Radioactivity in cathode ray tubes

Transcription

1 Operational Topic A discussion of the natural radioactivity in CRT monitors and a method for evaluating them are presented so appropriate disposal decisions can be made. Radioactivity in cathode ray tubes Nancy P. Kirner,* G.L. Troyer, R.A. Jones and E.W. Gray, Jr. Abstract: While surveying used computer equipment out of a zone posted as a Contamination Area, 100% of the computer monitors surveyed had levels of radioactivity that were significantly above background. The radioactivity was primarily on the front face of the cathode ray tube and was not amenable to decontamination. Hot spots were found also along the edges and seals of the cathode ray tube. Similar surveys of computer monitors that were never in Contamination Areas confirmed that radioactivity was incorporated into the monitor. Surveys were made of recently manufactured television sets with similar results. Gamma spectroscopy indicates that the radioactivity is due to naturally occurring radioactive materials. Since most surveys of cathode ray tubes in the literature were made while the units were energized and indicated low-energy x-rays, the use of naturally occurring radioactive materials in the manufacture of cathode ray tubes has not been widely recognized. This paper presents the results of these surveys, the results of gamma spectroscopy, and a method for releasing existing computer equipment having naturally occurring radioactive materials. Health Phys. 86(Supplement 1):S20 S24; 2004 Key words: operational topic; cathode ray tube; computers; naturally occurring radionuclides * Kirner Consulting, Inc., Tacoma, WA; Hanford Group, Inc., Richland, WA. Disposal of CRTs that do not have the potential for contamination or activation are typically not considered to be mixed waste even though they may contain very low-levels of naturally occurring radioactive material. When such clean CRTs are no longer useful they are recycled or, if waste, they are managed as dangerous waste, not mixed waste. SCOPE This paper applies to potentially contaminated cathode ray tubes (CRTs) in computer monitors and other similar items that would normally be dispositioned without regard to their radioactivity. Although the paper focuses on survey criteria for uncontrolled release at a specific facility in Washington State, the methodology may be useful elsewhere. REQUIREMENTS Requirements for the release of material and equipment to uncontrolled areas can vary, but they generally include the following six criteria: 1. Material must be shown to meet specified surface contamination limits, fixed and removable, and contamination has been subjected to the as low as reasonably achievable (ALARA) process. These contamination limits are generalized in Table Materials shall be considered potentially contaminated if it has been used or stored in radiation areas that could contain unconfined radioactive Nancy Kirner has over 25 years of progressively responsible experience in health physics and regulatory processes, with specialized expertise in radioactive and hazardous waste management and environmental management systems. Kirner began her health physics career performing laboratory health physics for the University of Michigan. Since then she has been a state regulator with the state of Washington s Agreement State program and served as Corporate Health Physicist for a major environmental restoration firm before starting her own consulting firm. For the past 4 years, she has been assisting Hanford s 222-S Laboratory s Radiation Control organization. She holds a B.S. in biology from Tufts University and a M.S. in environmental health sciences (radiological health) from the University of Michigan and is certified in the comprehensive practice of health physics by the American Board of Health Physics. Her address is Kirner@harbornet.com material or if it has been exposed to beams of particles capable of causing activation. 3. Surfaces of potentially contaminated property shall be surveyed using instruments and techniques appropriate for detecting the limits stated in Table Inaccessible areas. Where potentially contaminated surfaces are not accessible, material may be released after caseby-case evaluation and documentation based on both the history of its use and available measurements demonstrating that the unsurveyable surfaces are likely to be within the limits given in Table Records. The records of released property shall include (a) A description or identification of the property; (b) The date of the last radiation survey; (c) The identity of the organization and the individual who performed the mnonitoring operation; (d) The type and identification number of monitoring instruments; (e) The results of the monitoring operation; and (f) The identity of the recipient of the release material. 6. Volume contamination. No guidance is currently given in DOE Order for release of material contamination in depth, such as activated material or smelted contaminated metals. However, site guidance allows disposal as hazardous, dangerous, or Toxic Substance Control Act (TSCA) S20 February 2004

2 The Radiation Safety Journal Vol. 86, suppl 1 February 2004 Table 1. Typical Surface Contamination Limits Nuclide Removable Total (Fixed Removable) U-nat, U-235, U-238, and associated decay products 1,000 5,000 Transuranics, Ra-226, Ra-228, Th-239, Th-228, Pa-231, I-125, I-129 Beta-gamma emitters (except predominately Sr-90 and others noted above) 1,000 5,000 waste if it can be shown that concentrations of the radiological component of the waste do not exceed specified values. At Hanford, the most restrictive of these limits is for Soil, Other and is set at 2 pci g -1. BACKGROUND Direct radiation surveys at the DOE-RL (Hanford) 222-S Laboratory Complex have confirmed that computer monitors contain radioactivity, regardless of whether the monitors have been in Contamination Areas or are new and never placed into service. A literature search found that the U.S. Environmental Protection Agency (EPA) recognizes that desktop computer display monitors contain both lead and radioactivity (EPA-744-R a; Sokoloff et al.2001). EPA additionally recognizes that the quantities of lead present in CRTs make their disposal as waste subject to regulation under the Resource Conservation and Recovery Act (RCRA). Similarly, Washington State Department of Ecology can regulate the disposal of computer monitors as a dangerous waste. As a matter of policy, both Ecology and EPA prefer that computer monitors undergo recycling rather than disposal. According to EPA-744-R a, a typical monitor contains 1.02 Bq (60 dpm or 27 pci) of radioactivity, identified as Pb If the mass of the glass of each monitor is considered (7.2 kg per unit), a calculated concentration of pci/g results. Operational Radiation Safety However, other authors suggest that the larger source of radioactivity in CRT is from uranium and thorium contaminants present in zirconium silicate (zircon sand), an additive in the manufacture of glass for televisions and computer monitors. According to one trade publication, the worldwide use of zirconium silicate for this application has grown from 70,000 tons per year in 1997 to 80,000 tons in 2001, and is expected to reach 100,000 tons per year in (IMI 2002). Hipkin & Shaw also report typical concentrations of thorium-232 and uranium-238 in zirconium silicate are 0.6 and 3 Bq/g, respectively (Hipkin & Shaw 1999). According to EPA- 744-R a, 54.3 g of zircon sand (zirconium silicate) is used in the manufacture of a typical CRT. Combining the Hipkin and EPA information, an estimated 32.6 Bq of thorium-232 and 163 Bq of uranium-238 per typical CRT can be calculated. Using the reported mass of 7.2 kg per monitor, the calculated radioactive concentration of uranium and thorium in the CRT glass would be on the order of 0.73 pci/g. Adding in the EPA s reported Pb-210 concentration of pci/g does not change this calculated value. These values can change depending on the or igin of the zircon sand, the mass of sand used in production, and the distribution of the zircon sand within the CRT. Based on the literature, therefore, CRTs would appear to meet the most stringent release criteria of 2 pci/g and be eligible for management as not-mixed waste. However, actual measurements appear to conflict with published values, as discussed below. A series of measurements were made on CRTs that never encountered loose radioactive material or activation, as shown in Table 2. These levels are sufficiently elevated that new, unused and uncontaminated monitors would not qualify for unconditional release from a Contamination Area under standard survey procedures for release of radioactive material. The observed levels are also elevated compared to the literature values cited above. MEASUREMENTS To provide additional information on the radioactivity in CRTs, three CRT computer monitors and a modern flat panel display underwent gamma energy analysis (GEA) using a four-detector fixed array of HPGe detectors. The shielding cave for this system was adequate for small to medium sized CRTs. The calibrated measurement geometries were scaled to the bulk CRTs using Microshield (Ver. 5) code. A diagram of the counting geometry is provided in Fig. 1. The results of these analyses are provided in Table 3. Most of the radioactivity is due to 40 K. Since radioactivity in these monitors was not added by DOE operations, the disposal of the monitors should be possible without regards to radioactivity. This paper sets forth a radiological survey protocol that will allow comparison of measurements from a single monitor with those taken from a population of monitors that never had the potential for exposure to loose radioactivity or activation. In that way, background radiation levels can be established for the glass of CRTs. Any measurable deviation from background levels would suggest that the monitor had ad- S21

3 N. P. Kirner et al. Radioactivity in CRTs Table 2. Surveys of CRTs using P-11 GM Probe. Small monitors 19 inches Monitor type Monitor size Max net cpm NEC Multisync 4FGe NEC Multisync 5FG NEC Multisync 5FGp Dell Trinitron IBM DTK Computer NEC Multisync XE IBM 14V Micron Dell Trinitron AST Vision 7B AST Vision 7B Dell Ultrascan Dell Compaq AST Vision 7B Large monitors 19 inches or larger Monitor size Max net cpm Dell Trinitron Dell Trinitron AST Vision 20B Dell Trinitron Dell Trinitron Dell Trinitron Dell Trinitron Dell Trinitron Dell Trinitron (never used) R a ambient background count rate. R b i Average n, (2) where: Average average of net background count rate for a group of CRTs; n Number of CRT surveyed in the group. Stdev Average R b 2, n 1 (3) where Stdev standard deviation of net background count rates of CRTs. DL 1 Stdev, (4) where DL decision level (the value where a single measurement differs from background measurements) at the 84% confidence level. ditional radioactivity that would make it ineligible for release. Although sample size was limited, suggesting the use of Student s t-test, using a normal distribution provides more conservative results. A one-tailed decision level (DL) at 1 sigma can then be calculated. At a count rate of R b 1 standard deviation, there is an 84% chance that any measurement above this level is truly elevated. Similarly, there remains a 16% chance of unnecessarily disposing of CRTs as mixed waste when they are not truly mixed waste. This confidence level meets the minimum site criteria of 67% confidence level, since CRTs that have not shown contamination on their exterior plastic cases are not likely to be contaminated on the exterior of their impervious glass. If the minimum site criterion of 67% confidence level were chosen (i.e., R b 0.44 standard deviation), the cost of disposal would be unnecessarily high, as there would be a 33% chance of falsely designating mixed waste. If the 95% confidence level were chosen (R b sigma), the cost of disposal would be less, but there would be a greater chance of inadvertently releasing a potentially contaminated monitor. Thus, as a matter of maintaining radiation exposures as low as reasonably achievable (ALARA), the 84% confidence level was chosen (R b 1 sigma). The following equations are used: R b R max R a, (1) where: R b net background count rate for a single CRT surveyed; R max maximum count rate measured for a single CRT surveyed; RESULTS AND DISCUSSION Using hand-held health physics instrumentation (Eberline Model E-140 with P-11 probe and PAM), surveys were made of the entire exposed glass surface of several computer monitors (CRTs) of varying manufacturers and sizes, as shown in Table 2. Background for the glass of the CRT was established separately from background for the other surfaces of the computer monitor. This was accomplished by surveying computer monitors that had never entered a Contamination Area at Analytical Services. No alpha counts were identified. The survey technique for the beta-gamma survey consisted of scanning the CRT at no more than 2 inches per second and taking 20-s static count rates over areas of increased count rates. By recording the highest measure- S22 February 2004

4 The Radiation Safety Journal Vol. 86, suppl 1 February 2004 Analytical Services shows an average efficiency of 20% for 137 Cs, 30% for 90 Sr( 90 Y), and 14% for 99 Tc, for an overall average of 21%. Using the average efficiency of 21% and 6 probe areas per 100 cm 2, the limiting value becomes Contamination level Figure 1. Diagram of detector geometry. Table 3. Summarized radioactivity from direct gamma energy measurement, pci/g 2 %. 226 Monitor (type) Ra (U) 224 Ra (Th) 235 U 40 K Mitsuba (B/W) % % Wyse (color).90 24% % % 20 20% Mag Informix (color) % % % 27 20% Dell Flat Panel %.13 Banana % ment from a 100% survey of the front face of each computer monitor, the average and standard deviation of the population of CRTs can be calculated. The results for the beta-gamma survey appeared to separate into large and small groups: CRTs less than 19 inches and those 19 inches or larger. Therefore, separate criteria for large and small CRTs were developed, as shown in Table 4. Applying the results of this statistical evaluation of background for CRTs, a CRT would be considered to be indistinguishable from background if it did not exceed ambient background by a total of 128 cpm per probe area for a small monitor. Similarly, the DL for a large monitor would be 166 cpm per probe area. There is the remote possibility that a computer monitor does not include naturally occurring radioactivity. If these monitors were not contaminated and were Table 4. Results of statistical analysis. Monitor size Average (net cpm) surveyed, they would be below the limiting values noted above and would be unconditionally released. However, if the unusual (non-radioactive) monitor were surveyed under this protocol and showed radioactivity only on the face of the monitor, such radioactivity could be misinterpreted as being part of the naturally occurring background of the monitor. This is not a problem because the calibration efficiency of GM probes is higher than the assumed 10% efficiency used in routine surveys. A default efficiency of 10% is used because of the differences in laboratory vs. routine conditions. Because surveys of the front glass surfaces of computer monitors have minimal self-absorption or interference, the GM instrument can perform at laboratory conditions for these surveys. Therefore, the actual, not default, calibration efficiency can be used. A random review of three pancake probes at Stdev (net cpm) DL (net cpm) Maximum releasable net cpm Small ( 17 inches) Large ( 17 inches) Operational Radiation Safety 166 cpm 6 probe areas probe area 100 cm ,671 dpm/100 cm 2. Thus, a limiting value of 166 is likely to correspond to less than 5,000 dpm/100 cm 2 for the radionuclide mixtures in use at the laboratory, even in the rare circumstance when the computer monitor has no inherent radioactivity. These values apply only to the counting system and technique used for determining the CRT background. A separate analysis would be needed if different instrument models or survey techniques were used. SUMMARY By statistically determining a separate background for the glass of CRTs, a CRT would be considered to be indistinguishable from background if it did not exceed ambient background by 128 cpm per probe area for a small monitor ( 19 inches) or 166 cpm per probe area for a large monitor ( 19 inches). Under unusual conditions where there is no naturally occurring radioactivity in the monitors, these levels are sufficient to maintain compliance with releave criteria. In order to limit the confounding effects of naturally occurring radioactive materials incorporated in CRTs in the future, every computer monitor going into a Contamination Area should be surveyed and records maintained of its individual background. When the monitor is eventually removed from the Contamina- S23

5 N. P. Kirner et al. Radioactivity in CRTs tion Area, comparison should be made to the background of the individual monitor, not the default values determined in this position paper. REFERENCES Hipkin J, Shaw PV. Working with ores containing naturally occurring radioactive materials. 3rd European ALARA Network Workshop, November 15 18, Munich, Germany: BFS Facilities; Industrial Minerals Information. Zircon A review of global markets and supply. Available from Industrial Minerals Information, Park House, Park Terrace, Worcester Park, Surrey, KT4 7HY, United Kingdom; Socolof, M. L., Overly, J. G., Kincaid, L. E., Geibig, J. R., Desktop Computer Displays: A Life-Cycle Assessment, Volume 1, University of Tennessee for U. S. Environmental Protection Agency, Design for the Environment Branch, Washington, DC; EPA-744-R a.. U.S. Nuclear Regulatory Commission. Minimum detectable concentrations with typical radiation survey instruments for various contaminants and field conditions. Washington, DC: U.S. NRC; NUREG/CR-1507; S24 February 2004