Design PBMRs

Alternative reactor types are possible for the VHTR. China s HTR-10 [35] and South Africa s pebble bed modular reactor (PBMR) [41] adopted major elements of pebble bed reactor design including fuel element from the past German experience. The fuel cycles might be thorium- or plutonium-based or potentially use mixed oxide (MOX) fuel. 

The PBMR-268 model is derived from the PBMR-268 design. Numerous assumptions were made regarding the geometry of the PBMR-268 design in the benchmark specification. Some of the most important geometric simplifications are (Seker, 2005) ... 

Several reactors are candidates for use as a high temperature heat source for the S-I cycle. Candidates include the modular helium reactor (MHR) and pebble bed modular reactor (PBMR). One of the most thoroughly investigated candidates is the PBMR. Recent work has been performed in benchmarking the THERMIX code to the PBMR-268 design (Reitsma, 2004 Seker, 2005). 

Recently, the fluidized bed membrane reactor (FBMR) has also been examined from the scale-up and practical points of view. Key factors affecting the performance of a commercial FBMR were analysed and compared to corresponding factors in the PBMR. Challenges to the commercial viability of the FBMR were identified. A very important design parameter was determined to be the distribution of membrane area between the dense bed and the dilute phase. Key areas for commercial viability were mechanical stability of reactor internals, the durability of the membrane material, and the effect of gas withdrawal on fluidization. Thermal uniformity was identified as an advantageous property of the FBMR. 

A simplified ID model illustrated the general features of the PBMR and demonstrated significant differences between FBR and PBMR, due to the different dosing concepts applied. This simple model revealed the need for a careful design of PBMR. 

Altogether, the results presented indicate the overall potential of using optimized dosing concepts, (e.g. in a PBMR), in order to improve the performance of complex chemical reactions. The theoretical framework presented allows as for rapid first estimations in early development stages as for more detailed studies required for process design and optimization. 

Hinssen, H.-K., Ktlhn, K., Moormann, R., Schlogl, B., Fechter, M., and Mitchell, M. Oxidation experiments and theoretical examinations on graphite materials relevant for the PBMR. Nucl. Eng. and Design. 238, 3018-3025 (2008). 

The PBMR is very interesting from the safety point of view even if the design of the system appears rather complex. The intrinsic safety characteristics declared seem to be feasible, under the condition that the detaU system design is submitted to an attentive safety analysis, and this inelndes surveillance systems for structures and components. 

Caravella et al. (2008) PBMR - ID - Nonisothermal conditions - Plug flow - Membrane completely selective toward H2 permeation - Ergun s equation - Xu and Froment (1989a) The paper pointed out that the reactive/permeative stage distribution has to be considered an important reactor design parameter. 

The major resource advantage over other burner designs may seem surprising given that a conversion ratio of 0.8 does not appear that much superior to LWR and PBMR (both 0.5-0.6) or CANDUs (0.7). However, conversion ratios do not take into account the limited residency time of fuel in solid-fueled reactors. Perhaps, a new term of effective conversion ratio would be to compare fissile consumption versus needed annual fissile additions. By this metric, most other reactors on a once-through cycle have effective conversion ratio of near zero since they consume about 1000 kg/GWe-year but need to add 1000 kg of fissile 235 U per year. Even with Pu recycle, they do not improve significantly. Thus, the great... 

The SSTAR reactor is coupled to a supercritical carbon dioxide (S-CO2) Brayton cycle power converter. It provides higher cycle efficiency than a helium ideal gas Brayton cycle or a Rankine saturated steam cycle operating at the same core outlet temperature. A key contributor to the high efficiency is the low amount of work (PdV work) to compress S-CO2 immediately above its critical temperature - due to the high S-CO2 density. Table XXII-6 compares the densities of S-CO2 at cycle conditions versus those for helium in the Eskom Pebble Bed Modular Reactor (PBMR) as well as typical liquid coolants the S-CO2 density is more like that of an ordinary liquid. Thus, the S-CO2 temperature and pressure at the low end of the cycle are designed close to but slightly greater than the critical temperature (30.98°C) and pressure (7.373 MPa) to exploit the small PdV work of compression. 

PBMR-400 (South Africa) 400 278 Yes - Yes 2010 Preliminary design - 1.3% X Direct cycle gas turbine. 

Similar though untitled strategies are used in the designs of SMART, CAREM, PBMR-400, AHWR, HTR-F, and VHTR (Generation-IV). The SVBR-75/100 relies to a high degree on the inherent safety features, and so do the authors of a FBNR concept. 

The designs of CAREM, SMART, PBMR, SVBR-75/100, and HTR-F implement an approach where the reactor and safety systems are jointly optimized in order to ensure a cost effective safety design. The innovative methodology and tools were specially developed for this purpose. 

To facilitate the adjustment of safety requirements and regulations, the designers of IRIS have kept informed the IAEA of their activities from an early stage and are planning to have an IAEA safety review. The need of an early involvement of regulators was noted in the presentations on the advanced high temperature gas cooled reactors HTR-PM and PBMR-400. 

Many designers identified an intention to license their innovative SMRs with the reduced or eliminated off-site emergency planning requirements (IRIS, SMART, CAREM, SAKHA-92, ABV, KLT-40S with long-life core, VEER, RIT, FBNR, PBMR-400, HTR-PM, AHWR, SVBR-75/100, and the STAR family ). However, it was confirmed that no example of such licensing exists at the moment. 

The targeted deployment dates, when specified by the designers, are generally between 2010 and 2020. The earliest dates for a demonstration prototype operation start-up are specified for the projects of PBMR, GTHTR-300, and HTR-PM they are around 2010. 

Over the past four years, the PBMR has moved from a conceptual to the detailed design stage. The timeline of readiness for demonstration plant deployment was rescheduled from 2005 to 2010. 

Preliminary PSAs have been performed or are in progress for the designs of PBMR, GT-MHR and GTHTR300. The evaluated core damage frequencies are very low (10 1/year), which motivates all designers to specify reduced or eliminated off-site emergency planning requirements (Level 5 in Table 4). 

An underground or a half-underground location of the reactor cavity and modules is provided in the designs of PBMR, GT-MHR and GTHTR300 as an enhanced protection against the external events, including those of malevolent human-induced origin. 

Most of the innovative SMRs addressed in this report require a prototype plant to be built to demonstrate reliable operation and qualify certain innovative features. Many of the considered SMRs are still at the conceptual design stage and would require multiple further R D. Although there are examples of industry and utilities involvement in the design and technology development for innovative SMRs (e.g. PBMR, CCR, IMR, etc.), these are the commitments of governments that remain decisive for the progress in SMR development and deployment... 

Initial PBMR development is focused on completion of the detailed design and engineering for a demonstration unit to be located at the Koeberg NPP site north of Cape Town, South Africa. The full cost of the demonstration plant is estimated at US 1 billion. 

The South African government has designated the PBMR a national strategic project with a cabinet level committee appointed in February 2004. The demonstration plant site preparation is scheduled to begin at the Koeberg NPP site in the first quarter of 2007 with fuel loading anticipated for mid-2010. The commercial acceptance by ESKOM is scheduled for early 2011. 

Participation in the IAEA s coordinated research programme on Evaluation of high temperature gas cooled reactor performance is providing independent validation of the codes and models being used in the design of the PBMR. 

The PBMR is a land based nuclear power plant. All major components including the reactor and power conversion system vessels and associated internal components are being sized and designed for ofif-site prefabrication capability. The transportability design intent is for the PBMR to be commercially available to a broad latitude of customers worldwide. 

TABLE XW-l. MAJOR DESIGN AND OPERATING CHARACTERISTICS OF THE PBMR... 

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