Information Notice 2015-03, 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 |
| Revision: | 0 |
| From: | Mark Lombard NRC/NMSS/SFST |
| To: | |
| Tapp J E | |
| References | |
| IN-15-013 | |
| Download: ML14213A477 (5) | |
ML14213A477 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 problem However, suggestions contained in this IN are not NRC requirements; therefore, no specific action or written response is require
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 atmospher Stainless steel cover plates and thick carbon steel shielding material form the top and bottom end of the dry shielded caniste The top and bottom cover plates are double seal welded to the stainless steel cylindrical shell to form the dry shielded canister's containment pressure boundar The transfer cask is a non-pressure retaining double shell cylindrical vessel with a welded bottom plate and bolted top cover plat The space between the shells contains radiological shielding materia The transfer cask is designed for on-site transport of the dry shielded canister to and from the plant's spent fuel pool and the ISFS 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 modul 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 operation 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 shieldin The transfer cask outer cylindrical shell makes up the inner boundary of the neutron shield tan 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 wate 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 annulu This volume is filled and subsequently emptied during cask loading operation 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 dos 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 syste Cooper Nuclear Station In November 2010, a TN NUHOMS-61BT dry shielded canister was located in the reactor building's railroad airlock area on a transport trailer awaiting final preparations for transport to the ISFS Before transport, residual water is normally drained from the transfer cask annulu The drain hose was mistakenly connected to the neutron shield tank drain port instead of the transfer cask annulus drain por 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 por Approximately 40 percent (220.8 gallons) of the neutron shield volume drained onto the floor under the dry shielded caniste This resulted in higher than expected neutron dose rates near the spent fuel transfer cas 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 involve Twenty employees at CNS received an unplanned dos 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. ML12192A62 Susquehanna Steam Electric Station In August 2013, SSES began loading spent fuel assemblies into a TN NUHOMS-61BT dry shielded caniste 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 ful No water was observed to flow into the neutron shield tank and SSES personnel concluded that the neutron shield tank was ful The loaded transfer cask/dry shielded canister was moved out of the cask storage pit of the spent fuel pool after loadin The dry shielded canister inner top cover was welded in place and initial draining of the dry shielded canister commence Health Physics technicians observed neutron dose rates that were higher than expected and restricted access to the side of the transfer cas Subsequently, a secondary indication of increased dose rates was reported after chemistry personnel observed that radiation monitors in the area were reading higher than norma 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 ful SSES then filled the transfer cask neutron shield tank using a fill line that was already connected to the fill por Health Physics technicians surveyed and confirmed that neutron dose rates returned to expected level Dosimeters were collected from the workers and rea No appreciable amount of dose was distinguishable from background reading SSES determined that the neutron shield tank was not filled in preparation for this loading campaig 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 tan This caused a false indication that the neutron shield tank was ful 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 erro 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 radiatio 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 port 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 port The vendor did not provide any form of identification or marking to distinguish between these port 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 heliu 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 fue Unanticipated neutron dose at CNS occurred because of the lack of shield water in the transfer cask neutron shield tan 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 caniste The bottom of the transfer cask was configured with three fill and drain ports that were identical and not identified by labels or tag 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 complete 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 tan When workers opened the neutron shield tank drain port, water did not immediately flow out of the neutron shield tan This was because a siphoning action was transferring water to a pressurization tank through the transfer cask neutron shield tank vent por 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 por There are no other design features for this cask type that would allow workers to determine the transfer cask neutron shield tank leve After the siphoning action stopped, the neutron shield tank partially drained, which resulted in increased neutron dose rates and unanticipated neutron dose to worker 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 caniste 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 procedur However, SSES personnel failed to fill the transfer cask neutron shield tank before this loading campaign, as required by procedur 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 tan The neutron shield pressurization tank, once connected, provides the only means to verify adequate level in the neutron shield tan Maintenance personnel received a false indication that the neutron shield tank was full because the neutron shield pressurization tank level did not chang During operations, SSES noted that the pressurization tank emptied, which was not a normal occurrenc 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 fille 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 ful 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 caniste 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 determine 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 provide 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 vers CNS also performed procedural revisions that include verification of the proper Swagelok fittings during receipt of the transfer cas 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 tan 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 por SSES revised their procedures to incorporate the new method to verify the neutron shield tank is filled with wate 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 abov 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 wate
CONTACT
This information notice requires no specific action or written respons 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 manage /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 IN 2015-03