Practical considerations in developing nuclear detectors for tokamak harsh environments

Abstract

Abstract Fusion tokamaks (FT) and hybrid fusion-fission reactors (HFFR) present harsh working conditions characterized by intense neutron and gamma fluxes (>1012 cm−2 s−1), high working temperatures (up to 600 °C) and corrosive environment. The breeding blanket region (BB) of these plants are resulting very hostile to the detectors used to monitor/measure fundamental nuclear parameters such as neutron/gamma fluxes and energy spectra, and tritium production. Presently no detectors are ready for being hosted in the harsh environment of the BB and R&D activity is needed to develop and test the candidate detectors. Some important lessons can be learned from past activities carried out in the EU and devoted to studying and realizing nuclear detector prototypes for the European Test Blanket Modules (TBM) of ITER. Amongst the other, these studies pointed out the need for intense neutron fields and calibration facilities closely reproducing the expected working environments to be used for reliable testing and calibration of the prototypes. Accurate simulation by Monte Carlo technique of the proposed detectors allows to mimic and foresee the response and performances of the detectors pointing out several fundamental and critical aspects on the physical response of the detector so helping in understanding the detectors response. This can help in selecting the best performing detector. The selection is based upon a multi-step procedure. The lesson learned for ITER-TBM can be helpful to study and develop nuclear detectors to be used in HFFR reactors and in next fusion machines like DEMO this because, despite the difference, the ITER-TBMs and the BB of fusion devices and HFFR reactors experience a number of similarities in terms of radiation level, temperature and nuclear quantities to be measured. In this paper, after discussing the requirements to be fulfilled by the nuclear detectors that must operate in the harsh environments we will discuss an example of detector development by considering the case of a self-power neutron detector (SPND) with chromium emitter studied and developed for ITER-TBM. The detailed Monte Carlo analysis is also reported and the many issues not yet solved are highlighted and the possible follow up to HFFR instrumentation discussed.