Thermal transient computation for a CTR blanket following a major plasma disruption

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Virginia Polytechnic Institute and State University

This study concerns a neutronic and transient thermal study of the first wall and blanket region of a typical controlled thermonuclear reactor (CTR).

Previous studies assumed a neutron wall loading of 1 ~ 5 MW/m², and usually an infinite slab for a blanket region in calculating the transient thermal behavior following a major plasma disruption (MPD). Besides, neutrons with a kinetic energy of 14 MeV were usually assumed by ignoring the energy distribution of fusion neutrons. Furthermore, the cross-sections of the interaction between the incident neutron and the first wall were neglected by assuming that all these 14 MeV neutrons were absorbed by the first wall.

This study made use of a more accurate model involving a canister design and considered both the incident neutron and secondary gamma heating in calculating the volumetric heat source rate. With these modifications, the average value of the volumetric heat source rate was calculated to be 0.05 MW/m³ ~ 0.5 MW /m³.

The disruption times used in this analysis were assumed to be 24 ms and 10 ms. For each case, a constant velocity model and a Gaussian velocity model were assumed for the surface heat flux impinging on the first wall after an MPD with emphasis on the constant velocity model. Neutronic studies including a Diffusion Test Model (DTM), a 23-group cross-section library, and a 37-group neutron and 21-group gamma library from ORNL used in conjunction with the ANISN Code, provided different volumetric heat source rates which were used to do a thermal analysis for the blanket at the steady state. With these volumetric heat source rates obtained, a heat conduction code HEATING5 was run for the steady state temperature distribution.

Results show that the average temperature for the first wall and the blanket are ~160°C and ~200°C, respectively. This steady state temperature distribution remained almost the same no matter whether DTM, 23G or 37N-21G cross-section sets were used, since the difference in the volumetric heat source rates generated were so small they did not change the temperature distribution significantly.

With the steady state temperature distribution as an initial condition, HEATING5 was run again for the transient thermal study which included the surface heat flux due to the disruption, together with a volumetric heat source rate resulting from the eddy currents induced in the wall following an MPD.

Results show that there is a possibility of melting portions of the first wall if the disruption time of 10 ms is used, while no melting is possible for the 24 ms case; however, a maximum transient temperature of ~1000°C on the first wall does occur.

With respect to the temperatures in the blanket region, they remained the same as they were before an MPD. The transient took place so rapidly that the effects were mostly on the first wall.

For the case of 24 ms, the average number of abortions allowed for failure of the first wall was 200 thermal cycles.