Introduction and overview
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Transcript: English(auto-generated)
00:00
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00:22
Hello, my name is Professor Andrew Kadak. I am a professor of the practice here at the Massachusetts Institute of Technology in the Nuclear Science and Engineering Department. What you're about to see is a summary of a course that we are presenting here at MIT on operational reactor safety.
00:41
This course was prepared with a funding grant from the Nuclear Regulatory Commission in the hopes that this material could be used by other universities in teaching more advanced nuclear power engineering. But we're going to try to cover in this course the fundamentals and basically a review of the fundamentals of reactor physics, core design, heat transfer, thermal
01:04
hydraulics, power conversion systems, safety and what the implications are in the design for safety, as well as probabilistic risk assessments. One of the key parts of this course, once you've established the fundamentals, will be a visit to a simulator, both the Pilgrim Simulator, which is a boiling
01:22
water reactor, and the Seabrook Simulator, where you'll be able to test your knowledge and learning to see how reactors actually perform. We also will also look at some of the more common accidents that nuclear power plants have experienced, including the three mile island and the Chernobyl accident, and we'll address some key current regulatory
01:41
questions that are hot topics for the Nuclear Regulatory Commission and the industry. We hope that you'll be able to use this material in your classrooms or as self-study programs, and we look forward to presenting this to you. Thank you very much.
02:03
Good morning, and welcome to the new course on operational reactor safety. This course was developed with the support of the Nuclear Regulatory Commission and the hopes that it could be used for other classes and other universities to help you better appreciate what nuclear power plants are really like and how they're
02:22
regulating and operating. What I'm going to do this morning is go over what we have as a overall agenda and course objectives. We're going to try to focus on understanding the complete nuclear reactor system, which basically means the core, the balance of plant, all the support systems that are required for nuclear, and
02:43
the interdependencies of both, of all the systems in terms of how they affect the safety of the plant. We'll also touch a little bit on the regulatory oversight and how that functions if you're running a good plant versus what you're running is a bad plant. The topics that we will specifically cover are listed
03:02
below. Now, this course is intended for people who have already had fundamentals of reactor physics, heat transfer, thermal hydraulics, and can understand many of the important aspects of the technology. And what this course is intended to do is bring all
03:21
those together in the power plant to be able to appreciate how they're all related to performance of the system. Now, specifically, we're going to be doing reviews pretty fast, lecture by lecture, of certain fundamentals to make sure that we're all on the same page. The topics we'll be covering are reactor physics,
03:40
a review of reactor physics. We're going to be reviewing how we make power using the power conversion systems. We'll look at the basic safety systems and functions that are required in a nuclear power plant to keep it safe. We'll introduce the concept risk assessment in terms of not only in terms of design, but also in operations. And I think the highlight of the course will be
04:01
simulator exercise. We've arranged for you, as a class, to go to the Seabrook Nuclear Power Station, which is an industrialized board of reactors, and to the Bilbram Nuclear Power Station, which is a boiling room reactor, happily there within about an hour's drive of MIT. And in those exercises, and we'll be going to them after we get some of these fun metals under our belts,
04:23
to actually test your knowledge and to get an appreciation of what goes on in the control room. We'll also talk about technical testifications, which, as you may know, are the rules of the road in terms of nuclear power plant operation. They describe specifically what you're allowed and not allowed to do, and what you're committed to doing
04:42
in terms of tests and inspections during operation. And lastly, we're going to talk about a very important system, what I call a system, and that's the safety culture of the plant, which in fact determines whether you run a safe plant or an unsafe plant. Clearly there are many reactors that are all built of the same type and the same technology,
05:02
but some are run better than others, and safety culture is a clear difference. This is a slide that summarizes the overall course objectives, and this is by lecture by lecture, as topical areas. Today we're going to review the overall reactor types that are presently in existence.
05:22
On the next lecture we'll be talking about reactor physics, and then a review of reactor kinetics and control. We'll then study feedback effects and depletion. Then we're going to go to the MIT research reactor to do a reactor physics exercise. We'll be doing some changes of control rock position
05:41
to assess feedbacks, react period, and you'll get a chance to actually maneuver the reactor, which is just down the street. Then we'll talk about how we remove heat or energy from the reactor in terms of being able to convert fission process into musical energy.
06:02
Then we'll discuss specifically two types of power conversion systems. One is the steam cycle, which we call the ranking, and the other is the Brayton cycle, which is the gas cycle. Then we'll address safety systems and functions, what kinds of safety systems are incorporated in these plants, and what their basic functions are. We will then look at the safety analysis reports
06:23
that are typically prepared for reactors, and we'll review certain types of accident and transients. We'll have several lectures on the safety of the plant using the safety analysis report as a basis. We'll then touch on the probabilistic safety assessment as a tool. We're not gonna make you PRA experts,
06:42
but we will help you appreciate this tool that is available to us, and not only in design and also operations. Now, this lecture, which is 13, which is the integration of safety analysis into operational requirements. What I will try to do in this particular lecture is take all the safety analysis,
07:02
all of our fundamental understandings of the core and how we remove heat, and we're gonna translate that into what turns out to be operational requirements, which are specified in our technical specification, which, as I said, allow you to do certain things to the reactor that allows it to stay within a design basis,
07:23
which then leads us to two interesting questions. What is the licensing basis of the nuclear power station versus the design basis? The licensing basis is different than the design basis. We then will go to Seabrook and then Pilgrim, and we'll do various accident scenarios
07:41
in their plant simulators, which are, as you know, a replica of the control of each of those plants. And we'll do loss of coolant accidents. We'll do possibly steam line breaks and other types of transients for both a, b, and a BWR. These exercises will hopefully give you a good appreciation of how operators really respond to accidents
08:02
as opposed to simple textbook assessments. Then we're gonna look at some significant accidents, ours being the Three Mile Island accident. We'll talk about what were the causes and what were the consequences and what the industry has done to change, and also the Chernobyl accidents. These are the two dominant, publicly recognizable events
08:23
that have really caused nuclear power to be somewhat questioned in terms of the public eye. We'll also study the Davis Bessie event, the most recent being that the reactor vessel had degradation, which occurred in 2002, quite many, many years later after the Chernobyl TMI events.
08:43
We'll then discuss safety culture. Then we'll talk about what I've listed there. It's new safety challenges. Basically, this is what the Nuclear Regulatory Commission has on their hot list of issues that one needs to be able to resolve as the reactors are continuing to be operated.
09:01
These include terrorists, what are they gonna do to spend fuel, the pump, some question, but those will be very interesting because they're on current, everyday issues with the NRC destruction. And then we'll conclude with a summary discussion of what the industry has developed in terms of advanced reactor designs for the near term and the longer term.
09:23
So hopefully this will be an interesting course for you. We're gonna cover a lot of ground. A lot of it's gonna go pretty fast, but I think once you're done with this course, you will be well-versed in operational reactor safety. Now the way this course is gonna work is obviously you have to get grades,
09:42
and the grade structure is homework will be 15% of the grade and I'm scheduling three quizzes, but there might only be two, and then the final exam. If we only have two quizzes, we'll just a portion of 60% amongst the two quizzes. And of course late homework will receive half of the full prep.
10:04
So now with that as an introduction to the overview of the course, I'd like to now spend some time going over what we have in terms of present technology nuclear reactors. And the objective here is to gain a broader understanding of what we have relative to pressurized water reactors,
10:23
boiling water reactors, and high temperature gas reactors, which are now coming back in terms of interest because of the next generation nuclear plant being a high temperature helium-cooled gas reactor. Now to appreciate nuclear, you have to understand the nuclear fuel cycle
10:41
and where reactors fit into this mix. And going over the nuclear fuel cycle, we have several steps. Now this slide comes from the Neath textbook, which will be the textbook for the course. And you'll see homework assignments based on the Neath textbook. And at the end of each lecture, you'll see some homework assignments posted on the web.
11:03
Now, typically we're gonna be focused on the reactor, but there's so many other important steps that we need to understand. And it's broken up into what we classify the front end, which is the steps taken from mining of the ore, exploring and mining of the ore
11:21
to ultimately making the fuel that goes into the reactor. We also have this back end, which after the spent fuel is used up in the reactor, it is stored, possibly reprocessed, which we used to do, and recycled back into the reactor. As you know, the French do this, the Russians do this,
11:41
and the Japanese are now doing this. From the reprocessing plant comes the high-level waste that will be disposed. As many of you know, the current strategy in the United States is a once-through fuel cycle in which the fuel goes right from the reactor to storage, and then has spent fuel assemblies to be disposed.
12:04
The current policy is being reconsidered now where we might, in fact, go into this complete nuclear closed fuel cycle economy. In the mining, milling, and enrichment, enrichment, as you know, is an important step for light water reactors. We have to take the 0.7% uranium 235 ore
12:22
and enrich it to approximately 4% to 5% for fuel to be used in thermal light water reactors. So this is sort of a big-picture view of the nuclear fuel cycle. Now, to give you some graphics about what each one of these steps are,
12:41
this is the mining, obviously. Some of these mines are deep mines. Many of them are not necessarily strip mines, but open mines, where the uranium ore is mined, it is then milled, and ultimately then enriched as a gas. It is converted into fuel pallets. The pallets go into fuel pins,
13:02
where the fuel pins are assembled into fuel assemblies, which is right here, and then the fuel assemblies are placed into the reactor, and the core is operated for about a year to 18 months, or it has to be refueled, and fresh fuel has to be put in. Typically, in these reactors,
13:22
about one-third of the fuel is taken out, replaced with fresh, and the fuel is rearranged for neutronics purposes, so they can run for another 18 to 24 months. Now, this is a picture of some reactors. These are called Generation II. Two reactors were built in probably the 70s and 80s,
13:45
and you'll note that Diablo Canyon is in California, Minnesota, New York, South Carolina, Virginia, and you'll note here that these are mostly pressurized water reactors, and you'll also note
14:02
that what's missing in these photographs are the typical cooling towers, which have been used as a symbol for nuclear power. I deliberately chose not to include those, because I wanted to show you that nuclear plants have containment and reactor power conversion systems, which are typically steam turbines.
14:21
These are twin plants. Most of these plants are twin plants. Indian Point has three reactors on the site, and Surrey has two. And these are the locations if nuclear power starts, we start building more nuclear power plants, that they'll be building additional plants at existing sites, at least in the interim. Now, one of the key objectives, obviously,
14:43
is to make electricity for nuclear power. All the safety stuff, all of a sudden, understanding how these plants work is very important, but the objective is to make electricity. And in order to do that, we have to make heat. We've got to remove the heat using some kind of fluid, whether it be water or gas,
15:01
and we've got to pass this fluid through a turbine, that turbine turning a generator, making electricity. That's the whole goal of using nuclear power. We do it because we don't release any CO2, noxious gases in the environment, dust particulates. It's essentially a clean energy source which people are now recognizing
15:21
as a really important value. Now, in terms of the removal of heat, we've got to not only take that fluid, which possibly you can see here, a liquid metal, didn't mention this, but breeders are another form of reactor that can be managed to make electricity. But we want to take this,
15:40
and we want to pump this through the core, capture the heat of the fissioning, and then take it into a system that is circulating that core, and whether it's transferred directly to turbines as boiling steam, there's no steam, from like a BWR, or steam generators in a PW,
16:03
pressurized water reactor. Now, we then have to condense the steam and recirculate it back, either to a steam generator or back to the core. Now, the next slide gives you a simple version of a boiling water reactor, a schematic. Now, if you look at the reactor core,
16:22
you see that we're going to be allowing boiling to take place in the core. We're going to have certain types of systems like steam dryers and separators that will make the steam drier, in other words, eliminate the water that's remaining in the steam since there is a core. And we're going to send that directly to the turbine,
16:40
which is outside the containment structure. Now, this part of the nuclear plant essentially replaces a fossil boiler. Namely, instead of boiling water by burning gas on tubes, pressure tubes, housing this water, we're using this reactor to boil the water and send steam directly to the turbine.
17:03
You'll notice the turbine is connected to the generator, which the turbine spins, making electricity for the consumer. Now, once it comes out of the turbine, it's a mixture of steam and water which has to be condensed in what we call a condenser. It's basically a whole series of tubes
17:21
that takes water from the environment now, whether it's a river, lake, or ocean, and condenses the steam, makes it back into the water, and is re-comped back into the core. Now, this is the simplest type of nuclear power plant and exactly replicates what is done in the fossil plant. So, if you look at this part of the curve, this chart,
17:43
everything to the right is, in fact, what you might see in a conventional fossil oil or natural gas plant. Now, pressurized water reactors are a little different, and we have, and I'll get to that in a moment, but there are many reactor types that we can talk about.
18:02
I've just done the boiling water reactor, pressurized water reactor, which we'll get to, but we also have a heavy, naturally radium heavy water cooler reactors, which are developed in Canada. We have the Russian RBNK reactors, which are boiling water reactors, but they have, instead of water as a moderator,
18:21
they use graphite. We have fast reactors, which use a liquid metal in the form of sodium, or perhaps lead, and then we have gas cooler reactors, which could use supercritical carbon dioxide or helium, and then the last category, which had been developed in the past, but not recently, is organic cooled or molten salt reactors.
18:42
So, all of these are choices that the engineers and the utilities can make in terms of what they want to use in terms of electricity. Now, the two types that are common in the United States are the pressurized water and the boiling water, and we used to have helium cooled gas reactors,
19:01
but they were shut down pretty much in the 70s. Now, when you wanna make some heat, what we wanna do is use the fissioning of uranium atoms, or plutonium, which is a fissile material, to release approximately 200 million electron volts
19:22
of energy per fission. Now, as I mentioned earlier, we also need to enrich this material from 0.7% found in nature to three to 4%, maybe up to 5%. Gas reactors require the richness of oil in the order of up to 20%. So, we need to fabricate the uranium and the pellets,
19:42
which are clad in zirconium tubes, which are placed into the reactor core. Now, the process of fission is relatively simple, and most of you, I'm sure, are quite well aware of. Where you take a neutron at a certain proper energy,
20:01
uranium 235 atom, and you create several fission fragments and hopefully at least two and a half or so neutrons that can be used to create more fissions. And the release of these excess neutrons allows one to maintain what we call a critical reactor,
20:21
which is self-sustaining in the sense that it can continue to fission neutrons, fission uranium 35, with time. Now, the energy that we're trying to capture is kinetic energy of these fission products. The neutron energies are relatively small contributions to the energy that we can capture from fission products.
20:43
So, as these things release energy at our main, release energy, which we're trying to capture as heat. Now again, one fission releases 200 billion electron volts. Now, if you want to calculate how much,
21:02
if you fission one gram of uranium 235, you can, when you convert it to electricity, make 24,000 kilowatt hours of electricity, which is a lot. And one gram can essentially light a small city overnight. Now, to do that same amount, make that same amount
21:22
of power, you need 3.2 tons of coal and roughly 13 barrels of oil, which is obviously something we want to avoid doing. And the energy density of uranium 235, in terms of energy divided by mass, is about 28,000 times the energy density of coal.
21:43
This is an important metric because we can, using uranium 235, and in fact a small amount, produce the equivalent of 28,000 times the amount of energy produced from that same mass of coal. That's why we're here so, so desirable.
22:01
Now, you saw some of these slides where pellets go into pins, which are then made into assemblies. Now, when you look at this, you see these people handling the uranium pellets. Now, uranium in this form is very low in activity because this uranium 235 has a very, very long pathway.
22:23
So the activity of this uranium 235 is extremely low, which can be handled. Obviously, when you put this back into the reactor, it's another story because then you have all the radioactive fission products produced. So when you create this reactor core, what you want to do is take the fuel assemblies
22:41
and configure them into the core in such a way as to allow the number of neutrons created to be sufficient to continue the fissioning process, which is a critical reactor. Most, and what reactor physicists do is figure out how to arrange the fuel in this reactor
23:00
such that it maintains criticality for 18 to 24 months. So in order to do this, you have to understand what's in the reactor physically. So you need to model the uranium fuel, the reactor internals, which are typically metal. They absorb neutrons.
23:20
You have to figure out how the flow will affect the reaction rates, because remember, water becomes a moderator for us, and that moderator allows the thermalization and then we'll be slowing down the neutrons to be able to fission in a thermal reactor. We then have to then use reactive physics tools
23:43
and develop what we would basically calculate to be the flux distribution. That flux is a flux of neutrons that then will be able to create fissions that will then yield what we call power distribution. And as you might imagine, their power distribution is not flat,
24:03
but it's sort of like cosine shaped in the core because we have neutrons that can leak out of the reactor and also neutrons that will be captured in the metal. But this power distribution creates what we would call the heat source that we then have to remove from the reactor.
24:22
Now to give you a sense of what's in this reactor vessel, the water comes in, goes down the sides of the reactor and up through the core, and as it's heated up, this is for a PWR, it will then go to a steam generator. Above the core, you'll see control rods. Control rods are used in a PWR
24:43
typically to change power level and also to shut down the reactor. Now these control rods are on top of the reactor vessel head and upon a signal that the reactor should be able to shut down. They drop into the core by gravity and essentially shut down the reaction.
25:04
Now this is a picture of a reactor being in the fuel. You can see, now this is under about 40 feet of water and you can see the fuel elements being placed into the reactor or being removed from the reactor, it's hard to tell from this picture.
25:21
And you can see the blue glow into the shrink and shrink up radiation that fresh fuel assemblies coming out of the reactor would emit in the water. Now one of the things I think that we need to appreciate in terms of factors in the design are how and what considerations we have to incorporate
25:44
in the design of this reactor to be able to assure safety. Can you pause it for a moment? Okay now, these factors are part of what an analyst would have to go through and think about in terms of,
26:01
is the design that he's constructed a safe design? Obviously the first thing is the core design. And in the core design, you have to really understand the type of fuel that you want to use. The uranium 235, you have to be able to, as I said earlier, model the physics of the core and to be able to understand the core power distribution
26:21
because it's so important to have a flatter core power distribution such that your limits are typically based on the peak, the highest power distribution in the core, which if you can lower the peaks to make a flat power distribution, you are actually enhancing the efficiency of this reactor neutrally.
26:41
Then you have to figure out, once you design a basic power distribution, you have to make sure you have what is called reactivity control. And reactivity control affects the rate of the nuclear reaction and be able to, for sure, that you can shut down the plant under all circumstances. And the other important part of the reactive core design
27:04
is the safety analysis. The objective is having no fuel failures, or if there are fuel failures, they're limited, and two, you don't have a melting situation. And the requirements and the regulatory requirements are such that you have to make sure
27:21
and demonstrate under a wide variety of circumstances and transients, including a complete loss of coolant accident, that the plant has emergency systems that are capable of keeping the core in a condition that is A, coolable, and B, the releases are limited and minor, if any. And that's why they set certain temperature limits
27:42
on what you can do relative to the climate. For example, the peak cloud temperature, which we'll talk about in class. The other part is how you wanna remove the heat from the core, which obviously is related to the safety analysis. Now, in this area, the most important thing
28:00
is understanding the heat transfer from the fuel assemblies to the core, whatever that core is. And clearly, many different types of fuel assemblies, as you will see, a DWR and PWR fuel assemblies are different, and all the heats are generated in that one fuel pin need to be removed.
28:20
And obviously, in the safety systems, you have to decide what kind of emergency or cooling systems you need to put into the plant to be able to keep the plant within the condition of no fuel failure and such that you do not have to challenge the tank, which is the next one.
28:42
The confinement of radioactivity. If you have an accident that releases radioactivity into the core, which is permitted, you need to be sure that that radioactivity is not released, and the confinement of radioactivity, the function that's performed, is the container.
29:02
And of course, we can never forget, all of this is done to produce electricity. Now, this is a graphic of a typical reactor. Now, I'm gonna go over some of the fundamental systems just to communicate an appreciation of what we're talking about
29:20
and how interrelated the systems are and how important understanding their interrelationship is in the overall reactor and plant design. We've already talked about the core, the fuel, the reactor vessel control organs. This core sits in the middle of the container.
29:43
Now, you'll see a lot of stuff around them. You'll see steam generators, which are listed here. This is the vessel here. This is the steam generator, this is the PW core. They make steam, they take steam, and they send it to the turbine generator unit.
30:01
This is called, over here, what we call the balance of plant, or the secondary side of the plant. The primary side, which is the reactor to the steam generator, which are in the container. Now, you'll see a lot of other systems around the plant. Now, these other systems are what we call support systems.
30:21
They are aimed at either cleaning the water in the primary system, providing separate shutdown cooling functions, water chemistry, charging and volume control, the make-up water. So, the plant is much more complicated than simply the reactor and the steam and the power conversion system.
30:41
There's a lot of things that you need in this plant that you need to completely appreciate to be able to say, well, I'm running firm, so I need the safety of the power station. Now, this is a skivag of that particular plant. And again, what I hope you'll be able to do after this course is over
31:01
is be able to look at this diagram. I know it's like an eye chart, but look at this diagram and understand how the reactor is linked to the steam generator, is linked to the pressurizer, is linked to the main coolant pumps, and all of the other support systems that may affect the safety of this particular plant.
31:22
These are basically interdependent systems that one needs to appreciate such that if there's a failure, say in this area, what impact might that have on the ability to remove heat from the reactor and maintain the plant in safe condition.
31:41
So, this eye chart, you will be familiar with when you finish this course because you'll be able to understand, A, where the water goes, B, if you lose it, what happens to temperature, and what happens to the containment in the event of certain types of accidents. Now, everything is controlled in a nuclear power plant under control.
32:01
Now, I've just shown you a relatively modern advanced boiling water reactor control where you can see operators, the chip supervisor, control room operators, and a more modern control board that is used to control all the systems that we talked about in this previous slide.
32:22
Now, this is the heart of the plant. This is where everything is monitored and actions taken to shut down systems, turn on systems, and they're all done remotely from this location. And these operators, the training of these operators, and they're part, in my view, of a key safety system for this reef for nuclear power plants.
32:43
And when we get to the simulators, you'll see an older version of this, both at Seabrook and Piltmore. Remember, Seabrook started construction in the 70s, and Pilgrim came online, I believe, also in 1972. So you'll see generational issues
33:01
compared to these more modern nuclear power plants. Now, let me go over boiling water reactors and then we'll do pressurized water reactors. Stemmatically, here we have, you've already seen a simple diagram. You've seen the core, and what we have is a direct cycle,
33:21
which is steam coming from the reactor, going to a high-pressure turbine, the low-pressure turbine, going to the condenser, then they have reheaters and feed water heaters. Then that water is condensed ultimately and sent back into the reactor as feed water, and that's how they heat the water.
33:43
BWRs have recirculation pumps, which take a stream of water from the reactor and pump it through a jet pump to aid in the core flow through the reactor. But bottom line, this chart, at this point in time, what we'd like you to focus in is it's a direct cycle
34:02
where the water is allowed to boil in the core, steam is dried and separated, and then sent to the turbine to move straight forward. For a BWR, this is a more detailed view of the reactor core,
34:20
pointing out several other functions. The feed water from the condenser coming down the vessel and up through the core, these jet pumps recirculating water and essentially provide an automated velocity stream that will send the water to the core. They have steam separators and dryers,
34:40
and then at the end of this, the water will come back, the steam will be fairly dry and go to the turbine. We'll look at some analysis as we get later on in the course and show us how these things work and the mass balance is needed for us. Now, BWR fuel assembly is unique in the sense that because we have boiling going on
35:01
in the core, there needs to be some assurance that every fuel assembly is getting cooler than getting water. So the way the boiling water reactor designs has been made in terms of the fuel, the fuel pins, which are shown here, which contain the uranium pellets
35:22
with a spring to hold the pellets in place or down, and what we call a gas plenum. This gas plenum is used to capture the fission products, which are gaseous, such that the fuel pin does not over-pressurize. Now, this fuel pin is placed in this array
35:42
of bundles called the fuel assembly and the bundles are put in what we call a fuel can. Now, this fuel can is designed such that the water coming through there has no chance of going into another fuel assembly, which allows one to be assured that water coming in,
36:03
which is water, and water as it's boiling here will stay in this regime. This is a very important design difference between a BWR and a PWR. So BWR fuel assemblies are essentially can to make sure that we have an adequate supply of fuel.
36:22
We also have different types of control rods. If you look at the fuel assembly, these are four fuel assemblies, but they have our cruciform control blades, which are put in the core in between the fuel assemblies and they have various water rods and tie rods that are meant to assist in the control of reactivity
36:42
and power distribution for this fuel assembly. Now, these cruciform rods in a BWR are placed in the core during operation. BWRs do not have a boron in the water, such as a PWR.
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The boron is used to control the reactivity of the core, reactivity meaning the extent to which the core will go critical. Instead, the BWR fuel assemblies have burnable poisons, which are specific poisons put into the fuel, poison in the sense of neutron absorbers, that will be depleted over time or consumed,
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allowing the reactor to maintain certain levels of criticality. Now, these burnable poisons are designed to deal with what we call the excess reactivity, which is we're putting in more uranium than we need to allow the reactor to run for 18 to 24 months.
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And that excess reactivity needs to be balanced by these burnable poisons. In addition, we put in control blades during operation to control the power distribution in a BWR. Now, this is a relatively bad picture of the Pilgrim Nuclear Power Station on Cape Cod Bay,
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where you can see the inlet and this is now the discharge and this is the reactor and this is the power conversion system. These pictures are hard to come by these days because of 9-11. Now, pressurized water reactors, excuse me, are different. And this is a schematic of a PWR.
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Now, what we notice are the differences are that the water in the reactor vessel is not allowed to boil. Pressure in a BWR is about 1,000 pounds, which allows the boiling to take place in the core. In a PWR, the pressure of the water is about 2,000 pounds
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which prevents boiling in the core. Now, the water, again, from the main cooling pump is circulated into the core, goes down along the down comer and up to the core. It's heated and not allowed to boil. In the PWR, we also have what is called a pressurized.
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This is sort of a pressure regulating device that maintains the pressure in the system at about 2,200 pounds. If the pressure is too low, they have heaters here that are used to increase the pressure. If the pressure gets too high, they have sprays that will condense the bubble and pressurize it.
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So, this water then goes into what we call the steam generator. Now, this is a U-tube steam generator where the primary system goes into the tubes and it transfers its heat like a radiator to a second. Okay, the steam generator basically takes water from the core, which is, again, 2,200 pounds,
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and sends it through tubes and transfers the heat from the tube to the other side of the tube, which has the water that is used to make the steam. So, imagine a tube over which water is flowing, which is the blue water,
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and that water is allowed to boil in the steam generator and then create steam, which is then also dried, and then sent to the turbine and then ultimately to the condenser, which is then repumped back into the steam generator. You'll also note in this figure,
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we have the condenser taking water from a river, a lake, or an ocean, or a cooling tower, for that matter, that would condense the steam. Recall again, if you were to draw this line to the right here, this is the secondary part of the plant, which every plant,
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whether it be fossil or nuclear, has that uses steam. What you see also here is a containment wall line. The steam generators are inside the containment. And you'll also note in a PWR, we have three different cooling systems, the primary, the secondary,
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and the third system, which is the system that actually goes into the environment. So we have one, two barriers as compared to a single barrier for a PWR system. Again, we cannot not ever forget the generator making the electricity. So reviewing again, primary system, no boiling,
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goes into a steam generator, which is thousands and thousands of tubes to which the primary water passes, giving its heat up to the secondary water system that's allowed to boil, and then steam is made, which can condense. Now in a PWR, we have essentially boron
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to control reactivity. Obviously, we need to increase the enrichment of the core, such that it can run for, say, 18 months to two years. Now that enrichment, that extra enrichment, that reactivity margin is balanced by the inclusion of boron,
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which is the neutron absorber, into the coolant. So as the reactor is consuming the uranium, the boron concentration is reduced, and these reactors are typically run un-rod, namely no control rods in the core. So again, if you look at the schematic, we have the containment.
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We have the reactor core, cooling pumps, going to the steam generator, the steam generator pumping steam to the turbine, making the electricity, condensing it. Now here we see the steam generator, I'm sorry, cooling tower. Then you have various feed water heaters,
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and we'll send the water back to the steam generator, and the primary cooling pumps are in the containment, so we're going to use water. So the line is the primary side and the secondary side.
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Now, typically, a PWR, and this is typically a configuration for a PWR, we have about 200 fuel assemblies arranged in a roughly cylindrical pattern. We have what is called a thermal shield, and that thermal shield is meant to protect the reactor vessel from fast neutrons
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to keep the embrittlement of the vessel down. And the arrangement of the fuel in this reactor is the responsibility of the reactor physicists. Typically, you'll put fresh fuel on the outside, because that's the most reactive.
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Oldest fuel in the middle to keep the power peaks down, and recycle fuel in sort of the middle range to levelize the power distribution. Now these are some parameters for roughly a thousand megawatt electric plant. You're talking, it's anywhere between 32 to 3,400 megawatts.
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The amount of heat generated in the fuel is about 97%. There's some gamma heating. The nominal system pressure is 2,250 PSI. The flow rate is 138 million pounds per hour, which is a lot of water. The inlet temperature is typically 550 or so degrees.
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The outlet from the vessel, this is Fahrenheit now, the outlet from the vessel is 620 roughly, and the temperature rise average over the core is only 60 degrees. The diameter of the core is 11 feet, and the core height is about 12 feet as a typical standard fuel assembly, and it's about 86,000 pounds,
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or kilograms of uranium in the core. And that's, again, about 193 fuel assemblies in this particular example. So the design of these PWRs is, in terms of outputs for the steam side, it's about similar to that for the PWRs,
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but the reactor core. So if you look at a PWR assembly, what you'll see is an open grid with the same fuel pins, types of fuel pins, and you'll also see that you'll have, this is a picture of a control rod
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going right into the fuel assembly, not in between fuel assemblies. And these control rods, again, falling by gravity, and the control rods are used only really to shut down the reactor or to make gross power changes. And if you look at a typical grid, you can see here typical control rod locations
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that are needed for shutdown or power control. Now this is a picture of a PWR wire, two reactors, one big steam turbine ball, and cooling towers for heat removal. This is in Chicago.
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Each reactor is about 1,100 megawatts, and these two came in service in 80, 85, and 87. Most of the nuclear fleet came in service in the late 70s and early 80s. The next reactor I'd like to talk about is a gas cooled reactor. This is an example of a Fort St. Mary plant,
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which is a high temperature gas reactor that operated up until, I guess, late 80s. Perhaps early 90s, I just don't recall at this point. But it made 330 megawatts of power. Unfortunately, its operating record was very poor, and they converted the plant to a gas turbine plant.
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The gas turbine, gas reactors use what is called a drain cycle, which is a gas cycle. And it's probably one of the more simple cycles that we have, where helium gas is blown into the core,
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and the gas coming out of the core, in this case I think it was around 750 degrees centigrade, which is much hotter than a light-loader reactor, goes to a high-pressure turbine, low-pressure turbines, and to the power turbine, which is then made turning a generator. So it's a gas turbine reactor.
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The core gas goes to exactly a gas power turbine, which is then, needs to be compressed after it's expanded in the turbine, and sent back into the reactor after it's properly cooled as a gas, so that it can be pumped efficiently. So it's a simple cycle where the gas room,
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and this is helium gas now, is circulated through the reactor and goes right to the gas turbines. Now it's a gas turbine, not a steam turbine. And helium is the core, and it is, because they go to high temperatures, they go to much higher efficiencies, 40 to 50% range,
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and this is the technology we've been looking at for the future. Now the fuel is also dramatically different than you might see in a light-loader reactor. It's a ceramic fuel made of tiny microspheres called trisocoded fuel particles,
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that where the uranium is either a car, uranium oxide or carbide, surrounded by a porous buffer layer to capture the fission products. And then a pyrocarbon layer, pyrolytic carbon layer, which is hard, surrounded by silicon carbide and another pyrolytic carbon. And these are the little tiny fuel particles
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that go into, or are pressed into, in the general atomic space, a prismatic reactor, which is a fuel compact, which are then inserted into graphite blocks. Now these graphite blocks are, in fact, the fuel assemblies. About 10 of these are stacked high in the core,
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and there's, I don't recall exactly how many, but these become the fuel assemblies, which you can refuel in three dimensions, which is another interesting challenge. Another type of high temperature gas reactor is what we call a pebble bed reactor.
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And this technology has been also used in the past. Originally, I think invented in the United States, but developed in Germany at the Eulich Research Institute, where they had an operated pebble bed reactor now for over 22 years. What a pebble bed reactor is, is in fact the same coated particle
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with the uranium kernel, and there are about 10,000 of these particles that are put into a graphite pebble. And this graphite pebble is about the size of a billiard ball.
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And 10,000 coated particles in a graphite pebble that is literally dropped into the center of this reactor, and is allowed to drain out slowly as the reactor operates, and then is recirculated and put back
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into the top of the reactor. Now, the pebble bed reactor has the same basic safety features as a high temperature prismatic reactor, in the sense that it has a meltdown-free core. The power density is about a factor of 10, lower than the light water reactor. And the fact that graphite is there is a very high heat capacity medium
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that can absorb a lot of heat. And this technology is, as I said, has been used, and is now being developed in China, and they have an operating pebble bed reactor now, it's a small research reactor, and it's being proposed in South Africa. And this is one of the two candidates
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for the next generation nuclear plant that hopefully will be built in Idaho by 2021. The technology, again, this is an online refueling system which basically avoids the need to shut down the reactor and refuel. The pebbles are continuously recirculated.
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The temperatures are quite high. Again, you can go from 850 to 900 degrees centigrade. The thermal power, however, is small. It's only 250 thermal megawatts. The South African design is upwards of 400.
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And the power output, electrical power output, looking at 40 to 45% thermal efficiencies, ranges from 120 to 160 megawatts electric. And that's one of the disadvantages. These reactors are not designed to be big, but are suitable for very highly efficient electric production.
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The vessel, the core height could be 10 meters, eight to 10 meters. The core diameter is about three and a half meters. And you'll see in this picture, there's a center column of graphite pebbles, this being the fuel zone and this being the unfueled zone. And this is done because the control rods are not in the pebble bed.
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The control rods are in fact placed outside of the core. What you also see in this design is that there's graphite, a lot of graphite, which is the graphite reflector for this core. So inside the graphite, the pebbles are literally dropped in place and discharged through the body and then recycled.
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The way these pebble bed reactors work is the helium comes in up the side and down through the pebble bed. It's not a fluidized bed. And the hot helium comes out here that goes either to a direct cycle, direct rain cycle or indirect cycle, which can be also used to make steam.
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So a 60 millimeter diameter pebble, pebbles in about a one millimeter microsphere diameter. The coolant is in fact helium. Now this chart, which is one of the final charts, provides a summary of the major reactor types.
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And I wanna focus in on the boiling water reactor and the pressurized water reactor. You can look at the high temperature gas reactor, but now this is more or less dated information because we're going to a different technology than what has been proposed here. But in terms of the different types of reactors,
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the fuel form is the same, uranium dioxide. The enrichment is the same. The fertile material, meaning the non-fissionable is uranium 235. And we're also putting the fuel pins in the same zirconoid cladding tubes.
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Now what you'll note is in a boiling water reactor, the typical design is an eight by eight, which means eight fuel pins, eight by eight fuel pins in a typical assembly we're calling this now in a can. In a PWR, we're talking about a 16 by 16,
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17 by 17 arrays of fuel pins. And you'll notice the number of fuel assemblies in a PWR is up around 750 compared to around 200 to 240 fuel assemblies in a PWR. So the design differences are really driven by the mechanism of heat removal in the reactors
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and how that reactor is operating. If you wanna just take a quick look at a breeder, or at least a breeder reactor, we have either mixed oxide or plutonium oxide. There's a blanket region in which you can make plutonium
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using uranium 238. And the fuel pins are very, very, very tiny in the sense of the diameters. So this sort of gives you an overview summary of the reactor types that we're going to be talking about,
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the safety systems that will be discussed. And hopefully, as I said, at the end of this course, you'll gain a much better appreciation of how all this stuff fits together. This is the reading and homework assignments from NEAF, chapters one and chapter two.
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And we'd like you to read just chapter four to get familiar with the material in that for the next lecture. If you have any questions, now's a good time to ask them, but I hope most of this material has been of a review nature, and it sets the framework for our future discussion.
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Recall, our next lecture will be on reactive physics. Now, we've got one lecture we are going to try to cover everything you need to know that you took a whole semester on in an hour and a half. So it's gonna be fast. If you're uncomfortable with the material, let's try to get some help to bring you up to speed because we're gonna assume
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that you understand reactive physics, and this is basically a refresher course.