CANDU spent fuel pools contain fuel bundles in geometries not replicated in any other power reactor. Therefore, the phenomenological issues, accident progression pathways and effectiveness and even danger of potential mitigative actions are somewhat different and require a dedicated approach. The SFPs house densely packed fuel bundles stacked in about a hundred vertical stainless steel tray towers containing upto 24 spent fuel bundles in each of the sixteen horizontal fish basket style steel trays. Some of the upto 10 year worth of the on-line refuelled bundles in the SFP are at relatively high decay powers and fuel trays are prepped for the towers in near daily basis. The design is robust as long as water envelopes the spent fuel bundles. About 5m of additional water on top of the tray towers provide radiation protection and margins to fluid loss. On a sustained loss of water envelope, they however, are vulnerable to causing off site consequences greater than those from a reactor core severe accident.

 

The CANDU spent fuel storage horizontal tray geometries, while optimized for cost effective, compact, under-water storage of intact spent fuel bundles are not conducive to their very long term survival in absence of liquid water as a heat sink and cannot effectively promote any air circulation for passive heat removal as many other vertical geometries in LWRs spent fuel pools likely can. Heat removal from inner fuel bundle surfaces within the 4 ring, 0.1 m dia, 0.5 m long fuel bundles can easily become retarded in absence of water.  Zircaloy fire in one single tray location can propagate to the rest of the pool trays in a manner that can threaten release into atmosphere from the SFP industrial grade building of long-lived fission product inventory equivalent of over 7 operating reactor cores.

 

While there is some renewed publically stated perfunctory interest after Fukushima, there really have been no efforts to identify, reflect upon, quantify and take concrete additional measures to reduce the associated risk.  The industry position is based on incredibility of such an accident and a usual confidence in their accident management capabilities, coupled with significantly lower than LWR fuel decay powers.

 

Given that a series of interdependent phenomena occur during heatup of fuel bundles exposed over vast spent pool expanses, a dedicated approach to modelling of thousands of fuel bundles within hundreds of closely stacked fuel trays is required to model accident progression and consequences to contemplate potential mitigating action effectiveness.

The computer program FUELPOOL models a logically sorted, important thermal-hydraulic and thermal chemical phenomena.  It numerically predicts response of fuel bundles to an unmitigated loss of pool water inventory by pool drain and/or boil-off.  Ensuing deformations, debris formation and thermo-chemical interactions with steam and air can cause significant releases of fission products of high dose conversion factor, high risk and long lived species not considered for releases in design basis reactor core accidents. Major components of the FUELPOOL computer code  include :

 

Pool drain, water heatup and increased pool water evaporation

Fuel bundle tray Heatup

Air and steam oxidation: 

Fuel bundle sheath failures and fission product releases  

Debris formation  and relocation

Melt formation, relocation, quench and hydrogen generation:

Fission Product Releases from debris

SFP building structure response

Fission product releases from building into the environment, and

Effectiveness of mitigating actions

 

FUELPOOL evaluates fuel and debris heatup during various accident progression pathways and estimates time available to the operator for various mitigative actions. When operators’ actions cannot be credited or are ineffective, it provides fission product release estimates for off-site dose calculation using an external fission product dispersion and dose conversion code.


FUELPOOL: A Computer Program to Model CANDU Spent Fuel Pool Severe Accident Progression and Consequences

Division / Sponsors: 
Power Division & Nuclear Engineering Division (NED)

Power Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2020/volume/83747

International Conference on Nuclear Engineering
Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE2020/volume/83761

POWER-ICONE - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.