The prediction and optimisation of assembly power distribution is a key input to a thermo-hydraulic evaluation and safety assessment. From a nuclear physics point of view, PWR assemblies are less complicated than BWR assemblies. The analysis of the latter has to deal with the use of control rods during core depletion producing axial neutron flux and power variations and with the formation of a void (steam) in the assembly channel. Whole core codes (3D) must therefore be linked to a thermo-hydraulic code to provide a realistic thermo­hydraulic feedback of neutron behaviour. Details such as the exact distribution of steam and liquid water can have a considerable influence on the results (Jatuff et al., 2006).

Figure 9.4 shows the relative power distribution in a 10 x10 array of fuel rods in a BWR-type assembly; the numbers indicate the percentage of average power. In contrast to the description in Section 9.2, the central water rod is omitted. The control blade cross is assumed to be withdrawn, and the power distribution is point-symmetric to the centre of the assembly. The rod power, obtained with the Helios lattice code (Wemple et al., 2008), is highest at the corners (139%) and decreases towards the centre where it is lowest (80%). The reason for this behaviour is the inhomogeneous distribution of moderator and fuel. Fission

neutrons are moderated not only within the assembly, but also in the water slab around it from where they diffuse to the fuel and cause new fissions. Due to self­shielding, fewer neutrons reach the inner rods, and power is lower there.

Such an uneven power distribution is not desirable because of non-optimal fuel utilisation and power limitations imposed by the coolability of the highest rated rods. As illustrated in Section 9.2, BWR assemblies have interior water rods or water channels to improve neutron moderation and thus to obtain a more even power distribution. The effect of a particular design (Westinghouse SVEA 96) is shown in Fig. 9.5. The internal water cross divides the assembly into four equal parts, and the lowest power (89%) now occurs in the interior of a sub-assembly. The highest power (124%) is still generated in the corner rods, but overall, the power distribution is more even than in the first case.

Real BWR and PWR assembly designs are even more complicated and try to decrease power differences by also varying the enrichment and by adding a burnable poison such as gadolinium to the fuel. This is illustrated in Fig. 9.6 (Brunzell, 2006). A good designs will work for the entire in-core service time of

235и

1.6 w/o

2.6 w/o

3.4 w/o

3.9 w/o

4.4 w/o

4.9 w/o

an assembly from 0 up to 60 MWd/kgU burn-up or even more. Optimal solutions can be found with modern core physics and computing tools as demonstrated by Martin-del-Campo et al. (2007).

The cooling and moderation in a BWR assembly suffers from increasing voidage towards the top of the assembly. A design attempting to counteract this effect employs part length rods extending from the bottom of the channel part way to the top of the channel into the boiling region. They improve the flow distribution in the upper part of the assembly by channelling steam and enabling a high steam — to-water slip ratio. This increases the density of the moderating water around the remaining rods in the upper region of the assembly and improves the axial power distribution. The design also reduces the two-phase flow pressure drop in the upper part of the assembly and improves the core’s thermo-hydraulic stability.