Module 12 Light Water Cooled, Graphite Moderated Pressure Tube Reactors (RBMK)

Prof.Dr. H. Böck Vienna University of Technology /Austria Atominstitute Stadionallee 2, 1020 Vienna, Austria boeck@ati.ac.at Module 12 Light Water Cooled, Graphite Moderated Pressure Tube Reactors (RBMK) 1.5.2015

RBMK= Reaktor Bolshoi Moshnosty Kanalny= High Power Channel Reactor

RBMK Reactors Lithuania: Ignalina 1 & 2, 1 shut down 2004, 2- shut down 2009 Russia: Bilibino 1-4 built between 1974 and 1977 Kursk 1-4 built between 1977 and 1986 Leningrad 1 4 built between 1974 and 1981 Smolensk 1 3 built between 1983 and 1990 Ukraine: Chernobyl 1-4 (1-3 shut down, 4 destroyed)

Basic Design of RBMK Reactors Origin in Soviet weapons program for Pu production Graphite moderated, light water cooled, pressure tube reactor (LWGR or RBMK) Slightly enriched uranium as fuel with on-load refuelling Standardized twin blocks of 1000 MW e each Presently 11 in operation in Russia

Location of Chernobyl Distance to Kiew 100 km Minsk 320 km Moskwa 700 km Vienna 1030 km

Layout of Units 1 and 2 1. Auxiliary building 123/2. Reactor hall of unit 2 2. Common turbine hall 123/1. Reactor hall of unit 1 3. Intermediate building 392/1. Control room of unit 1 4. Main circulation pump 392/3. Control room of unit 2 5. Generator 392/2. Electrical equipment (SUZ reactor instrumentation 6. Main feed water pump 390/1. Electrical equipment (SKALA computer) 7. Auxiliary feed water pump 390/2. Electrical equipment (SKALA computer) 8. Main transformer 397. Central control room (external grid, fire detection) 9. Auxiliary transformer 10. Start-up transformer 11. NA-pump, service water system 12. Cables to diesel building, unit 2 13. Cables to diesel building, unit 1

Cutaway of the Nuclear Unit 1. Core 2. Piping of water lines 3. Lower biological shielding 4. Distribution headers 5. Side biological shielding 6. Drum-separator 7. Piping of steam-water lines 8. Upper biological shielding 9. Refuelling machine 10. Demountable plating 11. Fuel channel ducts 12. Downcorners 13. Pressure header 14. Suction header 15. Main circulation pump

Internal Structures 1. Graphite stack 2. Pressure tubes 3. Reactor cavity 4. Concrete vault 5. Feedwater channels 6. Lower biological shield 7. Upper biological shield 8,9. Lateral shield 10. Reflector cooling channels 11. Feedwater pipes 12.Top cover 13. Top plate 14. Sand fill

RBMK Operation Principle

Simplified Operation Diagram

Top Plate with 1690 Channels for Fuel Loading

Fuel Bundle 1. Hanger 2. Guide tailpiece 3. Main support rod 4. Upper fuel bundle 5. Lower fuel bundle 6. End cap

Fuel Rod 1. Top plug 2. Cladding (Zircaloy) 3. Spring and fission gas expansion volume 4. Fuel pellet column 5. Lower end plug 6. Bottom plug

Fuel Channel 1. Plug of the biological shielding 2. Upper biological shielding 3. Fuel hanger 4. Shielding plate 5. Lower biological shielding 6. Fuel channel 7. Fuel assembly 8. Bellows compensator 9. Coolant inlet pipeline 10.Coolant outlet pipeline

Reactions in the fuel and control rods

Absorber Rod with Water Displacer 1. Strip 2. Inner cavity of the rod 3. Absorber 4. Aluminium cladding 5. Displacer with graphite filling

RBMK Control Rod Design Control rod design: totally 211 CR Lower part of CR graphite to displace water when CR are fully out Therefore positive effect on reactivity in lower core volume before absorber part enters core centre Control rod speed:40 cm/s Minimum number of CR in core according to safety rules: 15 Actual number of CR in core: 6

Xenon Poisoning Xenon poisoning: Xenon-135 is a fission product with a high fission yield and a very high neutron absorbtion cross section. In steady state reactor operation there is an equilibrium between Xe production and Xe removal Xenon production: Directly through uranium fission and by decay of Iodine-135 (6,2h) Xenon removal: By radioactive decay (9,2 h) or by neutron capture and transmutation into Xe-136 If this equilibrium is disturbed (i.e.by power changes) the Xe-135 concentration increases or decreases in the reactor and disturbes the neutron flux

Xenon Production and Removal Xe-135 is produced by decay of I-135 Xe-135 removed by decay or by neutron absorption Half-life of I-135 is shorter than half life of Xe-135 Reactor power up: I-135 production up Xe concentration first down then up Reactor power down: I-135 production down Xe concentration first up then down

Xenon Concentration after Reactor Shut Down When a reactor is shut down I-135 decays according to it s half-life to Xe-135. As I-135 decays faster than Xe-135 the Xe-135 concentration increases as there is no removal by neutron absorption because the reactor is shut down!!! The so called Xenon peak is approx. after 10 to 12 hours of shut down. In power reactors it could be strong enough that reactor start-up during Xe-peak is impossible.

Positive Void Coefficient Positive void coefficient: This coefficient describes the feedback of steam bubbles (or empty volumes in the core) on reactivity. For a stable reactor the coefficient should be negative, i.e. if power increases more steam bubbles are produced then reactor power should decrease (=stabilizing effect). This is valid in BWR s where water is both coolant and moderator In RBMK reactors the main moderator is graphite and water is only the coolant. But water is also a neutron absorber, therefore if water evaporates to steam, absorption of neutrons is reduced (less H-nuclei per unit volume) and reactivity (= reactor power) increases. Both effects overlapped at Chernobyl at that night: The reactor core was highly Xenon poisoned due to the 10 hours operation at 1600 MWth, therefore all control rods were removed from the core to reach the desired power level

The Test April 25 th 1986: prior to a routine shut-down, the reactor crew at Chernobyl-4 began preparing for a test to determine how long turbines would spin and supply power to the primary pumps following a loss of main electrical power supply. As this test failed before due to automatic reactor shut down several automatic shut down mechanisms were disabled The turbine energy should bridge the electricity gap until the emergency diesel generators reach full power (40 to 60 seconds) The test should be carried out between 700 to 1000 MW th, it was well known that the reactor is extremely unstable at a very low power level

Accident Chronology April 25 th, 1 am: the power level was decreased slowly from 3200 MWth to 1600 MW th. One of the two turbine-generators was stopped 13h05: the network distributor in Kiev requested a stop of the power decrease due to electricity demand in the network until 23h10 (important factor for the accident ) 23h10: the power was decreased further but the operators were unable to stabilize the reactor due to Xenon poisoning around 1000 MW th, the power decreased down to 30 MW th The operators tried to bring the reactor back to the necessary power level by removing all but 6 control rods out of the core as far as possible.

Accident Chronology 26.4.86, 1:00 am: With more than 200 control rods (CR) (6 CR remain close to core) removed only 200 MWth was reached 1:03 am: Additional coolant pumps switched on, core subcooled by cold water, shut down signals (steam pressure and water level were bridged) 1:23:04 am: Start of experiment: Turbine inlet valve closed, turbine-generator and coolant pumps run down 1:23:31 am: Water starts to evaporate in core and power increases slowly, remaining 6 CR cannot compensate power increase 1:23:40 am: Shift supervisor orders insertion of all CR but insertion speed too slow 1:23:44 am: First a prompt nuclear transient, temperature above 3000 C, followed by a second explosion (steam or hydrogen explosion), 1000 ton reactor cover lifted away and all coolant channels destroyed

Operational Power Diagram

Steps to the desaster

Arial View of the ReactorBuilding

Situation after the Accident

Model of the destroyed Reactor

View of the Reactor Building

View of the Reactor Building

Sarcophagus

Shelter construction

New Shelter Costs and Time Schedule End of 2010 about 990 M from EBRD available for shelter construction Contributions from 23 countries, the EU and donations from 6 countries (Austria 7,5 M) Scheduled to be moved over the sarcophagus and confine the remains of the plant from the outside world for about 100 years. It is expected to be completed in 2015. World Nuclear News 13.7.2011 http://www.world-nuclear-news.org/wr-funds_in_place_for_chernobyl_shelter-1307114.html http://www.ebrd.com/downloads/research/factsheets/chernobyl25.pdf

Serious Accidents in Military, Research and Commercial Reactors (1) Reactor Windscale-1, UK (military plutoniumproducing pile) SL-1, USA (experimental, military, 3 MWt) Fermi-1 USA (experimental breeder, 66 MWe) Lucens, Switzerland (experimental, 7.5 MWe) 1952 Date 1957 Nil Immediate Deaths NRX, Canada (experimental, 40 MWt) 1961 Three operators Nil 1966 Nil Nil 1969 Nil Environmental effect Widespread contamination. Farms affected Very minor radioactive release Very minor radioactive release Follow-up action Repaired (new core) closed 1992 Entombed (filled with concrete) Decommissioned Repaired, restarted 1972 Decommissioned

Serious Accidents in Military, Research and Commercial Reactors (2) Reactor Browns Ferry, USA (commercial, 2 x 1080 MWe^) Three-Mile Island-2, USA (commercial, 880 MWe) Saint Laurent-A2, France (commercial, 450 MWe) Chernobyl-4, Ukraine (commercial, 950 MWe) Date Immediate Deaths Environmental effect 1975 Nil Nil Repaired 1979 Nil 1980 Nil 1986 50 staff and firefighters Minor short-term radiation dose (within ICRP limits) to public, delayed release of 2 x 1014 Bq of Kr-85 Minor radiation release (8 x 1010 Bq) Major radiation release across E.Europe and Scandinavia (11 x 1018 Bq) Follow-up action Clean-up program complete, in monitored storage stage of decommissioning Repaired, (Decomm. 1992) Entombed Fukushima ( 4 BWR ) 2011 None Contamination Cleaning

International Nuclear Event Scale INES

Energy Related Accidents since Chernobyl (1) Place Year Killed Comments Chernobyl, Ukraine 1986 50 nuclear reactor accident Piper Alpha, North Sea 1988 167 explosion of offshore oil platform Asha-ufa, Siberia 1989 600 LPG pipeline leak and fire Dobrnja, Yugoslavia 1990 178 coal mine Hongton, Shanxi, China 1991 147 coal mine Kozlu, Turkey 1992 272 coal mine methane explosion Cuenca, Equador 1993 200 coal mine Durunkha, Egypt 1994 580 fuel depot hit by lightning Seoul, S.Korea 1994 500 oil fire Minanao, Philippines 1994 90 coal mine Dhanbad, India 1995 70 coal mine Taegu, S.Korea 1995 100 oil & gas explosion Spitsbergen, Russia 1996 141 coal mine Henan, China 1996 84 coal mine methane explosion Datong, China 1996 114 coal mine methane explosion Henan, China 1997 89 coal mine methane explosion

Energy Related Accidents since Chernobyl (2) Place Year Killed Comments Spitsbergen, Russia 1996 141 coal mine Henan, China 1996 84 coal mine methane explosion Datong, China 1996 114 coal mine methane explosion Henan, China 1997 89 coal mine methane explosion Kuzbass, Siberia 1997 67 coal mine methane explosion Huainan, China 1997 89 coal mine methane explosion Huainan, China 1997 45 coal mine methane explosion Guizhou, China 1997 43 coal mine methane explosion Donbass, Ukraine 1998 63 coal mine methane explosion Liaoning, China 1998 71 coal mine methane explosion Warri, Nigeria 1998 500+ oil pipeline leak and fire Donbass, Ukraine 1999 50+ coal mine methane explosion Donbass, Ukraine 2000 80 coal mine methane explosion Shanxi, China 2000 40 coal mine methane explosion Guizhou, China 2000 150 coal mine methane explosion Shanxi, China 2001 38 coal mine methane explosion Sichuan, China 2002 23 coal mine methane explosion Jixi, China 2002 115 coal mine methane explosion

Comparison of Accident Statistics in Primary Energy Production Fuel Immediate fatalities 1970-92 Who? Normalised to deaths per TWy* electricity Coal 6400 workers 342 Natural gas 1200 workers & public 85 Hydro 4000 public 883 Nuclear 56 (+4 Fukushima) workers 8 *Electricity generation accounts for about 40% of total primary energy

References http://www.iaea.org/newscenter/features/chernobyl- 15/timeline.shtml http://www.chernobyl.info/default.aspx?tabid=120 http://www.chernobyl.info http://www.world-nuclear.org/info/inf31.html http://chernobyltwentyfive.org/ http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/cherno

What you should remember RBMK is a graphite moderated, light water cooled pressure tube reactor with it s origin in military applictaions (Pu-production) It is refuelled during reactor operation (on-load) The water acts here only as a coolant but not as moderator, in fact water is also an absorber of neutrons If water evaporates the neutron absorption is reduced and reactor power increase (= positive void coefficient) The accident is a combination of unsafe technical design and violation of procedures Two reactor physics reason lead to the accident: Xenon poisoning Positive void coefficient