Information Notice 2015-03, Improper Operation of Spent Fuel Storage Equipment Leading to Elevated Radiation Levels Adjacent to Spent Fuel Transfer Cask: Difference between revisions

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{{#Wiki_filter:UNITED STATES
{{#Wiki_filter:ML14213A477 UNITED STATES


NUCLEAR REGULATORY COMMISSION
NUCLEAR REGULATORY COMMISSION
Line 20: Line 20:
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS


WASHINGTON, DC 20555-0001 February 9, 2015 NRC INFORMATION NOTICE 2015-03:               IMPROPER OPERATION OF SPENT FUEL
WASHINGTON, DC 20555-0001  
 
February 9, 2015  
 
NRC INFORMATION NOTICE 2015-03:  
IMPROPER OPERATION OF SPENT FUEL


TRANSFER CASK NEUTRON SHIELD
TRANSFER CASK NEUTRON SHIELD
Line 38: Line 43:


==PURPOSE==
==PURPOSE==
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to:
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to:  
    *   Inform addressees of improper operation of spent fuel transfer cask neutron shield
 
*  
Inform addressees of improper operation of spent fuel transfer cask neutron shield


equipment that resulted in elevated area radiation levels and unplanned dose to
equipment that resulted in elevated area radiation levels and unplanned dose to
Line 45: Line 52:
personnel
personnel


*   Make addressees aware of vulnerabilities in procedures and equipment design that
*  
Make addressees aware of vulnerabilities in procedures and equipment design that


could inadvertently cause unexpected high levels of radiation from improper operations
could inadvertently cause unexpected high levels of radiation from improper operations
Line 51: Line 59:
The NRC expects recipients to review the information for applicability to their facilities and to
The NRC expects recipients to review the information for applicability to their facilities and to


consider actions, as appropriate, to avoid similar problems. However, suggestions contained in
consider actions, as appropriate, to avoid similar problems. However, suggestions contained in


this IN are not NRC requirements; therefore, no specific action or written response is required.
this IN are not NRC requirements; therefore, no specific action or written response is required.


==BACKGROUND==
==BACKGROUND==
Description of Spent Fuel Storage System


===Description of Spent Fuel Storage System===
The dry shielded canister used at the facilities discussed in this information notice is a high
The dry shielded canister used at the facilities discussed in this information notice is a high


integrity stainless steel welded pressure vessel that confines fuel assemblies from a boiling
integrity stainless steel welded pressure vessel that confines fuel assemblies from a boiling


water reactor and maintains an internal helium atmosphere. Stainless steel cover plates and
water reactor and maintains an internal helium atmosphere. Stainless steel cover plates and


thick carbon steel shielding material form the top and bottom end of the dry shielded canister.
thick carbon steel shielding material form the top and bottom end of the dry shielded canister.
Line 68: Line 76:
The top and bottom cover plates are double seal welded to the stainless steel cylindrical shell to
The top and bottom cover plates are double seal welded to the stainless steel cylindrical shell to


form the dry shielded canisters containment pressure boundary. The transfer cask is a
form the dry shielded canisters containment pressure boundary. The transfer cask is a


non-pressure retaining double shell cylindrical vessel with a welded bottom plate and bolted top
non-pressure retaining double shell cylindrical vessel with a welded bottom plate and bolted top


cover plate. The space between the shells contains radiological shielding material.
cover plate. The space between the shells contains radiological shielding material.


The transfer cask is designed for on-site transport of the dry shielded canister to and from the
The transfer cask is designed for on-site transport of the dry shielded canister to and from the


plants spent fuel pool and the ISFSI. The transfer cask provides the principal radiological
plants spent fuel pool and the ISFSI. The transfer cask provides the principal radiological shielding and heat rejection mechanism for the dry shielded canister and spent nuclear fuel


ML14213A477 shielding and heat rejection mechanism for the dry shielded canister and spent nuclear fuel
assemblies during handling in the fuel building, dry shielded canister closure operations, transport to the ISFSI, and transfer to the horizontal storage module. The transfer cask also
 
assemblies during handling in the fuel building, dry shielded canister closure operations, transport to the ISFSI, and transfer to the horizontal storage module. The transfer cask also


provides primary protection for the loaded dry shielded canister during off-normal and load
provides primary protection for the loaded dry shielded canister during off-normal and load


drop-accidents postulated to occur during the transport operations. The transfer cask includes
drop-accidents postulated to occur during the transport operations. The transfer cask includes


an outer steel jacket that makes up the outer boundary of the integral neutron shield tank, which
an outer steel jacket that makes up the outer boundary of the integral neutron shield tank, which


is filled with water for neutron shielding. The transfer cask outer cylindrical shell makes up the
is filled with water for neutron shielding. The transfer cask outer cylindrical shell makes up the


inner boundary of the neutron shield tank.
inner boundary of the neutron shield tank.
Line 96: Line 102:
overflow tank is connected to the vent port of the neutron shield tank to act as an expansion
overflow tank is connected to the vent port of the neutron shield tank to act as an expansion


volume and makeup source to account for volumetric changes in the shield water. The space
volume and makeup source to account for volumetric changes in the shield water. The space


between the outside surface of the dry shielded canister and the inside surface of the transfer
between the outside surface of the dry shielded canister and the inside surface of the transfer


cask is referred to as the transfer cask annulus. This volume is filled and subsequently emptied
cask is referred to as the transfer cask annulus. This volume is filled and subsequently emptied


during cask loading operations.
during cask loading operations.
Line 109: Line 115:
notably neutron radiation) that were higher than expected adjacent to spent fuel storage transfer
notably neutron radiation) that were higher than expected adjacent to spent fuel storage transfer


casks, and (2) unplanned personnel dose. Both facilities, the Cooper Nuclear Station (CNS)
casks, and (2) unplanned personnel dose. Both facilities, the Cooper Nuclear Station (CNS)  
and Susquehanna Steam Electric Station (SSES), utilized a Transnuclear (TN) NUHOMS-61BT
and Susquehanna Steam Electric Station (SSES), utilized a Transnuclear (TN) NUHOMS-61BT


CoC No. 1004 (Amendment 9) fuel storage system.
CoC No. 1004 (Amendment 9) fuel storage system.


===Cooper Nuclear Station===
Cooper Nuclear Station
 
In November 2010, a TN NUHOMS-61BT dry shielded canister was located in the reactor
In November 2010, a TN NUHOMS-61BT dry shielded canister was located in the reactor


buildings railroad airlock area on a transport trailer awaiting final preparations for transport to
buildings railroad airlock area on a transport trailer awaiting final preparations for transport to


the ISFSI. Before transport, residual water is normally drained from the transfer cask annulus.
the ISFSI. Before transport, residual water is normally drained from the transfer cask annulus.


The drain hose was mistakenly connected to the neutron shield tank drain port instead of the
The drain hose was mistakenly connected to the neutron shield tank drain port instead of the


transfer cask annulus drain port. Over time, the opening of the neutron shield tank drain port
transfer cask annulus drain port. Over time, the opening of the neutron shield tank drain port


caused water to be siphoned and discharged from the neutron shield tank out through the drain
caused water to be siphoned and discharged from the neutron shield tank out through the drain


hose connected to the neutron shield tank drain port. Approximately 40 percent (220.8 gallons)
hose connected to the neutron shield tank drain port. Approximately 40 percent (220.8 gallons)  
of the neutron shield volume drained onto the floor under the dry shielded canister. This
of the neutron shield volume drained onto the floor under the dry shielded canister. This


resulted in higher than expected neutron dose rates near the spent fuel transfer cask. The
resulted in higher than expected neutron dose rates near the spent fuel transfer cask. The


unintentional draining of the transfer cask neutron shield resulted in a temporary halt to ISFSI
unintentional draining of the transfer cask neutron shield resulted in a temporary halt to ISFSI


work and an increase in dose to the workers involved. Twenty employees at CNS received an
work and an increase in dose to the workers involved. Twenty employees at CNS received an


unplanned dose.
unplanned dose.
Line 146: Line 153:
(ADAMS) under accession No. ML12192A620.
(ADAMS) under accession No. ML12192A620.


===Susquehanna Steam Electric Station===
Susquehanna Steam Electric Station
 
In August 2013, SSES began loading spent fuel assemblies into a TN NUHOMS-61BT dry
In August 2013, SSES began loading spent fuel assemblies into a TN NUHOMS-61BT dry


shielded canister. Procedures directed SSES personnel to attach a neutron shield
shielded canister. Procedures directed SSES personnel to attach a neutron shield


pressurization tank to the neutron shield tank vent connection port on the transfer cask to
pressurization tank to the neutron shield tank vent connection port on the transfer cask to


ensure that the neutron shield tank was full. No water was observed to flow into the neutron shield tank and SSES personnel concluded that the neutron shield tank was full. The loaded
ensure that the neutron shield tank was full. No water was observed to flow into the neutron shield tank and SSES personnel concluded that the neutron shield tank was full. The loaded


transfer cask/dry shielded canister was moved out of the cask storage pit of the spent fuel pool
transfer cask/dry shielded canister was moved out of the cask storage pit of the spent fuel pool


after loading. The dry shielded canister inner top cover was welded in place and initial draining
after loading. The dry shielded canister inner top cover was welded in place and initial draining


of the dry shielded canister commenced. Health Physics technicians observed neutron dose
of the dry shielded canister commenced. Health Physics technicians observed neutron dose


rates that were higher than expected and restricted access to the side of the transfer cask.
rates that were higher than expected and restricted access to the side of the transfer cask.
Line 165: Line 173:
Subsequently, a secondary indication of increased dose rates was reported after chemistry
Subsequently, a secondary indication of increased dose rates was reported after chemistry


personnel observed that radiation monitors in the area were reading higher than normal. In
personnel observed that radiation monitors in the area were reading higher than normal. In


reaction to the increased dose rates, maintenance personnel exercised the fitting, repositioned
reaction to the increased dose rates, maintenance personnel exercised the fitting, repositioned


the neutron shield pressurization tank, and observed it drain into the neutron shield tank, indicating the neutron shield tank was not full. SSES then filled the transfer cask neutron shield
the neutron shield pressurization tank, and observed it drain into the neutron shield tank, indicating the neutron shield tank was not full. SSES then filled the transfer cask neutron shield


tank using a fill line that was already connected to the fill port. Health Physics technicians
tank using a fill line that was already connected to the fill port. Health Physics technicians


surveyed and confirmed that neutron dose rates returned to expected levels. Dosimeters were
surveyed and confirmed that neutron dose rates returned to expected levels. Dosimeters were


collected from the workers and read. No appreciable amount of dose was distinguishable from
collected from the workers and read. No appreciable amount of dose was distinguishable from


background readings. SSES determined that the neutron shield tank was not filled in
background readings. SSES determined that the neutron shield tank was not filled in


preparation for this loading campaign. A hydraulic lock or air binding developed in the hose
preparation for this loading campaign. A hydraulic lock or air binding developed in the hose


from the neutron shield pressurization tank which prevented the flow of water from the neutron
from the neutron shield pressurization tank which prevented the flow of water from the neutron


shield pressurization tank into the neutron shield tank. This caused a false indication that the
shield pressurization tank into the neutron shield tank. This caused a false indication that the


neutron shield tank was full.
neutron shield tank was full.
Line 198: Line 206:
In both events, radiation levels that were higher than expected, including levels of neutron
In both events, radiation levels that were higher than expected, including levels of neutron


radiation, were experienced near the spent fuel casks because of a combination of cask design, operation, and human error. The design of the transfer cask uses shielding that consists of
radiation, were experienced near the spent fuel casks because of a combination of cask design, operation, and human error. The design of the transfer cask uses shielding that consists of


layers of stainless steel, carbon steel, and lead to reduce gamma, beta, and alpha radiation.
layers of stainless steel, carbon steel, and lead to reduce gamma, beta, and alpha radiation.
Line 204: Line 212:
Neutron radiation is shielded by water in the neutron shield tank, which can be filled and drained
Neutron radiation is shielded by water in the neutron shield tank, which can be filled and drained


through the neutron shield tank vent and drain ports. The transfer cask is also equipped with an
through the neutron shield tank vent and drain ports. The transfer cask is also equipped with an


annulus drain port, which is identical in size and construction to the neutron shield tank vent and
annulus drain port, which is identical in size and construction to the neutron shield tank vent and


drain ports. The vendor did not provide any form of identification or marking to distinguish
drain ports. The vendor did not provide any form of identification or marking to distinguish


between these ports. After the spent fuel assemblies are loaded into the dry shielded canister
between these ports. After the spent fuel assemblies are loaded into the dry shielded canister


and it is sealed with the first of two cover plates, water surrounding the fuel is removed and the
and it is sealed with the first of two cover plates, water surrounding the fuel is removed and the


interior of the sealed dry shielded canister is vacuum-dried and backfilled with helium. Now that
interior of the sealed dry shielded canister is vacuum-dried and backfilled with helium. Now that


the water inside the dry shielded canister (which had protected personnel with its
the water inside the dry shielded canister (which had protected personnel with its
Line 224: Line 232:
Unanticipated neutron dose at CNS occurred because of the lack of shield water in the transfer
Unanticipated neutron dose at CNS occurred because of the lack of shield water in the transfer


cask neutron shield tank. Before being transported to the ISFSI pad, the transfer cask annulus
cask neutron shield tank. Before being transported to the ISFSI pad, the transfer cask annulus


drain valve was to be opened to remove any residual water in the annulus space between the
drain valve was to be opened to remove any residual water in the annulus space between the


transfer cask and the dry shielded canister. The bottom of the transfer cask was configured with
transfer cask and the dry shielded canister. The bottom of the transfer cask was configured with


three fill and drain ports that were identical and not identified by labels or tags. Two of the ports
three fill and drain ports that were identical and not identified by labels or tags. Two of the ports


were for the transfer cask neutron shield tank vent and fill lines and the third was to drain the
were for the transfer cask neutron shield tank vent and fill lines and the third was to drain the
Line 236: Line 244:
annulus space if water was still present after dry shielded canister loading and processing
annulus space if water was still present after dry shielded canister loading and processing


operations were completed. CNS personnel failed to connect the drain line to the annulus drain
operations were completed. CNS personnel failed to connect the drain line to the annulus drain


port, but instead, connected it to the neutron shield tank drain port, which resulted in the partial
port, but instead, connected it to the neutron shield tank drain port, which resulted in the partial


draining of the neutron shield tank. When workers opened the neutron shield tank drain port, water did not immediately flow out of the neutron shield tank. This was because a siphoning
draining of the neutron shield tank. When workers opened the neutron shield tank drain port, water did not immediately flow out of the neutron shield tank. This was because a siphoning


action was transferring water to a pressurization tank through the transfer cask neutron shield
action was transferring water to a pressurization tank through the transfer cask neutron shield


tank vent port. One CNS worker did notice that water had flowed into the neutron shield
tank vent port. One CNS worker did notice that water had flowed into the neutron shield


pressurization tank, but failed to realize that this indicated that the drain line was connected
pressurization tank, but failed to realize that this indicated that the drain line was connected


incorrectly to the neutron shield tank drain port. There are no other design features for this cask
incorrectly to the neutron shield tank drain port. There are no other design features for this cask


type that would allow workers to determine the transfer cask neutron shield tank level. After the
type that would allow workers to determine the transfer cask neutron shield tank level. After the


siphoning action stopped, the neutron shield tank partially drained, which resulted in increased
siphoning action stopped, the neutron shield tank partially drained, which resulted in increased
Line 264: Line 272:
properly drained as part of the Dry Fuel Storage demobilization work package and documented
properly drained as part of the Dry Fuel Storage demobilization work package and documented


as specified in the SSES procedure. However, SSES personnel failed to fill the transfer cask
as specified in the SSES procedure. However, SSES personnel failed to fill the transfer cask


neutron shield tank before this loading campaign, as required by procedure. When checking the
neutron shield tank before this loading campaign, as required by procedure. When checking the


level of the neutron shield tank, air binding of the line between the neutron shield pressurization
level of the neutron shield tank, air binding of the line between the neutron shield pressurization
Line 272: Line 280:
tank and neutron shield tank occurred, preventing the flow of water from the neutron shield
tank and neutron shield tank occurred, preventing the flow of water from the neutron shield


pressurization tank to the neutron shield tank. The neutron shield pressurization tank, once
pressurization tank to the neutron shield tank. The neutron shield pressurization tank, once


connected, provides the only means to verify adequate level in the neutron shield tank.
connected, provides the only means to verify adequate level in the neutron shield tank.
Line 278: Line 286:
Maintenance personnel received a false indication that the neutron shield tank was full because
Maintenance personnel received a false indication that the neutron shield tank was full because


the neutron shield pressurization tank level did not change. During operations, SSES noted that
the neutron shield pressurization tank level did not change. During operations, SSES noted that


the pressurization tank emptied, which was not a normal occurrence. They refilled the neutron
the pressurization tank emptied, which was not a normal occurrence. They refilled the neutron


shield pressurization tank, but failed to determine the cause, challenge existing conditions, or
shield pressurization tank, but failed to determine the cause, challenge existing conditions, or


recognize this as an indication that the neutron shield tank was not filled. After refilling the
recognize this as an indication that the neutron shield tank was not filled. After refilling the


neutron shield pressurization tank, air binding prevented outflow from the neutron shield
neutron shield pressurization tank, air binding prevented outflow from the neutron shield
Line 296: Line 304:
kept doses below the technical specification maximum allowable levels adjacent to the dry
kept doses below the technical specification maximum allowable levels adjacent to the dry


shielded canister. Proper monitoring programs and health physics procedures alerted the staff
shielded canister. Proper monitoring programs and health physics procedures alerted the staff


to unexpected radiological conditions and prevented further dose to workers before the initial
to unexpected radiological conditions and prevented further dose to workers before the initial
Line 304: Line 312:
Based on the event at Cooper, TN now provides a transfer cask with color-coded and keyed
Based on the event at Cooper, TN now provides a transfer cask with color-coded and keyed


Swagelok fittings at each site where the transfer cask is provided. The fittings are installed at
Swagelok fittings at each site where the transfer cask is provided. The fittings are installed at


the annulus and neutron shield tank connections so that one cannot connect the line for the
the annulus and neutron shield tank connections so that one cannot connect the line for the


neutron shield tank to the annulus and vice versa. CNS also performed procedural revisions
neutron shield tank to the annulus and vice versa. CNS also performed procedural revisions


that include verification of the proper Swagelok fittings during receipt of the transfer cask.
that include verification of the proper Swagelok fittings during receipt of the transfer cask.
Line 318: Line 326:
shield tank drain port and verify water exits from the vent port into the neutron shield
shield tank drain port and verify water exits from the vent port into the neutron shield


pressurization tank. When the neutron shield tank drain port is not accessible, TN
pressurization tank. When the neutron shield tank drain port is not accessible, TN


recommended that users should remove the neutron shield tank pressure relief valve opposite
recommended that users should remove the neutron shield tank pressure relief valve opposite
Line 324: Line 332:
the neutron shield tank vent line and add water into either the pressure relief port or the neutron
the neutron shield tank vent line and add water into either the pressure relief port or the neutron


shield pressurization tank and verify water exits the opposite port. SSES revised their
shield pressurization tank and verify water exits the opposite port. SSES revised their


procedures to incorporate the new method to verify the neutron shield tank is filled with water.
procedures to incorporate the new method to verify the neutron shield tank is filled with water.
Line 332: Line 340:
equipment to determine whether current neutron shield water level is susceptible to false
equipment to determine whether current neutron shield water level is susceptible to false


indications as described above. To achieve this objective, consider the human factors and design deficiencies to ensure that adequate controls and steps are in place to prevent the
indications as described above. To achieve this objective, consider the human factors and design deficiencies to ensure that adequate controls and steps are in place to prevent the


inadvertent loss or inaccurate indication of neutron shield water.
inadvertent loss or inaccurate indication of neutron shield water.


==CONTACT==
==CONTACT==
This information notice requires no specific action or written response. Please direct any
This information notice requires no specific action or written response. Please direct any


questions about this matter to the technical contact listed below or to the appropriate Office of
questions about this matter to the technical contact listed below or to the appropriate Office of
Line 343: Line 351:
Nuclear Material Safety and Safeguards project manager.
Nuclear Material Safety and Safeguards project manager.


/RA/
/RA/  
                                              Mark Lombard, Director
 
Mark Lombard, Director


Division of Spent Fuel Management
Division of Spent Fuel Management
Line 353: Line 362:


===Technical Contact:===
===Technical Contact:===
Jeremy Tapp, NMSS


===Jeremy Tapp, NMSS===
301-287-9218 E-mail: Jeremy.Tapp@nrc.gov
                      301-287-9218 E-mail: Jeremy.Tapp@nrc.gov


Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections


ML14213A477
ML14213A477  
* concurred via email
* concurred via email


OFC   NMSS/DSFM NMSS/DSFM                     NMSS/DSFM/LA NMSS/DSFM NMSS/DSFM NMSS/DSFM
OFC
 
NMSS/DSFM
 
NMSS/DSFM
 
NMSS/DSFM/LA NMSS/DSFM NMSS/DSFM NMSS/DSFM
 
NAME EAllen
 
JTapp
 
WWheatley
 
ELove*
MSampson
 
PSilva
 
DATE
 
08/05/2014
08/13/2014
08/28/2014
08/28/2014
09/24/2014
09/18/2014 OFC
 
ADM/DAS/PB NMSS/MSTR
 
RES/DRA
 
RI/DNMS
 
RII/DRS
 
RIII/DNMS
 
NAME CHsu*
AMcIntosh
 
SEPeters*
MFerdas*
SWalker*
ROrlikowski*
DATE
 
10/2/2014
10/28/2014
11/6/2014
12/16/2014
01/06/2015
01/09/2015 OFC
 
RIV/DNMS
 
NRR/DPR/PGCB/LA NMSS/DSFM
 
NMSS/DSFM


NAME EAllen          JTapp                  WWheatley            ELove*         MSampson        PSilva
NAME RKellar*  
ELee


DATE 08/05/2014      08/13/2014              08/28/2014          08/28/2014      09/24/2014      09/18/2014 OFC  ADM/DAS/PB NMSS/MSTR                    RES/DRA              RI/DNMS        RII/DRS        RIII/DNMS
AHsia


NAME CHsu*            AMcIntosh              SEPeters*            MFerdas*        SWalker*        ROrlikowski*
MLombard
DATE 10/2/2014        10/28/2014              11/6/2014            12/16/2014      01/06/2015      01/09/2015 OFC  RIV/DNMS        NRR/DPR/PGCB/LA NMSS/DSFM                    NMSS/DSFM


NAME RKellar*        ELee                    AHsia                MLombard
DATE


DATE 01/06/2015       02/18/2015             02/08/2015           02/09/2015}}
01/06/2015  
02/18/2015  
02/08/2015  
02/09/2015}}


{{Information notice-Nav}}
{{Information notice-Nav}}

Latest revision as of 17:36, 10 January 2025

Improper Operation of Spent Fuel Storage Equipment Leading to Elevated Radiation Levels Adjacent to Spent Fuel Transfer Cask
ML14213A477
Person / Time
Issue date: 02/09/2015
From: Mark Lombard
NRC/NMSS/SFST
To:
Tapp J
References
IN-15-013
Download: ML14213A477 (5)
v • d • e


ML14213A477 UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS

WASHINGTON, DC 20555-0001

February 9, 2015

NRC INFORMATION NOTICE 2015-03:

IMPROPER OPERATION OF SPENT FUEL

TRANSFER CASK NEUTRON SHIELD

EQUIPMENT LEADING TO ELEVATED

RADIATION LEVELS ADJACENT TO SPENT

FUEL TRANSFER CASK

ADDRESSEES

All holders of and applicants for an independent spent fuel storage installation (ISFSI) license or

a certificate of compliance (CoC) under Title 10, Energy, of the Code of Federal Regulations

(10 CFR) Part 72, Licensing Requirements for the Independent Storage of Spent Nuclear Fuel, High-Level Radioactive Waste, and Reactor-Related Greater Than Class C Waste.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to:

Inform addressees of improper operation of spent fuel transfer cask neutron shield

equipment that resulted in elevated area radiation levels and unplanned dose to

personnel

Make addressees aware of vulnerabilities in procedures and equipment design that

could inadvertently cause unexpected high levels of radiation from improper operations

The NRC expects recipients to review the information for applicability to their facilities and to

consider actions, as appropriate, to avoid similar problems. However, suggestions contained in

this IN are not NRC requirements; therefore, no specific action or written response is required.

BACKGROUND

Description of Spent Fuel Storage System

The dry shielded canister used at the facilities discussed in this information notice is a high

integrity stainless steel welded pressure vessel that confines fuel assemblies from a boiling

water reactor and maintains an internal helium atmosphere. Stainless steel cover plates and

thick carbon steel shielding material form the top and bottom end of the dry shielded canister.

The top and bottom cover plates are double seal welded to the stainless steel cylindrical shell to

form the dry shielded canisters containment pressure boundary. The transfer cask is a

non-pressure retaining double shell cylindrical vessel with a welded bottom plate and bolted top

cover plate. The space between the shells contains radiological shielding material.

The transfer cask is designed for on-site transport of the dry shielded canister to and from the

plants spent fuel pool and the ISFSI. The transfer cask provides the principal radiological shielding and heat rejection mechanism for the dry shielded canister and spent nuclear fuel

assemblies during handling in the fuel building, dry shielded canister closure operations, transport to the ISFSI, and transfer to the horizontal storage module. The transfer cask also

provides primary protection for the loaded dry shielded canister during off-normal and load

drop-accidents postulated to occur during the transport operations. The transfer cask includes

an outer steel jacket that makes up the outer boundary of the integral neutron shield tank, which

is filled with water for neutron shielding. The transfer cask outer cylindrical shell makes up the

inner boundary of the neutron shield tank.

During operation, a small 5-gallon tank called the neutron shield pressurization tank or an

overflow tank is connected to the vent port of the neutron shield tank to act as an expansion

volume and makeup source to account for volumetric changes in the shield water. The space

between the outside surface of the dry shielded canister and the inside surface of the transfer

cask is referred to as the transfer cask annulus. This volume is filled and subsequently emptied

during cask loading operations.

DESCRIPTION OF CIRCUMSTANCES

Distinctly different incidents at two nuclear facilities resulted in (1) levels of radiation (most

notably neutron radiation) that were higher than expected adjacent to spent fuel storage transfer

casks, and (2) unplanned personnel dose. Both facilities, the Cooper Nuclear Station (CNS)

and Susquehanna Steam Electric Station (SSES), utilized a Transnuclear (TN) NUHOMS-61BT

CoC No. 1004 (Amendment 9) fuel storage system.

Cooper Nuclear Station

In November 2010, a TN NUHOMS-61BT dry shielded canister was located in the reactor

buildings railroad airlock area on a transport trailer awaiting final preparations for transport to

the ISFSI. Before transport, residual water is normally drained from the transfer cask annulus.

The drain hose was mistakenly connected to the neutron shield tank drain port instead of the

transfer cask annulus drain port. Over time, the opening of the neutron shield tank drain port

caused water to be siphoned and discharged from the neutron shield tank out through the drain

hose connected to the neutron shield tank drain port. Approximately 40 percent (220.8 gallons)

of the neutron shield volume drained onto the floor under the dry shielded canister. This

resulted in higher than expected neutron dose rates near the spent fuel transfer cask. The

unintentional draining of the transfer cask neutron shield resulted in a temporary halt to ISFSI

work and an increase in dose to the workers involved. Twenty employees at CNS received an

unplanned dose.

Additional information is available in Cooper Nuclear Station Inspection of the Independent

Spent Fuel Storage Installation Report Nos. 05000298/2010009 and 07200066/2010001, dated

July 10, 2012 and available in the Agencywide Documents Access and Management System

(ADAMS) under accession No. ML12192A620.

Susquehanna Steam Electric Station

In August 2013, SSES began loading spent fuel assemblies into a TN NUHOMS-61BT dry

shielded canister. Procedures directed SSES personnel to attach a neutron shield

pressurization tank to the neutron shield tank vent connection port on the transfer cask to

ensure that the neutron shield tank was full. No water was observed to flow into the neutron shield tank and SSES personnel concluded that the neutron shield tank was full. The loaded

transfer cask/dry shielded canister was moved out of the cask storage pit of the spent fuel pool

after loading. The dry shielded canister inner top cover was welded in place and initial draining

of the dry shielded canister commenced. Health Physics technicians observed neutron dose

rates that were higher than expected and restricted access to the side of the transfer cask.

Subsequently, a secondary indication of increased dose rates was reported after chemistry

personnel observed that radiation monitors in the area were reading higher than normal. In

reaction to the increased dose rates, maintenance personnel exercised the fitting, repositioned

the neutron shield pressurization tank, and observed it drain into the neutron shield tank, indicating the neutron shield tank was not full. SSES then filled the transfer cask neutron shield

tank using a fill line that was already connected to the fill port. Health Physics technicians

surveyed and confirmed that neutron dose rates returned to expected levels. Dosimeters were

collected from the workers and read. No appreciable amount of dose was distinguishable from

background readings. SSES determined that the neutron shield tank was not filled in

preparation for this loading campaign. A hydraulic lock or air binding developed in the hose

from the neutron shield pressurization tank which prevented the flow of water from the neutron

shield pressurization tank into the neutron shield tank. This caused a false indication that the

neutron shield tank was full.

Additional information is available in Susquehanna Steam Electric Station, Units 1 and 2; Flood

Protection Measures, Surveillance Testing, and Drill Evaluation Report

Nos. 05000387/2013005, 05000388/2013005, and 07200028/2013001, dated

February 14, 2014 (ADAMS Accession No. ML14045A295).

DISCUSSION

In both events, radiation levels that were higher than expected, including levels of neutron

radiation, were experienced near the spent fuel casks because of a combination of cask design, operation, and human error. The design of the transfer cask uses shielding that consists of

layers of stainless steel, carbon steel, and lead to reduce gamma, beta, and alpha radiation.

Neutron radiation is shielded by water in the neutron shield tank, which can be filled and drained

through the neutron shield tank vent and drain ports. The transfer cask is also equipped with an

annulus drain port, which is identical in size and construction to the neutron shield tank vent and

drain ports. The vendor did not provide any form of identification or marking to distinguish

between these ports. After the spent fuel assemblies are loaded into the dry shielded canister

and it is sealed with the first of two cover plates, water surrounding the fuel is removed and the

interior of the sealed dry shielded canister is vacuum-dried and backfilled with helium. Now that

the water inside the dry shielded canister (which had protected personnel with its

neutron-shielding properties) is no longer present, the transfer cask neutron shield must be able

to shield neutrons originating from the spent nuclear fuel.

Unanticipated neutron dose at CNS occurred because of the lack of shield water in the transfer

cask neutron shield tank. Before being transported to the ISFSI pad, the transfer cask annulus

drain valve was to be opened to remove any residual water in the annulus space between the

transfer cask and the dry shielded canister. The bottom of the transfer cask was configured with

three fill and drain ports that were identical and not identified by labels or tags. Two of the ports

were for the transfer cask neutron shield tank vent and fill lines and the third was to drain the

annulus space if water was still present after dry shielded canister loading and processing

operations were completed. CNS personnel failed to connect the drain line to the annulus drain

port, but instead, connected it to the neutron shield tank drain port, which resulted in the partial

draining of the neutron shield tank. When workers opened the neutron shield tank drain port, water did not immediately flow out of the neutron shield tank. This was because a siphoning

action was transferring water to a pressurization tank through the transfer cask neutron shield

tank vent port. One CNS worker did notice that water had flowed into the neutron shield

pressurization tank, but failed to realize that this indicated that the drain line was connected

incorrectly to the neutron shield tank drain port. There are no other design features for this cask

type that would allow workers to determine the transfer cask neutron shield tank level. After the

siphoning action stopped, the neutron shield tank partially drained, which resulted in increased

neutron dose rates and unanticipated neutron dose to workers.

The event at SSES resulted in increased levels of neutron radiation above those expected in the

area adjacent to the transfer cask once the water was drained from the dry shielded canister.

When the previous loading campaign was completed, the transfer cask neutron shield tank was

properly drained as part of the Dry Fuel Storage demobilization work package and documented

as specified in the SSES procedure. However, SSES personnel failed to fill the transfer cask

neutron shield tank before this loading campaign, as required by procedure. When checking the

level of the neutron shield tank, air binding of the line between the neutron shield pressurization

tank and neutron shield tank occurred, preventing the flow of water from the neutron shield

pressurization tank to the neutron shield tank. The neutron shield pressurization tank, once

connected, provides the only means to verify adequate level in the neutron shield tank.

Maintenance personnel received a false indication that the neutron shield tank was full because

the neutron shield pressurization tank level did not change. During operations, SSES noted that

the pressurization tank emptied, which was not a normal occurrence. They refilled the neutron

shield pressurization tank, but failed to determine the cause, challenge existing conditions, or

recognize this as an indication that the neutron shield tank was not filled. After refilling the

neutron shield pressurization tank, air binding prevented outflow from the neutron shield

pressurization tank to the neutron shield tank and it was assumed, again, that the neutron shield

tank was full.

The cask shielding design and spent fuel assembly distribution within the dry shielded canister

kept doses below the technical specification maximum allowable levels adjacent to the dry

shielded canister. Proper monitoring programs and health physics procedures alerted the staff

to unexpected radiological conditions and prevented further dose to workers before the initial

cause was determined.

Based on the event at Cooper, TN now provides a transfer cask with color-coded and keyed

Swagelok fittings at each site where the transfer cask is provided. The fittings are installed at

the annulus and neutron shield tank connections so that one cannot connect the line for the

neutron shield tank to the annulus and vice versa. CNS also performed procedural revisions

that include verification of the proper Swagelok fittings during receipt of the transfer cask.

Based on the event at SSES, TN provided its users with guidance on how to verify that the

neutron shield tank is full of water by specifying that users should add water to the neutron

shield tank drain port and verify water exits from the vent port into the neutron shield

pressurization tank. When the neutron shield tank drain port is not accessible, TN

recommended that users should remove the neutron shield tank pressure relief valve opposite

the neutron shield tank vent line and add water into either the pressure relief port or the neutron

shield pressurization tank and verify water exits the opposite port. SSES revised their

procedures to incorporate the new method to verify the neutron shield tank is filled with water.

It is recommended that cask vendors and licensees evaluate their operating procedures and

equipment to determine whether current neutron shield water level is susceptible to false

indications as described above. To achieve this objective, consider the human factors and design deficiencies to ensure that adequate controls and steps are in place to prevent the

inadvertent loss or inaccurate indication of neutron shield water.

CONTACT

This information notice requires no specific action or written response. Please direct any

questions about this matter to the technical contact listed below or to the appropriate Office of

Nuclear Material Safety and Safeguards project manager.

/RA/

Mark Lombard, Director

Division of Spent Fuel Management

Office of Nuclear Material Safety

and Safeguards

Technical Contact:

Jeremy Tapp, NMSS

301-287-9218 E-mail: Jeremy.Tapp@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections

ML14213A477

  • concurred via email

OFC

NMSS/DSFM

NMSS/DSFM

NMSS/DSFM/LA NMSS/DSFM NMSS/DSFM NMSS/DSFM

NAME EAllen

JTapp

WWheatley

ELove*

MSampson

PSilva

DATE

08/05/2014

08/13/2014

08/28/2014

08/28/2014

09/24/2014

09/18/2014 OFC

ADM/DAS/PB NMSS/MSTR

RES/DRA

RI/DNMS

RII/DRS

RIII/DNMS

NAME CHsu*

AMcIntosh

SEPeters*

MFerdas*

SWalker*

ROrlikowski*

DATE

10/2/2014

10/28/2014

11/6/2014

12/16/2014

01/06/2015

01/09/2015 OFC

RIV/DNMS

NRR/DPR/PGCB/LA NMSS/DSFM

NMSS/DSFM

NAME RKellar*

ELee

AHsia

MLombard

DATE

01/06/2015

02/18/2015

02/08/2015

02/09/2015


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