ACCIDENT TOLERANT FUEL AND RESULTING FUEL EFFICIENCY IMPROVEMENTS

Transcription

1 Advances in Nuclear Fuel Management V (ANFM 2015) Hilton Head Island, South Carolina, USA, March 29 April 1, 2015, on CD-ROM, American Nuclear Society, LaGrange Park, IL (2015) ACCIDENT TOLERANT FUEL AND RESULTING FUEL EFFICIENCY IMPROVEMENTS Jeffrey Secker, Fausto Franceschini, and Sumit Ray Westinghouse Electric Company, LLC 1000 Westinghouse Drive, Cranberry Township, PA, 16066, USA seckerjr@westinghouse.com, francef@westinghouse.com, rays@westinghouse.com Keywords:, UN, U 3 Si 2, Fuel Efficiency ABSTRACT Fuel designs using advanced, accident tolerant fuel materials can improve fuel efficiency and extend fuel management capability in addition to improving safety margins for LWRs. The use of cladding material can reduce fuel cycle costs by about 2% if it can be manufactured to the current thickness of zirconium alloy based cladding in use in PWRs today. The increased pellet densities associated with the higher density U 3 Si 2 or UN material also can reduce fuel costs by an additional 4-6% beyond the cost reduction for 18 month cycles or 8-11% for 24 month cycles. Because of the increased density, the use of these materials also extends the energy output and cycle length capability for PWR fuel assemblies while remaining below the 5 w/o enrichment limit for commercial fuel and can make 24 month cycle operation economical for today s uprated, high power density PWRs. 1. INTRODUCTION Current PWR fuel designs using UO 2 pellets and zirconium alloy based cladding have a long history of safe operation. However, under severe beyond design basis accident conditions, zirconium alloys can react exothermically with steam under high temperature conditions degrading the fission product barrier and liberating hydrogen, which becomes an explosive hazard. UO 2 pellets can also melt when long term cooling is inadequate. Cladding improvements to reduce high temperature corrosion and fuel pellet improvements to increase thermal conductivity are under active investigation to further improve LWR safety while reducing fuel costs. 2. ACCIDENT TOLERANT FUEL Silicon Carbide 1 () has been identified as a potential cladding material that is much less susceptible to high temperature corrosion and does not react with water to liberate hydrogen gas. Fuel pellet materials 2 such as UN or U 3 Si 2 also show improved thermal conductivity compared to UO 2 and will operate at lower temperatures. In addition to improving safety, use of these materials can also increase fuel efficiency and reduce fuel cycle costs, primarily as a result of the higher uranium density associated with these pellet materials which ultimately allows better fuel usage (e.g. the ANS 2015, Topical Meeting ANFM 2015, p. 1/10

2 number of feed assemblies per reload can be reduced). The higher uranium density can also extend the core operating capability compared to current fuels while maintaining the current 5 w/o 235 U enrichment limit for commercial fuel and economically competitive fuel management. Fuel management analysis has been performed to determine the effect on fuel costs resulting from using these materials. The Westinghouse PARAGON 3 /ANC 4 PWR core analysis package was used to compare potential fuel management scenarios using the advanced materials to current PWR fuel management schemes relying on UO 2 fuel with Zr cladding for both 18 and 24 month cycles. A 4 loop, 193 assembly core at a 3587 MWt power rating was assumed as representative for the analysis MONTH CYCLE FUEL MANAGEMENT A reference 18 month cycle equilibrium core model was generated for current Zr/UO 2 fuel, assuming a corresponding energy requirement equivalent to 510 Effective Full Power Days (EFPD). This corresponds to 18 calendar months between refueling with an assumed 25 day outage and 98% capacity factor. Westinghouse 17x17 RFA fuel with a rod diameter of inches was modeled. The core design loaded 76 feed assemblies out of 193 total each cycle, which was the lowest number achievable consistent with the 5 w/o 235 U enrichment limit for commercial fuel. The fuel design contained 8 inch 3.20 w/o annular axial blankets on the top and bottom of the fuel rod and utilized ZrB 2 Integral Fuel Burnable Absorbers 5 in the central 128 inch fuel stack on selected fuel rods. The ZrB 2 coating was assumed to be compatible with the advanced pellet materials. This will need to be confirmed. This resulted in a central enrichment of w/o and an average enrichment of w/o to meet the 510 EFPD cycle length requirement using 76 feed assemblies. The core loading pattern is shown in Figure 1. A comparable model was generated replacing the zirconium alloy based cladding with of the same dimensions. The use of improves fuel efficiency compared to zirconium based cladding of the same thickness, due to the lower absorption cross sections of its constituents and the resulting improvements in neutron economy. The resulting average enrichment requirements declined by w/o and fuel cycle costs declined by 2.2%. However, manufacturing capability for fuel cladding remains under development and it is not clear that can be manufactured to the current inch cladding thickness used in Westinghouse 17x17 fuel. Additional sensitivity studies were performed with inch cladding thickness. For this case the pellet diameter and pellet-cladding gap was unchanged, so the rod diameter increased. The inch case increased average enrichment requirements to by w/o, a 0.4% cost increase compared to Zr/UO 2. This deterioration in neutron economy is caused by the loss in neutron moderation resulting from water displacement from the cladding and, to a lower extent, by the increase in parasitic captures following the increase in cladding volume. The fuel cycle cost economics of cladding will depend on how thin the cladding can be manufactured while retaining reliable, failure free operation of the cladding. ANS 2015, Topical Meeting ANFM 2015, p. 2/10

3 Figure 1 Reference 18 Month Cycle Loading Pattern 76 Feed UO 2 /Zr Pellet materials with increased density were also modeled. U 3 Si 2 increases the pellet density by about 11% compared to UO 2, while UN increased the density by about 30%. A challenge for UN is the need to enrich the nitrogen in 15 N to reduce neutron absorption by 14 N. A case using U 3 Si 2 with inch cladding reduced the feed assembly requirements by 8 assemblies, from 76 for UO 2 to 68 assemblies for U 3 Si 2, with the average enrichment decreasing from to w/o 235 U. The loading pattern is shown in Figure 2. This reduced fuel costs by 5.6% compared to UO 2 /Zr, and by 3.4% compared to UO 2 /. The corresponding UN case with inch cladding, using nitrogen 99% enriched in 15 N, required only 52 feed assemblies using an average enrichment of w/o. The loading pattern is shown in Figure 3. This resulted in a 7.6% reduction in fuel costs compared to UO 2 /Zr (5.4% compared to UO 2 /). The cases with increased cladding thickness followed the same trend as with UO 2 pellets and cladding. Both cases assumed the same pellet geometry as the current UO 2 pellet design and the same pelletcladding gap. The pellet design may need to be adjusted to achieve acceptable fuel rod performance. The assumed commodity costs are provided in Table 1 and the fuel management results are shown in Table 2. ANS 2015, Topical Meeting ANFM 2015, p. 3/10

4 Figure 2 18 Month Cycle Loading Pattern 68 Feed U 3 Si 2 / Figure 3 18 Month Cycle Loading Pattern 52 Feed UN/ ANS 2015, Topical Meeting ANFM 2015, p. 4/10

5 Table 1 Economic Input Assumptions Item Value U 3 O 8 Price ($/lb) $55 Conversion Price ($/kgun) $12 SWU Price ($/SWU) $ N Price ($/kg 15 N) $1,130 Fabrication Price ($/kgu) $275 Pre-Operational Interest (%/Yr) 6.0% Spent Fuel Cooling Time (Months) 120 Spent Fuel Disposal Charge ($/MWhre) $1 Spent Fuel Dry Storage Charge ($/Fuel Assembly) $50,000 Cycle Length (Months) 18 or 24 Rated Thermal Power (MW t ) 3,587 Rated Net Electric Output (MW e ) 1,112 Inflation Rate 2.0% Return on Fuel Investment (%/Yr) 8.0% For this study, the fuel fabrication cost was assumed to be constant for all of the fuel types, since the actual cost for the cladding and improved pellet materials have not yet been determined. For the UN pellets, there was an additional cost input for enriching nitrogen in 15 N. Actual manufacturing cost are expected to be somewhat higher than for current fuel designs, which will slightly decrease the fuel cycle costs benefits described in this study. Fuel fabrication costs account for less than 10% of the fuel cycle cost for these cases, so higher fabrication costs will reduce the benefits of the advanced fuel designs slightly. For this study, 95.5% of the theoretical density of the pellet material was assumed. ANS 2015, Topical Meeting ANFM 2015, p. 5/10

6 Table 2 Clad and thickness (in) ZIRLO Month Cycle Fuel Management Summary Pellet Pellet # Central Average Density Feeds Enrich Enrich (g/cm 3 ) (w/o) (w/o) Reload Cost ($M) Fuel Cycle Costs ($/MWhre) (% Diff) UO Ref. UO % UO % U 3 Si % U 3 Si % UN (99% 15 N) -7.6% UN (99% 15 N) -3.8% MONTH CYCLE FUEL MANAGEMENT For typical high power density PWR s, 24 month cycles are generally not economical compared to 18 month cycles. Typically more than one half of the core must be loaded with fresh fuel which results in high enrichment fuel being discharged after only one cycle of use. The fuel cost increase generally outweighs the savings in outage and replacement power costs that result from eliminating one refueling outage every 6 years. Assuming a 25 day refueling outage and 98% operating capacity factor, a cycle length of 692 EFPD was used. The reference UO 2 /Zr fuel requirements increased feed assembly requirements by 36 assemblies compared to the 18-month cycle 510 EFPD core design, for a total of 112 fresh assemblies with an average enrichment of w/o. The loading pattern is shown in Figure 4. Fuel costs increase by 13.4% compared to the reference 18 month cycle. ANS 2015, Topical Meeting ANFM 2015, p. 6/10

7 Figure 4 Reference 24 Month Cycle Loading Pattern 112 Feed UO 2 /Zr The use of higher density fuel can make 24 month cycles economical. The U 3 Si 2 case with inch thick cladding reduces the feed fuel requirements to 96 feed assemblies, so all assemblies can be irradiated for two cycles. The loading pattern is shown in Figure 5. The average enrichment was w/o which provides a 10.3% fuel cost reduction compared to the 24-month reference UO 2 /Zr case. The UN case with inch cladding required 80 feed assemblies, or 32 fewer than the UO 2 case. The loading pattern is shown in Figure 6. The average enrichment was w/o for a fuel cost reduction of 13.2 %. The fuel costs are then comparable to an 18 month cycle UO 2 /Zr case, so 24 month cycles will reduce overall electricity generation costs with the reduced outages and replacement power costs. If cladding thickness with must be increased, then the fuel cost performance of the higher density pellet material is deteriorated by the loss in neutron economy from the thicker cladding, but still presents significant fuel cost savings potential even for cladding thicknesses exceeding 0.03 inch. The 24 month cycle results are summarized in Table 3. ANS 2015, Topical Meeting ANFM 2015, p. 7/10

8 Figure 5 24 Month Cycle Loading Pattern 96 Feed U 3 Si 2 / Figure 6 24 Month Cycle Loading Pattern 80 Feed UN/ ANS 2015, Topical Meeting ANFM 2015, p. 8/10

9 Table 3 Clad and thickness (in) ZIRLO Month Cycle Fuel Management Summary Pellet Pellet # Central Average Density Feeds Enrich Enrich (g/cm 3 ) (w/o) (w/o) Reload Cost ($M) Fuel Cycle Costs ($/MWhre) (% Diff) UO Ref. U 3 Si % U 3 Si % UN (99% 15 N) -13.2% UN (99% 15 N) -10.0% 5. CONCLUSIONS Fuel designs using advanced, accident tolerant fuel materials can improve fuel efficiency and extend fuel management capability in addition to improving safety margins for LWRs. The use of cladding material can itself reduce fuel cycle costs by about 2% if it can be manufactured to the current thickness of zirconium alloy based cladding in use in PWRs today. The increased pellet densities associated with the higher density U 3 Si 2 or UN material also can bring significant fuel cost reductions, 4-6% beyond the cost reduction for 18 month cycles or 8-11% for 24 month cycles. Because of the increased pellet density, the use of these materials also extends the energy output and cycle length capability while remaining below the 5 w/o enrichment limit for commercial fuel and can make 24 month cycle operation economical for today s uprated, high power density PWRs. ACKNOWLEDGMENTS The authors would like to thank Ed Lahoda of Westinghouse Electric Co for his extensive research into Accident Tolerant Fuel designs for PWRs. REFERENCES 1. F. FRANCESCHINI and E. LAHODA, Neutronic Behavior and Impact on Fuel Cycle Costs of Silicon Carbide Clad, 2010 ANS Winter Meeting, Las Vegas, NV, November 7-11, ANS Transactions Vol. 103, American Nuclear Society (2010). ANS 2015, Topical Meeting ANFM 2015, p. 9/10

10 2. F. FRANCESCHINI and E. LAHODA, Advanced Fuel Development to Improve Fuel Cycle Cost in PWR, Proceedings of GLOBAL 2011, Makhuri, Japan, December , Paper No M. OUISLOUMEN et al., PARAGON: The New Westinghouse Assembly Lattice Code, ANS Int. Mtg. on Mathematical Methods for Nuclear Applications, Salt Lake City, Utah, USA, Y. S. LIU et al., ANC: A Westinghouse Advanced Nodal Computer Code, WCAP A, Westinghouse Electric Company, September J. SECKER and J. BROWN, Westinghouse PWR Burnable Absorber Evolution and Usage, 2010 ANS Winter Meeting, Las Vegas, NV, November 7-11, ANS Transactions Vol. 103, American Nuclear Society (2010) ANS 2015, Topical Meeting ANFM 2015, p. 10/10