Non-U.S. Nuclear Plant Workers Receive Internal
Alpha Radiation Exposures
In late 2009, approximately 200 workers at a non-US commercial nuclear power plant
received internal alpha radiation exposures. The exposure was associated with work
being done during an upgrade of the plant’s primary heat transport system. Sixty
workers received doses in excess of 200 millirem from alpha airborne contamination.
The highest exposure was approximately 1.6 rem. Assumptions made about the
radiation hazards and controls discounted possible alpha contamination at the job site.
Why this event was significant:

Many workers received unplanned internal radiation exposures, several received
exposures that exceeded the plant’s internal control levels.

Deficiencies in the plant’s alpha monitoring contributed to the work condition risks
not being recognized or mitigated.
Summary:
A non U.S. nuclear power plant was performing plant upgrades to two reactor units.
The work required that sections of primary heat transport piping be prepared for
welding. This preparation involves machining (milling) the ends of the feeder tubes.
Engineering controls established to control the spread of airborne particulate
contamination included use of an enclosed tool to perform the milling. Staff members
performing the work were used protective equipment appropriate for the analyzed
hazards.
Similar work had been completed previously on a companion unit at the plant. No
radiological problems were detected as a result of that effort. The same controls were
deemed appropriate for use in this second unit upgrade.
Work began to prepare the pipes for welding. Within the first 24 hours particulate
airborne activity was detected in the vault. The work area was the enclosed by tenting to
prevent particulate spread beyond the protected area. Initial radiological assessment of
the contamination identified it as long-lived activity and the contamination was
confirmed to be from cobalt-60. Subsequent off-site analysis of air samples later
identified that alpha contamination was present.
Radiological protection controls did prevent exposure to Co-60. However work controls
were inadequate to prevent exposure to airborne alpha contamination to workers
outside the tented enclosed work area. Even though initial field radiological monitoring
readings indicated that alpha contamination was present, these indications were not
heeded, the work continued and the alpha airborne contamination excursions continued
for several weeks until confirmed by off-site analysis.
Post event analysis:
Incident reviews found problems with alpha radiation detection equipment, radiation
protection decision making, and management oversight of the radiation protection
program. Concerns were identified that the radiation protection program did not enable
plant personnel to anticipate, monitor, evaluate, and react to an alpha airborne
particulate hazard. A particular concern was that personnel relied on invalid
assumptions that alpha contamination would not be present since no problems were
identified during similar work performed on the plants unit 1.
Important lessons:
1. It is a non-conservative practice to make assumptions about the potential for the
presence of alpha contamination on equipment and components that have been
shut down for an extended period.
2. Actual beta/alpha radiation ratios found in upgrade work and work on shut down
equipment may differ largely from ratios commonly encountered when the
equipment was in normal operation.
3. Radiation monitoring equipment such as counting equipment, whole-body
monitors, and so forth must have detection ranges capable of confirming the
presence of alpha contamination.
4. Unexpected indications in radiological conditions should be treated seriously and
considered as a basis for suspending work until through technical analyses are
conducted.