IRSN is currently participating in the preliminary review of the safety options for the NUWARD™ project by the French (ASN), Finnish (STUK) and Czech (SUJB) safety authorities with the support of their technical safety organizations (TSO) respective ones, including IRSN for France. 

​​The NUWARD™ project is a concept of electricity production unit, supported by EDF, consisting of two pressurized water nuclear reactors of 170 MWe each. From a safety point of view, this concept has characteristics common to other SMR concepts in development abroad, in particular its low power, its compactness and the implementation of passive safety provisions. For IRSN, SMRs m​ust meet safety requirements at least equivalent to those adopted for third generation reactors, their low power and the inherent safety characteristics put forward by their designers should benefit safety.

IRSN experts have been contributing for nearly ten years to thoughts on the safety requirements and approaches for these new concepts, in particular through their involvement in various international working groups of the IAEA and the SMR Regulators' Forum. [1]

IRSN also participates in several European projects which aim to acquire knowledge on these new concepts, both in support of the safety demonstration and in the validation of simulation tools, for pressurized water reactors but also for other types of reactors, for example high temperature reactors dedicated to cogeneration. 

The evaluation of the performance of passive safety systems being a central element of the safety demonstration of SMRs, a very good understanding of the physical ​phenomena that participate to their activation, their operation and those which can prevent them is necessary. IRSN is considering the development of a specific experimental research platform (PAssive Systems Thermalhydraulic Investigations for Safety) to support its expertise. This new experimental facility will provide knowledge in particular on the operation of a passive core cooling system as well as on the passive cooling of a metal containment immersed in a water pool. ​

For more information : our information memo on Small Modular Reactors​

Notes:

1. The SMR Regulators' forum brings together the safety authorities of France, the United States, Canada, the United Kingdom, Finland, Japan, South Korea, China, Russia, the Czech Republic and South Africa

​IRSN publishes a new information report providing details of the arrangements planned in the event of a total loss of external supplies at the Zaporizhzhya nuclear power plant.

​The plant is currently connected to the Ukrainian electricity grid by two of the four 750 kV lines available. Two lines are currently unavailable due to the fighting; a third one was temporarily unavailable but was repaired on the evening of March 18, 2022. The power plant is also connected to the Ukrainian 330 kV grid, to which the Zaporizhzhya thermal power plant and the Dnipro and Kakhovka hydroelectric power plants are connected nearby. This 330 kV line is currently available.

The Zaporizhzhya NPP provides electricity to the Ukrainian power grid but when its reactors are shut down, the Ukrainian power grid provides power for its monitoring and backup systems. The availability of these external power supplies is therefore an important issue for ensuring the safety of the reactors.

On the basis of the information available to IRSN, the resources planned for the Zaporizhzhya power plant would enable the site teams, in the event of failure of the islanding transient, to cope with a situation of total loss of external power supplies for a period of at least 10 days. This conclusion is subject to the reliability of the equipment implemented, their initial fuel supply, the availability of crews and the absence of other factors that could aggravate the situation.

Download IRSN information report​​ from March 22​​, 2022: Arrangements in the event of a total loss of external power supplies of the Zaporizhzhya power plant in Ukraine

On 9 March at 11.22am, the Ukrainian nuclear safety authority SNRIU informed IAEA that the external power supplies of the Chernobyl plant facilities had been cut off. The site's emergency generators would be supplying power to the facilities, with enough fuel for 48 hours.

​IRSN presents below an assessment of the risks associated with the loss of external power supplies for the site’s various facilities, including the loss of emergency power supply, which is a potential situation that could come about rapidly if power is not restored to the site because the generator fuel reserves run out.

Reactors

​Reactors 1, 2 and 3 at the Chernobyl site have been shut down for over 20 years. All the fuel assemblies of these reactors have been transferred to the site's storage facility (see below). There is no risk of releases from these facilities, which are not backed up by diesel generators.​

Reactor 4, which was damaged in 1986, was initially covered by a temporary sarcophagus as an emergency measure. Uncertainties concerning the structural resistance of this sarcophagus led to the construction of a containment structure (NSC for New Safe Confinement), which was completed in 2017 (250 m wide, 160 m long, 100 m high). The temporary sarcophagus is being dismantled. The NSC ventilation system is backed up by two dedicated generators. In the event of total loss of electrical power, facility containment will be provided solely by the static containment of the structure. The dismantling of the sarcophagus of the damaged reactor has probably been suspended due to the conflict, so this containment should be sufficient to prevent releases into the environment.

Spent fuel storage facility

The facility consists of a storage pool (ISF-1) with approximately 20,000 assemblies and a dry storage facility (ISF2). Spent fuel elements are gradually transferred from the pool to ISF-2.

• Underwater storage pool ISF 1

The safety systems of this facility are backed up by two diesel generators with fuel reserves to last 48 hours.

The studies carried out after the accident at the Fukushima Daiichi plant on the consequences of total loss of the pool cooling systems indicate a slow rise in pool water temperature to a temperature of around 60°C but no dewatering of the assemblies and therefore no radioactive releases into the environment.

• ISF 2 dry storage facility

​To date, about 2,000 assemblies have been transferred from ISF-1 to ISF-2.

This facility does not present a risk in the event of total loss of electrical power, as the power removal from thefuel assemblies is completely passive.

Loss of the facility’s control systems

​Although the loss of power at the Chernobyl site does not have consequences that could lead to environmental releases, it does imply the loss of the facility's command-control systems. Thus, all the technical data used by the site’s real-time monitoring (water level, temperature, radioactivity, etc.) and alarm systems will cease to be available; this could delay the reactions of personnel if an incident occurs on the facility.

The loss of power would also mean the loss of lighting, heating and certain communication systems, resulting in deteriorated working conditions for personnel, who are already suffering from the stress of the current situation.

Download IRSN information note​​ from March 10​​, 2022​​ (PDF)

​Status on 7 March 2022

The invasion of Ukraine by Russian troops demands careful monitoring of its nuclear installations. Ukraine has 15 Russian-designed VVER reactors in service, research reactors, storage sites for sources and waste, as well as the reactors at the Chernobyl site, the last of which was shut down in December 2000, and the various facilities required to manage the accident site.

The major risk in terms of radioactive release concerns the power reactors in operation and the spent fuel pools [1]. The 1,000 MWe reactors [2] have concrete containment structures. In these facilities, the spent fuel pools are located inside the containment.

According to the information available to the Institute, the fire that occurred on the night of 3-4 March 2022 at the Zaporizhzhya nuclear power plant site did not cause any deterioration of reactor safety.

On the morning of 4 March, the Institute received confirmation from SNRIU that the electric power supply to the plant had not been damaged by the fire. This power supply is necessary to keep the facilities in a safe state, whether they are in operation or shut down. In this respect, the safety of Ukrainian power plants has been significantly improved since the accident at the Fukushima Daiichi plant. The plants are equipped with emergency electric power sources (4 generators per reactor, one of which is bunkered), and mobile equipment that can be connected to the reactor concerned. The fuel reserves for the diesel generators are sufficient to provide cooling for seven to ten days, after which refuelling will be necessary.

There have also been reports of damage to the containment structure of reactor 1, which had been shut down before the conflict began. This information has not been confirmed; it is more likely that the shots fired damaged a footbridge near the building. Regarding the operational status of the plant, the Ukrainian nuclear safety authority (SNRIU) reports that two of the plant’s six reactors are in service.

Concerning the environmental radioactivity monitoring networks, the Ukrainian national network is operational, with the exception of a few stations. Based on the information collected by IAEA from SNRIU and the data transmitted by the measurement network, there has been no increase in radioactivity since the fire that night. The absence of radioactive release is also confirmed by the monitoring networks of the countries bordering Ukraine, which do not indicate any abnormal increase.

Download IRSN information report from Ma​rch 7, 2022 (PDF)​

Notes:

1. The spent fuel pool contains fuel assemblies used in the reactor core. They are stored in this pool for a few years before being transported to other pools.
2. This means all Ukrainian reactors except Rovno 1 and 2, which have a capacity of 400 MWe.

​Note: This information report has been published in French on February 25, 2022.

In view of the situation in Ukraine, IRSN has produced an information note presenting the nuclear facilities in Ukraine and an overview on the radiological monitoring of the country.​

An increase of the radiological atmosphere around the Chernobyl site was reportedly observed on the stations near the installations. The Ukrainian safety authority mentions a resuspension of contamination by the passage of military tanks.

​IRSN does not have any information to confirm or refute this information. It is advisable to remain very cautious about these measurements at this stage. No increase in radioactivity has been detected in the European countries with which IRSN is in contact.

Download IRSN information report from Ma​rch 1, 2022: Situation of nuclear facilities in Ukraine

During the ten-yearly in-service inspection of reactor 1 of the Civaux nuclear power plant, which began on August 21, 2021, EDF performed ultrasonic testing of several welds in the emergency core cooling system, in accordance with the applicable preventive maintenance program. The examinations revealed the presence of faults near the welds of some pipe elbows.

The emergency core cooling system (ECCS) is a safety system that njects borated water into the reactor main coolant circuit (also called main primary circuit) to cool the core in the event of a breach affecting the main coolant circuit. The objective is thus to maintain a sufficient water level in the core to cool the fuel assemblies.

The emergency core cooling system is made up of two independent safeguard trains connected to the primary circuit via a hot leg [1] and cold leg [2] connecting pipe of each of the four loops of the primary circuit.

Ultrasonic examinations carried out on Civaux reactor No. 1 revealed the presence of faults near the welds of some pipe elbows (see figure). In accordance with the applicable preventive maintenance program, examinations were then extended by EDF to adjoining welds. In order to determine the origin of these cracks, the pipes were cut, and the welds involved have been sent to the laboratory for expertise. By metallographic and microscopic examination, EDF was thus able to determine the nature and depth of the faults detected. The origin of faults seems to be stress corrosion cracking.

Stress corrosion is a mode of damage that typically results from the combined action of mechanical stress and an aggressive environment with respect to the material. In order to better understand the factors behind the observed corrosion, EDF has undertaken a verification of the manufacturing files. At the same time, it carries out a performance review of the control procedures used. These analyzes aim to develop a verification program for welds likely to be affected by the phenomenon.

If these faults develop in the emergency core cooling system piping, it could lead to a leak or a break. If this break occurs on a pipe, this leads to a loss of coolant accident, the damaged elbows being located downstream of the isolation valves of the emergency core cooling system. The emergency core cooling system train unaffected by the breach would then ensure the injection of water into the primary circuit and the cooling of the core. If, on the other hand, this rupture or leak occurs simultaneously on several pipes, the cooling of the reactor core could potentially no longer be ensured. Events such as an earthquake (generating mechanical stresses in the involved pipes) or start-up of the emergency core cooling system (causing cold water to enter hot pipes) can simultaneously stress these circuits.

EDF decided to preventively shutdown Civaux’s No. 2 reactor on November 20, in order to carry out early examinations on the welds, the ten-yearly outage of the reactor being scheduled in a few months. The first results of examinations on this reactor show the presence of defects at the same welds as on reactor No. 1. EDF therefore decided to unload the fuel assemblies from the core of reactor No. 2 to proceed to in-depth investigations and to the repairs that may prove necessary.

As a generic anomaly relating to the 1,450 MWe reactors cannot be ruled out at this stage, EDF has decided to shut down the two reactors at the Chooz B nuclear power plant in the Ardennes from December 16, which are of the same type as those of Civaux (1450 MWe reactors), in order to carry out checks.

IRSN considers that EDF's decision to shut down the two Chooz B reactors, in addition to the two Civaux reactors, is satisfactory from a safety point of view. Examinations on Chooz B's reactors will determine whether they are affected by the same defects. In-depth investigations must be carried out in order to determine the phenomena at the origin of stress corrosion crac​ks and to define the scope of the examinations to be carried out.

Specific examinations may also be necessary on other reactors in operation.

Download IRSN information report of December 16, 2021​: Detection of cracks in pipes of the emergency core cooling system of the reactors No. 1 and No. 2 of the Civaux nuclear power plant​​ (PDF)

Notes:

1. After the reactor pressure vessel outlet.
2. Before the reactor pressure vessel inlet .

Nuclear modular reactors of less than 300 MWe (or small modular reactors - SMRs) have sparked growing interest in the world for several years. In this information report, IRSN presents its position on the safety requirements for these type of reactors.

For their promoters, SMR constitute a means of producing energy that can meet various needs, such as cogeneration [1] or non-electrical applications (industrial heat, production of fresh water, of hydrogen etc.). They are a suitable solution for districts that are isolated or have limited infrastructure. Their designers also show off improved performance in terms of safety, thanks to intrinsic and passive safety systems. Some designs offer an architecture allowing the installation of several modules independent of each other to achieve a larger overall power (of the range of 600 - 800 MWe).

To answer the question of economic profitability, the SMR designers put forth a simplification of the design and a shorter duration of the construction phase by means of modular construction, standardization with the benefit of series production. Therefore, the SMR designers ask for harmonizing the safety requirements in force in countries wishing to acquire such reactors. Some of them believe that the safety requirements should be adapted due to the intrinsic safety features of these designs.

IRSN considers, on the contrary, that there is no need to revise downwards the safety requirements for SMRs. The simplification and the inherent safety features should benefit safety and the demonstration thereof through compliance with these requirements.

Download IRSN information memo of October 07, 2021: Small​​​​ Modular​ Reac​tors​ (PDF)​

Con​siderations on the performance and reliability of passive safety systems for nuclear reactors

Pressurized water reactors currently operating in France are equipped with active safety systems requiring a power source, such as an electrical power supply, and also include passive safety features (nuclear fission reaction control and shutdown rods, hydrogen recombiners, etc.).

Due to their lower power, Small Modular Reactors can use passive residual power evacuation systems to be evacuated in the event of an accident, which do not require energy input to operate. The aim is to bring the reactor to a safe shutdown state and to be able to keep it there without the need for human intervention for a long period of time.

​The evaluation of systems that could be implemented by designers raises certain difficulties related to the justification of their efficiency and reliability. In this document, IRSN presents the characteristics of passive safety systems and sets out the questions associated with the evaluation of new systems of this type as well as the lines of research to be developed to respond to them.

More information: Download IRSN report on passive safety systems for nuclear reactors (PDF)

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