Canadian Licensing Procedure and Experience with … Licensing Procedure and Experience with Mixed...

Canadian Licensing Procedure and Experience with Mixed Cores for CANDU Reactors

Janusz Kowalski Canadian Nuclear Safety Commission

Canadian Nuclear Safety Commission 2 Technical Meeting on Fuel Design and Licensing of Mixed Core for Water Cooled Reactors, Vienna, 12-14 December 2011

Outline

• CANDU reactors • New fuel bundle projects • Mixed reactor cores • Impact of the mixed core on fuel

thermalhydraulic performance and safety margins

• Summary and conclusions

http://nuclearsafety.gc.ca/


Canadian Nuclear Safety Commission

Established May 2000, under the Nuclear Safety and Control Act

Replaced the AECB, established in 1946, Atomic Energy Control Act

Canada’s Independent Nuclear Regulator -

65 Years Of Experience



CNSC Regulates All Nuclear-Related Facilities and Activities

• Nuclear power plants • Small reactors • Uranium mines and mills • Uranium fuel fabricators and

processing • Waste management facilities • Industrial and medical

applications • Nuclear research • Export/import control



Typical Configuration of a CANDU Reactor

Typically number of fuel

channels 380-480 Number of bundles in each channel 12/13



37-Element CANDU Fuel Bundle Arrangement



Proposed Replacement of Fuel Bundles in CANDU Reactors Two new fuel projects were/are undertaken by Canadian Utilities to replace the current CANDU 37-element fuel bundles (37R) with:

– the CANFLEX-LVRF 43-element bundles (LVRF) - BP project

– the modified 37-element bundles (37M) -OPG and BP projects



Proposed Replacement of Fuel Bundles in CANDU Reactors

A main objective of these projects is to improve safety margins by:

– offsetting the impact of the plant aging on margin to fuel dryout

(LVRF and 37M fuel bundles projects) – improving LLOCA safety margins (LVRF

fuel bundle project)



Schematic of 37R CANDU Bundle and 43-Element LVRF Bundle

LVRF 37R



Key Features of the LVRF Bundle Design

Main differences between the LVRF and 37R bundle designs:

number of fuel elements In LVRF central element is composed of natural uranium mixed with a burnable neutron poison 42 elements of LVRF are composed of SEU two different sizes of LVRF fuel elements in the bundle CHF enhancing buttons brazed to the elements thinner cladding thickness raised bearing-pad height



Modification to 37R Fuel Bundle



Key Features of the 37M Bundle Design

• Small decrease of the centre element diameter – increased flow areas in the centre subchannels where CHF

is most likely to occur for 37R

• Remaining 36 fuel elements, end plates, bearing

pads and spacers are unchanged

• Bundle components interfacing with other structures, systems and components (SSC) are unchanged



Proposed Approach for Converting Reactor Cores to Use of New Fuel Bundles

• LVRF Bundle Project – Demonstration Irradiation (DI)- 2 channels

used to irradiate 24 bundles over two years)

– Full-Core Implementation • 37M Bundle Project

- DI using two channels of Darlington or Bruce reactors

- Full-Core implementation



Mixed Core During Core Conversion

• DI: Two selected channels contain a mix of 37R and new fuel bundles during a significant portion of DI – Typically, either four or eight in a fuel channel

are replaced during refuelling operation – 2 bundles per shift were loaded during LVRF

refuelling

• Transition to full core: Mixed bundle configuration during the transition period



Regulating New Fuel in Canada

CNSC regulatory requirement for implementation of the new fuel bundles:

The licensee shall not load any fuel bundle or fuel assembly into a reactor unless the use of the design of the fuel bundle or fuel assembly has received prior written consent by the Commission, or a person authorized by the Commission

•The Utilities requested the CNSC consent to use of the LVRF and 37M fuel bundles in the Bruce Power and Darlington reactors



Topics reviewed by CNSC for LVRF Project

To assess the safety risk associated with the conduct of DI, CNSC reviewed the following topics:

– Fuel thermalhydraulics (Focus of presentation) – Safety analysis (Focus of presentation) – Reactor physics and nuclear criticality safety – Fuel bundle manufacturing – Operational procedures – System interaction – Human factors



Review Findings: Thermalhydraulic Performance during DI for the 37R and LVRF Mixed Core

• Objective: To confirm that the LVRF bundle provides the thermalhydraulic performance in accordance with design requirements and assumptions applied in safety analyses.

• Improvements in critical heat flux (CHF) for LVRF fuel accommodate adverse effects of plant aging on DBA such as: pressure tube diametrical creep effects on slow LOR and LOF events and SBLOCA – when the LVRF fuel reaches higher burnup, CHF improves

providing additional safety margins in the selected DI channels • Differences in the LVRF and 37R fuel single and two-phase

pressure drop in two DI channels do not affect CANDU header conditions

• To assure that the LVRF bundle is fully compatible with the 37-element bundle in mixed fuel bundle strings, experiments of pressure drop involving mixed junction were conducted



Review Findings: Safety Analysis for the 37R and LVRF Mixed Core

• DI safety analysis was performed to assess the change in the existing safety margins when the channels are loaded with the new fuel

• Assessment of safety analysis results indicates that the accidents most affected, and for which incremental analysis needs to be performed to quantify the effect of the new fuel are: Loss of Regulation (LOR) – to offset the adverse effects of plant

aging Loss of Flow (LOF) – to offset the adverse effects of plant aging Small Break LOCA (SBLOCA) – to offset the adverse effects of

plant aging Large Break LOCA (LBLOCA) – to quantify impact of change in CVR

• Other accidents were assessed qualitatively to demonstrate negligible impact of the new fuel




• Safety margins for LOF events were not negatively affected by the presence of LVRF bundles in DI fuel channels. – For DI channels, channel and bundle power limits

were bounded by the results of current safety analysis for 37R fuel

• For LOR events, there was no impact of the mixed fuel on trip setpoints during DI – fuel heatup during LORA was bounded by safety

analysis for current CANDU bundle




• The maximum LVRF fuel temperature for all analyzed SBLOCA was lower than that calculated for 37R fuel. – For DI case: safety margins were not affected

• During LBLOCA, a power peak was predicted to be lower in the channels containing LVRF bundles than in the channels with 37R fuel – Primarily due to lower fuel element ratings (heat

flux), which resulted in lower fuel centreline temperatures



Topics reviewed by CNSC for 37M Project

To assess the safety risk associated with replacement of the 37R bundle by 37M bundle, CNSC reviewed the following topics

– Fuel thermalhydraulics (Focus of presentation) – Safety analysis (Focus of presentation) – Reactor physics – Fuel bundle manufacturing – Operational procedures – System interaction – Human factors



Review Findings: Thermalhydraulic Performance during DI for the 37R and 37M Mixed Core • Objective: To confirm that the 37M bundle provides the

thermalhydraulic performance in accordance with design requirements and assumptions applied in safety analyses

• The main effect of the 37M bundle was an increase in CHF relative to 37R bundles, offsetting some of the adverse impacts of HTS aging – For DI assessment, it was assumed that more limiting fuel type (37R) is

present at the location of dryout during the postulated accident • Single-phase pressure drop across the 37M bundle is slightly lower

than for 37R. The DP correlation developed for 37R fuel is considered applicable to the mixed fuel string, based on similarity of – the pressure drops along the two fuel bundle types; – similarity of bundle-to-bundle junctions between 37M/37R and 37R/37R

bundles • The presence of 37M in a limited number of fuel channels is expected

to have an insignificant impact on system thermal hydraulic conditions



Review Findings: Safety Analysis for the 37R and 37M Mixed Core

For DI: – An impact of bundle mixing on safety margins for LOF, SBLOCA,

LOR and LBLOCA is negligible For transition to full core (preliminary assessment):

– There is no impact of core mixing on trip effectiveness and maximum sheath temperature for LOF events,

– For SBLOCA, maximum sheath temperature increases slightly in the remaining 37R bundles but there is no impact on trip effectiveness

– For LOR events, Critical Channel Power (CCP) decreased insignificantly for channels with 37R fuel

– For LBLOCA, hot bundle enthalpy for mixed core is similar to the channels containing the 37R bundles. However, peak core reactivity and neutronic trip times have slightly increased values for 37M fuel



Review Findings: Safety Analysis for the 37R and 37M Mixed Core

• DI: No or negligible change for all accidents

• Transition to full-core: Possible slight change in trip margin

– Slight reduction in flow in lower powered channels with fewer 37M bundles

• Full-core: Small increase in full-core void reactivity for 37M fuel. Needed additional calculations to confirm prior to initiating full implementation: – Re-analysis of trip coverage accidents to demonstrate

improvement in safety margins following full core implementation of 37M fuel (SBLOCA, LOF and LOR events)



Summary and Conclusions

• CNSC requires a submission of the safety case for a new fuel

• CNSC review of two DI submissions for the LVRF and 37M fuel has been performed – introduction of new fuel designs for DI purpose does not

have the negative impact on the current reactor licensing basis

• DI for the LVRF fuel has been completed • The Industry (OPG and BP) is moving towards full-

core implementation of the 37M bundles • To obtain CNSC authorization to proceed with the

full-core implementation, the full-core and transitional-core safety cases need to be re-assessed



Thank you


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