Small Liquid Hydrogen Source

Provided that the moderator volume is not large and the heating is not high, the simplest technique is a direct cooling of the moderator chamber by means of a cold helium circulation. This principle was used in the first hydrogen ultracold neutron source at the WWR-M reactor. The source was placed at the Be-reflector where the perturbed neutron flux was 6.1013n cm-2s-1 for thermal neutrons and 8.1012n cm-2s-1 for neutrons with energy E>1 MeV. The specific nuclear heating was 8 W/g for hydrogen and 0.3 W/g for the chamber material. The chamber contained 150 cm3 of liquid hydrogen and the total nuclear heating was 300W. The chamber was made like a heat exchanger that allowed us to keep liquid hydrogen subcooled up to a reactor power of 12 MW. However, at 18 MW just 15% of vapor was in the chamber. This ultracold neutron source gave a gain factor of about 30 and 4.104n s-1 of UCN flux integrated up to the velocity 7 m/s. It was operating from 1980 to 1985 producing, at the time, the highest UCN intensity in the world.

1- helium collector;2-helium jacket;3 - tubes with maderator;
4- hydrogen collector;5 - hydrogen input tubes;6 - helium input tube
7 - helium output tube; 8 -neutron guide

Probably the maximum capability of the method of direct cooling was demonstrated in that case. It certainly could be recommended for application as the simplest and the most efficient way of heat removal, when the radiation heating is not very high.