Influence of n,γ-field fluctuations on critical hydrogen concentration in the reactor primary coolant

摘要

One of the problems arising in operation of the NPP with reactors VVER/PWR are the consequences of the primary coolant radiolysis, namely, generation of the oxidizing particles intensifying the equipment corrosion rate. During operation of the reactor a decrease in concentration of oxidizing radiolysis products is provided with introduction of molecular hydrogen into the coolant. In this connection, the reliable estimation of Critical Hydrogen Concentration (CHC), sufficient for suppression of formation of oxidizing radiolysis products under specific in-pile conditions (reactor radiation dose rate, temperature, coolant chemical composition) is of practical interest. Unfortunately, the experimental data on CHC in-pile determination differ essentially from the values calculated [AECL EACL R&D Report, Coolant Radiolysis Studies in the High Temperature, Fuelled U-2 Loop in the NRU Reactor, A.J.Elliot, C.R.Stuart, 2008]. Critical hydrogen concentration is in the region of kinetic instability of radiation-chemical system. A slight change in hydrogen concentration leads to a sharp (by several orders) change in concentration of both short-lived (OH, HO_2) and stable (O_2, H_2O_2) oxidizing particles. In essence, when reaching the CHC, the radiation-chemical system changes over from one stable state to another. The paper deals with the results of the computer simulation of influence of short-term n,γ-field fluctuations on changing of the radiation-chemical system from the state with low concentration of oxidizing particles over to the state with their high concentrations. It is demonstrated that for the correct calculation of CHC in the primary coolant of VVER/PWR the non-uniformity of n,γ-field in the core shall be taken into account.