A Three-dimensional activation analysis for the divertor of the International Thermonuclear Experimental Reactor (ITER) design has been performed. Results of the activation analysis are needed for the environmental and safety assessment of ITER. The activation characteristics are dependent on the materials used as well as the operational scenarios assumed for the machine. This paper focus on the activation of the divertor's plasma facing component (PFC). The current design of ITER's divertor uses tungsten as PFC. The Monte Carlo code MCNP was used to obtain the neutron spectra needed for the activation calculation.

Natural tungsten has five isotopes. The neutron capture cross sections of three of these isotopes, 182W, 183W, and 186W, have significant resonances in the thermal energy range. Multi-group activation cross section libraries ignore the effect of self-shielding associated with these resonances. The effect of self-shielding was particularly important for the 186W(n,g)187W reaction whose cross section has a giant resonance at 20 eV. In this paper, activation calculations have been performed using the DKR-PULSAR1.5 code. We used spectrum averaged multi-group tungsten capture cross sections that were generated by the MCNP code. These cross sections take into account the effect of self-shielding caused by the thermal resonances and hence, the results provide accurate estimate of the radioactivity generated in the tungsten PFC of the divertor.

To examine the effect of pulsing sequences on the decay heat generation, the calculations were performed for two different operational scenarios that were suggested by the ITER Joint Central Team (JCT). The first scenario (SA1) corresponds to a machine total fluence of 0.3 MW.a/m2. The SA1 scenario has a long term availability of 25% and a final month availability of 50%. A burn pulse length of 1 hour is used for this scenario. The second scenario (M5) corresponds to a total fluence of 0.25 MW.a/m2. The M5 scenario has a long term availability of only 4% and a last month availability of 50%. A 1000 second burn pulse length is used for the M5 scenario.

Results of the activation analysis showed that the activity and decay heat generated in the tungsten layers of the divertor are mostly dominated by 187W during the first day after shutdown. For longer periods following shutdown, the activity and decay heat generated are dominated by impurities present in tungsten. Accurate calculations of the 187W inventory have an effect on other radioisotopes like 188Re and 188mRe which are produced by further neutron reactions with 187W. The rhenium isotopes could be of importance in off-site dose calculations.