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Title: Heat exchanger temperature response for duty-cycle transients in the NGNP/HTE.

Abstract

Control system studies were performed for the Next Generation Nuclear Plant (NGNP) interfaced to the High Temperature Electrolysis (HTE) plant. Temperature change and associated thermal stresses are important factors in determining plant lifetime. In the NGNP the design objective of a 40 year lifetime for the Intermediate Heat Exchanger (IHX) in particular is seen as a challenge. A control system was designed to minimize temperature changes in the IHX and more generally at all high-temperature locations in the plant for duty-cycle transients. In the NGNP this includes structures at the reactor outlet and at the inlet to the turbine. This problem was approached by identifying those high-level factors that determine temperature rates of change. First are the set of duty cycle transients over which the control engineer has little control but which none-the-less must be addressed. Second is the partitioning of the temperature response into a quasi-static component and a transient component. These two components are largely independent of each other and when addressed as such greater understanding of temperature change mechanisms and how to deal with them is achieved. Third is the manner in which energy and mass flow rates are managed. Generally one aims for a temperature distributionmore » that minimizes spatial non-uniformity of thermal expansion in a component with time. This is can be achieved by maintaining a fixed spatial temperature distribution in a component during transients. A general rule of thumb for heat exchangers is to maintain flow rate proportional to thermal power. Additionally the product of instantaneous flow rate and heat capacity should be maintained the same on both sides of the heat exchanger. Fourth inherent mechanisms for stable behavior should not be compromised by active controllers that can introduce new feedback paths and potentially create under-damped response. Applications of these principles to the development of a plant control strategy for the reference NGNP/HTE plant can be found in the body of this report. The outcome is an integrated plant/control system design. The following conclusions are drawn from the analysis: (1) The plant load schedule can be managed to maintain near-constant hot side temperatures over the load range in both the nuclear and chemical plant. (2) The reactor open-loop response is inherently stable resulting mainly from a large Doppler temperature coefficient compared to the other reactivity temperature feedbacks. (3) The typical controller used to manage reactor power production to maintain reactor outlet temperature at a setpoint introduces a feedback path that tends to destabilize reactor power production in the NGNP. (4) A primary loop flow controller that forces primary flow to track PCU flow rate is effective in minimizing spatial temperature differentials within the IHX. (5) Inventory control in both the primary and PCU system during ramp load change transients is an effective means of maintaining high NGNP thermal efficiency while at reduced electric load. (6) Turbine bypass control is an effective means for responding to step changes in generator load when equipment capacity limitations prevent inventory control from being effective. (7) Turbine bypass control is effective in limiting PCU shaft over speed for the loss of generator load upset event. (8) The proposed control strategy is effective in limiting time variation of the differential spatial temperature distribution in the IHX during transients. Essentially the IHX can be made to behave in a manner where each point in the IHX experiences approximately the same temperature rate of change during a transient. (9) The stability of the closed-loop Brayton cycle was found to be sensitive to where one operates on the turbo-machine performance maps. There are competing interests: more stable operation means operating on the curves at points that reduce overall cycle efficiency. Future work should address in greater detail elements that came to light in the course of this work. Specifically: (1) A stability analysis should be performed to identify the phenomena that control reactor outlet temperature stability when operating with the Reactor Outlet Temperature Controller. The goal is to identify a better performing controller. (2) Future simulations should be performed with multiple axial nodes. The single axial node model for the core used in this work gives rise to an initial core reactor outlet temperature perturbation that is a numerical artifact. (3) The tradeoffs referred to above regarding the dependence of Brayton cycle stability and efficiency on performance curve characteristics and use need to be better understood. (4) The role of xenon was neglected in this work and needs to be included in future work.« less

Authors:
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
951244
Report Number(s):
ANL-GENIV-114
TRN: US0902162
DOE Contract Number:  
DE-AC02-06CH11357
Resource Type:
Technical Report
Country of Publication:
United States
Language:
ENGLISH
Subject:
22 GENERAL STUDIES OF NUCLEAR REACTORS; NUCLEAR POWER PLANTS; SERVICE LIFE; HEAT EXCHANGERS; PROCESS HEAT REACTORS; CHEMICAL PLANTS; ELECTROLYSIS; TEMPERATURE CONTROL; REACTOR CONTROL SYSTEMS; STABILITY; TEMPERATURE DISTRIBUTION; THERMAL EFFICIENCY; THERMAL EXPANSION; THERMAL STRESSES; TRANSIENTS

Citation Formats

Vilim, R B, and Nuclear Engineering Division. Heat exchanger temperature response for duty-cycle transients in the NGNP/HTE.. United States: N. p., 2009. Web. doi:10.2172/951244.
Vilim, R B, & Nuclear Engineering Division. Heat exchanger temperature response for duty-cycle transients in the NGNP/HTE.. United States. https://doi.org/10.2172/951244
Vilim, R B, and Nuclear Engineering Division. 2009. "Heat exchanger temperature response for duty-cycle transients in the NGNP/HTE.". United States. https://doi.org/10.2172/951244. https://www.osti.gov/servlets/purl/951244.
@article{osti_951244,
title = {Heat exchanger temperature response for duty-cycle transients in the NGNP/HTE.},
author = {Vilim, R B and Nuclear Engineering Division},
abstractNote = {Control system studies were performed for the Next Generation Nuclear Plant (NGNP) interfaced to the High Temperature Electrolysis (HTE) plant. Temperature change and associated thermal stresses are important factors in determining plant lifetime. In the NGNP the design objective of a 40 year lifetime for the Intermediate Heat Exchanger (IHX) in particular is seen as a challenge. A control system was designed to minimize temperature changes in the IHX and more generally at all high-temperature locations in the plant for duty-cycle transients. In the NGNP this includes structures at the reactor outlet and at the inlet to the turbine. This problem was approached by identifying those high-level factors that determine temperature rates of change. First are the set of duty cycle transients over which the control engineer has little control but which none-the-less must be addressed. Second is the partitioning of the temperature response into a quasi-static component and a transient component. These two components are largely independent of each other and when addressed as such greater understanding of temperature change mechanisms and how to deal with them is achieved. Third is the manner in which energy and mass flow rates are managed. Generally one aims for a temperature distribution that minimizes spatial non-uniformity of thermal expansion in a component with time. This is can be achieved by maintaining a fixed spatial temperature distribution in a component during transients. A general rule of thumb for heat exchangers is to maintain flow rate proportional to thermal power. Additionally the product of instantaneous flow rate and heat capacity should be maintained the same on both sides of the heat exchanger. Fourth inherent mechanisms for stable behavior should not be compromised by active controllers that can introduce new feedback paths and potentially create under-damped response. Applications of these principles to the development of a plant control strategy for the reference NGNP/HTE plant can be found in the body of this report. The outcome is an integrated plant/control system design. The following conclusions are drawn from the analysis: (1) The plant load schedule can be managed to maintain near-constant hot side temperatures over the load range in both the nuclear and chemical plant. (2) The reactor open-loop response is inherently stable resulting mainly from a large Doppler temperature coefficient compared to the other reactivity temperature feedbacks. (3) The typical controller used to manage reactor power production to maintain reactor outlet temperature at a setpoint introduces a feedback path that tends to destabilize reactor power production in the NGNP. (4) A primary loop flow controller that forces primary flow to track PCU flow rate is effective in minimizing spatial temperature differentials within the IHX. (5) Inventory control in both the primary and PCU system during ramp load change transients is an effective means of maintaining high NGNP thermal efficiency while at reduced electric load. (6) Turbine bypass control is an effective means for responding to step changes in generator load when equipment capacity limitations prevent inventory control from being effective. (7) Turbine bypass control is effective in limiting PCU shaft over speed for the loss of generator load upset event. (8) The proposed control strategy is effective in limiting time variation of the differential spatial temperature distribution in the IHX during transients. Essentially the IHX can be made to behave in a manner where each point in the IHX experiences approximately the same temperature rate of change during a transient. (9) The stability of the closed-loop Brayton cycle was found to be sensitive to where one operates on the turbo-machine performance maps. There are competing interests: more stable operation means operating on the curves at points that reduce overall cycle efficiency. Future work should address in greater detail elements that came to light in the course of this work. Specifically: (1) A stability analysis should be performed to identify the phenomena that control reactor outlet temperature stability when operating with the Reactor Outlet Temperature Controller. The goal is to identify a better performing controller. (2) Future simulations should be performed with multiple axial nodes. The single axial node model for the core used in this work gives rise to an initial core reactor outlet temperature perturbation that is a numerical artifact. (3) The tradeoffs referred to above regarding the dependence of Brayton cycle stability and efficiency on performance curve characteristics and use need to be better understood. (4) The role of xenon was neglected in this work and needs to be included in future work.},
doi = {10.2172/951244},
url = {https://www.osti.gov/biblio/951244}, journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Mar 12 00:00:00 EDT 2009},
month = {Thu Mar 12 00:00:00 EDT 2009}
}