M. Tanaka

M. Tosaki

T. Kawano

T. Sugiyama

Y. Isozumi

National Institute for Fusion Science (NIFS-Repository)

  The exhaust gas at radiation facilities is regularly monitored for the purpose of certifying that the concentrations of radioisotopes in the exhaust gas do not exceed their legal limits. Thus, the monitor employed for this purpose must have sufficient sensitivity to be able to detect the concentration limits of the radioisotopes that are handled at the facilities. The legal concentration limits depend both on the kind of radionuclide and on its chemical form. For example, in the case of tritium, the legal concentration limit according to Japanese law for tritiated hydrogen is 70 Bq/cm3, while that for tritiated water vapor is 5×10-3 Bq/cm3. This legal concentration limit for tritiated water vapor is very low compared with the tritium sensitivity of a conventional gas monitor. For this reason, several monitors dedicated to monitoring tritium are being developed at various laboratories.  The tritium gas monitor developed at our laboratory is shown in Fig.1, employing ingenious techniques including a two-parameter-pulse height analyzer, in which two parameters are a pulse height and a rise time of signal arose from radiation detection. The tritium gas monitoring system developed was composed of five parts: (1) Tritium detector (PCTD-1), (2) Nuclear radiation analyzer for proportional counter (PCTA-1), (3)Two-parameters pulse height analyzer (PHAS2-1), (4) Control data analyzer (SDA-1), (5) Two-channel oscilloscope, which is schematically shown in Fig.1. The tritium detector (PCTD-1) is a cylinder shaped proportional counter that was manufactured of stainless steel with s 400 mm in length and 60 mm in inside diameter. An active volume is 1 liter. A gold-coated tungsten wire with 0.02 mm in diameter is stretched along the centerline of the cylinder of the detector. Pure methane is used as counting gas. The nuclear radiation analyzer for proportional counter (PCTA-1) contains circuits including a Pre-amp unit and HV power supplier. The HV supplier can apply necessary high voltage (up to 3000V) to the tritium detector. The Pre-amp receives pulses generated by the detector and sends them to the two parameters pulse height analyzer through the Gain & Amp unit. The two parameters pulse height analyzer consists of a PIC (Peripheral Interface Controller) control unit, a Signal processor and a Universal DC power supplier. The pulse sent from the nuclear radiation analyzer examines their height and rise time. The pulse height reflects the energy of radiation detected by the detector. The rise time reflects a rising speed of the pulse. A pair of the pulse height and the rise time is created for respective pulses and sent to Control data analyzer. In the case of tritium beta-rays, the pulse height is very law, because the energy of tritium beta-ray is 18keV at Max. And the rise time of pulse due to tritium beta-rays are very fast compared to other types of radiation because the low-energy beta-rays ionize a small amount of counting gas that occupies a narrow space. The ionized electrons are quickly collected by the center anode wire of the detector, causing a fast raising pulse. The developed high-sensitivity tritium gas monitoring system employs the two-parameter pulse height analyzer for strictly separating the signal caused by tritium beta ray from all other ones. The prototype system was constructed as far, and detail examination will be performed next year.      Signal from a main detector Proportional counter with pure methane gas(2) Nuclear radiation analyzer for proportional counter   (PCTA-1) (3)Two-parameters pulse height analyzer  (PHAS2-1) (5) 2-channel oscilloscope (4) Control data  analyzer (SDA-1) Gain& Amp.unit Pre-amp PIC control unit・AD converter ・Pulse-width counter ・Data transfer Signal monitor 100V(AC) LCD HV monitor (1) Tritium detector (PCTD-1)Signal processor ・Pulse-width  converter ・Timing circuit     unit Universal DC battery ±12V、 ＋5V、 ＋16VHV power supply & PIC display control unit Fig.1. High-sensitivity tritium gas monitor using two-parameters pulse height analyzer  Tungsten, which will be used as a divertor target in ITER D-T operational phase, has many advantages compared to CFC, such as low physical sputtering, low tritium retension, high thermal conductivity, high melting temperature(3695 K) and so on.   The usage of tungsten, however, brings another problem different from CFC.  The surface modification by hydrogen and/or helium plasma irradiation at elevated temperature has been studied extensively and it has been discussed what are benefits and drawbacks when tungsten is used as plasma-facing components1).   Study of the dynamic response of tungsten surface to the transient and extremely high plasma heat load in type-I ELM’s and disruptions(>100MW/m2)2) requires experimental approaches different from those in steady state and low heat flux experiments(<10 MW/m2).  Simulation experiments of material erosion in ITER using high power plasma guns have been intensively studied.  In the present experiments, high current stabilized arc plasmas with ~GA/m2 is used as a high heat flux pulse and steady state plasma source. A scematic diagram of the cutting arc plasma and diagnostics is shown in Fig. 1. The arc current flowing between hafnium(Hf) cathode and anode made of copper is controlled by a DC current source(Iarc=10~150 A). This type of arc plasma torch enables us to generate very high heat flux plasmas with good reproducibility. The plasma heat flux onto the cathode surface is several hundreds MW/m2 in steady state  and  several GW/m2 in arc ignition and arc quenching phases3).  These   (Keyence:VW-6000)  () Hf	 + 1.6 mm DC current sourceAnode Color high speed cameraQuartz windowHf cathodeCopper cathode NozzleOxygen gasproperties of high heat flux arc plasmas are very useful to study the transient behaviour of the divertor materials during ELMs and disruptions in fusion reactor complementally with other ELM/disruption experiments. In Fig. 2  the abrupt ejection of molten Hf from the  hot cathode surface observed by high speed color camera is shown. The bright area in the figure is the hot molten cathode with T=3,500 K ~ 4,000 K.  The cathode surface temperature is estimated from RGB intensities of the color camera image under black body approximation. Typical error of the present temperature measurement is estimated to be about 150~200 K.   In the experiments disruptive ejection of the cathode molten materials, which may related to some fluid dynamic instabilities, is observed in the steady state arc  in addition to the arc ignition phase with pulse ~GW/m2 heat flux.  When a new Hf cathode which has a flat cathode surface is used, a large Hf bubble grows up and then burst to release Hf vapor to the arc plasma column.  The generation of the Hf bubble might come from the boiling of Hf molten pool.  In near future we will test tungsten cathode with helium arc palsmas using this arc plasma torch.1) Brooks, J.N.,  Allain, J.P.,  Doerner, R.P.  et al.:Nucl. Fusion 49(2009)035007.2) Hassanein, A.,   Belan,  V.,  Konkashbaev,  I.  et al.:J. Nucl. Matter.  241-243 (1997)288.3) Yamaguchi Y., Yoshida, K., Uesugi, Y., Tanaka, Y., et al.:Journal of The Japan Welding Society  29(2011)010.Fig. 1  Schematic diagram of the plasma cutting arc torch and observation of the cathode hot spot.Fig. 2  Abrupt ejection of the Hf droplet from the molten  Hf pool as a cathode spot(a-1~4) and the change of surface temperature during the cathode spot instability(b-1~4).   Frame interval of each picture from the top to the bottom is  125 ms.   4.5 3.5 25002600270028002900300031003200330034003500360037003800390040004100420043004400500.5 (a)-3 T [kK] (a)- 4 T [kK] (a)-1 (a)-2 (b)-1 (b)-2 (b)-3 (b)- 4 [kK] 459§14. High-Sensitivity Tritium Gas Monitoring System Using Two-Parameters Pulse Height AnalyzerKawano, T., Tanaka, M.,  Isozumi, Y., Tosaki, M. (RRC. Kyoto Univ.), Sugiyama, T. (Nagoya Univ.)

High-Sensitivity Tritium Gas Monitoring System Using Two-Parameters Pulse Height Analyzer

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High-Sensitivity Tritium Gas Monitoring System Using Two-Parameters Pulse Height Analyzer

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