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MEC and the Man-Can

Down the Nuclear Rabbit Hole

The history of Dempsey's Mechanical Equipment Company, Lynchburg, VA and their work in the nuclear industry.

By Jock Dempsey, anvilfire guru

This story starts in the late 1960's and early 1970's with the design and construction of the majority of the nuclear power plants in the United States. Engineering reports on many components of these huge plants read like a Star Trek script, "forces and conditions never seen before". These reports recommended that these plants, designed with a 30 year life, be dismantled and every part carefully inspected after 10 years of operation. These recommendations to the licenses of the nuclear power plants and were quickly forgotten.

We enter this story when my father John David Dempsey (1925-2006) was made quality control engineer over primary collant pumps in the early 70's. In fact this position was more ram-rod than inspector. The primary coolant pumps are the pumps that move the superheated water through the nuclear core and heat exchangers that connect to the steam turbines. The pumps were behind schedule and having operating problems. Seals and bearings were failing, impellers cavitating. . . These were some of the largest pumps ever built -- 10,000 HP and 100,000 GPM working under the high temperature and pressure of the nuclear reactors. They are one of the most important parts of a nuclear reactor.

In the process of solving the pump problems my father was "lent" to one of the pump manufacturers where he discovered a basic principal of face seal design, moment imbalance. Up to that time these seals were made more or less randomly when it came to the relative proportions of the face sizes. What my father discovered led to one of the most important pump seal patents in modern history. The John Dempsey balanced moment seal patent defined the art of this type of seal engineering world wide. Pumps are still being retrofitted with his seal.

Other problems were fixed by minor changes and operational procedures (heat up and cool down rates, order of operations. . .).

My father worked on pumps through the plant testing and startup period. Then he moved on to other things and took early retirement in 1978.

After retirement Dad formed M.E.C., a privately owned family operated business. I went to work with Dad shortly after.

Trouble was brewing in the world of primary cooolant pumps. That 10 year inspection was looming and the pumps had gasket leaks. During all their operating time the gaskets had been leaking corrosive radioactive contaminated water. The borate compounds in the water had no effect on the stainless pump parts but it rapidly corroded the huge highly stessed carbon steel studs than held the pump flange together. Gaskets had been replaced and sealing compounds pumped into the spaces between gaskets at a cost of millions of dollars in many plants. It was time to inspect the pumps and repair them if necessary.

The problem was you just don't take contaminated nuclear equipment apart like any other machinery. The radioactive contamination can be enough to kill. Then there was the question of what do you do IF the inspection proved that the pumps need to be fixed. You had to be prepared for that contingency. The pump housing of these pumps is part of the primary coolant loop. They are permenanetly welded in place and you don't just replace a defective housing. There was good evidence from the gasket replacements that the pumps needed the gasket surfaces remachined flat. So it was a serious contingency.

Nuclear power plants make a LOT of electricity for their size. They are money making machines and there is a huge amount of pressure to keep them operating and to keep refueling outages as short as possible. (TOO much pressure.) Ideally all maintenance that requires a shut down is done during a 60 day refueling outage. The utilities wanted the inspection and any possible repairs done during refueling.

Through my father's contacts at the nuclear power plants his company new M.E.C., was asked to submit a proposal as to how to achieve these goals. His former employer, the builder of the plants also submitted a proposal. This was when I went to work in the family business. We had to figure out how to do the job, make drawings and models for a presentation and had a little over a month to do it. The presentation included preliminary machine drawings, cartoon diagrams and a rough procedure or order of opperations. I was the logitician and sketch artist.

The Man-Can

Dad had been working on how to inspect and repair the pump case for some time. This involved using a device he called the "Man-Can". The Man-Can was a lead lined stainless steel tank for a man to fit into that had an inflatable seal to fit the pump bore. The Man-Can was held down in the pump and the seals inflated so that the reactor vessel could be refilled with water and refueling continue in a normal manner. The man working in the can was part of a system that held back 30 feet of water in a 28" bore. The lead filled 7,000 pound plug was not heavy enough to resist the 17,000 PSI of lift so it had to be held down. The Man-Can was held in place by either a hold down device OR the Reactory Coolant Pump Resurfacing Macnine.

The Resurfacing Machine

Due to localized failures of the pump gaskets there was a good chance that the pump had warped and the gasket and closing lands needed to be machined. For the gaskets to work a tollerance of +/-.005" with an RMS 32 finish was required. To give the gaskets a lower chance of failure we actually held the tolerance to +/-.001" and optimized the gasket compression.

These machines produced a finish to a better tolerance than the OEM's equipment. The result was that pumps which had leaked from the day they were installed have not leaked ANY in the 30 years since we remachined them.

The most notable feature of this machine was it had a VERY large hollow spindle so that a man (the Machinist) could climb down through the machine into the Man-Can. This large hollow spindle necessitated a planetary gear drive for the feeds. This was also one of my father's areas of expertise. Figuring out how to control the feeds, the transmission and electrical controls became MY job.

The Controlls:

The first resurfacing machine had relay logic controls and took me months to fiqure out. It was a nightmare. In the end every terminal (12 each) on twelve 4 pole double throw ice-cube relays had wires attached to them!

The second machine was built with a SquareD PLC (Programmable Logic Controller). This was a very early (primative) controller and its expensive programmer had no storage medium for the programming. . . Even with its first generation quirks it was much much easier than high density limited hardware relays and allowed for more sophisticated logic.

As the first machine had not been delivered the customer asked for it to have the same PLC system installed on it. The last machine we built had a much more advanced PLC and used a big stepper motor for CNC feed rates that would allow feeds as coarse as .050" and as fine as .001" per revolution.


The pump repair manual called for hanging the pump from a crane and working directly under it using a lift table to remove the impeller. This was not suitable on many levels, general safety being one, the fact that now everythiing was radioactive was another AND pump impellers just don't drop off on their own. . And in my view, fighting gravity, the fact that every time you unbolted one of those heavy parts it would want to fall on you was NOT a good idea. Lastly, we were going to be machining the gasket surfaces and that is another job you don't want to do overhead. SO, the best and logical thing to do is turn the pump upside down.

The upended was comprized of the shielded support ring or "yoke", a thick walled spacer cylinder and the disassembly device a shielded work platform. The minimum thickness of these parts was 4" of cast ductile iron. The result was ZERO radiation exposure to workers working around the upender pump assembly. If fact, during dissasembly operations the exposure was less than background as the large mass shielded workers from the general plant background radiation.

The total weight of the shielding was about the same as the pump. This presented a dsign quandry as the empty shielding was necessarily out of balance. But balancing the load would cause the empty load to double. So an optimal center of gravity was found so that the imbalance was the same in both cases and thus the least possible.

Dissasembly Device

This was a 6" thick circular steel plate that rotated with an excentrict rotating plate that gave infintite coverage under the tooling window. The tooling window was a 12" diameter 12" thick lead glass assembly with a central bushing for tools (long handled wrenches, pickup tools, drill extensions). A drill press was mounted over the window that could be swung away to let long handled tools to fit.

The reason for the drill press was that all the bolts were pined in place and then the pins were welded. So the welds and pins had to be drilled out to remove the bolts. All this was done from behind shielding. One or two workers could work all day with almost no radiation exposure.

The industry norm is to hire dozens of workers to do this kind of job. One would run in and center puch the weld, another (maybe two) run in and drill it, and yet another take out the bolt. . . It is still the industry norm and the temp services (body shops including companies like Westinghouse) love it.

State of the Industry

Burning up workers, using them as consumables is the antithesis of good nuclear work practice. But this is where the money is. It is not in building equipment to the job without radiation exposure, and do the job safely with advanced planning. We proved it could be done and the powers that be did not like it. Our last job was sabotaged by one of these big companies. But we perservered and succeeded. But that was the end.

Where engineering logic should prevale the plants are more concerned about such machinery being a capital extense on their taxes. They are also under hughe financial pressure to get these jobs done as fast as possible. So the nuclear service industry just throws more expendable people at the problem and the plants just write it off as an expense.

When large sophicicated plants like these are built the first plant manager is very often the Senior Construction Engineer. But years later when they retire the new managers are most often MBA's (bean counters). SO, instead of maintenace decisions being directed by a knowledgeble engineer, they are based on finances. It is scary and the best reason to be anti-nuclear.

References and Links

Return to biography of Jock Dempsey

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