Fuel assembly design improvement

As shown in Fig.4-17, due to the large power gradient existing in peripheral core assemblies, the local power peaking at outside of core is significantly large, which limits the average outlet temperature achieving design goal. Therefore, the flow separation plates are applied in peripheral core assemblies to improve the outlet temperature by reducing the flow-power mismatch of peripheral core assemblies. On the other hand, due to the application of separation plates, the peripheral core fuel assemblies are different from the inner core fuel assemblies. They are designed separately.

Peripheral core fuel assembly

Fig.4-26 shows the horizontal cross section of a peripheral core fuel assembly and axial enrichment distribution. The double-row fuel assembly is adopted for obtaining uniform pin power distribution as applied in the Super LWR with double tube WR. The stainless steel separation plates

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with the thickness of 1.0mm are applied to reduce the power to flow rate mismatches caused by the large power gradient existing in peripheral core. Each water rod has the size of 5×5 fuel rods for enough moderation. The assembly parameters are summarized in Table 4.5. The thermal insulator 8YSZ-50%(8 mol% Y2O3/92 mol% ZrO2 and sintered porous pellet with 50% relative density) clamped by two SS plates as sandwich is applied in the water rod wall to keep the moderator temperature below psudocritical temperature as applied in the assembly design with double tube WR. Since the outlet temperature is supposed to achieve 500°C, the average density decrease much more than that in the design of low coolant single pass core. Meanwhile, the axial density variation is intensified. To handle these problems, the average enrichment is increased and four axial enrichment zones are used to flatten the axial power distribution. Low enrichment of 7.0% is applied for bottom and top zones to prevent large bottom axial power peaking and decrease MCST

Fig.4-26: Specification of peripheral core assembly and fuel rod^[88]

respectively. High enrichment of 7.9% is adopted for middle zone to compensate the coolant density decrease. The buffer zone with the enrichment of 7.5%, which is located between bottom zone and middle zone, is used to avoid large axial power peaking occurrence caused by power abrupt increase at the bottom-middle zone boundary. Considering pin power distribution and shutdown margin, gadolinia(Gd2O3) with concentration of 1% is used and mixed uniformly in the fuel rods located at the middle of water rod(WR) where pin power peaking is expected to be. The pin power distribution at BOL is shown Fig.4-27. Across the assembly, the pin power peaking of 1.095 is located at the middle of WR at the corner of assembly. The small pin power peaking can benefit the local power peaking decrease. Meanwhile, to simplify the assembly structure, the peripheral core assembly is not equipped with CR guide tubes.

Inner core fuel assembly

Inner core fuel assembly has the same dimension as peripheral core fuel assembly. As there are no CR guide tubes applying for peripheral core assemblies, the CR clusters quipped for inner core assemblies are supposed to bring the core to cold standby condition with enough margin.

Meanwhile, the relatively high enrichment which is respond for the average density decrease caused by high outlet temperature requires large CR worth for inner core CR clusters. Therefore, twenty-one control rod(CR) guide tubes with the diameter doubling fuel pitch are arranged in the assembly to obtain enough shutdown margin as shown in Fig.4-28. There are 9 CR guide tubes are located at the intersection of double-row fuel rods and 12 CR guide tubes are inserted in the water rods. Four corner water rods are not equipped with CR guide tubes for matching the upper core circle shape of CR drive. Due to the enrichment increase, twenty-four fuel rods with 8% concentration Gd2O3are placed at inner assembly for compensating burn-up reactivity. The same axial enrichment zoning as peripheral core assembly is applied to flatten axial power distribution. Radial enrichment zoning is account for to suppress local power peaking. Three different enrichments are adopted. The relatively low enrichment applied at the middle water rod as well as the position near CR guide tube

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a) without radial enrichment zoning b) radial enrichment zoning Fig.4-29: Pin power distribution at BOL of quarter assembly

MOL to EOL is limited since the flow rate of assembly, which determines outlet temperature, is determined by the largest power peaking through the burn-up cycle. The parameters comparing with that of peripheral core fuel assembly are given in Table 4-4.

Table 4-4: Peripheral and inner core assembly characteristics^[88]

Assemblies Peripheral assembly Inner assembly

Number of fuel rods 384 348

Number of CR guide tubes / 21

Number of water rods 16 16

Fuel rod active height[mm] 4200 4200

Fuel rod outer diameter[mm] 8.00 8.00

Cladding thickness[mm] 0.50 0.50

Gap clearance between pellet and cladding[mm] 0.17 0.17

Fuel rod pitch[mm] 9.00 9.00

Gap clearance between fuel rod and channel

box/water rod wall/separation plate[mm] 0.50 0.50

Water rod wall(Stainless steel+ thermal

insulator+ stainless steel) [mm] 0.20+1.80+0.20 0.20+1.80+0.20

Guide tube diameter[mm] / 18.0

Thickness of guide tube[mm] / 0.46

Separation plate thickness[mm] 1.00 /

Assembly pitch[mm] 258 258

Channel box thickness[mm] 1.00 1.00

Gap between assemblies[mm] 3.00 4.00

Hydraulic diameter[mm] 3.50 3.44

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Infinite multiplication factor of inner core fuel assembly with CR insertion

Since only the CR clusters equipped in inner core assemblies response for the cold shutdown and initial reactivity control, the K-INF after inserting the CR should be much lower than 0.99 for satisfying shutdown margin of 1%. Fig.4-31 shows the K-INF of inner core fuel assembly changing with burn-up for different axial enrichment zones. It is evaluated for the case without CR insertion in hot core condition(average density) and the case with CR insertion in cold standby condition with the average density of 1g/cm³. The K-INF is 0.75 for all cases after CR being inserted, which is able to bring the hot core to the cold standby condition.

4.4.3 Core design improvement