R.E. Miller, I. Erdebil and M. Solaimani
Ontario Hydro
Nuclear Waste and Environment Services Division
D.W. Evans and M. Carney
Ontario Hydro Technologies
ABSTRACT
Spent ion exchange (IX) resins from Ontario Hydro's CANDU (Canada Deuterium Uranium) pressurized heavy water reactors contain high levels of 14C in addition to the usual radionuclides. These resins constitute the major fraction of Hydro's intermediate level radioactive waste. The radiochemical and physical characteristics of the spent resins generated at CANDU stations are described. The current Ontario Hydro inventory of spent resin is about 2300 m3, of which about 65% is expected to contain significant amounts of 14C (i.e. >>8 Ci/m3). The cumulative waste resin arisings to end- of-life will be 7000 m3. Various options for handling these resins are under evaluation, with the objective of establishing the least costly life-cycle management practices for the spent CANDU resins.
ONTARIO HYDRO
Ontario Hydro is the largest electrical utility in North America, with 32 GW of generating capacity. Hydro serves about 3.3 million customers, directly or indirectly through 306 municipal utilities. Ontario Hydro relies heavily on nuclear generation, with about 60% of the base load supplied by 19 CANDU reactors at three sites throughout the province (1).
SPENT IX RESINS - CURRENT WASTE MANAGEMENT PRACTICES
Spent resins removed from service in heavy water (D2O) systems are first de- deuterated with H2O, for heavy water and tritium recovery, then slurried into in-station bulk resin storage tanks. Radioactive resins from light water systems do not require de-deuteration and are slurried directly to the spent resin storage tanks. Fuel-contact resins (eg. reactor coolant resins, fuelling machine resins, irradiated fuel bay resins) are generally stored separately from non-fuel contact resins (eg. moderator, liquid zone control resins, etc) although at some stations mixing of these two streams has occurred. In the in-station bulk resin storage tanks the resins are maintained submerged in water for a protracted period, allowing for radioactive decay and decay heat removal. The duration of in-station storage is not pre-defined and is limited only by the remaining tank storage capacity. When required, periodic shipments of resin are made to the Bruce Reactor Waste Operations Site (RWOS). The resin is slurried into 3 m3 carbon steel (epoxy-coated) shipping/storage liners and are superficially de-watered, but not dried. At the Bruce Reactor Waste Operations Site these liners are transferred to engineered and monitored in-ground storage containers (12 or 18 m3 total capacity per in-ground container) (2).
IX RESINS - SOURCES AND INVENTORIES
Ontario Hydro uses organic ion exchange resins (IX) for purification of both light (H2O) and heavy water (D2O) process streams at its CANDU nuclear stations. Historically, these resins are used on a one-time basis, without regeneration and re-use. The resins are typically mixed beds, comprised of strong acid cation and strong base anion components, in the gellular bead form. Active resins from heavy water systems are first de-deuterated to recover the D2O and tritium and are then slurried with light water to the appropriate bulk in-station storage tank. Residual 3H levels of up to 1.8 Ci/m3 (67 GBq/m3) have been measured in the de-deuterated waste resins.
Moderator System Resins
The heavy water moderator purification system is the largest single source
of spent resins. The moderator is the principal site of 14C
production in the CANDU system, largely via the reaction 17O(n,
)14C.
About 6260 Ci (232 TBq) of 14C are produced annually in the
moderators of Hydro's stations, with most of it remaining exchanged on resin as
carbonate species (3). Based on calculated 14C production rates and
resin usage data Hydro's moderator resins have an estimated average 14C
specific activity of 82 Ci/m3 (3.0 TBq/m3).
Apart from normal steady-state purification, the moderator resins are used to control the levels of soluble neutron absorbers, gadolinium nitrate and boric acid, used for reactivity control. Resins used for normal purification are characterized by higher levels of 14C activity, while the resins used for reactivity control will be extensively exhausted in the nitrate and borate form.
Reactor Coolant System Resins
The reactor coolant or primary heat transport (PHT) system operates at alkaline pH, maintained through the use of LiOD in the coolant. As a consequence, Li+/OD- form mixed bed resins are used in the side-stream purification loop for chemistry control. 14C generation rates are lower in the reactor coolant than in the moderator, so the resulting average calculated 14C specific activity on the resins is only 4 Ci/m3 (0.15 TBq/m3), assuming all the 14C is present as carbonate and is captured by the resin. Fission product activity on CANDU PHT resins is expected to be generally lower than for PWR resins, since on-power refuelling permits rapid failed fuel replacement.
Decontamination (CANDECON) Resins
Ontario Hydro has performed numerous full- system decontamination's of the reactor coolant system pipework, using a dilute, regenerative chemical decontamination process (CANDECON). A dilute mixture of organic acids (oxalic, citric and EDTA) is used for corrosion product and radionuclide dissolution. Iron and 60Co removal and reagent regeneration is performed with H+ form cation resin. At the end of the decontamination process the reagents are removed on anion-rich mixed bed columns. The decontamination resins are expected to be low in 14C activity. Decontamination resins are de-deuterated in situ in their dedicated service vessels and are shipped directly to the Bruce Waste Management Site, without interim storage in the bulk resin tanks at the station. The decontamination resins are therefore effectively segregated from other types of resin waste.
Miscellaneous Resins
These comprise about 24% of the average yearly resin waste arisings and come from a number of heavy and light water systems, such as the recovery D2O clean-up system, irradiated fuel storage bay and several systems unique to the CANDU design (fuelling machine cooling, liquid zone control, end shield cooling, shield tankcooling). These resins are mixed with the other process resins in the in-station bulk resin storage tanks and are at best segregated only as fuel-contact or non fuel-contact source resins.
Table I Average Annual Production of IX Resin Wastes in Ontario
Hydro Nuclear
Based on current resin usage trends the total estimated IX resin waste inventory by end-of-life for all current Ontario Hydro nuclear stations is 6000 m3, exclusive of decontamination resins. The projected inventory of decontamination resins by end-of-life is more uncertain, with estimates ranging from <740 m3 (based on historic trends) to 3800 m3 for a more conservative scenario.
Ontario Hydro's total spent resin inventory as of 1996 consist of 1500 m3 in storage at the Bruce Reactor Waste Operations Site and 800 m3 in bulk storage at the stations. Most of the resins at RWOS are stored in 3 m3 liners, although a small volume of resins are stored in their original 110 L and 170 L disposable vessels.
RADIOCHEMICAL CHARACTERISTICS OF RESINS
To date most of the radionuclide analysis has focused on freshly discharged spent resins, with very limited characterization of the stored resins. A tabulation of some radionuclide analysis of moderator, reactor coolant and CANDECON decontamination resins is given in Table II. It should be noted that these results are for relatively fresh resins and are not decay-corrected. Only the longer lived radionuclides, with t½ >= 2 years, are shown since these are most likely to affect disposal strategies.
A review of decontamination resin usage indicated that about 192 m3 of cation resin waste and 64 m3 of mixed bed (reagent removal) resin wastes have been generated to date. The bulk of the 60Co and inactive iron inventory resides on the cation resin. The current decay-corrected 60Co inventory on the cation resin is 279 Ci, for an average 60Co concentration of 1.45 Ci/m3 (estimated range: 0.13 to 3.6 Ci/m3).
Most resins are changed out on conductivity breakthrough and there is very little direct analysis of the non-active chemical speciation of Hydro's ion exchange resins. The chemical speciation of the resins is therefore projected, based on extensive experience with the operating chemistry for the various systems.
Moderator resin generally spend first on anions such as nitrate, borate and carbonate/bicarbonate. The lithiated mixed bed reactor coolant purification resin wastes are expected to have a cation component in the Fe2+,Fe3+ and Li+ form and the anion component mostly in the CO32- and OH- form. The CANDECON cation resins are generally exhausted in the Fe2+ form. Analysis has shown the presence of some organic residual reagents, with up to 49 g/L EDTA, 5.5 g/L citrate and 2.8 g/L oxalate (4). These are probably present as precipitates (e.g. EDTA free acid, ferrous oxalate, etc.) rather than as ion exchanged species. The CANDECON mixed bed and anion resins are exhausted as organic anions, EDTA, citrate and oxalate. This corresponds to high organic loadings on the anion resin, in the range 50-140 grams of organic acid per litre of resin waste. The miscellaneous resin wastes from systems such as heavy water clean-up, the irradiated fuel bays and various closed-loop cooling systems are loaded with a complex mixture, including iron, calcium, magnesium, sodium, chloride, sulfate, carbonate and nitrate.
Table II Summary of Long-lived Activity (t½ >= 2
years) on Ontario Hydro Resins
PHYSICAL CONDITION OF STORED RESINS
As noted earlier most of the spent resin samples evaluated to date are from
resins freshly discharged from the process purification systems, not from resins
in long term storage. The freshly discharged moderator and heat transport resins
have shown elevated fines content, consistent with mechanical damage from resin
handling, possibly exacerbated by some in-service radiolytic degradation. Apart
from irradiation by ion exchanged or particulate radionuclides the resins are
also exposed to
irradiation by tritiated moderator or reactor coolant D2O and by
gamma irradiation from the short-lived 16N species, while in
service. Total radiation doses experienced by Hydro's resins while in-service
and in storage have not yet been estimated. Some samples of freshly discharged
resin were found to be highly friable. Some external reactor operators have
reported extensive degradation of resin. This can potentially result in the
formation of a difficult-to-handle gelatinous material, but this has not been
observed to date with Hydro resins in storage at the station or at the Bruce
Site.
The one sample of resin taken to date from a storage liner at Bruce RWOS showed some small increase in the fines content compared to virgin resin. The type of damage noted was consistent with the damage that results from repeated freeze-thaw cycling of resin. This particular sample proved to be a CANDECON mixed bed resin and was not exposed to a significant radiation dose, either in service or in storage. Additional sampling of stored resins at the Bruce Site is planned, to obtain samples of older (15-20 years), more highly irradiated resins.
ONTARIO HYDRO OPTIONS FOR LONG TERM SPENT RESIN MANAGEMENT
As described earlier, the spent IX resin from the generating stations are being held in carbon steel containers and stored in liners. This practice is for short time purposes and is not consistent with Ontario Hydro's long term environmental and safety programs. Therefore, safer and more reliable options are being sought for long term spent resin waste management. Major factors in the planning are the potential impact of the 14C and tritium in about 65% of the total resin waste (moderator and reactor coolant resins) A number of the options presently being studied, prior to ultimate disposal of this waste are as follows:
Cementation of All Resin Waste and Disposal
This option is a proven practice which is being exercised in some nuclear waste management facilities abroad. The major difference between Ontario Hydro's spent resin and other nuclear industries is the presence of 14C in the moderator and reactor coolant resin wastes. This radionuclide has a major impact on processing and disposal plans and it may require disposal of the cementitious wasteform as intermediate or high-level radioactive waste, as opposed to low-level.
Cementation of 14C Bearing Resin and Volume Reduction of Balance for Disposal
This option entails cementation of the high 14C resins (moderator and reactor coolant resins), while processing the balance (35%) for volume reduction.
14C Stripping and Disposal of 14C-Free Resin in High Integrity Containers
Although there are no fully commercialized processes for 14C stripping from spent resins Ontario Hydro Technologies has done extensive development on two processes: 1) acid stripping and 2) supercritical CO2 extraction (5,6). Ontario Hydro has also developed cryogenic distillation (7) and gas chromatographic processes for 12C/14C isotope separation. Numerous methods are available for 14CO2 trapping on alkaline or alkaline earth hydroxides (8-16). A portion of the current 14C inventory on the resins may be recovered as a high specific activity material suitable for isotope sales. The high 14C- bearing resins, which comprise about 65% of the waste, would need to be identified and stripped. The stripped resins and the largely 14C-free resins would then be disposed of in high integrity containers.14C Stripping and Volume Reduction of 14C Resin Prior to Disposal: 14C stripping is likely a prerequisite to any volume reduction process to minimize subsequent emissions. Some volume reduction technologies under evaluation include: 1) molten metal catalytic extraction 2) pyrolysis 3) vitrification and 4) incineration. In all cases the residue will be contained in high integrity containers or otherwise immobilized to ultimate disposal.
These options are presently under study from technical, financial and regulatory aspects.
COMPARISONS OF OPTIONS
The key criteria which will be sought in comparison of option are as follows:
CONCLUSIONS
Ontario Hydro's policies for radioactive waste management are driven by its strong commitment to environmental and public safety. It has long being recognized that spent resin forms a complex type of radioactive waste which requires careful planning for its long term waste management activities. The Spent Resin Processing Team welcomes the opportunity to share and discuss these complex issues with the larger radioactive waste management community.
REFERENCES