User:Srgreene/SmAHTR

SmAHTR or Small modular Advanced High-Temperature Reactor is a fluoride salt-cooled high temperature reactor concept developed by Oak Ridge National Laboratory in 2010. Fluoride salt-cooled high-temperature reactors, or FHRs, are a new class of thermal-spectrum nuclear reactors defined by their use of liquid-fluoride-salt coolants, together with tri-isotropic (TRISO)-coated particle fuels and graphite moderator materials. FHRs operate with primary system pressures near atmospheric pressure and at coolant temperatures in the range of 600°C to ~1000°C. FHRs combine and leverage technologies and system architectures originally developed for molten salt reactors, gas-cooled reactors, and liquid-metal-cooled reactors to provide functionalities not otherwise attainable with traditional reactor concepts.

The SmAHTR concept draws upon work conducted between 2002 and 2006, during which time the original large FHR concept, the Advanced High-Temperature Reactor (AHTR), was developed. SmAHTR is a 125 MWt, integral primary system FHR concept (Figs. 1 and 2). SmAHTR employs a “two-out-of-three system” philosophy for operational and shutdown decay-heat removal. Transition from operational power production to shutdown decay-heat removal is accomplished without active components.

The design goals for SmAHTR are to deliver safe, affordable, and reliable high-temperature process heat and electricity from a small plant that can be easily transported to and assembled at remote sites. The initial SmAHTR concept is designed to operate with a core outlet temperature of 700°C, but with a system architecture and overall design approach that can be adapted to much higher temperatures as higher-temperature structural materials become available. The SmAHTR reactor vessel is transportable via standard tractor-trailer vehicles to its deployment location (Fig. 3). Tables 1 and 2 provide SmAHTR’s principal design and operating parameters.

Attributes
Figures
 * Fig. 1. SmAHTR integral primary system concept.
 * Fig. 2. SmAHTR dimensions.
 * Fig. 3. SmAHTR reactor vessel can be transported via tractor-trailer.

Abbreviations
Several fuel and core design options for SmAHTR were investigated during the design evolution. These designs included solid cylindrical fuel “pins” in stringer fuel assemblies (which was the baseline design for the AHTR), hollow annular fuel pins in stringer fuel assemblies, and solid plate or “plank”-type fuel elements. The most developed and analyzed configuration, and current baseline design, is based on the annular fuel pin design. However, the plank-type fuel assembly in a “cartridge core” configuration appears to offer many operational advantages and is being further investigated at this time. Additional work is needed to arrive at an operational fuel assembly design that simultaneously accomplishes all of the required functions. A pebble-bed variant is also possible.
 * aBOL = beginning of life;
 * IHX = intermediate heat exchanger;
 * PHX = primary heat exchanger;
 * DRACS = director reactor auxiliary cooling system.
 * bCore uranium loading depends upon the fuel concept employed and the refueling interval requirement. The range presented encompasses all fuel concepts and refueling intervals considered to date in the trade study.
 * PHX capacity, 3 operating/2 operatinga	MW	42/63
 * PHX secondary salt		FliNaK
 * PHX secondary flow, each, 3 operating/2 operating	kg/s	247/370
 * DRACS heat losses (per DRACS)	MW	0.45
 * DRACS heat losses, total (3 DRACS)	MW	1.35
 * aDRACS = direct reactor auxiliary cooling system; PHX = primary heat exchanger.

The SmAHTR design takes advantage of the existing safety philosophy of several small modular reactors. The reactor employs passive decay-heat removal systems relying on natural convection, and the core is designed with large negative reactivity feedback coefficients. The core and all primary components are contained in the reactor vessel (integral design). This design eliminates the large break loss-of-coolant accident (LBLOCA) scenario. Only intermediate-loop piping carrying non-radioactive coolant penetrates the vessel. The passive decay-heat removal design eliminates the reliance on off-site power, which is necessary if the reactor is to be sited in remote locations, and the need for safety-related emergency on-site AC power. The reactor can be seen as having several barriers to fission product release in the case of an accident: coated particle fuel, the graphite moderator, the reactor vessel/guard vessel, and the containment.

Applications
The SmAHTR concept has been developed with three potential operating modes and applications in mind: (1) process heat production, (2) electricity production, and (3) a combined cogeneration mode in which both electricity and process heat are produced. The capability to cluster multiple reactors to meet energy demands greater than those which can be met by a single reactor unit is an important design consideration for small modular reactors. This is certainly the case for any reactor concept designed for both electricity production and process heat applications. However, numerous questions and issues arise whenever multiple reactor units are interconnected. Only integration methods that do not compromise system safety or reliability can be considered. The interconnection or “ganging” of multiple reactor units to drive shared electrical power conversion systems has been widely discussed by reactor vendors and advanced concept developers for many years. However, the matter of the correct approach for clustering multiple small reactor units to meet intermediate-to-large process heat loads has received much less attention. The use of an innovative liquid-salt thermal energy storage system, or “salt vault,” notionally depicted in Fig. ES.4, expands the flexibility and applicability of the SmAHTR reactor for all applications. The salt vault offers three distinct functionalities: (1) the potential to combine multiple SmAHTR reactor modules to meet thermal energy and electrical power generation demands much greater than 125 MWt, (2) a robust capability to buffer the reactors and the process heat load from transients (such as reactor shutdown or time-varying heat demand) on either side of the salt vault interface, and (3) the ability to buffer multi-reactor module installations from upsets within a single reactor.

As a high-temperature system, SmAHTR is potentially compatible with several highly efficient (>40% thermal efficiency) power conversion technologies. The most attractive options for power conversion systems are Rankine and Brayton cycle technologies. One particularly appealing option is to couple the system to a high-efficiency closed-cycle supercritical carbon dioxide (S-CO2) power conversion system. S-CO2 power conversion is an emerging technology that can potentially provide a combination of small system components and high operating efficiency even at modest operating temperatures. Multi-reheat helium Brayton systems are also viable options, though they are less compact than the supercritical Brayton systems. Based on work to date, it appears small modular FHRs are technically feasible. Additionally, it appears small FHRs, such as SmAHTR, would provide siting and applications flexibility unparalleled by other types of reactors.

Fig. 4. SmAHTR salt vault thermal energy storage system.

Though a detailed SmAHTR cost and economic analysis cannot yet be performed due to the relative immaturity of the concept, there are reasons to expect that SmAHTR would be an economically attractive system from both capital cost and operating cost standpoints. The intrinsic virtues of FHR systems and technologies (low pressure, high volumetric heat capacity coolant, high-fuel-temperature thermal margins, etc.), when coupled with the potential for further capital cost reductions stemming from factory fabrication of small reactor modules, hold much promise. Significant potential offsetting considerations include the cost of high-temperature nuclear structural materials and the fabrication techniques required to produce complex components (such as heat exchangers) from them.

The SmAHTR concept presented here is but a snapshot of a complex technology and system architecture trade space. SmAHTR and the SmAHTR salt vault thermal energy system described in this report are not optimized systems. Additional concept definition work to be done includes the following.
 * Optimization of the fuel and core design
 * Further definition of the reactivity and instrumentation and control system
 * Expanded transient and safety analyses (including generation of a pre-conceptual phenomena identification and ranking table, or PIRT, as soon as the concept is sufficiently defined to enable the exercise)
 * Optimization of the in-vessel primary heat exchanger and direct reactor auxiliary cooling system heat exchanger designs
 * Optimization of the salt vault system design and its interface with the reactors
 * optimization of the SmAHTR electrical power conversion system design and the interface to it
 * Preliminary SmAHTR capital cost estimates