Entrapment of Krypton in Sputter Deposited Metals--A Storage Medium for Radioactive Gases
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PNL-2879 UC-70 ENTRAPMENT OF KRYPTON IN SPUTTER DEPOSITED METALS--A STORAGE MEDIUM FOR RADIOACTIVE GASES G. L. Ti ngey E. D. McClanahan M. A. Bayne R. W. Moss Apri 1 1979 Prepared for the U.S. Department of ,Energy under Contract EY-76-C-06-1830 Pacific Northwest Laboratory Richland, Washington 99352 SUMMARY Sputter deposition of metals with a negative substrate bias results in a deposit containing relatively large concentrations of the sputtering gas. This phenomenon has been applied as a technique for storage of the radio active gas, 85 Kr , which is generated in nuclear fuels for power production. Alloys which sputter to yield an amorphous product have been shown to contain up to 12 atom% Kr [42 cm3 of Kr(STP)/g of deposit; concentration equivalent to a gas at 4380 psi pressure]. Release from these metals occurs at so low a rate that extrapolation to long times yields ~ 85 Kr release at 300°C of about 0.06% in 100 years. A preliminary evaluation of the engineering feasibility and economics of the sputtering process indicates that 85 Kr can be effectively trapped in a solid matrix with currently available techniques on a scale required for handling DOE generated waste or commercial reprocesses fuels and that the cost should not be a limiting factor. i i TABLE OF CONTENTS SUMMARY. ............ if INTRODUCTION .......... 1 GAS ENTRAPMENT DURING SPUTTERING 2 EXPERIMENTAL PROCEDURES ..... 6 SPUTTER DEPOSITION. ..... 6 KRYPTON RELEASE MEASUREMENTS 9 RESULTS AND DISCUSSION ..... 10 SPUTTER DEPOSITION OF CRYSTALLINE DEPOSITS 10 SPUTTER DEPOSITION OF AMORPHOUS DEPOSITS 11 GAS RELEASE MEASUREMENTS. ......... 11 ENGINEERING FEASIBILITY AND ECONOMIC ANALYSIS. 20 CONCLUSIONS. .. 25 ACKNOWLEDGMENTS. REFERENCES . 27 APPENDIX A-1 iii INTRODUCTION Several inert gas isotopes are generated by the fission of uranium in nuclear reactor fuels. Of these isotopes, many are non-radioactive or have very short half-lives and decay soon after the fuel is removed from the reactor. Krypton-85, thus, rema i ns the on ly inert gas isotope of concern for long-term storage of nuclear wastes. Krypton-85 emits a beta particle with a maximum energy of 0.66 MeV and a very low flux of 0.52 MeV gamma rays. It has a half life of 10.76 years. The chemical inertness of krypton reduces markedly the biological effects of 85 Kr and, therefore, release of small quantities to the atmosphere has been permitted. Federal regulations, (1) to take effect in 1983, limit the release of 85 Kr to 50,000 Ci/10 9 Watt years of electrical energy or about 15% of the total 85 Kr produced in light water reactor (LWR) fuels. To meet this requirement, studies ar'e underway to deve10p the most appr-opriate means to collect, separate, and store the 85 Kr generated in nuclear power plants. Several methods have been proposed for storage of 85 Kr including gas bottle storage, immobilization in the pores of molecular seives, dissolution in glass or other solids, ion implantation into metals, storage in gas pockets underground, and others. An evaluation of those processes presently receiving the most attention was recently completed by Knecht. (2) One of these techniques, ion implantation in metals, is currently under investigation in our laboratory and is the subject of this report. The purpose of this document is to describe the rationale for this approach and the results of our investigation to date. Also included in this report is a preliminary evaluation of the economics of this process for comparison with other potential 85 Kr storage methods. GAS ENTRAPMENT DURING SPUTTERING Gases can be implanted into solids by bombarding their surfaces with energetic ions. (3-6) For Kr+ the depth of implantation is energy dependent and amounts to only a few atom layers at energies in the keV range. Thus, to produce bulk amounts of a solid containing high concentrations of Kr, it is necessary to perform the implantation during film growth by a vapor deposition process such as sputtering. Many examples of gas incorporation by these techniques have appeared in the literature during the past few years. (7-9) Sputtering of solids appears to be particularly well suited for gas incorporation where the sputtering ion is generated from the gas which is to be implanted into the solid. Early researchers have shown that sputtering is a momentum transfer process(lO,ll) as illustrated in Figure 1. Positively charged ions are accelerated to the surface of the target and sufficient energy is transferred to overcome the binding energy of the atoms in their lattice. Sputtering yields, defined as the ratio of the number of ejected atoms to that of incident ions, are dependent upon the binding energy of the target atoms and the mass and energy of the bombarding ions. These yields may vary over a wide range from much less than one to as high as 15 for silver bombarded with 10 keV K/. (12) In order to achieve a practical deposition rate, a dense plasma adjacent to the target is requi red. In our system, thi sis achi eved by produc i ng a thermionically supported plasma in the annulus between the target and a substrate as shown in Figure 2. The positive ions are formed by collisions between energetic electrons and krypton gas atoms, and are accelerated to the target surface by applying a large negative voltage to it. The sputtered atoms leave the target with approximately a cosine spatial distribution and deposit uniformly on the substrate when the proper geometrical relationship between the target, substrate, and shields has been achieved. In most applications of sputter deposited metals, attempts are made to minimize the gas incorporated in the deposit to obtain the desirable properties. However, in our process, where the purpose of sputtering is to 2 ~ 0 0 E U1 .~ C n:l .c U <J.J :E ffDOO !.~ U1 n:l co z_<.::> -+ 0:::,, I-l..l...l :::.::::_ °0°0 I ::Jz a.. 0 I00- I (/') - 3 VACUUM CHAMBER (PRESSURE 0.6 Po) 01 ~ L Ir .---=-====----.:.:.:..--. 0-+---'-, I TARGET] TUNGSTEN L-~+ I g FlU)MENT FIIRGET ,--- d? POWER SUPPLY + ¢8 /'-0 r7 (}·3 k VDC Lu~- .-H-I~ :.. ~~~~ C(~ 0-1 1'---, ¢8 FlLlIMfAiT .p. POWER 5L1PPL V _----'I + 5UBSTRA TE. 0-10 V!JC S/j[)S7RI1TE POWEI{ 5IJPPL V 0-300 VDC +E j- ® SPUTlD"{[J) llTO/VI • KRYPTOA/ II TO/VI PU)SMA POWER SUPPLY KJ;YPTOAI ION o 0-50VDC (-j [LE(TROM FIGURE 2. Thermionically Supported Plasma Sputtering/Ion Implantation Components trap gas in the deposited material, we must adjust the parameters to obtain maximum gas content. It has been shown(8) that the gas content is increased markedly by applying a negative bias to the substrate, thus accelerating Kr+ to the substrate as well as to the target. Of course, the substrate voltage, relative to the target voltage, must be maintained at a ratio which will yield a net deposit on the substrate. Other sputtering parameters such as deposit composition and microstructure, electrode voltages and current densities, substrate temperature, and system geometry also affect the gas trapping rate and are adjusted to yield the desired result. Recently, Cuomo and Gambino(13) have reviewed the status of inert gas sputtering and correlated the gas trapping coefficient with a number of the sputtering parameters. In particular, Cuomo and Gambino showed a large increase in gas trapping when sputtering amorphous films. They reported achieving gas loadings as high as 13 atom% for Ar and Kr and up to 28 atom% for Ne in an amorphous alloy of nominal composition Gd16.5C068.5Mo15' These data and others in the literature clearly show the potential for trapping inert gases and it was recognized that this approach may yield an excellent means of long-term storage of gaseous nuclear fission products. This approach has distinct advantages in that there are no high tempera ture structural materials problems and all operations are conducted at low pressure (~l Pal when compared to other possible alternatives which require systems capable of operation at temperatures of 500°C or above and pressure up to 20 MPa. This program was, therefore, undertaken with the major objectives as follows: 1) Adapt the current sputtering technology to a high rate sputtering/ion implantation process. 2) Find a suitable solid matrix for trapping the gas. 3) Determine the potential for release of the incorporated Kr as a function of temperature. 4) Determine the limits of scale up and perform meaningful cost! benefit studies. 5) Evaluate any detrimental effects from radiation or Rb buildup in the sputtered product. 6) Test the process using 6% 85 Kr in Kr. 5 This program was initiated in July 1976, and is continuing to accomplish these objectives. EXPERIMENTAL PROCEDURES SPUTTER DEPOSITION The details of the sputtering process have been reported earlier. (14) However, a brief description is included here to aid the reader'in under standing the process. The sputtering apparatus designed for this project was of the therm ionically supported discharge type. The general arrangement is shown in Figure 3. The cylindrical substrate, fabricated from six-inch copper pipe, was used as part of the vacuum wall to facilitate water cooling. The hardware shown in Figure 3 was mounted on our standard sputtering base assembly(14) containing the electron source, power and cooling water connections, and the pumpout port. The centrally located water-cooled target was fabricated either by drilling a hole in a cylindrical bar of the selected metal or by using a pipe of sufficient wall thickness and strength to maintain the vacuum during the entire run. Some binary metal targets were prepared by drilling holes into the wall of a cylinder of the same composition as the major component of the desired metal deposit.