Effect of fission betas, activated structures and hydrogen on Self Powered Neutron Detectors

It is shown that when vanadium Self Powered Neutron Detectors are placed too close to fuel rods, fission betas can cause a significant perturbation of the signal. It is shown that this effect can be used to measure the decrease of the fuel rod power over time due to the burn-up. Betas emanating from activated structures around the detector were also found to lead to significant signal perturbations when not enough water was present to attenuate these betas. Further, it is shown for the first time that hydrogen dissolved in the water around the detector can cause very large signal perturbations. All these effects were not observed on rhodium Self Powered Neutron Detectors. Methods to mitigate, or correct, these effects are discussed. (author)