A Review of Criticality Accidents A Review of Criticality Accidents

A REVIEW OF CRITICALITY ACCIDENTS
ABSTRACT
Criticality accidents and the characteristics of prompt power excursions are discussed. Sixty
accidental power excursions are reviewed. Sufficient detail is provided to enable the reader to
understand the physical situation, the chemistry and material flow, and when available the administrative
setting leading up to the time of the accident. Information on the power history, energy
release, consequences, and causes are also included when available. For those accidents that
occurred in process plants, two new sections have been included in this revision. The first is an
analysis and summary of the physical and neutronic features of the chain reacting systems. The
second is a compilation of observations and lessons learned. Excursions associated with large
power reactors are not included in this report.
INTRODUCTION
This revision of A Review of Criticality Accidents1,2 represents a significant expansion of the prior edition
with the inclusion of one Japanese and 19 Russian
accidents. In the first two parts of this report, 60
criticality accidents are described. These are divided
into two categories, those that occurred in process
facilities (22), and thus were unexpected, and those that
occurred during critical experiments or operations with
research reactors (38).
These two categories are based on the following
considerations. Process facilities carrying out operations
with fissile material avoid criticality accidents through
physical and administrative controls. These controls are
intended to prevent critical or near–critical configurations
from ever occurring in the facility. Operating
personnel are usually not technical experts in criticality
physics. Under normal working conditions, however,
operating personnel can be close to (arm’s length from)
potentially critical configurations. In contrast, reactor
and critical experiment research facilities purposefully
plan and achieve near–critical and critical configurations.
These operating personnel are usually experts in
criticality physics. Although they carry out hands–on
operations with fissile material under restrictions similar
to those found in process facilities, the planned near–
critical and critical configurations are performed under
shielded or remote conditions.
In most cases, the descriptions of the Russian
accidents are somewhat lengthier than those that
occurred in other countries. This is attributable to the
lack of generally available references for the Russian
accidents. In other words, the descriptive information in
this report is all that is effectively available for these
accidents. It has been gleaned from both the original
Russian notes and discussions with those who had
personal knowledge of the accidents. With the exception
of the addition of the six Russian critical experiment
accidents, Part II of this report is basically unchanged
from that of the second edition.
Although this edition is planned to be available in
both English and Russian language versions, decisions
had to be made concerning units and terminology,
particularly in the English language version. We have
attempted to conform to common international usage.
However, units as originally reported are sometimes
retained for historical accuracy. Also, we have chosen
generally understood terminology and avoided terms
that might be facility or industry specific. For example,
the word “vessel” is used to describe most solution
holding containers. Thus “filter vessel” is used instead
of “filter boat,” a common U.S. chemical industry
term.
In Part III, a brief discussion of analytical methods
and quenching mechanisms for power excursions is
reproduced from the first revision. Two appendices are
also new to this revision. Appendix A is a reproduction
of LA-11627-MS, Glossary of Nuclear Criticality
Terms 3 (supplemented with one additional definition)
to assist the reader. In Appendix B diagrams of each of
the 22 process vessels in which the accidents took
place are shown along with tabular summaries of the
system parameters (mass, volume, etc.).
The emphasis of this revision has been threefold.
First, it was to include the one Japanese and 19
Russian accidents. Fourteen of these took place in
process facilities and six happened during critical
experiments and research reactor operations. A second
focus of this effort has been to thoroughly review the
eight process facility accidents described in the prior
revision of this document and their supporting
references. This led to some technical corrections and
modest expansions of the process descriptions for the
benefit of the expanded audience that this report has
attracted over the years. Third, two analyses of the 22
process accidents are included. These are (1) physical
and neutronic characteristics with an emphasis on
understanding systematic features and (2) observations
and lessons learned from these accidents.
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