Developing experimental capability for investigation of free radical processes under light water reactor operating conditions

Water radiolysis at extreme conditions of high temperature and high
pressure (HTHP) is an important issue in a number of areas in nuclear reactor
technology, especially in the high temperature corrosion of structural materials
in the primary circuit of Light Water Reactors (LWRs), which is noticeably
enhanced by the oxidizing radical products of water radiolysis. Mitigation of
the detrimental effects of water radiolysis on reactor’s structural components
promotes the long-term viability of water-cooled nuclear systems. In addition,
the choice of promising materials for the next generation of nuclear reactors
will rely on the fundamental understanding of the radiation chemical processes
in water under extreme conditions of high heat, pressure and mixed radiation
fields.
Direct examination of the free radical processes in nuclear reactor cores
is extremely difficult due to the intense mixed radiation fields and HTHP [1].
One way to circumvent these limitations is to design an experiment in which
water (or aqueous solution) under study is irradiated in the high temperature/high pressure conditions using conventional radiation sources. In my talk
I will describe the design of experimental capability dedicated to the study of
radical processes arising from the radiolysis in HTHP flowing water using either gamma (Co-60) or proton beam irradiation.
In the heart of our experimental setup is a recirculation loop that is capable of maintaining water inside the irradiation cell (autoclave) at temperatures
up to 350°C and pressures up to 200 atm. Essentially, the flow system consists
of a feed tank with aqueous solution under study, low and high pressure loops
with associated pumps, a preheater, an autoclave and a cooler. Water chemistry control is of a great importance, hence, the automated data acquisition system continuously monitors pressure, temperature, dissolved hydrogen and dissolved oxygen content and water conductivity before and after irradiation. The
water chemistry can be easily modified by saturation/doping with the additives
of interest, such as hydrogen, oxygen or ionic species.
Тезисы докладов Пленарного заседания / Plenary Session Abstracts June 25-26,2015
4
During water radiolysis a number of radical transients and molecular
products are formed, as shown by simplified Equation 1:
H2O → eaq,·OH, ·H, H+
,·HO2, H2O2, H2 (1)

The “yields” of radical species (eaq,·OH, ·H) are functions of time because their diffusive escape is in competition with recombination processes.
At room temperature within about a microsecond the recombination reactions
are completed, while the surviving radicals can be measured with appropriate
scavengers as an “escape yield”. Scavenged yields of major radical products
formed during water radiolysis at room temperature are well established.
However, radiation chemical yields and reaction rates of primary radicals in
the radiolysis of water above 300°C are either not measured at all or inconsistent; current reactor models include calculations based on extrapolated
data [2].
Our first experiments will target radiation chemical yields for the free
radicals ·H and eaq [3]. Corresponding G values will be determined for low
and high linear energy transfer (LET) types of radiation using, respectively,
gamma and ion beam radiolysis. Water radiolysis in a wide range of high temperature and pressure conditions (up to 350°C and 200 atm) will be performed.
N2O and ethanol-d5 will be used to scavenge eaq and ·H, respectively. Very
sensitive mass spectroscopy technique (or its combination with gas chromatography) will be employed to measure N2, HD, H2 or O2 formed either as stable products of water radiolysis (H2, H2O2→O2) or as the products of scavenging reactions (N2O→N2, C2D5OH→HD).
Preliminary data will be used to demonstrate that the described rig can be
successfully employed to study free radical processes in aqueous systems at
HTHP conditions. The importance of the radiolysis experiments in water at
HTHP is hard to underestimate: described studies will produce essential thermodynamic and kine