it was found that once cooled down below the maximum working temperature the detectors performances were always reproduced. The best performances with metal contacts were obtained for a 500 μm thick SCD film equipped with two Ag annealed contacts which was perfectly working up to 240 °C [15]. Since almost the same behavior was observed for all the tested detectors, regardless of the metal contact used, it is supposed that it is due to some physical characteristics of the used diamond films (all commercial from Element-6). One of the hypotheses is that some traps are formed at the metal-diamond interface and that these traps are activated at temperatures > 220–230 °C. The observed differences among the achieved maximum working temperatures (of the order of 10–20 °C) can be achieved to the different metal contacts as well as to the fact that the contacts were or not post-annealed after deposition. In the attempt to improve the performances of the diamond detectors and to understand whether the type of contact impacts on the performances, a different type of electrical contact, made of a heavily doped boron layer 2–3 μm thick and deposited on one side of a 100 μm thick Element-6 diamond film was produced. The Boron contact was deposited by chemical vapor deposition (CVD) technique radiofrequency (RF) assisted. The boron concentration was of the order of 1020at/cm3. This allows to get an ohmic contact while the metal-carbon contacts can be either of the Schottky or ohmic type depending upon the construction procedure (e.g. by performing post deposition annealing at T > 500 °C of the metal contact, ohmic contact can be obtained). Details of the boron deposition technique are in [16]. The new detector was thus heated and irradiated with 14 MeV neutrons at FNG. The detector performed very well at temperature > 300 °C. Example of the results are in Fig. 5. The reasons why this detector shown so improved performances is not yet understood. Indeed, a second twin detector, still 100 μm thick, was realized but its behavior was similar to that of the other detectors made with two metal contacts. Its maximum working temperature was around 250 °C. Further analysis to investigate the problem is ongoing.
Acknowledgments The work leading to this publication has been funded partially by Fusion for Energy (F4E) under the Framework Partnership Agreement F4E-FPA-395. This publication reflects the views only of the authors and F4E cannot be held responsible for any use which may be made of the information contained therein. References [1] G. Rampal, et al., HCLL TBM for ITER-design study, Fusion Eng. Des. 75–79 (2005) 917. [2] L.V. Boccaccini, R. Meyder, U. Fisher, Test strategy for the European HCPB test blanket module in ITER, Fusion Sci. Technol. 47 (2005) 2015. [3] L. Leichtle, et al., The F4E programme on nuclear data validation and nuclear instrumentation techniques for TBM in ITER, Fusion Eng. Des. 89 (2014) 2169. [4] M. Angelone, et al., Neutronics experiments, radiation detectors and nuclear techniques development in the EU in support of the TBM design for ITER, Fusion Eng. Des. 96–97 (2015) 2. [5] The ITER tokamak, https://www.iter.org. [6] A. Klix, et al., Preliminary experimental test of activation foil materials with short half-lives for neutron spectrum measurements in the ITER TBM, Fusion Sci. Technol. 64 (2013) 604–612. [7] K. Tian, et al., Feasibility study of a neutron activation system for EU test blanket systems, Fusion Eng. Des. 109–111 (2016) 1517. [8] P. Raj, et al., Self-powered detectors for test blanket modules in ITER, 2016 IEEE NS Symp. (NSS/MIC/RTSD) (2016) 1–4, http://dx.doi.org/10.1109/NSSMIC.2016. 8069908. [9] M. Angelone, et al., Development of self-power neutron detectors for neutron flux monitoring in HCLL and HCPB ITER-TBM, Fusion Eng. Des. 89 (2014) 2194. [10] A General Monte Carlo N-Particle (MCNP) Transport Code. https://mcnp.lanl.gov/. [11] P. Raj: private communication. [12] S. Baccaro, A. Cecilia, A. Pasquali, Gamma Irradiation Facility at ENEA-Casaccia Centre (Rome), ENEA Technical Report RT/2005/28/FIS, (2005) ISSN/0393-3016. [13] M. Martone, M. Angelone, M. Pillon, The 14 MeV frascati neutron generator, J. Nucl. Mater. 212–215 (1994) 1661. [14] M. Angelone, et al., Spectrometric performances of monocrystalline artificial diamond detectors operated at high temperature, IEEE Trans. Nucl. Sci. 59 (5) (2012) 2416. [15] R. Pilotti, et al., Development and high temperature testing by 14 MeV neutron irradiation of single crystal diamond detectors, J. Instrum. (2016), http://dx.doi. org/10.1088/1748-0221/11/06/C06008. [16] M. Marinelli, et al., High performances 6LiF-diamond thermal neutron detectors, Appl. Phys. Lett. 89 (2006) 143509. [17] R. Pilotti, et al., High temperature long-lasting stability assessment of a singlecrystal diamond detector under high-flux neutron irradiation, Eur. Phys. Lett. 116 (2016) 42991.
5. Discussion and conclusion The results presented above indicate that some interesting and encouraging results were already achieved for SPND and diamond detectors developed for harsh environments. Some important points seems established such as the capability of the prototype SPND_Cr detector to be sensitive both to gammas and neutrons, the sensitivity to gammas resulting smaller compared to that of neutrons. As far as diamond detectors are concerned, up to now the most reliable operation range seems up to about 240 °C, at higher temperature the operation in pulse mode (PHS) seems questionable. Study of their response in current mode are ongoing to see whether under this mode of operation higher temperature can be withstood. To note that in [17] a study about the long-lasting behavior of a diamond detector
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