Pebble-bed reactors

The pebble-bed reactor is the most viable reactor design option for near-term deployment, say researchers at the Massachusetts Institute of Technology (MIT) and a South African consortium called the Pebble Bed Modular Reactor (Pty) Ltd., headed by Eskom of South Africa with U.K.-based British Nuclear Fuel, Exelon Corp., Chicago, and the IDC (South African Industrial Development Corp.).
Both group's designs are based on a German HTGR reactor built over 20 years ago. And both are radically different from pressurized or boiling-water reactors currently in use in the U.S. Instead of conventional fuel rods, these pebble-bed reactors employ 330,000 tennis-ball-sized fuel elements or pebbles. Each pebble consists of an outer graphite matrix covering and an inner fuel zone. Fuel zones contain about 9 gm of enriched uranium separated into 15,000 particles or kernels called Triso fuel.
Each 0.5-mm-diameter uranium kernel is surrounded by a porous carbon buffer layer that accommodates fission gas release. A protective outer layer of silicon carbide, sandwiched between pyrolitic carbon, contains the fuel and radioactive decay products produced during fission.
The Triso fuel is said to withstand temperatures on the order of 2,000°C, well above the working temperature inside the reactor core.
The pebble-bed reactor houses the fuel pebbles, and 110,000 similarly sized graphite balls that act as neutron reflectors in a central column of pebbles in the core. Helium gas passed over the pebbles removes heat from the chain reaction, raising gas temperature to about 900°C. The heated helium directly or indirectly drives gas-turbine-driven generators to produce electricity.
How helium is used in the secondary power generating system or balance of plant (BOP) differs between the Eskom Pebble Bed Modular Reactor (PBMR) and MIT Modular Pebble Bed Reactor (MPBR). The Eskom design uses a direct Brayton cycle. Here the hot, high-pressure helium exits the reactor, enters the BOP, and directly powers the turbine generating systems. The generator set weighs approximately 28 tons and rotates at 3,000 rpm. It could mount vertically and would be supported entirely by two radial magnetic bearings and an axial magnetic bearing.

A heat exchanger or recuperator improves plant efficiency by heating the helium as it cycles back to the reactor. Ten of these modules ganged together and run by a centralized control center would produce 1,100 MW of power. This pebble-bed park would fit in an area of about the size of three soccer fields.
The use of a closed cycle and a helium-powered gas turbine with magnetic bearings gives the Eskom PBMR a thermal efficiency of approximately 45% compared to 33% for a conventional pressurized-water reactor (PWR).
Contrast this with the MIT MPBR. It uses an indirect Brayton cycle and an intermediate heat exchanger (IHX). The helium in the reactor is an independent closed cycle. Heat transfers from the reactor helium to the closed BOP helium system via the IHX.
According to MIT Professor, Andrew Kadak, the success of the MIT pebble-bed project hinges on the ability to package the reactor, the IHX, and the remainder of the BOP in such a way to let all modules be trucked to the site rather than by barge. The plant must also be easily assembled with minimal tooling and rework and operate in the same size footprint as conventional power plants. The modules can't be longer that 60 ft, nor bigger than 8 × 12 ft. And weight must not exceed 200,000 lb (trailer-truck capacity).
The MIT pebble bed has already undergone a number of design revisions. For example, it originally employed a two-shaft (turbine) design — one high-speed high-pressure (HP) turbine runs all three compressors sets. A second turbine driven by HP turbine exhaust runs the power generator.
Now MIT is evaluating a four-shaft system where one low-speed turbine powers a generator and three separate turbocompressor sets. The change limits shaft power of any one turbine to less than 36 MW to match current state-of-the-art turbomachinery. In addition, reducing the length of each turbocompressor set makes for a simpler plant layout, as each shorter shaft can be positioned in adjacent modules, horizontally or vertically.
To limit the IHX weight, the group plans to split the single IHX into six smaller IHX modules, each with its own containment vessel. This will bring each module to within the 200,000-lb truck limit. Splitting the IHX into smaller modules also eases maintenance. The recuperator will also be split into six modules for easier placement near the IHX modules.
Each identical module will be built in a centralized factory and modules can be interchanged. MIT's modular approach is said to have the best chance of competing with an industry benchmark — a 200-MW natural-gas-fired plant with an efficiency of 50%. The MIT pebble bed should be about 45% efficient. It will run continuous thanks to an online refueling system. Short maintenance outages are expected every five years or so, says Kadak.
The pebble bed reactor will produce about 110 MW or about 10% that of a conventional nuclear or about a fourth that for a fossil fuel plant. And PBMR cost/kW-hr reportedly will be on par or less than existing fossil fuel systems. Modularity will let energy suppliers tailor power plants to meet current and future energy needs. Unlike conventional construction methods, the MIT modules will be factory built, then assembled onsite. This should lower construction costs, improve quality, and speed construction.