Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes A control approach investigation of the Xe-100 plant to perform load following within the operational range of 100 – 25 – 100% Yvotte Brits a, ⁎ , Frikkie Botha a , Herman van Antwerpen b , Hans-Wolfgang Chi c a X-Energy, LLC, United States b North-West University, South Africa c Technology Insights LLC, United States ARTICLE INFO Keywords: Load-following Pebble bed reactor Rankine 100 – 25 – 100% ABSTRACT This paper describes the Xe-100 plant load following analyses done in Flownex® thermal-fluid system simulation software for normal operation from 100 – 25 – 100% electrical power. The plant model encompasses all major components in the Xe-100 plant’s primary helium side including the pebble bed reactor, helium circulator, and steam generator, which transfers heat to the secondary steam side feeding steam to a steam-turbine in the Rankine cycle. The requirement for load following was to have a ramp rate of 5% per minute when the turbine- generator power level ramps down from full power to 25% power level and back to full power. The control approach, which pairs selected manipulated variables namely the: reactor control rods; helium circulator speed; feed water pump speed; turbine throttle valve to selected controlled variables namely the: reactor outlet tem- perature; main steam temperature; main steam pressure; turbine-generator power, is discussed in this paper. The controller actions on the manipulated variables and their response during the load following signal are illu- strated. The deviation of the controlled variables during load following are shown within their required limits. This paper shows that the Xe-100 plant can achieve a 100 – 25 – 100% electrical load ramp at a rate of 5% per minute. 1. Introduction A Transient Analyses was conducted with Flownex (NQA-1 com- pliant) to determine whether the Xe-100 Plant can perform load fol- lowing from 100% to 25% power level and back to full power at a ramp rate of 5% per minute which is the load following requirement for the Xe-100 plant specified by the Xe-100 Plant Design Requirements Document. A simplified diagram of the Xe-100 plant showing the manipulated components enabling the plant to perform load following are shown in Fig. 1 below: The five manipulated variables which are available to control the Xe-100 during load following are: 1. Turbine throttle valve. 2. Helium Circulator. 3. High Pressure Feed Pump. 4. Turbine Extraction Steam Pressure. 5. Control Rods. 2. Modeling of the XE-100 in Flownex 2.1. Introduction to Flownex X-Energy currently uses Flownex for the thermal fluid design of the Xe-100 plant, including transient analyses. Flownex is developed in an ISO 9001:2008 and NQA1 quality assurance system environment and is the only software of its kind to hold a nuclear accreditation. Flownex uses an iterative solver – real time simulation is not a requirement for this code due to its priority on detailed engineering calculation accu- racy. This section will only give a brief overview of the modeling of the pebble bed reactor and the steam generator. The complete Xe-100 plant modeling is described in the: “Plant-Wide Simulation Model For Transient Studies on the Xe-100” (van Antwerpen et al., 2016). 2.2. Reactor model The reactor model has been the subject of numerous publications, also at previous HTR conferences Fig. 2 and is therefore described only https://doi.org/10.1016/j.nucengdes.2017.11.041 Received 10 March 2017; Received in revised form 10 November 2017; Accepted 28 November 2017 ⁎ Corresponding author at: X-Energy, LLC, 7701 Greenbelt Road, Suite 320, Greenbelt, MD 20770, United States. E-mail addresses: ybrits@x-energy.com (Y. Brits), hva@mtechindustrial.com (H. van Antwerpen). URL: http://www.x-energy.com (Y. Brits). Nuclear Engineering and Design xxx (xxxx) xxx–xxx 0029-5493/ © 2017 Elsevier B.V. All rights reserved. Please cite this article as: Brits, Y., Nuclear Engineering and Design (2017), https://doi.org/10.1016/j.nucengdes.2017.11.041