 Good morning students. Last lecture we had a look at the first reactor in the world, Chicago pile which is made by Fermi and his colleagues in Chicago. Then we had a look at the constituents of a nuclear reactor, it should have a core, whether it needs a moderator, then the control rods, then you had the reactor vessel and so on. We also had a look at the different types of reactors, the pressurized water reactor, the boiling water reactor, the fast reactor, sodium cooled fast reactors, the gas cooled reactors which are not now present in big numbers. But then just as we call different generations, even these reactors have now been tagged with generation numbers, luckily the generations are not very many. These generation 1 reactors were those reactors which are developed in the very initial days like maybe the Chicago pile, the shipping port reactor in the USA and some of the initial reactor test reactors in the different countries. Then came the generation 2 reactors, the PWR, the BWR which are mostly in operation. Then the generation 3 reactors with some advanced features over the generation 2 reactors have been built in France and Japan till many are going to be built in the next decade. But now all the countries have felt that there is a very imperative need to consolidate the efforts of the research towards particular designs instead of focusing on many many many designs which have been reported, why not to come to your particular designs and then do more research on them so that it economics also would be nice, you know people could contribute to the same. So that is called as the generation 4 reactor which also called the Gen 4 and is having different member countries in this Gen 4 reactor forum. Similarly I was showing you the different generations. Generation 1 as I mentioned include the gas cooled Magnox reactors or the advanced gas cooled reactors. Then the shipping port reactor which is was a pressurized water reactor. Then the Dresden reactor is a boiling water reactor. Then you see in the beyond the 60s you had the generation 2 most of them pressurized water reactors and boiling water reactors. These 2 are generally also called as light water reactors because they use light water ordinary water as the coolant and the moderator. Of course then comes the Kandu reactor or the heavy water reactors. As I mentioned in my last lecture, heavy water reactors are cooled by heavy water and moderated by heavy water. Main advantage of heavy water is you can use natural uranium with heavy water but if you want to use light water you have to go with enriched uranium. Then the advanced gas cooled reactors which are improvement over the Magnox reactors. Then there I mean some evolutionary designs which have called as the generation 3 which took place in the 1990s. The advanced water reactor then the AP600 and 1000 reactor designs and of course the European pressurized reactor which is again was an evolutionary design. It is supposed to be about 1600 megawatt electrical and one of them is under construction in Europe. Then of course here some more technology developments have taken place. So we have a sub-module generation 3 plus, they have got the pebble bed modular reactor development in Africa and then IRIS is another type of reactor design which has been developed. And what is the focus of these generation 4 reactors? Same thing is economics but enhance the safety of course to objectives, not that safety is not costly but safety need not be made excessively costly. Then what else? You must have minimum waste production, that is another requirement of this generation 4. Then this proliferation, you always hear about this word proliferation. We have talked about proliferation of nuclear weapons, proliferation, what is proliferation? The fear is that some of the radioactive material or plutonium could be taken out and people could make bombs with that. So proliferation of nuclear weapons is one which is not desired. So that is should be proliferation resistant. So how you design? Then the main thing is how do you go with the proliferation resistant is not that you do not build reactors, of course you build reactors, you will generate plutonium but you see that there is minimal transport, anything can happen when it is getting transported from one place to another. So this generation 4 reactors aim at a site in which you have the reactor, you have the reprocessing plant, you have the fuel fabrication plant, you have the waste treatment plant, the fabricated fuel again goes into the reactor. So it is in a single place. So that is one type of proliferation resistance. Now let us look at the generation 3 reactors which are mostly present today, the PWR, the BWR and the Kando PHWRs. What are the main features of this third generation? Main to reduce capital cost has been one approach and reduce construction time. This construction time has a very important economic aspect. You might be aware that when you build a plant you take money from the consortium or the government, in India the government makes the money and this money you have to pay interest. If suppose you build a reactor in about 10 or 15 years it have to pay the interest on that amount. So that will surely reflect on your cost of the electrical power that you are going to generate. So what? So it has an impact on the interest during construction normally called as IDC. So this IDC is a very important factor to be considered in the economics and that is the reason why we are looking for short construction times. Of course not losing the quality in the construction but still you take approaches such that the construction time is less. So this requires a very good planning. The procurement activity should be such so that the equipment arrive at the right time so that your construction time, erection time everything is well matched. Then more rugged design, rugged design means it should be able to withstand the effect of different events. Let us say we have a power failure or a loss of coal and or a leak in all such conditions it must be able first this should not happen, even should not happen and should an event happen they should be easily you know can be easily handled. So the main thing is minimize the number of incidents so that they are not the design should be such that they are not vulnerable to such things. And another thing is which we look forward is whether we can increase the life to a larger number of years. Now if you look at the present reactors we had operating history for about 30 to 40 years. Now during this period we do have come about the effect of irradiation on materials. So how it affects and the irradiation does produce voids in the material and that can create the material weak. So there is need to develop new materials all these things have taken place in the last 2-3 decades. So the idea is that if I can use materials which can have long life under irradiation atmosphere whether I can use fuel again which is a longer life like that all these aspects we should we are bringing in into the designs. So the present designs with these improved features is still going on. Then other things we have we talk about we talked about internal events like pump trip etc. There could be external events like a bomb or an aircraft crash. So all these things do have an impact on the safety. So basically the present directions in the generation 3 reactors has been to improve all these factors and they already have some sort of passive features or inherent or intrinsic safety features to minimize the accidents which can happen due to malfunction. Now what is this passive or inherent safety? This requires a bit of clarification. Now examples of the generation 3 the wasting house not generation 3 to generation 3 plus the wasting house AP600 600 megawatts then the advanced boiling water reactor by General Electric. General Electric and Wasting House are the big you know companies in USA. Then the EPR EPR is by the French areva so they are supplying and you might recall that the Jaithapur reactor is basically a European pressurized reactor and to be supplied by areva that was is proposed to be built in the in Maharashtra, Jaithapur is in Maharashtra. Now the Kandu group or the Canadian atomic energy of Canada limited they have developed another reactor design Kandu 6 this is also having a good number of features then but out of all this ABWR is in no operation today mostly in Japan there is a so that is what is the status of the generation 3 plus. As I mentioned the word passive safety inherent safety needs some clarification because unfortunately the they have been used too much without the correct meaning and to be frank with you when people are talking this is inherently safe nothing is inherently safe for everything it is not that suppose say inherently safe I will walk in the middle of the road and the car come no nothing can be made inherently safe. So this international atomic energy agency the main body which makes the rules for design and safety of nuclear reactors they had a meeting just to clarify what are all this. So I have just taken these from there inherent safety characteristic is the safety achieved by the elimination of a hazard by proper choice of material and design okay. Now your material is prone to corrosion you have an improved material so with reference to the corrosion that is inherently safe but suppose you do not provide sufficient thickness and the pressure is more and it fails you cannot say it is so the inherent safety is something like intrinsic safety for feature of a particular design for a particular event this is very again everything for a particular event. Then what is a passive component of course the name passive itself you can say inactive so passive component is a component that does not need an external input or external energy to operate a very simple example now you have pumps which are driven by motors when the power supply is lost the motor loses motor starts reducing speed the pump will reduce speed and the flow will come down. Now but suppose I want to prolong the flow that is I want to have more still more flow as much time as possible I can have flow then there is a method you put a flywheel with a good amount of mass on the same shaft as the motor and the pump now when the power supply goes you who still have the stored energy of the flywheel and this can supply the energy to the pump and keep it running for quite some time. This is a sort of a passive component and a passive way we can deal with it then what is a passive system surely is a system which compose entirely of passive components or it could also mean use of certain active components you can wonder why this bit of addition of a certain active components now finally you have about 10 or 15 components one of them may be required to start the action very little energy but start the action you require. So that is why this active component is there but once that active component has triggered that is all then afterwards everything is passive things go on smoothly by itself that is called as a passive system. Then the other term which we call is a grace period now what is this grace period grace means you understand very well that okay gives you some time but what is that time you give this is the time in which your safety function is assured without necessity of the operating personnel in the case of an event in other words the operator is not acted but so it gives some time for the operator to act this grace period is a very very important thing which we keep in mind in the nuclear reactor designs. Now let us say some event is happening the operator gets lot of alarms in the control room so many signals and he has to operate he has to do the next step but as a human being he requires certain time to grasp the situation and take surely there are rules available but it does take some time suppose you are able to design a reactor which gives sufficient grace time that means the operator need not be in a hurried manner hurry sometimes makes can make a wrong decision. So how much grace time it gives is a very important factor as I talked to you about the flywheel the flywheel is able to give him some time because the coolant flow is reducing slowly so the temperatures won't rise fast. So this is called as a grace period. Now just to give some examples of passive heat removal systems which are there in the reactors here this is a loss of power has happened and the core decay heat needs to be removed so here you can see the steam generators and the heat going on to a exchanger above which atmospheric air is flowing this is in a passive manner and that heat is going. You see the pump has stopped but the flow is occurring because here natural air flow over these exchanger removes the heat once that heat is removed the cooler sodium comes gets back into the core again gets heated and then so this is one natural circulation path created by the another natural circulation here. So this is a passive system in the absence of power supply then passive hydrogen recombiner you might have heard that in the Fukushima accident there are some explosions it was not a nuclear explosion it was a hydrogen explosion. You know that hydrogen concentration in a particular area if it becomes more than 4% it catches fire so normally hydrogen should not be allowed to get stagnated and how this hydrogen is produced in the case of Fukushima what happened that the coolant flow was lost when the coolant flow was lost whatever coolant was remaining it started boiling and zirconium clad which was around the fuel element has a reaction with the clad at higher temperatures and the temperature went up the cooling was not there this whatever was the pool water that started reacting and that started producing hydrogen zirconium hydroxide and hydrogen this hydrogen came out because the primary containment itself was breached and they got into the containment building and the concentration was such that it became caught fire and it exploded and that is the reason why the roof came off. In all our reactors we have what is called it is we designed such that hydrogen in any compartment should not become and should it become also we do not allow it to become we have hydrogen recombiner by which hydrogen is mixed with oxygen and then it will become water. Now this should happen in a passive manner and we do have such hydrogen recombiners in the reactors we have two boiling water reactors similar to Fukushima design in Tarapur we need one and two or that boiling water reactors but we have these hydrogen recombiners there unfortunately in the Fukushima reactor these recombiners were not in operation. Then another passive feature which can be very easily used for decay heat removal or even shutting down the reactor you have control rods you want to shut down the reactor you drop the control rods inside the core it absorbs the neutrons and doesn't allow the a self-sustaining chain reaction fission reaction so the reaction stops but of course still the decay heat is there. Now you have as a diverse method you have a tank of water containing boric acid boron you know is an absorber and if I can flood the core with this flow from a tank maybe through a check valve or a non-return valve which can open on a very small pressure difference I am assured of a shutdown this is you can say a passive shutout. Now this figure shows you how a passive shutdown is getting achieved from the boric acid tank a quick acting valve injects high concentration of borated water into the primary coolant and here you remember this pump is driving is getting driven because of the inertia of the flywheel in a similar way this tank gravity tank can be used for dowsing the system with water and seeing to it or compensating for the loss of coolant so that your temperatures don't go high and decay heat gets removed to certain extent. Now let us come to the generation 4 this initiative as I mentioned was by a group of countries they got together and in the year 2000 they set up the generation 4 international forum. So which would comprise not only the reactor designers the fuel all the fuel cycle facilities as I mentioned all fuel cycle facilities in a single location is one of the objectives of the generation 4 and the idea was to deliver react or design reactors which will be safer which will be economical which will produce less waste not you know what you call able to proliferate the proliferation resistant so as compared to the present reactors now these reactors are still in the evolutionary stage so you can say that till about 2030 or 40 still we have the generation 3 and generation 3 plus reactors filling this gap. So in this in the interim period our current 3 3 plus reactors will continue. Now let us look at the concepts which have got evolved one is the gas cooled reactor the gas cooled reactors earlier have had a reasonably good experience but they had some problems with material technology basically the Magnot's reactor and the advanced gas cooled reactors built both in UK and France and subsequently some high temperature gas cooled reactors have also been built one or two and then some experience has been gained and with all around research and developments in the material technology it is felt that gas cooled reactors could be a very good approach for nuclear power generation in the gen 4. Now all this time we were telling that we could think of a steam production to run a turbine but you also could have a gas turbine the gas cooled reactor could be used to run a turbine directly and that way you can produce power or you could exchange it heat to water in a steam generator and then produce steam so there are different ways of doing it. Then what are the other concepts the let cool reactor. Now the using of this let cool reactor again the gas cooled reactor which I mentioned is a fast reactor and not a thermal reactor why that point of a gas was felt needed because the sodium cooled fast reactors which are present they have a problem of in case of a sodium leak or a sodium water reaction they can produce damage in the steam generator and in case if they leak out sodium leaks out it can catch fire. So people said why not we change so one was gas other was lead because lead is not having that much of a reaction with water or air so it is one of their preferred but mind you lead itself has got a high boiling point high melting point and or something like about 250 to 300 degree centigrade and then but a large boiling point you can go to very high temperatures. But then these two were also as a in the fast reactor spectrum these two were the concepts which are arrived at but the sodium was not left off because sodium per se the operation of the reactors has been reasonably good and it does not cost you know cost us very bad problems because of sodium cooled fast reactors these are surmountable. So you see there are three fast reactors spectrum reactors the gas cooled reactor the lead cooled reactor and the sodium cooled reactor. One more reason why the lead cooled fast reactor found its place lead cooled reactors have been used by the Russians in their submarines and lot of knowledge base from the Russians is available today and that also would come into the generation for forum because of which this lead got a place. Now if you look up the gas cooled reactors the temperatures as much as about 850 degree centigrade we are able to reach with about lead and lead bismuth we are able to go to about 550 to 800 then the with sodium cooled fast reactors of course we are able to go to about 550 degree centigrade the other one is the molten salt reactor. This molten salt reactor to be frank with you lot of work was done in ORNL in USA on molten salt reactors. At the same time in the 60s when the sodium cooled fast reactor development was taking place the molten salt reactors in the molten salt reactor the fuel is itself is a salt in a molten state the coolant the fuel everything is mixed together and then getting pumped. So this concept was there and there is a molten salt reactor experimental reactor was built in the ORNL it was functioning but it so happened that when they had to take a decision whether to go for a sodium cooled reactor or a molten salt reactor the sodium lobby had gained enough experience and was able to put up an economic case and then the sodium cooled fast reactors took the plunge. But it has been felt now that that from a breeding point of view as a breeder molten salt reactors are very good. So there is an interest events in having molten salt reactors as I mentioned uranium fluoride in salt and then you have the super critical water cooled reactor. The super cooled water critical reactor interest comes basically after you have got super critical boilers which are used in the fossil fuel plants what do you mean by the super critical. Now as you boil water at a particular pressure you have to give latent heat to convert it into steam as you go up in pressure the latent heat comes down and at a point which is called as a critical point there is the phase changes there is no phase change there is no two phase just changes to water to steam and you are able to go to high temperatures with high temperatures you are able to produce steam at high temperatures and you are able to have a better efficiency of the cycle that is the recent result and very high temperature reactors again based on the gas cooled reactor concepts it has been there we can go to high temperatures and this gives you and remember four of them are using closed fuel cycle or you can say five of them and the last one is an open fuel cycle. So you see that closed fuel cycle has been a preferred one in the generation four. Now let us get a brief idea about okay what are all this. This is the reference gas cooled fast reactor system wherein you have the core here and the controlled rods are from down operated from down in the bottom of the vessel the gas picks up the heat gas is helium goes to a turbine rotates it and this is a gas turbine and it generates power here you can see it is something like a boiling water reactor directly the gas goes cools and comes back. So this gas cooled fast reactor concept is one of the concepts and this Brayton cycle theoretically Brayton cycle can go to as much as 65% efficiency but concerning losses it is felt that we could come to about 48% and as I mentioned in my last lecture the actinides get very well burnt in these fast reactors. So waste is getting limited then the lead cooled fast reactor surely being a fast reactor this also gas got a very good actinide management so that actinides produced use produced in the light water reactors are utilized and broken into short lived and so you are able to have lesser waste lesser activity. Now here you have the reactor core lead is the coolant these are the controlled rods and here you have an heat exchanger wherein there is a gas flow and this gas again goes through a Brayton cycle and produces electricity. So basically it is a lead cooled reactor but heat is taken by the gas and then runs the turbine. So this idea of this sort of a reactor is that we are not using sodium but mind you lead itself per se has some issues to resolve basically corrosion. Many of the corrosion you know in nature happens because of oxygen you put a any iron or thing on the outside it reacts iron reacts with oxygen and form rest so iron oxide. Similarly you have sodium with higher oxygen content will cause corrosion. Oxygen is one of the important things for corrosion. So we must minimize the amount of oxygen in a coolant but now in the case of lead it has got a funny thing above a certain level it has more corrosion similarly below a certain level of hydrogen also it has got more corrosion. So very narrow band of oxygen control is necessary unlike sodium. So here lot of research is getting on into how to do that and how to see materials which will be able to. So lead cooled fast reactor does have some issues. One thing Gen 4 does not mean that all issues are resolved. Gen 4 has got but which areas we must probe and concentrate our efforts then you have the molten salt reactor that molten salt flows through here this is the core these are the control rods and you have the graphite channels. So the molten salt goes through that and then there is a intermediate heat exchanger and here you may have again like another fluid like lead which can exchange heat with a gas which can run a gas turbine and produce electricity okay or you could have replace it by a steam generator it could have a lead secondary coolant giving heat to water producing steam and running a steam turbine. And here the main advantage of these molten salt is you do not need to fabricate the fuel again you are just pushing fuel into the chamber and then that is all it continues it is not a any special fabrication plant to be made and then put. So it just allow us a bit of flexibility in the fuel loading that is fuel fabrication aspect is not required. The sodium cooled fast reactor it is just a replica of the pool type fast bidder reactors which I have shown you. So here you have the primary sodium system which this is the primary sodium which picks up the heat from the core and gives it to IHX this is a pump which pumps from the cold pool to the core it goes to IHX comes out to the pool cold pool again and this is the control rod this heat exchanger contains again sodium fluid it picks up the heat called as a secondary sodium gives heat to water in a steam generator then afterwards again pumped back to the IHX and here you have the steam production and turbine running a conventional steam cycle. So remember the old sodium cooled fast reactor is still not left out it is a important candidate for the generation 4. So now this actinate management I mentioned but let us just elaborate it a little bit actinates are elements with atomic numbers between 89 and 103 89 is actinium and 103 is Laurentian uranium you know has an atomic number of 92 it is also an actinite and normally beyond 92 there is another name we give transuranics beyond uranium transuranics. Now these actinates have a very long half life that means they will decay very slowly have you saw plutonium having uranium plutonium having long half lives potassium also having a long half life so it is very uranium also has so these are all very important that they have long half lives and when they have long half lives if they are in the waste they also have to be handled with care for a longer time and in these light water reactors basically the pressurized water reactor and the boiling water reactor the actinates are produced but not consumed and here the way the in the fast reactors because of the fact that these actinates can be broken up some of them can be fissioned and they break up into smaller or lesser half life elements they can really be a very good impetus for the fuel handling because they are going to produce less waste now this conversion of these high or longer half life elements into lower thing is actually called as transmutation and this transmutation occurs surely after it absorbs a neutron or then there is a fission this is to give you an example plutonium 242 when it absorbs a neutron becomes 243 then it produce americium then americium absorbs a neutron then gives 244 and then curium then like that samarium curium this thing and then go on finally it gives two fission products and releases energy so release of energy is there similarly for americium how the breakup is happening these are just to give you an idea that they can be used for fission and also the fission products which have lower half life so this is to tell you about the waste minimization as I mentioned suppose you do not use fast reactors you have just an open fuel cycle see how the actinides will decay to natural uranium over level it will take many years about 100,000 years but the same thing if you had used it in a fast reactor and close you reprocess it and use it in a fast reactor it comes down below within about 100 to 200 years so there is a very great incentive to go for this sort of an approach and basically fast reactors are better because it has got a higher fission to absorbs on cross section so more chance of fission in the material compared to absorption and the final waste is reduced and the radioactivity the activity which is related to that waste is also reduced and that waste is reduced the space is reduced finally when you are going to put the waste in dispose it in some place that is also requires less space this is a super critical water reactor as I mentioned it generates steam at very high pressures and high temperatures and this technology is a combination of the light water technology with supercritical fossil fuel boilers and efforts are still of course in India still we have to build a supercritical boiler this is just to give you a quick idea about how the cycles are this is a Rankine cycle which is normally used in power plants the water is pumped sent to the boiler picks up the heat becomes superheated becomes steam works in the turbine high pressure turbine then comes out again reheated in the steam boiler and then comes out and then goes this is if this shows you a Mollier's chart which is the shows the properties here this is the water going up becoming saturation taking the latent heat and becoming superheated now you see at this point which corresponds to about 221 bars and 373 degree centigrade there is no latent heat it is 0 just water it just becomes into steam so this is what we are talking about this is the supercritical below this is the subcritical so these are the basic differences between a normal subcritical boiler or a subcritical operation and the supercritical operation this technology we are going to use for the supercritical water reactor and now this is the cycle how the supercritical cycle water entry pumped in that is all becomes it becomes steam then expansion in the high pressure turbine coming out expansion then again heated up to the higher temperature again coming back and do this reheating of course is there in fossil fuel boilers reheating in nuclear reactors is not done because the liquid has to go back into the reactor so we may have only this portion this is again another figure now since supercritical is there that means there is no distinction between water and steam it is just a same then last is a very high temperature reactor this temperature is aimed at producing this reactor is aimed at producing both electricity and also hydrogen as I mentioned hydrogen economy is one of the most important hydrogen generation if I can store I can use it for transportation and many other things so this is a helium cooled graphite moderator thermal reactor and the process heat also could be used for other applications like coal gasification desalination and cogeneration also this again is in the stage and here you see the gas cooled helium cooled reactor it is exchanging heat with water producing steam at high pressures and this is a schematic of a hydrogen plant fuel cells as the name mentioned basically it uses specialized hydrogen gas where the hydrogen enters on the anode side gas is forced through the catalyst by the pressure and the hydrogen molecule comes in contact with the platinum catalyst it splits up into hydrogen and two electrons and the electrons are conducted to the anode and they go to the complete the circuit and that way the fuel cells work and here this hydrogen generation is where our gen 4 reactors would play a very important role and as I mentioned in the transportation sector hydrogen has got a very very important role in summary this lecture we have briefly brought out you know in the last lecture we brought out the different components in the reactors and the major types of reactors and now we have also seen how the different generations of the reactors have got evolved and what is our final focus now on the generation 4 reactors to make them more safe more economical but these reactors will really take shape after about 2 decades and all our present idea is to use as many passive safety features for shutdown and decay heat removal in these new generation reactors this is to give you a bibliography of the papers which you can see and as my usual practice I would like to give you an assignment which you can go through and maybe submit it in the next time thank you.