atw 2018-04v6

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atw Vol. 63 (2018) | Issue 4 ı April 246 RESEARCH AND INNOVATION Revised version of a paper presented at the Annual Meeting of Nuclear Technology (AMNT 2017), Berlin. Irradiation Tests of a Flat Vanadium Self- Powered Detector with 14 MeV Neutrons Prasoon Raj and Axel Klix Self-powered detector (SPD) represents a class of neutron and gamma monitoring instruments used in the fission reactor cores worldwide. This detector has inherent advantages of functioning without a bias voltage, simple measurement scheme, compactness, ease of maintenance, and high reliability. We are studying SPD for application as flux monitors in the European test blanket modules (TBM) of ITER, fusion reactor under construction in southern France. This paper presents results of experimental tests performed with 14 MeV neutrons for a flat SPD with vanadium emitter. Vanadium responds by beta emission from products of reactions (main routes: 51 V (n, γ) 52 V and 51 V (n, p) 51 Ti) with thermal and fast neutrons. Secondary electrons due to gammas from these reactions and neutron irradiation of surrounding materials are also important contributors to the signal. Thin foils of emitter, insulator and collector materials are used to construct the test SPD. The detector is irradiated with short and long pulses of neutrons and is found to respond in proportion with the incident neutron flux. Further experiments with simplified and better optimized design of detector are underway for thorough study of the signal-creation mechanism. 1 Introduction ITER [1] is an experimental fusion reactor based on tokamak concept, under construction at St. Paul lez Durance in southern France. It is an international project aimed at proving feasibility of fusion as a large-scale and carbon-free source of energy. One of the main scientific goals of this project will be to test and prove the concepts of tritium breeding blankets. Tritium is an important fuel component for devices based on D-T reaction, which is being considered as main reaction for fusion power plants. Because tritium is a rare element, it is required to breed it in the fuel cycle of the reactor. A blanket with lithium compounds will cover the inner wall of the plasma vessel. Fusion neutrons from the plasma will be absorbed by lithium nuclei, causing reactions to produce tritium. There are multiple breeding blanket designs proposed by scientists. To determine their efficiencies in a real fusion environment, test blanket modules (TBM) based on different concepts will be inserted into equatorial ports of ITER for experimental tests in different operational phases of ITER. The European Union is going to test two such concepts, namely the Helium-Cooled Lead-Lithium (HCLL) and Helium-Cooled Pebble Bed (HCPB) TBMs [2]. In the neutronics experiments, nuclear responses like tritium production rate, material activation, nuclear heating etc. are to be measured and compared with the calculations. This step will validate the advanced computational tools and nuclear data utilized for nuclear analyses for fusion devices. The neutron and gamma fluxes are important quantities to be measured for these experiments, for which detectors like neutron activation system, fission chambers and self-powered detectors (SPD) are under study. An SPD is a multi-layered electrical device, which produces direct current (DC) signal on irradiation with neutrons and/or gammas. It can be preferentially responsive to neutrons (self- powered neutron detector, SPND) or gammas (SPGD), or as it is in most of the cases, to both. Figure 1 shows a rough sketch of the cross- section of a traditional detector. Central material, called emitter produces fast electrons on irradiation. These fast electrons can be betas from the decay of neutron activation products, or secondary electrons due to interaction of gammas in the bulk of the material. They slow down in a layer of insulation and stop in the outer electrode called collector. This electron-movement creates a potential difference and thus, produces a current signal proportional to the incident particle flux. The current due to beta electrons is “delayed” because of the half-life of beta-emitters, e.g. SPND based on Rh, V or Ag emitters. Whereas that due to gamma-initiated photoelectric or Compton electrons is “prompt”, for example Co-based SPND [3]. An SPD responds in a sophisticated manner, with multiple factors contributing to the small current signals often totaling between 10 -12 and 10 -3 Ampere. Due to its inherent advantages of simplicity, compactness and high-reliability, they are highly desirable for flux monitoring in areas with restricted access like reactor cores. At KIT, we are studying SPDs for application in ITER TBM [4]. Vanadium based flat SPD is being tested with 14 MeV neutrons, to understand its behavior towards fast neutrons expected in fusion environment and ascertain the feasibility of its application as flux monitor for European ITER TBMs. 2 Experimental details Vanadium is a common emitter for fission reactor SPNDs. The response of the detector towards thermal neutrons is understood well. The material is relatively inexpensive and easier to handle. However, due to lower cross sections the sensitivity of vanadium- SPND towards fast neutrons reduces (Figure 2). Commercially available SPND cannot be directly used for measurement of fusion neutron fluxes, going up to approx. 14 MeV in energy. Characteristics of the two main beta- emitters from 51 V (99.75 % isotopic abundance) in case of fast neutron irradiation, are reported in Table 1. Cross-sections of the fast neutron reactions in 51 V for a pure | | Fig. 1. Cross-sectional sketch of a cylindrical SPD showing emitter (green), insulator (dotted white) and collector (black) layers, with connection to the lead cable and current measurement device. Research and Innovation Irradiation Tests of a Flat Vanadium Self- Powered Detector with 14 MeV Neutrons ı Prasoon Raj and Axel Klix
atw Vol. 63 (2018) | Issue 4 ı April | | Fig. 2. Cross sections of vanadium reactions and photon production under neutron irradiation. Reaction 51 V (n, p) 51 Ti 51 V (n, γ) 52 V Threshold Neutron Energy 1.72 MeV 0 MeV 14 MeV Cross-section 30 mb (approx.) 0.6 mb (approx.) Beta Emitter, Half-life 51 Ti- 5.76 m 52 V- 3.74 m Average Beta Energy 51 Ti- 0.87 MeV 52 V- 1.07 MeV SPND Current (14 MeV) 3.46 × 10 -12 A 6.92 × 10 -14 A SPND Current (TBM) 7.97 × 10 -9 A 3.44 × 10 -8 A | | Tab. 1. Beta-emitters and corresponding currents from fast neutron reactions in vanadium based SPND. 14 MeV source are shown. Neglecting the self-shielding of electrons in emitter material, effect of other materials and taking a saturation condition (considering the short half-lives of daughter nuclides), one can ascertain the orders of magnitude of currents possible with V-SPND, as reported. For this estimation, vanadium density of 6.1 g cm -3 , and a typical volume of 1 cm 3 are assumed. For a 14 MeV neutron source, a flux intensity of 1 × 10 10 cm -2 s -1 is considered, which is achievable with state of the art 14 MeV neutron generators. For TBM, activation calculation was done [5] with the HCLL neutron spectrum and typical flux intensity (up to 1 × 10 14 cm -2 s -1 ) using EASY-2007 [6]. With high-sensitivity ammeters, currents down to the order of 1 × 10 -14 A can be reliably measured [7]. Values in Table 1 show that a vanadium emitter based SPND will produce measurable signals in TBM. Due to its high neutron threshold energy, the (n, p) reaction can be utilized to measure fast neutron flux exclusively. Fast neutron reactions lead to high-energy gamma production. This phenomenon competes with the neutron absorption reactions (Figure 2). Photoelectric and Compton electron emission from emitter causes a prompt current which is expected to form the major component of the signal of V-SPND towards 14 MeV neutrons. Secondly, vanadium being a medium- Z nucleus can be a potential emitter for SPGD also. With optimized dimensions and choice of collector material, a vanadium SPD can be envisaged for monitoring of photon flux in TBM. Instead of the usual coaxial type cylindrical geometry, we designed our test SPD in sandwich-type flat geometry. This provides a relatively higher cross section area to the incident neutrons, and ease of access for testing various materials in the same device. Thin foils (0.5 to 2 mm) of emitter, insulator and collector are arranged to form an assembly in an aluminum case, which also serves as an electromagnetic shield. Central conductor of the signal cable is linked to the emitter plates of the detector. The collector plates, case and the cable sheath are shorted and securely connected to the ground. Schematic sketch and photograph of the test detector are shown in Figure 3 (left). With comparable cross sections of reactions in different materials, the insulator and collector materials also play an important role in SPD response. Behaviors of different material combinations are experimentally tested. Alumina (Al 2 O 3 ) or beryllia (BeO) is used as insulator and Inconel-600 or graphite is used as collector in our experiments. Effects of the change of geometry and dimensions are also studied. A Keithley 6485 Picoammeter (sensitivity range -20 fA to 20 mA) is used as the measuring device. A low-noise triax cable (Belden 9222) is used to reduce the interferences in low-current measurement. The tests are conducted at the 14 MeV neutron generator of Technical University of Dresden (TUD-NG), shown in Figure 3 (right). Here, deuteron beams are impinged on a tritiated titanium target causing D-T reaction which leads to production of neutrons with peak energy of approx. 14 MeV. TUD-NG offers neutron flux intensities up to 1 × 10 10 cm -2 s -1 . The detector is placed in front of the tritium- target of TUD-NG and tested under different conditions by varying flux levels and irradiation times. 3 Results The irradiation tests of flat sandwichtype vanadium SPD were performed at TUD-NG, with neutron flux intensities around 1 × 10 9 cm -2 s -1 . DC signals in the range of 100 fA to 100 pA were measured. In Figure 4, a plot shows variation of SPD signal with change in neutron flux. The detector was composed of 1 mm thick layers of vanadium emitter and Inconel-600 collector. The signal was found to be proportional to the incident flux, with approx. 90 pA at the highest flux level. At low fluxes and low currents, the measurements have high uncertainties. Interference from electromagnetic sources of stray currents, | | Fig. 3. (Left) internal design of the sandwich-type flat SPD: (top)- an engineering sketch of the geometry having sandwich of foils of emitter (green), insulator (grey) and collector (red), and (below) a photograph of the assembly with vanadium SPD. (Right) experimental setup showing TUD-NG beamline, tritium target, mounted SPD, and the lead cable. RESEARCH AND INNOVATION 247 Research and Innovation Irradiation Tests of a Flat Vanadium Self- Powered Detector with 14 MeV Neutrons ı Prasoon Raj and Axel Klix
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