Development of Hydrogen Treatment System in Severe Accident: Part 4 — Study of Fission Products and Steam Effect on Hydrogen Treatment Characteristics

Abstract

A large amount of hydrogen is generated by the metal water-reaction in the Primary containment vessel (PCV) of light water reactors in the severe accident (SA). In the present accident management for boiling water reactor (BWR), vent of mixing gas with filtered vent is regarded as the most likely method that prevents the PCV overpressure. However, it is difficult to vent in early stage of SA because of high radioactive dose. Then we have been developing the hydrogen treatment system to prevent excessive pressure without PCV vent. In focusing on the oxidation-reduction reaction of metal oxides (MOs) with high reaction rate, we have been studying hydrogen treatment system using MOs as effective device under oxygen deficit conditions like PCV of BWR. In the previous studies, we evaluated the hydrogen treatment rate using a couple of MOs, and confirmed that CuO, Co3O4, and MnO2 were effective for the hydrogen oxidation under the oxygen-free condition. We also found that granules of these three MOs could achieve the goal of hydrogen treatment rate with reactor of hydrogen treatment system. We predicted that the performance of MOs decreased with exposure to steam and fission products (FPs) in the PCV during the hydrogen treatment, and investigated their influence. The objective of the present research is to investigate how the steam and FPs, which is supposed to be a reaction-inhibiting-factor, influence hydrogen treatment rate. Then, we conducted hydrogen treatment experiments using a fixed bed reactor with MOs layer. As the results, we confirmed that the hydrogen treatment rate of MnO2 decrease from 70 g/s/m3 to 15 g/s/m3 when partial pressure of vapor went above 0.1 MPa-abs, though, that of CuO didn’t depend on the partial pressure of vapor and sustain the same rate about 40 g/s/m3. We also confirmed that the hydrogen treatment rate was decreased with the consumption of granulated MOs faster than our expectation estimated with unreacted-core model*. We also estimated that CsI selected as typical FPs could not affect the hydrogen treatment rate of CuO. From these results, we have evaluated the reaction rate equation including the steam influence in CuO, which could estimate the hydrogen treatment rate of reactor unit. *Gas reacts only on the surface of solid and generates shell of products around reactants core. The core shrinks with reaction.

Modeling of low-temperature reduction of metal oxide in hydrogen treatment system for severe accidents in nuclear power plants

“…It has been reported that at relatively low temperatures (< 250 °C) the oxidation of hydrogen by CuO has an induction period of tens of minutes, but as the temperature approaches 250 °C the induction period becomes progressively shorter (Kim, et al, 2003). It has also been reported that the reaction rate decreases in atmospheres containing water vapor (Yamada, et al, 2015). However, the mechanisms underlying the process are not fully understood and current models fail to accurately simulate the empirically observed induction period.…”