9th Pacific Basin Nuclear Conference
Sydney, Australia, 1994: May 1-4.
High neutron economy, on-power refuelling, and a simple bundle design provide a high degree of flexibility that enables CANDU (CANada Deuterium Uranium; registered trademark) reactors to be fuelled with a wide variety of fuel types.
Near-term applications include the use of slightly enriched uranium (SEU), and recovered uranium (RU) from reprocessed spent Light Water Reactor (LWR) fuel.
Plutonium and other actinides arising from various sources, including spent LWR fuel, can be accommodated, and weapons-origin plutonium could be destroyed by burning in CANDU.
In the DUPIC fuel cycle, a dry processing method would convert spent Pressurized Water Reactor (PWR) fuel to CANDU fuel.
The thorium cycle remains of strategic interest in CANDU to ensure long-term resource availability, and would be of specific interest to those countries possessing large thorium reserves, but limited uranium resources.
Fuel cycles have always been of key strategic importance to the nuclear industry (Green and Boczar (1)). Keen interest in fuel cycles that improve uranium utilization was originally driven by a belief that uranium resources would not support the requirements of a growing nuclear system. Reprocessing technology was developed to provide plutonium for fast breeder reactors to extend fuel resources. Similarly, early work in the thorium fuel cycle was motivated by uranium resource considerations. Interest in effective uranium utilization is now motivated by other considerations, such as environmental concerns far the front- and back- end of the fuel cycle, and national policies to secure the maximum benefit from nuclear energy resources, or to increase energy self-reliance. Reprocessing, and recycling the recovered uranium and plutonium back into thermal reactors, is a means of increasing the energy derived from the original mined uranium.
The fuel cycle is being increasingly viewed in the context of the overall waste management strategy. Hence, there is currently interest in actinide burning and transmutation as waste management options, even though disposal concepts, such as geological disposal, have been shown to effectively eliminate the radiological risk from long-lived actinides. A related interest in fuel cycles stems from the end of the cold war, and the enticing possibility of burning weapons-origin plutonium or high enriched uranium in nuclear power stations to generate electricity while enhancing world security.
Of course, economics remains an important consideration in assessing fuel cycle options. The fuel cycle can impact both the fuelling and capital costs of a nuclear plant.
In Canada, the desire to burn natural uranium fuel (so as not to depend on enrichment facilities) necessitated a reactor design (the CANDU) having high neutron economy, achieved through the use of heavy-water moderation, low parasitic absorption in the core (in the reactivity control devices and in structural materials), and on-power refuelling. While the capability of using natural uranium is still an important attribute in some markets, these features also provide an unsurpassed degree of flexibility that enables CANDU to utilize a wide variety of fuel types to meet local market requirements.
This paper examines some of the CANDU fuel cycle options that are currently of interest to AECL and its customers.
2. SEU [ Slightly Enriched Uranium ]
The use of SEU in CANDU has many attractions (Boczar et al (2)). Enrichment would reduce the quantity of spent fuel produced in CANDU, which may be perceived by the public as addressing the first of the environmental three-R's: reduce, recycle, reuse. A U-235 content of 1.2 percent would increase the burnup in CANDU by a factor of three, and hence result in a three- fold reduction in the quantity of spent fuel produced. Enrichment would alleviate pressure on interim storage requirements at the reactor as well.
While the natural-uranium-fueIled CANDU is the most neutron-efficient of all commercial reactors in operation today, SEU would further improve the uranium utilization (the energy derived from the mined uranium). An improvement of about 30 percent in uranium requirements is achieved for an enrichment of 1.2 percent. Enrichment would preserve the advantage in uranium utilization that CANDU enjoys over the LWR, as the latter goes to higher burnups, employs uranium-conserving fuel-management techniques, or employs lower tails enrichment that could eventually be made economical through advanced enrichment technology. Uranium utilization is an important consideration for some countries that have few indigenous uranium resources, and which have a strategic interest in energy self- reliance.
In operating CANDU stations, significant cost reductions can be achieved by using SEU fuel. Fuel cycle costs are minimized at an enrichment of around 1.2 percent, and are about 30 percent lower than with natural uranium fuel. Both front- and back-end fuel cycle costs would be reduced with SEU.
SEU offers greater flexibility in reactor design. In new reactors, or in existing reactors where there is sufficient heat removal capacity, SEU can be used to uprate reactor power without exceeding existing limits on bundle or channel power, by flattening the channel power distribution across the reactor core (Chan and Dastur (3)). This option involves trading-off the extra burnup potential of SEU for more power. In a new reactor design, the use of power flattening to obtain more power from a given-sized core has an advantage in capital costs over simply adding more channels to the reactor. One option for AECL's new CANDU 9 reactor uses enrichment of around 0.9 percent to flatten the channel power distribution in the core to obtain 1030 MWe from a 480-channel, Darlington-size core, nominally rated at 935 MWe (Hart (4)). SEU could be used to reduce the capital cost of new plants by increasing the pressure tube thickness to upgrade the primary heat transport system (PHTS) conditions, thereby achieving higher thermodynamic efficiency, or by reducing the moderator inventory by decreasing the moderator and reflector volumes (Dastur and Chan (5)).
The use of enrichment in CANDU also offers greater flexibility in fuel bundle design. One example is the Low Void Reactivity Fuel (LVRF) bundle, in which the use of enrichment and neutron absorber materials allows any value of void reactivity and discharge burnup to be designed (Boczar et al. (6)). This has the potential for increasing the degree of passive safety in the CANDU design, as well as reducing capital costs (by allowing a simplification of the PHTS).
CANDU's on-power refuelling offers flexibility in fuel management that facilitates the use of SEU and other advanced fuel cycles. This flexibility extends from the equilibrium core, where, for example, different fuel management strategies could be used to accommodate different levels of enrichment, to the transition from one type of fuel (such as natural uranium) to another (such as SEU). Fuel management strategies have been identified for both the equilibrium core, and for the transition from natural uranium to SEU (Charles Dastur (7); Younis and Boczar (8, 9); Boczar et al. ())
No reactor physics obstacles have been identified, and no significant changes are required to accommodate SEU in CANDU. An advanced fuel bundle is being developed as the optimum carrier of enriched fuels in CANDU (Lane et al. (11)). This new bundle, called CANFLEX, (CANDU FLEXible Fuelling) is more subdivided than other CANDU bundles, having 43 elements with two pin sizes. When operated at current bundle powers, peak linear element ratings are reduced by 15-20 percent, depending on burnup. The lower ratings, along with optimized internal fuel-element design, will facilitate the achievement of extended burnup in CANDU by reducing fuel temperatures, and hence fission-gas release within the fuel element. Critical heat flux and critical channel power will be increased, due to the optimization of the number and location of flow-disturbing appendages in the bundle. This feature can also be used to increase operating margins in operating reactors.
The use of enrichment is the logical first step from natural uranium fuel in CANDU.
3. CANDU / PWR SYNERGISM
The basis for the synergism between CANDU and PWR arises from the fundamental characteristics of the two reactor types: PWR fissile requirements are higher than for CANDU, because of the good neutron economy of the latter (Boczar and Dastur (l2)). The higher parasitic loads in the PWR lattice need to be compensated for by extra fissile material, both in the fresh and spent fuel. As a result, PWR spent fuel has a high fissile content -- about 0.9 percent by weight of U-235, and about 0.6 percent fissile plutonium, depending on the initial enrichment and exit burnup. In CANDU, the fissile content in the fresh fuel is low because of good neutron economy. Moreover, the initial U-235 and the self-generated plutonium are burned to low levels in CANDU. CANDU fresh natural uranium contains 0.7 percent U-235, while the spent fuel contains 0.2 percent U-235 and 0.2-0.3 percent fissile plutonium. Even with 1.2 percent SEU, there is only about 0.4 percent fissile material in the spent CANDU fuel. Hence, spent PWR fuel has about 1.5 percent fissile material, compared to about 0.4 percent fissile material in spent CANDU fuel. Spent PWR fuel, therefore, can be viewed as a source of fissile material for CANDU.
CANDU's excellent neutron economy means that about twice as much energy can be extracted from the fissile material in spent PWR fuel by recycling it in CANDU rather than in a PWR (Boczar et al. (13); Hastings et al. (14)). In conventional reprocessing, fission products are removed, and the uranium and plutonium are separated. The plutonium can be mixed with uranium (either natural, depleted, or the recovered uranium from the reprocessing plant) to form MOX fuel, which can be effectively utilized in CANDU. AECL has performed extensive studies on the use of MOX fuel in CANDU in collaboration with an overseas client. No technical obstacles have been identified, and in fact there is considerable potential for optimizing the plant design to reduce capital costs through the use of MOX, as with SEU.
The uranium from reprocessing is referred to as "recovered uranium" (RU). It has a U-235 content of around 0.9 percent, and its use in CANDU without re-enrichment is a very attractive fuel cycle option; it is discussed in greater detail in the next section.
A chemical decontamination process could be used to separate fission products and unwanted actinides from the unseparated uranium/plutonium mixture, which would then be co-converted into MOX fuel, and used either as-is in CANDU, or diluted with natural or depleted uranium (depending on the desired burnup). This is the conventional TANDEM fuel cycle. The advantage of chemical decontamination over conventional reprocessing lies in the potential of a cheaper, simpler process that is more proliferation-resistant and easier to safeguard, since plutonium is not separated from uranium. AECL and the Korean Atomic Energy Research Institute (KAERI) investigated the TANDEM cycle in the early 1980's.
A fuel cycle currently under extensive consideration has an even greater degree of safeguardability than the TANDEM cycle. This is the DUPIC cycle (Direct Use of Spent PWR Fuel In CANDU), discussed in greater detail in section 5.
The use of CANDU to maximize the energy potential of the fissile material from spent PWR fuel offers several benefits, including increased overall uranium utilization, and a reduction in the total quantity of spent fuel.
4. RECOVERED URANIUM (RU)
RU is a by-product of conventional reprocessing of LWR fuel. With a nominal U-235 concentration of 0.9 percent, RU is a subset of SEU that is particularly attractive for currently operating and future CANDU reactors. Its use without re-enrichment in CANDU offers many of the benefits of SEU. Uranium utilization (the amount of energy derived from the mined uranium used in the original PWR fuel) would be improved by about 25 percent. Double the energy can be extracted from the RU by burning it in CANDU rather than re-enriching it as fuel for a PWR. Fuel burnup in CANDU would be about twice that of natural uranium, resulting in a two- fold reduction in the volume of spent fuel and a commensurate reduction in back-end disposal costs. By flattening the channel power distribution across the reactor core so that all channels produce nearly the same power, RU offers a power uprating capability.
The suitability of RU as a rector fuel for CANDU was recently assessed in a joint program between AECL and COGEMA of France (Boczar et al. (15)). Pellets were pressed from the RU powder, and both powder and pellets met CANDU fuel specifications. One issue that had been identified in an earlier assessment was whether trace amounts of cesium-137 in the RU powder would be released during sintering, and if so, whether this would condense in the cold part of a sintering furnace in a commercial fuel fabrication plant, leading to a build-up in fields over time. This was assessed by sintering 4000 RU pellets in a furnace that had been designed with a cold-trap in which volatile cesium released during sintering would condense. It was concluded that volatile cesium-l37 would not pose a radiological problem in a commercial fuel fabrication plant.
Fuel management with RU should be particularly simple. A simple four-bundle shift, bi-directional fuelling scheme would result in good axial power profiles, and a refuelling rate in bundles per day that is half that for natural uranium. Alternatively, a two-bundle shift fuelling scheme could be used that would result in smaller refuelling ripples, but a higher refuelling rate. Peak channel and bundle powers would be comparable, or lower, than for natural uranium fuel. Peak element ratings with CANFLEX would be below 45 kW/m in a CANDU 6 reactor, which would facilitate good fuel performance at extended burnup, with low fission-gas release within the fuel element. Significant power boosting during refuelling would occur only for relatively fresh fuel, which is tolerant to power boosts. The reactivity worths of control devices would be acceptable for safety and control functions.
Fuel cycle economics were recently assessed for RU and SEU in CANDU, and for re-enriched RU in a PWR (Boczar et al. (15)). The potential savings in CANDU fuel cycle costs with RU are striking. Over a range of reasonable cost assumptions, front-end fuelling costs for RU are reduced relative to natural uranium by between 28 percent and 67 percent, and by 15 percent to 30 percent compared to fuelling costs for 1.2 percent SEU.
In summary, excellent neutron economy and fuel cycle flexibility creates a niche in which CANDU is uniquely suited for burning RU without re-enrichment.
5. DUPIC [ Direct Use of Spent PWR Fuel In CANDU ]
The DUPIC fuel cycle exploits the CANDU neutron economy and fuel cycle flexibility in a manner that maximizes the safeguardability of recovered fissile material from spent PWR fuel (Keil et al. (16)). The various DUPIC options do not use reprocessing or wet chemical processes, only dry processes, to utilize the energy content of spent PWR fuel in CANDU.
In 1992, AECL, KAERI and the U.S. Department of State completed Phase I of an assessment of the DUPIC cycle. Five mechanical reconfiguration options were assessed, involving rearranging the spent PWR elements into CANDU bundles, with or without double cladding. Two conceptual CANDU fuel-bundle designs were evaluated to maximize fuel utilization: 61- and 48-element bundles having either single- or double-clad element sheaths. These bundles were chosen to make use of the smaller PWR-size elements while maximizing the fuel content of CANDU bundles.
Two powder-processing concepts were also evaluated. In the OREOX option (Oxidation, REduction of enriched OXide fuel), spent PWR pellets would be subject to successive oxidation/reduction cycles to produce a sinterable UO2 powder that would be pressed into pellets, sintered, loaded into CANDU sheaths, and fabricated into conventional CANDU bundles. The second powder-processing option was "VIPAC" (VIbratory comPACtion), in which PWR pellets would be ground into small, dense granules and vibratory-packed into sheaths.
All of the options were assessed against a set of selection criteria, which included retrofitability to CANDU and to PWR, safeguardability, licensability, reactor physics, fuel performance, fuel handling, fuel fabrication, and waste management.
Both the mechanical reconfiguration options and the powder-processing options were found to be feasible. For the mechanical reconfiguration options, the low ratings (and consequently lower peak center-line temperatures) resulting from greater subdivision with the 48-element or 61-element bundles compensated- for both the greater variation in fissile composition due to axial or rod-to-rod variations in fissile content, and the greater heat resistance of double-cladding. The primary appeal of the VIPAC option is its inherent simplicity, since no sintering is required (part of the fuel sinters in-core), and the specifications on the granules are much less stringent than for pellets. The main disadvantage is that it results in lower fuel densities than pellet fuel, and there is much less world-wide experience with VIPAC than with pellet fuel.
It was concluded that OREOX is the most promising option, largely because of the homogeneity of the resultant powder and pellets. One of the advantages of this process is that it removes a high fraction of gaseous and volatile fission products, thereby improving fuel burnup. The CANDU burnup with the OREOX option is about 18 MWd/kg, using spent fuel from the reference Korean PWR which has an average discharge burnup of 35 MWd/kg (initial U-235 enrichment of 3.5 percent).
The DUPIC cycle is particularly attractive in Korea, which has both CANDU and PWR reactors. In an equilibrium system in which the spent PWR fuel would provide the fuelling needs of CANDU, the DUPIC cycle would improve uranium utilization by about 25 percent, compared to an open cycle in which CANDU was fuelled with natural uranium. In this scenario, the total quantity of spent fuel produced by both CANDU and PWR will be reduced by a factor of three.
Although a large fraction of the gamma radioactivity would be removed from the recycled fuel, fields would still be high enough to require all refabrication and handling to be done remotely in a shielded facility. While this makes the fabrication of the CANDU fuel bundles more costly and difficult, it increases the diversion-resistance of the cycle. The OREOX process should result in good fuel performance, since the pellet and bundle design would be close to that of the reference CANDU fuel. The safeguards assessment concluded that the proliferation risks of the DUPIC cycle are relatively small, and presently known safeguards systems and technologies can be modified or adapted to meet DUPIC safeguarding requirements (Pillay et al. (17)).
The workscope for Phase II of the DUPIC program is now being defined. This is a multi-year experimental verification program, involving optimization. of the OREOX process, and fabrication of DUPIC elements and bundles from spent PWR fuel for subsequent test irradiation in a research reactor, followed by post-irradiation examination, development of remote fabrication technologies, and development of appropriate safeguard technology.
6. ACTINIDE BURNING AND PLUTONIUM DESTRUCTION
Fuel cycle options are being proposed internationally that reduce the radiotoxicity of spent fuel arising from the long-lived actinides. Radiotoxicity is a measure of the hazard of ingesting or inhaling a substance. Radiotoxicity is not a measure of long-term risk from spent fuel in a waste management system, in which natural and man-made barriers are designed to isolate the waste from the biosphere. In fact, the environmental review of the Canadian geological disposal concept shows that the actinides pose negligible risk, because of their immobility in the disposal vault. The largest contributors to long-term dose are from the long-lived fission products, I-129, C-14 and Tc-99 (Dormuth et al. (18)), but the doses associated with these species are well below regulatory limits in a properly designed disposal vault.
Nonetheless, there is interest internationally in assessing the feasibility of burning the plutonium and transuranic actinides from reprocessing in-reactor, as a waste management option. Because of its high neutron economy, CANDU can be effective in this role (Dastur et al. (19)). The traces of fissile material in the transuranic mix from the reprocessing of spent LWR fuel provide sufficient reactivity in a CANDU lattice for use as fuel. The absence of uranium in such fuel prevents the formation of plutonium and the higher actinides. Without plutonium formation, the fissile content or the mix depletes rapidly with irradiation and constant reactor power output is maintained by using the on-power refuelling feature of CANDU to shift the targets into increasing flux. The high neutron flux facilitates the transmutation and annihilation of the higher actinides. About 3.6 GW(e).a of LWR actinide production could be annihilated annually in a CANDU 6 reactor of current design. No adverse effects on reactor dynamic behavior have been identified.
High operating neutron flux, high neutron economy and on-power refuelling also make CANDU particularly suitable for the annihilation of weapons-grade plutonium (Pitre and Dastur (20)). The plutonium would be irradiated in an inert matrix, such as zirconia or beryllia. The fissile content is maximized by using gadolinium to suppress-excess reactivity. Calculations show that an annihilation rate of 2.5 kg/FPD (Full Power Day) can be achieved in a CANDU 6 reactor that is rated at 680-MW(e). Significant fractional annihilation is achieved by shifting the fuel into locations of increasing flux level as the fissile content depletes. This is facilitated by the on-power refuelling capability of CANDU. Due to the abundance of heavy water, which is the main component of the CANDU lattice, the effect of plutonium in the absence of U-238 on the reactor dynamics is shown to be acceptable with current reactor control technology. Furthermore, the reactivity coefficients of the CANDU lattice can be adjusted by the judicious placement of certain burnable poisons in the fuel. This technique is used to ensure that the reactor fuel temperature and void reactivity coefficients are negative.
Another option being proposed for disposing of weapons-grade plutonium is "spiking": burning the plutonium in the form of a mixed oxide fuel, with the result that the radiation field from the resulting fission products is high enough to discourage diversion. Again, the on-power refuelling capability represents a significant advantage for CANDU in this option.
7. THORIUM FUEL CYCLES IN CANDU
Thorium is an alternate fuel to uranium, but since it has no fissile isotopes, it is necessary to provide fissile material (uranium or plutonium). The U-233 produced by irradiation of Th-232 has the highest eta value (ratio of neutrons produced to neutrons absorbed) for thermal neutron fission of any of the fissile nuclides. It is thus a very good fuel in the soft CANDU spectrum. Moreover, the equilibrium concentration of U-233 in spent thorium fuel (about 1.5 percent U-233) is about five times that of fissile plutonium in spent natural uranium fuel, and so it should be a cheaper source of recycle fuel than plutonium (aithough this will be offset by higher fuel fabrication costs with recycled U-233, compared to recycled plutonium). The flexibility in fuel management provided by on-power fuelling is another CANDU advantage in burning thorium.
The fissile material can be provided in several ways, and these options define the various thorium fuel cycies (Milgram (21)). In most variants of the conventional once-through thorium cycle, ThO2 and SEU are burned in separate channels, and the U-233 that is produced from neutron capture in Th-232 is burned in-situ. The conventional once-through thorium cycles require high thorium burnups, 40-100 MWd/kg Th (compared to 7 MWd/kg U for natural uranium fuel). Re-insertion of the spent ThO2 fuel after a cooling period can further utilize the energy from the decay of Pa-233 to U-233 while in storage. A major challenge in the once-through thorium cycles is to devise appropriate fuel management strategies.
Other thorium fuel cycles employ reprocessing to optimize the energy potential from U-233, and these are of longer-term strategic interest. These reprocessing cycles mix ThO2 with either enriched uranium, or plutonium. U-235 can be provided as either high enriched uranium (around 92 percent U-235, as a vehicle for burning weapons-material U-235), or as medium enriched uranium (less than 20 percent U-235, for non-proliferation considerations). If plutonium were used to initiate the cycle, it would be obtained from reprocessing conventional PWR or CANDU spent fuel, or from dismantled weapons.
If further improvements are made to the CANDU neutron economy (such as removal of adjuster rods, and use of enriched zirconium for structural materials), the self-sufficient thorium cycle is feasible. This requires no fissile makeup once the equilibrium concentration of U-233 (1.5 percent) has been achieved.
Thorium has an additional potential benefit of lower radiotoxicity of the spent fuel than for uranium.
Several options are being examined for exploiting the ability of CANDU reactors to burn a variety of fuels. The direction of CANDU fuel cycle development will be driven largely by local considerations, such as the availability and cost of fuel resources (uranium and thorium), the presence (or lack) of a high-technology infrastructure, and the reactor mix in the particular country. The flexibility exists with CANDU technology to optimize the fuel cycle to meet the needs of our customers.
AECL-XXXX is an Atomic Energy of Canada Limited published report.
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