A Review of Criticality Accidents A Review of Criticality Accidents

Figure 64. Energy model computation of power vs time. The initial reactivity is 1.0 $ above delayed critical. Neutron
lifetime values are 10 -8 , 10 -6 , and 10 -4 seconds. The bottom panel shows the corresponding curves of reactivity vs
time.
t 1/2 = 3.52 l / ∆k p
(3)
where l = the neutron lifetime and the maximum power
is
dE/dt max ≈ 2 ∆k p 2 / 3.5 bl (4)
The experimental systems that have been intensively
studied and that exemplify the data in Figures 63
and 64 are the Godiva, KEWB, and SPERT reactors,
and the CRAC experiments.
Godiva I and II were near solid uranium (93% 235 U)
metal critical assemblies, pressed into service as
irradiation facilities. At a few cents above prompt
criticality, controlled prompt excursions provide an
excellent experimental picture to complement the
curves of Figures 63 and 64. The prompt negative
temperature coefficient of reactivity of about
4.3 × 10 3 $/°C (depending on the model) arises from
thermal expansion and is directly related to the
deposition of fission energy. The transient proceeds so
rapidly that no heat is lost from the system. When the
step change of reactivity is increased to 4 or 5 ¢ above
prompt criticality, a new effect sets in. The power rises
to such high values that the thermal expansion lags the
energy deposition and the simple ratio of E and ∆k p in
Eq. (2) is no longer true. At still higher step changes,
the energy release becomes proportional to the square
and eventually to the cube of the initial excess reactivity.
Structural damage from shocks commences at 10
or 11 ¢, thus providing a limit for planned repetitive
bursts.
The transient behavior of solution systems has been
studied with the two KEWB reactors. The KEWB–A
core was a 13.6 l stainless steel sphere containing
11.5 l of highly enriched UO 2 SO 4 solution; the
reflector was thick graphite. This reactor provided a
means of studying transients in solution systems during
which the period was as short as 2 milliseconds. The
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