NRC Bulletin 79-26, Boron Loss From BWR Control Blades
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Bulletin 79-26: Boron Loss From BWR Control Blades
SSINS: 6820
Accession No.:
7910250475
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
November 20, 1979
BORON LOSS FROM BWR CONTROL BLADES
Description of Circumstances:
The General Electric Company (GE) has informed us of a failure mode for
control blades which can cause a loss of boron poison material. Hot cell
examinations of both foreign and domestic blades have revealed cracks near
the upper end of stainless steel tubing and loss of boron from the tubes.
The cracks and boron loss have so far been confined to locations in the
poison tubes with more than 50 percent Boron-10 (B10) local depletion.
Observed crack sizes range from a quarter to a half inch in length and from
one to two mils in width.
GE has postulated that the cracking is due to stress corrosion induced by
solidification of boron carbide (B4C) particles and swelling of the
compacted B4C as helium and lithium concentrations grow. Once primary
coolant penetrates the cladding (i.e., the cracking has progressed through
the cladding wall and the helium-lithium pressures are sufficient to open
the crack), boron is leached out of the tube at locations with more than 50
percent B10 local depletion (local depletion is considered to be twice the
average depletion). It was further found with similar cracking but with less
than 50 percent local depletion of B10, that leaching did not occur even
though primary coolant had penetrated the cladding.
The cracking and boron loss shorten the design life of the control blade.
According to the GE criteria the end of design life is reached when the
reactivity worth of the blade is reduced by 10 percent,, which corresponds
to 42 percent B10 depletion averaged over the top quarter of the control
blade. Because of the leaching mechanism, GE has reduced the allowance for
B10 depletion averaged over the top quarter of the control blade from the 42
percent value to 34 percent.
The safety significance of boron loss is its impact on shutdown capability
and scram reactivity. Although shutdown capability is demonstrated by
shutdown
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margin tests after refueling, the calculated control blade worths used in
the tests are based on the assumption that no boron loss has occurred.
Reduction in scram reactivity due to boron loss could increase the severity
of Critical Power Ratio (CPR) reductions during the plant transients and
could increase the consequences of control rod drop accidents.
Because the locations of limiting Linear Heat Generation Rate (LHGR), CPR,
and Average Planar LHGR (APLHGR) are not in controlled cells, local power
limit monitoring is not affected by boron loss.
GE has evaluated the potential effect of boron loss on shutdown capability,
CPR reduction and the consequences of control rod drop accidents. GE's
evaluation is based on the hot cell result that no boron loss is observed
until 50 percent local B10 depletion is attained. For each B4C tube,
complete loss of B4C was assumed when the calculated B10 depletion needed 50
percent locally. For any blade expected to reach a B10 depletion greater
than 34 percent during a cycle, GE assumed a B10 depletion distribution
typical of blades at the previously defined end of design life.
Based on these evaluations GE arrived at the following conclusions:
(a) Control rod drop accident consequences are not sufficiently sensitive
to small reductions in scram reactivity to be affected by boron loss
before the end of design life of the blades involved.
(b) If no more than 26 percent of the control blades have experienced a 10
percent reduction in projected worth taking boron loss into
consideration, there is a negligible effect on transient CPR reduction
and MCPR limits.
(c) If any control blades have experienced more than 10 percent reduction
in projected worth, taking boron loss into consideration, the shutdown
margin
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IE Bulletin No. 79-26 November 20, 1979
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should be demonstrated to be at least the sum of the shutdown margin
required by Technical Specifications plus an increment sufficient to account
for the potential for boron loss.
We have examined the bases for GE's conclusions, including the hot cell
tests and the calculational assumptions. the preferred action is to replace
all blades expected to have greater than 34 percent B10 depletion averaged
over the upper one-fourth of the blade. However, based on our review we
believe the relation between boron loss and B10 depletion (i.e., the
observations to date show that boron loss does not occur until 50 percent
local of B10) is sufficiently understood to justify BWR operation on an
interim basis provided the following actions have been taken by licensees.
Action to be Taken by Licensees:
For all BWR power reactor facilities with an operating license:
1. The operating history of the reactor is to be reviewed to establish a
record of the current B10 depletion averaged over the upper one-fourth
of the blade for every control blade; the record is to be maintained on
a continuing basis. This action is required on all reactors whether
shutdown for refueling or operating.
2. Identify any control blades predicted to have greater than 34 percent
B10 depletion averaged over the upper one-fourth of the blade by the
next refueling outage.
a. Describe your plans for replacement of identified control blades.
b. Describe measures which you plan to take justifying continued
operations until the next refueling specifically addressing (1)
any blade with
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IE Bulletin No. 79-26 November 20 , 1979
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greater than 42 percent depletion averaged over the upper
one-fourth of the blade; and (2) the condition where you find
greater than 26 percent of the control blades calculated to have
greater than 34 percent depletion averaged over the upper
one-fourth of the blade.
3. At the next cold shutdown or refueling outage, conduct shutdown margin
tests to verify that:
a. full withdrawal of any control blade from the cold xenon-free core
will not result in criticality; and
b. compliance with the shutdown margin requirement in a manner that
accommodates the boron loss phenomenon (i.e., by including a plant
specific increment in the shutdown margin that takes the potential
loss of boron from control blades identified from evaluation of
Item 1 into consideration).
4. Perform a destructive examination of the most highly exposed control
blade at the end of the next cycle and provide results of the
examination within one calendar year after removal of the blade. The
results to be reported should include:
a. Tube number or identification.
b. The evaluation of each crack in the tubing.
c. The calculated B10 depletion versus elevation for each tube.
d. The measured B10 loss versus elevation for each tube.
e. The maximum local depletion for -tubes having no cracks.
f. The maximum local depletion for tubes having no loss of boron.
Alternately, the results of a destructive examination of a blade of
similar fabrication and operational history may be provided within one
year of the
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date of issuance of this Bulletin. If the highest local B10 depletion
is less than 50 percent, this examination can be deferred until the
next refueling.
5. Submit within 45 days of the date of issuance of this Bulletin, a
written report of the findings as to Items (1) and (2). for facilities
in a refueling outage, and all other facilities at their next refueling
outage, submit the written report on Item (3) within 30 days after
plant startup following the outage. A written report on Item (4) is
requested within one year after removal of a control blade for
destructive examination.
Reports should be submitted to the Director of the appropriate NRC Regional
Office and a copy should be forwarded to the NRC Office of Inspection and
Enforcement, Division of Reactor Operations Inspection, Washington, D.C.
20555.
For all BWR facilities with a construction permit and all other power
reactor facilities with an operating license or construction permit, this
Bulletin is for information only no written response is required.
Approved by GAO B180225 (R0072); clearance expires 7/31/80. Approval was
given under a blanket clearance specifically for identified generic
problems.