20. How Nuclear Energy Works
20. How Nuclear Energy Works
MIT 22.01 Introduction to Nuclear Engineering and Ionizing Radiation, Fall 2016
Instructor: Michael Short
View the complete course: https://ocw.mit.edu/22-01F16
YouTube Playlist: • MIT 22.01 Introduction to Nuclear Eng…
Ka-Yen’s lecture on how nuclear reactors work is expanded upon, to spend more time on advanced fission and fusion reactors. Lots of topics related to reactor operation are conceptually introduced - moderation, absorption, leakage, fast vs. thermal spectrum, breeding fuel, neutron poisons, and temperature/density feedback. This sets the stage for student control of the MIT reactor to come shortly.
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21.79 -> so today I wanted to give you some
23.59 -> context for why we're learning about all
25.78 -> the Neutron stuff and go over all the
27.34 -> reactor types that until this year the
29.86 -> first time you learned about the non
31.3 -> light water reactors at MIT was once you
33.31 -> left MIT I remember that as an undergrad
36.97 -> as well the only exposure we had to non
39.129 -> light water reactors is in our design
41.02 -> course because we decided to design one
42.58 -> so I wanted to show you guys all the
44.5 -> different types of reactors are out
45.82 -> there how they work and start generating
48.37 -> and marinating in all the different
50.86 -> variables and nomenclature that we'll
52.36 -> use to develop the neutron transport and
54.97 -> Neutron diffusion equations the nice
57.129 -> part is now until quiz 2 you can pretty
59.8 -> much forget about the concept of charge
61.6 -> so 802 can go back on the shelf because
64.18 -> every interaction we do here is neutral
66.46 -> charge neutral there'll be radioactive
68.86 -> decays that are not the case but
70.78 -> everything Neutron is neutral doesn't
73.33 -> mean it's going to be simple it's just
74.62 -> gonna be different but in the meantime
76.84 -> today is not going to be particularly
79.33 -> intense but I do want to show you where
81.19 -> we're going and this goes with the
84.07 -> pedagogical switch that we made in this
85.9 -> department starting this year and you
87.79 -> guys are the first trial of this we're
90.31 -> switching to context first in theory
92.23 -> second I personally find it much more
94.33 -> interesting to study the theory of
96.33 -> something for which I know the
98.05 -> application exists who here would agree
102.12 -> just about actually everybody ok yeah
105.64 -> that's what I thought too so in the end
108.07 -> we had arguments amongst the Faculty of
110.86 -> all well you have to learn the theory to
112.48 -> understand the application and that
115.21 -> works really well when you say it behind
116.95 -> the closed office door by yourself but
118.87 -> the fact is I'm in it for yeah I'm in it
122.47 -> for a maximum subject matter retention
124.93 -> so in whatever order that works the best
127.09 -> and sounds like for you guys this works
129.099 -> the best that's what we're doing with
130.72 -> the whole undergrad curriculum not just
132.28 -> this class so let's launch into all the
136.239 -> different methods of making nuclear
138.04 -> power both fission and fusion and to
140.95 -> switch gears since we're dealing with
142.45 -> neutrons I don't know what happened with
144.58 -> the oh there we go the idea here is that
147.129 -> neutrons hit things like uranium and
149.5 -> plutonium the fizzle isotopes that you
151.359 -> guys saw in the exam and caused the
153.34 -> release of other neutrons
155.04 -> as we come up with these variables I'm
157.439 -> gonna start laying them out here it
159.48 -> might take more than a board to fill
161.73 -> them all and I'll warn you ahead of time
163.739 -> this is the only time in this course
165.299 -> that we're gonna have V and knew the
167.76 -> Greek letter nu on the board at the same
169.409 -> time and I'm gonna make it really
171.36 -> obvious which one is nu and which one is
174.9 -> V so this parameter that describes how
184.95 -> many neutrons come out from each fission
187.079 -> reaction we refer to as nu or the
188.79 -> average number you'll see in the data
190.92 -> tables as nu bar and so as we come up
193.859 -> with these sorts of things I will start
195.989 -> going over them and the idea here is
197.849 -> that each uranium-235 or plutonium or
201.51 -> whatever nucleus B gets two to three
203.31 -> neutrons the exact number for which is
205.59 -> still under a hot debate and I don't
207.15 -> think it actually matters we'll make a
209.099 -> couple of fission products that take
210.51 -> away most of the heat of the nuclear
212.67 -> reaction and I just want to stop there
215.069 -> even though you know there's going to be
216.78 -> a chain reaction and that's what makes
218.28 -> nuclear power happen and we can go over
220.829 -> the timeline of what actually happens in
222.9 -> fission and what kind of a nuclear
225.06 -> reaction it really is so in this case
227.79 -> this is a reaction where a neutron is
230.34 -> heading towards this time are actually
232.41 -> gonna give it a label a uranium-235
235.47 -> nucleus and it very temporarily like I
238.889 -> showed you yesterday forms a compound
240.989 -> nucleus some sort of large excited
244.349 -> nucleus that lasts for about ten to the
247.829 -> minus fourteen seconds so it doesn't
251.28 -> instantly fizz apart there's actually a
253.109 -> neutron absorption event some sort of
255.449 -> nuclear instability at which point your
260.01 -> two fission products break off
265.289 -> notice you don't have let's call them
267.789 -> fission product one and fission product
269.47 -> to notice you don't quite have any
271.72 -> neutrons yet Neutron production is not
274.63 -> instantaneous for the following reason
278.28 -> if you remember back to nuclear
280.419 -> stability when we plotted let's say n I
283.569 -> think that was maybe Z and this was N
286.78 -> and I think this was a homework problem
288.16 -> and you had to come up with some sort of
290.74 -> curve of best fit for the most stable
293.759 -> come combination of n and Z for a
297.13 -> nucleus it was not a straight line it
298.96 -> was something on the order of like N
302.009 -> equals like what is it
304.259 -> 1.00 5 5 z plus some constant something
310 -> with a rather small slope well if you
313.27 -> have a heavy nucleus like uranium-235
316.389 -> and you split it apart evenly let's just
320.68 -> pretend it splits evenly for now you're
323.38 -> kind of splitting that nucleus along a
326.169 -> rather unstable line and as you saw in
329.44 -> the semi empirical mass formula a little
331.81 -> bit of instability goes a really long
333.25 -> way towards making the nucleus extremely
335.889 -> unstable so let's say you'd make a
338.02 -> couple of fission products that just
340.09 -> cleave that just cleave that nucleus
342.82 -> with the same proportion of protons and
344.62 -> neutrons how would they decay or how can
347.65 -> they decay there's a couple different
349.21 -> ways what do you guys think
352.26 -> it can emit neutrons if it's really
356.34 -> unstable at which point it would just go
358.74 -> down a neutron number or how else could
360.42 -> it decay
363.4 -> alpha decay let's see yeah a lot of
366.25 -> those will the heavier ones tend to do
368.29 -> alpha decays what would it do at alpha
369.88 -> decay for alpha you guys have we go in
372.55 -> that direction right yeah you know what
375.01 -> I'm not gonna rule that out yet so let's
376.479 -> let's go with that how else could they
378.34 -> decay yeah through beta decay let's say
381.58 -> in that direction
385.26 -> pretty much all these happen just not
388.06 -> necessarily in this order when you have
390.22 -> a really really asymmetric nucleus a lot
393.58 -> of these fission products will emit
396.729 -> neutrons almost instantaneously in the
400.479 -> realm of like 10 to the minus 17 seconds
403.03 -> some incredibly short time line you'll
406.69 -> start to decay downwards a little bit
409.26 -> but you're not quite at the stability
411.699 -> line which is why a lot of the fission
413.71 -> products then go on and they deposit
418.18 -> their kinetic energy by bouncing around
419.74 -> the different atoms and material
421.12 -> creating heat but a lot of them will
423.49 -> also send off betas or gammas and it may
433.51 -> take you know ten to the minus thirteen
435.13 -> seconds for them to whatever the
437.86 -> half-life of that particular isotope is
439.599 -> and after around like ten to the let's
443.169 -> say 10 to the minus 10 to 10 to the
444.82 -> minus six seconds depending on the
447.46 -> isotope in the medium those two fission
450.28 -> products will stop and let's just say
455.949 -> that they stop there so the whole
458.74 -> process of fission it's actually quite a
460.24 -> compound process first the neutron is
463.33 -> absorbed forming a compound nucleus then
466.12 -> it splits apart then those individual
468.22 -> fission products undergo what ever
469.66 -> decays suit them best and that's the
471.94 -> source of the neutrons in fission
473.76 -> sometimes one of those fission products
475.93 -> might be particularly unstable and it
478.15 -> might send off two neutrons in other
480.31 -> cases that I don't know of it one off
481.99 -> the top of my head it might be none but
483.97 -> this is the whole timeline of events in
485.53 -> fission and the justification for why
487.93 -> this happens straight from the first
490.06 -> month to 2201 and I wanted to pull up
493.03 -> some of the nuclear data so you can see
494.89 -> what these values tend to look like
497.199 -> and also where to find them I've got to
499.84 -> do that screen cloning thing again hey
506.68 -> there we go
507.31 -> so I've already pre pulled up the Janus
511.3 -> library I've already clicked on
512.709 -> uranium-235 thanks to you guys I have
514.839 -> all the data now on my shirt but so you
517 -> can see a little better I also have it
518.56 -> on the screen so let's look at this
520.779 -> value right here a new bar total neutron
523.839 -> production and I'll make it bigger so
526.269 -> it's easier to see did I click on the
528.97 -> right one yeah so take a look at that
531.94 -> then total number of neutrons produced
534.79 -> during u-235 it's for most energies it's
538.899 -> hovering around the 2.4 or so there's
542.199 -> been arguments about whether it's two
543.55 -> point four three or two point four four
545.529 -> and that's a linear scale that's not
549.1 -> very helpful let's go to a logarithmic
550.6 -> scale that's more like what I'm used to
552.91 -> seeing most of the fission happens for
556.269 -> u-235 in the thermal region in the
559.029 -> region where the neutrons are at values
560.829 -> let's say the cutoff usually about one
562.75 -> electron volt or lower in average energy
565.06 -> and noubar is fantastically constant at
569.35 -> that level then as you go up and up in
572.11 -> energy you start to make more and more
574.39 -> neutrons why do you guys think that
575.829 -> would be the case
582.07 -> what are you doing to that compound
583.839 -> nucleus as you increase the incoming
586.149 -> Neutron energy it's gonna have more
590.17 -> energy itself you might excite other
592.48 -> nuclear states that can then lead to
594.1 -> other sorts of decays or other neutron
596.139 -> emission so to me that's the reason why
598.87 -> once you hit about 1 MeV you can start
601.3 -> to see a lot more neutrons being given
603.97 -> off the reason we usually treat this as
606.73 -> a constant notice I haven't given it an
609.009 -> energy dependence is because most of the
611.44 -> fission that happens is at thermal
613.269 -> energies for that I want to show you the
616.329 -> fission cross-section there's a lot of
620.05 -> cross-sections and it's probably gonna
623.05 -> be in a different graph because it's in
624.639 -> different units and this gives you a
626.8 -> rough measure per atom what's the
628.48 -> probability of fission happening as a
630.67 -> function of incoming Neutron energy at
633.009 -> those high energies you have relatively
635.649 -> low cross sections or low probabilities
638.17 -> of fission happening then there's this
639.97 -> crazy resonance region that looks like a
641.949 -> sideways mustache but then as you get
644.41 -> down to the lower energy levels it gets
646.839 -> much more in fact exponentially more
649.06 -> likely that fission will happen so
650.889 -> almost all the fissioning in a light
652.51 -> water reactor or any sort of other
654.04 -> thermal reactor happens at thermal
656.079 -> energies and that's why we take nu bar
658.029 -> as a constant you don't have to
660.569 -> especially if you're analyzing what's
662.8 -> called a fast reactor or a reactor whose
665.199 -> Neutron population remains fast on
667.329 -> purpose and so with that I want to
670.18 -> launch into some of the different types
671.829 -> of reactors that you might see you guys
675.639 -> already did those calculations in
677.319 -> problem set 1 so I don't have to repeat
679.269 -> them for you let's get right into the
682.569 -> acronym so if you haven't figured this
684.339 -> out already nuclear is a pretty acronym
686.829 -> dense field does anyone can anyone say
689.829 -> they know all the acronyms on this slide
693.63 -> you're gonna know about 90% of them in
695.86 -> about 90 minutes so it's okay or you'll
698.589 -> have seen them at least and you look
702.069 -> completely unfamiliar
705 -> most of them well let's knock them off
708.31 -> so KN last Thursday already showed you
710.95 -> the basic layout of a boiling water
712.6 -> reactor one of the types of light water
714.94 -> reactors and the reason that this is a
716.98 -> thermal reactor is because it's full of
719.2 -> water water as we saw in our old Q
721.9 -> equation argument is very good at
724.27 -> stopping neutrons because if you guys
726.79 -> remember this the maximum change in
730.12 -> energy that a neutron can get is related
734.11 -> to alpha times its incoming energy where
736.9 -> this alpha is just a minus 1 over a plus
740.83 -> 1 squared and I think this should
744.94 -> actually be a 1 minus right there a is
749.35 -> that mass number of whatever the
751.66 -> neutrons are hitting and that one comes
755.32 -> directly from the neutron mass number if
762.07 -> you remember this was the simplest
764.47 -> reduction of the Q equation the
766.42 -> generalized Q equation or kinematics
769.42 -> that we looked at when I said let's do
771.13 -> the general form and okay let's take the
772.66 -> simplest form Neutron elastic scattering
774.76 -> here's where it comes back if a neutron
779.11 -> hits water which is made mostly a
780.58 -> hydrogen and a is one then it can
783.07 -> transfer a maximum of all of its energy
785.08 -> to the let's say to that hydrogen atom
788.43 -> therefore given the neutron no energy
790.39 -> and thermal izing it or slowing it down
792.1 -> very quickly to show you what one of
795.49 -> these things actually looks like that's
797.68 -> the underside of a BWR I don't know if K
799.78 -> did K and show you this before okay
801.52 -> you've already seen what this generally
803.74 -> looks like what about the turbine does
806.11 -> anyone actually seen a turbine this size
808.9 -> close up gigawatt electric turbine
811.589 -> trying to see which one of those pixels
813.64 -> is a person so I'm person-sized
818.8 -> I don't see anything persons oh there's
820.45 -> a ladder that looks to be about 6 feet
822.16 -> tall so give you guys a sense of scale
825.61 -> of the sort of turbines that we say oh
827.86 -> yeah we draw a turbine on our diagram
830.17 -> well it's not actually not simple these
833.56 -> things take up entire hallways they're
835.27 -> kind of airport hangar size buildings
836.98 -> never seen one of the you
838.42 -> but I've seen one in Japan there was a
840.16 -> lot cleaner than this but otherwise it
842.11 -> looked pretty much the same and the way
844.15 -> this actually works for those who
845.68 -> haven't taken any thermo classes yet is
848.59 -> this turbine is full of different sets
851.86 -> of blades that are curved at an angle so
854.02 -> that when steam shoots in it transfer
856.09 -> some of its energy to get the turbine
857.89 -> rotating and there's going to be a
859.84 -> generator found of an eye like an
862.15 -> alternator to generate the electricity
863.68 -> there which looks to be roughly a
866.2 -> hundred feet away just to give you a
868.33 -> sense of scale for this stuff as Kayne
871.39 -> showed you a pressurized water reactor
872.98 -> it's another kind of light water reactor
874.93 -> with what's called an indirect cycle so
877.21 -> this water stays pressurized it also
879.37 -> stays liquid which is good for Neutron
882.07 -> moderation or slowing down because in
884.5 -> addition to the probability of any
886.51 -> interaction some probability Sigma if
889.93 -> you want to get the total reaction
891.76 -> probability you have to multiply by its
893.74 -> number density to get a macroscopic
897.28 -> cross-section this is why I introduced
899.89 -> this stuff Y at the beginning of class
901.33 -> so you'd have time to marinate in it and
903.73 -> then bring it back and remember what it
905.89 -> was all about and so every single
907.96 -> reaction that goes on in a nuclear
910.9 -> reactor has got its own cross section
913.9 -> will probably need half the board for
917.41 -> this one you can say you have a total
922.95 -> microscopic cross-section these are all
926.17 -> going to be as a function of neutron
928.24 -> energy what's the probability of
930.19 -> anything happening at all and these are
932.41 -> actually tabulated up on the janice
934.81 -> website so let's unclick that get rid of
938.89 -> neutron production and go all the way to
940.57 -> the top n comma total so all this stuff
944.92 -> is written in nuclear reaction parlance
946.87 -> where if you have let's say n comma
948.97 -> total that means a neutron comes in and
952.09 -> that's the reaction that you're looking
955.57 -> at so this data file here once I open it
959.8 -> up will give you the probability that
962.44 -> anything at all will happen you can see
966.19 -> as the neutron energy gets higher the
968.92 -> probability of anything happening at all
970.9 -> gets less and less
972.01 -> less and it follows the shape of most of
974.32 -> the other cross-sections and I'm going
976.03 -> to leave this up right there you've also
978.16 -> got a few different kinds of reactions
980.2 -> like you can have a scatter let's call
985.12 -> that scatter which we've already said
989.02 -> can either be elastic or inelastic it
998.02 -> may not matter to us from the point of
999.85 -> view of neutron physics whether the
1003.12 -> collision is elastic or inelastic all
1006.75 -> that matters is the neutron goes in and
1008.82 -> a slower Neutron comes out because what
1011.1 -> we're really concerned with here is
1013.1 -> tracking the full population of neutrons
1017.13 -> at any point in the reactor so we'll
1020.01 -> give this a position vector R which has
1022.74 -> just got x y and z in it or whatever
1028.44 -> other coordinate system you might happen
1030.18 -> to you is I prefer Cartesian because it
1031.98 -> makes sense at every energy going in any
1036.69 -> direction so we now have a solid angle
1039.51 -> vector that's got both theta and Phi in
1043.02 -> it at any given time and the whole goal
1047.25 -> of what we're going to be doing today
1048.329 -> and all of next week is to find out how
1050.55 -> do you solve for and simplify this
1053.22 -> population of neutrons
1059.48 -> let's see make sure to fill that in as
1062.07 -> velocity
1065.64 -> yes
1070.04 -> and so a lot of let's see let me get
1073.04 -> back to the cross sections and stuff if
1075.29 -> we want to know how many neutrons are in
1077.87 -> a certain little volume element in some
1082.13 -> D volume in some certain little
1085.16 -> increment of energy de traveling in some
1088.34 -> very small solid angle D Omega
1091.96 -> supposedly if you have this function
1094.13 -> then you know the direction and location
1096.32 -> and speed of every single Neutron
1098.21 -> everywhere in the reactor and this is
1100.37 -> eventually what the goal of things like
1101.99 -> Ben and cords group does the
1103.52 -> computational reactor physics group is
1105.47 -> solve for this or a simplified version
1107.45 -> of it over and over and over again for
1109.76 -> different sorts of geometries and in
1111.98 -> order to do so you need to know the
1114.08 -> rates of reactions of every kind of
1116.96 -> possible reaction that could take a
1118.58 -> neutron out of its current position like
1121.28 -> if it happens to be moving which most of
1122.99 -> them are out of its current energy group
1125.5 -> which pretty much any reaction will
1128.6 -> cause the neutron to lose energy what's
1131.09 -> the only reaction we've talked about
1132.65 -> where the neutron loses absolutely no
1134.72 -> energy it's a type of scattering
1143.48 -> yep exactly forward scattering so for
1147.08 -> forward scattering for that case where
1149.08 -> theta scattering equals zero again
1151.97 -> that's you missed the neutron didn't
1155.99 -> actually change direction at all and
1157.49 -> therefore it didn't transfer any energy
1159.139 -> but for everything else for every other
1161.659 -> possible reaction there's gonna be an
1163.94 -> energy change associated with it and
1165.679 -> probably some corresponding change in
1168.08 -> angle because a neutron can't just be
1170.12 -> moving and hit something and continue
1172.76 -> moving more slowly there's got to be
1174.74 -> some change in momentum to balance along
1177.98 -> with that change of energy and it might
1180.59 -> slightly move in some different
1181.85 -> direction
1182.51 -> and all this is happening as a function
1184.49 -> of time as you can see this gets pretty
1186.889 -> hairy pretty quick and that's why we put
1189.08 -> the full equation for this on our
1190.429 -> department t-shirts but no one ever
1192.38 -> solves the full thing what we're gonna
1193.73 -> be going over is how do you simplify it
1195.889 -> into something you can solve with like
1197.45 -> pen and paper or possibly a gigantic
1199.909 -> computer but it's not impossible so
1203.75 -> inside this Sigma total we talked about
1205.94 -> different scattering and then you could
1208.279 -> have absorption in all its different
1215.929 -> forms
1217.059 -> what sort of reactions with a neutron
1220.13 -> would cause it to be absorbed
1227.13 -> yes vision thank you so there's going to
1231.7 -> be some Sigma fission cross-section as a
1234.43 -> function of energy and if it doesn't
1238.51 -> fizz but it is absorbed we'll call that
1242.68 -> capture
1248.88 -> but capture can mean a whole bunch of
1251.16 -> different things to right there could be
1252.99 -> also a whole bunch of other nuclear
1254.61 -> reactions like there could be a reaction
1257.76 -> where one Neutron comes in two no
1261.27 -> neutrons go out like we looked at with
1264.21 -> beryllium in the Chadwick paper from the
1266.67 -> first day
1267.33 -> we're like what actually does exist for
1270.6 -> this stuff so Janice doesn't like
1275.01 -> multi-touch so have to bear with me on
1277.26 -> the small print on the screen but there
1280.14 -> should be up here it is cross-section
1282.51 -> number 16 there is a probability that
1285.24 -> one Neutron goes in that Z right there
1288 -> is whatever your incoming particle
1291.03 -> happens to be and in this case we know
1292.68 -> it's a neutron because we picked
1294.39 -> incident Neutron data and two n means
1296.85 -> two neutrons come out let's plot that
1299.28 -> cross-section you can see that the value
1302.94 -> is zero until you hit about four or five
1306.09 -> oh it's actually five point two nine
1309.33 -> seven seven eight one MeV so that's the
1312.75 -> Q value at which this particular
1314.49 -> reaction happens to turn on might be
1317.46 -> responsible for a little bit of the blip
1319.8 -> in the total cross-section so
1321.81 -> technically if we were to turn on every
1324.3 -> single cross-section in this database it
1326.61 -> should add up to that red line right
1329.7 -> there so you can start to get an idea
1332.28 -> for how much of all the reactions of
1335.16 -> uranium-235 are due to fission that's
1337.86 -> the one we want to exploit
1339.53 -> so let's find fission right down there
1343.83 -> oh wow there's a 3n reaction I want to
1346.65 -> see that that doesn't happen until 12
1350.19 -> MeV yeah so neutrons don't typically
1354.81 -> tend to hit 12 MeV in a fission reactor
1358.23 -> so this is a flute and the perfect
1360.09 -> flimsy pretext to bring in another
1362.01 -> variable it's called the Chi spectrum or
1366.29 -> what's called the fission birth spectrum
1373.9 -> yeah we've already talked about the
1380.179 -> neutrons being born and how many there
1382.19 -> were but we're didn't say at what energy
1385.28 -> they're born in fusion reactors this is
1388.429 -> pretty simple
1390.669 -> you've already looked at this case what
1394.34 -> is it fourteen point seven MeV that's a
1397.16 -> lot simpler
1398 -> that's the fusion for fission it's not
1401.929 -> so simple for the case of fission if you
1407.54 -> draw energy versus this Chi spectrum it
1412.88 -> takes an interesting looking curve from
1416.09 -> about one MeV to about ten MeV with the
1421.46 -> most likely energy being around to
1423.62 -> anything so you aren't really going to
1426.35 -> get neutrons at the energy required for
1429.26 -> a three n reaction in a regular fission
1432.02 -> reactor just not gonna happen but it's
1435.02 -> good that you know that that exists so
1436.429 -> let's go and answer my original question
1438.95 -> how much of the total cross-section is
1441.26 -> due to fission most of it especially at
1448.85 -> low energies so let me get rid of those
1451.64 -> two n and 3n ones because they're kind
1453.38 -> of ruining our data it's making it
1456.679 -> harder to see
1463.28 -> that's better so you can see at energies
1466.85 -> below around let's say a KETV or so
1470.75 -> almost all of the reactions happening
1472.94 -> with neutrons in uranium-235 are fission
1475.31 -> this is part of what makes it such a
1477.08 -> particularly good isotope to use in
1479.36 -> reactors the other one is you can find
1480.92 -> it in the ground
1482.18 -> unlike most the other fissile isotopes
1485.32 -> unlike I think any of the other fissile
1487.7 -> isotopes thorium you got to breed and
1490.43 -> turn it into uranium 233 have to think
1493.25 -> about that one but then you can start to
1495.98 -> look at what are the other components of
1497.69 -> this cross-section like ZN prime in
1501.41 -> elastic scattering which doesn't turn on
1504.79 -> until about 0.002 MeV but later on is
1510.53 -> one of the major contributors and
1512.24 -> actually is responsible for and I've
1513.95 -> brought this for a reason is responsible
1516.56 -> for that little bump in the total
1518.51 -> cross-section so eventually all these
1520.58 -> things do matter but let's think about
1523.13 -> which ones we actually care about at all
1526.39 -> because what we eventually want to do is
1529.45 -> develop some sort of Neutron balance
1532.4 -> equation if we can measure the change in
1536.3 -> the number of neutrons as a function of
1537.92 -> position energy angle and time as a
1545.21 -> function of time and that would probably
1547.55 -> be a partial derivative because there's
1549.35 -> like seven variables here before I write
1553.1 -> any equations it's just going to be a
1554.81 -> measure of the gains minus the losses
1557.68 -> and while every particular reaction has
1562.1 -> its own cross section there's only going
1564.2 -> to be a few that we care about like
1565.88 -> they'll only be one or two types of
1569.27 -> reactions that can result in a gain of
1571.64 -> the neutron population into a certain
1574.43 -> volume with a certain energy with a
1575.84 -> certain angle and for losses there's
1579.11 -> only one we really care about total
1581.63 -> because any interaction with a neutron
1583.91 -> is going to cause that Neutron to leave
1587.06 -> this little group of perfect position
1590.3 -> energy and angle
1594.19 -> so that's where we're going we'll
1596.72 -> probably start down that route on
1598.249 -> Tuesday because I promised you guys
1600.309 -> context today you've all been to the MIT
1606.529 -> research reactor a couple of you are you
1608.72 -> running it yet awesome okay yeah say
1613.46 -> that Bo so yeah so Sarah and Jared's
1615.289 -> doing that anyone else
1616.429 -> training or trained no I'd say the folks
1621.08 -> usually pretty scared when they find out
1622.58 -> mit has a reactor and they're even more
1624.229 -> scared when they find out you guys run
1625.7 -> it what they don't realize is there's
1627.859 -> been basically no problem since 1954 the
1631.07 -> only one I know of as someone fell
1632.479 -> asleep at the controls once and forgot
1634.249 -> to push the don't call FoxNews button
1636.379 -> and it called Fox News or something so
1639.109 -> there was a big story about asleep at
1640.729 -> the helm
1641.47 -> Nora knew alarms and passive safety
1645.47 -> systems and backup operators and
1647.359 -> everything else that actually made sure
1649.159 -> that nothing happened but nowadays
1651.019 -> correct me if I'm wrong you actually
1652.46 -> have to get up every half hour reach
1654.139 -> around a panel and hit a button right so
1661.97 -> you want to hit it before it beeps at
1663.289 -> you
1665.26 -> it's reminding you okay
1672.91 -> okay yeah I'd heard the buttons every
1674.9 -> half-hour gotcha cool yeah so for all
1679.61 -> use watching on camera whatever just
1682.07 -> know that these guys got it under
1683.45 -> control so onto some gas cooled reactors
1687.65 -> and to explain some of these acronyms
1689.24 -> there are some that use natural uranium
1691.36 -> though all the ones pretty much all the
1693.53 -> ones in this country you need to enrich
1695 -> the uranium to get enough u-235 to turn
1697.94 -> the reaction on but that's not actually
1700.4 -> you don't have to do that in every case
1702.11 -> and you'll also see these acronyms
1704.33 -> Leu meu or h EU standing for low medium
1708.35 -> or high enrichment the accepted standard
1711.32 -> for what's low enriched uranium is 20%
1713.75 -> or below an interesting fact though you
1716.929 -> can't have something at 19 what at 19
1719.72 -> point 99 percent enriched uranium and
1722.96 -> expect it to be low enriched uranium
1724.46 -> because every measurement technique has
1726.5 -> some error and what really determines if
1728.57 -> it's Leu is when an inspector comes and
1730.7 -> takes a sample it better be below 20%
1733.6 -> including their error so you'll usually
1735.98 -> see 19 point 75% given as the Leu limit
1740.54 -> because there's always some processing
1742.49 -> error in homogeneities measurement error
1744.53 -> head your bets
1745.76 -> pretty much like in england or the UK
1749.27 -> the advanced gas reactors have been
1750.89 -> churning along for decades they actually
1753.559 -> use co2 as the coolant which is
1756.29 -> relatively inert and they use graphite
1758.9 -> as the moderator so in this case the
1761.179 -> coolant in the moderator are separate
1762.679 -> unlike the light water reactors we have
1764.32 -> so this way the graphite right here just
1767.72 -> sits in solid form and slows down those
1769.7 -> neutrons not quite as good as water but
1771.98 -> pretty good there is an issue though
1774.17 -> that co2 just like anything has a
1778.04 -> natural decomposition reaction where co2
1783.22 -> naturally is in equilibrium with CO and
1786.8 -> O 2 and O 2 plus graphite yields co2 gas
1794.5 -> graphite was solid and talking with a
1798.32 -> couple folks from the National Nuclear
1799.73 -> Laboratory they said that 40 years later
1802.04 -> when they took the caps off these
1803.63 -> reactors a lot of that graphite was just
1806.6 -> gone with a good explanation it
1810.41 -> vaporized very very very slowly over 40
1813.71 -> earth years or so
1814.64 -> due to this natural recombination with
1817.85 -> whatever little bit of o2 is an
1819.799 -> equilibrium with co2 and possibly some
1822.08 -> other leaks I'm sure I wouldn't have
1823.58 -> been told that if there was a leak so
1825.41 -> I'd say the feasibility is high because
1827.57 -> they've been running for almost half a
1829.22 -> century the power density is very low
1832.34 -> why do you guys think that's the case
1836.89 -> yeah
1843.23 -> mm-hmm
1847.03 -> absolutely so well let's say you need
1849.65 -> the same cooling capacity but you're
1851.18 -> right co2 even if pressurized is not a
1853.94 -> good at heat transfer medium as water
1855.559 -> water's dense it's also got one of the
1858.17 -> highest heat capacities of anything
1859.67 -> we've ever seen the other reason is
1862.7 -> right here if you want enough reaction
1865.37 -> density it not only matters what the per
1868.1 -> atom density is but what the number
1870.92 -> density is and if you're using gaseous
1873.44 -> co2 coolant even if it's pressurized
1875.27 -> there are fewer reactions happening per
1877.94 -> unit volume because there are a few co2
1880.1 -> molecules per unit volume than water
1882.559 -> would have so that's why we pressurize
1884.96 -> our light water reactors to keep water
1887.6 -> in its liquid state where it's a great
1889.19 -> heat absorber takes a lot of energy to
1891.53 -> boil it and it's really dense so it's a
1893.51 -> very effective dense moderator these
1898.01 -> have been around forever I think when
1899.72 -> did Windscale happen when scale was also
1903.02 -> the source of an interesting fire that's
1906.08 -> you guys might want to know about it's
1907.58 -> one of those only nuclear disasters that
1910.07 -> hit seven on the arbitrary unit scale I
1912.65 -> don't quite know how they determine
1914.72 -> what's a seven but there was a fire at
1917.21 -> the wind scale plan due to the build-up
1918.95 -> of what's called Wigner energy it turns
1921.95 -> out that when neutrons go slamming
1923.99 -> around in the graphite they leave behind
1926.53 -> radiation damage and when my family
1929.51 -> always explained what do you do for a
1932.15 -> living and I just can only think well
1933.98 -> they don't know radiation damage they've
1935.9 -> watched Harry Potter I like to say
1937.82 -> radiation like dark magic leaves traces
1939.95 -> well it leaves traces in the graphite in
1942.44 -> the form of atomic defense which took
1945.47 -> energy to create so by causing damage to
1948.53 -> the graphite you store energy in it
1951.02 -> which is known as Wigner energy and you
1952.97 -> can store so much that it just catches
1955.52 -> fire and explodes sometimes that's what
1958.1 -> happened here at Windscale eleven tons
1960.679 -> of uranium ended up burning because all
1963.08 -> of a sudden the temperature in the graph
1964.49 -> I just started going up for no reason no
1966.47 -> reason that they understood at the time
1968.059 -> it turns out that they had built up
1969.95 -> enough radiation damage energy that it
1972.86 -> started releasing more heat and
1974.17 -> releasing more heat caused more of that
1976.82 -> energy to be released and it was
1978.429 -> self-perpetuating
1980.24 -> until it just caught fire and burned 11
1982.94 -> tonnes of uranium out in the countryside
1984.44 -> this was 1957 so again a seven on the
1988.64 -> scale with no units of nuclear disasters
1992.17 -> argue it's probably not as bad as
1994.49 -> Chernobyl so they might want a little
1995.929 -> bit of sort of resolution in that scale
1999.01 -> there's another type of gas cool reactor
2001.57 -> called the pebble bed modular reactor a
2003.7 -> much more up-and-coming one or each fuel
2006.88 -> element you don't have fuel rods you've
2008.65 -> actually got little pebbles full of tiny
2010.9 -> kernels of fuel so you've got a built in
2013.39 -> graphite moderator tennis ball sized
2015.52 -> thing with lots of little grains of sand
2017.95 -> of uo2 cooled by a bed of flowing helium
2022.36 -> or something like that and then that
2024.429 -> helium or the other gas transfers heat
2027.22 -> to water which goes into make steam and
2029.559 -> goes into the turbine like I showed you
2031.3 -> before so this is what's the fuel
2034.84 -> actually looks like inside each one of
2036.73 -> these tennis balls spheres of mostly
2039.28 -> graphite there's these little kernels of
2041.679 -> uranium dioxide about a half a
2043.45 -> millimeter across covered in layers of
2046.12 -> silicon carbide a really strong and
2048.52 -> dense material that keeps the fission
2050.169 -> products in because the biggest danger
2052.899 -> from nuclear fuel is the highly
2055.419 -> radioactive fission products that due to
2058.75 -> their instability are giving off all
2060.55 -> sorts of awful for anywhere from
2062.97 -> milliseconds to mega years after reactor
2066.73 -> operation and so if you keep those out
2068.83 -> of the coolant then the coolant stays
2070.389 -> relatively non radioactive and it's safe
2072.85 -> to do things like maintain the plant
2076.109 -> then there's the very high temperature
2078.429 -> reactor the ultimate in acronym
2080.32 -> creativity it operates at a very high
2082.659 -> temperature which has been steadily
2085.24 -> decreasing over time as reality has
2088.419 -> caught up to expectations when I first
2090.85 -> got into this field they were saying
2092.59 -> we're gonna run this at 1,100 Celsius
2094.75 -> then I started studying material science
2096.94 -> and I was like yeah nothing wants to be
2098.47 -> at 1,100 Celsius by that time they
2100.869 -> downgraded it to a thousand now they've
2102.94 -> asked some toda at around 800 850 due to
2106.27 -> some actual problems in operating things
2109.03 -> in helium it's not the helium itself but
2111.85 -> the impurities in the helium that could
2113.29 -> really mess
2113.71 -> you up and the sorts of alloys that they
2116.26 -> need to get this working these nickel
2119.05 -> super alloys like alloy 230 they can
2121.96 -> slightly carburized ich arbor eyes
2124 -> depending on the amount of carbon and
2125.32 -> the helium coolant either way you go you
2127.839 -> lose the strength that you need so I'll
2131.47 -> say feasibility is low to medium because
2133.42 -> well I haven't really seen one of these
2135.04 -> yet then onto water-cooled reactors has
2138.73 -> anyone heard hear heard of the reactors
2140.2 -> they have in Canada the CANDU reactors
2143.29 -> that's my favorite acronym hope that was
2145.93 -> intentional is what yeah it's not like
2150.46 -> the well they're not sorry about
2151.99 -> anything but whatever
2153.28 -> at any rate one of the nice features
2155.68 -> about this is you can actually use
2157.63 -> natural uranium because the moderators
2160.9 -> heavy water you have to look into what
2163.15 -> the sort of cross-sections are even
2164.83 -> though deuterium won't slow down
2168.67 -> neutrons as much as hydrogen will or my
2171.339 -> alpha thing oh it was right here all
2172.81 -> along even though a is two instead of
2175.45 -> one for deuterium its absorption
2177.97 -> cross-section or specifically yeah
2179.56 -> because it doesn't fish in its
2181.39 -> absorption cross-section is way lower
2183.67 -> than that of water
2184.599 -> so you actually it actually functions as
2187.21 -> a better moderator because fewer of
2188.89 -> those collisions are absorption and
2191.2 -> because you have a better Neutron
2193.03 -> population and less absorption you don't
2194.98 -> need to enrich your uranium you also
2197.109 -> don't need to pressurize your moderator
2199 -> so you can flow some other coolant
2201.849 -> through these pressure tubes and just
2204.07 -> have a big tank of close to something
2206.38 -> room temperature unpressurized e2o
2208.63 -> as your moderator problem with that is
2211.83 -> d2 is expensive anyone priced out
2216.359 -> deuterium oxide before probably have it
2220.359 -> the reactor because I know you have
2221.349 -> drums of it
2227.25 -> a couple thousand a kilo it's an
2229.17 -> expensive bottle of water
2230.52 -> it'll also mess you up if you drink it
2232.73 -> because a lot of that even if it's you
2234.9 -> know crystal-clear filtered d2o
2236.67 -> a lot of what if sell your machinery
2238.95 -> depends on the diffusion coefficients of
2241.26 -> various things in water those solutes in
2244.26 -> water and if you change the mass of the
2246.9 -> water than the diffusion coefficients of
2249.24 -> the water itself as well as the things
2250.86 -> in it will change and if you depend on
2253.17 -> let's say exact sodium and potassium
2254.84 -> concentrations for your nerves to
2256.56 -> function a little change in that can go
2258.69 -> a long way towards giving you a bad day
2260.51 -> and there's actually we have a little
2263.46 -> piece of one of these pressure tubes
2264.75 -> upstairs if anyone wants to take a look
2266.43 -> there's all these sealed fuel bundles
2268.64 -> inside what they call a calandria tube
2271.2 -> just a pressurized tube that's
2273.33 -> horizontal the problem with some of
2275.46 -> these is if these spacers get knocked
2277.95 -> out of place which they do all the time
2279.45 -> those tubes can start to creep downward
2282 -> and get a little harder to cool or touch
2285.21 -> the sides and change thermal and now
2287.43 -> getting into material signs it's it's a
2289.77 -> mess then there's the old RBMK the
2293.64 -> reactor that caused chernobyl you can
2296.19 -> also use natural uranium or low enriched
2298.95 -> uranium here the problem though that led
2301.92 -> to turn out one of the many problems led
2303.96 -> to Chernobyl was you've got all this
2306 -> moderator right here so if you lose your
2308.04 -> coolant let's say you had a light water
2310.26 -> reactor and your coolant goes away your
2312.6 -> moderator also goes away which means
2315.33 -> your neutrons don't slow down anymore oh
2318.72 -> that one reaction is messing up there we
2321.9 -> go which means your neutrons don't slow
2323.94 -> down anymore which means the probability
2325.95 -> of fission happening could be like
2327.92 -> 10,000 times lower so losing coolant and
2331.5 -> a light water reactor might temperature
2333.75 -> might go up but it's not going to give
2334.8 -> you a nuclear bad day in the RBMK
2337.41 -> reactor it will and it did and in
2340.53 -> addition the control rods which was
2343.11 -> supposed to shut down the reaction made
2345.45 -> of things like boron for carbide or
2347.49 -> hafnium or something with a really high
2349.67 -> capture cross-section we're tipped with
2352.89 -> graphite to help them ease in so you've
2355.38 -> got moderator tipped rods which induce
2358.2 -> additional moderation which helps slow
2360.93 -> down the
2361.319 -> neutrons even more to where they fission
2364.319 -> even better and that's what led to
2365.88 -> what's called a positive feedback
2367.14 -> coefficient so the more you tried to
2369.569 -> insert the control rods and the more you
2371.039 -> tried to fix things the worse things got
2372.749 -> in the nuclear sense and in something
2375.299 -> like a quarter of a second the reactor
2377.759 -> power went up by like 35,000 times and
2380.489 -> we'll do a millisecond by millisecond
2382.229 -> rundown of what happened in Chernobyl
2384.239 -> after we do all this Neutron physics
2387.18 -> stuff when you'll be better equipped to
2388.319 -> understand it but suffice to say there
2390.599 -> were some positive coefficients here
2392.549 -> that are to be avoided at all costs in
2394.529 -> all nuclear reactor design and the
2398.549 -> actual reactor Hall you can go and stand
2400.289 -> on one of these things very different
2402.9 -> design from what you're used to I don't
2404.099 -> think anyone would let you stand on top
2405.66 -> of a pressure vessel first your shoes
2407.519 -> would melt because they're usually at
2409.17 -> like 300 Celsius or so and second of all
2412.049 -> you probably get this a little too much
2413.64 -> radiation but this is actually what an
2415.559 -> RBMK reactors all looks like for one of
2418.829 -> the units that didn't blow up there were
2421.38 -> multiple units at that site then there's
2424.229 -> the supercritical water reactor let's
2426.449 -> say you want to run at higher
2427.559 -> temperatures than regular water will
2429.9 -> allow you to you can pressurize it so
2432.479 -> much that water goes beyond the
2435.239 -> supercritical point in the phase sense
2437.219 -> and starts to behave not like a slick
2439.559 -> wicked liquid not like a gas but
2441.809 -> somewhere in between something that's
2443.699 -> really really dense so getting towards
2446.489 -> the density of water not quite which
2448.44 -> means it's still a great moderator but
2450.239 -> still can cool the materials quite well
2452.249 -> to extract heat to make power and so on
2454.739 -> and so on yeah
2460.54 -> ah good question for a supercritical
2464.02 -> water reactor it most definitely refers
2466.24 -> to the coolant it's the phase of the
2468.79 -> coolant words beyond the liquid gas sort
2471.64 -> of separation line and it's just
2473.65 -> something in between any of these
2475.66 -> reactors can go supercritical where
2478.03 -> you're producing more neutrons than
2479.44 -> you're consuming and that is a nuclear
2481.69 -> bad day but the supercritical water
2484.21 -> reactor does not refer to neutron
2486.01 -> population just a coolant good question
2489.07 -> it's never come up before but it's like
2491.88 -> should've thought of that and so then my
2494.89 -> favorite liquid metal reactors like LBE
2498.28 -> or lead bismuth eutectic it's a low
2501.49 -> melting point alloy of lead and bismuth
2503.56 -> lead melts at around 330 Celsius bismuth
2507.49 -> 200-something
2508.48 -> put them together and it's like a low
2510.76 -> temperature solder it melted 123 point 5
2513.22 -> Celsius you can melt it in a frying pan
2515.31 -> this is nice because you don't want your
2517.66 -> coolant to freeze when you're trying to
2519.85 -> cool your reactor because imagine that
2522.1 -> you something happens you lose power the
2524.62 -> coolant freezes somewhere outside the
2526.15 -> core you can't get the core cool again
2527.98 -> that's called a loss of flow accident
2530.35 -> that can lead to a really bad day and
2531.76 -> the lower your melting point is the
2533.56 -> better sodium potassium it's already
2536.53 -> molten to begin with sodium melts at
2538.45 -> like 90 C and when you add two different
2541.06 -> metals together you almost always lower
2543.31 -> the melting point of the combination in
2546.25 -> this case forming what's called the
2547.39 -> eutectic or a lowest possible melting
2549.4 -> point alloy so the sodium fast reactor
2553.75 -> has a number of advantages like you
2555.94 -> don't really need any pressure as long
2558.04 -> as you have a cover gas keeping the
2559.63 -> sodium width from reacting with anything
2562.15 -> like the moisture in the air or any
2564.07 -> errant water in the room you can just
2566.32 -> circulate it through the core and liquid
2568.81 -> metals are awesome heat conductors they
2571.51 -> might not have the best heat capacity as
2573.52 -> in how much energy per gram they could
2575.8 -> store like water but they're really good
2577.87 -> conductors with very high thermal
2579.4 -> conductivity they also are really good
2581.89 -> at not slowing down neutrons so these
2584.86 -> tend to be what's called fast reactors
2586.81 -> that rely on the ability of other
2589.96 -> isotopes of uranium like uranium 238 to
2593.14 -> undergo what's called fast
2594.43 -> vision and I want to show you what that
2596.65 -> looks like let's pull up u-238 and look
2600.91 -> at its fission cross-section and you
2603.73 -> might find it should look a fair bit
2605.74 -> different so we'll go down to number 18
2609.52 -> to fission cross-section very very
2617.77 -> different so u-238 is pretty terrible at
2622.21 -> fission at low energies it's pretty good
2625.39 -> at capturing neutrons this is where we
2627.28 -> get plutonium 239 like you guys saw in
2629.74 -> the exam but then you go to really high
2631.72 -> energies and all of a sudden it gets
2633.42 -> pretty good at undergoing fission on its
2636.91 -> own and so the basis behind a lot of
2639.79 -> fast reactors is a combination of making
2642.01 -> their own fuel and the fact that uranium
2644.65 -> 238 fast visions even better than at
2648.01 -> thermal visions so something good for
2649.99 -> you to know even though it's not a
2651.13 -> fissile fuel that's light water reactor
2653.859 -> people talking you can get it to fission
2656.589 -> if the neutron population is higher now
2659.98 -> there's some problems with this it takes
2662.349 -> some time for neutrons to slow down from
2666.25 -> 1 to 10 MeV to about 0.025 Evie if your
2671.98 -> neutrons don't need to slow down and
2673.809 -> travel anywhere it pretty much all they
2675.73 -> have to do is be born and absorbed by a
2677.619 -> nearby uranium atom the feedback time is
2681.069 -> faster in these sorts of reactors
2683.17 -> they're inherently more difficult to
2684.76 -> control and you can't use normal physics
2687.73 -> like thermal expansion of things that
2690.25 -> might happen on the order of micro to
2691.839 -> nanoseconds if it takes less time than
2694.15 -> that for one Neutron to be born and find
2696.67 -> another uranium atom you can still use
2698.98 -> it somewhat but not quite as much so
2701.65 -> it's something to note backed up by
2703.809 -> nuclear data that's what one of them
2706.839 -> actually looks like these things have
2708.369 -> been built that's a blob of liquid
2710.74 -> sodium on the Monju reactor in Japan and
2713.71 -> where I was all last week in Russia they
2716.29 -> actually have fleets of fast reactors
2718.48 -> they're bien 300 and bien 600 reactors
2721.839 -> are 3 and 600 megawatt sodium cool
2724.569 -> reactors one of them in the Chelyabinsk
2727.24 -> region they used for
2728.17 -> much for desalination down in the center
2730.63 -> of Russia where there's no oceans nearby
2732.579 -> and probably dirty water they actually
2735.28 -> use that to make clean water they also
2738.94 -> use this for power production and for
2740.559 -> radiation damage studies so when it
2743.41 -> comes to radiation material science
2745.75 -> these fast reactors are really where
2747.67 -> it's at yeah you just notice the bottom
2751.54 -> I went to Belgium to their National
2754.54 -> Nuclear labs where they have a slowing
2756.28 -> sodium test loop it's not a reactor but
2758.619 -> it's like a thermal hydraulics and
2759.94 -> materials test loop and I asked a simple
2762.04 -> question where is the bathroom
2763.9 -> and they started laughing at me they
2766.809 -> said we're not putting any plumbing in a
2768.49 -> sodium loop building you'll have to go
2770.98 -> to the next building over and that's
2773.079 -> when I noticed there weren't any
2774.069 -> sprinkler systems or toilets but every
2776.26 -> 15 or 20 feet there was a giant barrel
2778.75 -> of sand that's the fire extinguisher for
2780.97 -> a liquid metal fire is you just cover it
2783.369 -> with sand absorb the heat keep the air
2785.26 -> out of the moisture out wick away the
2787.75 -> moisture whatever else and does I don't
2789.609 -> know but you can't use normal fire
2792.339 -> extinguishers to put out a sodium fire
2797.39 -> ah I don't know if that would work guess
2802.55 -> it's worth a shot with glasses and
2805.67 -> safety and stuff of course and the ones
2807.92 -> that I spent the most time working on
2809.33 -> like I showed you in the paper yesterday
2810.65 -> is the LED or LED bismuth fast reactor
2813.29 -> this one does not have the disadvantage
2815.87 -> of exploding like sodium it does have
2818.6 -> the disadvantage like I showed you
2819.77 -> yesterday of corroding everything pretty
2822.2 -> much everything and so the one thing
2824.51 -> keeping this thing back was corrosion
2827.06 -> and I say the outlet temperatures medium
2828.95 -> but hotter soon hopefully someone picks
2831.23 -> up our work and like yeah that was a
2832.82 -> good idea because we think it can raise
2834.65 -> the outlet temperature of a lead bismuth
2836.93 -> reactor by like a hundred Celsius as
2839.24 -> long as some other unforeseen problem
2841.04 -> doesn't pop up and we don't quite know
2842.87 -> yet these things also already exist in
2846.17 -> the form of the Alpha class attack
2848.81 -> submarines from the Soviet Union these
2851.81 -> are the only subs that can outrun a
2853.25 -> torpedo so you know that old algebra
2855.83 -> problem if personated leaves pittsburgh
2857.96 -> at 40 miles an hour and person B leaves
2859.76 -> Boston at 30 miles an hour
2861.23 -> word of the trains collide or I forget
2863.84 -> how it actually ends well in the end if
2866.51 -> a torpedo leaves an American sub at
2868.94 -> whatever speed and the alpha class
2871.25 -> submarine notices it how close do they
2873.32 -> have to be before the torpedo runs out
2875.66 -> of gas so what I was told by the
2877.82 -> designer of these subs fellow and by the
2880.79 -> name of George Ito schinsky
2882.11 -> when he came here to talk about his
2884.36 -> experience with his lead bismuth
2885.62 -> reactors is there is a button on the sub
2888.2 -> that's the forget about safety it's a
2890.9 -> torpedo button because if you are have a
2893.6 -> if you're underwater it'll lead bismuth
2895.49 -> reactor and a torpedo is heading at you
2897.29 -> you have a choice between maybe dying in
2900.56 -> a nuclear catastrophe and definitely
2902.36 -> dying in a torpedo explosion well that
2904.82 -> button is the I like those odds
2907.1 -> button and you just give full power to
2910.49 -> the engines and whatever else happens
2913.13 -> happens the point is you may be able to
2914.66 -> outrun the torpedo and quite popular
2918.17 -> nowadays especially in this department
2920.36 -> is molten salt cooled reactors that
2922.37 -> actually use liquid salt not dissolved
2924.89 -> but molten salt itself as the coolant
2927.53 -> that doesn't have as many of the
2929.12 -> corrosion problems as lead
2931.01 -> the exploding problems is sodium it does
2933.92 -> have a low a high melting point problem
2935.99 -> though they tend to melt at around 450
2937.91 -> degrees Celsius but there's one pretty
2940.04 -> cool feature you can dissolve uranium in
2942.59 -> them so remember how in light water
2944.63 -> reactors the coolant is also the
2946.19 -> moderator in molten salt reactors the
2949.1 -> coolant is also the fuel because you can
2951.8 -> have principally uranium and lithium
2953.93 -> fluoride salt Co dissolved in each other
2956.42 -> and the way you make a reactor is you
2958.52 -> just flow a bunch of that salt into
2960.98 -> nearby pipes and then you get less
2964.22 -> what's called Neutron leakage or in each
2966.77 -> of these pipes once in awhile uranium
2968.48 -> will give off a few neutrons most of
2970.58 -> them will just come out the other ends
2971.69 -> of the pipes and you won't have a
2972.71 -> reaction when you put a whole bunch of
2974.66 -> molten salt together
2975.59 -> most of those neutrons find other molten
2978.2 -> salt and the reaction proceeds and it's
2982.01 -> got some nice AIF tea features like if
2983.48 -> something goes wrong just break open a
2985.79 -> pipe all the salt spills out becoming
2989.84 -> some critical because leakage goes up it
2992.06 -> freezes pretty quickly and then you must
2994.43 -> deal with it but it's not a big deal to
2997.34 -> deal with it if it's already solid and
2999.08 -> not critical so it's actually five of
3002.89 -> its 0 of 5 of I'll stop here Tuesday
3007.09 -> we'll keep developing the many many
3009.1 -> different variables we'll need to write
3010.75 -> down the neutron transport equation at
3012.85 -> which point you'll be qualified to read
3014.38 -> the t-shirts that this department prints
3015.94 -> out and then we'll simplify it so you
3017.8 -> can actually solve the equation
3026.45 -> you
Source: https://www.youtube.com/watch?v=RW2DPHAoXiQ