P-LOFC = loss of forced circulation with primary loop remaining at pressure.4.1 P-LOFC: The reference case P-LOFC for the PBMR is similar to the corresponding GT-MHR
accident., with a peak fuel temperature of 1266°C occurring at ~37 hours, with a maximum
reactor vessel temperature of 501°C at 77 hr. Sensitivities to variations in the emissivities of the
vessel and RCCS are nearly identical to those for the GT-MHR.
4.2 D-LOFC: In the D-LOFC reference case “conduction-heatup” accident, T(fuel)-max peaks at
1517°C 77 hr into the accident, and for this configuration, maximum temperatures for the reactor
vessel (SA 508) and core barrel (316 SS) are not of concern.
4.4 P-LOFC with ATWS: In this PBMR design, recriticality occurs at about 28 hours, and T(fuel)-
max reaches 2127°C at 103 hr. Maximum vessel temperatures are also higher, 711°C at 145 hr.
Fuel failure after 7 days was 57%. Variations in this accident are sensitive to fuel and moderator
temperature-reactivity feedback coefficients. As with the GT-MHR, if after recriticality the SCS
is started (with still no scram), peak fuel temperatures would exceed limits even more due to the
4.5 D-LOFC with ATWS: Recriticality occurs at 31 hr. In this case, T(fuel)-max is 2166°C at 137
hr, and the maximum vessel temperature (496°C at time = 168 hr) was still rising slowly after a
week. Fuel failure at the end of the week was 59%.
D-LOFC = loff of forced circulation with primary loop failed (depressurized).
ATWS = anticipated transient without scram. (control rod or logic/electronic failure).
This is pretty serious. I'd thought the pebble bed helium cooled reactors were inherently safe. Clearly such statements are too bold.