Methodology to Ascertain Core Recriticality Following a Severe Fission Reactor Accident

International Nuclear Safety Journal, Vol. 1, No. 1 (2013)
Methodology to Ascertain Core Recriticality Following a Severe Fission Reactor
J. J. Bevelacqua*

Bevelacqua Resources, 343 Adair Drive, Richland, WA 99352 USA

Abstract: Recriticality concerns have been expressed following severe power reactor accidents, involving fuel melting and possible core relocation out of the reactor vessel.

Analyses of reactor coolant liquid samples must carefully evaluate fission isotope
activities when assessing the possibility of a recriticality. Assessments should consider
the criticality duration, sampling time, sampling location, and core operating history to
provide an accurate determination of a recriticality. By considering these factors, the
131 137I/Cs activity ratio can lead to a determination if a recriticality occurred…..

At the Fukushima Daiichi Nuclear Power Station (FDNPS) Units 1, 2, and 3, core
geometry was not only lost, but reactor pressure vessel (RPV) and containment vessel
(CV) integrity may have been compromised [5]. Fuel apparently melted through the
bottom of the RPV and CV. It is believed that quantities of fuel reside on the bottom of
the RPV, bottom of the CV, and possibly on the reactor building floor [5].

The loss of core geometry and loss of fission product barrier integrity enhances
the possibility of core recriticality, and complicates it detection. This condition is
exacerbated because the status of the control rods is uncertain, the core cooling flow
paths are dynamic, fuel may have been relocated out of the reactor vessel, and changing
plant conditions permit the potential for deboration of the water cooling the fuel. Each of
these events could result in a subsequent recriticality [2].

A criticality would normally be detected by the neutron count rate, but the loss of
well-defined core geometry with fuel relocation in a severe accident may limit the
effectiveness of nuclear instrumentation [1]. This instrumentation may have been
damaged during the accident or become inoperable as defueling proceeds [1]. The
unavailability of nuclear instrumentation would require an alternative method to
determine if a recriticality occurred. This paper outlines an alternative approach to
determine if a recriticality occurred and its effectiveness in assessing the nature of the

2.0 General Approach
A recriticality could be recognized from the production of short-lived fission
isotopes (e.g., 131I) if the event occurred during recovery operations after the core’s
fission product inventory of short-lived nuclides had decayed. In the case of TMI-2, in
core instrumentation was damaged when the fuel melted [1]. As such, recriticality
detection depended upon other measurements such as reactor coolant samples and
temperature measurements.

Gas samples could also detect a recriticality, but these samples might not be
definitive since fission gases will be trapped in undamaged fuel as well as melted fuel.

Noble gases were liberated during TMI-2 defueling operations, particularly during
plasma arc cutting operations [1]. In view of the potential issues with nuclear
instrumentation and possibility of noble gas emissions during defueling operations, this
paper examines the analysis of liquid samples and their potential to determine if a
recriticality occurred.

Matsui [3] performed an analysis of Fukushima Daiichi liquid samples to
ascertain the possibility of a recriticality following the accident. However, the analysis
only considered radioactive decay as a fission product removal mechanism and did not
examine details of the core operating history. Based on experience at TMI-2, other
removal mechanisms and the specific sampling location must be also addressed to
determine if a recriticality occurred.

3.0 Formalism

A criticality results in a nuclear reaction that produces neutrons and fission
products. For the purpose of this paper, neutron detection is assumed to be unavailable or
unreliable. This is a reasonable assumption, because the neutron count rate determination
is affected by core alterations that occur during defueling operations and original nuclear
instrumentation and associated systems may have been damaged during the accident or
recovery activities [1]. In addition, fuel may have been relocated out of the reactor
vessel, which limits the usefulness of original nuclear instrumentation.
The quantity of fission products produced in a criticality depends on a number of
factors including the number of fission events, nuclide that fissions, the duration of the
criticality, time the liquid sample is drawn following termination of the criticality, and the
sampling location.

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