KMACS – Core simulator

The GRS simulation program KMACS allows the calculation of core states of pressurised water reactors within one operating cycle, from loading to shutdown. Taking the scheme of the reactor core loaded with fresh and partially spent fuel elements as a basis, the code calculates, for example, detailed temperature and power distributions (see Figure 1).

Figure 1: Power of the fuel elements in the reactor core (standardised to the mean fuel element power) for a Pre-KONVOI-reactor at the start of the cycle. Depending on the local release of fission heat and the energy of the neutrons, the nuclide inventory develops spatially in different ways. KMACS allows e.g. the precise tracking of fuel elements with the highest performance or decay heat. Based on this, a realistic initial situation of the reactor core can be determined for an incident or accident simulation. The nuclide inventories calculated with KMACS can also be used for disposal analyses.

With the release of energy during normal operation, the inventory of fissile material decreases and thus also the excess reactivity that must be absorbed by the control rods or by the boric acid in the coolant. Consequently, there is also a decrease in the critical boron concentration over the operating cycle. This can be calculated using KMACS (see Figure 2).

KMACS (Core Simulator, a Modular Adaptable Core Simulator) is based on external and GRS-owned software (QUABOX/CUBBOX, ATHLET) and contains elements of programs for the generation of cross sections, the calculation of the reactor state and the determination of burn-up and nuclide inventory. In contrast to commercial core simulators, KMACS is based on other programs, making it a diverse alternative to them.Figure 2: Critical boron concentration in ppm during the operating cycle of a pressurised water reactor. The values determined with KMACS (red) compared to the actual measurements (blue).