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Dependence of Decontamination Performance
on Co distribution in Test Pieces
in Dry Surface Decontamination with Low-Pressure Arc PlasmaShizue FURUKAWA1,2,*, Makoto ICHIMURA2, Tadashi AMAKAWA1
and Kazuo ADACHI11Central Research Institute of Electric Power Industry
2Graduate School of Pure and Applied Sciences, University of Tsukuba
We have been studying dry surface decontamination technology with low-pressure arc plasma (LPAP). Thedependence of the decontamination performance on Co distribution in the test piece was investigated. The Coremoval ratio became lower when Co was distributed more deeply in the metal oxide film, although all the metal oxidefilm was removed. However, a 60Co removal ratio exceeding 90% was eventually achieved by repeated LPAPtreatment.
Keywords: low-pressure arc plasma, dry surface decontamination technology, decontamination performance, cobalt,corrosion product film
1. IntroductionA clearance level  system for radioactive waste was
standardized on December 1, 2005 in Japan. Therefore, itis becoming vital to reduce the radiation dose level of thiswaste to the clearance level by decontamination.
Some researchers have been studying various kindsof decontamination technology using plasmas [2-4]. Wehave also been studying dry surface decontaminationtechnology with low-pressure arc plasma (LPAP) for thecorrosion product films formed in the cooling watersystem of an atomic power plant . LPAP is atransferred-type DC arc plasma operated under reducedpressure. One of the advantages of LPAP is the fact thatthe cathode spots formed inside LPAP can eliminate themetal oxide film on the metal substrate withoutsignificant damage to the metal substrate when an objectto be treated is set as a cathode [2,5]. This generates smallamounts of secondary radioactive waste because nochemical solution is utilized. These advantages suggestthe strong potential for practical decontaminationtechnology. To realize this technology, the applicabilityshould be investigated from various viewpoints. However,the dependence of decontamination performance on theCo distribution in the depthwise direction of the object tobe treated remains unclear. The Co distribution in realradioactive waste differs in each case, because it dependson the contamination process, for example the formationof a radioactive corrosion product film under a coolingwater system at an atomic power plant or the adhesion ofradioactive nuclides to the tools etc. Therefore, in thisreport, we investigated the dependence of the LPAP
decontamination performance in the case of different Codistributions in a depthwise direction inside the metaloxide film.
Object to be treated(Cathode)
Object to be treated(Cathode)
Fig. 1 Photograph of LPAP.
Fig. 2 Schematic process of removing Co by LPAP.
J. Plasma Fusion Res. SERIES, Vol. 8 (2009)
2009 by The Japan Society of PlasmaScience and Nuclear Fusion Research
(Received: 29 August 2008 / Accepted: 13 April 2009)
2. Experimental Method2.1 Principle of LPAP decontamination
Figure 1 shows a photograph of LPAP, in which wecan see illuminating cathode spots.
Figure 2 shows the schematic process of removingCo. The metal oxide film on the metal substrate can bemelted and evaporated due to the heat flux from thecathode spots. When the film is removed and the metalsurface is exposed, the cathode spots move and rapidlyfind a new area that is still covered with the metal oxidefilm ((2) in Fig. 2) [2,5]. By repeating this process, theentire metal oxide film on the metal substrate can beremoved. One of the reasons for this may be the fact thatthe work function of the metal covered with the metaloxide film is lower than that of the metal without the film.However, the details of these phenomena have not beencompletely clarified to date [2,5]. It is noteworthy thatcathode spots can be formed, even on metal substrate,after all the metal oxide film has been removed, as shownin (3), (4) and (5) of Fig. 2.
2.2 LPAP Decontamination System andExperimental ConditionsA schematic diagram of an LPAP decontamination
system is shown in Fig. 3. The system comprises ananode and cathode set in a chamber, a DC power supplysystem (no-load voltage: 200 V, load voltage: 100V). Thewater-cooled anode is made of copper.
The test procedure is as follows. First, the air in thechamber is replaced with argon gas after setting the testpiece as the cathode. Next, LPAP is ignited after fillingup the gas to a set pressure. The main conditions of LPAPare listed in Table 1.
The arc voltage was almost 20 V, while thein-chamber pressure was set at a value whereby the bestdecontamination performance was obtained within therange from a few Pa to several hundred Pa .
2.3 Test Pieces and Evaluation of the Co DepthRatioThe specifications of the test pieces in the
experiments are listed in Table 2. Two kinds of test pieces,namely corrosion product film and the metal oxide filmby thermal oxidation are used. 60Co is one of the typicalradioactive nuclides in low-level radioactive wastegenerated from an atomic power plant.
It was decided that the shapes and surface areas ofthe two test pieces were sufficiently large to makesignificant measurement of Co contained inside. Detailsof the measurement methods of Co are described insection 2.4.
A corrosion product film was formed on its surfaceby immersing the metal substrate in the water. The
Table 1 Main LPAP conditions.
Arc current 60 AGap length 2 mm
Pressure 50 Pa
Type of gas Ar
Treatment time 1-4 s
Fig. 3 Schematic diagram of the LAPAdecontamination system.
Table 2 Specifications of test pieces.
Type of test piece Corrosion product filmcontaining 60CoMetal oxide film formed by thermaloxidation with non-radioactive Co
Shape of metal substrate 16*16*8t (mm) 30*30*1t (mm)
Concentration of Co about 0.1(kBq) about 40 (g/cm2 )
Main chemical compounds Fe2O3, FeCr2O4, etc. FeCr2O4, Cr2O3 etc.
60Co distributed both in the corrosion product film and metal substrate at a ratio of 70:30. 60Co distributed at a depthof about 20 m from the surface of the metal substrate.
S. Furukawa et al., Dependence of Decontamination Performance on Co Distribution in Test Pieces in Dry Surface Decontamination with Low-Pressure Arc Plasma
conditions of this water imitated those of the coolantwater of an atomic power plant.
In the case of metal oxide film by thermal oxidation,an MOD Type Dip coating solution, Co-03 (KojundoChemical Laboratory Co., Ltd.,) was used as a cobaltsource. Three production methods are applied to vary theCo distribution in the depthwise direction of the metaloxide film shown in Table 3 (The word Co depth ratio isused from now on.). The thickness of the metal oxidefilm itself is almost 1-2m. The Co depth ratio in themetal oxide film was measured and evaluated by GDS(Gas Discharge Sputtering) analysis. Figure 4 shows anexample of the GDS measurement result of the test piecemade using the No. 1 method in Table 3. It represents thedistribution of each component (O, Fe and Co) in thedepthwise direction from the surface of the test piece.
In Fig. 4, point PO is the mid-point of the maximumand minimum values of O signals intensity. The arrowwas drawn from the surface to PO, with its length definedas AO. AFe and ACo are similarly defined. The thickness ofthe metal oxide film was defined as the average of AFeand AO. The Co depth ratio (%) is defined by the ratio ofACo relative to the thickness of the metal oxide film.
2.4 Evaluation Method of DecontaminationPerformance
The Co removal ratio when only the metal oxide filmwas removed, a0, is defined as,
a0 = (1 0/F) 100 (%) (1)where F is the amount of Co in the film before LPAPtreatment. And 0 is the amount of Co remaining in thesurface of the metal substrate after removing only themetal oxide film.
Additionally, the Co removal ratio when Q (A s) ofLPAP treatment was applied, aQ, is defined as,
aQ = (1 Q/N) 100 (%) (2)where N is the amount of Co in the test piece beforeLPAP treatment and Q is the amount of Co remaining inthe test piece after Q (A s) of LPAP treatment.
For radioactive test pieces, were evaluated fromthe radiation dose level of the test piece with a Gesemiconductor detector (SEIKO EG&G). Fornon-radioactive test pieces, F was evaluated from theweight of the Co solution applied to the metal substratewhen the forming metal oxide film shown in Table 3. 0was evaluated as follows: The surface of the metalsubstrate, including Co, was dissolved in the solution byelectrolytic polishing. Subsequently, the concentrations ofCo and Fe in the solution were measured by ICP(Shimadzu, ICPS-7500). Based on this measurementresult of Fe concentration in the solution, the polisheddepth of the metal substrate can be evaluated. Themeasurement results of the Co concentration mean thetotal weight of Co included in the polished metalsubstrate. 2 to 3 times of electrolytic polishing of 1-2mdeep, was repeated, because Co in the metal substratesdecreases with the depth of the metal substrate. Theelectrolytic polishing method can polish only metal, andcannot polish and dissolve metal oxides.
a0 and aQ were used to evaluate the decontaminationperformance of LPAP.
Table 3 Production methods of metal oxide film test pieces with varying Co depth ratio in the film.No. Making me