atw 2018-09v3


atw Vol. 63 (2018) | Issue 8/9 ı August/September

FUEL 445

| | Fig. 4.

Hottest core node for TMI-2 accident where coolant is restored

at ~9,900 seconds.

| | Fig. 5.

Hottest core node for PWR station blackout.

­leakage of fission products into the

primary loop since it will not balloon

and burst. Due to the short timespan

before coolant was re-introduced to

the system, the SiC cladding would

have had no adverse consequences

from a TMI-2 type accident (Figure 4).

5 Transition cycle analysis

for optimum ATF

implementation in

current PWRs

5.1 U 3 Si 2 fuel

As previously noted, one of the

­primary benefits of U 3 Si 2 is that it

increases the uranium density by up

to 17 percent as compared to UO 2 .

This yields an effective enrichment

of 0.84 weight percent U-235 as

compared to 0.71 weight percent

U-235 found in natural uranium. This

increase in density will support

improved fuel cycle economics and

reduce the total number of fuel

bundles that need to be inserted into

a reactor, resulting in significant

savings. Because of the increased

density, the use of U 3 Si 2 also extends

the energy output and cycle length

capability for PWR fuel assemblies,

while remaining below the 5 weight

percent enrichment limit for commercial

fuel. The Westinghouse ATF can

thus either decrease the fuel cycle cost

of 18-month cycles by reducing the

number of feed assemblies and increasing

fuel utilization, or it can

make 24-month cycles economical for

today’s uprated, high-power density


Economic analysis shows that the

Westinghouse EnCore Fuel has very

favorable economics, not only at the

ATF equilibrium cycle, but also during

the transition cycles from UO 2 to ATF.

This is especially applicable when

transitioning to a 24-month cycle

operational regime, which thus represents

the recommended path forward

for implementation. The higher

thermal conductivity of the U 3 Si 2 also

provides a very high tolerance for

transients while operating at higher

linear heat generation rates than is

possible for UO 2 – which will increase

plant operating margin. In addition,

the higher uranium density can

extend the core operating capability

compared to current fuels, while

maintaining the current 5 weight

percent 235U enrichment limit for

commercial fuel; yet enable economically

competitive fuel management

schemes for the longer cycles.

In particular, the introduction of

ATF in a current 18-month cycle

high-power density PWR to accomplish

a transition from UO 2 to ATF by

either maintaining the currently predominant

18-month cycle operational

regime, or extending it to a 24-month

cycle has been analyzed. Implementing

the Westinghouse ATF to achieve a

more cost effective 18-month cycle

will deliver fuel cost savings due to

fewer fresh assemblies per reload

and improved fuel utilization. Implementing

the Westinghouse ATF in

conjunction with a transition to

24-month cycle will yield economic

benefits due to the resulting reduced

number of outages and related

savings, which offset the slightly

higher fuel costs (as compared to

18-month cycle fuel costs). Analyses

have shown that the economic impact

of the transition cycles to implement a

24-month cycle operation with ATF is

significantly better than the economic

impact of transition cycles which implement

ATF and maintain an

18-month cycle operation.

It is anticipated that the fabrication

costs to make the U 3 Si 2 powder could

increase as compared to existing

UO 2 fabrication. However, after the

powder is made, only minor cost increases

are expected to occur in the

rest of the fuel manufacturing process.

Therefore, the overall cost increase

is anticipated to be offset by

the safety, economic and operational


| | Fig. 6.

Total hydrogen generated for PWR station blackout.

5.2 Chromium-coated

zirconium cladding

Chromium-coated zirconium alloy

offers a higher accident temperature

capability, compared to uncoated

zirconium alloy cladding, of between

1,300 ˚C and 1,400 ˚C. The coated

cladding also reduces corrosion and

hydrogen pickup. Resistance to rod

wear is another benefit of this cladding

type. The potential for exothermic

reactions is greatly reduced

during LOCA or transient events

that lead to high-temperature fuel

transients. These attributes provide

both safety and economic benefits

that support licenseability and economically

viable transition scenarios.

5.3 SiC cladding

SiC cladding provides 25 percent lower

thermal neutron cross-sections

than current Zr cladding. This would

afford even greater neutron economy.

Additionally, the fuel and cladding

would be able to withstand temperatures

~2,000 ˚C in the event of a

beyond design basis accident. This

temperature increase could result in a

rise in design basis operating margins.

6 Licensing

To get EnCore Fuel licensed and

loaded into commercial reactor cores

in region quantities by 2027, Westinghouse

has initiated a program to

­significantly compress the licensing

timeframe from initial testing to


Westinghouse EnCore® Accident Tolerant Fuel ı Gilda Bocock, Robert Oelrich, and Sumit Ray

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