In D-T burning fusion reactors such as ITER, cooling water in the blanket is activated by high energy neutrons through the reaction of ¹⁶O(n,p)¹⁶N. It is necessary to assess the nuclear heating due to high energy gamma-rays (6.13, 7.11-MeV) emitted from ¹⁶N to determine the required shielding around the cooling pipes in order to avoid excessive nuclear heating in cryogenic systems. The major uncertainty in calculating nuclear heating arises from estimating the fraction of gamma-rays that deposit energy in the cryogenic temperature components. In this paper, results of a detailed 3-D Monte Carlo calculation are described. Accurate modelling of the toroidal (TF) and poloidal (PF) field coils, blanket and vacuum vessel is necessary. In addition, detailed representations of the (a) shielding blanket cooling pipes that pass through the upper ports (normal temperature), (b) the inter-coil structure between TF coils (cryogenic temperature), (c) break boxes and current- and cryogenic feeder lines to the TF and PF coils (cryogenic temperature), (d) biological shield (normal temperature) and (e) the cryostat (normal temperature) are also needed.

The total gamma-ray energy emitted by the decay of ¹⁶N in the cryostat is about 60 kW. More than half of the energy is deposited in the cooling pipes themselves. About 60 % of the gamma-ray energy that escapes from the cooling pipe bundle is deposited in the cryostat and biological shield. The remaining ~40 % of the escaping energy (16 % of the total energy) is deposited in cryostat with ~1/3 deposited in cryogenic temperature components and 2/3 in room temperature components. As a result, the heat deposition in the cryogenic temperature components is about 3 kW which is marginally acceptable considering other smaller additional contributions from the water coolant pipes that are located in the equatorial ports. The nuclear heat density in the cryogenic superconducting components and absorption dose rates to the magnet insulators are compared with design limits and found to be acceptable. Calculations of the energy deposition with a 13-cm-thick guard pipe of (8 cm stainless-steel and 5 cm water) were also performed. A factor of ten reduction in the nuclear heating in the cryogenic temperature components was observed suggesting that it may be prudent to have a guard pipe around the cooling pipes with dimensions of a few centimeters.