Decay heat experiment featuring low-energy neutron induced tungsten-187 production in ITER baffle plates and its analysis

Abstract In safety design of the international thermonuclear experimental reactors (ITER) against loss of coolant/flow accidents, a target accuracy of 15% in decay heat prediction for baffle plates made of tungsten is required. To judge whether the target accuracy was achievable or not, a decay heat experiment featuring low-energy neutron induced 187W production was performed as a part of ITER/EDA task T-339. Tungsten foil samples were irradiated in an ITER-like neutron field, and decay heat produced in the samples was measured with experimental errors of ∼5%. The experiment analyses were performed with the MCNP transport code. It was found that the use of an effective activation cross-section calculated by MCNP was needed to consider the self-shielding effect. When a best calculation with the ITER relevant FENDL cross-section library was compared with the experimental data, the calculated decay heat was 21% less than the experimental data. Therefore, it turned out that the target accuracy of 15% could not be fulfilled. In conclusion, the safety margin of 15% presently used in the design has to be increased to ∼25–30%, as far as the ITER baffle plates are concerned.