The panel notes that for this option vir-.
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Japan also has a heavy-water moderated thermal reactor at Fugen, with a significantly different design from the CANDU, which was designed for and has operated with full-MOX cores. This reactor, however, is too small MWt, MWe to play a major role in the plutonium disposition mission. Construction of larger reactors of this type in the United States or other countries for the WPu disposition mission has not been proposed, and is not analyzed further in this report.
An experimental MOX fabrication facility was installed at Chalk River in , and operated until placed on standby in ; this facility is now being reopened, and a resumption of MOX fabrication is expected in late In the early s, six element fuel bundles similar to the bundles now proposed for plutonium disposition, but containing only 0. Thus the existing MOX irradiation experience includes the plutonium loadings and burnups relevant for plutonium disposition, with performance described as comparable to that of uranium fuel.
Issues identified for further study are the effects on performance of the metal-to-oxygen ratio in the fuel and of the particle size AECL , pp. The fuel design has evolved toward the current use of thin cladding, with a minimum thickness of 0. This thin cladding improves neutron economy an important factor in a reactor fueled by natural uranium , but the lifetime of the cladding may limit the burnups that could be achieved with fuels enriched with plutonium. The vendor concludes that: "There should. The vendor suggests that the four MWe units located at the Bruce A site would be particularly suitable for WPu disposition.
The Plutonium Surplus
Ontario Hydro, the utility that owns these reactors, has expressed interest in studying the plan. The utility has taken part in an initial study funded by DOE, and it sponsored a public meeting near the plants in June to discuss the idea of using plutonium there AECL , pp. AECL has also pursued preliminary discussions with the Russian government regarding this concept.
While new CANDU reactors could also be built in Russia to consume the plutonium there, this would be slower and more expensive than using existing reactors, and would not appear to have great advantages compared to constructing reactor types with which Russia is more familiar, such as light-water reactors.
Compared to the use of U. In normal CANDU operations with natural uranium fuel, more than half of the energy is provided by fissioning plutonium produced in the fuel as the reactor operates. As a result, adding plutonium to the initial fuel would represent a smaller change in the physics of the reactor core than in the case of LWRs.
Moreover, the structure of the CANDU reactors allows plenty of space for added controls, and additional neutron absorbers could be dissolved in the heavy-water moderator used in the reactors. Thus the vendor. Several factors should be mentioned in this regard. First, CANDU reactors offer greater flexibility for maintaining control-absorber worth with plutonium fuels. In a calandria-type heavy-water reactor most of the reactor-core volume is occupied by the relatively cool, low-pressure, heavy-water moderator. There is ample space to increase the size of each absorber, or increase the number of absorbers, as necessary to counteract the higher neutron absorption in plutonium fuel and to maintain control-reactivity worth.
The CANDU design also provides for boron absorber dissolved in the moderator for shim control of reactivity. The well-thermalized neutron spectrum provides greater control worth for a given amount of boron than in a plutonium-fueled LWR.
The danger of mothballing the MOX
The movable and soluble absorbers are also more effective because they are located in the higher-flux region where thermal neutrons are formed. The designers expect that the reactivity control would even be sufficient for a full loading of nonfertile plutonium fuel.
The heavy-water moderator is at low pressure and is thermally insulated from the hot fuel and coolant in the pressure tubes. Therefore, there is only a weak coupling between an assumed sudden increase in power and temperature of the fuel and the temperature of the moderator. Also, in the CANDU, dissolved boron for shim reactivity control is in the moderator rather than in the coolant. In a pressurized-water reactor, with boron dissolved in the coolant, thermal expansion of the coolant accompanying an assumed sudden increase in local fission rate reduces the amount of boron near the fuel and tends to add to the reactivity.
Third, prompt neutrons have a considerably longer lifetime in a CANDU reactor than in other reactors because of the relatively large volume of heavy water with weak absorption of thermal neutrons. This results in a much longer. Moderator heating tends to make a positive contribution to reactivity when using Pu fuel, because it shifts the thermal spectrum toward the 0. The increase in the fission cross-section of Pu is more important than the increase in the capture-to-fission ratio of Pu in this region.
In an LWR it is a licensing requirement that the boron concentration not be large enough to make the moderator coefficient of reactivity positive. Thermal shock is less likely, and more time is available for safety-system actuation. Finally, the addition of dysprosium in the MOX fuel has a beneficial effect on the reactor's performance in a loss-of-coolant accident LOCA. The plutonium-dysprosium fuel is designed to counter this positive reactivity coefficient, so that the reactor will become less reactive in the same scenario.
Analysis by the vendor shows in the event of a large-break LOCA, power output in a uranium-fueled reactor could increase to over four times its nominal value, but would not increase at all in the MOX fueled system AECL , p.
Simplified Fuel Fabrication. The specifications that CANDU fuel must meet, in such areas as granularity, pellet shape, and the like, are less stringent than those required for LWR fuel. Experience with LWR fuel, however, suggests that the need for tight specifications increases with greater enrichment, so adding plutonium to CANDU fuel might require specification tolerances closer to those of typical LWR fuel.
Operating with natural uranium fuel requires frequent replacement of fuel, so CANDU reactors use mechanisms for continuous refueling while at power. The refueling mechanisms require relatively short 60 cm fuel-assembly cartridges. As irradiated fuel cartridges are removed from one end of a pressure tube, fresh cartridges are inserted at the other end. Within a given pressure tube the cartridges can be moved along the pressure tube at a rate proportional to the average neutron flux in the pressure tube.
All discharge cartridges can be irradiated to the same burnup, not possible in LWRs refueled by periodic batch replacement. The ability to refuel without shutdown offers additional options for the "spiking" option.
While the "spiking" approach would still require added capital expenditures for a larger fuel fabrication facility, it would not decrease revenue as a result of reactor downtime for refueling. Moreover, the remotely operated refueling machines would reduce concerns regarding possible worker exposures to radiation in reloading "spiked" fuel to finish burning it to "spent" fuel.
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The small cartridges and remotely operated refueling machines also make it possible to consider recycling discharged cartridges for further irradiation, providing a possible means of greater annihilation of plutonium than otherwise obtainable in once-through irradiation. Fresh first-cycle plutonium-fueled cartridges could supply neutrons to overcome the neutron-absorbing fission-. Irradiation of individual cartridges could proceed to the material limits of the fuel material, rather than being limited by reactivity considerations.
Similar operation of a plutonium-burning LWR would be difficult because of the very long approximately 5-meter fuel assemblies and the need to shut down and depressurize for refueling. The high neutron economy of the CANDU makes it possible to irradiate fuel of a given fissile concentration to much greater burnup, and to a greater degree of annihilation in one irradiation cycle, than is possible in other thermal reactors, such as the LWR or high-temperature gas-cooled reactor HTGR.
A measure of the neutron economy is the conversion ratio, defined as the number of neutrons absorbed in fertile material per fissile atom destroyed. High neutron economy would be important for plutonium-burning fuel cycles that sought high burnup during an irradiation cycle using fertile-free plutonium fuels. It still may not be possible, however, to design fuels that can achieve the very high burnups claimed for HTGRs, described below.
In designing thermal reactors for efficient utilization of plutonium as a fuel, it is important to seek a well-thermalized neutron spectrum to avoid the higher capture-to-fission ratio of Pu that results when the thermal-neutron spectrum is shifted to higher energies. Calandria-type heavy-water reactors have characteristically lower-energy and better-moderated spectra of thermal neutrons than do LWRs, particularly with plutonium fueling. Although Pu is consumed by both neutron capture and fission, the nonfission capture produces additional plutonium isotopes such as Pu, , and If disposition of WPu emphasizes the destruction of all plutonium isotopes, nonfission capture of neutrons in Pu is less productive in destroying plutonium than using those neutrons for more extended fission of Pu during an irradiation cycle.
Uncertain Canadian Acceptance. The Canadian government has reportedly suggested to U. But further discussions between the U.
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Canada has previously avoided using either enriched uranium or plutonium fuels in CANDU reactors, and might reject this plutonium-use option as well. Yet Canada has also traditionally played an active role in disarmament; playing a central role in disposition of materials resulting from nuclear arms reductions might well be appealing enough to overcome the resistance to use of weapons materials.
Canadian public acceptance is also an open question. Large-Scale International Plutonium Transport. The distance over which plutonium would have to be transported to be burned in CANDU reactors would be greater than that in using U.
The attendant controversies and risks of theft would be correspondingly larger. Possibly more important in political terms than the sheer distances is the need for the material to be shipped across international borders, to a non-nuclear-weapon state. This would require larger quantities of fuel to be produced than in the case of LWRs, increasing costs and countering the advantage of simpler fuel fabrication described above. Lower Radioactivity and Small Size of Bundles. Because of the relatively low burnup even when enriched with plutonium and small size of the CANDU MOX bundles, the gamma-radiation dose rates from them would be somewhat lower than those from LWR spent fuel of equal age.
Safeguards Issues of Online Refueling. Fuel can be removed from CANDU reactors at any time without shutdown of the reactor, and the fuel elements are substantially smaller and more portable than is the case for LWRs. For fuel containing more plutonium, still more intensive safeguarding would be needed. In addition, the task of accounting for and securing complete fuel assemblies for either a CANDU or an LWR is substantially easier than that of accounting for bulk plutonium at a MOX fabrication plant.
Therefore, the net additional security risks. The dose rate also falls off more rapidly with distance for the CANDU fuel bundle, because of its more compact size. Since the units also produce excess steam for local heating, the total gross electrical equivalent output is MWe per unit. Four of these units are in operation at the Bruce A site. The reference case of natural uranium fueling i. The first option for plutonium disposition, using fuel elements similar to those now used with natural uranium, would have an average plutonium loading of 1.
The innermost pin and the first ring of pins surrounding it 7 of the 37 pins in the bundle would consist of 5-percent dysprosium-oxide burble absorber in depleted uranium. These absorber pins would compensate for the greater reactivity of the plutonium fuel compared to natural uranium; in a CANDU spectrum, the dysprosium absorption would be reduced at approximately the same rate as the plutonium reactivity, helping to flatten the reactivity of the fuel over its life.
The next ring would have 2-percent plutonium oxide, and the outermost ring 1. With this approach, each reactor would consume 1. No hardware changes to the reactor system would be needed to operate within the existing safety envelope, according to the manufacturer. Demonstration irradiations of this design in power reactors are expected to begin in AECL , p. The panel was informed by representatives of the vendor that this figure was arrived at not by analysis designed to estimate the maximum plutonium loading that could be safely accommodated in CANDU reactors, but rather because this was the loading required to meet DOE's speci.
The primary potential advantage of this latter approach is not speeding the process, but drastically reducing the number of MOX fuel bundles that would have to be fabricated, thereby potentially reducing the cost of the operation. Licensing reactor operations with plutonium would probably be a less difficult issue than securing agreement on the basic approach. In general, the process in Canada relies more on co-operation between licensees and the board, and less on an adversarial process.
Related Mixed-Oxide Fuel Fabrication Plant and Plutonium Disposition: Management and Policy Issues
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