MYRRHA, nouveau réacteur de recherche polyvalent

What is SCK•CEN? How long have you been working in this research center?

Hamid Ait Abderrahim – SCK•CEN (Studiecentrum voor Kernenergie – Centre d’Étude de l’énergie Nucléaire) is the Belgium nuclear research center. It is located in Mol, in Antwerpen Province. SCK•CEN is equivalent to the French CEA. It has roughly 700 employees, half of which have an academic degree, PhDs or engineers. There are on yearly average 70 PhD students doing their research work and 100 young people completing training for their master’s and bachelor’s degrees. The first time I worked for SCK•CEN was in 1983, back when I was still a student in nuclear engineering, completing my engineering thesis there. Then, I studied at INSTN and Orsay Paris XI University in France to obtain a postgraduate diploma in reactor physics. 

Afterwards, I came back to Mol to work on my PhD thesis and definitely stayed at SCK•CEN. In my career, I have worked in different research areas: core physics, reactor dosimetry field for surveillance program of power plants, fuel research, thermomechanics, fuel performance, experimental reactor studies and modeling. Finally, in 1998, I have been appointed head of the MYRRHA project. Since then, I am leading this fantastic project with a team of about 100 people. Since 2010 I am also the deputy director general for international affairs of SCK•CEN.

What led you to work on this field? Why did you choose nuclear engineering in the first place? 

HAA – Well, that is kind of a romantic story! I fell in love with nuclear engineering since the very first course on radioactivity in secondary school. I found that so fascinating that I decided that I would pursue nuclear studies. It is merely falling in love with the subject.  

The MYRRHA project is about subcritical reactors. What is a “subcritical reactor” and what are the different projects going on?

HAA – In the majority of nuclear reactors throughout the world, nuclear reactions are self-sustaining within a chain reaction, with perfectly balanced production and consumption of neutrons: this neutron balance is called “criticality”. In subcritical reactors, production of neutrons is lower than consumption; nuclear reactions are sustained thanks to an external source of neutrons, beamed into the core. The neutron source is generated thanks to an accelerator producing high energy protons that are impinging on a heavy metal target which produces neutrons through what is called a “spallation” process. That is why they are called ADS, for Accelerator-driven systems.  

The interest for ADS is originally due to the idea of H. Takahashi, from Brookhaven National Laboratory (USA), who imagined a concept of accelerator-based transmutation of nuclear waste, which reduces their toxicity. In 1993, Carlo Rubbia, from CERN in Switzerland, suggested to use ADS as an energy amplifier, i.e. to produce more energy with fission than the energy needed to power the accelerator. 

Beyond Europe and the US, many countries are also pushing the ADS program as part of their program closing the fuel cycle: India uses ADS as a way to support the Thorium fuel cycle. The Japanese ADS program is designed to transmute transuranics and generate nuclear energy, in the frame of the OMEGA project. Finally, since 2011, the Chinese Academy of Science (CAS) is also developing an ADS program. 

So these are the two main purposes of having ADS: reducing nuclear waste by transmutation or produce energy. However, at SCK•CEN, neither of them was our initial purpose.  

We started working on ADS in 1994. It was my honor to be one of the first persons at SCK•CEN to work on it. The initial idea was to synthesize Molybdenum-99, the most commonly used radioisotope in nuclear medicine. At that time we wanted to test if we could produce it by bombarding a solid target with a proton beam and the target being surrounded with highly enriched uranium plates.

We started this project because of a strike at a molybdenum plant in Canada that used to produce over 80% of the world’s supply of 99Mo. As a consequence, we realized the problem of being dependent on one single machine in the world. Even though at that time 99Mo was already produced in the Belgian Reactor 2 (BR2) in Mol, as a research centre, we wanted to look forward and build something else than a thermal reactor; ADS are Fast Neutron Reactors.  

What makes ADS more attractive than thermal reactors at producing radioisotopes?

HAA – Contrary to BR2, MYRRHA will be able to perform other tasks, additionally to producing radioisotopes, hence the name of MYRRHA (Multi-purpose hybrid research reactor for high-tech applications). 

Contrary to BR2, MYRRHA will be able to perform other tasks, additionally to producing radioisotopes, hence the name of MYRRHA (Multi-purpose hybrid research reactor for high-tech applications). 

We have to prove that transmutation can be done in an efficient way in these subcritical systems and this by allowing a large quantity of minor actinides to be loaded in the machine, without jeopardizing the safety of the installation. 

Handling large amounts of minor actinides is practically impossible for critical reactors, because the fraction of delayed neutrons is too small and it would make the whole thing unstable. That explains the advantage of subcritical cores to transmute minor actinides in large quantities. 

How many countries and companies are involved in MYRRHA?

HAA – Since the beginning in 1998, MYRRHA project has gathered 55 partners, in Belgium and in Europe such in France, Germany, Italy and Spain and even further in Japan, South Korea and Kazakhstan.  

The Belgium government committed to finance 40% of the budget. In addition to that, we have received letters of intent from many countries that expressed their willingness to commit for the project. Certainly, the Fukushima accident had some consequences on MYRRHA: the safety constraints are much higher and more demanding than what it used to be before.  

The cost of the project is 1 Billion Euros as of 2009. However it is being reassessed following the Fukushima accident, we added safety improvements to our design, which implies more expenses.  

Concept technique du réacteur MYRRHA – Copyright SCK•CEN

 

What are MYRRHA’s specificities in terms of safety?

HAA – The licensing of MYRRHA was engaged in 2010. It is basically governed by the same safety requirements as those of Fast Reactors. However, there are other aspects that needed to be taken into account due to the safety rules of accelerators. 

One of the main challenges is the reliability issue: in case of a rupture of the neutron beam from the accelerator lasting longer than 3 seconds, the reactor may not be restarted right away. This is due to the induced thermal shocks that adversely affect structural materials of the reactor and possibly causing safety issues. 

Another issue is the dependence of the power level of the reactor on the intensity of the neutron beam.

If the beam intensity is increased, then over-power of the reactor may be induced; so we have to prevent that sort of initiating events. The trouble is that the accelerator is a machine by itself: the neutron beam is obtained by a technique called “spallation”, i.e. a material is itself bombarded with some particles that make it in turn beam neutrons. 

So we have to design the accelerator so that its beam power cannot double or triple without any reason. These are the kind of safety concerns induced by the accelerator component.  

Fortunately, it also has advantages to design an ADS: since the reactor is subcritical, shutting down the reactor is merely ensured by stopping the neutron beam. Of course, as in all reactors, decay heat remains and needs to be removed. For MYRRHA, we have implemented a passive design, based on natural circulation.  

What are MYRRHA’s main technological challenges?

HAA – We identify 4 main issues :

• The coolant we chose for MYRRHA is a mix of lead and bismuth, which is not a wide-spread technology. It is a challenge by itself to use liquid metal as a coolant for a nuclear core. This technology was first used in some submarines in Russia; it has been followed by 20 years of developments in Europe, the USA, Japan and Russia again. So we have now 20 years of experience with various facilities: high temperature loops, technologies for filtering of leadbismuth coolant, quality control of oxygen content, etc. 
• Once we manage to use the mix of liquid metals, we have to take into account its corrosion effect on the structures of the reactor. The way we chose to deal with this issue is Oxygen control. 
• • We also have an issue of the accelerator reliability: today the largest accelerator like the one we have considered for MYRRHA typically trips 2 000 times a year, which implies that my ADS would never work. In order to improve the availability of the accelerator, two main strategic principles were adopted: we implemented redundancy in the design, i.e. some components (at injection, see Scheme above), duplicated (in the form of backup) will reduce the number of beam trips by a factor of 2. The second principle we implemented in the design is fault tolerance: if one or several components of the linear accelerator fail (for instance an accelerating cavity), the others can be adjusted to compensate for such a defect. Even though the reliability of the accelerator seems an outstanding challenge for MYRRHA, implementing innovative ideas will help us reaching the final requirement of less than 1 beam trip larger than 3s/10 days. 
• Fuel qualification: Mixed plutonium-uranium OXide fuel (MOX) was selected as the main candidate for MYRRHA. An enrichment of 30 to 35% of plutonium is envisaged, but there is today not any place in the world that produces this kind of fuel. So we are looking worldwide for the possibility to get it manufactured. We also want ADS to transmute minor actinides. This raises other issues as this type of waste has to be separated from the rest of the waste. Advanced separation methods were developed in Europe, for instance in the ATALANTE facility in Marcoule, France. This delivered good results at the lab scale. Now, we need to demonstrate these techniques in a semi-industrial scale (increase the separation capacity of the used fuel from 50 Kg to 1 ton per year). Once we have both MOX and separate minor actinides, we need to assemble them up. The demonstration phase in a pre-industrial scale needs to be achieved. Again, in Europe, namely in Karlsruhe (ITU – Germany), we have demonstrated its feasibility. But this remains a lab-scale demonstration. The challenge is to go to a larger scale (pilot engineering scale) which means up to 100 kg of heavy metal, which is a big step compared with the manipulation of a few grams only over the last years. 

   

Could other candidates (fast reactors, molten salt reactors, etc.) compete with the Accelerator Driven System technology? 

HAA – As explained, subcritical reactors have a real advantage compared to critical reactors, because they can be loaded with very large amounts of minor actinides. In other words, in order to burn with critical reactors the same quantity of MA as one ADS will do, one need to spread it into small fractions over several critical reactors. But this will multiply all the measures to ensure the protection of workers and the public with respect to the transports. 

Another alternative consists in using molten salt reactors. But for the moment, liquid fuel circulating can induce lots of reactivity issues. Of course, a core melt accident cannot happen in such a reactor, because the fuel is already under a liquid form, but I personally think that lots of safety issues have to be dealt with. It is not impossible to solve them but there are a lot of challenges there.  

Do you have an advice for the Nuclear Young Generation?

HAA – Yes !YOU SHOULD DARE TO CHALLENGE THE SENIORS. 

Changes need a new blood, therefore I am encouraging lots of young people to do research, to look for innovation. What I am doing in MYRRHA project, is to attract a lot of young people to allow them to innovate, to come with new ideas because this is the key for success.  

For the young generation, my advice is first of all: when you have new ideas, don’t hesitate to shake the tree, to shake the establishment, because new ideas can come from you and you should dare to fight for them. You will find a lot of people to tell you “this is idiot”, “this is not a good idea”, “look if this is such a good idea, why haven’t we done it yet?”  

Secondly, you should have dreams because it allows you to be innovative and that means you are not scared by your environment. 

Thirdly, I recommend to the young generation, especially in the nuclear field: YOU SHOULD DARE TO CHALLENGE THE SENIORS and don’t think they know everything or master everything, you can come with a new idea which is very valuable but you have to dare to fight for it.