THE FUKUSHIMA ACCIDENT: CAUSES, RESULTS AND IMPLICATIONS FOR EUROPE

Transcription

1 THE FUKUSHIMA ACCIDENT: CAUSES, RESULTS AND IMPLICATIONS FOR EUROPE Steven C. Sholly, Senior Scientist Institute of Safety and Risk Sciences Department of Water, Atmosphere & Environment University of Natural Resources & Life Sciences Course (Winter 2011) 1

2 Fukushima Accident Update 2

3 Fukushima Update 1 New calculation of accident progression in Unit 4 spent fuel pool by Dr. John Luxat (Research Chair in Nuclear Safety Analysis, Department of Engineering Physics, McMaster University, Canada Presented at the NURETH-14 conference in Toronto, 27 September

4 Fukushima Update 2 Recall from Block Lecture Number 2, Japanese government reports suggest that hydrogen was transported from Unit 3 through the stack release ductwork, and flowed backwards into Unit 4 But why didn't Unit 4 explode when Unit 3 detonated? And note the ductwork at Unit 3 was blown apart in the Unit 3 hydrogen detonation, so no further hydrogen could have moved from Unit 3 to Unit 4 after the Unit 3 detonation You will recall the NGOs and independent scientists suggested that the Unit 4 spent fuel pool was the source of the hydrogen in Unit 4 4

5 Fukushima Update 3 On 17 March 2011, US NRC Chairman Gregory Jaczko announced that the US had indications that the Unit 4 spent fuel pool was dry Spraying of fresh water onto the Unit 4 spent fuel pool did not take place until 20 & 21 March 2011 Concrete pumping truck started pumping sea water to the spent fuel pool on 22 March; pumping of fresh water began on 30 March 5

6 Fukushima Update 4 When cameras were finally able to film the Unit 4 spent fuel pool, the pool was found to have the fuel covered by water that was steaming There is no visible evidence of fuel damage (i.e., no warping of fuel channels, no warping of fuel assemblies, no charring of pool walls due to hydrogen combustion, no settling of fuel assemblies due to slumping from melting Pool water water samples provide no evidence of fuel damage, etc.). 6

7 Fukushima Update 5 Water volume = 1,425 m 3, 2.3 MWt of heat generation in Unit 4 spent fuel pool # of spent fuel assemblies in pool = 1,331 (large number due to full core offload to spent fuel pool to allow replacement of core shroud) Newly calculated timeline for Unit 4: 11 March, 1447 hrs Earthquake offshore 11 March, 1538 hrs Tsunami strikes plant resulting in station blackout, loss of spent fuel pool cooling, loss of capability to pump additional water to spent fuel pool to make up for losses due to boiling of pool 12 March, 2100 hrs Uncovery of fuel in Unit 4 spent fuel pool starts (calculated) 7

8 Fukushima Update 6 Newly calculated timeline for Unit 4 (continued from previous page): 14 March, 1101 hrs Hydrogen detonation in Unit 3 causes damage to reactor building wall of Unit 4 14 March, 2100 hrs Hydrogen generation begins from oxidation pf spent fuel in Unit 4 spent fuel pool (metal water reaction of spent fuel cladding with steam) (calculated) 15 March 0600 hrs Hydrogen deflagration/detonation at Unit 4 No significant release of radioactivity from fuel in spent fuel pool (calculated) 8

9 Fukushima Update 7 If Unit 3 was so closely connected to Unit 4 via the exhaust piping, when Unit 3 exploded so would Unit 4 likely have exploded simultaneously Instead, the Unit 4 hydrogen detonation/deflagration occurred 19 hours after the Unit 3 detonation It appears that even if some hydrogen migrated from Unit 3 to Unit 4, the quantity was not enough to cause a hydrogen deflagration at Unit 4 when Unit 3 exploded (and note that the Unit 3 explosion was violent enough to damage walls at Unit 4) 9

10 Fukushima Update 8 The calculations by Dr. Luxat indicate that hydrogen production from the Unit 4 spent fuel pool started 10 hours after the Unit 3 explosion The Unit 4 explosion occurred 9 hours after hydrogen generation is calculated to have begun in the Unit 4 spent fuel pool My tentative conclusion is that the Japanese Government report is incorrect about the source of the Unit 4 hydrogen it came primarily from metal/water reaction in the Unit 4 spent fuel pool, not from Unit 3 via the exhaust piping 10

11 Introduction 11

12 Lecture Scope This lecture covers similarities between the Fukushima reactors and reactors in Europe. The similar reactors include General Electric BWRs, Asea Atom BWRs, Siemens BWR/69 & BWR/72 reactors, and VVER-440/213 PWRs. 12

13 European "Cousins" There are no exact duplicates of the Fukushima reactors in Europe. However, there are what one might refer to as European "cousins" to the Fukushima reactors. These reactors fall into three groups: BWRs supplied by General Electric. BWRs supplied by Asea Atom and Siemens. PWRs of the type VVER-440/213 supplied by Atomstroyexport and which have pressure suppression confinements and reactor compartment end caps. 13

14 GE BWRs in Europe 14

15 GE BWRs in Europe There are four General Electric BWRs in Europe: Santa María de Garoña (BWR/3 Mark I) and Cofrentes (BWR/6 Mark III) in Spain. Mühleberg (BWR/4 Mark I) and Leibstadt (BWR/6 Mark III) in Switzerland. 15

16 Santa María de Garoña (Spain) (1) Santa María de Garoña is a 1,381 MWt/446 MWe (net) BWR/3 Mark I unit operating since 1970, and is similar to Fukushima Unit 1 The plant is located in the Tobalina Valley on the bank of the Ebro River The plant is operated by Nuclenor This plant is scheduled to be finally shut down on 6 July 2013 according to its operating permit from the Ministry of Industry, Tourism and Trade (MITYC) 16

17 Santa María de Garoña (Spain) (2) The most recent reported PSA results for Santa Maria de Garoña are a CDF of /a, and a large early release frequency (LERF) of /a (these are internal events results only) 17

18 Santa Maria de Garoña NPP, Spain 18

19 Cofrentes (Spain) (1) Cofrentes is a BWR//6 Mark III BWR with power levels of 3237 MWt/1092 MWe (gross) The plant is located in Valencia province on the bank of the Júcar River on the Embarcaderos reservoir The plant is operated by Iberdrola The Mark III containment, unlike the Mark I and Mark II containments, is not inerted Hydrogen igniters are provided in the wetwell only at Cofrentes 19

20 Cofrentes (Spain) (2) The most recent reported PSA results for Cofrentes are a CDF of /a, and a large early release frequency (LERF) of /a (these are internal events results only) 20

21 Cofrentes NPP, Spain 21

22 Cofrentes NPP Cross Section 22

23 Mühleberg (Switzerland) Mühleberg is a BWR/4 Mark I General Electric BWR producing 1097 MWt/373 MWe (net) The plant is located on the Aare River, 1.2 km downstream from a dam that impounds the Wohlensee Reservoir The plant is operated by BKW FMB Energie AG Mühleberg is expected to cease operation in

24 Mühleberg Design 1 Although Mühleberg is a BWR/4 Mark I plant like Fukushima Units 2-5, it has major design differences compared to these units making it unique among BWR/4 Mark I units in the world Mühleberg has a full, flood-proof, double containment; the secondary containment is a steel-lined, reinforced concrete structure with walls 0.9 meters thick at the base, and 0.6 meters thick above +29 meters elevation 24

25 Mühleberg Design 2 Mühleberg also has a redundant "outer" suppression pool in the secondary containment The pool contains a multi-venturi scrubber system (MVSS) that filters air vented from the drywell and from the wetwell airspace The vent lines can be opened manually, but open automatically via rupture disc at a pressure of 0.7 MPa 25

26 Mühleberg Design 3 Mühleberg also has a bunkered, floodproofed, reinforced concrete safety complex known as SUSAN SUSAN houses 2 of the 3 diesel generators; a separate control room; a redundant RCIC system; a redundant 2- train low pressure injection system; and a redundant 2-train suppression pool cooling system 26

27 Mühleberg Design 4 The condensate storage tank at Mühleberg is twice as large as the much higher power level Peach Bottom BWR/4 Mark I The overall primary containment at Mühleberg is approximately double in volume compared with Peach Bottom on a per MWt basis, as is the suppression pool water volume 27

28 Mühleberg External View 28

29 Mühleberg NPP Cross Section 29

30 Mühleberg Design 5 The result of these design changes is that a full scope probabilistic safety assessment (PSA) of Mühleberg estimates the core damage frequency (CDF) at /a. 64.4% of all core damage accidents are estimated in the PSA to end with the reactor vessel intact, the containment intact, and only normal leakage 30

31 Mühleberg Design 6 Another 32% of the CDF is estimated to end with the RPV failed, but the containment intact due to manually-initiated filtered venting In another 0.5% of the CDF, the RPV is failed, but the containment is intact the the filtered venting system is actuated by the rupture disc bursting The remaining 0.5% of the CDF are moderate to large release accidents involving containment isolation failure, early containment failure, drywell melt-through, and containment bypass The large release frequency is this estimated at about /a 31

32 Leibstadt (Switzerland) (1) Leibstadt is a BWR/6 Mark II General Electric BWR producing 3600 MWt/1165 MWe (net) The plant is located on the Rhine River The plant is operated by AXPO AG Final shutdown is scheduled for

33 Leibstadt (Switzerland) (2) Leibstadt is unique among BWR/6 Mark III units in having a full double pressure suppression containment, as well as a bunkered safety complex and a filtered venting system A 2009 PSA estimated the internal events CDF at /a, and external events at /a (dominated by earthquakes) Most likely large release is from early failure of the containment airlock, with an estimated frequency of /a 33

34 Leibstadt External View 34

35 Other BWRs in Europe 35

36 Other BWRs in Europe ASEA-ATOM produced BWRs that are operating in Finland (Olkiluoto 1 & 2) and Sweden (Forsmark 1, 2 & 3; Oskarshamn 1, 2 & 3; Ringhals 1). See next viewgraph. Siemens produced two lines of BWRs (BWR/69 and BWR/72) in Germany. All of the BWR/69 units are now closed. Only the BWR/72 units at Gundremmingen B & C remain in operation. 36

37 Asea Atom BWRs 37

38 Asea-Atom BWRs (1) Asea-Atom produced BWRs that are operating in Finland (Olkiluoto 1 & 2) and Sweden (Forsmark 1, 2 & 3; Oskarshamn 1, 2 & 3; Ringhals 1). Asea-Atom merged with another company to form ABB. ABB purchased Combustion Engineering, and was later merged with BNFL. BNFL subsequently sold its nuclear operations to Westinghouse, and then Westinghouse was acquired by Toshiba. 38

39 Asea-Atom BWRs (2) The Asea-Atom BWRs were equipped with MVSS filtered venting systems after the Three Mile Island accident. Note that the end cap in the Asea-Atom BWR design is under water during normal operation. This feature, combined with the filtered venting system, could significantly reduce radiological releases to the environment ("source terms") PSA results for Olkiluto 1 & 2: CDF estimated at 1.3E-5/a; large early release frequency (LERF) estimated at 6E-6/a (corresponding to 0.1% or greater release of Cesium-137 or the equivalent) 39

40 ASEA-ATOM BWR Containment 40

41 Siemens BWR/69 & BWR/72 41

42 Siemens BWRs Siemens built four BWR/69 units that operated in Germany (Brunsbüttel, Isar Unit 1, Krümmel, and Philippsburg Unit 1)l the never operated Zwentendorf plant in Austria was also a BWR/69 design All BWR/69 units in Germany were permanently closed following the Fukushima accidents Only the BWR/72 units at Gundremmingen B&C remain in operation No recent publicly-available PSA results for Gundremmingen have been located so far 42

43 Gundremmingen External View 43

44 Zwentendorf External View 44

45 VVER-440/213 PWRs 45

46 VVER-440/213 (1) VVER-440/213 units are operating in the Czech Republic (Dukovany 1-4), the Russian Federation (Kola 3 & 4), Slovakia (Bohunice V2 3 & 4, Mochovce 1 & 2), and Ukraine (Rovno 1 & 2). Two more units are under construction at Mochovce. These reactors are six-loop PWRs with horizontal steam generators housed in a pressure suppression confinement. 46

47 VVER-440/213 (2) There are three main similarities between GE Mark I BWRs and VVER-440/213s (despite the fact that the latter are PWRs and not BWRs): First, VVER-440/213 plants rely on a pressure suppression system to reduce steam pressure in case of an accident (via a so-called "bubbler-condenser" tower). Second, the reactor shaft is closed by an end cap similar in concept to the BWR Mark I drywell closure cap. Third, the structure above the refueling deck is simple industrial construction, and is not designed either to retain pressure or radioactivity released in an accident. 47

48 VVER-440/213 (3) Most (if not all) VVER-440/213 units have been retrofitted with passive autocatalytic recombiners (PARs) to reduce the threat of hydrogen combustion Other retrofits include reactor cavity flooding provisions to attempt to retain core debris in the reactor vessel, additional water sources for confinement spray and vessel injection, and extra protection for the reactor cavity door 48

49 VVER-440/213 (4) VVER-440/213 units are vulnerable to severe accidents occurring when the reactor shaft end cap and reactor vessel heads are removed (for refueling). Such accidents result in rapid failure of the industrial structure above, and a large direct release path to the environment (as well as contamination of the adjacent unit) Other VVER-440/213 vulnerabilities are steam generator tube ruptures and collector failures, unmitigated core debris interaction with the reactor cavity door, and unmitigated high pressure melt ejection & direct confinement heating 49

50 VVER-440/213 (5) Recent probabilistic safety assessment (PSA) studies for VVER-440/213 units indicate core damage frequencies in the range of about /a to /a 50

51 VVER-440/213 Design 51

52 Dukovany Design Cutaway 52

53 Passive Autocatalytic Recombiner (PAR) 53

54 Next Lecture 54

55 Next Lecture Block Lecture 4 can take place on either 14 November or 28 November from 18:30 20:00. We will discuss the Japanese Government's two official reports on the Fukushima accidents, as well as the IAEA's "Fact-Finding" Mission report. Fukushima_ pdf cn200_final-fukushima-mission_report.pdf 55

56 Contact Information 56

57 Contact Information Direct line: ; Secretariat: Institute URL (Deutsch): Course URL (English): Course URL (Deutsch): Lecture Details (date, room, time): Viewgraphs: 57