1 The Contemporary Materials Cycle for Radioactive Cesium-137 in the United States “Restricted copy. Under publication review as of Oct.8,2004. Do not quote, cite or duplicate” T. Okumura and T.E. Graedel Center for Industrial Ecology Yale University New Haven, CT 06511 ABSTRACT The materials cycle for 137 Cs, a low-level radioactive material of interest from a security perspective (as a possible source for “dirty bombs”) as well as for its extensive industrial and medical uses, has been characterized for the United States for the year 2000. The focus is on products utilizing the isotope rather than on isotope production and subsequent disposal as a result of nuclear power generation. The results indicate that, during 2000, of the 1.5 PBq of 137 Cs that entered use, 94% was contained in sources in imported devices; the amounts in domestic source material recycling (4%) or as imported source materials (2%) were trivial by comparison. Losses from use were about 0.5 PBq; of this amount 86% was by radioactive decay, 11% was active source material that was recovered and recycled, and 3% was source material sent to low-level disposal sites. The current stock of 137 Cs in use is 20 PBq; this stock is currently growing by more than 1 PBq per year (the difference between inputs to and losses from use). As a result, the security challenge related to monitoring stock in use is increasing by around 5% per year. Key words: Material flow analysis, dirty bombs
Transcript
1
The Contemporary Materials Cycle
for Radioactive Cesium-137 in the United States
“Restricted copy. Under publication review as of Oct.8,2004. Do not quote, cite or
duplicate”
T. Okumura and T.E. Graedel
Center for Industrial Ecology
Yale University
New Haven, CT 06511
ABSTRACT
The materials cycle for 137
Cs, a low-level radioactive material of interest from a security
perspective (as a possible source for “dirty bombs”) as well as for its extensive industrial
and medical uses, has been characterized for the United States for the year 2000. The focus
is on products utilizing the isotope rather than on isotope production and subsequent
disposal as a result of nuclear power generation. The results indicate that, during 2000, of
the 1.5 PBq of 137
Cs that entered use, 94% was contained in sources in imported devices;
the amounts in domestic source material recycling (4%) or as imported source materials
(2%) were trivial by comparison. Losses from use were about 0.5 PBq; of this amount 86%
was by radioactive decay, 11% was active source material that was recovered and recycled,
and 3% was source material sent to low-level disposal sites. The current stock of 137
Cs in
use is 20 PBq; this stock is currently growing by more than 1 PBq per year (the difference
between inputs to and losses from use). As a result, the security challenge related to
monitoring stock in use is increasing by around 5% per year.
Key words: Material flow analysis, dirty bombs
2
INTRODUCTION
“Restricted copy. Under publication review as of Oct.8,2004. Do not quote, cite or
duplicate”
Incidents involving commercial radioactive sources that have escaped from monitoring
and control have been reported for many years. In some cases, sources entering scrap
processing facilities have contaminated steel mills, resulting in millions of dollars in
cleanup costs (Lubenau and Yusko, 1995). In other cases, sources have been breached,
exposing the public to harmful radiation. In 1987, for example, a sealed source containing
cesium-137 was taken from abandoned medical equipment by scrap metal collectors in
Brazil. The sealed capsule was opened and people played with it, rubbing it on their bodies
as “carnival glitter.” This resulted in four deaths, one arm amputation, and 50
hospitalizations or placements in a temporary dispensary under medical care (Petterson,
1988).
In the wake of the terrorist attack on the United States on 11 September 2001, there is
a further security concern over radioactive materials. Traditionally, most commercial
radioactive materials have been managed less stringently than is uranium or plutonium
because these materials cannot be sources of nuclear weapons. However, the terrorist
attack raised the concern that commercial radioactive materials could cause social disorder
through their use in terrorist weapons such as “dirty bombs” (Solomon, 2003). The U.S.
government and the International Atomic Energy Agency (IAEA) had previously shown
interest in a more stringent management system for commercial radioisotope materials, and
their plans were accelerated by the terrorist attack. Among commercial radioactive
materials, cesium-137 is one of those generating substantial concern because of its beta and
gamma emissions (gamma emission through its transitional daughter isotope barium-137)
and relatively long half-life (Ferguson, et al. 2003).
3
Cesium-137 is a reactor-produced material, and its uses are based on its radioactivity.
They include industrial irradiation, several types of monitoring and calibration gauges, and
cancer treatment. 137
Cs disappears naturally by radioactive decay, with a half-life of 30.17
years (Chase and Rabinowitz, 1967). Thus, its annual decay rate is about 2.3%. Half of
any stock of 137
Cs decays in 30 years, and by 200 years only 1% of the original amount
remains radioactive.
Fabricators of devices that employ 137
Cs typically seal the radioisotope in a lead
capsule, which is then incorporated into the final product. Because of the relatively long
half-life, sources rarely need to be replaced. The rate of decay is rapid enough, however,
that recovery and recycling of the material may or may not be practical. Legally, devices
are supposed to be return to the device manufacturer for disposal in an approved radioactive
material disposal site, but for various reasons some are not recovered and recycled.
Abandoned devices, of course, pose a potential risk of exposure.
Although concerns regarding low-level radioactive material are increasingly discussed,
little is known about their entire life cycle and material flow. Portions of the necessary
information are owned by parties concerned with different product life stages –
governmental agencies, manufacturers, waste management site operators – and these parties
have different interests. Each has good knowledge of its own sector, but not of the others.
The present study addresses a portion of this challenge by integrating the available
information and characterizing the comprehensive material flow of cesium-137 in the
United States in the year 2000.
METHODS OF CHARACTERIZATION
The general framework used to characterize the cycle of 137
Cs is shown in Fig. 1.,
following the framework used for cycles of the industrial methods (Graedel et al., 2002).
The cycle has four principal reservoirs: production, fabrication & manufacturing, use, and
waste management. Losses to the atmosphere or geosphere may occur at each stage. For all
but global cycles, 137
Cs may be imported or exported at any stage. The interval of our
analysis is one year, with year 2000 being selected as the focus.
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Data were gathered from a variety of government, industry, and literature sources.
The numbers on Fig. 1 refer to
specific stocks (reservoir contents of 137
Cs) or annual flows of 137
Cs in the United
States in the year 2000.
1. Production
1.1. Commercial U.S. production
Cesium-137 has not been commercially produced in the U.S. in recent years.
Historically, during the 1950s and 60s the facilities of Department of Energy (DOE) at
Hanford, Oak Ridge National Laboratory, and Savannah River produced cesium-137.
More than 6000 cesium-137 devices with an activity of 1.816 EBq were stored at DOE sites
as of January 1999 (Hamlin, 2002) and could be used commercially if desired. As will be
seen, this amount is very large compared to that used annually in the United States. Since
there was no commercial production of cesium-137 in the U.S. in 2000, Flows 1, 3 and 8 on
Fig. 1 are zero.
1.2. Cesium-137 in waste from commercial power plants
Cesium-137 is produced by commercial power reactors as a residue of the nuclear
fission of uranium. The material generated under these circumstances is part of the mixture
of nuclear wastes in spent fuel, and does not go into industrial circulation. The amount that
is generated can be roughly estimated. To do so, reactor capacity data are taken from
Information Digest 2001 Edition (USNRC, 2001, Appendix A). For the 104 reactors in
nuclear power plants in the U.S., the maximum dependable capacity (MDC) is multiplied
by the capacity factor of each reactor to determine the actual production capacity in the
year 2000. The result is 86,080MW. From that figure, the amount of cesium-137 that is
generated each year can be calculated, based on the assumption that reactors annually
produce 9 g of cesium-135 per 1 MWt capacity (Garwin and Charpak 2001), and that the
yields of cesium-135 and cesium-137 that is generated each year are about 7% and 6%
respectively in terms of number of atoms (INEEL 2001). 9 g of cesium-135 equals 0.067
mol (cesium-135: 135 g∙mol-1
). 0.067 mol is 7% of the yield; thus, the amount of cesium-
137, which is 6% of the yield, is 0.057 mol or 7.8 g.
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Therefore, 7.8 g of cesium-137 is produced per 1 MW of reactor capacity. As a
result, 671 kg of cesium-137 is estimated to have been produced in commercial power
plants in the U.S. in 2000. This is equivalent to approximately 2.186 EBq because the
specific activity of cesium-137 is 3300 GBq∙g-1
. This amount, Flow 6 on Fig. 1, is
admittedly a rough an estimate, but it is sufficient to determine the order of magnitude of 137
Cs production.
1.3. Cesium-137 in waste from research reactors
Cesium-137 is also contained in waste material from nuclear fission conducted in
research reactors. The 36 university research reactors in the U.S. are listed in Information
Digest 2001 Edition (USNRC, 2001, Appendix E). In addition, there are 16 reactors
licensed by DOE (of which 15 are owned by DOE and one by DOD), and one reactor
owned and licensed by DOD. Thus, 53 research reactors were in operation in the year
2000. The operation data from IAEA (2003) is used as the primary basis for estimating
cesium-137 production. For the reactors for which “MW Days per Year” is not reported,
“MW year” is calculated by multiplying reactor capacity by a utilization ratio, which is the
percentage of time the reactor is in operation. The result is that a total of 340 MW was the
approximate capacity of research reactors in 2000. Using the same conversion rates as
commercial reactors, 7.8 g of cesium-137 per 1 MW and 3300 GBq∙g-1
, about 2.7 kg or 8.6
PBq of cesium-137 is estimated to have been produced by research reactors; this is
generation Flow 2 on Fig. 1. Since none of this cesium-137 is used commercially, it all
goes to waste management as Flow 4, and Flow 3 is consequently zero.
1.4. Cesium-137 in waste from nuclear powered ships/submarines
The U.S. Navy operated 84 nuclear-powered ships and submarines in 2000 (Sharpe, 2001),
and the reactors in these vessels also produce cesium-137 as a fission waste product. Since
the size of reactors is given by Sharpe (2001) in thermal output rather than power, those
figures have been converted to thermal output using a thermal efficiency of 32.1%
(USDOE, 2002).
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The total production capacity of reactors used in nuclear-powered vessels
is thus estimated as 12,400 MW, and thus about 97,000 g or 316 PBq of cesium-137
is estimated to have been produced by nuclear-powered vessels, as represented by Flow 5
in Fig. 1.
1.5. Other sources
Cesium-137 can also be introduced into the environment by nuclear weapons. The
most recent atmospheric test occurred in 1980 in China, and the most recent underground
tests occurred in 1998 in India and Pakistan (NRDC 2002a, b). No nuclear weapons were
tested in 2000, either in the U.S. or elsewhere in the world, nor was any cesium-137
produced from this source.
Cesium-137 can be released accidentally, such as in the Chernobyl case in the Ukraine in
1986. However, there was no accident involving a release of cesium-137 in 2000, and
Flow 7 in Fig. 1 is thus set to zero.
1.6. Summary of Production
The current production of cesium-137 in the U.S. is only in the form of mixed waste
material. The production estimates given here for cesium-137 from commercial power
plants, research reactors, and nuclear powered ships/submarines are very crude. They are
based solely on reactor size, and there are no independent data available for verification.
Even from these crude estimates it is clear that the amount of cesium-137 produced by
these facilities is more than a thousand times that of commercial cesium-137 flows, as will
be shown below. However, the mixed waste materials go directly to waste management
sites and are not a concern from the perspective of monitoring and control of in-use
material.
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2. Fabrication and manufacture
2.1. Material input and Source fabrication
Cesium-137 is imported into the United States entirely from Russia, although several
other countries have modest capacity to produce commercial amounts of the isotope
(Ferguson et al., 2003). This imported material is then fabricated into “sources” of various
sizes, largely by AEA Technology of Massachusetts and Isotope Products Laboratory of
California. Interviews with these manufacturers provide the amount of cesium-137
contained in devices sold by them: about 37 TBq. Thus, Flow 9 of Fig. 1 is 37 TBq 137
Cs,
as is Flow 10, the sum of the flows from U.S. fabrication of 137
Cs sources to U.S. device
fabrication.
2.2. Device fabrication
Devices containing cesium-137 may either be manufactured in the U.S. from
imported radioactive material, or the devices may be manufactured elsewhere and imported
as finished products. Since neither production nor sales data for cesium-137 devices are
available, the number of devices and the amount of cesium-137 employed must be
estimated from the current use trend. First, the inventory – the amount of cesium-137
which has been distributed and is currently in use – is examined and then the annual input
of devices is estimated from the inventory data and the life cycles of 137
Cs- containing
products.
The inventory data are taken from USNRC (2000). In the following sections,
cesium-137 inventories are examined by use. Unless otherwise indicated, estimates of
typical activity and number of devices are from USNRC (2000) and inventories are
calculated by multiplying the typical activity of cesium-137 in a device by the number of
devices. The results are shown in the Table 1 column “Total inventory”.
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2.2.1. Irradiators
An irradiator using cesium-137 is of the self-shielded type, relatively small in
volume, and used to irradiate blood, cosmetics, and other products, or to calibrate radiation
measuring devices. The typical source strength of a cesium-137 irradiator is 74 TBq with a
maximum strength of 92.5 TBq. The estimated number of such devices in the U.S. in 2000
is 300 (USNRC, 2000). The total 137
Cs inventory in irradiators is therefore calculated to be
22.2 PBq.
2.2.2. Fixed gauge small calibrators
Fixed gauges are primarily used for the purpose of quality control in manufacturing
processes. They measure, for example, the thickness of paper, the density of coal, the level
of materials in vessels and tanks, and volumetric flow rates. The source strengths of
cesium-137 for these devices ranges from 370 KBq to 4.1 TBq. The NRC estimates the
number of units of this type to be 19,000 under general license, with unit activity of about
6.3 GBq, and 9,500 under specific license, with unit activity of about 30 GBq. Thus, the
total inventories are calculated as 120 TBq and 320 TBq under general and specific
licenses, respectively, or 440 TBq in total.
2.2.3. Portable gauges
Portable gauges are primarily used to measure the density or other properties of soil,
concrete, and other materials in a field setting. Some special types are also used in
fluoroscopes for nondestructive inspection. The number of these units using cesium-137 is
estimated to be 19,000 under general license and 9,500 under specific license. Unit activity
is estimated as 6.29 GBq for both licenses. Additionally, some neutron source units
employ both cesium-137 and americium-241. The number of these units is estimated to be
14,000, with cesium-137 unit activity estimated as 0.3 GBq (USNRC, 2000).
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The total inventories of cesium-137 for these several uses are calculated as 120
TBq, 60 TBq, and 4.1 TBq for general license units, specific license units, and neutron
source units, respectively, and as 184 TBq in total.
2.2.4. Brachytherapy
Brachytherapy is a treatment for cancer in which a radioactive source is placed close
to a tumor in the body of the patient. The treatment using cesium-137 requires
emplacement of the source, followed by removal after the treatment period, which usually
lasts from 48 to 120 hours. The cesium is typically contained in a 2.5 mm-wide stainless
steel capsule containing beads of cesium-137. Brachytherapy is done either manually or by
automation. Manual brachytherapy (manual afterloading) typically employes 20 GBq for a
single treatment and sources comprising up to 150 GBq may be stored at any one time.
Automated brachytherapy (low dose rate remote afterloading) typically employs 4 GBq for
a single treatment and a facility typically stores sources comprising 300 GBq. There are
approximately 500 licensees of manual after loading and 130 licensees of low dose rate
remote afterloading (USNRC, 2000). The total inventory is calculated using the amount in
storage. Assuming all licensees store the maximum amount of cesium-137, the inventory
of cesium-137 in manual brachytherapy is calculated as 74 TBq and that in automated
brachytherapy as 34 TBq. The total inventory for brachytherapy use is 108 TBq.
2.2.5. Well logging
Cesium 137 is used to collect geophysical information in well drilling operations.
The data include porosity, hydrogen content, and bulk density, which are used to determine
the potential availability of oil and natural gas. Cesium-137 used in this application is
doubly encapsulated, with typical source strength of 60 GBq. The estimated number of
sources in use is 300 (USNRC, 2000). The total inventory is thus calculated as 18 TBq.
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2.2.6. Other uses
There are other minor uses of cesium-137, each of which accounts for less than 0.1
% of total cesium-137 usage; they are briefly described below.
Cesium-137 is used as a calibration source for liquid scintillation counters and
other loose calibration devices. The typical source strength for liquid scintillation counter
calibration is 1.5 MBq and the number of units is 12,000. The typical source strength for
loose calibration devices is 3.7 MBq and the number of units is 6,000 (USNRC, 2000).
The total inventory is calculated as 18 GBq and 20 GBq, respectively.
Bulk material analyzers use radioactive materials to analyze the composition of
various materials. Most employ californium-252, but some use cesium-137 with a typical
activity of 10 GBq. The number of units using cesium-137 is not known but is probably
significantly less than 100, which is the number of units employing californium-252.
Depleted uranium collimators also use cesium-137. The typical activity of cesium-
137 used is 10 GBq; the number of units is not known, but is thought to be very small
(USNRC, 2000).
Teletherapy is a treatment of cancer using an external beam of ionizing radiation.
This use requires a large source; for example, a cobalt-60 teletherapy unit uses 200-450
TBq on average, depending on the type of unit. Cesium-137 is a possible source of
teletherapy units, but in the U.S. cobalt-60 is the predominant source (USNRC, 2000).
2.2.7. Summary of device fabrication
The total stock of 137
Cs in devices in use in the U.S. in 2000 is calculated by adding the
inventories for the individual uses, as described above. The information, collected in Table
1, demonstrates that the stock in use is about 20 PBq.
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The most stringent annual occupational intake limit on adult oral ingestion of 137
Cs is
40 MBq. Among cesium-137 uses on Table 1 the unit activity of each of four categories -
electron tubes, ionizing radiation measurement instruments, liquid scintillation counter
calibration, and loose calibration sources is typically less than 40 MBq. Devices employed
in these uses therefore pose only minimal security risk associated with terrorism (US Code
of Federal Regulations, 2003).
The recommended lifetime of a typical cesium-137 sealed source for industrial use is
about 15 years (Nuclear Technology Products 2003). The activity of a 15 year old source is
about 70% of the original. Assuming that users replace devices every 15 years, one
fifteenth of the devices in inventory is replaced every year, by replacing either the devices
themselves or the sealed sources in them. This leads to the estimation that new devices
containing about 1.5 PBq of cesium-137 are entering inventory annually, as shown in Table
1. This figure is Flow 13 on Fig.1.
3. Use
3.1. Flow into in-use stock
Flow into in-use stock is estimated as the difference between input to and output from
the product use stage. The input is 1.5 PBq from Flow 13, and the output is 74 TBq into
Flows 17-19, details of which are discussed later. Therefore, the flow of stock (Flow 14) is
about 1.4 PBq and the direction of flow is towards increasing the in-use stock reservoir
contents.
3.2. Orphaned sources
Some of the devices containing cesium-137 are improperly managed, and thus can be
lost or stolen (Solomon, 2003; USNRC, 2004). Most are found or recovered later, but
some are not. Those materials not found or recovered become “orphaned sources”. The
data for orphaned sources are gathered from the Nuclear Material Events Database
(NMED), owned by NRC and maintained by INEEL.
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From the NMED database, the cesium-137 events under “Lost not found”, “Stolen
not found”, “Material found” were examined. The first two categories represent materials
lost or stolen that have become orphaned. The last category represents material that was
supposedly lost or stolen from somewhere unidentifiable, and where those responsible are
not aware of the loss or theft. The information from the NMED database for 1999-2002, is
shown in Table 2.
In 2000, a net flow of about 31 GBq of cesium-137 in total were reported under
these categories. This is Flow 15 in Fig. 1. It should be noted that this figure represents
only reported events. There may be additional orphaned sources that are not reported, or
whose absence is not realized by the device licensees.
3.3. Radioactive decay
As mentioned earlier, the strength of cesium-137 sources diminishes as a result of
radioactive decay. Based on the 2.3% annual decay rate, the estimated annual consumption
of 1.5 PBq, and a source lifetime of 15 years, the current inventory and the amount of
radioactive decay is calculated in Table 3. About 20.0 PBq of cesium-137 are estimated to
have been active in 2000, and approximately 440 TBq are estimated to have been
diminished by radioactive decay in that year. This quantity is Flow 16 in Fig. 1.
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4. Waste management
4.1. Recycling
Some source/device manufacturers recycle the sources in used devices. However, the
possibility of recycling depends on the demand for source materials. For example, gauge
manufacturers did not recycle cesium-137 sources due to unfavorable market conditions in
2002 (Product Stewardship Institute 2003).
There are three principal U.S. manufacturers of cesium-137 irradiators: CIS, Inc,
MDS Nordion, and J.L. Shepherd and Associates. As of 2001, MDS Nordion and J.L.
Shephard would accept a spent irradiator for a fee. MDS Nordion only accepts irradiators
of its own manufacture; J.L. Shephard accepts its own and competitors’ units. The fee the
manufacturers charge their customers for accepting used devices is considerably lower than
that charged to the manufacturers for disposal of these devices at a waste disposal site
(Kirk, 2001). Therefore, the manufacturers are obliged to recycle used devices in some
way in order to avoid losing money in the transaction. In the absence of actual data for
irradiator recycling, the amount is estimated here by assuming that one irradiator of typical
strength is recycled per year, since recycling of large sources such as those found in
irradiators happens occasionally due to market conditions. (Note that, at steady state, an in-
use stock of 300 irradiators (NRC, 409-464) with a 15-year life would produce a discard
flow of about 20 per year; we are thus assuming only about a 5% recycling rate based on
discussions with manufacturers.) After 15 years of use, a typical irradiator source of 74
TBq has become approximately 53 TBq by radioactive decay. Therefore, 53 TBq of
cesium-137 is assumed to be recycled from irradiators.
As for the recycling of 137
Cs in gauges, the Product Stewardship Institute (PSI)
reports that two major manufactures of fixed gauges say that 200-350 gauges are returned
to each of them annually. This means that they receive a total of perhaps 550 gauges – of
which those containing cesium-137 comprise a large share. One manufacturer says that
65% of returned items were recycled, and, again, gauges containing cesium-137 are more
likely to be recycled. However, recyclable material may go to waste management sites if
the market situation is not good (Product Stewardship Institute 2003).
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Since detailed data about these returned gauges – such as the number of gauges
containing cesium-137, or the type of licenses – are lacking, the flow is estimated as
follows. First, it is assumed that half of the 550 returned gauges contain cesium-137, and
the activity of each gauge is a weighted average of typical strength under general and
specific licenses. We know that the typical source strengths of gauges under general and
specific licenses are 6.3 GBq and 30 GBq respectively, and the numbers of devices are
19,000 and 9,500. Therefore, the weighted average becomes 15 GBq, which will decay to
about 11 GBq after 15 years of use. Because 550 gauges are returned, 5.4 TBq of cesium-
137 is estimated to be contained in them. Of that amount, 65% (3.5 TBq) is assumed to be
recycled and the rest (1.9 TBq) is assumed to go to waste management sites for disposal.
Although the recycling option may be available for other uses, other recycling
practices are not assessed here because data about them is unavailable. Since irradiators
and fixed gauges comprise more than 98% of total cesium-137 usage, characterizing the
recycling of these two uses is a reasonable representation of the flow of 137
Cs recycling.
Therefore, it is assumed that 58 TBq of cesium-137 is returned to manufacturers (Flow 17),
of which 56 TBq is recycled (Flow 12) and 2 TBq goes to waste management sites (Flow
20).
4.2. Disposal
As mentioned previously, high-level wastes are temporarily stored at their production sites
or at three designated storage sites Barnwell (state), Hanford, WA, and Clive (state). About
2.51 EBq of cesium-137 produced by commercial power reactors, research reactors, and
nuclear-powered vessels are contained in high-level wastes and are currently in temporary
storage. For low-level wastes, site operators indicate that the amount of cesium-137
received in the year 2000 at the Barnwell site was 16.5 TBq, and at the Hanford site, 570
GBq. The yearly data for 1998 to 2002 are shown in Table 4. The amount of cesium-137
brought to the Clive site should be negligible, since that site only accepts Class A waste:
i.e., large-volume, bulky, or containerized soil or debris, such as radiologically
contaminated paper, piping, rocks, or slag, or personal protective equipment (Envirocare of
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Spent radioactive sources in devices are not included among the items accepted at
this site. Therefore, the flow to waste management sites: is estimated to be the total of
wastes accepted by the Barnwell and Hanford sites, 17.1 TBq in 2000, this is Flow 22 on
Fig. 1. We note that this amount is far less than that entering the U.S. market. There is no
information that any of this material is in temporary storage, so Flow 23 on Fig. 1 is set to
0. Additionally, since flows 22 and 20 are now quantified, they establish Flow 18 at 15.1
TBq.
4.3. Cesium-137 at scrap metal facilities
Some cesium-137 is found in scrap metal when radioactive devices are improperly
disposed of and brought to scrap metal facilities. In the worst case, metal capsules of
sealed sources may be opened at scrap metal facilities, and the leaking cesium-137 may
cause severe contamination. In the NMED database, there were five cases with which
cesium-137 sources were found in metal scrap factories and their total activity was 0.3 GBq
(INEEL), which is expressed as Flow 19 in Fig. 1. If cesium-137 is smelted with other
scrap metals, it volatilizes and most of it ends up in the furnace dust (Lubenau and Yusko,
1995). According to Lubenau and Yusko (1998), information from the scrap metal
industry, and the NMED database, 17 melting accidents involving cesium-137 occurred
between 1984 and 2001 in the U.S. In these accidents where the activity is known, the
average activity of cesium-137 per accident is 13 GBq, and the maximum 56 Gbq. No
accident was reported for 2000 (Lubenau and Yusko 1998, INEEL). Therefore, all sources
found in metal scrap factories in 2000 were assumed to go to proper waste management
sites. Flow 21 in Fig. 1 is thereby determined to be 0.3 GBq and Flow 24 to be zero.
5. Closing the Cesium-137 Cycle
The quantification of flows described above permits the U.S. cycle of cesium-137 to be
closed by difference calculations, of which the first is for the amount of 137
Cs in imported
devices, Flow 11 in Fig. 1.
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The difference between the amount of source fabricated and the amount of source
used is accounted for by sources imported and/or exported. Since devices containing 1.5
PBq are estimated to enter use while only 93 TBq of sources are estimated to be
manufactured or recycled in the U.S., Flow 11 in Fig. 1 is calculated to be 1.4 PBq of net
import.
How accurate is this estimate? One clue comes from the rate of installation of
irradiators, shown above to be the largest in-use reservoir of 137
Cs by far. An employee
from a company manufacturing irradiators (whom we are not permitted to identify)
estimates that, on average, six irradiators, each with 260 TBq of cesium-137, are installed
in the U.S. every year (Hamlin, 2002). This means that 1.6 PBq is entering the inventory
annually, which is within 5% of the estimate of 1.5 PBq calculated on Table 1. (Note,
however, that the irradiator source strength used in this estimate, 260 TBq, is more than
three times bigger than that used in the NRC’s estimate, 70 TBq. This difference should be
further assessed if additional data become available).
The second flow computed here is Flow 14, the addition of cesium-137 to in-use stock.
That amount is the difference between the amount entering use in 2000, 1.5 PBq (Flow 13),
and the exiting use (74 TBq, the sum of flows 17-19). Thus, Flow 14 in Fig.1 is 1.4 PBq.
6. The Completed Cycle
With all stocks and flows on Fig. 1 now quantified, the components of the cycle can be
examined. The first of those, the Production stage, is shown in Fig. 2. This figure makes it
clear that although large amounts of 137
Cs are produced in nuclear reactors in the U.S., the
material is entirely contained in mixed waste and all sent to waste management. For 137
Cs
in the U.S. in 2000, there was no connection between this stage of the cycle and any of the
other stages.
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The fabrication and manufacturing stage is shown in Fig. 3. A small amount of source
material is imported into this stage, but it is dwarfed by the importation of 137
Cs in
fabricated devices, all or most from Russia. These two flows, plus a small flow of recycled 137
Cs combine to form Flow 13, the flow into the use stage.
Figure 4 presents the use stage of the US 137
Cs cycle. The flow of material into
products is some twenty times that discarded, the difference being a large new flow into in-
use stocks. Stock loss by radioactive decay is six or seven times that loss from stock to
discard.
The waste management stage is shown in Fig. 5. The best characterized flow is the
input to low-level waste repositories (Flow 22). Several of the other flows are estimated on
the basis of typical device stock and lifetimes.
The complete 137
Cs cycle is constructed by combining the information on Figs. 2-5;
this cycle, for the US in year 2000, is given in Fig. 6.
7. Discussion
Cesium-137 is unique in that it is totally anthropogenic. Unlike some other
radioisotopes, cesium-137 is not produced from a non-radioactive parent isotope, but from
uranium. Its production results from any incidence of nuclear fission, and the current
production magnitude is large compared to the amount that is commercially used.
However, the cesium-137 that accounts for this difference in the U.S. goes directly to waste
management sites.
Recycling is a potentially important component of the cesium-137 cycle, but the
amount of cesium-137 recycled is estimated to be small (56 TBq) relative to the amounts
used (1.5 PBq).
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Only 3.7% of the cesium-137 used in the U.S. comes from recycled sources. Either
major recycling flows are not being captured in the data, or major opportunities exist for
enhanced recycling, or both.
In Fig. 7, the total amount of 137
Cs is plotted as a function of number of devices of
different types. The large total inventory of irradiators and the large number of gauges are
noticeable. The inventory of 137
Cs in irradiators is about two orders of magnitude bigger
than that of fixed gauges, which has the second biggest inventory. The inventory of
irradiators consists of a small number of devices, each with a very large amount of cesium-
137. On the other hand, the inventory of gauges consists of numerous devices, each with a
small amount of cesium-137.
The completed cycle, even if admittedly seriously constrained by data limitations,
demonstrates that substantial potential exists for improving the oversight of devices
containing cesium-137. There is, of course, the question of how much control is
appropriate. It seems reasonable that devices containing radioactive materials should be
treated much as problematic chemicals are, with requirements for tracking, bills of lading
when ownership is transferred, annual reports of stocks in use or stocks in storage (so-
called “hibernating” stocks), and so forth. As with proprietary chemical data, this
information could be provided in confidence.
8. Conclusions and Recommendtions
From the perspective of the data that are available, and on the analyses described
above, several conclusions and recommendations can be presented. Three conclusions are
straightforward. First we note that cesium-137 is produced as a fission byproduct, in
quantities determined not by the demand for cesium-137, but by the demand for electricity.
Therefore, reducing the demand for cesium-137 does not reduce the amount produced.
Second, in the U.S. all commercial cesium-137 is imported, so reducing the use of cesium-
137 in the U.S. results in reducing cesium-137 in the import flows.
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Third, it is highly beneficial to encourage recycling and reuse of devices, since such
actions will reduce the amount of cesium-137 entering the U.S. At the same time, they
diversify disposal options and help reduce unwanted stock, which can become orphaned
sources.
Three recommendations can also be made. First, from a security point of view, device
use and circulation should be better controlled. Non-commercial cesium-137 is classified
as high-level waste, while commercial cesium-137 is used at various places, with various
levels of management and relatively little oversight. Second, most of the government data
concerning radioisotopes consist of estimates of cumulative stock and not annual flows.
Data archiving of flows on an annual basis will be helpful in future analysis. Third,
because radioactive materials are a national concern, regular and complete compilations of
national data, including information about all types of devices, would be helpful in creating
appropriate management policies. The gathering of basic information and construction of a
database at the national level is therefore encouraged. Information gathered should include
data on number of devices, their use, and their radioactivity.
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