Sources de Rayonnement Réacteurs Nucléaires de puissance Jour 4 – Presentation 3

Objectif Discuter le sujet des réacteurs nucléaires de puissance, y compris leurs types et éléments de base

Contenus Types du Réacteurs Nucléaires PWRs BWRs CANDU Réacteurs Nucléaires avancés Composantes d'une centrale nucléaire

Le début Enrico Fermi led the team which produced the first sustained controlled nuclear chain reaction.

Fossil vs nucléaire

Réacteurs Nucléaires Types des réacteurs nucléaires: Réacteurs à Eau Légère (LWR) Réacteurs à Eau (HWR) Réacteurs refroidis au gaz à haute température Neutrons rapides surgénérateurs

Nucléides Primordiaux Demi-vie Activité Naturelle 235U 7.04 x 108 ans 0.711% de tout l’U naturel 238U 4.47 x 109 ans 99.275% de tout l’U naturel; 0.5 to 4.7 ppm du total U dans les roches communes 232Th 1.41 x 1010 ans 1.6 à 20 ppm dans les roches communes Three very important naturally occurring terrestrial radionuclides include U-238, U-235, and Th-232. They are actually parents of three long radioactive decay chains, which end in stable lead. Some nuclides, like Th-232 have several members in their decay chains. You can roughly follow the chain starting with Th-232 Th-232 --> Ra-228--> Ac-228 -->Th- 228 --> Ra-224 --> Rn-220--> Po-216 --> Pb-212--> Bi-212 --> Po-212 --> Pb-208 (stable) Some other primordial radionuclides are: V-50, Rb-87, Cd-113, In-115, Te-123, La-138, Ce-142, Nd-144, Sm-147, Gd-152, Hf-174, Lu-176, Re-187, Pt-190, Pt-192, and Bi-209.

1n + 235U  produits de fission Interaction des neutrons lents Fission 1n + 235U  produits de fission disponible pour d'autres fissions A very important neutron interaction mechanism is fission, which is the basis for nuclear power reactors. More neutrons are relased in this reaction than are absorbed (the mean number of neutrons released per fission for U-235 is 2.5). This leads to a self-sustaining chain reaction or “critical mass.” le nombre moyen de neutrons libérés par la fission de l‘U-235 est de 2,5). Cela conduit à une réaction en chaîne auto-entretenue ou "masse critique".

Réacteur Nucléaire à Eau Bouillante (BWR) In a BWR the water is boiled by the core, turned to steam and that steam is used to drive the turbines which generates the electricity. The spent steam is cooled back to liquid and recycled through the core.

à Eau Pressurisée (PWR) Réacteurs Nucléaires à Eau Pressurisée (PWR) In a PWR, water is heated in the core and converted to superheated steam. This is a closed system and is called the primary loop. This contaminated water/steam does not exit the containment. The heat from the steam in the primary loop is transferred to a separate water supply (the secondary loop) causing it to boil and turn to steam. This is done by using “steam generators” which have many small tubes inside. The steam from the primary loop travels through the tubes giving up heat to the water surrounding the tubes. The steam in this secondary loop is used to run the turbines to generate the electricity. In this way, the contaminated water supply is always maintained inside the containment unless of course the steam generator tubes leak causing cross contamination in the secondary loop. After passing through the turbines, the spent steam in the secondary loop is cooled back to water and run through the steam generators again.

Composantes d’une Centrale Nucléaire Les cinq prochaines diapositives présentent les principaux éléments d'une centrale nucléaire: Bâtiment de contrôle Bâtiment de confinement Bâtiment des turbines Bâtiments du combustible Bâtiment des générateurs diésel Bâtiment auxiliaire Building Control confinement du bâtiment bâtiment des turbines Bâtiment carburant Générateur diesel bâtiment bâtiment auxiliaire

Salle de contrôle From this location, the operator controls the reactor.

Bâtiment de confinement This is the the location of the core and primary components including the steam generators if it is a PWR.

Bâtiment des Turbines This is where the steam is converted to electricity. In a PWR it is “clean” whereas in a BWR, the steam is contaminated since it is produced from water which has been in contact with the core. Thus the turbine floor in a BWR has elevated radiation levels.

Bâtiment du Combustible This is where the spent fuel is stored onsite in a pool.

Bâtiment des auxiliaires Générateur Diésel et Bâtiment des auxiliaires This is the location of the generators which supply emergency power and the other components which support the water/steam system.

Barrières de Protection The fuel pellets are protected by the fuel rod which is in turn protected by the reactor vessel which is in turn protected by the reactor containment. This affords 3 levels of “containment” for the radioactive material.

Générateurs de vapeur For a PWR, heat from the steam in the primary loop is transferred to the secondary system via a heat exchange system. The primary steam (which may contain radioactive contamination) travels through “U” tubes similar to the ones pictured here (these are actually from a heat exchanger rather than a PWR steam generator). The tubes are immersed in clean water from the secondary loop which is then turned to steam. Since this device generates steam in the secondary loop it’s called a “Steam Generator”

Réacteurs Nucléaires Reactor building don’t always look the same. Many people in the US believe the cooling towers are the reactors. They are not. They provide cooling for the lake or river water which is used to condense the spent steam back to water in the closed system. They typically exhaust condensation from the cooling process. This condensation cloud is sometimes mistaken for leakage from the reactor.

Réacteurs avancés Le premier réacteur avancé est en fonctionnement maintenant au Japon Neuf nouveaux modèles de réacteurs nucléaires approuvés ou à un stade avancé de planification Amélioration de la sûreté et plus simples à utiliser, à inspecter, à entretenir et à réparer Advanced Reactors Today's nuclear reactor technology is distinctly better than that represented by most of the world's operating plants, and the first advanced reactors are now in service in Japan. Reactor suppliers in North America, Japan and Europe have nine new nuclear reactor designs either approved or at advanced stages of planning, and others at a research and development stage. These incorporate safety improvements including features which will allow operators more time to remedy safety problems and which will provide greater assurance regarding containment of radioactivity in all circumstances. New plants will also be simpler to operate, inspect, maintain and repair, thus increasing their overall reliability and economy.

Réacteurs avancés La nouvelle génération des réacteurs a: une conception standardisée afin d'accélérer l'octroi de licences, de réduire le coût du capital et de réduire le temps de construction une plus grande disponibilité et une plus longue durée de vie, seront économiquement compétitives dans une gamme de tailles, de réduire davantage la possibilité des accidents avec fusion du cœur burn‑up élevé et réduction du taux de consommation du combustible et réduction de la quantité de déchets. The new generation reactors: have a standardised design for each type to expedite licensing, reduce capital cost and reduce construction time, -are simpler and more rugged in design, easier to operate and less vulnerable to operational upsets, -have higher availability and longer operating life, -will be economically competitive in a range of sizes, -further reduce the possibility of core melt accidents, -have higher burn‑up to reduce fuel use and the amount of waste.

Réacteurs avancés Plus de fonctionnalités «passives» de sûreté qui reposent sur ​​la gravité, la convection naturelle pour éviter les accidents Deux grandes catégories: Évolution - essentiellement de nouveaux modèles de conceptions existantes et éprouvées Développement - départ plus important des installations d’aujourd’hui et exige plus de tests et de vérification avant le déploiement à grande échelle The greatest departure from most designs now operating is that many new generation nuclear plants will have more 'passive' safety features which rely on gravity, natural convection, etc, requiring no active controls or operational intervention to avoid accidents in the event of malfunction. The new designs fall into two broad categories: evolutionary and developmental. The evolutionary designs are those which are basically new models of existing, proven designs. The developmental designs depart more significantly from today¹s plants and require more testing and verification before large‑scale deployment.

Réacteurs CANDU CANDU signifie "Canada Deutérium Uranium“ s'agit de l’eau lourde sous pression, réacteur de puissance à uranium naturel conçu à la fin des années 1950 par un consortium du gouvernement canadien et l'industrie privée Tous les réacteurs de puissance au Canada sont du type CANDU Le concepteur du réacteur CANDU est AECL (Atomic Energy of Canada Limited), une société d'État fédérale

Réacteurs CANDU The CANDU reactor uses natural uranium fuel and heavy water (D2O) as both moderator and coolant (the moderator and coolant are separate systems). It is refuelled at full‑power, a capability provided by the subdivision of the core into hundreds of separate pressure tubes. Each pressure tube holds a single string of natural uranium fuel bundles (each bundle half a meter long and weighing about 20 kg) immersed in heavy‑water coolant, and can be thought of as one of many separate "mini‑pressure‑vessel reactors" ‑ highly subcritical of course. Surrounding each pressure tube a low‑pressure, low‑temperature moderator, also heavy water, fills the space between neighbouring pressure tubes. The cylindrical tank containing the pressure tubes and moderator, called the "calandria", sits on its side. Thus, the CANDU core is horizontal.

Réacteurs CANDU In the CANDU design, as with the PWR design, the heat of fission is transferred, via a primary water coolant, to a secondary water system. The two water systems "meet" in a bank of steam generators, where the heat from the first system causes the second system (at lower pressure) to boil. This steam is then dried (liquid droplets removed, since they can damage turbine blades) and passed to a series of high‑pressure and low‑pressure steam turbines. The turbines are connected in series to an electrical generator. The primary water system, which becomes radioactive over time, does not leave the reactor's containment building. It is a highly complex system from start to finish, involving a series of energy transformations with associated efficiencies. The potential energy of nuclear structure is converted first to heat via the fission process, then steam pressure, kinetic energy (of the turbine and generator), and ultimately to electrical energy Fueling is accomplished by a fuelling machine which visits each end of the core, one fuelling and the other de‑fuelling, allowing operators to insert fresh fuel at alternate ends for neighbouring fuel channels. From six to ten bundles are "shuffled" each day. Flux‑shaping is provided by fuel management. Long‑term reactivity control is also achieved through fuel management. Short‑term reactivity control is provided by controllable light‑water compartments, as well as absorber rods. Thermalhydraulically, the core of most CANDU reactors is divided into two halves, with the divider line running vertically down the centre of the reactor face. Each half represents a separate coolant circuit. Heavy water coolant is supplied to the pressure tubes in each circuit via large headers at each end of the calandria, one pair of headers (inlet/outlet) for each half of the core. The subdivision of the core into two circuits, plus the fine subdivision into hundreds of interconnected pressure tubes, greatly reduces the effect of a potential LOCA (Loss‑of‑Coolant Accident).

Réacteurs refroidis par gaz haute Température

réacteurs à haute température (refroidis au gaz) Building on the experience of several innovative reactors built in the 1960s and 70s, development is proceeding on new high‑temperature gas‑cooled reactors which will be capable of delivering high‑temperature (up to 950oC) helium either for industrial application or directly driving gas turbines for electricity. The small High‑Temperature Test Reactor (HTTR) in Japan started up at the end of 1998. Its fuel is ceramic‑coated particles incorporated into hexagonal graphite blocks or 'prisms', giving it a high level of inherent safety. China's HTR‑10 demonstration reactor started up in 2000 and has its fuel particles compacted with the graphite moderator into spherical balls, collectively known as a 'pebble bed'. South Africa's Pebble Bed Modular Reactor (PBMR) is being developed by a consortium led by the utility Eskom, drawing on German expertise, and aiming for a step change in safety and economics. Modules with a direct‑cycle gas turbine generator will be of 110 MWe and thermal efficiency about 42‑50%. Fuel consists of billiard ball sized pebbles of graphite moderator each containing 1500 particles of 8% enriched UO2 and coated with silicon carbide. Up to 450,000 fuel pebbles recycle through the reactor continuously until they are expended, giving an average enrichment in the fuel load of 5‑6% and burn‑up of 80,000 MWday/t U. Each unit will finally discharge about 19 tonnes of spent pebbles per year to ventilated on‑site storage bins. Construction cost (for clusters of 10 ‑ 14 units) is expected to be US$ 1000/kW and generating cost 1.6 US cents/kWh. Eskom and the South African Industries Development Corporation hold 55% the project, with BNFL 20% and Exelon (USA) 12.5%. A prototype is due to be built in 2002 for commercial operation in 2006. A larger US design, the Gas Turbine ‑ Modular Helium Reactor (GT‑MHR), will be built as modules of 285 MWe each. It has prismatic fuel elements like the HTTR and will directly drive a gas turbine at almost 50% thermal efficiency. It is being developed by General Atomics in partnership with Russia's Minatom, and initially will be used to burn pure ex‑weapons plutonium in Russia. In 1996‑97 Framatome (France) and Fuji (Japan) joined the development consortium. The detailed design stage is complete and some componenet testing has started. Plant costs are expected to be less than US$ 1000/kW.

Réacteur au lit de boulets En 1950s, Dr Rudolf Schulten («Père» du réacteur à lit de boulets) a eu une idée. L'idée était de compacter le silicium de carbure revêtu des granules d'uranium en boule dure comme celle de billard sphères de graphite à utiliser en tant que combustible pour une nouvelle haute température, le type de réacteur est refroidi à l'hélium. L'idée a pris racine, et en temps opportun , l'AVR, a 15 MW (mégawatts) réacteur de démonstration à lit de boulets, a été construit en Allemagne. Il fonctionne avec succès depuis 21 ans.

Réacteur au lit de boulets The pebble bed modular reactor (PBMR) is designed to skirt some of the biggest headaches of nuclear power: Pausing to refuel (which takes on average, about 40 days), complex piping and melting down. But the design does not address every objection to nuclear, and it raises some problems while solving others. Small pebble bed reactors ran in Germany in the 1970s, and China has recently started one. A larger version is now being designed by South Africa's state utility, with investments from British Nuclear Fuels, owner of the reactor maker Westinghouse, and Exelon Corp., the largest U.S. operator of power reactors. The design uses advances that appear to produce a small reactor that can be built cheaply and operated safely. Instead of the typical rod-shaped fuel, the fuel is formed into "pebbles" about the size of a tennis ball. Each pebble is made of grains of uranium sheathed in heat-resistant graphite and silicon carbide. The 100 million-watt reactor is supposed to use 310,000 fuel pebbles.

Réacteur au lit de boulets Problèmes potentiels (selon certains groupes) Il n'a pas de bâtiment de confinement Il utilise le graphite inflammable comme modérateur Il produit des déchets nucléaires plus haut niveau que les modèles de réacteurs nucléaires actuelles Although the arguments against current nuclear reactors have been addressed for some time, the PBMR is relatively new. The comments on this and the next slide are taken from a web site sponsored by an anti-nuclear group indicating that opposition is already forming to this new technology. 1. The lack of a containment building is a necessity because cooling is by natural convection. Also, a containment building would hinder the modular design - that is - no additional reactors could be added onto the plant after initial construction. This modular capability is what is so appealing to utilities because it requires less investment from the beginning. Frankly, this single point is enough to conclude that this reactor design is unsafe. The United States has criticized Soviet reactor designs for not having containment buildings. It is the last line of defense for containing a radiological release. Furthermore, the lack of a containment building leaves the reactor(s) wide open to a terrorist attack. 2. The uranium is covered by a layer of graphite. The graphite is covered by several other layers of materials including a silicon carbide. The graphite could burn if defects in the fuel defeat the outer coverings. The industry acknowledges that there is approximately 1 defect per pebble associated with these layers. There are approximately 370,000 pebbles in a pebble bed reactor. One tennis ball sized pebble comes out the bottom of the reactor every 30 seconds. It can be returned to the top of the reactor for additional use. The 1957 Windscale accident and the 1986 Chernobyl accident both involved burning graphite. The burning graphite dispersed radioactivity. At Chernobyl, the burning graphite released radiation for ten days. 3. Although the volume by "configuration for long term storage" is lower than current design, the actual amount of high level waste by weight is higher. The pebbles are less radioactive than conventional fuel assemblies and more pebbles are required to produce the needed heat inside the reactor. There will be many more truck and railroad transports needed to remove the wastes. This will increase the numbers of vehicle accidents and the odds of another radiological accident involving these vehicles traveling across the country.

Réacteur à lit de boulets Problèmes potentiels (selon certains groupes) It relies heavily on nearly perfect fuel pebbles Il s'appuie fortement sur les boulets du combustible presque parfaits It relies heavily upon fuel handling as the pebbles are cycled through the reactor Il s'appuie fortement sur la manipulation du combustible que les cailloux sont recyclés dans le réacteur 4. The industry acknowledges that "fuel pebble manufacturing defects are the most significant source of fission product release." Recent history shows that some companies have falsified fuel quality. In fact, there have been instances of fuel sabotage and tampering over the last few decades. Germany and Japan have shut down plants or refused fuel shipments once the problems were discovered. The industry can't produce "defect-free" fuel and therefore it is a certainty that a pebble bed reactor will experience an accident. The industry acknowledges that there is approximately 1 defect per pebble associated with these layers. This shows an actual photograph of defective fuel pebble cross-section - nuclear industry reports admit the US had been unable to manufacture satisfactory pebbles 5. & 6. There was a pebble bed reactor accident at Hamm-Uentrop West Germany nine days after the Chernobyl accident. On May 4 1986, a pebble became lodged in a feeder tube. Operators subsequently caused damage to the fuel during attempts to free the pebble. Radiation was released to the environs. The West German government closed down the research program because they found the reactor design unsafe. Il y a déjà eu un accident dans un réacteur à lit de boulets en Allemagne en raison de problèmes de manutention du combustible

Où trouver plus d’Information Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009) More information at: http://www.pbmr.co.za/index.htm