Monday, 6 January 2020

All About the Active Gas Handling System

Many across UKAEA, not just amongst the graduates, will never get to see inside the building housing the active gas handling system and the labyrinth of equipment that makes JET the only fusion device in the world that can currently run with tritium. The invitation for a visit was therefore eagerly snatched up by graduates wishing to see inside JET’s fuel processing annex. James O’Callaghan and Hannah Todd, 2nd year graduates, have previously given tours to their graduate peers last year and now it was time to induct the new cohort into the mysterious world of tritium. 

The Active Gas Handling System or ‘AGHS’ is currently readying itself for DTE2 (the 2nd deuterium-tritium experiment). This has involved an immense amount of effort from the H3AT department (Hydrogen-3 Advanced Technology) who are upgrading and commissioning systems that have not been operated in full since 1997 when the first DT campaign broke the record for fusion power produced at 16.1 MW. With tritium capability restored, JET is poised to break the record for fusion power in the DTE2 campaign by the end of 2020. 

But first, a quick 1-0-1 on tritium. Tritium is an isotope of hydrogen. In tritium world, ‘normal’ hydrogen (1H) is termed protium and heavy hydrogen (2H) is called deuterium. Super heavy hydrogen so called because of its extra two neutrons is tritium. These extra neutrons make tritium nuclei unstable and radioactive. Hydrogen-3 decays to Helium-3 by β- decay the conversion of a neutron to a proton and emission of an electron and an antineutrino. Hydrogen’s small nuclei can permeate through the atomic lattice of materials. This causes tritium to be retained in pipework and vessels, reducing the efficiency of the process and increasing the tritium inventory in the material. Additionally, decay to 3He can cause structural holes and the emitted electron (Beta particle) can go on to irradiate the rest of the material. Tritium preferentially exchanges with protium in H2O to form HTO. Use of oils and plastics is avoided in AGHS as tritium exchanges with the protium in the hydrocarbon chains. These are a few of the challenges faced by those designing a tritium compatible fuel cycle.

The Active gas Handling system contains subsystems all with different roles to play in the fusion fuel cycle. There are some key systems that must be present. The first system is responsible for taking the gas from the JET tokamak back to AGHS. For this we use a cryogenic pump much quicker than mechanical pumps at low pressures. The second is impurities processing. Exhaust gas from the JET tokamak must be processed to remove everything that isn’t hydrogen such as water, hydrocarbons, nitrogen, helium and oxygen. This is achieved using a variety of chemical modules that can process and remove a specific type of impurity. To obtain the optimum 50:50 D:T plasma, an isotope separation system is needed. This consists of cryogenic distillation columns and a gas chromatograph. The pure deuterium and tritium can then be stored ready for re-injection into the tokamak.  

Due to its radiological properties the systems must be designed to limit any release of the radioisotope to personnel and the environment. The plant purges system secondary containments, detritiates exhaust gases before release to the environment and monitor the composition of the gas at each stage. This is achieved by our Over Under Pressure Protection System, which purges outer containments, and our Exhuast Detritiation System, which removes tritium by producing tritiated water. This water is processed to recycle the tritium back into the fuel cycle by our new system, the Water Detritiation System. AGHS is continuously staffed during a D-T campaign. Operators in the control room must process the exhaust ready for the next pulse, respond to any alarms and know the protocol for different emergency situations.  

But what is it like to work in such a specialised chemical plant? Tritium engineers are faced with very different problems to the rest of the chemical industry. Oils and plastics are out; 316 stainless steel is in. Parts are bespoke and their tritium capability unknown. The control room is intense and busy. Operators must be aware of everything that is going on to protect people, environment and plant. Tritium scientists are advancing the technology for future fusion machines like ITER, DEMO and STEP. Small scale experiments are performed to prove viable technologies; tritium’s unique properties are not easily modelled. Working with tritium is just as weird, rare and volatile as the isotope itself. 

What did the graduates think of this one-of-a-kind experience? 

A very informative tour. And by far the cleanest chemical plant I've ever been in. 

I really enjoyed this tour the guides were very knowledgeable and were able to answer all my questions about an area of plant I was very unfamiliar with.  This area of plant has cryogenics, radiation and stainless steel galore all wondrous to behold. 

The storage solution for tritium was interesting and unexpected. 

I remember being amazed by the amount of Chemical Engineer work that went into a physics experiment. Learning about the Gas Chromatography and the Cryogenic Distillation to recycle unused deuterium and tritium was fascinating.