The LBNF Cryostats and
Cryogenics Systems

Configuration of the four six-story-high cryostats proposed to house the DUNE detector modules at 4,850 feet underground. A central utilities cavern houses cryogenics and other equipment. Credit: FNAL

Massive Tanks a Mile Underground

The DUNE far detector is expected to be the largest cryogenic instrument ever to be placed deep underground. It is planned as a modular structure housed in a set of four massive insulated tanks (cryostats) that are each designed to be about as tall and wide as a six-story building (or a brachiosaurus) and longer than a football field (18m high by 19m wide by 66m long each). Each cryostat will contain about 17,000 metric tons of liquid argon at a temperature of 89 K (-184°C or -299°F) and the DUNE particle detector elements will be immersed in the liquid argon.

Design of the steel outer frame of a LBNF cryostat that will house one of four DUNE detector modules. Credit: FNAL

LBNF is designing both the cryostats and a comprehensive cryogenics system for receiving the liquid argon and nitrogen, initially purging and cooling down each cryostat, filling it, achieving and maintaining the liquid argon temperature, reliquefying the boil-off gas, and continuously cycling the liquid through a purifying system over the experiment's entire lifetime — at least ten years.

Design of cryostat layers. Credit: GTT

The Cryostat Walls

The cryostats are designed as free-standing steel-framed vessels with an insulated double-membrane system to provide containment of the liquid argon. The membrane system is based on the technology used for liquefied natural gas (LNG) storage and transport ships. It consists of an inner corrugated thin membrane of stainless steel surrounded by thermal insulation with a secondary thicker aluminum membrane placed between insulation layers. The structural steel frame provides the necessary support for the considerable hydrostatic pressure of the contained liquid.

Handling 68,000 Metric Tons of Liquid Argon

Imagine the infrastructure that would be needed to collect and transport 4 x 17 kilotons of even a simple room-temperature liquid a mile underground! First, you would need to receive and store it. LBNF plans for about five 18-ton deliveries by truck per day during the initial filling period. Add to that the requirements of keeping the liquid extremely cold and extremely pure (impurities diminish the particle detection capabilities), the capability of piping it down in gas form, recondensing it underground, running a 24 x 7 purification cycle, and most importantly guaranteeing safety — it's complex. Look at the schematic diagram and imagine following a drop of liquid argon through the system!

Schematic design of the LBNF cryogenics system. GAr is gaseous argon, LN2 is liquid nitrogen (used for refrigeration), GN2 is gaseous nitrogen. Credit: FNAL