Calculating streaming with the 'tally source' method, as applied to IFMIF

Shielding calculations for a thick shield with lots of penetrations can be a very challenging task. An example of this is the test cell cover of IFMIF, consisting of 270 cm of heavy concrete penetrated by about 40 ducts with a diameter of several centimeters, each of them with bends to limit neutron streaming. A proposal by Hendricks to tally the neutron current at intermediate surfaces and then model it as a source, with appropriate angular biasing, is described and tested. Although this method is laborious and leads to some loss of accuracy, it is found to give solutions to otherwise intractable problems