Paolo Ferroni

IRIS safety-by-design? and its implication to lessen emergency planning requirements

International Reactor Innovative and Secure (IRIS) is an integral configuration pressurised light water reactor that has been in development since late 1999 by an international consortium. Its design and safety characteristics have been amply reported. In this paper the safety-by-design? IRIS philosophy is reviewed to show how the projected safety performance (most accidents either eliminated or inherently mitigated, Core Damage Frequency (CDF) due to internal events of the order of 10−8 events/year) exceeds the current norm of nuclear reactors. The IRIS project plans to use this enhanced safety response to explore the possibility of lessening, or even eliminating, the off-site emergency planning requirement. A review is given of previous attempts to attain this relaxation of licensing regulations and of current goals for advanced reactors. Finally, the proposed methodology is outlined. It consists of a combined deterministic and probabilistic approach, including a review of the defence in-depth, and a risk informed analysis of a wide spectrum of accidents, rather than an evaluation limited to a few design-based accidents.

On the Use of Reduced-Moderation LWRs for Transuranic Isotope Burning in Thorium Fuel—I: Assembly Analysis

Multiple recycle of transuranic (TRU) isotopes in thermal reactors results in degradation of the plutonium (Pu) fissile quality with buildup of higher actinides (e.g., Am, Cm, Cf), some of which are thermal absorbers. These phenomena lead to increasing amounts of Pu feed being required to sustain criticality and accordingly larger TRU content in the multirecycled fuel inventory, ultimately resulting in a positive moderator temperature coefficient (MTC) and void reactivity coefficient. Because of the favorable impact fostered by use of thorium (Th) on these coefficients, the feasibility of Th-TRU multiple recycle in reduced-moderation pressurized water reactors (PWRs) and boiling water reactors (BWRs) has been investigated. In this paper, Part I of two companion papers, the analysis is limited to a single assembly, with full-core models presented in Part II. Spatial separation of TRU from bred uranium is found to greatly improve neutronic performance. A large reduction in moderation is necessary to allow full actinide recycle. This will pose thermal-hydraulic challenges, which are discussed in Part II. In addition, the harder neutron spectrum resulting from the reduced moderation also reduces the control rod worth, while there is a neutronic incentive to use increased mechanical shim to maintain a negative MTC. It may therefore be desirable to increase the number of rod cluster control assemblies. Superior burnup is achievable in a reduced-moderation BWR as a larger reduction in moderation is feasible, although the incineration rate is reduced relative to a PWR due to a higher conversion ratio.

Predictive Modeling of Acoustic Signals From Thermoacoustic Power Sensors (TAPS)

Thermoacoustic Power Sensor (TAPS) technology offers the potential for self-powered, wireless measurement of nuclear reactor core operating conditions. TAPS are based on thermoacoustic engines, which harness thermal energy from fission reactions to generate acoustic waves by virtue of gas motion through a porous stack of thermally nonconductive material. TAPS can be placed in the core, where they generate acoustic waves whose frequency and amplitude are proportional to the local temperature and radiation flux, respectively. TAPS acoustic signals are not measured directly at the TAPS; rather, they propagate wirelessly from an individual TAPS through the reactor, and ultimately to a low-power receiver network on the vessel’s exterior. In order to rely on TAPS as primary instrumentation, reactor-specific models which account for geometric/acoustic complexities in the signal propagation environment must be used to predict the amplitude and frequency of TAPS signals at receiver locations. The reactor state may then be derived by comparing receiver signals to the reference levels established by predictive modeling. In this paper, we develop and experimentally benchmark a methodology for predictive modeling of the signals generated by a TAPS system, with the intent of subsequently extending these efforts to modeling of TAPS in a liquid sodium environment.

Thermoacoustic power sensors: Principles and prediction

Thermoacoustic Power Sensor (TAPS) technology can be used to wirelessly measure the temperature and radiation flux conditions in a nuclear reactor core. A TAPS is a self-powered, standing-wave thermoacoustic engine, enclosed in a cylinder, and placed in the reactor core (for example, inside instrumentation tubes). TAPS utilize heat from a radiation-powered heater and cooling from the reactor core coolant to generate acoustic waves; these waves have amplitude proportional to the local radiation flux and frequency proportional to the local coolant temperature. The acoustic waves propagate physically into the reactor coolant and structure, and are detected with receivers (e.g. accelerometers) placed on the outside of the reactor vessel. TAPS signals are interpreted (and the reactor state conditions measured) by comparison of the received signals to a reference generated by predictive modeling. Since TAPS are wireless and self-powered, they offer advantages in safety (e.g., by reducing the required number of ...

I 2 S-LWR Concept Update

Pressurized water reactor of integral configuration (iPWR) offers inherent safety features, such as the possibility to completely eliminate large-break LOCA and control rod ejection. However, integral configuration implemented using the current PWR technology leads to a larger reactor vessel, which in turn, due to the vessel manufacturability and transportability restrictions, limits the reactor power. It is reflected in the fact that there are many proposed iPWR SMR concepts, with power levels up to approximately 300 MWe, but not many iPWR concepts with power level corresponding to that of large traditional PWR NPPs (900 MWe or higher). While SMRs offer certain advantages, they also have specific challenges. Moreover, large energy markets tend to prefer NPPs with larger power. The Integral Inherently Safe Light Water Reactor (I2S-LWR) concept is an integral PWR, of larger power level (1000 MWe), that at the same time features integral configurations, and inherent safety features typically found only in iPWR SMRs. This is achieved by employing novel, more compact, technologies that simultaneously enable integral configuration, large power, and acceptable size reactor vessel. This concept is being developed since 2013 through a DOE-supported Integrated Research Project (IRP) in Nuclear Engineering University Programs (NEUP). The project led by Georgia Tech includes thirteen other national and international organizations from academia (University of Michigan, University of Tennessee, University of Idaho, Virginia Tech, Florida Institute of Technology, Brigham Young University, Morehouse College, University of Cambridge, Politecnico di Milano, and University of Zagreb), industry (Westinghouse Electric Company and Southern Nuclear), and Idaho National Laboratory. This concept introduces and integrates several novel technologies, including high power density core, silicide fuel, fuel/cladding system with enhanced accident tolerance, and primary micro-channel heat exchangers integrated with flashing drums into innovative power conversion system. Many inherent safety features are implemented as well, based on all passive safety systems, enhancing its safety performance parameters. The concept aims to provide both the enhanced safety and economics and offers the next evolutionary step beyond the Generation III + systems. This paper presents some details on the concept design and its safety systems and features, together with an update of the project progress.