CANDU Owners Group

The Canadian CANDU Industry is implementing a composite analytical solution to demonstrate, with high confidence, adequate safety margins for Large Break Loss of Coolant Accidents (LBLOCA) in existing CANDU reactors. The approach involves consolidating a number of individual approaches in a manner that alleviates reliance on any single analytical method or activity. Using a multi-layered approach, the objective of this composite solution is to use a variety of reinforcing analytical approaches such that they complement one another to collectively form a robust solution. The composite approach involves: i) systematic reclassification of LBLOCA to beyond design basis events based on the frequency of the limiting initiating events; ii) more realistic modeling of break opening characteristics; iii) application of Best Estimate and Uncertainty (BEAU) analysis methodology to provide a more realistic representation of the margins; iv) continued application of Limit of Operating Envelope (LOE) methodology to demonstrate the adequacy of margins at the extremes of the operating envelope; v) characterizing the coolant void reactivity, with associated uncertainties; and vi) defining suitable acceptance criteria, accounting for the available experimental database and uncertainties. The approach is expected to confirm the adequacy of existing design provisions and, as such, better characterize the overall safety significance of LBLOCA in CANDU reactors. This paper describes the composite analytical approach and its development, implementation and current status.

Most nuclear plants around the world are roughly 15 to 30 years old. The design and procurement of CANDU plants took place from the late 60's to mid 80's (i.e., 20 to 30 years vintage). Most equipment originally installed in these plants is obsolete or the manufactures are out of business or their production has been discontinued due to technological evolution. In order to maintain operation of nuclear plants with safety integrity and commercial viability, certain spare parts must be available at the plant all the time. The objective of this paper is to identify an optimum, cost-effective approach that solves obsolescence problem efficiently and without duplicating efforts. The Nuclear Utility Obsolescence Group (NUOG) has embarked upon the following major tasks: Developing a Guideline for use by the utilities that addresses obsolescence; Collection of obsolescence data in a database (Web-based) to be shared by all members; Motivation of the suppliers to engage them in obsolescence solutions; Increase in awareness among the utility management to consider obsolescence as a priority issue and allocate funds to address them pro-actively; Coordination with other industry groups (EPRI, INPO, NEI, BWROG etc.) to avoid duplication of effort in obsolescence resolution process. The NUOG strategy is based upon the principles of sharing. It advocates sharing of obsolescence solutions and concerns among the utilities. Candu Owners Group Inc. (COG) has initiated self-assessment of obsolescence in the members' plants. The purpose of self-assessment is to provide baseline information that would help identification of obsolescence and coordination of their solutions. The following areas are covered in the self-assessment initiative: Identification of obsolete components in selected systems in the plant; Assess effectiveness of the current obsolescence identification process and in resolution of obsolescence Issues in the plant; ldentification of common Candu plant design specific obsolescence issues; Benchmarking of good obsolescence practices followed in the plant. The benefits of Obsolescence Self-assessment would be shared among the COG members. It will also provide an opportunity to solve common obsolescence issues collectively. The author believes that the combination of initiatives taken by NUOG and COG would lead to cost-effective and pro-active identification of obsolete components and sharing of solutions among all participating nuclear utilities. The nuclear industry will be the net beneficiary in achieving safer, productive and competitive operation.

Each CANDU reactor is monitored internally in both horizontal and vertical positions, by numerous self-powered neutron flux detectors during high power conditions. After approximately 9 -11 years they start to degrade and eventually cannot function as designed. Although flux detector assemblies contained spare wells to accept new detectors as old detectors started to fail, many CANDU reactors have reached a point where they no longer have spare wells left. Therefore the old detectors need to be removed, safely disposed and replaced with new ones.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has had an unprecedented impact on the people of Ireland as waves of infection spread across the island during the global coronavirus disease 2019 (COVID-19) pandemic. Viral whole-genome sequencing (WGS) has provided insights into SARS-CoV-2 molecular mechanisms of pathogenicity and evolution and contributed to the development of anti-virals and vaccines. High levels of WGS have enabled effective SARS-CoV-2 genomic surveillance on the island of Ireland, leading to the generation of a sizeable data set with potential to provide additional insights into viral epidemiology. Because Ireland is an island, accurate documentation of travel rates to and from other regions, both by air and by sea, are available. Furthermore, the two distinct political jurisdictions on the island allow comparison of the impact of varying public health responses on viral dynamics, including SARS-CoV-2 introduction events. Using phylogenomic analysis incorporating sample collection date and location metadata, we identify multiple introduction and spreading events for all major viral lineages to the island of Ireland during the period studied (March 2020-June 2022). The majority of SARS-CoV-2 introductions originated from England, with frequent introductions from USA and northwestern Europe. The clusters of sequences predicted to derive from discrete introduction events ("introduction clusters") vary greatly in size, with some involving only one or two cases and others comprising thousands of samples. When introduction cluster samples are mapped sequentially by collection date, they appear predominantly in previously affected or adjacent areas. This mirroring of the phylogenetic relationships by the geospatiotemporal propagation of SARS-CoV-2 validates our analytic approach. By downsampling, we estimate the power to detect introductions to Ireland as a function of sequencing levels. Per capita normalisation of both sequencing levels and detected introductions accounts for biases due to differing sequencing efforts and total populations. This approach showed similar rates of introductions for all major lineages into Northern Ireland (NI) and Republic of Ireland (RoI) with the exception of Delta, which was higher in NI which is likely attributable to higher travel per capita. However, there were similar rates of Delta infection within NI and RoI, suggesting that although travel restrictions will reduce risk of introducing novel variants to the region, they may not substantially decrease total incidence. Our generalisable methodology to study introduction dynamics and optimal sequencing levels will assist public health authorities to select the most appropriate control measures and viral sequencing strategy.