Simulation of helium gas cooled pebble bed reactor.

Tom Keheley,
Advisory Engineer, Framatome ANP

Gas cooled reactors, utilizing helium as the coolant, are seen by many as the future of nuclear power generation. The safety features offered by this technology together with zero CO2 emission levels make it an environmentally attractive alternative to fossil based power generation technologies. South Africa has been at the forefront of development of this technology over the past 6 years. The Pebble Bed Modular Reactor (PBMR) project is a commercial venture utilizing this technology and is currently nearing completion of its conceptual design phase. As part of the engineering design and licensing process, the inherent safety features embedded in the PBMR design have to be demonstrated. This is accomplished by utilizing a unique coupling between thermo-hydraulic CFD simulations and the neutronic calculations used to calculate the nuclear power production in order to predict the temperature profiles inside the reactor. The thermohydraulic calculations are performed using STAR-CD and the neutronic calculations using the VSOP neutronics code.

Figure 1 shows a schematic of the composition of pebble fuel elements. One of the main advantages in using pebble fuel is that the reactor can be continuously refueled during operation. This means that no outage is required for refueling, as is the case with fixed fuel reactors. The main and most important safety feature of this type of fuel element design is that the silicone carbide coating of the fuel kernel, embedded in the pebble fuel element, provides the first level of containment as it encapsulates the fission products within the fuel kernel itself. Another advantage of this type of fuel lies in the composition and makeup of the fuel, which exhibits a negative temperature coefficient. In normal operation, this effect automatically stabilizes the nuclear reaction. In upset conditions, where the main coolant flow is lost, the fuel temperature increases due to insufficient heat removal. However, at high temperatures the nuclear reaction decays due to enhanced neutron absorption. This fuel characteristic results in a self-regulating nuclear reaction, which automatically reduces power with an increase in temperature. These are some of the most important inherent safety features of this type of nuclear power generation technology and make it attractive for commercial use.

The reactor unit

The reactor unit consists of a barrel-shaped pressure vessel that contains the reactor core structures. These consist of a core barrel that supports the ceramic structure and the pebble bed core assembly. Figure 2 shows asectional view of the reactor unit. Due to the complexity of the geometry, only a 15° slice of the reactor is simulated in order to minimize the number of computational cells required (see Figure 3).

The reactor is coupled to a helium-driven Brayton cycle PCU rendering overall power station efficiencies in excess of 42%. Helium is used as a coolant due to its favorable physical characteristics, in that it is chemically inert, has a high thermal capacity and has good neutron physics properties.

The reactor unit heat transfer poses a challenge in that it includes complex three dimensional geometries combined with fourteen different material and fluid types involving a combination of convective, conductive and radiative heat transfer (see Figure 4). CFD is the ideal tool to analyze this type of thermo-hydraulic system. However, the complex nature of the design requires a significant amount of additional coding to be combined with a commercial CFD code to achieve a credible solution. Figure 5 shows a typical temperature distribution through the reactor.

Neutronic coupling

Figure 6 illustrates the mapping process between STAR-CD and VSOP. An initial power profile from VSOP is mapped to the STAR-CD mesh. STAR-CD uses this power profile to calculate the initial temperature field. Once this has been established the STAR-CD temperatures are mapped to the VSOP mesh. VSOP then uses the new temperature field to re-calculate the power profile. This process is iterated to convergence. CFD results are directly imported into the FEA codes for thermal stress analysis (see Figure 7). In this manner, localized high-stress areas can be identified and the design can be adapted accordingly.

Conclusion

CFD has proven invaluable as a design tool for the thermohydraulic development of the reactor unit. Progress in the design would have been severely delayed if cutting-edge technological tools such as CFD and FEA were not available to accurately and efficiently test the details of such complex design.

STAR-CD’s approach to modeling, the stability of the solver and the ability to add user coding further enhances the capabilities of this design tool and makes it an integral part of the design process.