1 Nuclear Energy – “But the blow up!”My sequence of lecture topics has been a little strange: I started with basic science I then described almost all of the ways we traditionally produce electrical power I followed this by descriptions of up and coming power technologies Then, seemingly about to exhaust possibilities, described exotic long shots And only today am I looping back to our biggest carbon-free technology: Nuclear I followed this path because I suspect many of you are uneasy with nuclear So am I An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm

2 And I probably have more reason to be uneasy than you:Early in my marriage, when my wife and I were hoping for a first child A nuclear reactor called Three Mile Island blew up 125 miles directly upwind from our home And we had to decide whether to evacuate my possibly pregnant wife So yes, I am uneasy about nuclear, but following the path I've taken you along, I've reluctantly concluded that greener technologies may not be ready to have a big enough impact, in a short enough time This has led me and many others (including major environmentalists) To ask, not only if we might be able to live with nuclear, but if it can improved to the point that we feel comfortable living with it

3 Yes they (or at least three of them) have (sort of) blown up"But they blow up!" Yes they (or at least three of them) have (sort of) blown up So in this lecture we are going to learn how nuclear reactors blow up AND, for comparison, how nuclear bombs blow up Starting with a quick review of nuclear physics: Nuclear physics is all about nuclei, which consist of protons plus neutrons But protons and neutrons are schizoid, and capable of changing identities Neutron => proton + electron + DE or the reverse reaction And that DE can be HUMONGOUS – Which is what draws us to nuclear And it all comes directly from Einstein's E = mc2 Which says that mass can actually be converted to immense energy as transforming protons & neutrons slightly shift their masses

4 Keeping track of atom's protons, electrons and neutrons:Atoms start with equal numbers of protons and electrons, balancing charge Their count is encoded in the atom's name, and in it's atomic number Most carbon atoms have 6 protons (6 p) + 6 neutrons (6 n) Giving carbon an atomic number of 6 (≠ its atomic mass of ~ 12) The number of nucleons = number of protons + neutrons in atom's nucleus But the number of neutrons in an atom varies => isotopes of an atom In light atoms, numbers of protons and neutrons tend to be equal In heavier atoms, neutrons tend to outnumber protons Nucleon count is given by a leading superscript, as in 13C for carbon From this, number of neutrons = [number of nucleons – number of protons]: For 13C, neutron count = 13 – 6 = 7 For 12C (the more common isotope of carbon), neutron count = 12 – 6 = 6

5 Showing all of that schematically for 12C and 13C:12C: C: Protons = 6 = Electrons = Atomic Number Protons = 6 = Electrons = Atomic Number Neutrons = 6 Neutrons = 7 But natural atomic abundances are 98.93% 12C, and only 1.07% 13C So (averaged) atomic mass in nature works out to be Electrons Electrons

6 In nuclear reactors (and bombs) a few atoms play major rolesUranium (U), plutonium (Pu) and, perhaps in the future, thorium (Th) Uranium, with an atomic mass of , is currently the major player Its mass suggests its main isotope is 238U, which is indeed the case: 238U: % Half-life: 4.6 billion years 235U: 0.72% Half-life: million years Plus other much less abundant isotopes (<0.01%) Finite lifetimes => They ARE radioactive, eventually falling apart (releasing energy) Extremely long lifetimes mean that very few decay in a given amount of time So in reactors OR bombs something must vastly speed up the process of decay An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm

7 Decay is stimulated by capture of neutrons of particular energies:Dominant 238U isotope captures high kinetic energy / "fast" neutrons 238U + 1n (hot/fast) => 239U => 239Np + b => 239Pu + b where b ("beta") = a released high energy electron Significantly: This decay sequence does NOT produce more neutrons So while a neutron can CAUSE 238U to fission, that neutron is thereby consumed And because it is not replaced, you cannot get a 238U chain reaction Helping to explain 238U's surviving abundance However, 238U's reaction DOES produce plutonium Which works so well in bombs Attracting would be members of the "nuclear club"

8 Whereas for 235U: 235U captures low kinetic energy / "slow" / "thermal" neutrons 235U + 1n (slow/thermal) => 236U => 89Kr + 144Ba + 2 1n MeV With many other possible, but less likely, decay paths: 1 235U + 1n (slow/thermal) => 236U => 92Kr + 141Ba + 3 1n MeV 235U + 1n (slow/thermal) => 236U => 94Zr + 139Te + 3 1n MeV Weighted average => 235U fission produces ~ 2.4 neutrons These neutrons tend to have lots of kinetic energy = hot / fast And they thus don't strongly stimulate other 235U atoms to decay But as hot / fast neutrons, they CAN stimulate 238U to decay 1)

9 Summary schematics of 238U and 235U interaction with neutrons:238U: Slow neutron incident Fast neutron incident 235U: Slow neutron incident Fast neutron incident 238 239 b etc. 238 235 235

10 However, hot / fast neutrons can be slowed down:Simplest way is by bouncing them off light atoms Those atoms are accelerated, taking part of the neutron's kinetic energy Light atoms => NEUTRON MODERATORS (absorbing energy) Whereas a neutron can hardly budge a very heavy atom and thus Heavy atoms => NEUTRON MIRRORS (neutrons ricocheting off) And another important player: NEUTRON ABSORBERS / POISONS / SINKS Which, because they absorb but do not emit more, eliminate neutrons Xenon (Xe), Iodine (I), Boron (B) are examples of neutron poisons/sinks To sustain nuclear fission there must be very little of these around!

11 Summary schematic of things affecting neutrons:Before: After: Neutron Moderator (light atom that absorbs some of neutron’s kinetic energy): Result: Neutron Mirror (heavy atom that absorbs ~ none of neutron’s kinetic energy): Neutron Poison (special atom that absorbs neutron into its own nucleus):

12 Putting this all together, a reactor's chain reaction goes like this:3) hot/fast neutron moderating into slow/cool neutron 1) 235U = "spark" 235 238 239 b etc. H O 4) Another 235U propagating the chain reaction 2) 238U's = "fuel"

13 But things really break down even further:235 U fission fission path (displayed horizontally across the figure) Ba & Kr fission paths ( & ) 238 U fission path (to lower right) 23.5 minutes But all of these paths take time 2.35 days And they continue right off the page! BOTTOM LINES: All SORTS OF THINGS continue fissioning LONG after initial 235U / 238U fission stops => Sustained heat + radiation! => 2 of 3 accidents I'll soon describe Modification of figure found at:

14 On to how nuclear fission bombs and reactors are made:BOTH set up a sustained chain reaction of nuclear fission decay Huge, extremely fast heat liberation => bomb Moderate, slow heat liberation => nuclear reactor For earliest bombs, and most reactors to date, key fission reaction is: 235U + 1n (slow/thermal) => (byproducts) + ~ 3 1n (fast/hot) If output neutrons are slowed down => input stimulating other 235U's to decay 235U's decay then becomes a self-sustaining chain reaction But to become chain reaction, EACH liberated neutron must FIND other 235U If probability of finding another 235U <1, reaction is NOT self-sustaining If probability of finding another 235U = 1, reaction becomes self-sustaining If probability of finding 235U >1, reaction is self-sustaining and FAST

15 Leading to important (but poorly named) concept of Critical MassWhich defines mass above which a lump of radioactive material will explode WRONG! Its actually much more complex and much simpler Simpler in that it’s really about probability of a liberated neutron finding another 235U Say that a fissioning 235U emits exactly 3 neutrons: High mass / NO CHAIN REACTION: Low mass / CHAIN REACTION: BECUASE of the tighter packing!

16 So its more about the critical concentration?Partly: Indeed reactors use ~4% 235U while nuclear bombs use ~80% 235U But it's also about shape: Two objects with identical concentrations AND same total mass of 235U: NO chain reaction: YES, chain reaction: Similar to heat, shape with lower surface to volume ratio traps more neutrons So "critical mass" is ACTUALLY about concentration, mass, shape = It’s about PROBABILITY of neutron collision with another 235U

17 Nuclear bombs require rapid assembly of supercritical masses"Rapid" because even as you approach critical mass, chain reaction accelerates beginning to yield vast amounts of heat That heat then quickly, fractures, melts and vaporizes things Which are thus propelled rapidly apart! If/when fissioning material spreads too far apart, you loose criticality Reverting to one of the above too dilute / too spread out configurations Chain reaction then dies out, long before all available fissile material fissions And long before all available nuclear energy is released Which was given the apt name of a FIZZLE

18 Beating that fizzle required this (over Hiroshima): "Little Boy"

19 So named because it was little and relatively simple: Tube of 80% 235U SHOT (by cannon!) into position around cylinder of 80% 235U With Neutron Mirror then also bouncing back neutrons leaking outward from tube ONLY in this way could they BEAT the initial heat starting to push things back apart Avoiding fizzle, getting MOST of 235U to fission => ~ Complete energy liberation

20 They didn't even test this design in advanceReason #1) Because they were almost certain it would work Reason #2) Because they had so little 235U Why? Because 235U is SO HARD TO ENRICH: 235U is chemically identical to 238U. So it bonds to all the same things! To separate, must exploit ~1% mass difference between 235U and 238U Via gas diffusion OR mass spectrometers OR high speed centrifuges Plutonium, obtained from 238U decay, is much easier to separate: 238U + 1n (hot/fast) => 239U => 239Np + b => 239Pu + b Pu and 238U have different number of electrons, so bond to different things So 239Pu can be chemically separated

21 They had PLANNED to use plutonium in same Little Boy designBut plutonium fission reaction started so much faster That mass would have begun blowing back apart too early Before cannon could fully merge tube / cylinder => FIZZLE So were driven to "Fat Man" = sphere of explosives surrounding sphere of Pu Were still so unsure of it, that THIS is what was tested at Alamogordo NM And then dropped on Nagasaki Shaped conventional explosive Hollow Plutonium Sphere

22 Comparison of nuclear bombs and reactors:SIMILARITY: Most common reactor designs DO use the SAME 235U fission reaction DISSIMILARITY: In bombs, when fissile masses are merged, they are critical Facilitated by extremely rapid merge + 20X more concentrated 235U In reactors, even if fissile masses are merged, they are subcritical HOLD IT! But how then does a nuclear reactor continue working? That is, how do 235U's continue fissioning at a rate higher than the natural rate of 50% probability per million years? ANSWER: By deliberate addition of NEUTRON MODERATORS Which slow down (thermalize) neutrons liberated by one 235U's decay increasing likelihood that they will cause another 235U to decay So when I say MODERATOR think ENHANCEMENT of 235U fission!

23 Reactor = Subcritical mass + Accelerator + BrakeAccelerator is the above mentioned neutron moderator Brake is added neutron poison (absorbers) contained in the "control rods" GOAL: Balance competing effects to so that: Exactly one of the neutrons ejected by first 235U stimulates another 235U Then decays/second (and energy release) stays ~constant Balancing is helped by by details of neutron emission: Only few neutrons (~0.65%) are "prompt" = released extremely quickly Most, in full decay scheme, take milliseconds to minutes to emerge Giving control rods much more time to react

24 Details of the full control scheme vary by reactor designMajority of U.S. and worldwide reactors are of two basic types: Boiling Water Reactor (BWR): Pressurized Water Reactor (PWR): Both use heat of 235U + 238U decay to boil water and drive turbine generator But details of control scheme (and safety containment structures) are as follows: An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm

25 Details of boiling water reactor (BWR) design:Simpler control scheme which is, in a subtle sense, more sophisticated: 4% 235U + 238U fuel pellets inside zirconium tubes (1-2 cm dia. / 3-4 m long) = "Fuel Rods" Plus movable "Control Rods" containing neutron poison/sink (= "brake") Plus neutron moderator supplied by surrounding water (= "accelerator") Water ALSO absorbs heat, boiling into the steam that drives the turbine Fuel Rod Control Rod

26 But water can be both a neutron moderator and absorber:Water moderates neutrons because it contains all of those light hydrogen atoms Normal hydrogen IS just a proton + electron = 1H So colliding neutron (of ~same mass) can transfer LOT of energy to it Where it would just ricochet off much more massive atoms Thus water transforms hot/fast neutrons into slow/thermal neutrons So output of one 235U fission becomes ideal input for next 235U fission Based on moderation alone: More water should accelerate reaction Conventional “light” water (with neutron-free hydrogens) can also absorb neutrons H nucleus absorbs neutron converting from p to n+p (= 2H = deuterium) Based on absorption alone: More water should decelerate reaction An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm

27 Most reactors are designed so that water "moderation" dominates:Then, if reactor overheats, water first expands then (if allowed to) boils: This spreads out the water molecules, making it harder for Hot/fast neutrons to moderate into slow/thermal neutrons Fewer slowed neutrons make it much harder for 235U to fission Which automatically turns the reactor back down! A second level of control is added via the control rods Which, absorbing neutrons, diminish the likelihood of fission Third level of control added via emergency ("scram") shutdown E.G. by injecting boric acid, the boron's of which strongly absorb neutrons Because boiling water reactors DO allow water to boil (and thus spread out) industry insiders tend to view it as being the most stable type of reactor

28 But there is a potential problem with boiling water reactors:The turbine generators are located OUTSIDE reactor containment structure Because they must be more easily accessible for servicing, meaning: Water from reactor (as steam) is allowed to exit the containment Fortunately, pure water can become only slightly/mildly radioactive: Some 1H => 3H (tritium) which decays relatively slowly and benignly An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm

29 Alternate Pressurized Water Reactor (PWR):Inspired, in part, by concern about reactor cooling water exiting containment If water picks up impurities, THEY could become strongly radioactive Or if fuel rods leaked, water could become extremely radioactive So instead of one water loop, there are two: Primary loop enters reactor core then, via heat exchanger, transfers heat ONLY secondary water/steam loop exits containment to drive turbines

30 Subtleties of Pressurized Water Reactor (PWR):Primary loop's job is to supply enough heat to boil water in secondary loop It can carry a lot more heat energy if water in it remains a dense liquid (vs. vapor) But it still has to reach temperatures ABOVE boiling so it must be pressurized Keeping that water liquid even well above 100°C However, the water in that primary loop is ALSO a NEUTRON MODERATOR But, under pressurization, its water cannot expand much and can't vaporize So degree of neutron moderation (which accelerates 235U fission reaction) Will not automatically decrease sharply when reactor core heats up So you loose some of the negative feedback that enhances the stability of competing boiling water reactor (BWR) designs

31 Putting basic schematics of these two designs side by side:Boiling Water Reactor: Pressurized Water Reactor:

32 Adding a bit more technical detail:Boiling Water Reactor: /www.world-nuclear.org/info/Nuclear-Fuel-Cycle/Power-Reactors/Nuclear-Power-Reactors/ Pressurized Water Reactor:

33 Finally: Different "hot zones" => Different containment strategies:Boiling Water Reactor: Strong reactor vessel containment Weaker overall building containment (=> conventional flat walls & roofs) Pressurized Water Reactor: Strong building containment of reactor vessel & steam generator (=> signature concrete domes) No turbine building containment Source: CRS Report to Congress – "Power Plants: Characteristics and Costs" (November 13, 2008) - Order Code RL34746

34 But we need to include one more type of reactor:RBMK (Reaktor Bolshoy Moshchnosti Kanalnyy) reactor – as used at Chernobyl

35 RBMK Reactors RBMKs use partially pressurized cooling water, that is allowed to boil Putting them somewhere between previous BWR and PWR reactors But they use water only for heat transfer, NOT for neutron moderation Instead, fuel rods rest in oversized metal-lined holes in blocks of graphite With thin layer of cooling water flowing between rods and liners Plus gas flow for heat transfer between liner and block / block to block The graphite (alone!) produces ~ complete neutron moderation/slowing Graphite blocks with holes/liners for fuel rods and control rods Fuel rods containing uranium Control rods containing neutron poison

36 Unique goals/characteristics of RMBK reactors:Design goals were to: - Use much cheaper un-enriched natural uranium: 0.72% 235U % 238U - Produce BOTH electrical power PLUS plutonium for weapons - Build unusually large high power reactors, at unusually low costs Which was accomplished via: - Complex heat transfer scheme combining thin layers of water w/ inert gas flows - Constant, heavy, neutron moderation provided by (flammable) graphite blocks With neutrons already moderated, water's moderation is unimportant! - WITHOUT a heavily reinforced reactor containment vessel As used in western reactors including both BWR and PWR designs above

37 With this background, let's figure out WHY three reactors "exploded"An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm

38 THREE MILE ISLAND (TMI) – Eastern Pennsylvania - 28 March 1979Reactor involved (TMI #2) = Pressurized Water Reactor (Babcock & Wilcox Corp.) Initial fault was in secondary water cooling loop (outside reactor containment): Filter clogged, operators tried to clean by injecting compressed air Resulting over-pressurized water leaked into air control line Hours later compromised air control line caused pumps to trip off => Secondary loop could no longer fully remove heat from primary loop Primary loop then overheated, reactor automatically initiated "scram" shutdown Ramming in control rods to absorb neutron flux But there was still HUGE amount of heat energy in the reactor core Which was no longer being carried away by the cooling loops An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm

39 The TMI blow by blow analysis (continued):But with the scram, three emergency pumps automatically turned on to cool core But two of their valves had been left closed after earlier maintenance So effectiveness of emergency cooling system was vastly reduced Primary loop then heated so much that pressure relief valve was energized to open When excess pressure was vented, that valve should then have closed Limiting loss of water from that primary cooling loop But valve instead stuck open, allowing more water to escape Dark control room light indicated that power to open valve had been removed But there was no light indicating whether or not valve HAD actually closed Operators misinterpreted dark "open" light as indicating valve closure

40 The TMI blow by blow analysis (further continued):Operators had NO instrument to directly read level of water around core But they knew water was high in the "pressurizer," located above the reactor So they assumed that reactor core below was still fully immersed in water Because of pump vibrations, and fearing pressurizer would overfill (and fail): Operators shut down pumps adding more water to the primary loop But reactor’s core was NOT fully covered by cooling water Water pumps were vibrating because they were pumping steam Confusion reigned for four hours New shift of operators finally figured out situation and began to correct By then half of the reactor core had melted down and, driven by hydrogen combustion, some radioactivity had already escaped from the containment vessel

41 Partial list of faults and errors:Equipment failures: Stuck primary loop vent valve Indicator giving only intended state of that valve and not its true state Lack of dedicated indicator giving water level in core Control system which produced over 100 alarms in first minutes of failure Management / operator / training errors: Initial procedure for cleaning out clogged filter Emergency cooling system valves left closed after earlier maintenance Misinterpretation of above (badly designed) relief valve indicator Operator mistrust of automatic safety systems (for cause?), including: Operator override of automatic water cooling system Repeating error that almost caused earlier accident elsewhere TMI management knew of that near miss, but had not told operators!

42 Report Of The President's Commission On The Accident at Three Mile Island:"We have stated that fundamental changes must occur in organizations, procedures, and, above all, in the attitudes of people. No amount of technical "fixes" will cure this underlying problem. There have been many previous recommendations for greater safety for nuclear power plants, which have had limited impact. What we consider crucial is whether the proposed improvements are carried out by the same organizations (unchanged), with the same kinds of practices and the same attitudes that were prevalent prior to the accident. As long as proposed improvements are carried out in a "business as usual" atmosphere, the fundamental changes necessitated by the accident at Three Mile Island cannot be realized." ("Kemeny Report," Overview, p. 24) In light of the above, note that in 2014 I found TMI "information webpages" posted by BOTH a key industry association AND a key federal agency that still fail to mention central critical errors, including failure to reopen emergency cooling valves after earlier maintenance. An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm

43 CHERNOBYL – then USSR now Ukraine – 26 April 1986Chernobyl's RBMK reactor used masses of graphite as a neutron moderator This solid does not expand and then boil away as temperature increases So, as reactor power increases, its neutron moderation does not diminish Vs. moderating water whose loss would have dampened fission The graphite core produced strong, continuous, neutron moderation: Initially hot neutrons with extremely high kinetic energy => Many, many collisions with cooler graphite (carbon) atoms => Neutron kinetic energy approached that of the ambient From then on, these cooled neutrons were almost as likely to gain energy from collisions as lose energy from collisions An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm

44 Leading to Chernobyl's 1st positive feedback loop:Water no longer strongly moderated these already slowed neutrons However, water did still absorb neutrons, slowing nuclear fission reaction But then, when reactor began to overheat and water started to boil There was less water per volume => There was less neutron absorption per volume => Leaving more neutrons to accelerate nuclear fission This acceleration of fission, upon creation of steam bubbles, is called a: Positive void coefficient “Positive” in the sense that it provides positive feedback, stoking the fission reaction So when Chernobyl started to overheat, this further accelerated the heating An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm

45 Chernobyl’s 2nd positive feedback loop: Its strange control rodsReactor core Neutron absorber Displacer Water Control rod's job is to slow nuclear fission when it's pushed into the reactor core But before control rod enters reactor core, its hole is filled with water Which (per discussion above) already absorbs some neutrons Designers wanted strongest possible drop in neutrons when absorber entered So they decided to kill off the initial absorption of the neutrons in water, by first pushing out water (via a "Displacer" extension of the control rod) But that meant as control rod entered reactor, neutron population changed as: Medium (due to water) => High (no loss in displacer) => Low (due to absorber) In the middle (with only displacer inserted) nuclear fission accelerated because they made displacer out of neutron moderating graphite

46 Chernobyl’s 3nd positive feedback loop: Neutron "poisons"I mentioned earlier that things like Xenon, Boron & Iodine are neutron poisons Absorbing but not re-emitting any neutrons (taking them out of play) Fission reactions themselves produce "poisons" such as these As poisons accumulate, reactor control rods must be withdrawn farther Compensating for poisons by reducing rod's neutron absorption But neutrons from reactor can also make some of these poisons radioactive Causing them to fission into new non-poison elements Sort of like a hot furnace burning soot out of narrowing chimney But when reactor is turned down, neutron poisons tend to build back up Which, in turn, drives nuclear fission rate down even further => Positive feedback loop trying to shut reactor down

47 But this then works in reverse when turning up reactorIf reactor has been off, or running very low, neutron poisons build back up Normal withdrawal of control rods will then not accelerate fission as intended So they pull out more rods than normal (or rods further) to get running But as fission reaction finally accelerates, neutrons begin to "burn off" poisons Causing fission reaction to further surge upwards (NOTE: These surges are also an issue in non-RBMK reactors) Safe way start reactor (done elsewhere, and normally done at Chernobyl): Leave IN enough control rods that surge cannot go supercritical But doing a much delayed test, missing key reactor experts, they were in a hurry And withdrew many more than the recommended number of rods

48 Three positive feedback loops => Instability => Sudden spike in fissionAnd, due to their abnormal procedures, they’d left themselves no margin for error Likely leading to ("likely" because witnesses were dead / damage overwhelming): - Steam explosion blowing lid off reactor - Which was enough to effectively open things up Because RBMK's were built without western-style containment - Allowing air (w/ oxygen) to reach super hot graphite moderator blocks Which had, to that point, been bathed in inert cooling gasses - Causing them to near instantaneously catch fire - Producing strong smoke plumes and thermal updrafts Distributing radioactive debris and dust far and wide Exceptionally bad reactor design? Or (once again): Key role of the "human factor?"

49 FUKUSHIMA DAI ICHI – 11 March 2011Which is a location (140 miles Northeast of Tokyo) where there are SIX reactors Four of which were involved in the accident (and critically damaged) While the other two were shut down for maintenance at the time I came up with reams of data on this accident, much more than on TMI or Chernobyl But it really wasn't necessary, because this accident was easy to figure out: It wasn't due to unpredictable equipment breakdowns It wasn't due to operator errors Both instead worked essentially as intended and as hoped for It was instead due to design shortcomings That were longstanding and well known (indeed known for decades!) But accepted by designers, utility company, and government regulators

50 Fukushima design shortcoming #1 (shared by ~ all reactors):Turning a reactor off doesn't really turn it off A reactor is turned off (including in emergency "scram") by inserting control rods => Neutron poisons absorb so many neutrons that 235U stops fissioning But firstly: There is still a huge amount of heat in the reactor core And while the core itself may be able to withstand these temperatures E.G. by employing exotic/expensive high temperature materials Steels of reactor shell and piping may not withstand such temperatures And secondly: Fission is not an instantaneous process 235U does not => End products in one quick energy releasing step It instead decays into something else, which decays to something else With each radioactive decay along the way releasing more energy

51 Meaning that while control rods stop 235U + 238U fission:The overall fission decay process continues Until ALL radioactive products Have decayed into final NON-radioactive elements Which means control rods cannot instantaneously cut energy release to zero Instead, energy release may only fall by ~ 95% With the remaining 5% (due to radioactive decay of fission products) then taking hours or days to fall away STORED ENERGY in core + FISSION ENERGY STILL BEING PRODUCED => Reactor MUST be actively cooled for additional day/days "Active cooling" = Electrically powered, fully functioning cooling pumps

52 So with days of active cooling essential, where was Fukushima built?On the edge of one of the world's most seismically active / tsunami prone coasts: And where were the back-up generators for these essential pumps placed? In the basements (i.e. as close to sea level as you could possibly put them) Why locate plant and pumps essentially AT sea level? To save a little money by using smaller water cooling pumps and piping? An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm

53 Tsunami protection (?) Immediately offshore a system of barriers WAS built (see photo) With design goal of blocking tsunami's of up to 10 METERS in height But subsequent studies suggested that risk of larger tsunamis was too high And that barrier height should be significantly increased TEPCO considered these studies but decided against higher barriers Fearing that admission of error might lead to calls for similar barriers, or barrier heightening, at nuclear plants elsewhere (Including sites where tsunami threat was less acute) Instead they moved SOME of the backup generators to the top of the hill But they left power lines / circuit breakers in the oceanside basements Where they were flooded when the 14 METER tsunami hit So three now uncooled reactors began to melt-down

54 A Short Digression: To fully understand my empathy for the Fukushima plant operators, and my animosity toward TEPCO and Japanese Government "regulators" I STRONGLY RECOMMEND this superb PBS Nova documentary: Available to Public TV members via their PBS station Or viewable by all at this YouTube link

55 Have we in the U.S. been smarter, wiser, or less penny pinching?Beachfront / at sea level: Just above sea level: SAN ONOFRE CALIFORNIA (closing) DIABLO CANYON CALIFORNIA (closure planned) Beachfront / at sea level: Minor natural protection (Google Earth) X HUMBOLDT BAY CALIFORNIA (closing) en.wikipedia.org/wiki/Humboldt_Bay_Nuclear_Power_Plant

56 Fukushima design shortcoming #2 (shared by ~ all reactors):Spent nuclear fuel is stored inside the reactor enclosure "Spent fuel" is really not all that spent: Atoms are still fissioning (in ever decreasing numbers) for hours, days, years, centuries and millennia afterwards And no country has yet agreed upon a long term storage site for this "spent" fuel Further, ≤ 25% of 235U & 238U fissions over the ~ 2 years it's in the reactor Providing a reason to hold on to it for later re-enrichment and reuse With no place to go, it is now generally stored AT the reactor site, indefinitely It exits the reactor still highly radioactive, so one wants to minimize its handling Leading to common practice of storing it in INSIDE the reactor building Until, less radioactive, it's moved to another facility at the site

57 This increases radioactive material within the reactor buildingStored spent fuel can easily exceed the amount INSIDE the reactor And thus total amount of radioactive material doubles, triples, quadruples . . . That "spent" fuel, still fissioning, must also be cooled, so it is held in water pools: Spent fuel storage pools Vermont Yankee Nuclear Plant with same GE BWR design as Fukushima

58 Or diagrammatically: Crane for fuel rod loading / unloading"Spent" fuel rod storage pool Reactor vessel Reinforced reactor enclosure High position of storage pool DOES make it quicker and easier to reach But it is already outside of the main reinforced reactor enclosure And, being high above reactor, it is susceptible to damage and water loss

59 Fukushima design shortcoming #3 (shared by ~ all reactors):High temperature catalytic decomposition of H2O by zirconium Fuel rods consist of enriched 235U held in zirconium metal alloy tubes Because it's one of very few materials that can withstand full reactor heat! But at the 2000°C temperatures of an approaching/ongoing meltdown Zirconium catalyzes steam/water decomposition: 2 H2O => 2 H2 + O2 These gases accumulate inside reactor until they reach an explosive level And then an abundance of hot things can cause them to ignite: 2 H2 + O2 => 2 H2O + large amount of energy (=explosion) Despite the four meltdowns, radiation HAD been confined to reactor buildings Because "Containment structures" had been doing their job! But hydrogen + oxygen explosions now blew open the containments!

60 Dirty Bomb: Radioactive materials dispersed by conventional explosives Were these explosions "nuclear explosions?" NO! Energy was due to chemical bonding and NOT nuclear fission! And their power was immensely less than even early nuclear bombs But nuclear fission HAD supplied necessary heat to generate the hydrogen and oxygen gases that fueled these explosions And net effect DID end up being widespread dispersion of radioactive materials = Dirty Bomb: Radioactive materials dispersed by conventional explosives

61 A hydrogen explosion also occurred at Three Mile Island (thirty two years earlier)High temperature zirconium catalysis of water was also identified as the cause! E.G. in the Presidential Commission's Report on TMI That hydrogen chemical explosion also moved the accident from a contained meltdown to an external (environmental) radiation release That is, hydrogen explosions converted problem inside a single reactor building Into the beginnings of a large area environmental disaster But, fortunately, the TMI hydrogen + oxygen explosion was much, much smaller And the damage to the containment vessel was proportionally reduced Such that radiation leakage at TMI was minimal And it took a 2nd go round (at Fukushima) to fully play out this disaster scenario:

62 Fukushima: Before and AfterBarely discernible four seaside reactors + surrounding countryside: Barely discernible four seaside reactors + massive clean-up / waste-storage facility Left: Right:

63 To close: All THREE of Fukushima's critical shortcomings were well known Two (plant site / spent fuel storage) had easy (but not inexpensive) fixes Third (zirconium catalysis of H2O) had also caused TMI's radiation release And while its elimination may indeed be difficult 32 years passed without any significant effort to eliminate it! Making that 32 year old TMI Presidential Commission Report seem prophetic: "No amount of technical 'fixes' will cure this underlying problem. As long as proposed improvements are carried out in a 'business as usual' atmosphere, the fundamental changes necessitated by the accident at Three Mile Island cannot be realized"

64 Other class lectures on nuclear energy:The follow-on to this "Nuclear Power – But they blow up!" lecture: Next Generation Nuclear Power (link) The follow-on to that lecture: Other Gen IV Nuclear Reactors (link) A bonus mini-lecture describing something truly remarkable: Prehistoric Natural Reactors? (link)

65 Credits / AcknowledgementsSome materials used in this class were developed under a National Science Foundation "Research Initiation Grant in Engineering Education" (RIGEE). Other materials, including the "Virtual Lab" science education website, were developed under even earlier NSF "Course, Curriculum and Laboratory Improvement" (CCLI) and "Nanoscience Undergraduate Education" (NUE) awards. This set of notes was authored by John C. Bean who also created all figures not explicitly credited above. Copyright John C. Bean (However, permission is granted for use by individual instructors in non-profit academic institutions) An Introduction to Sustainable Energy Systems: WeCanFigureThisOut.org/ENERGY/Energy_home.htm