The reactor, (APRFR), is designed for both self-limited,
super-prompt-critical pulse operation and steady state operation. The
maximum available pulse has a yield of ~2.1 × 10¹⁷ fissions, while
steady state operation is limited to about 10 kilowatts by the reactor
core cooling system and activation of the core.
[Illustration: High Yield Prompt Pulse Shape]
The APRFR is an advanced version of the Health Physics Research
Reactor (HPRR) at Oak Ridge National Laboratory (ORNL), which has
been operating since 1962. ORNL has played a key role in the design
and testing of the APRFR. In pulse operation, the power level may
rise on periods as short as 10 microseconds. Electro-mechanical scram
systems are too slow to terminate such an excursion. Shutdown results
from increased neutron leakage due to fuel expansion, resulting in
a large prompt negative temperature coefficient of reactivity. This
self-limiting feature depends almost entirely on the thermal expansion
of the fuel alloy, and thus it is regarded as completely reliable and
safe.
Following a pulse, additional reactor shutdown capability is provided
by a safety block which, when ejected from the core, reduces the
reactivity to about 20 dollars below delayed-critical. At lower yield
pulses, below about 6 × 10¹⁶ fissions, the safety block is ejected by
the electro-mechanical scram system in about 0.1 seconds after a pulse.
At higher yield pulses, the safety block is ejected in much shorter
times due to thermo-mechanical shock forces which cause the safety
block to bounce out. The large shutdown margin provided by the safety
block is also the primary design device for preventing accidental
criticalities during periods of reactor shutdown.
The APRFR core is an unmoderated cylindrical assembly containing about
125 kilograms of an alloy of uranium 235 containing 10% molybdenum. The
actual core mass varies with the experiment. The core is cylindrical
and consists of two concentric annuli: a fixed outer shell of stacked
fuel discs bolted together with nine fuel bolts and Inconel nuts and
a movable inner safety block, also of fuel alloy. The 1½-inch OD
“glory hole” runs vertically through the center of the safety block.
Key reactor data is summarized in Tables I and II. The APRFR has been
operated during tests at ORNL at more than twice its design yield.
Table I.