CANDU reactors and advanced fuel cycles.
Recently, there has been considerable discussion in the media concerning the potential use of CANDU[R] reactors to destroy weapons-derived plutonium. What is there about CANDU Pressurized Heavy Water Reactors (PHWRs) that would allow them to be used for such an application? The answer to this question defines a wide variety of options for using various fuels in CANDU reactors.
Nuclear reactors, of course, normally "burn" U-235, which has an isotopic abundance in nature of only 0.7%. The heat from the fission reaction heats coolant water passing through the reactor's core, and the heat is used to produce steam for driving a turbine-generator. In a CANDU reactor, the fuel is cooled with heavy water and is contained in fuel channels that are positioned horizontally in large tank, called a calandria [ILLUSTRATION FOR FIGURE 1 OMITTED]. The calandria is also filled with heavy water to moderate the neutrons. The power output from the reactor can be increased by increasing the number of fuel channels. For example, a 700 MWe class CANDU reactor contains 380 fuel channels, while a 900 MWe class reactor contains 480.
This configuration results in "high neutron economy", such that more of the neutrons produced are available for sustaining fission. A major reason for this is that heavy water has a low cross-section for neutron absorption compared to light water. This means that CANDU reactors are highly efficient burners of fissile and fertile (isotopes that can be converted to fissile isotopes) material, and all CANDU reactors operating today use natural uranium as fuel. By contrast, light water moderated reactors (LWRs) use fuel that is enriched to about 3.5% in U-235 to allow for neutron loss.
Another key feature of CANDU reactors is on-power refueling. A fueling machine pushes fuel in from one side of the fuel channel, and a similar machine accepts the spent fuel bundles that are ejected from the other end. No other reactor in widespread use has this facility.
The fuel design [ILLUSTRATION FOR FIGURE 2 OMITTED] is relatively simple - a 0.1 m diameter, 0.5 m long fuel bundle that can be easily manufactured and handled. The bundle consists of a number of zircaloy-sheathed elements (for example, 28 or 37 in the fuel in common use today) arranged in concentric rings. The fuel material itself consists of high density U[O.sub.2] pellets.
It is the combination of high neutron efficiency, on-power fueling, and simple bundle design that gives CANDU reactors the flexibility to use several different fuels and fuel cycles. The exploitation of this flexibility results in fuel cycles that optimize the use of uranium resources, that can exploit LWR/PHWR synergism, and that secures long-term fuel supply even if uranium resources become scarce or expensive. The various potential fuel cycles for CANDU reactors are summarized in Figure 3.
Natural Uranium and Slightly Enriched Uranium
Currently operating CANDU power reactors use a once-through natural uranium fuel cycle, which avoids the need for securing a supply of enriched uranium. The low fissile content of natural uranium means that this cycle will only work for a reactor having very high neutron economy. Also, the high conversion ratio (about 0.8) provides a high fissile material production rate, which converts fertile U-238 to fissile Pu-239. In fact, about 50% of the fission energy, from natural uranium fuel comes from plutonium that is produced and burned in the fuel during irradiation. Overall, uranium resource consumption is about 30% lower than the enriched fuel cycle used in LWRs for the same power generation.
Even higher efficiencies can be gained by using slightly enriched uranium (SEU) fuel, at U-235 enrichments in the range 0.9 to 1.2%. With an enrichment level of 1.2%, fuel burnup (burnup = energy produced by the fuel per unit weight) is increased by almost a factor of 3, and uranium consumption drops by about 25% compared to natural uranium fuel.
There are several options for the destruction of weapons-derived Pu in CANDU reactors. The options available in the short term all make use of depleted uranium (0.2 wt% U-235) as the ceramic matrix in which the Pu[O.sub.2] is mixed. The main objective is not to destroy all the Pu, but to convert it to a form (i.e., radioactive spent fuel) that would make it difficult to reapply to weapons manufacture, while at the same time producing electricity. Such a fuel is called a "mixed oxide fuel" or MOX (MOX = U[O.sub.2]Pu[O.sub.2]) . One scheme would use conventional 37-element fuel. The inner 7 elements would contain U[O.sub.2] with 5% dysprosium (a neutron absorber used to adjust the fuel's reactivity), while the 3rd and 4th rings would contain 2.0 wt% and 1.2 wt% Pu[O.sub.2], respectively. This graded enrichment minimizes the peak element power rating by flattening the power distribution across the bundle. The net destruction of fissile Pu would be almost 60%. The disposition rate in a 900 MWe class CANDU reactor would be about 1.0 Mg of Pu per year. Other schemes use somewhat higher Pu content fuel and would result in an increase in the dispositioning rate of about 50%.
A different option is to maximize the amount of energy output that could be extracted from the MOX fuel. Such an option would use a more advanced fuel bundle, consisting of 43 elements, arranged in rings of 1,7, 14, and 21 elements, respectively. This more advanced bundle (called CANFLEX) would contain more than twice as much Pu (4.6 wt% in ring 4 and 2.6 wt% in ring), and would extract about 3 times more energy out of the fuel.
Finally, more advanced options are to use an inert fuel matrix material, such as SiC. Such a fuel could be used to destroy about 94% of the fissile Pu. The use of Th[O.sub.2] with Pu[O.sub.2] would result in a similar net destruction of Pu and would increase the energy extracted from the Pu by 50%.
The high neutron economy in CANDU reactors creates a natural synergism between LWR and PHWR fuel cycles. LWRs are designed to burn enriched uranium (about 3.5% U-235) fuel down to a fissile content of 1.5% (0.9% U-235, 0.6% Pu-239) at the end-of-life of the fuel. CANDU NU fuel starts with 0.7% U-235, which is burned down to a fissile content of about 0.2% U-235 and 0.25% Pu-239. Therefore, CANDU reactors are in a unique position to take advantage of the relatively high fissile content of spent LWR fuel. Several options for the use of spent LWR fuel in CANDU reactors are possible.
In conventional reprocessing, uranium and plutonium are separated from the fission products and other actinides in the spent fuel. The recovered uranium (RU) from conventional reprocessing still contains valuable U-235 (typically around 0.9%, compared to 0.7% in natural uranium fuel). This can be burned as-is in PHWRs, without re-enrichment, to obtain about twice the burnup of natural uranium fuel. Also, approximately twice the energy would be extracted using CANDU reactors, compared to re-enrichment of RU for recycle in an LWR. The U-235 would be burned down to low levels (i.e., 0.2%) in PHWRs compared to LWRs (0.9%) so there may be no economic incentive for further recycle of this material.
Mixed Oxide (Pu, U)[O.sub.2] Fuel (MOX)
The other major product from conventional LWR reprocessing is plutonium. Plutonium is mixed with depleted uranium to form MOX fuel, which is recycled by loading up to (typically) 1/3 of a LWR core with the MOX fuel. However, MOX fuel can also be used in PHWRs using a full core load. While MOX fuel fabrication will be much more expensive than natural uranium, the simplicity of the CANDU fuel bundle would result in cheaper MOX fuel fabrication costs compared to LWR MOX. A high burnup CANDU MOX fuel, therefore, has the potential of considerably lowering fuel cycle costs. Up to 50% more energy could be extracted from the fissile uranium and plutonium in spent LWR fuel through recycling in CANDU compared to recycle in an LWR. This has important advantages in improving uranium utilization, reducing enrichment requirements, and in reducing the amount of spent fuel for ultimate disposal.
TANDEM Fuel Cycle
In the TANDEM fuel cycle, the uranium and plutonium from spent PWR fuel are co-precipitated in a conventional reprocessing plant without separation. This fuel cycle uniquely takes advantage of the fact that the fissile component in spent LWR fuel (about 1.5 %) can be used directly in PHWRs, without adjustment of the enrichment. This cycle is potentially much cheaper than conventional Pu separation and recycle into LWRs, since relatively simple steps can be used to remove only the fission products from the spent fuel.
Direct Use of Spent PWR Fuel
The Direct Use of Spent PWR Fuel in CANDU (DUPIC) involves converting the spent PWR fuel into CANDU fuel without any wet chemical processing. Only dry processes are used, in which there is no selective element removal. This, along with the high radiation fields associated with the fuel, offers a very high level of proliferation resistance. The Korean Atomic Energy Research Institute (KAERI), AECL, and the US Department of State have been examining a relatively simple thermal/mechanical process. The process involves a series of oxidation/reduction steps to produce a sinterable ceramic material suitable for fabrication of CANDU fuel pellets.
CANDU reactors could potentially be efficient eliminators of other separated nuclear wastes. Detailed fuel management simulations have been performed for CANDU reactors fueled with a mixture of plutonium and actinide waste in an inert matrix carrier. Over 63% of the actinides can be destroyed in a single pass through the reactor, and over 91% of the initial fissile plutonium. Refueling rates, and bundle and channel powers are within the natural uranium operating envelope.
All fissile material for nuclear reactors is ultimately derived from U-235. The use of thorium as an alternative fuel to uranium could secure and extend nuclear fuel supplies indefinitely.
Natural thorium consists of the fertile isotope Th-232, which is converted to fissile U-233 in a reactor. Therefore, neutrons must be initially provided by adding a fissile material, either within or outside tire Th[O.sub.2] fuel material. Those same CANDU features that provide fuel cycle flexibility also make possible many thorium fuel cycle options.
One option for a "Once-Through Thorium" (OTT) cycle involves the irradiation of thorium fuel bundles separately from "driver" fuel, such as SEU. The thorium and driver fuel would be irradiated at different rates, with the thorium fuel typically residing in the reactor much longer than the driver fuel. The fissile U-233 produced reaches an equilibrium level of around 1.5%, and would be burned in-situ. There are various options for the driver fuel in addition to SEU, such as recycled material from spent PWR fuel (e.g., DUPIC fuel) or even natural uranium fuel.
Alternatively, the fissile material can be mixed directly with the Th[O.sub.2] fuel material. The fissile "topping" material used and the burnup define a wide range of thorium fuel options.
Even higher energy production from thorium fuel can be achieved by recycling the U-233. Such recycling would require the development of simple, inexpensive means for fission product decontamination of the fuel that does not produce any separated fissile material.
In the very long term, with improvements to the neutron economy (e.g., by using higher purity heavy water and lower cross section materials in the core), the Th-232/U-233 cycle can be closed and operated with total independence of external fissile material. In this cycle, as much U-233 is produced in the spent fuel as is required in the fresh fuel. This approach would obviate the need to develop cycles based on expensive LMR (Liquid Metal Reactor) Pu breeding technology, should uranium resources ever become limited.
In conclusion, CANDU reactors will have a sustainable supply of fuel no matter what fuel cycle strategy is followed in the future. This is a consequence of the flexibility provided by the combination of high neutron economy, on-power fueling, and simple fuel design.
David F. Torgerson, FCIC is the vice-president of research and product development at Atomic Energy Canada Limited, Chalk River, ON.
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|Comment:||CANDU reactors and advanced fuel cycles.|
|Author:||Torgerson, David F.|
|Publication:||Canadian Chemical News|
|Date:||Jul 1, 1999|
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