As the demand for economical and clean energy increases, more utilities desire long-term, consistent, and secure uranium supply to ensure fuel resource sustainability. This is especially the case for countries that do not have abundant sources of uranium and do not wish to be reliant on foreign imported supplies.
To meet these requirements, better utilization of existing uranium fuel resources and the use of alternative fuels, such as recycled uranium (RU)-based fuels in CANDU reactors are proposed strategies.
CANDU® reactors can efficiently and economically utilize alternative fuels, as shown in Figure 1, due to their characteristics of neutron economy, versatility of fuel channel design, and simplicity of fuel bundle design. A heavy water moderator and heat transport system results in favourable reactor physics for the use of advanced fuel cycles. CANDU reactors use low-temperature heavy water as the moderator in order to slow neutrons without absorption and allow capture by fissile isotopes. Also, compared to other reactor technologies, CANDU reactors have a large moderator-to-fuel ratio which strengthens the advantage further. The neutrons are highly thermalized and at a lower temperature than in other reactor designs, therefore the neutron spectrum is ‘softer’ than that in other reactor technologies, resulting in fewer losses of neutrons through absorption by non-fissile materials. A versatile pressure tube design allows for online refuelling and allows the reactor operator to shape the core properties for optimization of fuel utilization. This feature gives CANDU reactors the ability to manage alternative fuel cores effectively without the need to shut the reactor down, as would be the case in a batch-fuelled reactor, allowing for a smoother transition from one type of fuel to another. This capability also means that the reactor can be operated just above the criticality threshold and large amounts of burnable poison, which decrease neutron economy, are not required as they would be for light water reactors. The CANDU fuel bundle is simple and small (approximately 20 kg weight, 500 mm length and 100 mm diameter), which allows for easy and low-cost adaptations to accommodate alternative fuels. Alternative fuels can simply be loaded into the fuel bundle with minimal changes in the reactor design.
Recognizing this advantage, Candu Energy Inc. (Candu) in partnership with China National Nuclear Corporation’s (CNNC) Third Qinshan Nuclear Power Corporation (TQNPC), China North Nuclear Fuel Company (CNNFC), and the Nuclear Power Institute of China (NPIC) developed Natural Uranium Equivalent (NUE) fuel for implementation in the CANDU reactors in China. NUE fuel offers the simplest, quickest, and least-expensive path to efficient utilization of alternative fuel sources in existing CANDU reactors without modifications to the reactor design, to operations or to licensing basis. NUE fuel is the first step in demonstrating advanced fuel cycles in operating CANDU reactors.
Natural uranium equivalent (NUE) fuel
NUE fuel is a mixture of recycled uranium (RU, also known elsewhere as RepU) and depleted uranium (DU) combined in a 37-element fuel bundle such that the resulting fuel will behave similarly to natural uranium (NU) fuel. That is, the reactor performance as a result of using NUE fuel will be similar to the performance with NU fuel. RU is recovered from recycled light water reactor (LWR) fuel (originally enriched to between 3-5% wt% U-235) and has a nominal U-235 concentration in the range of 0.85-0.99 wt%, concentrations higher than natural uranium used in CANDU reactors (0.7 wt% U-235). DU, the ‘tails’ of the uranium enrichment activity, has a typical U-235 concentration range of 0.2-0.3 wt%. RU and DU are blended in such a manner that NUE will have an effective U-235 concentration similar to natural uranium found in nature. In practice, the U-235 content of RU and DU will vary and the ratios of RU and DU required to produce NUE will also vary.
The use of RU and DU to manufacture NUE fuel for CANDU reactors has multiple benefits. It reduces the burden faced by facilities for conditioning, monitored storage, and/or costly re-enrichment and handling for reuse in LWRs. It improves the utilization rate and supply longevity of NU. It results in increased electricity production from otherwise waste materials. It has potential economic benefits with lower expected front fuel cycle costs. The cost of RU and DU materials is expected to be lower than NU. It creates a symbiotic relationship with other nuclear reactor technologies (three-to-four LWRs can fuel one CANDU reactor). It is the first and necessary step to achieving a closed fuel cycle.
There is considerable industrial-scale experience in the civilian recycling of used fuel in several countries. There is currently a global recycling capacity of 3800 tonnes/year of used LWR fuel in countries such as France, United Kingdom, Japan, and Russia [1]. According to [2], more than 15,000 tonnes of RU from spent commercial LWR fuel is expected to be available by the end of 2013. Furthermore, China is also planning to build a full-scale recycling plant. Appropriate measures are used to ensure the safe operation of recycling facilities and plants. Purification and conditioning for storage, re-enrichment, and/or direct utilization are well-controlled. Due to the high cost of re-enrichment of RU for LWR plants, the majority of RU inventory is placed in temporary storage.
The concentration of the isotopes in RU depends on a number of factors such as:
- Origin of the fuel (that is, whether it is from natural, enriched, or RU-based fuels)
- The type of reactor the fuel had been used in
- The degree of initial U-235 enrichment
- The level of fuel discharge burnup
- The ageing period of the discharged used fuel in cooling ponds.
The even-numbered uranium isotopes in RU, U-234 and U-236, act as neutron absorbers in the fuel. Therefore, the U-235 concentration in the NUE will be slightly higher than that in NU to account for the presence of these neutron absorbers. The degree of extra enrichment required for NUE fuel is an order of magnitude less than what is required when RU is re-enriched for use in LWRs.
Another under-utilized nuclear fuel resource, DU, is derived as a by- product of enrichment processing. World stocks indicate abundance of DU availability, estimated at 1.2 million tonnes [3], with an expectation to grow further as nuclear power capacity is increased. Historically, DU has been viewed as a waste product because considerable enrichment processing is required to bring the U-235 concentration to a useful level. Currently, the only significant commercial use for DU is as a shielding material.
NUE demonstration irradiation at Qinshan
In 2008, Candu initiated a collaborative programme with TQNPC, CNNFC, and NPIC to develop and demonstrate the use of NUE fuel in the Qinshan CANDU reactors. The demonstration irradiation consisted of loading two channels in the Qinshan CANDU Unit 1 with 24 NUE fuel bundles following the regular refuelling cycle over a period of approximately one year. Technical analyses were performed to support the licensing submission required for regulatory approval of the demonstration irradiation. The scope of work also included the activities required for manufacturing the NUE fuel bundles.
Various studies were done to confirm that NUE fuel behaves similarly to NU fuel. In particular, the studies demonstrated that NUE fuel stays within the current Qinshan CANDU analyzed safety case such that little or no changes to the licensing, reactor design, and reactor safety parameters would result from the NUE fuel implementation. Reactor physics analyses determined the composition of the NUE fuel and corresponding characteristics. The impact of NUE fuel on interfacing reactor systems, including fresh and spent fuel handling, cooling systems, reactor monitoring systems and radiation dose assessment was analysed. Fuel design and technical specifications were investigated. A safety analysis of the limiting safety cases in the Qinshan Revised Final Safety Analysis Report was carried out. The results from these studies indicate that the use of up to 24 NUE fuel bundles in the operating CANDU reactor would not have an impact on the existing licensing case.
In addition to the technical analyses, a plan was developed that outlined how the demonstration irradiation would be performed and defined the acceptance criteria for its success.
Considerable effort was given to manufacturing the NUE fuel bundles for the demonstration irradiation. Compared to manufacturing NU fuel, NUE requires more than one input resource material. RU and DU have to be blended in a specific manner to meet the specified fuel composition. This operation was performed using the existing equipment at the CNNFC fuel manufacturing facility. The remainder of the fuel manufacturing process remained the same as for NU fuel. In total, 26 fuel bundles were manufactured; 24 bundles for the test irradiation and two bundles for archiving.
In March 2010, testing of the NUE fuel started in Unit 1 of the Qinshan CANDU reactor. During the NUE demonstration irradiation, the performance of the fuel was monitored to ensure that it met acceptance criteria, and NUE data was compared against data from comparable NU fuel operation.
The NUE fuel bundles were loaded following the normal operating procedures used for NU fuel. Levels of CANDU light-water moderator compartments, called liquid zone controllers (LZC), and the channel/ zone power did not indicate any abnormal levels. In addition, the power of the channels and test channel bundle containing NUE fuel was found to be within the corresponding operational limits for NU fuel. The average zone level of the liquid zone controllers in the reactor with NUE were found to correspond to those of a fully NU-fuelled reactor. These results indicate that NUE fuel does not impact reactor operations and performance and that the fuel behaves as predicted.
At the end of the refuelling cycle and after a period of time for cooling, the irradiated bundles underwent a thorough in-bay visual examination for any defects or anomalies. The scope of the in-bay inspection included bundle inspection, bundle disassembly, and element inspection. The NUE demonstration irradiation bundles were subject to inspection along with reference NU bundles from similar locations for direct comparison.
Throughout all of the testing, results of the inspection of the NUE fuel bundles were consistent with the reference NU fuel bundles. There were no defects or evidence of sheath swelling observed during the inspection. The results also indicated that non-homogeneity issues did not arise during the NUE mixing process in fuel manufacturing. All fuel element sheath-to-endcap welds appeared clean and defect free. Pellet interface circumferential ridging was distinct and consistent with those observed from the reference NU fuel bundle. All of the visual evidence, based on the in-bay inspection, suggests that the NUE fuel bundle performance is the same as that of the NU fuel bundle. Further confirmation of other performance parameters will be provided through post-irradiation examination (PIE).
A number of irradiated elements have been transported to a facility where PIE will be performed, again to confirm the fuel performance was as predicted by the analyses/assessments. Multiple PIE inspections were planned. Visual examination was carried out to record any anomalies or unusual features. Visual examination includes overall appearance, sheath oxide condition, visual indication of defects, bearing pad wear, and spacer pad wear. Element profilometry measured the axial element profile and strains along three angles with one reference angle through bearing pad centreline. Average diameters and residual sheath strains for the mid-pellet and the pellet-pellet interface are available for the element profilometry measurements. Fission gas analysis measured fission gas pressure and volumes, and gamma spectral analysis was also carried out. Pellet grain growth was observed to measure the pellet grain growth to disposition fission gas measurement anomalies (UO2 ceramography). Burnup estimates were made by measuring chemical burnup at selected location in selected elements. Uranium isotopic abundances were also to be measured.
At the time of writing, the results from the PIE had not yet been obtained. However, it is expected that the PIE results will confirm the observations obtained through the in-bay inspection and demonstration irradiation; namely that there are no differences between the NUE fuel and the NU fuel.
Full-core implementation
Based on the success of the NUE fuel demonstration irradiation, Candu and TQNPC signed a commercial contract to implement NUE fuel in the entire core of both Qinshan CANDU reactors. Candu’s scope in the project was to complete the technical analyses for TQNPC to use in their licensing application submission to the Chinese regulatory body. The licence application package will contain analyses demonstrating that NUE fuel does not cause any changes to the reactor performance and safety and that the station’s existing analyzed safety case will remain applicable.
The start date for Candu’s scope of work in the NUE Full Core Implementation project was March 2011; it was to last two years. Once licensing approval is obtained, it is estimated that an additional 18-24 months are required to fully implement NUE fuel in the Qinshan CANDU reactors.
The licensing application submission required multiple technical analyses. They are discussed below.
The first activity in the NUE fuel scope of work was to determine the mixing ratio of RU and DU that resulted in the maximum burnup while staying within the limits of the analyzed safety case. The mixing ratio calculation took into consideration the quantities of uranium isotopes present in each constituent fuel input. To provide the utility with the flexibility to utilize RU within a range of isotopic concentrations, a combination method software was developed. The combination method software uses the isotopic characteristics of the RU and the DU feed stocks, determines the appropriate ratio of each fuel required, and provides the isotopic content of the resulting NUE fuel, with the corresponding tolerances. Tolerances were set on the uranium isotopic values and are required to be measured at the end of the mixing portion of the manufacturing line in order to ensure that the manufactured fuel meets the required composition. The fuel design and technical specifications are based upon the physics results and the isotopic tolerances required.
Fresh NU fuel at CANDU stations is handled by hand and is visually inspected prior to insertion of the fuel into the reactor. Fuel manufacturing is also mostly done manually. Unlike NU fuel, NUE fuel will have some gamma activity due to the daughter products of U-232 uniquely present in RU. The dose rates on contact with the NUE fuel bundles have been estimated to be up to a factor of six times higher than with NU fuel bundles. Calculations indicate that the radiation exposure to operators is still below the regulatory limits and is acceptable provided that minor localized shielding is implemented in the new fuel storage room and that exposures are reduced in the new fuel loading room. The spent fuel decay power at short decay times following reactor core discharge was found to be equivalent to NU fuel and it is only at long decay times (that is, more than six years) that the decay heat was calculated to be slightly higher (4%) than NU fuel. These increases are minor and the full core implementation of NUE fuel will result in changes to spent fuel handling and spent fuel storage that are manageable.
Full-core simulations were conducted in order to demonstrate that refuelling in various configurations would meet the current analyzed envelope for safe operation given in the Qinshan revised final safety analysis report (RFSAR). A number of core-following simulations were performed and the results indicate that the refuelling scenarios are feasible and will not exceed the current Qinshan CANDU operational limits.
The regional overpower protection system is designed to detect overpower conditions in the reactor that could impact fuel integrity and initiate a timely reactor shutdown. The ROP system must protect the reactor against overpowers in the fuel, whether due to localized peaking within the core or a general increase in core level. In general, the NUE equilibrium core will be no more restrictive than the NU equilibrium-fuelled core in terms of the ROP margin-to-trip. The operating reactor power will not be affected by the change of fuel. In addition, there is a slightly higher margin-to-dryout for some cases in the NUE core which could result in a gain in setpoint for the NUE full core if desired.
The limiting NUE fuel safety cases were analysed to confirm that a full NUE core implementation would have a negligible effect on the current safety analysis results. The limiting cases considered include:
Large loss-of-coolant-accident (LOCA) with 100% pump suction break (large LOCA event)
Small LOCA with 2.5% reactor inlet header (RIH) break (small LOCA event)
Loss of forced circulation: single pump trip for initial core power of 90% full power (loss of heat transport system (HTS) flow event)
Steam and feedwater circuit event: loss of feedwater pumps (loss of feedwater flow event).
The results indicate that changing the core to NUE fuel has no significant impact on the results of the limiting safety analysis cases, compared with a NU-fuelled core. For each of the limiting cases, the corresponding acceptance criteria are met and sufficient margins to the safety limits are maintained.
A systematic review of all reactor systems was performed in order to assess and evaluate the potential impact of a full core of NUE fuel on CANDU systems. The results from the review indicate that no significant impact is expected on these systems. Due to the difference in isotopic composition of the fuel material, the NUE fuel will possess a higher radiation dose rate than NU fuel. Additionally, spent NUE fuel will produce a slightly higher decay heat when compared with NU fuel. These differences will be managed by implementing minor localized shielding in the new fuel storage room and by ensuring that procedures resulting in exposures are reduced in the new fuel loading room.
A major component of the licensing submission to load NUE fuel into the reactor is the revised final safety analysis report (RFSAR). The RFSAR requires updated data to cover analysis of NUE fuel. For each of the limiting cases, the corresponding acceptance criteria are met and sufficient margin to the safety limits are maintained. The analyses performed for the NUE full core implementation indicate that NUE fuel will behave in a similar way to NU fuel such that the NUE fuel remains within the analyzed safety case for the NU fuel, the corresponding safety acceptance criteria are met, and sufficient margin to the safety limits are maintained.
About the authors
Valeh Aleyaseen, Catherine M. Cottrell and Sermet Kuran, Candu Energy Inc. Mississauga, Ontario, Canada
This paper was first presented at the Women in Nuclear Conference 2013, September 29-October 1, Pembroke, Ontario, Canada.
The authors wish to acknowledge colleagues at TQNPC, Candu, and Atomic Energy of Canada Limited (AECL) for their innovative and high- quality contributions to the NUE fuel project.
This article was first published in the January 2014 issue of Nuclear Engineering International magazine.
References
[1] ‘Processing of Used Nuclear Fuel’, World Nuclear Associaton Information Library, http://www.world-nuclear.org/info/inf69.html (accessed 6 December 2013).[2] IAEA, Management of Reprocessed Uranium – Current Status and Future Prospects, TECDOC-1529, Vienna, 2007.
[3] OECD Nuclear Energy Agency, "Management of Depleted Uranium: A Joint Report by OECD Nuclear Energy Agency", OECD, 2001.
