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General description

General description of the nuclear unit

The nuclear unit is composed of only one civil engineering structure supporting two zones with different containments: the reactor building (RB) and the nuclear auxiliary building (NAB). The objective of this single structure is to contain all the radioactive materials in one place.

The reactor is a pool type reactor. The maximum thermal power is 100 MW. This power is dissipated via the primary and the secondary circuit to the external cold source during irradiation ; the core, the primary circuit and experimental rigs, are completely enclosed in the RB. The Reactor pool is connected to several storage pool and hot cells located in the NAB through a water block


Aseismic bearing pads

Protection against severe earthquake has led to a specific design of the nuclear unit consisting on two basements separated by about 160 anti-seismic pads (concrete plots and elastomeric absorbers) able to maintain the complete integrity in such a case.
Complementary Safety Assessment of JHR

The reactor core

The core (600 mm fuel active height) is cooled and moderated with water. It will be operated, as a reference solution with a high density low enriched fuel (U enrichment lower than 20%), density 8 g.cm-3, requiring the development of UMo fuel.

The fuel element is of circular shape (set of curved plates assembled with stiffeners) and comprises a central hole. The reference UMo fuel, is under development within an international collaboration (UMo/Al dispersion solutions and monolithic UMo solution) and is not at the time being an industrial product.
Consequently, as a back-up solution, the JHR will start with an
U3Si2 fuel with a larger enrichment (typically 27%).

The core area is surrounded by a reflector which optimizes the core cycle length and provides intense thermal fluxes in this area. The reflector area is made of water and beryllium elements.

Irradiation devices can be placed either in the core area (in a fuel element central hole or in place of a fuel element) or in the reflector area.

In core, experiments will address typically material experiments with high fast flux capability up to 5.1014 n.cm-2.s-1 perturbed fast neutron flux with energy higher than 1 MeV.

In reflector, experiments will address typically fuel experiment with perturbed thermal flux (lower than 0.625 eV) up to 5.1014 n.cm-2.s-1.

Experiments can be implemented in static locations, but also on displacement systems as an effective way to investigate transient regimes occurring in incidental or accidental situations.

This provides a flexible experimental capability able to create up to 16 dpa/year for in-core material experiments (with 260 full power operation days per year) and 600W.cm-1(on 1% 235U enriched fuel) for in reflector simple fuel experiments.

Consequently, as a back-up solution, the JHR may start with U3Si2 fuel using a slightly higher enrichment depending on the requested power

Typical Loading

The JHR Experimental Capacity

JHR is a 100MW tank pool reactor. The core area is inserted in a small pressured tank (section in the order of 740 mm diameter) with forced coolant convection (low pressure primary circuit at 1.5 MPa, low temperature cooling, core inlet temperature in the order of 25°C). Reactor primary circuit is completely located inside the reactor building.

The reactor building is divided into two zones. The first zone contains the reactor hall and the reactor primary cooling system.

The second zone hosts the experimental areas in connection with in pile irradiation (eg., typically 10 loops support systems, gamma scanning, fission product analysis laboratory etc.).

The Fission Product Laboratory will be settled in this area to be connected to several fuel loops either for low activity gas measurements (HTR, …) or high activity gas measurements (LWR rod plenum, …) or water measurements (LWR coolant, …) with gaseous chromatography and mass spectrometry.

The hot cells   The NAB/RB Interconnexion  
The hot cells
The NAB/RB Interconnexion

Bunkers and laboratories in the experimental area will use 300m² per level on 3 levels.

Pools in the reactor building are limited to the reactor pool (including neutronography for experiments) and an intermediary deactivation pool (for temporary storage of fuel elements, reflector elements or replaced core mechanical structures).

During reactor shutdown, experimental devices can be temporarily stored in a dedicated rack in the reactor pool.

Hot cells, laboratories and storage pools are located in the nuclear auxiliaries building.

The experimental process will make use of two hot cells to manage experimental devices before and after the irradiation. Safety experiments are an important objective for JHR and require an “alpha cell” to manage devices with failed experimental fuel.

A fourth hot cell will be dedicated to the transit of radioisotope for medical application and to the dry evacuation of used fuel.

Three storage pools are dedicated respectively to spent fuel, experimental devices and mechanical components management.


1. What is a MTR?

Material Test Reactors (MTRs) are necessary for the development and qualification of materials and nuclear fuel used in the nuclear industry. The related studies contribute to the optimisation of existing or coming nuclear power reactors and to the developments of the future reactors.

Most of the irradiation tools utilised by industry are now fairly old in the western world. The sustainability of a high performance experimental capacity and the related  expertise for the coming decades are mandatory. There is a consensus on the necessity to study and build a new Material Test Reactor (MTR) to support  different power reactor existing and future technologies systems.
Coping with this context, the Jules Horowitz Reactor (JHR), built on the Cadarache site, will be a major infrastructure of European interest in the fission domain, open to the international collaboration.

The OSIRIS Reactor (Saclay—FRANCE)   The Jules Horowitz Reactor (Cadarache—FRANCE)   The BR2 Reactor (Mol—Belgium)

The OSIRIS Reactor

The Jules Horowitz Reactor (Cadarache—FRANCE)

The BR2 Reactor

The HBWR Reactor (Halden—NORWAY)


The LVR-15 Reactor (Rez—Czech Republic)


The HFR Reactor (Petten—The Nederlands)

The HBWR Reactor


The LVR-15 Reactor
(Rez—Czech Republic)


The HFR Reactor
(Petten—The Nederlands)

2. Where will be located the Jules Horowitz Reactor

The Jules Horowitz Reactor will be implemented in the CEA Cadarache Center located in the South of France.
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3. Who manages the JHR Project?

The JHR project has been promoted and discussed at the international level. For example, a JHR International Advisory Group has been settled within the OECD/NEA framework to assess the project and to promote it as an international users-facility.

As a result, a JHR Consortium has been set up to finance the JHR construction and to provide to funding Members a secured and guaranteed access to the JHR experimental capability. This Consortium gathers research institutes from several European Member States such as CEA (France), SCK (Belgium), NRI (Czech Republic), VTT (Finland), CIEMAT (Spain), and the European Commission. Major industrial companies such as EDF, AREVA, VATTENFALL are also Members of the JHR Consortium.

The CEA has also bilateral agreements upon the JHR with two associated partners: DAE from India and JAEA from Japan. Discussions are ongoing with other European and international partners to enlarge the JHR Consortium.

Members contributing to the financing of JHR construction will have guaranteed and secured access rights to experimental locations in the reactor in order to perform their Proprietary Experimental Programs. In parallel, a Joint Program will be opened to international collaboration in order to address issues of common interest.

The establishment of the JHR Consortium together with the networking of relevant research laboratories is a most important step in the building of the coming generation of R&D competences and infrastructure. This is required to cope with R&D needs to support present and future power reactors.

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4. Which measures are taken against seismic risk ?

The JHR lays on specific seismic pads designed in order to allow a disconnection of ground movement and building movement. This technique, well known for bridges construction is also already applied on other nuclear facilities.

Seismic pads are 90 by 90 centimetres “sandwiches” made of six layers of 2 centimetre-thick rubber (elastomer), embedding metal plates. Placed atop 2.2 metre-high concrete columns that rise from the lower raft of the structure – the “groundmat” –, they support the upper raft – the “basemat” –, which is the actual “floor” of the installation.

On the JHR, each pad bears a weigh of 550 tons. Pads are arranged in such a pattern that the total charge of the installation is uniformly distributed. This means 110,000 tons in JHR’s case.

Seismic pads are the key to what engineers call “aseismic base isolation”. Their flexible structure filters the shake, rattle and roll – or more appropriately, the accelerations… – that a strong earthquake would cause. The system is simple, robust and requires little maintenance. It can reduce a potential acceleration of 0.7G to a mere 0.13 G.

Antisismic pads

5. Do The Japanese events (Spring 2011) have consequences on JHR project ?

Following the Fukushima-Daïchi accident last March 2011, the French Nuclear Safety Authority has requested , for all French nuclear facility, a complete Safety Assessment (called stress-tests) based on WENRA (Western Europe Nuclear Regulator Association) recommendations (and this in agreement with many regulators in European Union).

WENRA European Stress Test

This complementary Safety Assessment of JHR has been done in a very tight schedule and the report has been given to the regulator by mid-September 2011. It demonstrates the robustness of JHR facility versus natural events with level higher than the ones used for dimensioning.

Nevertheless, supplementary measures will be taken by the Cadarache centre to improve this robustness and so having more Safety margins considering the dimensioning of the Facility.

Complementary Safety Assessment report (French version)

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Revision : 2016-04
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