Information Notice 2014-14, Potential Safety Enhancements to Spent Fuel Pool Storage

Potential Safety Enhancements to Spent Fuel Pool Storage

ML14218A493
Person / Time
Issue date:	11/14/2014
From:	Michael Cheok, Kokajko L, Mark Lombard Office of Nuclear Material Safety and Safeguards, Office of New Reactors, Generic Communications Projects Branch
To:
A Popova, DPR/PGCB, 415-2876
References
IN 2014-14, TAC MF4562 IN-14-014
Download: ML14218A493 (7)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NEW REACTORS

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS

WASHINGTON, DC 20555-0001 November 14, 2014 NRC INFORMATION NOTICE 2014-14: POTENTIAL SAFETY ENHANCEMENTS TO

SPENT FUEL POOL STORAGE

ADDRESSEES

All holders of an operating license or construction permit for a nuclear power reactor under

Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of

Production and Utilization Facilities, including those that have permanently ceased operations

and have spent fuel stored in spent fuel pools (SFPs).

All holders of and applicants for a power reactor early site permit, combined license, standard

design approval, or manufacturing license under 10 CFR Part 52, Licenses, Certifications, and

Approvals for Nuclear Power Plants. All applicants for a standard design certification, including

such applicants after initial issuance of a design certification rule.

All holders of and applicants for an independent spent fuel storage installation license under

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

licensees of insights associated with the storage of spent fuel in SFPs gained through study of a

reference boiling-water reactor SFP. The insights discussed in this IN may help optimize

operating practices and event mitigation capabilities to further enhance the safety of spent fuel

storage in pools.

Addressees

should review the information for applicability to their facilities and

consider actions as appropriate. However, suggestions contained in this IN are not NRC

requirements; therefore, no specific action or written response is required.

BACKGROUND

On March 11, 2011, the Thoku earthquake and subsequent tsunami in Japan resulted in

significant damage to the Fukushima Dai-ichi nuclear power station. The Fukushima Dai-ichi

SFP-structures remained intact, and the spent fuel assemblies stored in the pools remained

cool and water-covered throughout this event and subsequent recovery. Nevertheless, ML14218A493 uncertainty regarding the status of the pools during the event raised questions about the safe

storage of spent fuel and whether the NRC should require expedited transfer of spent fuel to dry

cask storage at nuclear power plants in the U.S.

Subsequently, in the summer of 2011, the NRC staff initiated a research project, Consequence

Study of a Beyond-Design-Basis Earthquake Affecting the Spent Fuel Pool for a U.S. Mark I

Boiling Water Reactor, referred to as the SFP study or the SFPS. The results of the SFPS are

published in NUREG-21611. The SFPS examined the consequences of a hypothetical SFP

accident initiated by an unlikely, beyond-design-basis seismic event. The SFPS concluded, consistent with earlier generic NRC studies, that the reference plants SFP was a robust

structure that is likely to withstand severe earthquakes without leaking. Nevertheless, the NRC

staff analyzed the pool structure to determine the most likely location and size of leaks that

could develop as a result of such an extreme earthquake. From that information, the NRC staff

determined the conditions that would result in fuel overheating, considering both a low-density

and high-density storage configuration, and the radiological consequences of any predicted

release of radioactive material into the environment. In the unlikely event of a leak, and

subsequent emptying of the SFP, this study showed that (for the scenarios and SFP studied)

the spent fuel was only susceptible to overheating and a radiological release within a few

months after it was moved from the reactor into the SFP. If a leak develops after those first few

months from when the fuel was moved from the reactor into the SFP, then the study found that

air cooling was sufficient to prevent overheating of the spent fuel (for the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> time period

analyzed in the SFPS). The SFPS demonstrated that the period in which fuel could overheat

could be further reduced by dispersing the hottest assemblies among a larger number of colder

assemblies and by the effective deployment of equipment and strategies implemented pursuant

to the requirements of 10 CFR 50.54(hh)(2).

The SFPS analyzed cases with and without successful deployment of 10 CFR 50.54(hh)(2)

equipment and strategies. For the included human reliability analysis, the SFPS assumed that

there was sufficient staff to deploy the SFP mitigation systems and access was not impaired by

damage to the reactor core and primary containment. If the earthquake had damaged multiple

reactors and SFPs, some of these assumptions may be invalid.

In response to the Fukushima Dai-ichi accident, the NRC is currently implementing regulatory

actions to further enhance nuclear reactor and SFP safety For example, on March 12, 2012, the staff issued Order EA-12-0512, Issuance of Order To Modify Licenses with Regard to

Reliable Spent Fuel Pool Instrumentation, which required that licensees install reliable means

of remotely monitoring wide-range SFP levels to support effective prioritization of event

mitigation and recovery actions in the event of a beyond-design-basis external event. Also on

March 12, 2012, the staff issued Order EA-12-0493, Order Modifying Licenses with Regard to

Requirements for Mitigation Strategies for Beyond-Design-Basis External Events, which

required licensees to develop, implement, and maintain guidance and strategies to maintain or

restore core cooling, containment, and SFP cooling capabilities following a

1 Available in the Agencywide Documents Access and Management System (ADAMS) at

Accession No. ML14255A365.

2 Available in ADAMS at Accession No. ML12054A679.

3 Available in ADAMS at Accession No. ML12054A735. beyond-design-basis external event. These requirements ensure additional mitigation capability

is in place (beyond that assumed in the SFPS) in the unlikely event in which degrading

conditions occur in the SFPs.

The NRC used insights from the SFPS to perform a regulatory analysis4 of the fuel storage

practices at all U.S. operating nuclear reactors to help determine if expedited transfer of spent

fuel to dry casks was warranted. A regulatory analysis is the standard method for evaluating the

costs and benefits of a proposed Federal agency action. As part of its regulatory analysis, the

staff first conducted a safety goal screening evaluation using the Commissions safety goal

policy statement. The safety goal screening evaluation concluded that SFP accidents are a

small contributor to the overall risks for public health and safety (less than one percent of the

Commissions safety goal). Although the agencys guidance would normally allow the staff to

stop the evaluation upon determining that the proposed action does not provide a sufficient

safety enhancement to meet the threshold of the safety goal screening, the staff proceeded to

perform a cost benefit analysis to provide the Commission additional information. The staff

concluded that the expedited transfer of spent fuel to dry cask storage would provide only a

minor or limited safety benefit (i.e., less than safety goal screening criteria), and that its

expected implementation costs would not be warranted. Based on the regulatory analysis, including the NRC staffs review of operational experience, the NRCs oversight history, and

other SFP studies, the NRC staff recommended to the Commission that further regulatory action

not be pursued because the current regulatory framework is sufficient to ensure adequate

protection of public health and safety. The Commission approved this conclusion in SRM-

COMSECY-13-0030.5

DISCUSSION

The results of the SFPS and previous generic studies indicate that the current spent fuel

storage provides adequate protection of the public health and safety. In addition, recent

regulatory analyses have demonstrated that the safety benefits of further changes to SFP

operating practices would be limited, largely as a result of the low frequency of challenges that

could damage the SFP structure. However, the SFPS provided insights into operating practices

and mitigation capabilities that could enhance defense-in-depth by further reducing the

likelihood of fuel assemblies overheating in the event of substantial SFP damage.

Storing Spent Fuel in a More Favorable Loading Pattern

Spent fuel can be arranged in a dispersed pattern (e.g., 1 x 4 or a 1 x 8) that provides a more

favorable response to a loss of cooling water. In a dispersed pattern, recently discharged (hot)

assemblies are surrounded by older assemblies with less decay heat (cold). In some

circumstances, other patterns which do not satisfy the definition of a dispersed pattern may be

used for a limited period of time when other factors prevent directly discharging the assemblies

into a dispersed pattern. See the illustration below for examples of the 1 x 4 and 1 x 8 arrangements.

4 Available in ADAMS at Accession No. ML13273A628.

5 Available in ADAMS at Accession No. ML14143A360. Illustration of SFP patterns

From left to right: 1 x 4; 1 x 8 Red = a recently discharged assembly (hot); Blue = an older, lower decay heat assembly (cold);

Black outline = repeating pattern

An air coolable fuel assembly is one where, in the unlikely event of a loss of cooling water from

the SFP, natural circulation of air combined with radiative and conductive heat transfer between

the fuel and the storage rack structures will prevent overheating of the fuel. From a risk

perspective, a reduction in the time between when an assembly is added to the SFP and when

it is air coolable is advantageous.

Although variability in the SFP loading configurations was not a focus of the SFPS, Section 9 of

the SFPS, Considerations of Uncertainty, cataloged sensitivity analyses, where the NRC staff

compared the thermal response of spent fuel stored in contiguous and 1 x 8 patterns with the

1 x 4 pattern (baseline configuration used in the SFPS). In the unlikely event of a loss of cooling

water in the SFP, natural circulation of air combined with radiative and conductive heat transfer

between the fuel and the storage rack structures was found to reduce the likelihood of

overheating of the fuel. In the 1 x 4 pattern, fuel was found to be air coolable for at least

72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> for all but roughly the first 10 percent of the operating cycle. When the 1 x 8 fuel

pattern was evaluated, air coolability for at least 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> was achieved earlier in the operating

cycle. As such, to further enhance air cooling of spent fuel, licensees may choose to configure

the SFP with a 1 x 8 loading pattern as an improvement over the standard 1 x 4 loading pattern.

If licensees choose to configure the SFP in a 1 x 8 pattern, licensees may consider integrating

the fuel movement necessary to achieve the chosen fuel configuration with necessary

operational fuel movement and implementing over multiple operating cycles to minimize overall

fuel transfers and the associated risk. See IN 2014-09 for recent examples of SFP misloading

issues.

Directly Offloading Fuel from the Core into Dispersed Patterns in the SFP

The SFPS demonstrated that storing spent fuel in a dispersed pattern in SFPs promotes air

coolability of the spent fuel in the unlikely event of a loss of water. In addition, the SFPS

showed that minimizing the time that spent fuel is stored in a less favorable pattern could further

reduce the likelihood of a release if the SFP were to completely drain. Licensees may choose

to optimize spent fuel transfer into the SFP by direct placement in a dispersed pattern to further

enhance the safety of SFPs. Enhancing Mitigation Strategies

In addition to SFP loading patterns, the SFPS considered the benefit gained from the effective

deployment of the strategies implemented under 10 CFR 50.54(hh)(2) in the event of complete

SFP draining. While increasing the dispersal of the hottest fuel assemblies (from 1 x 4 to 1 x 8)

significantly reduced the rate of temperature increase following a loss of coolant, the effective

deployment of these strategies implemented under 10 CFR 50.54(hh)(2) was found to have the

largest impact on the frequency of release of radioactive material. Effective implementation of

these strategies reduced the frequency of release from the SFP.

The SFPS identified that these strategies can be challenged during periods of relatively higher

SFP heat load. In some cases, the SFPS found that existing strategies required by 10 CFR

50.54(hh)(2) may not be effective, either because available equipment would not provide

sufficient mitigation flow rate or radiation levels on the refueling floor would preclude access of

responders to provide cooling water to the SFP. At the time of the SFPS, the actions being

taken to comply with Order EA-12-049 were not fully developed and thus were not considered in

the SFPS. In light of the SFPS, licensees may choose to provide additional mitigation

capabilities through, for example, pre-deploying mitigation equipment during times of high SFP

heat load, moving connection points and operating controls for spray nozzles to areas of lower

dose, and providing additional water sources and connection points. Some or all of these

additional mitigation capabilities may already be planned to comply with Order EA-12-049.

As discussed above, Order EA-12-049 requires, in part, actions associated with restoring and

maintaining SFP cooling capability following a beyond-design-basis external event. For

example, the NRC-endorsed industry guidance for compliance with this order, NEI 12-06, Diverse and Flexible Coping Strategies (FLEX) Implementation Guide (ML12242A378),

includes a provision for connection points for SFP makeup that do not require access to the

refueling floor and additional provisions for the reasonable protection of the associated

equipment from external events. These enhancements may provide additional capability for

mitigating events that result in SFP draining, beyond those required by 10 CFR 50.54(hh)(2)

and considered in the SFPS.

CONCLUSION

The NRCs studies continue to show that current SFPs are effectively designed to prevent

accidents affecting the safe storage of fuel. The SFPS identified potential improvements that

could help licensees further manage the risk of significant radiological releases associated with

SFPs. This IN discusses insights from the SFPS regarding an unlikely, beyond-design-basis

seismic event. Storing spent fuel in more favorable loading patterns, placing fuel in dispersed

patterns immediately after core offload, and taking action to improve mitigation strategies when

the SFP heat load is high may help licensees further reduce the risk associated with the SFP.

CONTACT

S

This IN requires no specific action or written response. Please direct any questions about this

matter to the technical contacts listed below.

Michael C. Cheok, Director Lawrence E. Kokajko, Director

Division of Construction Inspection Division of Policy and Rulemaking

and Operational Program Office of Nuclear Reactor Regulation

Office of New Reactors

Mark Lombard, Director

Division of Spent Fuel Management

Office of Nuclear Material Safety

and Safeguards

Technical Contact:

Steve Jones, NRR Don Algama, RES

(301) 415-2712 (301) 251-7940

e-mail: Steve.Jones@nrc.gov e-mail: Don.Algama@nrc.gov

ML14218A493 TAC No. MF4562 OFFICE NRR/JLD/PPSD NRR/JLD/PPSD QTE RES/DSA/FSCB NRR/DSS/SPBP NRR/JLD/PPSD

NAME RBernardo* KWitt* CHsu* DAlgama* SJones* GBowman*

DATE 08/25/2014 08/25/2014 08/29/2014 09/17/2014 10/01/2014 10/02/2014 OFFICE NRR/DSS/SPBP RES/DSA/FSCB NMSS/SFST/CSDAB NRR/DE/ESGB NRR/JLD RES/DSA

NAME GCasto* RLee* MRahimi* GKulesa* JDavis* MCase*

DATE 10/02/2014 10/03/2014 10/01/2014 10/02/2014 10/08/2014 10/07/2014 OFFICE NRR/DSS OGC NRR/DPR/PGCB NRR/DPR/PGCB/LA NRR/DPR/PGCB NRO/DCIP

NAME TMcGinty* HBenowitz* TKeene* CHawes* SStuchell MCheok*

DATE 10/09/2014 11/03/2014 11/03/2014 11/03/2014 11/04/2014 11/05/2014 OFFICE NMSS/MSTR/DD NRR/DPR NMSS/SFSM NRR/DPR

NAME PHenderson AMohseni MLombard LKokajko

DATE 11/07/2014 11/12/2014 11/13/2014 11/14/2014