SOLID STATE RADIATION DETECTORS FOR FUSION FUEL CYCLE APPLICATIONS

Current fusion devices, such as the Joint European Torus (JET) operated by the UKAEA, operate using batch process fuel cycles with tritium (3H) flow rates in the order of 10s of grams per day. Future planned fusion devices, such as ITER and DEMO, would operate with continuous fuel cycles and tritium flow rates in the order of 1 to 2 kg per hour. Existing tritium measurement and accountancy instruments employ ionisation chambers which whilst barely sufficient for the needs of JET would not meet the requirements of ITER or DEMO, both of which would also likely be nuclear licensed sites. The inaccuracy of ionisation chambers stems from the variability of unknown gas species within the detector volume and/or errors in assessing the pressure and temperature of the gas stream containing the tritium, all of which effect the final reading. Some work has been done, and devices built and operated, that measure the tritium content of a volume by measurement of x-rays generated by the beta decay of the tritium. These devices have been employed in either the measurement of bulk materials or on gas streams of very pure and concentrated tritium. The systems to date measure the total counts generated (across the full energy range) which is heavily influenced by the broad spectrum bremsstrahlung x-ray signal. In this work the possibility of using the energy spectrum of x-rays generated by the tritium beta decay to better understand the sample containing the tritium, and potentially correct for any signal loss or modification of the energy dependence of the x-ray spectrum detected. Such a device would be based on the characteristic x-rays of materials impacted by the tritium beta decay rather than the gross count rate (including bremsstrahlung). A Monte Carlo code called PENELOPE is used to compare simulated results with experimental data for a series of configurations. Whilst the initial experimental tests are performed, a failure in the safety systems of the JET Active Gas Handling System (AGHS) prohibited the running of higher activity gaseous source tests. Nevertheless, the simulated and experimental data collected informs a series of more comprehensive PENELOPE simulations that indicate that if the tritium beta spectrum is modified by attenuating gases as described by literature, then an energy sensitive detector based on x-ray emission induced by beta decay could compensate and correct for errors induced in a detector by gas compositions and other factors. Such a detector would be greatly beneficial to the operation of large-scale fusion devices such as ITER and DEMO