Towards the Direct Observation of Xenon-135 Poisoning in a Zero-Power Reactor via Gamma Spectroscopy

¹ École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
² Paul Scherrer Institut (PSI), 5232 Villigen, Switzerland
³ Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, Michigan, 48105, USA

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Published online: 6 November 2025

Abstract

Xenon-135 (Xe-135) is a high-yield fission product with high neutron-capture cross section. it is a commonly encountered reactor reactivity poison. Direct observation of Xe in reactors typically relies on off-gas measurement techniques, which can be challenging or costly to implement and often require high-flux reactors. Gamma spectroscopy of irradiated U or Pu samples provides an alternative method for obtaining fission yields and population estimates. When predicting the Xe populations of a given reactor, we rely on calculations and models to estimate the instantaneous reactivity worth and post-shutdown dynamics of Iodine-135 (I-135) and Xe-135 decay that leads to the so-called ‘Xenon pit’. In this work, developing towards a direct estimate of the total Xe-135 population in an operating reactor, we measured the gamma-ray emissions of Xe-135 and I-135 emanating from the CROCUS zero-power reactor using a high-purity germanium detector in an irradiation channel. CROCUS is a uranium fueled, light water moderated, zero-power reactor operated by the Laboratory of Reactor Physics and Systems Behavior at EPFL, Switzerland. We placed an Ortec GEM-15180-P detector in the irradiation beam port of CROCUS at roughly 4.3 meters from the core center. We measured the gamma ray spectra using a CAEN DS5730 digitizer to obtain gamma-ray time stamps and pulses for the start-up, at-power operation for 1 hour at 20 W, and subsequent shutdown of the reactor for 100 hours. At power, the spectrum is dominated by the gamma ray emission from fission and neutron capture in water and aluminum. Following shutdown, the spectrum transforms, displaying a dense set of characteristic line emissions. To observe Xe-135, we used its 250 keV emission, whilst for I-135 we observed the 1260 keV emission. We binned the spectra in time for 60-minute intervals to accumulate sufficient counts under the photopeak of interest. The time dependent Xe-135 population was observed to follow the expected ‘Xenon pit’ shape, i.e. we were able to directly observe the Xe-135 and I-135 populations in the reactor. These preliminary data are currently only processed into count rates over time. Future work includes determining the detector efficiency in counts per emission in the fuel to finally estimate the whole-core Xe-135 and I-135 populations. Given the published IRPhE geometry of CROCUS, this dataset holds potential as a validation benchmark for reactor poison prediction models.