In discussing LMFBR thermal-hydraulic analysis, this paper focuses on the heat transport system a... more In discussing LMFBR thermal-hydraulic analysis, this paper focuses on the heat transport system and its impact on the predicted core behavior, particularly during oil-normal or protected accident transients. Following a brief background of related work in the area of system simulation for both loop and pool-type LMFBR designs, modeling considerations for individual components such as reactor core, piping, pumps, heat exchangers and check valves, together with the overall integrated approach to system simulation, are discussed. The need for, and current approaches to, modeling pool stratification are also examined. The role of buoyancy forces in the system is clarified, with particular emphasis on its increasing influence during flow decay. Sample results are presented to illustrate the influence of system modeling details, and selection of component parameters and operational mode, on predicted core thermal-hydraulic response during protected loss-of-flow transients. From a systematic study of the effect of pump inertia for a flow coastdown to natural circulation event in a loop-type design, it is found that certain combinations of primary and secondary pump inertias can lead to core flow reversal for a sustained period, and eventual boiling in the hot fuel channel. This et~ect, based on its impact on core flow, is even more pronounced in pool-type designs.
The decay heat removal capability of LMFBR plants is inverstigated in two steps: the transition p... more The decay heat removal capability of LMFBR plants is inverstigated in two steps: the transition phase and the established phase of natural circulation. Major parameters affecting the transition phase are briefly reviewed. This paper, then, considers two analytical approaches to study plant behavior for the established phase, i.e., the long-term heat dissipation capability of the plant. Advanced thermohydraulic system simulation codes (SSC-L and SSC-P) are used to study the response Of the CRBRP, FFTF and PHENIX plants. Simplified analyses were also performed for these plants, as well as to interpret the actual whole plant test data from PFR, PHENIX and EBR-II. All of these studies have shown that the mixed mean reactor core temperature rise is proportional to the 0.58 exponent of the shutdown steady power level.
... The authors gratefully acknowledge access to facilities provided by Thinking Machines Corp., ... more ... The authors gratefully acknowledge access to facilities provided by Thinking Machines Corp., and the help and encourage-ment of Dr. Rolf Fiebrich and Ken Crouch of Thinking Machines Corp. 'Connection Machine is a trademark of Thinking Machines Corp., Cam-bridge, MA. ...
In this paper, we give the first polynomial time approximation algorithms for two problems in com... more In this paper, we give the first polynomial time approximation algorithms for two problems in combinatorial optimization. The first problem is single-processor scheduling to minimize weighted sum of completion times, subject to precedence constraints. The second problem, interval graph completion, is finding a minimum-size interval graph containing the input graph as a subgraph. Both problems are NP-complete; our algorithms output solutions that are within a polylogarithmic factor of optimal. To achieve these bounds, we make use of a technique developed and first applied by Leighton and Rao [12], together with a technique of Hansen [5].
We give the first approximation algorithm for the generalized network Steiner problem, a problem ... more We give the first approximation algorithm for the generalized network Steiner problem, a problem in network design. An instance consists of a network with link-costs and} for each pair {i, j} of nodes, an edge-connectivity requirement Tij. The goal is to find a minimum-cost network using the available links and satisfying the requirements.
We give the first approximation algorithm for the generalized network Steiner problem, a problem ... more We give the first approximation algorithm for the generalized network Steiner problem, a problem in network design. An instance consists of a network with link-costs and} for each pair {i, j} of nodes, an edge-connectivity requirement Tij. The goal is to find a minimum-cost network using the available links and satisfying the requirements.
In this paper, we prove the first approximate max-flow min-cut theorem for undirected multicommod... more In this paper, we prove the first approximate max-flow min-cut theorem for undirected multicommodity flow. We show that for a feasible flow to exist in a multicommodity problem, it is sufficient that every cut's capacity exceeds its demand by a factor ofO(logClogD), whereC is the sum of all finite capacities andD is the sum of demands. Moreover, our theorem yields an algorithm for finding a cut that is approximately minimumrelative to the flow that must cross it. We use this result to obtain an approximation algorithm for T. C. Hu's generalization of the multiway-cut problem. This algorithm can in turn be applied to obtain approximation algorithms for minimum deletion of clauses of a 2-CNF≡ formula, via minimization, and other problems. We also generalize the theorem to hypergraph networks; using this generalization, we can handle CNF≡ clauses with an arbitrary number of literals per clause.
The first approximate max-flow-min-cut theorem for general multicommodity flow is proved. It is u... more The first approximate max-flow-min-cut theorem for general multicommodity flow is proved. It is used to obtain approximation algorithms for minimum deletion of clauses of a 2-CNF≡formula, via minimization problems, and other problems. Also presented are approximation algorithms for chordalization of a graph and for register sufficiency that are based on undirected and directed node separators
In discussing LMFBR thermal-hydraulic analysis, this paper focuses on the heat transport system a... more In discussing LMFBR thermal-hydraulic analysis, this paper focuses on the heat transport system and its impact on the predicted core behavior, particularly during oil-normal or protected accident transients. Following a brief background of related work in the area of system simulation for both loop and pool-type LMFBR designs, modeling considerations for individual components such as reactor core, piping, pumps, heat exchangers and check valves, together with the overall integrated approach to system simulation, are discussed. The need for, and current approaches to, modeling pool stratification are also examined. The role of buoyancy forces in the system is clarified, with particular emphasis on its increasing influence during flow decay. Sample results are presented to illustrate the influence of system modeling details, and selection of component parameters and operational mode, on predicted core thermal-hydraulic response during protected loss-of-flow transients. From a systematic study of the effect of pump inertia for a flow coastdown to natural circulation event in a loop-type design, it is found that certain combinations of primary and secondary pump inertias can lead to core flow reversal for a sustained period, and eventual boiling in the hot fuel channel. This et~ect, based on its impact on core flow, is even more pronounced in pool-type designs.
The decay heat removal capability of LMFBR plants is inverstigated in two steps: the transition p... more The decay heat removal capability of LMFBR plants is inverstigated in two steps: the transition phase and the established phase of natural circulation. Major parameters affecting the transition phase are briefly reviewed. This paper, then, considers two analytical approaches to study plant behavior for the established phase, i.e., the long-term heat dissipation capability of the plant. Advanced thermohydraulic system simulation codes (SSC-L and SSC-P) are used to study the response Of the CRBRP, FFTF and PHENIX plants. Simplified analyses were also performed for these plants, as well as to interpret the actual whole plant test data from PFR, PHENIX and EBR-II. All of these studies have shown that the mixed mean reactor core temperature rise is proportional to the 0.58 exponent of the shutdown steady power level.
... The authors gratefully acknowledge access to facilities provided by Thinking Machines Corp., ... more ... The authors gratefully acknowledge access to facilities provided by Thinking Machines Corp., and the help and encourage-ment of Dr. Rolf Fiebrich and Ken Crouch of Thinking Machines Corp. 'Connection Machine is a trademark of Thinking Machines Corp., Cam-bridge, MA. ...
In this paper, we give the first polynomial time approximation algorithms for two problems in com... more In this paper, we give the first polynomial time approximation algorithms for two problems in combinatorial optimization. The first problem is single-processor scheduling to minimize weighted sum of completion times, subject to precedence constraints. The second problem, interval graph completion, is finding a minimum-size interval graph containing the input graph as a subgraph. Both problems are NP-complete; our algorithms output solutions that are within a polylogarithmic factor of optimal. To achieve these bounds, we make use of a technique developed and first applied by Leighton and Rao [12], together with a technique of Hansen [5].
We give the first approximation algorithm for the generalized network Steiner problem, a problem ... more We give the first approximation algorithm for the generalized network Steiner problem, a problem in network design. An instance consists of a network with link-costs and} for each pair {i, j} of nodes, an edge-connectivity requirement Tij. The goal is to find a minimum-cost network using the available links and satisfying the requirements.
We give the first approximation algorithm for the generalized network Steiner problem, a problem ... more We give the first approximation algorithm for the generalized network Steiner problem, a problem in network design. An instance consists of a network with link-costs and} for each pair {i, j} of nodes, an edge-connectivity requirement Tij. The goal is to find a minimum-cost network using the available links and satisfying the requirements.
In this paper, we prove the first approximate max-flow min-cut theorem for undirected multicommod... more In this paper, we prove the first approximate max-flow min-cut theorem for undirected multicommodity flow. We show that for a feasible flow to exist in a multicommodity problem, it is sufficient that every cut's capacity exceeds its demand by a factor ofO(logClogD), whereC is the sum of all finite capacities andD is the sum of demands. Moreover, our theorem yields an algorithm for finding a cut that is approximately minimumrelative to the flow that must cross it. We use this result to obtain an approximation algorithm for T. C. Hu's generalization of the multiway-cut problem. This algorithm can in turn be applied to obtain approximation algorithms for minimum deletion of clauses of a 2-CNF≡ formula, via minimization, and other problems. We also generalize the theorem to hypergraph networks; using this generalization, we can handle CNF≡ clauses with an arbitrary number of literals per clause.
The first approximate max-flow-min-cut theorem for general multicommodity flow is proved. It is u... more The first approximate max-flow-min-cut theorem for general multicommodity flow is proved. It is used to obtain approximation algorithms for minimum deletion of clauses of a 2-CNF≡formula, via minimization problems, and other problems. Also presented are approximation algorithms for chordalization of a graph and for register sufficiency that are based on undirected and directed node separators
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