Steam Generators: Design and details

A recent NRC blog post (see here) brings up a matter that has come into focus of late at San Onofre Nuclear Generating Station in California. The problem, briefly, is this; leaks have been detected in the steam generators at Unit 3 at the site, and Unit 2, with identical steam generators and which has been shut down for work anyway, was also inspected and the operator, Southern California Edison, has made the decision to restart neither plant until the cause of the leaks at No. 3 plant and assurance it won't happen at No. 2 plant are both secured. While No. 2 plant did have some steam generator tubes plugged as a part of normal work, and there appears to be no commonality between the few leaking tubes at No. 2 and the leaks at No. 3, SCE has decided to be cautious. (For those interested, No. 1 unit has been shut down and dismantled for some time.)

Now, what makes this story a bit more frustrating for all involved is the fact that all of the steam generators we're talking about here are brand new. These steam generators are Mitsubishi built replacements for the originals, built by Combustion Engineering, which are no longer available. This author has made some requests in the industry to find out some specific details on the replacement generators but so far these have not come back. Click here for an SCE release on MarketWatch.

Now, the NRC blog post gave perhaps the briefest possible explanation of what a steam generator is. We can do better than that here at APR - and we will.

Let's first, right off the bat, mention that there have historically been three manufacturers in this country of large, commercial, pressurized water reactor plants; Westinghouse was first, followed by Combustion Engineering and by Babcock & Wilcox. Each had its own design theories on everything; comparing two of the plants is like comparing a 1970 Camaro to a 1970 Mustang. Let's take a look, then, at some general characteristics of steam generators - we'll look at operative features and the differences between the original equipment built and installed by the vendors.

BASIC STEAM GENERATOR FUNCTIONS AND PARTS

For this portion, we'll use Westinghouse steam generators. Below, our first illustration which is very basic and which depicts major parts of a Westinghouse steam generator.

The function of the steam generator is to transfer heat from the primary system (which is water that goes through the reactor core) into water in the secondary system (water that is turned into steam, used to perform work, condensed, and returned to the steam generator, never mixing with reactor water.) So then we need a piping system inside the steam generator to carry the primary water - which is at very high pressure and temperature - and efficiently transfer heat to the secondary water. This is best done with many thousands of small tubes, made of a special material (Inconel 600, very often) that will not corrode in this type of environment. As we see in the drawing above, thousands of tubes form the "tube bundle" so clearly visible taking up most of the volume of the lower 2/3 of the inside of the steam generator. These tubes are all mounted into a thick, heavy tube sheet (sometimes called a tube plate) and, as we can see, the area below these is divided in half. Hot primary water enters the inlet plenum, then goes through the tubes transferring energy to the secondary water outside the tubes; it then exits the tubes into the outlet plenum, and then goes on to the cold leg of the primary system. Since the primary tubes are shaped kind of like an upside down letter "U" they are widely just called "U tubes."

Now, before we get too far, it's important to note that the temperature difference between the water going in and the water coming out of the steam generator on the primary side isn't as high as most people think. Generally, pressurized water reactors have a differential temperature, at full power, between the water going into the steam generator and the water coming out well under 100 degrees. For example, the "hot leg" temperature at Shippingport Atomic Power Station, the first commercial US pressurized water plant, was 537F while the "cold leg" temperature was 509F, for a difference of just 28 degrees. However, the plant was moving about 7,350,000 lbs of water per hour through each of three operating loops; so, we can begin to understand that while the temperature change of the water was not that great, the total energy transferred WAS because of the massive amount of water being moved.

In later plants of course the temperature difference gets wider and the amount of water being moved becomes truly massive. By the mid 1980's the Westinghouse Model 412 design (4 loop, 12 foot core height, 3411 MWt core power) was being advertised as having a hot leg temperature of about 619F, cold leg temperature of about 558F for a difference of 61 degrees. This plant design had four loops, each of which had a seven thousand horsepower electric motor-driven pump moving about 34,600,000 lbs of water per hour.

Now that we've had an idea of what's going on inside the primary side of the steam generator, let's take a look at the really complicated portion - which is the secondary side. This area of the steam generator has to admit feedwater from the feed and condensate system, allow for heatup and mixing of the incoming feedwater (so as not to thermally shock the U-tubes), provide for an area to allow water to rise up through and boil in the tubes, separate out the water droplets from the steam (and return the water to the steam generator while allowing only steam to exit), provide for blowdown (letting out steam or water for cleaning), allow for measurement of water level in the steam generator, allow for sampling of water (to control chemistry to prevent corrosion), and some other things. It's a massive, complicated piece of equipment! Let's look at a really detailed illustration which comes from a visitor's center brochure covering Connecticut Yankee - an old Westinghouse PWR plant. As with all illustrations here, click to enlarge.

On the right side of this illustration we see the feedwater inlet; this supplies a ring that runs around the steam generator, spraying the water downward into the downcomer region that is outside of the tube bundle, but surrounds it completely. As the feedwater goes down, it both mixes with the water raining from the steam separators (more on those in a moment) and heats up. At the bottom, it makes the turn and moves up into the tube bundle area where it is heated to the boiling point; it blasts upward toward the top of the tube bundle. (Follow the arrows.) The steam, laden with water droplets carried along, enters a set of cyclonic or swirl type moisture separators that force the mixture to swirl rapidly; the steam can make the turns, but the water cannot and is thrown off and rains back down into the downcomer area. After this, a second set of moisture separators is encountered in this design which is labeled as the "mist extractor assembly" at the very center top of the steam generator. Again, steam is allowed to continue on to the outlet to the steam system, while water returns to the downcomer region. All this water returning isn't just a gentle rain - it's a rainforest gully-washer of Biblical proportions. Well, anyway, it's a lot of water. Many other design features are also shown.

Let's now take a brief look at some of the basic differences between the Westinghouse, Combustion Engineering, and Babcock & Wilcox steam generator designs and plant layouts. Again -- this is a basic introduction.

WESTINGHOUSE: In general, Westinghouse plants have either two, three or four steam generators depending on how powerful the plant is. Each steam generator has its own specific hot leg, cold leg and reactor coolant pump. Below is a general basic view of a Westinghouse four-loop plant (as was described second above when we were talking about temperature differences and flow rates.) The steam generators, internally, are quite like those already shown.
Below, a typical Westinghouse steam generator as depicted in WASH-1082.


COMBUSTION ENGINEERING: CE plants always have two steam generators, no matter how powerful the plant is. Here is a view from WASH-1082 of a typical Combustion Engineering reactor plant. Note that while there are only two steam generators, and two hot legs, there are two cold legs and two pumps for each steam generator.

Below, from a Combustion Engineering - Consumers Power specification brochure covering Palisades is a view of one of that plant's steam generators.

Below, from WASH-1082 is a slightly more detailed illustration of a typical CE steam generator.

BABCOCK & WILCOX: These are the most unusual steam generators. These steam generators do not use U-tubes; instead, they use straight tubes - and the primary coolant passes vertically down through the steam generator in these tubes, from the top to the bottom. These steam generators are very tall - and the run of primary coolant pipe that forms the hot legs is enormous. Below, a general perspective of a Babcock & Wilcox reactor plant.
Below, the "Raised Loop" configuration of the very late Babcock & Wilcox plant design.

Below, from WASH-1082, we see a partial cutaway of a Babcock & Wilcox steam generator. These are generally known as "single pass" steam generators, or else as "once through" steam generators. Note also that on Babcock & Wilcox plants there are always only two steam generators, with one hot leg and two cold legs (and thus two pumps.)


These single-pass steam generators do not have anywhere near the amount of water on the secondary side at all times that Westinghouse or Combustion Engineering steam generators do; according to "The Second Nuclear Era," Combustion Engineering steam generators have a secondary water inventory five times that of the B&W steam generators. This makes the pressurized water plants with U-tube type steam generators less rapidly responsive to secondary side transients .. such as loss of feed, or overfeed after scram .. since there's more secondary water to absorb transients caused by rapidly changing temperature of feedwater.

On the other hand, though, the single-pass steam generators are much more efficient and actually allow a tiny degree of steam superheating -- there is no superheat to the output steam at all on U-tube type steam generators. Superheats of between 30 and 60 degrees are achievable with vertical, single pass steam generators; while this does not allow the use of "dry steam" turbines without multiple stages of separation and reheat, it does drive up overall plant efficiency.

STEAM GENERATOR MANUFACTURING

As you have probably already guessed from looking at the illustrations which include dimensions, steam generators are exceedingly large and heavy components. They are also difficult, time consuming and exceedingly expensive to manufacture.

The three major reactor vendors who offered pressurized water reactor plants all manufactured their own steam generators (except for the earliest Westinghouse plants.) A generic description of a 250 MWe steam generator in "The Nuclear Industry, 1969" (USAEC) gives rough dimensions of 68 feet in height, 14 feet in diameter and a weight of 330 tons. Westinghouse, in the middle 1980's, specified its Type F steam generator as weighing 346 tons dry, 422 tons normal operating weight and 560 tons flooded full. This steam generator could produce 3,813,000 lbs/hr of steam at between 920 psia and 1000 psia.

Because of the complexity and mass of this piece of equipment, it had to be one of the earliest ordered during the construction process for a plant. According to WASH-1174-71 ("The Nuclear Industry, 1971") the time that steam generators had to be ordered from the manufacturing facility was slightly over four and a half years before the expected criticality date. (Only the reactor vessel, turbine generator, and condenser & auxiliaries had to be ordered earlier.) This same volume mentions that in 1971 the three companies manufacturing steam generators (Westinghouse, with facilities at Tampa, Florida and Lester, Pennsylvania; Combustion Engineering, with plant at Chattanooga, Tennessee, and Babcock & Wilcox, with plant at Barberton, Ohio) could together supply about 60 steam generators total among them per year which at that time was enough to supply the expected build rate for the next ten years.

SAN ONOFRE

It suffices to say that the worst thing that can happen in a steam generator is for the U-tubes to leak or rupture, allowing primary coolant to get into the secondary side water and steam. This is called a primary to secondary leak, and is the reason we're talking about San Onofre at all. I should mention that very tiny leaks are detectable - they're then found by hydrostatic testing when the plant is shut down, and the normal practice is to plug the tubes. Steam generators are always built with so many tubes that even plugging a hundred of them won't affect the steam generator's ability to provide the rated amount of power.

There are a few ways to get U-tubes to leak. Poor chemistry control on the secondary side can lead to corrosion, and leakage. Poor design can cause crushing of the tubes under flow impingement. Poor fabrication and manufacturing can lead to leaks in anything man ever made - and this could be true for steam generators as well. I could go on, but it's important to note here that SCE does not yet know why there are tube leaks on the San Onofre generators and we'll just have to wait for their report or something more from the NRC before we can make any kind of judgement call.

In Depth: Dan Yurman at IDAHO SAMIZDAT wraps up the entire situation at San Onofre.

SOME HISTORICAL STEAM GENERATORS

The first commercial pressurized water reactor plant that had unitized, vertical, U-tube steam generators was Yankee Rowe (Yankee Atomic Electric Company.) That plant went on line in 1960. Prior to that, steam generators in commercial plants had separate heat exchangers and steam drums. Let's take a look at some of these.

First, we'll take a look at two different designs that were used on the Shippingport Atomic Power Station. Two of that plant's four loops used Foster-Wheeler steam generators, while the other two loops used Babcock & Wilcox steam generators. (Note: As first built, Shippingport only required three loops to operate at full power and these are the figures quoted earlier.)

These two illustrations are from the complete Shippingport Atomic Power Station press package in the APRA collection. Note the many spidery pipes that are used to pipe steam/water mixture up to the drum or separator section, and the many downcomer pipes used to bring returned saturated water down from the drum to the heat exchanger.

The Foster-Wheeler design used straight, once-through or single pass design on the primary side with straight tubes and straight shell. The Babcock & Wilcox design was also single pass, but the whole heat exchanger portion was bent into a "U" so that both the tubes and the shell were "U" shaped. The different designs were fabricated this way to allow investigation of the relative merits of the two designs.

Below, one of the lower heat exchanger sections being removed from Foster-Wheeler Corporation's plant at Mountaintop, Pennsylvania.


Babcock & Wilcox used the same general design that they supplied for Shippingport in their own Indian Point 1 and NS Savannah plant designs before changing over to the once through design. Below is an illustration of one of the Savannah's two steam generators.

The illustration below is from the US AEC's photographic essay for the 1958 Geneva conferences entitled "Atoms for Peace / USA 1958" and shows what is described as a heat exchanger for Indian Point being drilled. This kind of operation, on a larger scale, had to be performed to make the tube sheets (or, "tube plates") of every steam generator. The large number of penetrations through the plate weakens it; the plates are made extremely thick to compensate.


We have mentioned Foster-Wheeler; this company did not stick around in the fabrication of large primary plant components, but did manage to get an order to build the secondary steam generators for the dual-cycle Dresden-1 which was constructed by General Electric. These were vertical U-tube type steam generators, and were in service prior to those at Yankee.. so that these might be considered the first vertical U-tube steam generators in any commercial plant, while those at Yankee Rowe were the first at any commercial PWR plant. Below is one of the Foster-Wheeler steam generators built for Dresden-1.


The advantages of having the heat exchanger and the steam/water separation performed inside a single, more compact unit seem obvious, so that there is no need to explain the transition to vertical U-tube steam generators as soon as they were developed.

I hope this has been a good, basic introduction to PWR steam generators; now, people everywhere can get a handle on just what is going on inside one of these things, how they're built, and perhaps begin to understand what's being described about San Onofre's primary-secondary leaks.

6:00 PM Eastern Tuesday March 20, 2012
ATOMIC POWER REVIEW