ACCIDENT TOLERANT FUEL AND RESULTING FUEL ......Fuel designs using advanced, accident tolerant fuel materials can improve fuel efficiency and extend fuel management capability in addition

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)

© ANS 2015, Topical Meeting ANFM 2015, p. 1/10

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 [email protected], [email protected], [email protected]

Keywords: SiC, UN, U3Si2, 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 SiC 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 U3Si2 or UN material also can reduce fuel costs by an additional 4-6% beyond the SiC 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 UO2 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. UO2 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 Carbide1 (SiC) 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 materials2 such as UN or U3Si2 also show improved thermal conductivity compared to UO2 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

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J. R. Secker, et. al., Accident Tolerant Fuel and Resulting Fuel Efficiency Improvements


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 235U 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 PARAGON3/ANC4 PWR core analysis package was used to compare potential fuel management scenarios using the advanced materials to current PWR fuel management schemes relying on UO2 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.

3. 18 MONTH CYCLE FUEL MANAGEMENT

A reference 18 month cycle equilibrium core model was generated for current Zr/UO2 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 0.374 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 235U 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 ZrB2 Integral Fuel Burnable Absorbers5 in the central 128 inch fuel stack on selected fuel rods. The ZrB2 coating was assumed to be compatible with the advanced pellet materials. This will need to be confirmed. This resulted in a central enrichment of 4.870 w/o and an average enrichment of 4.722 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 SiC of the same dimensions. The use of SiC 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 0.109 w/o and fuel cycle costs declined by 2.2%.

However, SiC manufacturing capability for fuel cladding remains under development and it is not clear that SiC can be manufactured to the current 0.0225 inch cladding thickness used in Westinghouse 17x17 fuel. Additional sensitivity studies were performed with 0.0300 inch cladding thickness. For this case the pellet diameter and pellet-cladding gap was unchanged, so the rod diameter increased.

The 0.0300 inch case increased average enrichment requirements to by 0.019 w/o, a 0.4% cost increase compared to Zr/UO2. 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 SiC cladding will depend on how thin the cladding can be manufactured while retaining reliable, failure free operation of the cladding.



Figure 1 Reference 18 Month Cycle Loading Pattern – 76 Feed UO2/Zr

Pellet materials with increased density were also modeled. U3Si2 increases the pellet density by about 11% compared to UO2, while UN increased the density by about 30%. A challenge for UN is the need to enrich the nitrogen in 15N to reduce neutron absorption by 14N.

A case using U3Si2 with 0.0225 inch SiC cladding reduced the feed assembly requirements by 8 assemblies, from 76 for UO2 to 68 assemblies for U3Si2, with the average enrichment decreasing from 4.722 to 4.591 w/o 235U. The loading pattern is shown in Figure 2. This reduced fuel costs by 5.6% compared to UO2/Zr, and by 3.4% compared to UO2/SiC. The corresponding UN case with 0.0225 inch SiC cladding, using nitrogen 99% enriched in 15N, required only 52 feed assemblies using an average enrichment of 4.591 w/o. The loading pattern is shown in Figure 3. This resulted in a 7.6% reduction in fuel costs compared to UO2/Zr (5.4% compared to UO2/SiC). The cases with increased cladding thickness followed the same trend as with UO2 pellets and SiC cladding. Both cases assumed the same pellet geometry as the current UO2 pellet design and the same pellet-cladding 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.



Figure 2 18 Month Cycle Loading Pattern – 68 Feed U3Si2/SiC

Figure 3 18 Month Cycle Loading Pattern – 52 Feed UN/SiC



Table 1 Economic Input Assumptions

Item Value

U3O8 Price ($/lb) $55

Conversion Price ($/kgUn) $12

SWU Price ($/SWU) $142

15N Price ($/kg15N) $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 (MWt) 3,587

Rated Net Electric Output (MWe) 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 SiC cladding and improved pellet materials have not yet been determined. For the UN pellets, there was an additional cost input for enriching nitrogen in 15N. 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.



Table 2 18 Month Cycle Fuel Management Summary Clad and thickness (in)

Pellet Pellet Density (g/cm3)

# Feeds

Central Enrich (w/o)

Average Enrich (w/o)

Reload Cost ($M)

Fuel Cycle Costs ($/MWhre) (% Diff)

ZIRLO 0.0225

UO2 10.47 76 4.870 4.722 125.351 9.209 Ref.

SiC 0.0225

UO2 10.47 76 4.750 4.613 122.545 9.003 -2.2%

SiC 0.0300

UO2 10.47 76 4.890 4.741 125.858 9.247 +0.4%

SiC 0.0225

U3Si2 11.65 68 4.722 4.591 118.298 8.691 -5.6%

SiC 0.0300

U3Si2 11.65 68 4.830 4.690 120.803 8.873 -3.6%

SiC 0.0225

UN (99% 15N)

13.68 52 4.873 4.723 115.834 8.511 -7.6%

SiC 0.0300

UN (99% 15N)

13.68 56 4.752 4.618 120.575 8.857 -3.8%

4. 24 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 UO2/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 4.980 w/o. The loading pattern is shown in Figure 4. Fuel costs increase by 13.4% compared to the reference 18 month cycle.



Figure 4 Reference 24 Month Cycle Loading Pattern – 112 Feed UO2/Zr

The use of higher density fuel can make 24 month cycles economical. The U3Si2 case with 0.0225 inch thick SiC 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 4.894 w/o which provides a 10.3% fuel cost reduction compared to the 24-month reference UO2/Zr case. The UN case with 0.0225 inch cladding required 80 feed assemblies, or 32 fewer than the UO2 case. The loading pattern is shown in Figure 6. The average enrichment was 4.607 w/o for a fuel cost reduction of 13.2 %. The fuel costs are then comparable to an 18 month cycle UO2/Zr case, so 24 month cycles will reduce overall electricity generation costs with the reduced outages and replacement power costs. If cladding thickness with SiC 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.



Figure 5 24 Month Cycle Loading Pattern – 96 Feed U3Si2/SiC

Figure 6 24 Month Cycle Loading Pattern – 80 Feed UN/SiC



Table 3 24 Month Cycle Fuel Management Summary Clad and thickness (in)

Pellet Pellet Density (g/cm3)

# Feeds

Central Enrich (w/o)

Average Enrich (w/o)

Reload Cost ($M)

Fuel Cycle Costs ($/MWhre) (% Diff)

ZIRLO 0.0225

UO2 10.47 112 4.980 4.980 192.874 10.444 Ref.

SiC 0.0225

U3Si2 11.65 96 4.950 4.894 172.931 9.364 -10.3%

SiC 0.0300

U3Si2 11.65 100 4.950 4.881 180.239 9.760 -6.6%

SiC 0.0225

UN (99% 15N)

13.68 80 4.740 4.607 167.431 9.066 -13.2%

SiC 0.0300

UN (99% 15N)

13.68 80 4.935 4.785 173.540 9.397 -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 SiC 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 U3Si2 or UN material also can bring significant fuel cost reductions, 4-6% beyond the SiC 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).



2. F. FRANCESCHINI and E. LAHODA, “Advanced Fuel Development to Improve Fuel Cycle Cost in PWR,” Proceedings of GLOBAL 2011, Makhuri, Japan, December 11-16 2011, Paper No. 464820.

3. 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, 2001

4. Y. S. LIU et al., “ANC: A Westinghouse Advanced Nodal Computer Code”, WCAP-10966-A, Westinghouse Electric Company, September 1986

5. 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)

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ACCIDENT TOLERANT FUEL AND RESULTING FUEL ......Fuel designs using advanced, accident tolerant fuel materials can improve fuel efficiency and extend fuel management capability in addition