Models, Methods, and Software Research at Olde Camp NRTS: Part 0

by Dan Hughes

This post is Part 0 of a few that summarize some of the activities that occurred not long after I started work for the Atomic Energy Division of Phillips Petroleum Company at Camp NRTS. The cast includes George Brockett, Larry Ybarrondo, and Chuck Solbrig, with appearances by a host of others.

All are welcomed to jump in and fix my screw-ups.

How I got to Camp NRTS
In 1968, some nine years after Miss Hallum, history teacher at Pickens High School, had mentioned that I should consider college, I was nearing the end of my graduate studies. Larry Ybarrondo, representing the Atomic Energy Division of Phillips Petroleum Company, the operating contractor at the National Reactor Testing Station (NRTS) in Idaho Falls, Idaho, visited the campus for interviews with graduate students who were looking to soon join the workforce. I was one of those he interviewed and was fortunate to be offered a visit to the company and interviews with potential co-workers.

Later in 1968, I am not certain of the date but it was cold and snow season when I arrived in Idaho Falls, I went out for an interview. The flight schedule included a layover and plane change in Salt Lake City. That layover was sufficiently long for me to rent a car and ride up one of the canyons, probably Emigration Canyon, or up Interstate 80. Having spent my entire life in the southeast, I had never seen so much snow. Flying over the snow covered Rocky Mountains was an unforgettable experience.

I do not now recall if I took the infamous last flight to Idaho Falls from Salt Lake City. A flight that I took many times in the following years, usually arriving close to midnight and a very, very quiet town. In the olde days, the 5 pm traffic rush in Idaho Falls was over at 5:15 pm. We could always count on Western Air Lines (We’re Always Late) to hold the flight in Salt Lake for all late-arriving flights that had connecting passengers. At the time, the plane was a propeller-driven DC-6. During my flight the pilot walked back through the cabin to check something, maybe checking the door, maybe to check with a stewardess, and they were all young women back then, about activities later that night, and he was wearing sunglasses. I finally figured out that he was probably wearing the sun glasses to assist his adjustment to the dark outside when he returned to the flight controls.

Larry’s group that I interviewed was located out on the semi-arid high plains about 60 miles from Idaho Falls on the NRTS at Test Area North, TAN. I do not recall who I chatted with other than I’m sure that Carl Hocevar was among them. And possibly Jay Larson, George Brockett, and Don Curet.

Following the interviews I was offered employment and at the time I was wrapping up my studies and my dissertation. The latter would prove to almost be a show stopper. Larry called me a couple of times over the months to find out my status and to remind me that I had an opportunity for full-time gainful employment waiting. These calls made me very anxious because I did not have a back-up employer; I had put all-in for the NRTS job.

I finally left Raleigh in February 1969, now almost 10 years after Miss Hallum’s suggestion, after passing my oral exam in math, completing, defending, and getting approval of my dissertation. I think it was February, but nonetheless it was early in 1969. By this time, Larry’s group had moved into Idaho Falls and had access to an IBM 370 mainframe computer that was located in the same building on the northern edge of the town. Damn near cutting-edge technology at the time. I think the building was named the Computer Science Center (CSC)??

My initial assignments including getting to know what was going on in an organization, unknown at the time, that would prove to be the nucleus of the world-wide water reactor safety industry. The projects underway included development of mathematical models, numerical solution methods, and computer software for transient, compressible two-phase flows of water in complex engineered equipment, and several experimental projects. The combination of theoretical and analytical developments with corresponding experimental projects is to this day considered to be the standard approach for attacking inherently complex physical problems.

New Code Development Background
At that time, the software at NRTS included RELAP3 for system-wide modeling and analysis, CONTEMPT for containment response, THEATA for fuel-rod thermal response, MOXY for radiative energy exchange in rod bundles, nuclear reactivity models and codes (I don’t do neutrons, so I don’t know these codes), and several supporting software, e.g. environmental routines like the steam tables.

My primary initial assignment was associated with the experimental data and empirical correlations for Critical Heat Flux/Departure from Nucleate Boiling (CHF/DNB), a phenomenon of critical importance for many off-normal states in light-water nuclear reactors. At steady state conditions, CHF/DNB is predicted by use of sub-channel models of fuel rod arrays. The primary non-vendor code for this calculation was COBRA which was developed at the Pacific Northwest Laboratory by Don Rowe and colleagues. As part of my duties I was a consultant to the Advisory Committee on Reactor Safety (ACRS), Thermal-Hydraulic sub-committee for sub-channel models and software.

There are several aspects of the water-cooled reactors of those days that required that special attention be applied to; (1) different geometrical regions and components of the systems, (2) different physical processes that occur within different regions and components, and (3) the response of each phase, liquid and vapor, of the two-phase mixture.

The main-line piping is basically one-dimensional, but the downcomer and plena are basically three-dimensional, for example. The response of the fuel rods is critically important and is the focus of licensing limitations. The response of the containment, which is basically a huge compartment with the exterior of the components that make up the reactor system exposed to the atmosphere within, generally required that some approximation to the distributions of the fluid be attempted. Some rapid neutronic/reactivity matters required a focus solely on that processes. And, while the piping is basically one-dimensional, the distribution of the liquid and vapor within horizontal segments can have important consequences, for the mass flow and energy leaving the system, for example. The downcomer is three-dimensional and counter-current flow of the liquid and vapor, and the distribution of the phases across the geometry is important. There are many other examples not mentioned here.

To address the issues at an appropriate level, different models, methods, and software were developed. Some of these have been mentioned previously; RELAP3, THEATA, MOXY, CONTEMPT, &etc. At the time, computer capabilities were very limiting relative to incorporating all the critical geometries, components, and physical phenomena into a single code. The approach was to couple the output from one focused code to the input of another code. The coupling was one-way and prevented feedback from the second code to the first. Basically, the second code simply read a file that was produced by the first code.

To overcome the limitations and improve applications of the approach, our analytical development group would later propose an Executive Code that would automate the one-way, no-feedback coupling method of those times. The Executive could also be used to initiate investigations of two-coupling feedback.

The LOOP Project gets Started
I do not recall the circumstances under which Chuck Solbrig and I initially somehow mutually agreed that I would work directly with him on formulation of a system of model equations for two-phase flows. I vaguely recall that he arrived at the NRTS in Idaho Falls having already started thinking about and doing some initial development. Chuck arrived in late 1970 or early 1971 and I recall that we started working together not long after that.

Chuck’s concept was that the model would be significantly more detailed than the homogeneous models that were in use at the time. And the equation system would form the basis of new models and codes for water reactor safety analysis at the system level. The new codes would eventually replace the existing codes that were in the public domain. I think at the time the number of organizations and codes was limited to about two or three; the RELAPSE/RELAP series at the NRTS and the FLASH codes at Bettis. Although the vendors of the reactor systems very likely had equivalent codes.

The basic concept was to describe each phase in a mixture of liquid and vapor water with its own set of model equations for mass, momentum, and energy conservation, and Equation of State (EoS). Whatever the process that we got started working together, we pushed through and as time passed and new people joined Chuck’s group they, too, contributed to the effort.

Chuck has got a copy of the original hand-written manuscript into the Office of Scientific and Technical Information (OSTI) database:

Two phase flow equations which account for unequal phase velocities and unequal phase temperatures October 1971, Aerojet Nuclear Company, Charles W. Solbrig and E. Daniel Hughes.

I think there’s an old ANCR report somewhere. Yep, found it.

The nomenclature for the various two-phase flow model equations developed at the time included shorthand acronyms. The standard model in universal use at the time, the Homogeneous Equilibrium Mixture Model, was labeled HEMM. The liquid and vapor phases were assumed to exist at the same temperature and to move with the same velocity; both thermal and mechanical equilibrium. The HEMM model had been developed and applied since the earliest days, ca. the 1950s, of the nuclear power industry. That research was mainly directed toward natural circulation Boiling Water Reactors (BWRs) for applications to propulsion of naval vessels.

A model based on an assumption of thermal equilibrium for the two phases, but allowing for mechanical (momentum) non-equilibrium, we labeled Unequal Velocity Equal Temperature, UVET. A model based on both mechanical and thermal non-equilibrium we labeled Unequal Velocity Unequal Temperature, UVUT. Some 50 years later, the nomenclature continues to appear in the peer-reviewed literature. Not frequently but from time to time.

Later the concept of a constrained, or limited, thermal non-equilibrium model was developed:

M. P. Paulsen and E. D. Hughes, “RETRAN Nonequilibrium Two–Phase Flow Model for Operational Transient Analyses”, Nuclear Technology, Vol. 61, pp 153–166, 1983

The vapor phase was assumed to be at the saturation temperature while the liquid could be either subcooled below the saturation temperature or at the saturation temperature. The model could use either an equal or unequal velocity approach, and a model based on the drift-flux concept was also developed:

E. D. Hughes, M. P. Paulsen, and L. J. Agee, “A Drift–Flux Model for Two–Phase Flow for RETRAN”, Nuclear Technology, Vol. 54, pp 410–421, 1981

The model has been proven to be especially valid for boiling at steady state conditions. But the limitations must be recognized relative to applications that involve wide ranges of mixture states.

Today these concepts, first developed in the early to mid 1970s and considered revolutionary at the time, have been significantly extended and have become the standard modeling approach for transient, compressible, boiling two-phase flows in complex engineered systems. The concepts have been extended to include modeling the several distinct fluid regions that occupy the flow field; liquid film, vapor core, entrained droplets. Today the concepts have been extended to include multi-field, multi-physics, multi-scale thermal sciences. And the water reactor safety field is the driving force behind much of the leading-edge research. The concept has appeared in a variety of industries.

Among the first applications of a somewhat general multi-fluid modeling approach was for modeling and prediction of liquid film dryout (Critical Heat Flux) by Saito et al (1978).

The New Analysis Development Activity (NADA) ca. early 1970s
In time, Chuck had built up a team that was investigating the several aspects of model and software development, including the continuous equations, the discrete approximations to the continuous equations, numerical solution methods for the discrete equation system, engineering models and correlations for mass, momentum and energy exchange between the phases and between each phase and the wall structures bounding the mixture, and coupling of the system code to other codes such as conduction and neutronics. And while we had lead personnel expertise in each area there was very significant cross cultivation between the areas. At its peak the team was operating like a well-oiled machine and the camaraderie was a joy to behold and an even bigger joy to be a part of. We would party like there was no tomorrow, including Thursday night beer and pool gatherings that shut down the bar at about 1 am and we would be at work by 7:30 am Friday mornings. We were a younger, mostly in our 30s, group back then.

The group eventually grew to include the following and these are arranged very roughly into the primary areas of research.

George Brockett and Larry Ybarrondo: Management

Chuck Solbrig, Dimitri Gidaspow, Dan Hughes, Bob Lyczkowski, Glen Mortensen: Fundamentals and generalists.

Jim McFadden, Dick Farman, Jim Mills, Bill Yuill, Kent Richert, Carl Hocevar: Engineering models and empirical correlations for wall-to-phase and inter-phase heat, mass, and momentum exchange.

Bob Narum, Chuck Noble: Mathematics and software tools.

John Trapp, John Ramshaw, Walter Wnek: Fundamentals and three-dimensional modeling.

Bill Suitt, Gerry Jayne, Gill Singer: computer science and coding.

Bob Grimesey, Ross Marsden: Neutron physics and application calculations.

This list has been made from memories that are over 45 years old, so it might not be either complete or accurate. (At one time the name of the group was New Analysis Development Activity, NADA. I was always reminded of New Automobile Dealer Association whenever I saw that. Our group being a software parts store.)

The equation system was so new and somewhat complex that software tools were frequently needed in order to carry out investigations into the properties and characteristics of both the continuous equation system and the discrete approximations. Additionally, test-bed software was developed and built to test out many aspects of the model, and, importantly, the structure of the new code, including the coupling with other codes.

Coding rules and guidelines were developed and implemented, with one objective being to avoid, at even the cost of CPU cycles, many of the non-standard coding practices that were in widespread use at the time. Good, clean, well documented, structured, easily extendable software was the goal. Everyone who worked on the project had significant coding experience and the purely personal coding prejudices had to be brought under control. All the codes were built using SI units. COMMON was to be eradicated along with bit operations and all non-standard FORTRAN.

The test-bed codes included those developed to check out mass, momentum, and energy exchange engineering models and empirical correlations. The UVUT approach requires significantly more detailed modeling of these exchanges than do the more standard approaches. In particular, the mass, momentum, and energy exchanges between the phases had not been considered to any detailed level at all prior to our original two-fluid model of boiling two phase flows. I recall that Jim McFadden and Jim Mills were the leads in these efforts. They were later joined by Bill Yuill, Dick Farman, Kent Richert, and Bob Grimesey.

The team had expertise and experience in each of the physical domains e.g. transient, compressible single- and two-phase fluid flow, boiling heat transfer, conduction heat transfer, and neutron physics; mathematicians to handle both the continuous and discrete domains; and software expertise in the coding domain. Chuck, Bob Lyczkowski and I were the initial expertise in the thermal-hydraulics and continuous equations. John Trapp, John Ramshaw, and Glenn Mortensen later joined us. Chuck Noble and Bob Narum handled the mathematics, and Bill Suitt provided significant software and coding domain expertise.

These are rough boundaries because there was a tremendous amount of cross-fertilization, and that was a result of the high level of camaraderie between everyone working on the various problem areas. I think to a certain extent we all realized that we had been given a very significant opportunity to make new, and important, contributions. Oh, and it was fun!

Bob Lyczkowski has summarized the events that led to dispersion of NADA team members to private companies that were springing up around Idaho Falls: The History of Multiphase Computational Fluid Dynamics. The paper also summarizes the genesis and growth of the science and engineering that has been built on the developments at Camp NRTS in the early 1970s.

As mentioned above, these are memories that are more than 4 decades old. Amplifications and clarifications will be appreciated.

The Present
Some of the companies that sprang from Camp NRTS back in the 1970s, or at least remnants of these, and some NADA team members, have been active in the Nuclear Power and other Industries up through the present time. The results of the concepts developed, and implementations of these into software, by the personnel and companies to which they dispersed continue to be heavily utilized throughout the world-wide power generation industries.

The results from projects that built on and followed the NADA work at Camp NRTS, and the associated personnel, some of whom remain active, have also made important contributions in both theory and software.

In general, the results of the research that George Brockett, Larry Ybarrondo, and Chuck Solbrig initiated and supported at Camp NRTS, and the associated personnel, made, and continue to make, extremely significant contributions to both theoretical and practical aspects of the science and engineering of multi-phase thermal hydraulics and water reactor safety. Additionally, and equally important, the science and engineering has been, and continues to be, adapted by other industries.

Not to mention, we had lots o’ fun!

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