Long-time behaviour of regenerating in-core neutron detectors with 238U−239Pu electrodes during power cycling

First-order numerical optimization of fission-chamber coatings using natural uranium and thorium

“…During extended exposure to neutrons, the fissile 235 U atoms deplete, which leads to decreased neutron sensitivity [5]. A typical fission chamber becomes 10% less sensitive to neutrons after a fluence of 1.5 × 10 20 n cm -2 in a typical Light Water Reactor (LWR) (mean neutron flux of 1 × 10 14 n·cm -2 ·s -1 ) [5].…”

“…Previous studies have focused on regenerative 234 U and 240 Pu coatings because of the absence of intermediate isotopes [5]. Using a mixture of 238 U and 239 Pu, calculations show that fission chamber coatings can maintain operation within ± 5% sensitivity deviation up to a neutron fluence of 6.2 × 10 21 n cm -2 [5]. However, by utilizing the smaller neutron capture cross section of 232 Th and the absence of a fission cross section below 2 MeV, stable device operation should be possible at significantly higher neutron fluences (based on the fertile isotopes listed in Table I).…”

“…A typical fission chamber becomes 10% less sensitive to neutrons after a fluence of 1.5 × 10 20 n cm -2 in a typical Light Water Reactor (LWR) (mean neutron flux of 1 × 10 14 n·cm -2 ·s -1 ) [5]. In order Manuscript to extend stable detector lifetime, fertile isotopes can be added to the neutron sensitive coating [5]. A fertile isotope is one that is not initially fissile but will become fissile after absorbing (capturing) a neutron.…”

“…Conversely, systems employing 234 U and 240 Pu as fertile isotopes utilize the production of 235 U and 241 Pu, respectively, immediately after neutron absorption [7]. Previous studies have focused on regenerative 234 U and 240 Pu coatings because of the absence of intermediate isotopes [5]. Using a mixture of 238 U and 239 Pu, calculations show that fission chamber coatings can maintain operation within ± 5% sensitivity deviation up to a neutron fluence of 6.2 × 10 21 n cm -2 [5].…”

“…A fertile isotope is one that is not initially fissile but will become fissile after absorbing (capturing) a neutron. Numerous fertile isotopes are of interest for fission chamber coating optimization, including the isotopes listed in Table I [5]. The use of 232 Th and 238 U as fertile isotopes yields the fissile isotopes 233 U and 239 Pu, respectively, after neutron absorption and subsequent decay of the intermediate isotopes 233 Pa (T 1/2 = 27 days) and 239 Np (T 1/2 = 2.35 days) [7].…”