Primary circuits of nuclear power plants (NPP) with VVER reactors use corrective water chemistry. Weakly alkaline reducing coordinated potassium-ammonia-hydrazine water chemistry with boron control is used for water coolant during power operation of the reactor unit (Subbotin et al. 1970, GOST 24693-81 1981, Avdeyev et al. 1992, Roshchektayev 2010, Technical Justification 2016). Boric acid has a number of advantages in the reactor operation conditions: it is chemically stable in the primary circuit’s radiation and temperature fields, is highly soluble in water, and do not form sludge and scale. However, boric acid reduces the coolant’s hydrogen potential (pH) value which leads to more intensive corrosion of the structural materials of the circuit components. Corrective additives are introduced into the coolant in the form of alkali metals (lithium, potassium, sodium), ammonia and hydrazine hydrate (N₂H₄∙2H₂O) to make up for the acidic qualities of boric acid and to maintain the optimum water chemistry parameters.

According to the coolant quality standards, the total concentration of alkali metal ions in the hot state of a VVER unit is ~ 3.0 mg/dm³ and is achieved by introducing alkali metal hydroxides into the primary circuit coolant in the amount depending on the concentration of boric acid and enabling the hydrogen potential to be kept at a level of pH = 5.8–10.3 (Yefimov et al. 1983, Shkolnikov et al. 2008, Klinshpont et al. 2012, Milinchuk et al. 2014, 2015). No influence of alkali metals on the hydrogen formation in the process of the water coolant interaction with the reactor structural materials has been investigated as part of this problem. The purpose of this study is therefore to determine the influence of lithium, potassium and sodium hydroxides on the process of hydrogen formation as a result of the coolant interaction with the reactor structural materials. For research, aluminum in the form of powder used in hydroheterogeneous compositions was chosen as the model metal (simulating the reactor structural materials). Aluminum was used in the form of powder to increase the contact area of the reagents (aluminum and aqueous solution with a corrective additive) and the hydrogen yield so that it would be possible to estimate the difference in the generation of hydrogen using aqueous solutions with different corrective additives of alkali metal hydroxides. Aqueous solutions with corrective additives of alkali metal hydroxides simulate the VVER reactor primary circuit water chemistry (Legasov 1978, Dmitriyev and Prokhorov 2003, Sheyndlin and Zhuk 2006, Tarasov and Lototsky 2006, Milinchuk et al. 2012, 2013, 2016, Milinchuk and Roshchektayev 2014, Aminov and Bayramov 2016, Salakhova et al. 2018).

Fig. 1 presents a laboratory setup of the Department of General and Special Chemistry at the OINPE for investigating the formation of hydrogen by chemical decomposition of water in hydroheterogeneous compositions. The facility makes it possible to study the generation of hydrogen by compositions in a temperature range from indoor temperature to 100 °C. The hydrogen formation reaction is controlled by measuring the quantity of the liberated hydrogen in cylinder 7. The investigation target was a reaction mixture consisting of aluminum powder with a specific surface area of 1.6 m²/g in the form of flakes of 25 to 50 μm with a thickness of 0.25 to 0.50 μm (PAP-2 grade, GOST 5494-95) and lithium hydroxide (MRTU 6-09-4597-57 grade, GOST 8595-83), potassium hydroxide (ChDA grade, GOST 24363-80), and sodium hydroxide (ChDA grade, GOST 4328-77). Boric acid was also used as the additive (B grade, GOST 18704-78).

The hydrogen production process includes a number of procedures. Portions of aluminum powder 8, alkali metal hydroxides and boric acid are weighed on an OHAUS Adventurer analytical balance (accuracy, ± 0.0002 g). Distilled water is measured off using volumetric cylinders of 10 and 50 ml (GOST 1770-74, accuracy, ± 0.2 ml). The portions of aluminum powder, metal hydroxides, boric acid and distilled water taken in a particular proportion are loaded into glass reaction flask 3 with a volume of 100 ± 0.2 ml (GOST 1770-74) and mixed in a careful manner. The flask is placed in boiling water bath 2 fitted with thermometer 4 and heated to the required temperature on electric hot plate 1 which makes it possible to run the hydrogen generation process in a controlled mode at an acceptable rate. Reaction flask 3 is fitted with tube 5 to remove the hydrogen formed through water 6 into volumetric cylinder 7 with a volume of 1000 ± 10 ml (Lavoisier method). A pH-150 MA рН meter was used to measure the hydrogen potentials of the hydroheterogeneous compositions.

It follows from the obtained results that the nature of the hydroxides of alkali metals (Li, K, Na) has a major influence of the process of hydrogen formation in hydroheterogeneous compositions with aluminum, since these metals are strong activators of the hydrogen formation process. Lithium hydroxide is the most chemically active activator in this process. Hydrogen forms at a high rate at indoor temperature in an exothermal mode in compositions with lithium hydroxide. In compositions with potassium and sodium hydroxides, hydrogen accumulates at a rate acceptable for recording when heated to 60 °С. The accumulation rate and the hydrogen yield in these compositions depend on the content of boric acid, e.g. the hydrogen yield is ~ 1000 ml per 1 g of the reacted aluminum with the boric acid content of ~ 0.03 g/l, and there is no hydrogen formation at 0.6 g/l due to the potassium hydroxide neutralization by boric acid. In these compositions, therefore, alkali metals are efficient activators of the hydrogen formation process and boric acid is the inhibitor of this process.

According to the coolant quality standards, in the hot state of a VVER-1000 unit and in the reactor state at the minimum controlled power level, the total concentration of alkali metals is about 1 mg/dm³, i.e. two to three orders of magnitude as small as in the compositions under investigation (Roshchektayev 2010). However, the discovered influence of alkali metal hydroxides on the formation of hydrogen, involving structural materials, as considered on the example of aluminum that simulates the reactor structural material, allows one to suggest that hydroxides of these metals, which are present in the coolant in small amounts, may take part in the hydroheterogeneous process of formation of small quantities of hydrogen. The potential for the hydrogen formation in such a way needs to be taken into account, specifically during continuous long-term periods of the nuclear reactor operation, as well as in conditions of incidents and accidents at NPPs (GOST 24693-81 1981, Avdeyev et al. 1992, Technical Justification 2016).