Prismatic core reactor design

Figure 3.5 GTHTR300 s prismatic core 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. 

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. 

A prismatic block core, with fuel and reflector blocks essentially identical to the GT-MHR design, was built and operated to power the Fort St. Vrain generating station. Open cycle gas turbines are operating at temperatures enveloping the GT-MHR conditions. Component testing will provide additional experience and understanding in areas not covered by Fort St. Vrain and open cycle gas turbines. However, Fort St. Vrain differed from the GT-MHR in important aspects (e.g. use of a pre-stressed concrete reactor vessel vs. a steel reactor vessel. 

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