Tritium storage beds at ITER are likely to use uranium as a getter material with a total inventory of 150g T2 at 75% stoichiometric composition of UT3. We propose a storage bed design directly extrapolated from the JET uranium beds which already have a U238 inventory of 4.284kg. Three alternative approaches to implement calorimetry for in situ tritium inventory accounting are discussed. The favoured solution uses a microporous thermal insulation operating in a hydrogen atmosphere. This design is presented in detail and is shown to meet all operational and safety requirements. The accuracy of calorimetric assay to ±0.1% requires only the measurement of a temperature difference to ±0.1 K and stabilisation of the ambient reference temperature of 300K to ±0.1K.