To set the main specifications such as numbers of fuel assemblies and rods, and fuel rod size, the design parameters referred to as power peaking factors are defined, and roughly investigation is made that the maximum linear heat generation rate and assembly power which fuel rods experience during reactor operation satisfy the thermal design conditions.

(i) Radial power peaking factor FR = ratio between the maximum ^) and average values of fuel assembly — averaged power in the core

(ii) Axial power peaking factor FZ = ratio between the maximum and

average values of fuel assembly cross section — averaged power (3.7)

in the axial direction

(iii) Local power peaking factor FL = ratio between the maximum and

(3.8)

average values of fuel rod — averaged power in the fuel assembly

(iv) Total power peaking factor FP = ratio between the maximum and average values of local power in the core

=f Fr XFZXFL (3.9)

Table 3.6 shows examples of the power peaking factors which are evaluated by detailed design calculations in the step when reactor specifications and operating conditions are determined. However, generally the factors are set based on the design and operating experiences and then investigate them.

Using the power peaking factors above, the average fuel rod linear heat generation rate (power per unit length of fuel rod) qave, the maximum linear heat generation rate qmax, the average fuel assembly power PBave, and the maximum power PBmax can be given by the following equations.

Qave — Q/ GYb X Nrod X Lrod)
Qmax Pp X qaue

PBave = Q/NB

PВшах = PrX Рваие

The maximum linear heat generation rate qmax is the dominant factor in the thermal and mechanical integrity of fuel such as the maximum fuel tempera­ture and the maximum heat flux on the fuel rod surface. A low value of qmax is desirable to secure the reactor safety. In order to decrease qmax, the following measures can be considered.

(i) Flatten the power distribution in consideration of enrichment zoning in the fuel assembly or fuel loading pattern so as to reduce the power peaking factor.

(ii) Reduce the fuel rod diameter Drod and change the fuel rod array (8 x 8, 9 x 9, or 10 x 10) so as to increase the number of fuel rods per fuel assembly.

(iii) Increase the number of fuel assemblies.

(iv) Lengthen the active height of fuel rods.

Since (iii) and (iv) increase the core size, (i) and (ii) are usually investigated first. Too thin a fuel rod gives rise to difficulties such as its bending and an increase in fuel processing cost. Therefore, the fuel rod size is naturally limited. The main specifications of BWR fuel rod design are shown in Table 3.7. The diameter of the fuel rods was as large as about 14 mm early in BWR development. The number of fuel rods in the same-sized fuel assembly was then increased from the viewpoint of increasing volumetric power and improved safety margins. A thinner fuel rod of about 10 mm is currently used.

Table 3.7 Examples of main specifications of BWR fuel design Fuel type 7×7 type Improved 7×7 type 8×8 type New 8×8 type New 8×8 Zirconium liner type (Step I) High bumup 8×8 type (Step П) 9 x 9 A type (Step III) 9 x 9 В type (Step III) Maximum liner power (kW/m) 57 (17.5 kW/ft) 61 (18.5 kW/ft) 44 (13.4 kW/ft) 44 (13.4 kW/ft) 44 (13.4 kW/ft) 44 (13.4 kW/ft) 44 (13.4 kW/ft) 44 (13.4 kW/ft) Average dis­charge bumup (GWd/t) 21.5 27.5 27.5 29.5 33.0 39.5 45.0 45.0 Pellet material uo2 U02 or Gd203 added U02 U02 or Gd203 added U02 U02 or Gd203 added U02 U02 or Gd203 added U02 U02 or Gd203 added U02 U02 or Gd203 added U02 U02 or Gd203 added U02 Diameter (mm) 12.4 12.1 10.6 10.3 10.3 10.4 9.6 9.4 Length (mm) 22 12 11 10 10 10 10 10 Stack height (mm) 3,660 3,660 3,710 3,710 3,710 3,710 Standard 3,710 Partial Length 2,610 3,710 Cladding Zircaloy-2 Zircaloy-2 Zircaloy-2 Zircaloy-2 Zircaloy-2 Zircaloy-2 Zircaloy-2 Zircaloy-2 material Stress-relief Recrystallization Recrystallization Recrystallization Recrystallization Recrystallization Recrystallization Recrystallization Annealing Annealing Annealing Annealing annealing annealing annealing annealing

(Zirconium (Zirconium (Zirconium (Zirconium liner) liner) liner) liner) 12.3 12.3 11.2 11.0 0.86 0.86 0.71 0.70 -0.1 -0.1 -0.1 -0.1

Outer diame­ter (mm) 14.3 14.3 12.5 12.3 Thickness (mm) 0.81 0.94 0.86 0.86 Zirconium liner thickness (mm) No. of fuel rods per assembly 49 49 63 62 No. of water rods 0 0 1 2 No. of water channels — — — — Spacer type Grid type Grid type Grid type Grid type Gas filled Helium Helium Helium Helium in rod gap (pressure) (0.1 MPa) (0.1 MPa) (0.1 MPa) (0.3 MPa)

62 60 74 72 2 1 (large diameter) 2 (large diameter) — — — — 1 (Square) Grid type Helium (0.3 MPa) Circular cell type Helium (0.5 MPa) Circular cell type Helium (1.0 MPa) Ring type Helium (1.0 MPa)

The maximum fuel assembly power must be below the assembly

power which meets the limit of MCPR to avoid boiling transition. can

be reduced through the following measures.

(i) Improve the radial power peaking factor considering fuel loading pattern and control rod pattern.

(ii) Increase the number of fuel assemblies NB.