Prismatic reactor design

Figure 3.1 Pebble bed reactor design and prismatic reactor design.

Accident evaluations specific to the GT-MHR confirmed that the passive safety characteristics of the previous steam cycle modular high temperature gas-cooled reactor designs were maintained. Events initiated by one or more turbine blade failures were assessed. It was found that the resulting differential pressure forces across the prismatic core did not exceed the allowable graphite stresses. Since the dominant risk contributor for the steam cycle design were initiated by water ingress from the steam generators, the GT-MHR is expected to have a lower risk profile to the public. References 4 and 5 provide more information on the GT-MHR safety evaluations. 

Molten reactor designs have been configured within a wide range of concepts. Similar to gas-cooled reactors (pebble bed, prismatic. He, CO2, etc.) this variety can be a drawback as it can tend to spread effort and support. Early efforts examined both a breeder approach termed the molten salt breeder reactor (MSBR) and a burner approach termed the denatured MSR (DMSR) while currently there is also renewed interest in fast spectrum designs. The following sections attempt to show the varied potential as well as differentiating factors for a variety of MSR designs. 

The excellent heat transfer properties of molten fluoride salts, compared with those of helium, reduce the temperature drops between (1) the fuel and molten salt and (2) the molten salt and any secondary system. Comparable calculations for a typical prismatic geometry were made of the temperature drop between the centerline prismatic fuel temperatures and coolant for helium and molten-salt coolants. The temperature drops for helium and molten-salt coolants were 415 and 280 C, respectively. The better heat transfer capabilities of molten salts (a liquid) compared with those of helium allow reactor designs with higher coolant exit temperatures and power densities than in gas-cooled systems for the same maximum temperature limit in the fuel. 

The GT-MHR shares certain technologies and design approaches with other prismatic block or pebble bed fuel high temperature gas cooled reactor designs described in this report, e.g. GTHTR300 (Japan), PBMR-AOO (South Africa), HTR-PM (China), etc. 

Figure 3.5 GTHTR300 s prismatic core reactor design.

Pebble bed and prismatic reactor are the two major design variants. Both are in use today. In either case, the basic fuel construction is the TRISO-coated particle fuel. Uranium, thorium, and plutonium fuel cycle options have been investigated and some have been operated in the reactors. Spent fuel may be direct disposed or recycled. The unique constmction and high bumup potential of the TRISO fuel enhances proliferation resistance. 

The HTTR is an experimental helium-cooled 30 MW(t) reactor. The HTTR is not designed for electrical power production, but its high temperature process heat capability makes it worthy of inclusion here. Construction started in March 1991 [47] and first criticality is expected in 1998 [48]. The prismatic graphite core of the HTTR is contained in a steel pressure vessel 13.3 m in height and 5.5 m in diameter. The reactor outlet coolant temperature is 850°C under normal rated operation and 950°C under high temperature test operation. The HTTR has a primary helium coolant loop with an intermediate helium-helium heat exchanger and a pressurized water cooler in parallel. The reactor is thus capable of providing... 

On the preliminary phase of the design development two variants of the core were analyzed on the basis of pebble bed and prismatic fuel blocks. As u result of caiculational, design and engineering analysis the pebble bed core was chosen for further development. The pebble bed core option for this reactor was made taking account of the following considerations ... 

Prismatic graphite-block fuel with traditional refueling. The LS-VHTR would be fueled with prismatic fiiel and refueled when shut down. This particular fuel geometry provides a large latitude for the reactor core designer in the choice of (1) fuel-moderator-coolant ratios and (2) core geometry. 

Refueling differences exist between sodium-cooled reactors and the AHTR. For the AHTR, refueling temperatures are somewhat higher, the fuel geometry is different, the power density of the prismatic-block fuel-type SNF is 1 to 2 orders of magnitude lower, the vapor pressures of the liquid salts are much lower than those of sodium, and the liquid salt is transparent whereas the sodium is opaque. This section provides discussions of design considerations for the LS-VHTR fuel-handling system relative to sodium-cooled fast reactors. 

The main design variants of this type of reactor have depended on whether fuel is of the pebble bed type or of the prismatic type. The pebble bed type consists of a large number of spherical fuel elements. The fuel element matrix is graphite and the fiael kernels are imbedded within the inner layer of the matrix. A fuel free graphite reflector shell is located inside the RPV. 

Non-conventional designs MARS - a fixed-bed fuel molten salt coolant reactor CHTR - a prismatic block fuel lead bismuth coolant reactor... 

Of the twenty-six concepts and designs addressed, 13 (50%) are water cooled SMRs, 6 (23%) are gas cooled SMRs-high temperature gas cooled reactors (HTGRs), 6 are sodium or lead-bismuth cooled fast reactors, and 1 is a non-conventional very high temperature reactor concept, a liquid salt cooled reactor with HTGR type prismatic fuel. 

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