Self-radiolysis of tritiated water. 2. Density dependence of the yields of primary species formed in the radiolysis of supercritical water by tritium beta -particles at 400 degree C

• Published in: RSC advances 2014-06, Vol.4 (44), p.22980-22988
• Main Authors: Butarbutar, Sofia Loren, Sanguanmith, Sunuchakan, Meesungnoen, Jintana, Causey, Patrick, Stuart, Craig R, Jay-Gerin, Jean-Paul
• Format: Article
• Language: English
• Subjects: Chemical reactions; Computer simulation; Density; Fast neutrons; Free radicals; Mathematical models; Radiolysis; Tritium
• ISSN: 2046-2069
• DOI: 10.1039/c4ra02761b

Supercritical water (SCW) has attracted increasing attention after the Generation IV International Forum selected the supercritical water-cooled reactor (SCWR) as one of six concepts for further investigation. The reference design for the SCWR calls for an operating pressure of 25 MPa and a core outlet temperature as high as 625 degree C. Tritium is of special interest in these proposed systems, because of the appreciable quantities that would be produced. Regarding the water chemistry in SCWR systems, there is however a complete lack of information on the radiolysis of SCW by tritium beta -particles. Because direct measurement of the chemistry under such extreme conditions of high temperature, pressure, and mixed neutron and beta / gamma radiation fields is difficult, chemical models and computer simulations are important for predicting the detailed radiation chemistry of the cooling water in a SCWR core and the impact on materials. In this study, Monte Carlo simulations were used to predict the yields (or G-values) for the primary species e super(-) sub(aq), H&z.rad; , H sub(2), &z.rad; OH, and H sub(2)O sub(2) formed from the radiolysis of deaerated SCW (H sub(2)O) by the low-energy beta -electrons ( similar to 18.6 keV maximum) of tritium at 400 degree C as a function of water density in the range of similar to 0.15-0.6 g cm super(-3) ( similar to 24-56 MPa). The objective was to elucidate the (time-dependent) mechanisms involved in the self-radiolysis of tritiated water under supercritical conditions. Calculated yields were compared with data obtained for low-"linear energy transfer" (LET) radiation (such as super(60)Co gamma -rays or high-energy electrons) and fast neutrons. Our simulations revealed that there was a strong resemblance between the density dependences of the different yields for the radiolysis of SCW with tritium beta super(-) particles and fast neutrons, corroborating very well with a model of tritium beta radiolysis mainly driven by the chemical action of "short tracks" of high local LET. As for the effect of density on the various yields, there was an increased "cage" escape of free radicals at low-density SCW. In contrast, these density effects acted in the opposite sense in the high-density liquid-like region where the caged free radical products were forced to remain as colliding neighbors and recombine, thereby increasing the molecular yields. Finally, the occurrence of the reaction of H&z.rad; atoms with water in the homogeneou