Primary circuit in PWRs

Diagram of three-circuit water-steam reactor

Pressurized water reactors (PWR) are generally designed with a third water circuit in addition to the two found in other thermal power plants (e.g., boiling water reactors (BWRs) or coal-fired and geothermal power plants). In this so-called primary circuit, the water absorbs the heat generated by nuclear fission and transfers it to the secondary circuit. This additional circuit ensures that radioactive materials remain contained in it and do not disperse to the secondary circuit and therefore potentially to the environment.

This additional water circuit entails some specific requirements regarding chemical analysis and monitoring.

Determining boric acid …

The water in the primary circuit of PWRs contains dissolved boric acid. The purpose of the boric acid, and of the 10B isotope in particular, is to act as a moderator: The boric acid captures the fission neutrons, which keep the nuclear chain reaction going and are responsible for the reactivity of the reactor. Since its concentration plays such an important role, the determination of boric acid in the primary circuit is extremely important in view of reactor safety and efficiency.

… in the lab

Boric acid titration curve, with and without mannitol

Boric acid, being a weak acid with an acidity constant of Ka1 5.75·10-10, is difficult to titrate. To enable easy titration, polyalcohols, e.g., mannitol, need to be added to the boric acid, leading to the formation of complexes with a greater acidic strength that behave like a monovalent acid which can be titrated with a NaOH solution.

If boric acid is to be titrated manually, this complexation involves a great deal of work and requires exact pipetting of the sample, distilled water, and mannitol solution in order to yield accurate results. Automated titration is therefore certainly the better choice.

Metrohm offers a fully automated system for boric acid analysis.

> Learn more about the Robotic Acid Analyzer

… and in the process

Online process analyzer for titration, Karl Fischer titration, colorimetry, and more

The fuel assemblies cannot be exchanged during operation in PWRs. This makes it necessary to have a fuel reserve in place at the start of an operating cycle. The excess activity in the reactor associated with this reserve fuel is controlled by adjusting the boric acid concentration in the reactor. As the fuels burn up, the boric acid concentration has to be lowered to keep the reactor running. Setting the correct boric acid concentration in the primary circuit is therefore crucial for safe and efficient operation of a PWR.

This can be achieved by closely monitoring the boric acid concentration. The Metrohm Process Analytics 2026 Titrolyzer and ADI 2045TI Analyzer enable you to determine the boric acid concentration quasi-continuously by way of potentiometric titration.

> Learn more about the Metrohm Process Analytics 2026 Titrolyzer

> Learn more about the Metrohm Process Analytics ADI 2045TI Analyzer

Determining lithium

Chromatogram of lithium determined with ion chromatography

The boric acid that is added to the primary circuit of the PWR lowers the pH value and thus increases the corrosion potential. To counteract this effect, an alkalizing agent, in most cases monoisotopic lithium hydroxide (7Li), is added to the primary circuit. The lithium concentration therefore also has to be monitored. This can be done with ion chromatography in conjunction with intelligent partial loop injection (MiPT).

> Learn more about the determination of lithium with ion chromatography

> Learn more about Metrohm ion chromatography

Determining metal cations in the primary circuit

Chromatogram of nickel, zinc, calcium, and magnesium determined with ion chromatography

Some metal cations have to be monitored in the primary circuit of a PWR as well. Nickel is an important alloying metal that increases the corrosion resistance of steel. However, in ionic, dissolved form, nickel promotes corrosion, making regular checks of the nickel concentration necessary.

Depleted zinc is often added to reduce the radioactivity on component surfaces and to reduce corrosion of metal surfaces. This cation, too, has to be monitored regularly.

These cations can also be determined down to the sub-µg/L range using ion chromatography. Because of the presence of boric acid and lithium hydroxide, the determination of these cations requires combined Inline Preconcentration and Matrix Elimination.

> Read the application

> Learn more about Metrohm ion chromatography


Further applications and products

Keeping turbines free of rust and corrosion with chemical analysis

Turbine and lubricating oils

Turbine and lubricating oils are exposed to extreme conditions in power plants. Numerous international standards define the requirements and test procedures for in-service maintenance of the turbines.

The ASTM D 4378 standard specifies that the acid and base numbers are to be determined by potentiometric titration and that the water content is to be determined by Karl Fischer titration.

Metrohm titration Karl Fischer titration
Tubes in an industrial production plant

Process Analytics Applications

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