Heavy water as upper reflector in MNSRs

The use of heavy water as upper reflector in MNSRs is investigated in this paper. Risks associated with the addition of top beryllium shims is avoided by pouring heavy water in a polyethylene tube which connects a polyethylene container posted in the ST to the top of the reactor. The fuel cycle lifetime is shortened to about 5–6 years, but a constant volumetric reactivity is obtained. A less safe reactor is obtained whenever beryllium or heavy water is used as reflector. The presence of the upper reflector decreases the control rod worth anyway.