The pressuriser is a vital part of a pressurised water reactor (PWR). We are now investigating a weld of a pressuriser that has been in operation for 29 years. The long time at about 345C has degraded the material properties. Studies using atom probe tomography (APT) have shown that Cu-particles have formed (at dislocations). The particles are unevenly distributed on the sub-micron scale studied by APT. By using SANS it will be possible to obtain statistically relevant data on the Cu particle size distribution and number density. In addition, it will be possible to measure variations through the thickness of the weld and observe the influence of the slight temperature gradient from the inside to the outside of the pressuriser. Such information is practically impossible to obtain with other methods, and is important for understanding ageing and embrittlement of reactor components.