Steam Generator Nozzle Dam Jumping

Last year I wrote an in depth article on my work at nuclear plants in the Ice Condenser systems,  I also work as a steam generator technician with most of my jobs being Nozzle Dam jobs. I have been asked by several people to describe what I do, which is a difficult proposition. Hopefully this article will shine some light:

The Purpose of Steam Generator Nozzle Dams:

In a pressurized water reactor water is heated by the reactor in a closed primary loop under pressure and sent to steam generators. The steam generators are a heat exchanger. The steam generators are composed of tubes that the primary water travels through.  Outside of these tubes is a secondary loop where water is heated by the tubes and then turns to steam to spin the turbines.

In a refueling outage, the primary activity is removing the used fuel and installing the new fuel.  Inspections and maintenance also have to take place on the steam generator.  In order to move fuel there has to be water above the reactor.  In order to service the steam generators, water has to be drained from them.  Since this is a single system either fuel movement and steam generator services can not happen in parallel, OR a way needs to be found to isolate the steam generators from the rest of the primary system.  Nozzle Dams are the solution.

By moving fuel and performing steam generator inspections and maintenance at the same time utilities can save between 5 days and 2 weeks on their outages depending on the scope of their steam generator inspections and maintenance.  When each day out of service costs the plant over a million dollars, it’s easy to see why most PWR plants would choose to use nozzle dams.

To isolate the steam generators the plant will lower the inventory level of water in the primary system to what is called “Mid-Loop”.  This is the point at which water is completely below the steam generators. The generators are then opened up and nozzle dams are installed in the inlet and outlet pipes of the generator. The operators of the plant then “flood up” the primary system to allow for fuel movement.  The nozzle dams keep water from entering the steam generators and inspections can begin as well.


Normally the generator and reactor are filled with water. At mid-loop the water is drained to just below the pipe connecting the reactor to the generators. This is when the nozzle dams are installed. the green lines are the nozzle dam locations. Once installed the water level can be raised in the reactor and the steam generator stays empty.

During mid-loop operations plants will limit the amount of work that is going on to ensure the focus is on getting the mid loop operation completed efficiently.  This is because of the safety concerns with mid-loop. At mid-loop configuration the plant is in a vulnerable spot. Typically without core cooling there is enough water in the primary system to go over 24 hours before the water starts to boil; at mid-loop this is reduced to 15 minutes.  Obviously you don’t want anything to happen during mid loop activities that can jeopardize core cooling.

The second reason the plant is concerned with nozzle dam installation is that it is always a critical path evolution on the project.  Any delays we encounter delay fuel movement, which delays the outage.  The third reason is that it is a high risk evolution and an infrequently performed task.  When installing nozzle dams technicians also referred to as generator jumpers enter the steam generator.  This is an activity that is rarely done because the dose in the generators is about as high as you can get. Across the nuclear fleet these are all locked high radiation areas, which means the radiation dose is so high that plants are required to keep them locked so no one can inadvertently access them. A locked high radiation area is any area that has dose rates in excess of 1,000 millirem per hour.

Some plants only service their steam generators every other outage. For a single unit plant this means they go 3 years without anyone entering a steam generator. It is also a short evolution so it is entirely possible that the plant staff involved have not seen a steam generator jump in 6 years, or even in their entire careers. This is why they hire specialists to perform this work. In contrast, I’ve been doing this part time for 6 years and have worked on 18 nozzle dam jobs across a dozen different plants. Had I been doing this full time without working ice condensers or other jobs, I’m sure my experience would be about 3 times what it is.

What are Steam Generator Nozzle Dams?

Nozzle DamNozzle Dams are an active and passive method to prevent water from entering the steam generator.  Nozzle dams are designed to fit at the nozzle of the pipes entering and exiting the steam generator and prevent water flow.  They are typically composed of 3 sections, a middle metal section attached to the rubber seal and two metal side sections that fit into the seal to create a full circle barrier.  Nozzle Dams lock into place using several different methods depending on the design of the dam.  The most common are throw pins operated by an allen key with a quarter turn.   There are also designs that require bolts to be installed to lock the dam into place.

The seals of nozzle dams contain 3 seals. The first seal is a passive seal that by itself will prevent the bulk of the water from coming out of the dam. The second seal is an inflatable seal called the wet seal, which is the seal closest to the water, then there is a dry seal. If the wet seal failed, the dry seal would keep water from coming through the dam.  In between the wet seal and the dry seal is a gap called the annulus.  All three of these seals are inflated and attached to monitoring consoles that read out what the pressure is at.  Under normal circumstances these readings will stay constant.  If there is a problem with a dam, the readings on the console will show pressure variations and the technician or engineer monitoring the consoles can make appropriate adjustments based on this data.


Training To Install Steam Generator Nozzle Dams:

When you first get to a plant you go through training.  In addition to the everyday stuff that all workers need such as radiation worker training, foreign material exclusion, and plant access training, nozzle dam workers also need confined space training, respirator training, and tag out training.

Classroom/Computer Based Training

We go through respirator training even though most of the aspects of respirator training don’t apply to us and we don’t need a fit test. This is because we use air supplied bubble suits for generator jumps.  The suits are made of a thick plastic and are connected via a hose to our breathing air source far away from the generator itself.  Bubble suits restrict movement quite a bit and it takes practice to get used to working in a way that won’t get you all tangled up with your co-workers hoses.  If the suit starts fogging up on the inside that is a sign you aren’t getting enough air.  It could be something as simple as a kinked hose, or your buddy standing on your hose.  In an emergency the bubble suit can be opened by the wearer or other techs on the platform.  The atmosphere is not hazardous to life, but it could result in internal contamination.  Internal contamination is much less of a problem than running out of air.

Steam Generators count as confined spaces because there is limited means of ingress/egress (only through the manway, there is the potential for a hazardous atmosphere, and it is not designed for continuous occupancy.  Even though we are jumping generators in supplied air suits, we still ensure that the atmosphere is safe to breathe through atmospheric testing.  During mock up training we practice extracting a co-worker from inside the generator in a worst case scenario that someone goes down while working in the generator.  One major aspect of confined space training is that if ANYONE tells you to exit a confined space, you do so immediately, no arguments.

Tag out training covers isolating components to ensure workers are protected from hazardous sources of energy.  In our case, flooding up while we are working inside of the generator would be a huge safety hazard.  The tag out system ensures that while we are signed on to needing protection under the tags other workers can not energize the system. Tag outs MUST be signed on to by anyone who will be, or MAY be working somewhere that would be dangerous if someone operated the system.  This means that our Radiation Protection techs on the platform with us and our backups must sign on as well. Before leaving the site for the day the tags must be signed off on. The tagging system ensures that a red tag labelled “Danger – Do Not Operate” is attached to every valve or other device that could allow energy to pass into the system.


Mock Up Training

Installing Nozzle Dam 1On a typical nozzle dam job the workers will practice the evolution of installation dozens of times.  Most sites have a mockup building with a replica steam generator bottom and nozzle dams for us to practice on.  We start out by practicing the actual installation of the dam, then we work on the choreography of the entire evolution with the teams we will be working with.  Depending on the site we may also go through an evolution in full bubble suits.  During mock up training the guys who aren’t actively participating at the time watch the other workers to learn from what they are doing and where they are having problems.

In the image above a technician is installing the center section in the pipe.  This is of course the mockup.  In the actual generator this camera angle would be from inside the pipe.


Installation and Removal of Steam Generator Nozzle Dams:

Setting Up Our Equipment

After our classroom training and mockup training we get access to the plant. We mobilize all of our equipment into the plant and stage the jump platforms. We need to make sure that our equipment is staged out of the way of the guys removing the manways.  On some jobs we do the manways as well, but that is another article. We have to run hoses from the generator platforms to our monitoring consoles and from our consoles to plant air. We also need to hook up back up air, usually in the form of nitrogen bottles to our consoles as well.  We wait as long as we can, usually until the day before installation is scheduled to stage our dam segments on the platform.  This helps to minimize the risk that the seals will be damaged.

On jump day we will hang out and wait for the plant to get ready. There are some plants that will call us in when the manways are coming down so that we don’t have to wait as long.  I have been on sites where we have sat around for over 12 hours and then jumped, I have also been on sites where we walked in the door, everything was ready and we were off site in under 6 hours.

Briefing and Getting To The Platform:

We have a final brief with the Radiation Protection Manager to make sure we are all of the same page. Each team is composed of 3 people and depending on the platform we may install anywhere from 1 dam to 4 dams (some sites have shared platforms where two generators can be reached at once.) The three techs and 1 backup are all issued jump packs, bubble suits and a headset. Jump packs are a set of dosimeters that are placed all around our bodies to measure different amounts of radiation to different body parts. Normally nuclear workers only have 1 dosimeter, with jump packs we can have up to 7.  We go to lower containment and get our headsets tested and start getting suited up.  We will have our suits on halfway and wait by the RP desk in the basement until site is ready for us to go up.  Once we have the go-ahead the RP techs will finish dressing the first jumper and attach his air hose.  The first guy up ensures all of our tools and equipment are accessible and in the right place. He makes sure that the show is ready to roll once the other two techs are dressed.  An RP tech is usually on the platform with us as well. The RP tech will unlock the shield doors over the opening the generator and we will begin our evolution. The platforms are usually very tight on space and since we all have supplied air hoses attached to us we have to be conscious not to tangle ourselves up.

Communication and Staging

Constant professional communication is needed during the evolution.  We need to ask the RP manager who is on the headsets with us for permission any time we open the shield door, or breach the plane of the manway in any way.  Right before we jump the 3 nozzle dam pieces will be staged by the two platform technicians. The platform guys are on each side of the manway and the jumper will position himself by the manway ready to jump, but outside of the shine.  The shine is the area of space continuing out of the manway where dose rates are substantially higher.  The jumper will verbalize to the lead who is on headsets back at the console exactly what he plans to do inside the generator step by step to install the dam.  Once this is done he will ask for permission from the RP to remove the shield door. He then will ask permission to wipe the throat of the manway with a damp rag. This cuts down on contamination and makes it a bit easier for the rubber suit to slide through.  He then asks permission to put the stick light in the generator. The generator is dark, so without the stick light we are working in the dark. Recently we have begun jumping with headlamps as well (They are attached inside of our bubble suit). The jumper then verifies that his platformers are ready and asks RP for permission to enter the generator.

Entering The Generator:

Generator jumping 3The jumper will deflate his suit by pinching his hose and jump in, with a boost from his platform guys. These suits have a lot of air in them and it is difficult to fit into the manway opening when fully inflated.  He has to announce the second he breaks the plane of the manway, at which point the RP manager watching remotely will activate a stop watch to keep track of his time. This is a secondary way of tracking our dose. The RP station tracks our dose through our dosimeters we are wearing but those can lose signal inside the generator.  Stay times are calculated using the highest dose rate in the bowl divided by our available dose. For example, if dose rates are 200 MR per minute and I have 1,000 MR available, my stay time would be 5 minutes.

Foreign Material Check

As he enters the jumper looks for any foreign material in the generator. If he sees any he can not touch it and must immediately exit the bowl.  It is extremely rare to find anything but it has happened.  A couple years ago I found a bolt head in the bottom of the bowl when jumping at Salem Nuclear Plant. The reason we can’t touch an object is because it could have come in contact with fuel and have a hot particle.  Hot particles can have dose rates way higher than general area dose rates.  If an object is found, RP would need to survey it and we would need to come up with a retrieval plan.  I have removed objects from a steam generator with duct tape on a stick and with a vacuum system. At Salem it was found that the bolt heads were being sheared off from the Reactor coolant pumps and were also discovered in other parts of the primary system.  Ultimately the utility had to remove and refurbish all of their RCP’s and then re-install them.   Foreign material in the primary system is a major issue.  Anything in the system could potentially make its way to the fuel and cause fuel leakage.  Even though it would be contained in the primary system leaking fuel is a problem because it increases dose rates for all plant workers.

Installing The Dam

Once the bowl is determined to be clean (in real time this is only a couple seconds from when the jumper entered) he turns around and is handed the center section, which the platform tech has wrapped up to fit through the manway.  He announces everything that comes into the bowl so the FME monitor working remotely in the RP trailer by the RP manager can write down everything that comes in.   He lines up the center section and then starts receiving the side sections.  It is imperative to install the center section correctly, because if it is off the entire dam will not go in. He installs both side sections and ensures they are flush with each other and line up correctly.  The platformer will send the T handle allen tool inside of the second side section.  The jumper will then announce as he locks each pin: “Locking 1, 2, 3, 4 etc.” Once he has everything installed he will inspect his work and verbalize his inspection before leaving the bowl.  As he exist the bowl he will turn his body around and look inside the bowl and verify the bowl is clean on his way out.

From the time the jumper entered the bowl to the time he exited is generally between 1 minute and 5 minutes depending on the dam and any problems encountered.  If the jumper runs into problems he is on the headset with his lead who can help walk him through whatever obstacle he has.  One of the two platform techs is also able to look into the generator without being in the shine based on the layout of the bowl.  That platform tech can sometimes see what the jumper is having a problem with and help him by telling him.  In the event that the jumper can not get the dam installed or RP needs him to exit for another reason, one of the platformers will jump to finish the revolution. Once the dam is installed, another one of the techs will jump to verify that the dam is properly installed. The crew rotates who is doing what job across all of the dams that need to be installed.  For example, on a shared platform with 2 generators, each person will install once and one person will install twice. We usually install cameras that mount to the shield door as well after installation. This keeps the eddy current techs from having to wear a bubble suit and dosimetry packs, which are typically required if someone needs to reach into the generator.


Installed Nozzle DamOnce the dams are in the lead technician at the consoles will energize the dams and check to verify they have a successful installation. Once all of the dams on the platform are installed the jumpers will be cut out of their bubble suits and exit containment. Once out of the RCA (radiological control area) the jumpers need to sign off of the tag out, letting the plant know that they are no longer in need of protection under that tag.  Once everyone is off the tag out the operators are clear to flood up the reactor.  The plant can then flood up, exiting mid-loop.  Radiation protection will decontaminate the platform and then the eddy current technicians will get their equipment set up to start inspecting the steam generator tubes.


Nozzle Dam Removal:

After the dams are installed we typically leave site until it is time for the removal. We either go home for that time period, which is usually 5 – 14 days, or we go to another site to install or remove there.  Occasionally some of us stay to help to provide platform support for the Eddy Current testing, once again, that’s another article.

When we come back for the removal we do a day of mockup training for a refresher. The removal process is exactly the same as the installation process.  Once the plant reaches mid-loop we go to remove the nozzle dams.  We first remove the cameras then we go through our process of jumping into the generator.  Once all dams are removed we will use the cameras to do a close out inspection of the generator.  After the closeout inspection we will put on the diaphragm, which is a heavy metal disc about an inch thick that attaches over the manway entrance with 3 screws.  These only take a couple minutes each to put up.  This closes up the generator and allows the guys who will be installing the manways to come up in a more comfortable level of dress.

As we pull the dams out of the generator two guys will get everything set up to jump the next bowl, while one of the techs does a quick wipe down of the parts to get the bulk of the contamination off of them.  Once all the diaphragms are installed we get the pieces off the platform, where they get a full decontamination later on.  As we exit the manway guys will enter the platform to install the manways.  Once the manways are installed the plant can flood up and generator servicing is compete. We then pack up all our equipment (usually the next day) and get it loaded outside of containment.

Between the end of the outage and the start of the next one the dams will undergo refurbishment and testing to ensure they are in good working order.  The metal segments are re-used and the seals are generally throw away each outage.  Some sites will save the seals and get them to last for two cycles.  Since the seals are made of rubber they don’t decontaminate well and they are also much more prone to damage than the metal sections.



Isn’t getting a lot of dose dangerous?

Yes, radiation is dangerous, however the administrative limits set by the NRC and by Utilities ensures that even generator jumpers are not exposed to dangerous amounts of radiation.  The NRC limits occupational dose to 5 Rem (5,000 Millirem or MR) per year.  I have yet to find a utility that doesn’t have an administrative limit set vastly lower than this.  Most sites will not allow you to reach 2,000 MR per year.  By comparison, you would need to have over 50 REM administered at once before any noticeable health effects would take place. Dose limits used to be set higher, but then again doses were also higher.

Dose rates in steam generators have been greatly reduced across the nuclear fleet over time.  Utilities have improved on plant chemistry and have greatly reduced the amount of leaking fuel in reactors. Replacement steam generators have also greatly reduced dose throughout the nuclear fleet. Improved nozzle dam designs, increased time training and the retention of experienced generator jumpers also reduces the total time spent on the platform and inside the steam generator, further reducing total dose.

I personally have been a nuclear worker for 10 years and half of that has been spent as a part time steam generator technician.  My lifetime does is just under 5,000 MR.

How Much Do Generator Jumpers Earn?

This is the million dollar question and the real answer is it varies widely.  On my first job I earned $19 per hour. My rates have since increased substantially.  I have heard of more experienced techs earning in the mid 30s. We get a decent amount of overtime, and occasionally are paid for on-call time to sit at the hotel.  Our hotels and rental cars are covered, and we get $35 – $50 per day in per diem, depending on the employer.

How Many Outages Do You Do A Year?

Every season is different.  There are 65 PWR units in the United States. Some of these units do not do nozzle dams at all and some of the sites that do nozzle dams have their in house people do them rather than contract them out.  The majority of units are on 18 month fuel cycles, while some are on 24 month fuel cycles, this cuts the total number in any given season to about 1/3. As mentioned above some units skip generators every other cycle, meaning it could be 3 years between jobs at that plant. For the handful of PWR units that go down each season and service their generators and install nozzle dams, there is a high likelihood that their outages will overlap to some degree, reducing the number of potential jobs further.  Each utility chooses different contractors to provide nozzle dam services as well.  I have had seasons with only 1 nozzle dam job and I have had seasons with 4 nozzle dam jobs.  It just depends on how everything lines up. Some techs are able to go between multiple employers during the same season, but if a job pulls up or pushes out can put the tech in a tough position.

Can’t This Be Done With Robots?

There are patents for this, and it has been done, however there are more uncertainties with a robot doing this type of task. Humans are much better at problem solving than robots. The robots also need humans on the steam generator platform to install them and support them.  Standing on the platform by the steam generator for 30 minutes can result in a higher total dose than being in the generator for 1 minute.  The robots also take way more time to install them than humans do, prolonging mid-loop. To the best of my knowledge there are no sites in the U.S. that routinely practice nozzle dam installation with robots.  Click here to see an image of a nozzle dam robot arm.

Will You Hook Me Up With A Job?

No.  Nozzle Dam jumping should not the the introductory job for a nuke worker. These jobs are also done with small crews so openings for new workers are scarce. If you would like to start working in the nuclear field, Ice Condensers is a better way to start. It is a no dose job working in a non contaminated area and it requires way less training.  We will be having a job fair in February for the next D.C. Cook outage.  Starting rates are $14 per hour with $70 per day per diem. Due to the scale and temporary nature of these jobs we have been bringing in around 60 new to nuclear people from our job fairs every outage. There are dozens of different niches for both full time and outage work in the nuclear industry.  The book The Essential Guide to Getting a Job in the Nuclear Power Industry: How To Secure Full-Time Employment or Contract Work may be able to help you get a job in nuclear power as well.

Any more questions about nozzle dams, steam generators, or nuclear power in general?


John C. started Action Economics in 2013 as a way to gain more knowledge on personal financial planning and to share that knowledge with others. Action Economics focuses on paying off the house, reducing taxes, and building wealth. Check out the Action Economics archives section for all past posts.

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