Universal Subcooled Neutron Source

To obtain the maximum of cold neutron intensity at the WWR-M reactor, we had to place a large hydrogen source into the center of the reactor core. In this case, the extremely high specific heating required a high power method of heat removal. It was found that the best way would be a natural circulation of subcooled liquid hydrogen between the chamber and the external heat exchanger. This method is known as a thermosiphon, but it has not been used regularly with subcooled liquids before. It can maintain the moderator in the chamber a few degrees below the boiling point at the highest reactor power.

Schematic diagram of the universal liquid hydrogen source
This new neutron source has been operating at the WWR-M reactor since 1986. It is a universal source since it produces both ultracold and polarized cold neutrons. The chamber with the moderator is placed inside the flux trap in the center of the reactor core where the flux is (1.5 - 2).1014n cm-2s-1 for thermal neutrons and 2.1013n cm-2s-1 for neutrons with energy E> 1 MeV The chamber, made of zircalloy, has a volume of 1 liter. The specific nuclear heating was 18-20 W/g for hydrogen and 0.7 W/g for zircalloy. The total nuclear heating with 100 percent hydrogen was 2.8 kW. The liquid mixture of 40 percent of hydrogen and 60 percent of deuterium is used as the moderator. In this case the total nuclear heating is 1.8 kW.

An attractive feature of the source is that the thermosiphon circuit is placed entirely in the cold helium flow, the hydrogen being surrounded by two walls with helium between them and then by vacuum containment. Such an approach to the source design improves heat transfer, allows independent operation of the reactor and of the source and substantially increases the reliability of the construction and the hydrogen safety.