Ken Moore on the Origins of RELAPSE-1

Introduction by Dan Hughes
Ken Moore and co-workers formed the foundation of the tremendous success of generations of thermal-hydraulic model, methods, and T/H software development at Camp NRTS. Ken’s career spans T/H code history from RELAPSE in the early 1960s through RETRAN in the 2000s.

Updated May 5, 2017, to include copy of Ken’s presentation at the 10th International RETRAN Conference in Jackson, Wyoming in 2001.

Ken was involved with development of the RELAP series of codes from RELAPSE-1 through RELAP2, RELAP3, and RELAP4. I have found online versions of

W. H. Rettig, G. A. Jayne, K. V. Moore, C. E. Slater and M. L. Uptmor, RELAP3 — A computer program for reactor slowdown analysis, Idaho Nuclear Corporation Report ANCR-1321, 1970


K. V. Moore and W. H. Rettig, RELAP4 — A computer program for transient thermal-hydraulic analysis, Aerojet Nuclear Company Report ANCR-1127, 1973.

Ken has hard copies of the RELAPSE-1 and RELAP2 reports. Maybe we can get them scanned to pdf and uploaded. I don’t know if copies of the RETRAN manuals are publicly available. As Quality Documents, distribution of these is generally very tightly controlled.

EPRI honored Ken in 2001 at the Tenth International RETRAN Conference in Jackson, Wyoming, for his important contributions to these software tools. The EPRI citation, presented by Jack Haugh and Lance Agee, reads:


Ken has sent a copy of the presentation that he made at the 10th International RETRAN Conference at Jackson, Wyoming in 2001.

You can read the whole thing by clicking on this image:


A few hand-written notes are included.

Ken and co-workers developed the models, methods, and software. As with all the system-analysis codes, a very large number of people are involved in using the codes for validation with experimental data and applications to plant analyses. Results from these generally provide feedback to code developers for making improvements. A Google Schalor search turned up this representative report from 1976:

RELAP4/MOD5: a computer program for transient thermal-hydraulic analysis of nuclear reactors and related systems. User’s manual. Volume III. Checkout applications. [PWR and BWR], Aerojet Nuclear Company Report, ANCR-NUREG-1335, Septeber 1976.


C. F. Obenchain J. H. Ramsthaler E. P. Eales
T. R. Charlton F. W. Childs M. M. Giles
E. G. Good G. E. Gruen J. Guttman
G. W. Johnsen K. R. Katsma C. D. Keeler
T. W. Lawford C. M. Mohr G. L. Singer
W. C. Townsend

There are a lot of familiar names in that list.

Development of RELAP4 continued for decades after Ken left Camp NRTS; I have seen citations to RELAP4/MOD6. The various versions of the base code have been adapted and applied and modified by many organizations throughout the world-wide nuclear power industry. These are likely too numerous to count. The base code has also been used to couple a systems-wide model to other specialized models such as neutronics and core subchannels. The RELAP codes, counting the various base versions and all the many modifications and adaptions, is highly likely to be the most widely used system T/H codes on the entire planet.

The importance of the successes by Ken and co-workers to subsequent work that was built upon the early RELAP codes cannot be overstated. It goes without saying that many of us owe a substantial part of our successes to these pioneers.

Ken has provided the following post to begin his history of some of the RELAP codes developed at Camp NRTS. This first post focuses on Ken’s start at Camp NRTS and the RELAPSE-1 code.

by Ken Moore
I was hired by Phillips in February 1957 and worked at the Material Test Reactor until the early ’60s when we were moved the Central Facilities. I was a lowly engineer with computer skills having first used the IBM-650 to do various analyses such as a control rod-ejection events and Xenon transients in reactor operations. I was also assigned the task of calculating how far the main control rod in SL-1 had to move to cause the fatal accident. I also worked in an analog computer group with Joe Sielinsky, Fred Hyer, and headed by Serge Gossmann.

In the late 60’s or early 70’s, MTR was fueled at one time by Plutonium and at another time by Uranium-233. The normal design core was highly enriched U-235. I actually carried one of the fresh Pu fuel elements on my shoulder. I was given the assignment to measure Beta over L for each of these fuels under the direction of Deslonde deBoisblanc. So far as I know, these were the only times any reactor was fueled by Plutonium-239 or Uranium-235. We directed the reactor operator to actually oscillate the control rod in a snusoidal pattern to obtain a transfer function for Pu-239. Other work included assisting in the initial criticality of the Engineering Test Reactor Critical Facility. Also published several reports on the kinetic effects of scram rods, Xenon transients in reactor operation, and Tables of reactivity for U-235, Pu-239, and U-233. My specialty at that time was reactor kinetics.

The RELAPSE story began in 1965/1966 just after I returned from Stanford. At that time I was in a division headed by Deslonde deBoisblanc, the greatest boss in my career. Deslonde was instrumental in getting me an AEC fellowship. I could pick any university in the US and chose to stay in the west at Stanford. Had to resign from Phillips and move to Palo Alto in August of 1963. Was in the Mechanical Engineering school that also had the option of Nuclear Engineering. Lived on campus in newly constructed married student housing with wife and two young children.

I really enjoyed the computer classes where I was introduced to Algol. Took a number of courses including Complex Variables where I pulled a C. Stanford does not inflate grad students grades to a B average and the AEC was really upset. To them a C was failing but not at Stanford. Lost my fellowship, graduated with a masters degree, was offered a deal by Stanford to stay but turned it down to return to a full-pay job. At that time the job market was really tight, but I called Deslonde and he said come on back.

If I had stayed at Stanford to earn a PhD my entire future life would have been completely different. Someone in Idaho would have developed the T-H codes needed for LOFT, but the code RELAPSE would never have be invented.

In the mid ’60s, commercial power reactors were becoming a reality and the NRC was very concerned with the Loss-Of-Coolant accidents. An experimental reactor LOFT was being built to study these events but no one really knew how a LOCA event would unfold. Someone in the nuclear community had heard that there was a computer program in the nuclear submarine program that began to address this type of event.

I was assigned to visit the NRF (Navel Reactor Facilities) to begin the process of obtaining an unclassified version of this computer program called FLASH and adapt it to help study the “blowdown” event to aid in the design of LOFT.

The resulting computer program was RELAPSE-1 written in Fortran using the IBM 7040 and CDC 6600 computers The name was intended to be a semi-humorous acronym for REactor Leak And Power Excursion with the implication that computer costs were really very large and would send the user into relapse. In later versions the name was shortened to RELAP since Fortran words were limited to 6 or less characters.

At this point let me stress that computer programs, especially ones that addressed engineering/physical modeling were new concepts. Computers were the tools used by bean counters. Math models of physical processes were initially very simple and many decision makers had little or no confidence in these calculations. Today the models are much more complex but are still only models of complex processes.

I think the best way to understand the first version of RELAPSE is to quote from the published Phillips Technical Report PTR 803. So here goes.

K. V. More, L. C. Richardson and J. W. Sielinsky, RELAPSE-1 — A DIGITAL PROGRAM FOR REACTOR BLOWDOWN AND POWER EXCURSION ANALYSIS, Phillips Technical Report PTR 803.

RELAPSE is a digital program, coded in FORTRAN IV, which calculates flow, mass inventories, temperatures, pressures, in addition to reactivities and power for a reactor primary system during a reactivity accident or a loss-of-coolant accident. Basically, RELAPSE is a modified version of the FLASH blowdown code.

The primary System is represented as three lumped volumes: (a) a pressurizer, (b) a hot volume, and (c) a cold volume. Pressure dependent coolant pumps and a flow dependent heat exchanger are included in the primary loop.

The reactor core is represented by a two-point heat transfer model and a one-point reactor kinetics model. Three modes of heat transfer are considered along with several types of reactivity functions.

Two-phase coolant flow is calculated with the aid of steam tables covering the range of 1 to 3200 psia. A constant velocity steam bubble rise model is included as part of the two-phase calculations.

Various control options describing reactor scrams, fill systems, and pump shutdown characteristics are available.

RELAPSE retains most of the calculational features of FLASH, but differs mainly in the reactor kinetics, reactor control options, and input/output form.

The basis for RELAPSE is the FLASH code developed by S. G. Margolis and J. A. Redfield of Westinghouse Bettis Power Laboratory. We wish to thank Dr. Redfield for his special assistance in providing us with a detailed understanding of FLASH.

We also received a great deal of help from the Phillips staff, especially Dr. R. P. Rose, Sid Cohen, G. E. Gruen, and R. J. Wagner.


Thus RELAPSE was born.

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