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All issues Volume 11 (2025) EPJ Nuclear Sci. Technol., 11 (2025) 37 Full HTML
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EPJ Nuclear Sci. Technol.
Volume 11, 2025
Euratom Research and Training in 2025: ‘Challenges, achievements and future perspectives’, edited by Roger Garbil, Seif Ben Hadj Hassine, Patrick Blaise, and Christophe Girold
Article Number 37
Number of page(s) 5
DOI https://doi.org/10.1051/epjn/2025037
Published online 22 July 2025

© A. Laborda, Published by EDP Sciences, 2025

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

This paper introduces a new technology rather than engaging in a detailed scientific discussion of the issues caused by non-condensable gases in a nuclear reactor. Although it does not strictly meet all the requirements for a scientific publication, its diffusion is valuable to show this new and original solution to the Nuclear community.

There is a serious complication with a high probability of occurrence during a Station Blackout (SBO) or a Loss of Coolant Accident (LOCA): the inflow of nitrogen into the Reactor Coolant System (RCS) from the accumulators.

The nitrogen inside the system can seriously disturb core cooling by disrupting the natural circulation in the Steam Generators (SGs), among other complications.

The current strategies to avoid this injection rely on active elements and operator actions:

  • Closing the accumulator output isolation valve.

  • Opening the accumulator relief valve.

  • Maintaining the RCS pressure above the residual nitrogen pressure.

The ASVAD valve [1] is an innovative safety valve that can easily avoid this problem. This valve is a passive and automatic element that can precisely detect the correct moment to act by monitoring the residual pressure in the accumulator. Only when the accumulator is empty, does the valve opens and vents the nitrogen into containment, making sure that nitrogen will never reach the RCS pipes.

The ASVAD valve provides the following main advantages:

  • it is a passive device and does not require any external energy to actuate.

  • It is fully automatic and does not need any operator assistance.

  • It actuates at the required moment–just when the accumulator is nearly depleted.

  • It is available at all times, protecting the RCS from nitrogen.

  • It allows depressurization of the RCS to lower pressures, thus diminishing the leakage rate and facilitating recovery.

2. The nitrogen threat

Nitrogen is a gas widely used in PWR pressurized accumulators, make-up tanks, or similar equipment systems. It allows passive water injection into the RCS when a LOCA depressurizes the system. However, once its function is complete, it must be isolated to prevent it from entering the RCS pipes. If this isolation is not performed in time, the gas can seriously threaten core cooling and fuel integrity.

Once nitrogen reaches the RCS, it unleashes several complications (see refs [2–17]):

  • natural circulation in the SGs diminishes or even stops, and heat transfer is seriously reduced by the presence of nitrogen.

  • RCS pressure rises–and remains high–due to nitrogen expansion caused by heating. This will hinder or even impede the low-pressure pumps from injecting into the RCS.

  • Nitrogen can even reach the Residual Heat Removal (RHR) pumps, rendering them inoperable.

  • If the temperature rises enough, nitrogen will accelerate fuel cladding oxidation [7].

In the event of an SBO accident, nearly all plant equipment is lost. Under these circumstances, only passive elements operate. The principal way to maintain core cooling is through the SGs by natural circulation. Unfortunately, this SBO accident triggers another serious accident: a LOCA. As this leak is permanent, sooner or later the water in these accumulators runs out, and at that moment, the nitrogen threat begins. If the RCS pressure cannot be maintained above the nitrogen pressure, nitrogen will start to enter the RCS pipes, soon reach the core, and disseminate throughout the system, eventually reaching the SGs and rendering them useless for cooling the RCS.

Why does this nitrogen injection have a high probability of occurring?

  • Because the LOCA is permanent, and the RCS pressure will drop constantly (unless operator actions can temporarily stop this drop).

  • Because nitrogen is already inside the system–and in large quantities!

  • Because it has an open and direct path to the reactor.

  • Because it is constantly attempting to enter the RCS; it only needs a moment of low pressure.

  • Because the only element able to stop it–the accumulator isolation valve–is an active element that requires energy to close.

Therefore, it is only a matter of time before nitrogen enters the RCS and disturbs core cooling. These are not merely assumptions or opinions; there have been some notable events in the past related to nitrogen intrusion in the pipes [6].

3. Current strategies

To prevent this nitrogen from reaching the RCS pipes, only three strategies may be employed [18, 19]:

  1. closing (on time) the accumulator output isolation valve.

  2. Venting (on time) the residual nitrogen to the containment building.

  3. Maintaining the RCS pressure above the nitrogen pressure.

All these strategies have common drawbacks or weaknesses during an SBO accident.

They require the deployment and proper actuation of active elements such as FLEX AC generators, FLEX pumps and compressors, isolation or venting valves, and all their support systems. Since all these elements are active, they need energy to operate.

They also require operator intervention. These elements are not automatic; they must be deployed and operated by recovery personnel. This necessitates not only significant organizational effort, but also sound procedures and a well-trained team to implement the recovery strategies.

These strategies have another important drawback: TIME. It is not easy to determine when isolation should occur. If done too soon, some water will be wasted in the accumulator. If done too late, nitrogen injection will occur. Moreover, these actions must be performed simultaneously for all accumulators, as they work in parallel, making these strategies timecritical.

All these strategies rely on a long chain of active components (operators included). If something fails in this chain, nitrogen injection will occur. A good example is the isolation valves: even when closed, they leak nitrogen throughout the recovery because they are not gas-tight.

The last strategy–maintaining the RCS pressure–is only a temporary measure. Sooner or later, the RCS will be fully depressurized, and then one of the two previous strategies must be employed (with all its drawbacks).

4. The ASVAD valve

The (ASVAD) Automatic Safety Valve for Accumulator Depressurization [1] is the response to the nitrogen injection issue. It is a unique kind of safety valve, with one main difference from standard safety valves: the ASVAD only actuates when the pressure drops below a present level. This level can be adjusted to correspond to the point at which the accumulator becomes empty of water.

Figure 1 is an internal view of the valve's components. Figure 2 is a simplified representation of the ASVAD valve. It has a pressurized chamber (1) connected to the accumulator's nitrogen side. This chamber is sealed by a hollow obturator (2) and a gasket. A preloaded spring (3), whose preload is adjusted by an adjustment disc (4) threaded over the obturator, is also present.

thumbnail Fig. 1.

Inner view of the ASVAD valve internal elements.

thumbnail Fig. 2.

ASVAD valve simplified operation diagram.

The operation of the ASVAD valve is based on the imbalance between the forces exerted on the obturator. On the bottom side, the accumulator's internal pressure exerts a force that firmly pushes up the obturator, keeping it closed. On the upper side, the preloaded spring exerts a constant force that pushes the obturator downward, attempting to open it. As long as the force exerted by the accumulator's normal pressure (thick arrow) is greater than the force exerted by the spring (thin arrow), the obturator remains firmly closed. During normal operation, the force ratio is 3:1.

The spring is pre-set to match the force of the nitrogen pressure when the accumulator is empty. When these conditions are with the accumulator nearly depleted, the spring overcomes the force exerted by the residual pressure and suddenly pushes down the obturator. This opens a communication channel from the pressure chamber (1) through the obturator holes and its hollow body to the outside. Once the obturator has left its seat, the pressure in the chamber falls further, leaving the valve permanently open due to the continuous spring action.

This is a very simple principle, with a passive and automatic operation. Simplicity means also reliability.

Additionally, the valve has also other optional elements to allow its remote manual operation (open & close), but usually is not needed.

The ASVAD Valve has the following advantages.

Main advantages:

  1. the ASVAD Valve will be available at all times after its installation, always waiting for its moment to act.

  2. It completely prevents nitrogen injection.

  3. It is based on two simple and universal physical principles (force and pressure).

  4. It is fully passive; it does not require any external energy.

  5. It is fully automatic; it does not need any operator assistance.

  6. It acts at the right time and across all accumulators. No time-critical operations are needed–it does so automatically by sensing the accumulator pressure. It is also able to self-adapt its opening point to the existing environmental conditions, maximizing the water volume injected from the accumulator.

  7. It completely vents the accumulator, so no further nitrogen injections will be possible.

  8. It allows depressurization of the RCS to lower pressures, which will greatly facilitate further accident recovery (this could be its best advantage).

  9. It will save the organization's efforts, allowing them to focus on other recovery tasks.

  10. It does not interfere with normal operation.

Secondary advantages:

  1. it is highly reliable due to its simple and robust design and physical operating principle.

  2. It is robust enough to withstand the post-LOCA environment; it is made of stainless steel.

  3. It is very easy to install in the system and does not require significant modification.

  4. It does not add any new failure modes beyond those already analyzed.

  5. It is intrinsically safe, with no electromagnetic issues. It's fireproof, does not add any fire load, it is not sensitive for radiation, moisture, or flooding.

  6. It also has a manual operation feature; it can be remotely actuated when required.

  7. It's maintenance is simple. There is no wear. It just requires a few spare parts. Minimum maintenance cost.

  8. The desired actuating pressure is easy to adjust and check.

  9. It is economical and does not require complex or expensive system modifications.

  10. It's qualified life is very long.

About the ASVAD Valve problems:

no specific problems are known yet, as its simplicity implies that there are few elements that can fail. A possible problem during operation could occur if the valve leaks nitrogen to the atmosphere.

However, this kind of problem is extremely unlikely and very easy to fix by isolating the valve. It must be taken into account that as long as there is enough pressure in the accumulator, the closing force will keep the obturator firmly closed. Another unlikely problem is if the obturator gets stuck in the closed state. The obturator geometry makes this kind of failure extremely improbable, as there is no way for any pollution to enter the valve.

5. Termohidraulic analysis of ASVAD behaviour

Few analyses of the ASVAD valve performance have been carried out yet. The main one is reference [1] which studies the beneficial effect of the valve during a Reactor bottom LOCA. This study evaluates how the ASVAD valve can solve a former experiment, test 6–2 from the OECD/NEA ROSA-1 project, carried out at the Large Scale Test Facility (LSTF) in Japan.

This experiment had to be aborted to protect the installation due to the negative effects of nitrogen. In the conclusion, the simulation shows that using ASVAD, the core damage would have been avoided.

Other more recent analyses [20] were done by the Spanish Nuclear Regulator (Consejo de Seguridad Nuclear). They thoroughly evaluated both, the constructive aspects of the valve, and its expected effects on design accidents at Westinghouse PWR-type power plants. They have given their approval to the use of the ASVAD valve in the Spanish Nuclear Plants.

Unfortunately, this study is not published due to its proprietary nature. Nevertheless, it can be directly provided by the author when asked.

6. Conclusions

Nitrogen injection into the RCS is a complication that can seriously threaten core cooling and fuel integrity during an SBO/LOCA accident. It is highly likely to occur during an SBO.

The current strategies to avoid it are too weak to be relied on. All of them use active components that require energy and multiple operators to perform time-critical actions simultaneously on different elements. This is like a chain: if any link fails, the whole operation fails.

Using ASVAD, nitrogen injection into the RCS can be easily avoided. ASVAD allows operators to be freed from dealing with this issue. This way, they can remain focused on core cooling or other recovery tasks. By allowing further RCS depressurization, it can facilitate accident recovery, giving a longer coping time.

ASVAD is a passive device that automatically vents the nitrogen at the correct moment. Overall plant safety can be improved by installing this valve on each accumulator.

Acknowledgments

The author acknowledges the help of the company Ringo Valves (Samson group) who made real this project, by building and testing the first ASVAD prototype. He also acknowledges the contribution of the staff from the Universidad Politecnica de Cataluña (UPC) and the Consejo de Seguridad Nuclear (CSN) who evaluated the ASVAD's performance.

Funding

All the work presented in this paper was carried out by the author without any financial support from third parties.

Conflicts of interest

All the work of this paper was done in accordance with the ethical standards of scientific communications. There are also other interests besides improving PWR reactors safety. The ASVAD valve is patented in different countries. Patent rights protect all the work involved in its development. The author controls the ASVAD patent, and this represents his legitimate interest.

Data availability statement

Any additional data about the aspects of the non-condensable behaviour inside the reactor cooling systems, or the current strategies to avoid it, can be found in the referenced documents. Additional data, detailed descriptions, and test results about ASVAD valve can be found at www.asvad-nuclear.com or directly asking the author.

Author contribution statement

As there is only one author, all the work was done by Arnaldo Laborda.

References

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Cite this article as: Arnaldo Laborda. ASVAD. A new safety element to avoid the complications of the undesired nitrogen injection to PWR reactors, EPJ Nuclear Sci. Technol. 11, 37 (2025). https://doi.org/10.1051/epjn/2025037

All Figures

thumbnail Fig. 1.

Inner view of the ASVAD valve internal elements.
In the text
thumbnail Fig. 2.

ASVAD valve simplified operation diagram.
In the text

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