113
Journal of Nuclear Materials 149 (1987) 113-116 North-Holland, Amsterdam
LETTER TO THE EDITORS
AIR CONTAMINATION BY CESIUM IN A CANISTER CONTAINING NUCLEAR WASTE GLASS
1. Introduction High-level nuclear waste (HLW) glass containing hazardous fission products and actinides will be stored in a facility for many years before geologic disposal. If HLW glass in the storage facility is subjected to high temperatures by accidents such as a fire, then volatile radioactive elements can be released into the surrounding air through any cracks in the canisters. The authors previously studied the air contamination by cesium-bearing materials in a canister containing simulated HLW glass after reheating up to a maximum of 1000” C. This experiment showed that such contamination was significant even at waste storage temperatures of less than 500” C if the glass contained a realistic amount of radioactive cesium expected in commercial HLW glass [l]. However, the authors have not yet characterized the cesium-bearing materials. It is very important to know the characteristics of the cesiumbearing materials such as their size and composition since they provide basic information useful for the understanding of and protection against the air contamination. In the present study, as the first step for that purpose, the size and the composition of the cesium-bearing materials are examined by a scanning electron microscope (SEM) with wave dispersive X-ray analyses (WDX). The results imply that the production of dust from the corroded canister plays an important role in the air contamination inside the canister.
2. Experimental Simulated HLW glass for the present study was borosilicate glass containing about 12.7 wt% of simulated HLW [2], the composition of which is given in table 1. The reagents for the glass additives and the simulated HLW indicated in table 1 were mixed with a solution of 3.0 X 10” Bq of 134Cs. This mixture was calcined at 750 ’ C, melted at 1200 o C for 2 h, poured into an 8.1-cm-ID., 24.4-cm-high stainless steel canister
0022-3115/87/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
(SUS 304L), kept at 600 o C for 2 h, and cooled to room temperature. A cooling rate of less than 40”C/h was used in order to prevent the glass cracking. After the vitrification, the amount of ‘34Cs in the glass was measured to be 2.2 x 10” Bq by a gamma-scanning method [3]. The rest of 134Cs was lost from the glass matrix to the off-gas line during the vitrification. The volume of the glass in the canister was about 490 cm3 which was calculated from the weight and the density of the glass. The canister containing the simulated HLW glass was heated in an electric furnace at 1000 o C for 4 days which was sufficient time for cesium-bearing materials to reach a constant contamination level in the air above the glass in the canister [l]. Part of the air was collected in a sampling bottle and the radioactivity was measured by a multichannel analyzer equipped with a NaI(T1) detector. The amount of ‘34Cs dispersed in the air inside the canister at 1000°C was 6.7 x lo2 Bq/(cm3 air) from which the amount in the air is calculated at 4.4 X 10’ Bq; approximately 2 X 10m3’% of the total amount in the canister. Subsequently it was cooled to room temperature in the same furnace by cutting off the electricity. This cooling did not crack the glass due to the large heat capacity of the furnace. The canister containing the glass had two pipe lines; an inlet and an outlet. Through the inlet, air was forced to pass into the canister at a flow rate of about 6 I/mm, and from the outlet, the air was collected by an air sampler (Dylec, MPS-3) for about 20 min. Fine particles trapped by the air sampler were observed by SEM-WDX shielded with heavy metal for protection against ‘34Cs radiation. The accelerating voltage of the electron beam had a constant value of 20 keV, and the probe current was about 3 X 10m9 A throughout the observation. A higher probe current of more than 5 x 1O-9 A caused the particles to change their morphology and a lower probe current of less than 1 x 1O-9 A was not sufficient to obtain significant X-ray signals. Therefore, the probe current of 3 x lop9 A was chosen as a compromise.
B.V.
114
H. Kcrmrzono et ul. /AU
3. Results and discussion 3. I. Size
contamination
by cesium
l(b), and it is said that the size of particles l(a) tends to increase due to agglomeration. 3.2. Composition
of simulated
Component
(Added
SiO,
(SiO, ) (Na,B,O,) (Al(OH),) (CaCO,)
B,O, A’,O, CaO
high-level
of cesium-hearing
materials
Fig. 2(a) shows a typical example of cesium-bearing materials trapped by the air sampler. Their shape is irregular. Eleven elements; i.e., Ni, Fe, Cr, Cs. Te, Cd, Ag, Zn. Si, Na and 0 were identified by WDX. The elements of Ni, Fe and Cr may come from the corroded stainless steel canister for the reason described in section 3.3 The elements of Cs, Te, Cd and Ag are those from simulated HLW. The elements of Zn. Si and Na
waste glass
reagent)
Content
45.20 13.90 4.90 4.00
(wt%)
Component
(Added
Na,O ZnO Li,O
Wa,CO,) (ZnO) (Li,CO,)
6.10 2.50 2.00
La,& CeO, Pr,O, 1 Nd 8, Sm203 Eu,O, Gd $4
(La(NO,),~6H,O)
0.51 1 .Ol 0.49 1.65 0.33 0.06 0.04 0.02 0.80 0.12 0.15 0.43
reagent)
Content
Swnulated (nonradiouctroe) high-leuel waste Oi As fi.rsion products Rb,O (RbNO,) sro (SrNO,) (Y(NO,),.6H,O) Y*O, (ZrO(NO,),.6H,O) ZrOz (H,MoO,.H,O) MoO, MnO, (Mn(NO,),.6H,O) Ag,O Cd0 SnO,
( AgNO,)
SW, cs,o
WC1 3) (TeO, ) (CsNO,)
BaO
(Ba(N$),)
TeO,
(Cd(NO,),) (SnCl) )
0.12 0.34 0.20 1.64 1.73 0.26 0.03 0.03 0.02 0.004 0.23 0.98 h’ 0.62
As actinide elements CeO, UW’JO,),-6H,O)
0.90
As Zircolloy fillings zro, Pro,)
1 .oo
(WNO,),.6H,O) (PrWO,),~6H,O)
PW’JO,),~6H,O) (Sm(N0,),~6H,O) (EuWO,),~6H,O) (‘WNO~)x~6H,O)
sea,
(SeO,)
RuO, RU Rh Pd
(RuCI,.3H,O) (Ru) (Rh) (Pd)
As corrosion products Fe,O, NiO
in fig.
distribution
Fig. l(a) shows the size distribution of the fine particles which were collected by the air sampler. At a glance, most of the particles are in the range of 0.5 to 5 pm in diameter. However, fig. l(b), which is an enlargement of the one particle indicated by an arrow in fig. l(a), shows that this particle consists of about ten smaller particles of 0.2 to 0.7 pm in diameter. More than 50% of the particles trapped by the air sampler were in the form of aggregates and similar to that in fig.
Table 1 Composition
shown
(Fe(NO,),~9H,O) (Ni(N0,),,6H,O)
As the other inerts Na,O (NaCO,)
2.90 0.40
Cr,O,
0.50
3.70
p20,
0.30
” This simulated high-level waste corresponds to what is called JW-A in our notation [2]. h, The amount of cesium added as ls4Cs was negligible in comparison with 0.98 wt% of nonradioactive
cesium.
(wt%)
H. Kamizono
et al. / Air contamination
Fig. 1. Scanning electron microphotographs of the fine particles trapped by the air sampler.
are those from glass additives. Oxygen is everywhere in the glass and in the surrounding air. Since the electron beam tends to diffuse up to an area of about 3 pm in diameter, a resolution of less than 3 pm is impossible in our experiments. Therefore, when a variety of particles of less than 3 pm in diameter are agglomerated into a larger particle, many elements are detected in the same area by one analysis. Fig. 2(b) shows another example of a cesium-bearing material. The elements identified by WDX were Fe, Cs and Zn. Since its shape is like a bar (sometimes intersected by another bar), the material is considered to be crystallized to a certain extent. Asano et al. [4] have examined the vapor species over Na,O-B,O,-SiO,Cs,O glass and showed that Cs vaporizes in the form of borates. However, B was not found in the cesium-bearing material. Further examination, for example by X-ray diffraction, will be necessary to determine the phase. 3.3. Role of zinc
It should be noted that Zn is a volatile element in the glass and tends to adhere to the stainless steel
by cesium
115
Fig. 2. Scanning electron microphotographs of the cesiumbearing materials trapped by the air sampler. Compositional analyses were carrier out by WDX within the circles.
canister through the vaporization. Moreover, Stahl et al. have noted that Zn, as well as Si, is a element which tends to attack and corrode the stainless steel canister, judging from the pit formation [5]. Since in our experiments, compared with the outside of the canister, the inside was observed to be markedly corroded and a relatively large amount of Ni, Fe and Cr was generally found with Cs, it is speculated that fine particles that peel off from the inside of the canister (to which Cs adheres) easily fly up into the air above the glass. This is in line with the description by Klein et al. [6] that part of the cesium released is entrained by dust. Thus, Zn is an unfavorable element which works to increase the air contamination by the production of dust from the inside of the canister. Note that the existence of Zn in the glass is avoidable since it is not a fission product.
116
H. Kamizono
Ed al. / Au contammation
4. Concluding remarks Once HLW glass in a canister has been subjected to high temperatures, various volatile elements are released from the HLW glass. The elements condense into fine particles on the one hand, and on the other hand some elements such as Zn and Si attach the stainless steel canister. When engineered barriers are damaged, they provide possible pathways for the radioactivity in the air inside the canister to escape with the dust produced from the corroded canister. In these cases, a filtration method which can efficiently collect particles of 0.2 pm or less is necessary for the storage facility.
Acknowledgements The authors would like to thank U. Shiota and T. Tsuboi for their preparation of the glass containing lX4Cs. It is also a pleasure to acknowledge the helpful
Received 31 July 1986; accepted 12 February 1987
by cesrum
discussion with S. Muraoka excellent technical assistance Chiba.
and M. Kumata of S. Kamoshida
and the and H.
References S. Kikkawa, S. Tashiro and H. Nakamura, Nucl. Technol. 72 (1986) 84. PI T. Banba, H. Kimura, H. Kamizono and S. Tashiro. Report Ill H. Kamizono,
JAERI-M 82-088 (Japan Atomic Energy Research Institute, Tokai, Ibaraki, 1982). [31 H. Otsuka, Y. Tamura, M. Nomura and S. Tashiro. Report JAERI-M 84-067 (Japan Atomic Energy Research Institute, Tokai, Ibaraki, 1984). [41 M. Asano and Y. Yasue. J. Nucl. Sci. Technol. 22 (1985) 1029. by D. Stahl and N.E. Miller, Report ISI Compiled NUREG/CR-3427, BMI-2113, Vol. 4 (Battelle Columbus Laboratories, Columbus, Ohio, 1984). [61 M. Klein, C. Weyers and W.R.A. Goossens, Rad. Waste Manage. Nucl. Fuel Cycle 6 (1985) 255.
Hiroshi Shingo
Kamizono, Tashiro
and
Shizuo Haruto
Kikkawa, Nakamura
Department of Enwronmental Safety Reseurch Japan Atomic Energy Research Instrtute Tokui-mura, Iharukr-ken, .slY-II Jupun and Hiroyuki
Kanazawa
Department of Reactor Fuel Exammation Japan Atomic Energy Research Institute Tokai-mura, Ibaraki-ken, 319.I I Jupun