 Good morning students, in the last lecture, we talked about passive safety and in this lecture we will continue with some more features. Just to recapitulate what you heard in the last lecture, I introduce you to the difference between passive safety and active safety. In passive safety, active components like based on mechanical energy or based on electrical energy are not involved and the process happens through natural loss. I had also given you that an absolute passive system is difficult because you do require some energization, some signal at some point. So this has led to the classification of different types of categories of passive systems starting from the most passive to the minimum passive and this basically helps in assessing the reliability of the system. Now, when we talk about safety, I mentioned to you that we are talking about two important things in the safety of a nuclear reactor. One is the shutdown, that is the reactor must be shut down in case of any event. Similarly, the decay heat removal system must be there, so you must have some coolant which could pass through the core and take the heat out of the core to another environment. So here last time we saw some of the shutdown systems and also the decay heat removal systems which were acting in a passive manner. Now today if I look up, we will talk about the passive safety systems for containment cooling. Now why is containment cooling essential? You know in case of a loss of coolant, let us say a pipe is ruptured, the water has come out and it flashes into steam inside the containment. So there is a increase of pressure and heat which is being taken to the containment. So the heat if removed from the containment would help in two ways. It will maintain the temperature of the containment, not only that, it will be able to help you in reducing the pressure within the containment. So let us look what are. So there are two types of containment. One is a containment pressure suppression pool. So here basically we are looking for taking the containment cooling whatever is there. So the whole thing whatever which ever cools the containment is taken to a pressure suppression system. The other is the containment passive heat removal and the third is passive containment spray. We just will see the three how they are. We take up the containment pressure suppression pool. This is basically has been used for the boiling water reactor designs for many years. Following a loka as I said the steam is generated in the primary containment that is essentially because the water flashes into steam and then it really pressurizes the thing. So this steam coming out into the containment is forced through lines which are vent lines and they are submerged. If you see here these lines are submerged in a suppression pool. What is a suppression pool? Basically it is a pool of water through which this steam is passed and the steam will condense. The moment steam condenses automatically the pressure will come down in the containment. Here it is not completely passive because there needs to be a system to vent it to the suppression pool. So it is partly a category B and partly a category C passive system as we shall see. Next we talked about containment passive heat removal. Here what we do? We have a tank at a good elevation and this contains a pool of water and here the steam which is vented into the containment actually it condenses on the, this will have a water circuit. The steam will heat up this water in this pipeline. Once the water gets heated up it will reduce in density and it will go up and that will push the water down and here when the water gets cooled it will push the water down. So this way there is a natural circulation setup and the heat which is coming into the containment is getting continuously removed. Of course this was an indirect means by having a heat exchanger. There could be the other means of having the steam directly go through this coil to the pool and then this pool would condense and the condensate would come here to the suppression pool. So essentially as I mentioned earlier for any natural convection your sink of the heat sink must be at a higher level, must be at a higher level than the heat source. So this is a very important point to be kept in mind. Now we go to the passive containment spray system. As the name indicates the passive containment spray means the you are spraying water. So what let us have a look after a loss of coolant accident steam is in contact with the inside surface of the steel containment. This is the steel containment and steam is in contact whatever steam is produced is in contact with this. Now so heat is transferred to this wall. This wall can conduct the heat to the external air here in the space and this external air picks up the heat and goes up. In addition to this there is a pool at the top and this from this pool you have a spray of water. So here basically if you see you have air flow cooling the annulus besides water also. So this way it is able to effectively remove heat and this at the top there is a spray. So the spray needs to be activated you require a signal. So this containment vessel spray could be categorized as a category D passive system while the air cooling could be put in a category B. So you see that this sort of thing is actually has been used in most of the many of the pressurized water reactors and here I just show you what is the passive containment cooling system adopted in the Kodangulam reactor which is in operation in Tamil Nadu. So here exactly as I mentioned there is a air going through and in fact if you look in the actual building there is a lot of annulus space. So atmospheric air is entering like this picks up the heat from the containment and then goes out. So this way it is able to do even when there is a power failure there is no power still you are able to do that means you are not dependent on any electrical power or mechanical power you are able to do by natural convection. So it is not that we are talking in the air it is actually happening and if you go to the Kodangulam power plant you can have a real nice look at it. All of you know that then you have a loss of coolant accident it is quite likely that the clad will get overheated because the heat is not getting removed. So the clad zirconium will react with the steam and at a temperature above 350 to 400 degrees centigrade we saw that it forms zirconium hydride and releases hydrogen also. So this hydrogen should would be there in the containment and we know that hydrogen concentration in an area if it goes more than about 4 to 9% it can catch fire. So one of the important things or which is safety should be regarding how to remove the hydrogen or to see how the hydrogen can be burnt ourselves so that it does not cause an explosion. Now why this point is being talked about you know both in the 3 mile island reactor accident and the Fukushima accident hydrogen was produced luckily for us in the 3 mile island the there was no explosion but in the case of Fukushima there was a hydrogen explosion it was not a nuclear explosion mind you it was a hydrogen explosion. So people have done research and already lot of autocatalytic recombiners have been developed and now since we wanted to act without any of the energy source either electrical or mechanical there has been a development of passive autocatalytic recombiners. Let us see what these are. Now this passive autocatalytic recombiners are basically a means to see that the hydrogen comes in contact with the air so that the hydrogen and oxygen are recombined catalytically. What does this mean? An exothermic reaction occurs at the surface of the catalyst plates when hydrogen and oxygen are present in the atmosphere. The heat of the reaction combined with the vertical arrangement and spacing of the catalyst promotes natural convective flow. I mentioned no if it is a horizontal there cannot be in a vertical way there is a natural convection through the recombiner and warm humid air and the unreacted hydrogen are exhausted. So this is a passive autocatalytic recombiner basically it is platinum put on a charcoal substrate is there and here the hydrogen is coming water and whatever water it becomes water it condenses down. So this sort of passive hydrogen recombiners have been designed and they operate over a wide range of temperature and humidity levels and they are very good for converting hydrogen to water and they are not deactivated because of the flow of water or water vapor and these developments have taken place in Canada and France and today most of the reactors are provided with passive autocatalytic recombiners. Now these recombiners will be mounted onto the concrete and so that and not only that remember in case of a seismic condition this should not fall down because that is the time when you can have a real accident. So and that condition these should so they are also seismically qualified. So it is a passive system and also seismically qualified. So these are important aspects if you just see in safety we look for as absolute safety as possible even though absolute safety is not possible. Now just to give an example of reactor designs which are already developed there is a reactor called as AP600 it is an advanced pressurized water reactor. The original design was 600 megawatt electrical. Now we do have a AP1000 design also. In fact AP1000 design construction is going on for some plants in China. So what does this AP600 passive related safety systems? Let us look at it. It has a passive residual heat removal system as I mentioned based on natural convection and cooling I will give you the details later. It consists of 2 core make up tanks that means it has 2 tanks which can which have got enough water for cooling the core then your 4 stage automatic depressions system. Suddenly what is this depressions system coming? You know these core tanks can put in water into the thing they are at a low pressure there is no force flow. So the pressures will be less so that means you have to depressurize the system then only you can put water into the system. So that is why we talk about automatic depressurizing systems. Then accumulator tanks so that you have enough water inventory. Then not only that you see it contains a refueling water storage tank. This is a very important you must note here refueling water storage tank that means this has essentially got to do with the spent fuel. So the spent fuel as I mentioned to you produces some heat because of the fission product decay. So under the case of that case the spent fuel cooling has to be continuous even though the heat level is less maybe initially about 5% of the full power and then gradually coming down nevertheless this has got to be cooled. In fact in the case of Fukushima accident the spent fuel storage was not cooled it did not get the enough cooling supply that was also one more reason. So they have put in the loop the refueling water storage tank. Then a sum a lower containment sum so that the thing is getting collected and of course not the least the passive containment cooling system which we saw sometime back. Let us see what further here you see the figure of the AP600 core cooling. So this is the refueling water storage tank and this goes you here you have the break. So this refueling water storage tank is normally used to flood the core when it is shut down for refueling and it is also used for emergency decay heat removal. So what is there inside this tank you have a heat exchanger and this heat exchanger is part of a natural convection loop like this like this then like this back. So this is one this is the passive residual removal system and here you have got the depressurization systems to bring down the pressure. Besides thus these are the accumulators which can supply water to the reactor core again to cool. Now this heat exchanger needs to be activated they are all activate that valves are there which are activated by air driven valves. So this gives you a figure of the emergency core cooling as well as the containment cooling. This is the emergency core cooling as we saw it comes from the refueling water storage tank there is a small low pressure injection pump which pumps into the core then you have accumulator injection also here basically what is happened you have gas pressure. The moment the pressure in the reactor comes down as a consequence of reduction of pressure this water also is able to enter. So this way emergency cooling is assured in the AP600. Coming to the containment cooling exactly what we saw about the kurungulum you have got a steel containment the steam comes and this is getting cooled. And you have a gravity driven water tank from where water is also falling on the containment. So this way the heat is getting removed. I can tell you you might wonder why this sort of a cooling this sort of a cooling can help you to remove some of the decay heat not a complete decay heat because the idea is this is actually to protect the containment. This cannot be the main path for decay heat removal. This is to basically protect the containment because if the containment integrity must not be lost at any stage because if the containment integrity is lost then you can have the fission products coming out to the atmosphere. So basically this is to keep the temperature of the containment within limits yeah. So having looked at some of the passive features of the AP600 I would be failing in my duty if I do not give you some ideas about the advanced heavy water reactor which we are developing in India and it is quite gone a quite a long way and we should be able to have the start of the construction in a year or two and this advanced heavy water reactor idea is we have tried to put as many passive safety features as possible. So that is the idea and we have taken the experience of the pressurized heavy water reactors. So what it is this advanced heavy water reactor is a 300 megawatt electrical is a boiling light water cooled and heavy water moderated vertical pressure tube type reactor. This vertical pressure tube unlike our all our PHWRs are horizontal pressure tubes. This is a vertical pressure tube and the core consists of thorium uranium 233 in oxide form and also thorium plutonium in oxide form. So basically we are using the uranium 233 from which is obtained from the other reactors like the pressure water reactors and the plutonium which are getting from the heavy water reactors as well as our fast reactors. Now one of our main idea is how to use this advanced heavy water reactor with thorium breeding with the self sustained thorium breeding. It is something like you put a fuel into the reactor and the thorium the thorium breeds generates fuel and it sustains. So that is our idea. Now here the thorium based fuel is basically advantages because it is a negative wide coefficient of reactivity unlike uranium. Then we also in this advanced heavy water reactor we have passive systems for core heat removal both under normal operating and shadow conditions. Just underline the words under normal operating. So really speaking under normal operation itself we do not have a pump. We have only natural convection. We set up the natural convection slowly and go upon power. Yes the rising of power would be slow but it is a passive. So it is one of the unique designs in the world and it also has passive containment cooling and passive containment isolation. And what are the other features? Direct injection of emergency core cooling system water into the fuel bundle. Then we have an accumulator as we saw other everywhere you needed an accumulator for high pressure ECC. Then we have again a gravity driven water pool at a high elevation because as I said then only the water will be able to flow inside. Let us look at how the AHWR passive core cooling works. As I mentioned natural circulation is used to remove heat from the reactor core under normal as well as shut down conditions. So then from the reactor you get a two-phase steam mixture and it flows through pipes. Then it flows through the stream drum where it gets separated from water. The water comes through this downcomer back into the inlet header and the steam goes to the turbine and this is a bypass across the turbine in case turbine is not available and rest of the system is similar to a conventional or a any other nuclear power plant. So if you see here this is by natural that is the water getting heated rising up this itself is by natural circulation. So basically we will raise the heat in the core and by raising the controlled rods and slowly the power will rise the power will rise then the temperature will rise the flow would develop like that. Now I can tell you this being a new feature we have already set up lot of test loops to see how this process can be optimized. In the Indian Institute of Technology at Mumbai we already have set up a 2 megawatt loop which is in operation and has given lot of feedback for us in deciding how we will start up the reactor. So as I mentioned the large density difference between the hot leg which is the core and the cold leg which is the steam generator they are all possible to be achieved in 2-phase flow systems. So everything is the why why not you go for a pump the idea is if you have a pump again you must have a standby pump so that you can assure that it will be there not only that you have a pump then you have to consider all the events related to a pump trip pump seizure and pump trips are one of the most important when case when you lose power. So it may be a power supply failure or any other failure so you get rid of all these transients so the absence of pumps not only reduces operating cost but also eliminates all postulated transients and steady state flow will prevail in a natural circulation through there is no difficulty as long as your buoyancy force is balanced by the retarding friction forces. So it sets up yes the natural circulation flow will be much lower you require a large driving force. So one of the ways is how to minimize the frictional losses in the pipe. So we have to design the system in such a way that frictional losses must be minimized so we use big diameter pipes larger the diameter lesser is the friction pressure drop we try to reduce the number of bends we reduce the number of valves not only that we also eliminate mechanical separators in the steam drum because mechanical separators again they have a pressure drop they are adding the emergency core cooling system of the AHWR in the event of a rupture of any pressure primary coolant boundary. So initially you have the water flow from the high pressure accumulators then later this gravity driven water pool will continue to cool the reactor for nearly 3 days there will be a small low pressure injection pump which can pump this water into the core. So this of course is the passive decay heat removal here you see already it is passive even under normal conditions. So it is also passive under the any case of a power failure condition having talked about passive safety then why not all people build reactors which are passively safe so can you can say that okay I have achieved the safety but then there are some issues which are important to be resolved one and most important is how do you quantify the reliability of a passive system then how a passive system can fail very simplest example I can give you suppose I have a pump which is running and going through the core water flow is going through the core I have a standby pump if I want to check whether the standby pump is going to operate I can just need to put it on and see whether flow is coming that is all so I can monitor that system is a force circulation but I can I can monitor a force circulation system but if it is a natural circulation system how do I monitor yes you have the flows but that flows will come in a system which is already in operation a system which has to develop natural circulation how do I do it so one of the important issues is in what ways failure can be there and how to measure the reliability so inclusion of failure modes and reliability estimates of passive components is one of the important things which has done attention and these are in progress today many methodologies have been evolved and consensus is getting reached based on how we should make a probabilistic safety assessment of such passive systems and most important is a natural circulation for which still there is not an agreed approach but still it is in progress then what are the important phenomena based on which natural circulation will happen yes one is steady state operation of natural circulation systems how they suppose it is a single phase surely you have no difficulty but suppose it is a two phase natural circulation then you have to think about what is called as the instability of natural circulation two phase flow systems are prone to flow instabilities so this is one area where enough attention needs to be given the other one is I talked about startup of natural circulation systems as I talked to you about the advanced heavy water reactor where we want to start the reactor based on a natural circulation system so it needs not it is not difficult so only thing is it has to be given enough attention then of course design and validation of computer codes which you use for natural circulation systems believe here under natural circulation flow the velocities are low so you must have enough data of friction factors and pressure drop correlations for such low flows and when you have a low flow you have you know when you have a pool you have a thermal stratification when a low flow that would not be good mixing and when there is no good mixing the hot will be at the top and the cold will be at the bottom so it will be stratification and this itself can oppose your natural convection flow then what are the uncertainties we talk about uncertainties first and foremost as I mentioned to you the friction pressure drop so in the case of a pump you would have some margin you can always have a increase of speed of the pump and then get a higher head and go ahead but here it is not possible so it should not happen that you design a reactor for 300 megawatt electrical and you are not able to reach the flow beyond 200 megawatt electrical so lot of experimental data and experience is required to do this so in important is identify the uncertainties in terms of the critical parameters the modes of failures and how it happens in the presence of non condensables if air were there non condensables are there the condensation process also will get affected so how this and as I mentioned thermal stratification so these add some sort of a I would not say that it is a fear it needs attention that is all because for everything you have to pay a price for passive safety also you need to pay a price so in a comparative sense let us look at the advantages and disadvantages or drawbacks of passive systems advantage no external power supply so no loss of power incident can be conceived because there is no external power supply no human factor because it is based on natural loss but as I mentioned there is a degree of human factor may be involved in case one has to actuate a button better impact on public acceptance the idea is that public would be able to accept such reactors with ease because you are dependent on the natural forces like gravity and buoyancy so it is going to happen gravity is not going to fail you but then what are the drawbacks or the disadvantages low driving force because the buoyancy forces are low so it requires a very large height between the source and the sink so more means it also increases your pressure drop so you are relying on a system and just because you want to minimize the pressure drop you have large pipes then what about the licensing requirement this is a very open issue this needs a lot of so in fact you would be happy to note that the at present our advanced evo reactor design is with the atomic energy regulatory board where they are going through the licensing aspects and how to license this also will be a one of the issues which will be resolved then need for operational tests now testing of the system for operation as I mentioned monitoring monitoring the thing how will be able to monitor so here again the person who's to monitor the systems and say that's working or not here again a human factory is getting involved and of course last but not the least the reliability assessment and as I mentioned earlier the flows are so low may not be difficult to make quite difficult to monitor sometimes because the flow is very much less yes before summarizing I would just compare the existing force systems or active systems to passive systems in a very simple way active systems I can say are known devil's while passive systems are the unknown angels so in this lecture last two lectures let us see what we have covered we covered the passive safety what is passive safety why this attractive basically because of simplicity public appeal and high reliability but mind you reliability and passivity are not same that is a major thing which I indicated in the beginning of my lecture it is not necessary that the passive system is reliable it needs to be made reliable then many designs evolutionary designs have been incorporated these enhancements of course while retaining the economic aspect we talked about two reactors AP 600 in the USA and our reactor in India the advanced water reactor so one is today it is not a proven technology to show it has to be a proof proving the technology is going to be the major challenge and yes if this challenge is met it is will be surely possible to have passively safe reactors in the next generation as usual I have put here a bibliography for some of the systems this is an IAA passive safety systems and natural circulation in water cold nuclear reactors is a very good document for understanding how the different developments are going on this is the first one is the safety related terms which I had explained to you in the first lecture this gives you an overview of the AP 600 design and we do have what innovative design concepts we are going to use for the future fast-meter reactors because we have two types of reactors in India the precious hay water reactor and the advanced hay water reactors so this paper deals with the passive safety of future fast-meter reactors and this with the advanced hay water reactors is a very good paper this is a small assignment for you to see whether you have understood and you may really probe this further thank you