Modeling and Validation of an Advanced Pressurized Water Reactor using Monte Carlo Technique

Abstract

The validation of reactor core design modeling codes is very essential, and can be achieved by comparing the code results with the available experiments or other computational models.

In this paper, the reactor core of the advanced pressurized water reactor, AP1000 was simulated, verified and modified using Monte Carlo Computer Code, MCNP. The physical and mathematical models of the MCNP-Code include, the Boltzmann neutron transport equation and the time dependent number densities of the depleted fuel inventory equations. The reactor core of AP1000 includes 157 fuel assemblies, and each assembly contains 264 UO₂ fuel rods arranged in 17x17 square array, with three different batches of fuel enrichment. The initial core contains two types of burnable absorbers, discrete burnable absorbers (PYREX), and Integrated Fuel Burnable Absorbers (IFBA) for the compensation of the initial excess reactivity and for increasing the reactor operation cycle length.

The simulated results were compared with the reference design parameters and validated with other calculations which were performed by other authors using SCALE and WIMS Codes.

The modeling results showed that the criticality (K_eff) for cold core was 1.204 for our model and 1.205 for the reference design, and for the clean assembly was 1.32728 for our model and 1.328 for the reference design which showed a good agreement. In addition to, some of thermal hydraulic safety parameters as; critical heat flux, actual heat flux, Departure from Nucleate Boiling Ratio (DNBR) and fuel power density were calculated using different correlations and other conservative equations. The results were compared with the reference design parameters, which showed a good acceptance, and confirm the safe design and efficient modeling. 

Also, AP1000 core were modified by two approaches: the first, by using Uranium oxide fuel composition with three different enrichment regions (3.5, 4.5 and 4.95) % respectively and including burnable absorbers with the purpose of reaching high burnup and long cycle length. The modified core was modelled using MCNPX computer Code, and the results of the burn up calculations at cycle length of 21 months reached 25.3 GWd/MTU. So, in the three cycles it reaches to 75.9 GWD/MTU and preserving the fuel integrity by using high performance ZIRLO cladding material. The second core modification was by using Mixed Oxide fuel (MOX).This core consist of three different regions; first region contains MOX fuel (U+Pu) O₂ with 7% Pu- fissile content, second and third regions consist of two different enrichment of UO₂ (4.5 and 4.95) %. In addition to, the MOX core include the same types and number of BAs. as in the reference design core of the AP1000