While SuperChooz is a new project whose conception started in 2018, especially during the COVID era, some of its building blocks pre-date. The most notable goes back to the ’60s when the first PWR nuclear reactor was built by France and Belgium at Chooz.
Indeed, much of the SuperChooz project relies on a unique opportunity for the reuse of the EDF former Chooz-A nuclear reactor site, where 2 huge existing underground caverns may become available for fundamental science. Together, the caverns amount to about 50,000 m3 with up to 100m overburden. The original Chooz-A reactor was built in the ’60s, right under the Ardennes mountains close to the Chooz hamlet (France) in one of the meanders of the Meuse river. This was in fact a major milestone for the foundation of the nuclear industry in France and Belgium during the post-war era.
The Chooz-A reactor successfully ran until the ’80s and is under dismantling since. In fact, Chooz-A remains one of the most important deconstruction sites of a nuclear reactor in France; a major operation for the DP2D department of EDF. The decommissioning of Chooz-A in ’80s coincided with the construction and commission of the Chooz-B double-core N4 EPR AREVA nuclear reactor in the ’90s. While effectively the same site, the Chooz-B NPP was built on the bank of the Meuse river on the opposite shore of Chooz-A. Hence, Chooz-A is located at about 1km baseline from the Chooz-B reactor cores, thus defining one of the most important features of the Chooz site for reactor neutrino research. In fact, it is a small tunnel to the huge Chooz-A underground facilities that effectively sheltered the small experimental laboratory hall used by the CHOOZ and Double Chooz (far detector) experiments in the decades 1990 and 2010, respectively. That hall is no longer available for science, now exploited by EDF for Chooz-A dismantling. This site has constituted one of the first key experimental halls of the Laboratoire Neutrino de Champagne-Ardennes (LNCA) founded in 2008 for the Double Chooz experiment under the direction of Dr H. de Kerret until 2014.
Upon the culmination of the Double Chooz experiment, around 2017-2018, the CNRS team working at Chooz, led by Dr A. Cabrera as responsible for both the Double Chooz experiment (nationally and internationally) and the director of the LNCA laboratory (2014-2021), was charged to address one question difficult question: could more fundamental physics be done at the Chooz site after Double Chooz? If the outcome was negative, the Chooz site was deemed to close its doors for a long time – even forever. In fact, this question was not easy as the mood for most scientists after Double Chooz was that the site was rather uncompetitive due to its low overburden (high cosmic-based background) and small size of the LNCA experimental hall. These features limited Double Chooz significantly. Moreover, a hypothetical scenario of further underground civil construction for a possible expansion was considered unviable by the major difficulties and delays exhibited during the LNCA near-detector hall construction (2010-2013). So, the prospect of more science at Chooz, after Double Chooz, was indeed rather grim. Instead, against all odds and expectations, SuperChooz was conceived as the outcome.
One of the main reasons for this major change is that in parallel, the same CNRS team, has conceived the novel LiquidO technology, incidentally originally instigated by the major background challenges of the Chooz site, during the Double Chooz era, as highlighted in LiquidO’s history. Indeed, if LiquidO detection technology worked as designed and expected (still under demonstration), Chooz may enjoy a new era of fundamental physics opportunities. This scenario is additionally boosted by the opportunity to recover the EDF Chooz-A site for science, implying new experimental large halls to host larger detector(s) therein. Together, this makes SuperChooz a considerable reality, thanks to the coincidence in timing being perfect, including the lifetime of the Chooz-B nuclear reactors, as neutrino source, due to stop by 2040-2050.
Naturally, SuperChooz was first explored in light of its potential for reactor neutrino physics. This was done by the sane CNRS/IN2P3 teams, constituted by LAL (Orsay), LNCA (Chooz), and Subatech (Nantes), along with the University of California Irvine (USA). The first sensitivity studies, also defining the main experimental constraints of the SuperChooz articulation, included several novel ideas needed for the control of systematics well beyond today’s capabilities by almost one order of magnitude. This study was first reported at the EPS-HEP-2019 Conference (Belgium), including the participation of the EDF Chooz-A team, led by Mr J.-N. Cachia (EDF). Since then the EDF team for the SuperChooz exploration has been reinforced, Mr P. Vieu (leading the team now) along with Mr B. Gannaz and Mr F.-G. Hochard. Today’s SuperChooz definition owes much to all these EDF scientists and engineers.
The reactor neutrino exploration demonstrated that the needed LiquidO detector was rather large: order 10kton. This is a priori not impossible given the pertinent precedent of the NOvA detector experiment, exploring similar technology (scintillator, fibres, etc. but segmented) to yield a demonstrated 14kton size. Coincidentally, the SuperChoooz size is perfectly matching the largest reactor Chooz-A cavern geometry of about 30,000 m3. However, a large order-10kton LiquidO detector could open for far more physics potential in SuperChooz, and indeed this has been explored since, especially in the 2020-2022 period, where the potential for solar and supernovae neutrino detection has been considered as well as geoneutrinos. Even further physics potential appears possible and is slowly being considered, while progress has been awaiting the official approval of the SuperChooz Pathfinder to endow the effort with the necessary support behind it.
Indeed, on the 7th of September of 2022, the national directions of EDF and CNRS/IN2P3 signed the SuperChooz Pathfinder project collaboration agreement, thus officialising a major joint effort to address and quantify the feasibility exploration of the SuperChooz experiment potential and feasibility. This opens a new era of cooperation between both EDF-CNRS, where EDF is for the first time effectively involved in neutrino science. Reciprocally, the CNRS scientists are engaged in testing whether the LiquidO-based technology may have pertinent innovation applications that may benefit directly the EDF’s agenda. This is indeed the main goal of the AntiMatter-OTech project, approved and funded now. While the SuperChooz experiment articulation does no longer involve any of the existing experimental premises of the LNCA, now directed by Dr J.-F. Le Du (since 2022), this laboratory remains a key facility for its validation and experimental feasibility during the SuperChooz Pathfinder phase.
The SuperChooz Pathfinder agreement implies that EDF is committed to launching all the necessary feasibility and costing studies needed to assess how and when the Chooz-A cavern(s) could be delivered to CNRS to start the SuperChooz project if proved feasible. CNRS is instead committed to exploring and consolidating the possible physics programme, completing the feasibility studies of the LiquidO detection technology, and supporting EDF for the definition of the Chooz-A cavern(s) as possible new underground laboratories for neutrino fundamental research. If successful, Chooz may become one of the European largest world-leading facilities for neutrino fundamental research.