4 FUEL MANAGEMENT STRATEGY

At the present, the Atucha-1 on power fuel reloading scheme for natural uranium applies a subdivision of the core on three concentric-annular zones numbered from the center (zone 1) to periphery (zone 3) as is shown in Figure 7.

The FAs exchange is done by a one-radial route and each FA is moved two times (or sometimes three times) to different locations. Fresh fuel is loaded in zone 2 (Intermediate power density) up to get an average FA bumup of about 2,7 MWd/kg U. After that, the fuel is moved to zone 1 (central, higher power density) suffering a positive power ramp. The FA stays in zone 1 up to about 5,1 MWd/kg U and then is reshuffled to zone 3 (peripheral, lower power density) up to about 5,9 MWd/kg U (peak pellet bumup of 8,4 MWd/kg U). Later, the FA is discharged of the reactor, as is showed in Table 6.

This fuel management scheme was foreseen to obtain maximum FA average discharge burnup fulfilling operational limits for local linear power density, channel critical heat flux ratios and power ramps.

CHANNEL CLOSURE

FILLER BODY

FUEL ASSEMBLY

COUPLING

COUPLING

TIE PLATE

SPACER

FIG. 5. Total fuel column of ATUCHA 1.

ON

- SPRING

L-UPPER ENDPLUG

"-ISOLATING PELLET

L- GAS PLENUM

L ISOLATING PELLET

L FUEL PELLET

-CLADDING

WEIGHTs 5,6 Kg

L-WEAR PADS

LOWER ENDPLUG

TABLE 5.

CNEA-UPESN-IEC TABLE 5

ATUCHA-1 FUEL ROD TECHNICAL DATA FUEL ROD OUTSIDE DIAMETER:

CLADDING WALL THICKNESS:

ACTIVE LENGHT:

U02 PELLET DENSITY:

11,90 mm 0,55 mm 5300 mm 10,55 g/cm3

10,62 mm Zircaloy-4 232 W/cm FUEL PELLET DIAMETER:

CLADDING MATERIAL:

AVERAGE FUEL ROD HEAT RATE:

MAXIMUM FOR STATIONARY CONDITION: 531 W/cm MAX. FOR NON-STATIONARY CONDITION: 600 W/cm MAXIMUM DESIGN PEAK POWER ( + 15 %): 690 W/cm INTERNAL PRE-PRESSURIZED: 17 bar (helium)

5 DIFFERENCES BETWEEN PRESENT FUELS FOR LWRs AND ATUCHA-1

The most remarkable differences between Atucha-1 fuel and modem LWR fuels (see Table 7 and 8) are as following:

Low burnup

The average FA discharge burnup is about 5,9 MWd/kg U and the peak pellet discharge burnup currently is below 9 - 1 0 MWd/kg U.

High linear power density

The core average linear power density is 232 W/cm and the design value for the local peak of linear power density under stationary conditions is 531 W/cm. The design value for the local peak of linear power density for non-steady state operational fluctuations is 600 W/cm (the hot channel factor is 2,59 to design verification).

Short dwelling time

The average residence time of the FA in the core is about 195 full power days (fpd).

Power ramps on fuel loading and shifting under power

Power ramp experienced by the FA when it is moved from intermediate core zone (2) to central zone (1) is of special concern because of the risk of fuel failures by pellet cladding interaction assisted by stress corrosion cracking (PCI - SCC). The most of the local power increments are in the range between 150 - 250 W/cm at an average FA burnups around 2,5 - 3,3 MWD/kg U and a peak pellet bumups of 3,6 - 4,7 MWd/kg U. The procedure of FA movement from zone 2 to zone 1 is subjected to operative restraints based on PCI - SCC fuel rod failure thresholds.

>0 11 it '11 tt H '« IT -I •* » Jl J7 7J .'t TSs. » ?7

I i i I I i I ! ' ! I i ! L

SPENT FUEL POOL

REFUELLING SCHEME:

FRESH FUEL —— ZONE 2 —— ZONE 1 —— ZONE 3 —— POOL

FIG. 7. Section ofATUCHA 1 core with refuelling zones.

TABLE 6. NATURAL URANIUM FUEL RELOAD AND SHUFFLING ON ATUCHA-1 CORE

CORE ZONE

2 (intermediate)

1 (central)

3 (external)

LOADING BURNUP [MWd/kgU]

Fresh

2,7

5,1

POWER RAMP AP AP > 0

AP > 0

AP < 0

REMOVAL BURNUP [MWd/kgU]

2,7

5,1

5,9

DISCHARGE TO SPENT FUEL STORAGE POOL

Additionally, other differential features between the Atucha-1 and LWR fuels that should be pointed out are:

• Cladding wearing pads, each fuel rod has 45 wearing pads resistance welded to the cladding at spacer locations,

• Different channel coolant flows, as was indicated, the coolant velocity and mass flow of each channel depends on the different radial power distribution zones of the core,

The worst hydraulic conditions for the FA depend on the combinations of coolant mass flow and the velocity profile at the channel inlet. The FA is required on different grades by each throttle type.

6 ATUCHA-1 NPP Overall Fuel Performance

More than 7300 fuel assemblies (< 2,63 x 105 fuel rods) from three different suppliers have been irradiated in the Atucha-1 reactor since 1974. The overall performance is shown in Table 9

The fuel failure rate was generally very low with some periods of enhanced failure frequency.

The reasons for these enhancements were: fuel operation beyond limits, manufacturing flaws concentrations and interaction with reactor internals damaged (in 1988) and debns.

PCI - SCC failures were related with high power ramps either as result of fuel movement from intermediate to central core zone or during reactor start-up operations. Operation restrictions for fuel shuffling and reactor start-up were introduced to reduce the nsk of PCI failures.

The overall rate of FA failures because of manufacturing faults is 0.56%. In our experience only in a few cases there were more than one fuel rod failed in each FA because of manufacturing flaws.

Therefore the overall fuel rod manufacturing failure rate is 0.016% and almost the same, 0.014%, for the last 5 years.

Regarding the manufacturing failures, since hot cell examinations are not available, the primary causes of the failures were never exactly identified. There are some concerns on the wearing pad welding process and on the soundness of the fuel sheaths. Some recommendations to improve fuel manufacturing process and QC procedures were introduced. At the present, additional recommendations are being developed.