Astonishing axial zone divsion effects

Validation and Verification
1

The reactor core consists of pin-type cylinders with an axial length of 80 cm and is composed of fuel rods, burnable poison rods, and control rods. Using the OpenMC code, the following three types of burnup calculations were performed:

  1. No cell division: When no cell division was applied, the Keff vs. EFPD curve obtained was similar to the results from MCNP.
  2. Radial cell division: The burnable poison rods were divided radially into 10 cells, and the same calculation was performed. The results were also similar to MCNP.
  3. Axial cell division: Both fuel rods and burnable poison rods were divided axially into 8 segments. Surprisingly, the results were unexpected. The fuel cycle length in case (2) was about 3% longer than in case (1), which is reasonable. However, in case (3), the fuel cycle length was about 30% shorter than in case (1).

This result is logically and physically difficult to understand. Could you provide an explanation for this discrepancy?

2

Hai Okum, welcome to the openmc community.
Have you check the volume of each material division? Maybe you want to double check your input because 30% deviation is big in term of core cycle length. Are you using keff (excess reactivity) to determine the cycle length or using other approach such as fuel discharge burnup?

I think the initial heavy metal loading or initial fuel mass should be same regardless of material division methods. Also, have you tried to check the material composition at each material region? You can try to use get mass feature from depletion results file.

3

Thank you very much for your time and very kind response.
Oyeon