Hello.

We are preparing two group neutron cross sections for VVER fuel assembly.

Then we compare k_inf computed by OpenMC and k_inf_xs calculated using the prepared by OpenMC two group neutron cross sections as follows:

R_inf = S12/Sa2

k_inf_xs = (nuSf1 + nuSf2*R_inf)/(S12+Sa1)

where the notations are clear.

Between two k_inf we have difference (delta_k_inf) about 0.1%

( delta_k_inf = 100*(k_inf_xs - k_inf)/k_inf_computed ).

The difference does not depends on statistics.

We can not understand why do we have such difference (0.1 %).

We expected to see a difference of about 0.01%

Below we demonstrate our results as the function of coolant density:

coolant density(g/cm3) k_inf_xs k_inf delta_k_inf

0.200 0.9416 0.9420 -0.046

0.224 0.9691 0.9702 -0.104

0.248 0.9927 0.9934 -0.072

0.272 1.0136 1.0144 -0.079

0.296 1.0318 1.0329 -0.111

0.320 1.0478 1.0489 -0.096

0.344 1.0620 1.0628 -0.070

0.368 1.0744 1.0754 -0.099

0.392 1.0853 1.0864 -0.099

0.416 1.0950 1.0961 -0.106

0.440 1.1030 1.1040 -0.088

0.464 1.1104 1.1113 -0.083

0.488 1.1166 1.1175 -0.079

0.512 1.1224 1.1233 -0.079

0.536 1.1269 1.1280 -0.092

0.560 1.1311 1.1323 -0.106

0.584 1.1346 1.1357 -0.095

0.608 1.1376 1.1390 -0.114

0.632 1.1401 1.1415 -0.119

0.656 1.1419 1.1430 -0.091

0.680 1.1434 1.1447 -0.110

0.704 1.1447 1.1457 -0.088

0.728 1.1453 1.1461 -0.071

0.752 1.1457 1.1472 -0.135

0.776 1.1460 1.1470 -0.083

0.800 1.1458 1.1472 -0.124

0.824 1.1455 1.1465 -0.081

0.848 1.1447 1.1458 -0.095

0.872 1.1438 1.1447 -0.075

0.896 1.1428 1.1436 -0.070

0.920 1.1416 1.1429 -0.114

0.944 1.1401 1.1410 -0.079

0.968 1.1386 1.1398 -0.098

0.992 1.1369 1.1382 -0.112

We will be grateful for any explanation of such differences.

Thank you in advance!