Wolf Creek (USA)

Map of Wolf Creek

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1200 MW PWR; constructed by Westinghouse; operation since 1985

Facilities in Wolf Creek

plantreactor typconstruction startoperation startshut down
Wolf Creek-1PWR19771985

Westinghouse, which has assigned 100 employees to investigate recent control rod drop problems, has not yet determined the root cause of rod mispositioning.
Five control rods at Wolf Creek and seven South Texas-1 didn't properly drop into the cores after plant trips in December and January, respectively.
Westinghouse does not consider the problem a significant safety issue.
In both cases, the rods were housed in 17 x 17 " assemblies with burnups exceeding 42,000 megawatt-days per metric ton uranium (MWd/t).
Though thimble tube distortion is seen as the apparent cause at Wolf Creek, company officials believe there are myriad other factors that must be explored before the root cause can be determined.
"We're working very hard to understand what's happening." Westinghouse speaker N. Marshall said last week.
King told NRC staff that an incident at Virginia Powers North Anna-1 in February is not part of the investigation because it did not involve control rod insertion problems. Crews at North Anna had trouble retrieving control rods that were temporarily stored in used fuel in the spent fuel pool. The assemblies involved at North Anna, as at South Texas and Wolf Creek, had high burnups.


The January ice-induced trip of Wolf Creek Nuclear Operating Corp.´s Wolf Creek station has been rated a Level 2 event on the IAEA´s Nuclear Event Scale (INES).
At Wolf Creek in January, operators manually tripped the plant from 80% power after ice - clogged intake screens and trash racks, and hindered water intake on the circulating and service water intake systems. Following the trip, five control rods failed to properly drop into the reactor core.( it took them up to 20 minutes to fall in their proper positions!)
The IAEA said the "failure potential introduced by the frazil ice phenomena" and the fact that a control room operator did not properly align essential service water system (ESW) valves, when the plant went to that system for decay heat removal, warranted a Level 2 rating. Ice also challenged the one working train of the ESW system.
After this incident the plant started the refueling outage a month early. The plant was down for 63 days. Modification of the ESW system is planned, that will increase the flow od warm water andprevent a recurrence of the icing problem.


On 17 September 1994, operators at the Wolf Creek pressurized water reactor in
Burlington, Kansas made mistakes as they opened and closed valves. The reactor had
been shut down 28 hours earlier for refueling. The residual heat removal system was
being used to remove the large amount of decay heat still being produced by the
irradiated fuel in the shut down reactor core. The erroneous valve line-up allowed
nearly 9,200 gallons (35 m3) of reactor cooling water to flow to the refueling water
storage tank. The inadvertent drainage of reactor coolant water was stopped after
about one minute by an operator who closed a valve.
The NRC investigated the event and concluded that, had operator intervention not
occurred, the reactor core cooling by the residual heat removal system would have
failed in about 3 ½ minutes. The NRC reported that restoration of reactor core cooling
would have been complicated because the water in the piping for the cooling pumps
would have been replaced by steam in further 2½ minutes. The operators would have
had to vent the piping and refill it with water before restarting the pumps needed to
restore reactor core cooling. The NRC estimated that the water level inside the reactor
vessel would have dropped below the reactor core in about 30 minutes had the
operators been unable to restore cooling water flow. The NRC calculated the severe
core damage risk from this event to be 3 x 10-3 or 0.3% per reactor year and rated it
Level 2 on the INES scale.
(source: Residual risk)


Inspection of hafnium control rods reveals unexpected swelling of the control rods cladding. (diametrical swelling of 20 to 25 mils) Concern was raised that a control rod could swell beyond the inner diameter of its guide tube, thereby occluding a rod drop.