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Radiochemistry in Nuclear Power Reactors
6.3RADIOLYTIC GAS PRODUCTION IN BWRS
In an operating BWR, most of the radiolysis occurs in the high flux core region where boiling takes place. The net average production rate of radiolytic gases (containing approximately stoichiometric mixtures of H2 and O2) in BWRs has been determined to be 0.041 SCFM/MWt (19.35 cc/s/MWt) (Figure 6–2). The majority of the data shown in Figure 6–2 were determined by measuring the radiolytic gas (H2+O2) content in a gas sample vial and the offgas flow rate. As shown in Figure 6–2, considerable variation in the measured radiolytic gas production rate can be seen. The variation may be partly attributed to analytical error and the difficulty in accurately calibrating the gas flow rate in the off-gas line. However, the data measured in steam samples appear to be more consistent with the average production rate. It should be noted that the actual production rate in the BWR core may slightly vary due to differences in design and/or operating characteristics (e.g., core power density, steam void fraction, operating pressure, coolant flow rate, and impurity levels in the coolant).
For the average offgas production rate of 0.041 SCFM/MWt in BWRs, the “apparent” H2 and O2 yields are G(H2)BWR=0–0056 and G(O2)BWR=0.0028, respectively. These apparent yields are produced by a mixture of neutron and gamma radiation in the core region. It is entirely possible that most of the radiolytic gases are produced in the boiling region and quickly partition between the steam phase and the liquid phase. The steady-state concentrations of dissolved gases in non-boiling water are expected to be relatively low. In the presence of excess H2 in water, the production of O2 may be effectively suppressed in non-boiling water.
6.4SUPPRESSION OF WATER RADIOLYSIS BY H2 ADDITION
In the presence of excess H2 in water, the water decomposition and production of O2 can be suppressed through a chain reaction which rapidly reduces the concentration of OH and H2O2, in the reactions.