Challenge of tritium contamination monitoring
Sheila Liddle , SRP Past President , is an RPA and RWA with over thirty years ’ experience of working in the nuclear and non-nuclear industries . Here she discusses the practical challenges of tritium monitoring .
Unless you work in the nuclear or life sciences industries you may not have had dealings with tritium monitoring and contamination . Tritium is still used in beta lights , fishing rods , emergency exit signs and
some jewellery , even though it is frowned upon . It is popular in emergency situations as , when used with a phosphor , it creates a bright light without the need for a battery . Large items such as emergency exit signs frequently contained several tubes of glass containing the tritium . Although tritium is one of the less toxic radioactive materials , beta lights ( small glass tubes ) can be hazardous in a confined space as they contain giga-becquerels of tritium and tritiated water .
Tritium is a radioactive isotope of hydrogen with a nucleus made up of a proton and two neutrons . Tritium is a beta emitter that undergoes radioactive decay to form a stable helium-3 atom , a beta particle and a neutrino . β- 3 H 1
3 2 He + e - + ν - e
The beta particle from tritium has an average energy of 5.7 keV with a maximum energy of 18.6 keV , which is low compared to other beta particles , making it difficult to detect in the field . Tritium combines and / or exchanges with hydrogen in the atmosphere or materials it contacts and behaves chemically the same as a hydrogen atom . Tritium gas is very often in the form of tritiated water . It can also be present as organically-bound material or as solid tritides .
Atmospheric monitoring Monitoring for tritium in the atmosphere can either be carried out directly using a pump and an ion chamber , a proportional counter , or indirectly using a bubbler system . The problem with the ion chamber system is that where there are significant levels of tritiated water present , the chamber becomes contaminated and is not easily cleaned .
The most common technique is to pass the gas through a bubbler system and analyse the liquid using the Liquid Scintillation Counting ( LSC ) technique . In the laboratory environment , this is typically used to measure gaseous discharges . Several companies such as Ludlum and Berthold manufacture real-time tritium gas monitors .
Monitoring liquids and solids Monitoring tritium in liquids is relatively easy , especially for water samples , as this can be analysed directly using LSC . The difficulty comes when the liquid to be monitored is a solvent , oil or a very dirty aqueous sample such as sewage sludge . Non-aqueous liquids or dirty samples absorb some of the scintillation emissions , a process known as quenching . Such samples need to be analysed using either pyroliser techniques or calorimetry . Similar techniques are also required for the analysis of solid samples . Pyrolisers heat the sample to drive off tritiated water ( HTO ) which is then trapped and analysed by LSC .
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26 Radiation Protection Today www . srp-rpt . uk