A Review of Criticality Accidents A Review of Criticality Accidents

C. OBSERVATIONS AND LESSONS LEARNED FROM PROCESS CRITICALITY ACCIDENTS
There have now been 22 reported accidents in fissile
material process operations. Significant, and often
painful, lessons have been learned from these accidents.
These lessons are associated with the following
design, managerial, and operational attributes: communications;
procedures; fissile material accountability
and accumulation; vessel geometry and volume;
operator knowledge; new restarted, and one–of–a–kind
operations; equipment malfunction; and unanticipated
movement of solutions. This review has also revealed
the actual magnitude and breadth of accident consequences
and the value of criticality alarms. While not
always readily apparent or emphasized during accident
investigations, other significant factors that influence
accident risks are: (1) senior management awareness
and involvement in safety in general and criticality
safety in specific; (2) regulatory agency personnel
awareness and involvement; and (3) national and
international consensus standards and regulations that
are both corporate and governmental.
It is important to note that there have been no
accidents that were caused by a single failure. That is,
there were always multiple causes for each of the 22
accidents. It is also noteworthy that equipment failure
or malfunction was either a minor or a non-contributing
factor in all of the accidents.
That lessons have been learned from past criticality
accidents is made clear from their time histogram,
Figure 1. For about the first decade of operations with
significant * quantities of fissile materials, there was not
a reported accident. This was likely associated with the
relatively small scale of individual operations and the
relatively small amounts of fissile material (almost
exclusively plutonium and enriched uranium) that was
available.
However, between the late 1950s and the middle
1960s there was about one accident per year in both
the R.F. and the U.S. During this time there was a very
large increase in the production of fissile material and
in the scale of operations at the process sites. Since the
middle 1960s, the frequency of accidents dropped by a
factor of about 10, to approximately 1 per 10 years.
This drop can be attributed to several factors. First
there were significant lessons learned from the earlier
accidents such as the need to avoid unfavorable
geometry vessels. Secondly, there was a significant
increase in management attention to criticality safety,
particularly the presence of staff devoted specifically to
controlling this hazard. These accidents also prompted
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those with criticality and operational responsibilities to
begin to document critical mass data and operational
good practices. The first compilations of data began
appearing in the late fifties, and the first national
standards in the mid-sixties.
From a review of all the process accidents, we can
summarize the findings into two categories: observations
and lessons learned. The former are simply facts
observed at the time; while the latter are conclusions
that can be used to provide safety guidance to enhance
future operations. Both categories are discussed in the
following sections.
Observations
The following are factual observations from the
22 reported process accidents with some elaboration as
they may apply to the lessons learned.
• The accident frequency rose from zero in the first
decade of operations with significant quantities of
fissile material to a high of about one per year in
both the R.F. and the U.S. during the years around
1960. The frequency then dropped noticeably to
about one per ten years and has seemingly remained
there. It has been suggested that in the second decade
there was a significant increase in both the
production of fissile materials and in the scale of
operations at process sites, without commensurate
attention to criticality safety. Certainly lessons
learned from these earlier accidents contributed to
the later improved record.
• No accident occurred with fissile material while in
storage. This should not be surprising considering
the relative simplicity of this operation and the ease
of controlling criticality.
• No accident occurred with fissile material while
being transported. This should not be surprising
given both national and international transport regulations.
These regulations specify defense in depth
in criticality safety that goes far beyond what would
be practical and cost–effective for plant operations.
• No accident resulted in significant radiation consequences
beyond the facility site, either to people or
to the environment. This reinforces a commonly
held contention that criticality accidents are similar
to small, bench–top scale, chemical explosions in
their personnel and environmental consequences,
i.e., they are worker safety issues.
* The term “significant” as used here refers to having sufficient fissile material to sustain a chain reaction. The actual quantity of
material being processed during the first decade was much less than during subsequent decades.