Corrium — Wikipedia

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The leather is a metallic and mineral magma made up of melted elements of the heart of a nuclear reactor, then the minerals that he can absorb during his journey.

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The term “Corium” is a neologism formed of core (in English, for the heart of a nuclear reactor), followed by the suffix -ium Present in the name of many elements of the periodic table of elements: lithium, calcium, uranium, plutonium, helium, strontium, etc.

Initially made up of nuclear fuel (mainly enriched uranium oxide), elements of the fuel assembly and various heart equipment (control bars, instrumentation) or the wall of the reactor tank with which it between In contact, it forms at very high temperature (about 3 000 °C , fusion temperature of uranium oxide) when the heart is no longer cooled, as in nuclear accidents such as those of Three Mile Island, Chernobyl, or Fukushima.

The reactor n O 2, fade, from the Three Mile Island nuclear power plant after the accident of Three Mile Island:
1: Arrival pipe 2b;
2: 1A finish pipe;
3: cavity;
4: upper layer of partially melted heart fragments;
5: crust;
6: melted material;
7: Lower layer of uranium oxides and partially melted zirconium;
8: potentially depleted region of uranium;
9: instrument guidance sleeve inside the heart, damaged;
10: hole in the partitioning enclosure;
11: layer of material melted in the structure of the partially bypassed enclosure;
12: damaged pipes and upper grid

The Corium emits a large residual thermal power, that is to say that unlike the lava of a volcano which ends up cooling in contact with air, the Corium continues to emit heat for years, which gradually decreases due to the disintegration of fission products, after stopping the reactor [ first ] .

Highly toxic, radioactive, extremely dense and extremely warm, if it is not refrigerated and if it remains concentrated, it can melt most of the materials and unravel everything that is under it.

In recent years, in order to control potential accidents with the formation of a Corium picking up a reactor tank after merger of the heart, it has been planned to equip the new power plants or existing nuclear power plants with recovery and cooling devices of the Corium.

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Fall of Corium, with debris, and steam production and possibly hydrogen [ 2 ]

Corium formation is the consequence of a removal of the heart of a nuclear reactor resulting in overheating of all the elements constituting it.

Indeed, after stopping the chain reaction, there is no more heat due to the fission but that due to radioactivity (thermal effect of the natural disintegration of the different fission products).

This power, known as residual, depends primarily on the power history of the reactor preceding the accident, but also on the nature and enrichment of the fuel, the exhaustible of the fuel (advancement of the cycle) and the possible period of deactivation.

The residual power, if it is not evacuated by the cooling circuits, increases the temperature of the fuel. Beyond a certain threshold ( 1 200 °C approximately), an oxidation reaction occurs on the zircaloy sheaths of the combustible pencils. This reaction is very exothermic (increased temperature of 1 to 10 K/s [ 4 ] ) which causes the fusion of the fuel assembly. The next step is the rupture of assembly ducts and therefore the release of fission gas which contaminate the heart and the primary circuit (it is a rupture of the first protective barrier). If the temperature rise continues, the combustible elements melt and flow at the bottom of the tank.

The fuel fuel, mixed with partially oxidized zirconium and part of the lower interns of the tank accumulate at the bottom of the reactor tank [ 4 ] .

If it is an unpowering reactor (up to 600 You ), the tank can be cooled by flooding the tank well to avoid its drill.

For reactors from 1,300 to 1,600 You , the weak sprawl of the corium at the bottom of the tank makes cooling by flooding the tank well insufficient to prevent the piercing of the tank. In addition, it generates a risk of vapor explosion in the event of rupture of the tank. The flooding of the tank well is not a standard nuclear safety security device and falls under “ultimate” procedures for limiting the impacts of a nuclear disaster. The priority given to the restoration of a heart cooling, via water injection into the primary circuit (safety injection) much more efficient than a clock of the tank well.

If the corium at the bottom of the tank is not sufficiently cooled, it continues its temperature rise until reaching the melting point of the tank, piercing it and spreading in the reactor building (it is a rupture of the second protective barrier). The confinement enclosure then only plays the role of 3 It is Barrier, backup devices (sprinkling, U5 ultimate device) make it possible to avoid breaking.

According to CEA and IRSN [ 5 ] , when the Corium attacks concrete soil (and/or other materials in height if it has been expelled explosively), “A significant amount of inconsolable gases is released causing a progressive rise in the confinement enclosure. In order to avoid the rupture of the speaker that could result, a building-filtration device (U5 device) has been installed on pressure water reactors and can be implemented 24 hours after the start of the accident, in the event of failure of the pregnant spraying system » [ 5 ] .

A catastrophe scenario nicknamed “Chinese syndrome” envisages the case where the Corium pierces or explodes under pressure the reactor building and then sinks into the soil, breaking the third and ultimate protective barrier and spreading in the natural environment.

With the increase in the power of the reactors, recovery devices and spreading of the Corium are developed to ensure their cooling in the event of the tank piercing: Corium recuperators.

The « core-catcher Or Corium recuperator, as provided for in the EPR.
The Corium recuperator is a “spreading chamber” kept dry in normal operation; It is automatically filled with water by a passive system only after spreading the possible Corium in the cavity.

In the European pressurized reactor (EPR), the ATMEA1 reactor and the Russian reactor Vver-1200 (AES-2006), a particular device (the “Cendrier” or core-catcher [ 6 ] ) Composed of refractory ceramic elements [ 7 ] Was planned to contain and cool the Corium, if it could be pierced the reactor’s tank, in order to prevent it from sinking into the ground.

Regarding the American project AP1000, according to its designers, it is planned to maintain the Corium inside the tank and to cool it from the outside [ 8 ] .

They are mainly those of the following historical accidents:

Accident de Three Mile Island [ modifier | Modifier and code ]

An incident on the main water supply pumps of the secondary circuit of the Three Mile Island nuclear power plant the led, following numerous failures and errors, to the formation of a steam bubble at the top of the 900 reactor’s heart MW electric (2,722 MW thermal) on sales service three months earlier. This leading coolant bubble the top of the combustible elements for several hours, around 45% of the heart melted and formed a corium which flowed at the bottom of the tank [ 9 ] . According to the IRSN, the fuel had started to melt less than 3 hours after the start of the accident [ ten ] . The tank was not pierced and the reactor building played its role as a final confinement barrier. The only external contamination to be deplored occurred following liquid effluent handling errors.

Chernobyl disaster [ modifier | Modifier and code ]

Corium production took place during the Chernobyl disaster in Ukraine in an RBMK reactor of 1000 You (3 200 Booth ). The concrete slab supporting the reactor threatened to be pierced by the Corium formed as a result of the accident. Professor Vassili Nesterenko had diagnosed that if the molten heart reached a water table accumulated by the intervention of firefighters, an explosion of vapor was likely to occur and disseminate radioactive elements at a very long distance. A new team of employees from the power station is formed to evacuate this water by opening the drain valves of the removal pool located under the floor of the reactor cavity.

Approximately 400 minors from the surroundings of Moscow and the Donbass coal basin dug a gallery of 150 m length up to the reactor to cool the heart. This gallery, which was originally to house a liquid nitrogen cooling system, was ultimately filled with concrete to isolate the corium from the external environment.

During inspections made within ten years that followed, 1,370 tonnes (± 300 tonnes) of Corium [ 11 ] were found in the various premises of the reactor building pierced on three levels. Borovoi & Sich [ 11 ] et pazukhin (1997) [ twelfth ] estimated that this Corium had thus progressed by piercing thick walls and concrete floors to the basement in a few days (four according to Borovoi and nine according to Pazukhin).

Sand spills on the heart in the course of the accident, the presence of serpentinitis and a large amount of concrete decomposition products mixed with corium (its mass contained only about 10% uranium), reducing its volume power. This decrease in power plus its dispersion interrupted its progression inside the reactor building before it sinks towards the tablecloth [ 13 ] .

Accident by Fukushima [ modifier | Modifier and code ]

In , during accidents which concerned four of the six reactors of the Fukushima Daiichi nuclear power plant, following the tsunami and the earthquake of March 11, 2011 (of magnitude 9) which devastated the northeast of the island of Honshū, The hearts of three of the six reactors of the power plant began to melt following the loss of their cooling.

The , the operator Tepco admitted that the reactor 1 fuel bars melted only five and a half after tsunami [ 14 ] . But according to the IAEA inspectors, the calculations indicate that the heart of the reactor n O 1 would have melted three hours after the earthquake, then pierced the tank two hours later. The heart n O 2 would have started to melt 77 hours after the earthquake by piercing the tank three hours later. Finally the heart n O 3 would have melted 40 hours after the earthquake and pierced his tank 79 hours after [ 15 ] , [ 16 ] .

CONTENTS in radioelements and radioactivity of a Corium [ modifier | Modifier and code ]

These two factors vary according to the type of reactor, the content in initial fuel (enriched uranium, Mox, etc.) and the age of the fuel at the time of the accident; In the example below, Corium only contained 10% (in mass) of uranium.

Many works have focused on the behavior of concrete at high temperature [ 17 ] , other reactor materials [ 18 ] , and especially on the thermophysical properties of Coriums [ 19 ] , [ 20 ] , [ 21 ] and individually [ 22 ] materials that make them up (including zirconium [ 23 ] and uranium dioxide [ 24 ] and various alloys containing uranium (eg U-Fe and U-GA) [ 25 ] ).

These studies have focused on many factors: viscosity [ 26 ] and rheology of fusion metals (and in the process of solidification [ 27 ] , density, emissivity, thermal conductivities, initial temperature, radioactivity, erosive capacity, vaporization, thermal, physico-chemical and rheological layers, calorie transfers from liquids to solid [ 28 ] , etc. ).

To produce or stall sufficiently credible models, we studied the rheological behavior of basalts (different basalt compositions and basalt mixture containing up to 18 %m of UO 2 ), as well as mixtures of different compositions (mainly UO 2 , Zro 2 , Fexoy and fe for the scenarios in tank, more sio 2 and CAD for off-covenous scenarios) [ 29 ] . Various authors have shown that the viscosity of Coriums cannot be described by conventional models, for example, of non-interactive spheres suspensions [ 29 ] ; A Arrhenius type law [ 30 ] has been proposed, with a multiplicative factor such as n = exp (2.5cφ) [ 29 ] , C being between 4 and 8. It is more important in the case of low shear and cooling speeds.
Sotern samples have been the subject of structural analyzes which have shown that this factor depends on the morphology of the particle. Finally, this type of rheological law with a factor C of 6.1, made it possible to satisfactorily recalculate an test of sprawl in corium to 2 100 K SOUTH A HORIZONTAL PLAN [ 29 ] .

It is a question of understanding and modeling [ thirty first ] To anticipate or better control the behavior of the Corium during its training, its flow, its spread [ 32 ] and its cooling. We must also understand the complex chemical kinetics of the Corium during its evolution.

This need stems in particular from the Demonstration that a serious nuclear accident with rupture of primary containment was more likely that it had not initially calculated. [Ref. necessary]

These studies are generally done under the aegis of the IAEA and in Europe, with the support of the European Commission, for example in:

  • le projet CSC (Corium Spreading and Coolability) ;
  • le projet ECOSTAR (European Core Stabilization Research) ;
  • le projet ENTHALPY (European Nuclear Thermodynamic database for Severe Accidents) ;
  • the R&D analysis group on Corium recovery);
  • The Common ISPRA Center for Research and Faro Installation [ 33 ] ;
  • The test laboratory for controlling serious accidents (LEMA).

Calculation codes and specific software have been developed ( ex. : CEA Crust Software to model the mechanical behavior of the crust which forms on the surface of a Corium, and which interferes with its displacement, the cooling of the flow (see insulating crust braking the relaxation of the latent heat) of the melted corium and its influence) (Gatt et al., 1995)

“Korium prototypique» [ modifier | Modifier and code ]

To avoid exposing to the risks and dangers of a real Corium, nuclear physicists use in the context of their research a false Corium (known as “prototypical Corium”), substitute whose characteristics are supposed to be quite close to the real.

It is with this “prototypical corium”, brought to very high temperature that are carried out by the tests judged by their promoters as being the most credible to test various scenarios of major accidents (all involving the melting of the heart of a reactor), especially in France by the CEA Center of Cadarache, in collaboration with EDF, IRSN, Areva, Cerdan [What ?] , the Promes-CNRS laboratory, many researchers, in connection with the “high temperatures” group of the French thermal company.

This “prototypical corium” has a density and rheological properties close to those of the real Corium, and of the largely comparable physical properties. It differs, however, thermodynamically (it is not a source of self-diatalytic heat, that is to say self-entering by radioactivity) and has a different isotopic composition since it is composed of depleted uranium or natural uranium in replacement of enriched uranium. Some fission products, when they are present, then present a natural isotopic composition, etc.) which make it much less dangerous than a real Corium [ 34 ] .

  1. ↑ Five years after the stop, the residual power of the heart of a 1,000 reactor You – 3 200 Booth Having operated a year without interruption is close to 100 kW by higher value; After 15 years she is around 6 kW (a large electric radiator)
  2. ↑ NRC training document concerning nuclear security, page 100 Section 4.3.6, Fig 4.3.1 , NRC.GOV, accessed March 12, 2021
  3. ↑ NRC training document concerning nuclear security, page 112 Section 4.4.6, Fig 4.4.1 , NRC.GOV, accessed March 12, 2021
  4. ↑ a et b Sehgal, 1999
  5. ↑ a et b Tenaud et al. (2006), R&D relating to serious accidents in pressurized water reactors: assessment and perspectives , Joint report IRSN-CEA (with EDF contribution for chapter 8) referenced IRSN-2006/73 Rev 1, CEA-2006/474 REV 1 and succinct presentation
  6. ↑ Core Catcher
  7. ↑ Core-Catcher components for EPR
  8. ↑ Corium baths at the bottom of the tank of a pressure water reactor (REP) during a serious accident – IRSN report of 2005 .
  9. ↑ (Broughton et et al., 1989 ; akers et et mccardell, 1989 ; Libmann, 2000 ; Osif et al., 2004)
  10. ↑ http://www.irsn.fr/fr/laCherche/publications-documentation/colection-ouvrages-irsn/documents/15_lag_chap07.pdf
  11. ↑ a et b (in) A.A. Borovoi, A.R. SICH, « The Chornobyl accident revisited, Part II: State of the Nuclear Fuel located within the Chornobyl Sarcophagus », Nucl. Safety , 36, 1995 p. 1- 32.
  12. ↑ (in) E.M. Pazukhin, « Fuel containing lavas of the Chernobyl NPP fourth block: Topography, Physicochemical properties, and formation scenario », Radiochem . 36, 1994, p.109-154.
  13. ↑ Christophe Journeau, Corium spreading: hydrodynamics, rheology and solidification of a high temperature oxide bath [PDF] , Doctoral thesis in mechanics and energy, supported on June 21, 2006.
  14. ↑ Merger of three reactors in Fukushima: the French media look elsewhere , Images stop, 2011. Accessed on 05/20/2011
  15. ↑ (in) ‘ Occurrence and Development of the Accident at the Fukushima Nuclear Power Stations » [PDF] , on japan.kantei.go.jp.
  16. ↑ IAEA international fact finding expert mission of the Fukushima Dai-Ichi NPP accident following the great east Japan earthquake and tsunami [PDF] , AIEA, June 16, 2011.
  17. ↑ Harmathy, T.Z. (1970), Thermal properties of concrete at elevated temperatures , J. Mater. 5, 47-74.
  18. ↑ Hohorst, J. K. (1990), SCDAP/RELAP5/MOD3 code Manual Volume 4: MATPRO – A Library of Materials Properties for Light Water Reactor Accident Analysis , Rapport EG&G Idaho NUREG/CR 5273
  19. ↑ Journereau, c. and bagaccio, e. cogrever, j., yégou, which, Jegou, which, Jegou P., Monerris, J. (2003), Ex-vessel corium spreading : results from the VULCANO spreading tests , Nucl. One. Due. 223, 75-102.
  20. ↑ Journeau, C., pyluso, p., frolov, k. N. (2004), Corium physical properties for Severe Accident R&D , Proceedings of Int. Conf. Advanced Nucl. Power Plant ICAPP ’04, Pittsburgh, Pennsylvanie
  21. ↑ Cognet, G., 2003, Corium Spreading and Coolability (CSC) Final Summary Report, EU cosponsored research on reactor safety/severe accidents: Final summary reports – ‘EXV’ cluster projects, Office Official Publication European Communities, Luxembourg, EUR 19962 EN.
  22. ↑ Cleveland, J., 1997, Thermophysical properties of materials for water-cooled reactors , AIEA TECDOC-949 technical report, Vienna, Austria
  23. ↑ Paradis, J.F., Rhim, W.K (1999), Thermophysical properties of zirconium at high temperature , J. Mater. Res., 14, 3713-3719
  24. ↑ Fink, J. K., Pietri, M. C., 1997, Thermophysical properties of uranium dioxide , Argonne National Lab. Report ANL/RE-97/2.
  25. ↑ Gaudie, P. (1992), Contribution to the thermodynamic study of U-Fe and U-GA alloys by high temperature mass spectrometry, and the wetting of the Yttrium oxide by uranium , Doctoral thesis, National Polytechnic Institute, Grenoble.
  26. ↑ (in) M. Bornaccotti says, C. Journeau, F. – Sauveau, G. cognet, « Viscosity models for corium melts », Nucl. One. Due. , n°204, 2001, p. 377-389.
  27. ↑ (in) M. Ramacciotti, C. Journeau, G. Abbas, F. Werozub, G. Cognet, Rheological properties of mixtures under solidification , Cahiers Rheol., XVI, 1998, p. 303-308.
  28. ↑ J.P.Bardon, Heat transfer at solid-liquid interface, basic phenomenon , recent works, Proc. 4th Eurotherm Conf. , 1988, vol.1, Nancy, September 1988.
  29. ↑ A B C and D Muriel Ramacciotti, study of the rheological behavior of mixtures from the Corium/Concrete interaction, thesis led by Robert Blanc, University of Aix-Marseille 1, 1999, 214 pages ( Inist-CNRS sheet , Cote INIST : T 130139).
  30. ↑ Arrhenius law describes the variation in the speed of a chemical reaction with temperature.
  31. ↑ (in) Gatt, J.-M., Buffe, L., Marchand, O., 1995, Numerical modelling of the corium-substratum system , 13th Int Conf Structural Mech. Reactor Technol. (SMIRT 13), Porto Alegre, Brazil
  32. ↑ (in) T.N. Dinh, M.J. Konovalikhin, B.R. Sehgal, Core Melt Spreading on a reactor Containment Floor , Progr. Nucl. Energ., 36, 4, 2000, p. 405-468.
  33. ↑ (in) Trum, Wit, Wit, J. J., DGG, D.. 2000, Dry and wet spreading experiments with prototypic materials at the FARO facility and theoretical analysis , What. Ber. FZKA, 6475, p. 178-188.
  34. ↑ Christophe Journeau, Contribution of tests in prototypical materials on the Plinius platform to the study of serious accidents of nuclear reactors [PDF] , Habilitation memory to direct research in energy mechanics (University of Orleans), atomic energy police station, Cadarache, Lemag, June 2008, CEA-R-6189, (ISSN  0429-3460 ) , 227 pages, p. 20 notably.

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