As analysts still grapple with understanding core damage accident progression at Three Mile Island and Fukushima that caught the nuclear industry off-guard once too many times, one notices the very limited detail with which the large reactor cores of these subject reactors have been modelled in their much hyped severe accident simulation code packages. At the same time, modelling of CANDU severe accidents have largely borrowed from and suffered from the limitations of the same LWR codes (see IAEA TECDOC 1727) whose applications to PHWRs have poorly caught critical PHWR design specifics and vulnerabilities. As a result, accident management measures that have been instituted at CANDU PHWRs, while meeting the important industry objective of publically seeming to be doing something about lessons learnt from say Fukushima and showing that the reactor designs are oh so close to perfect and the off-site consequences of severe accidents happily benign. 

Integrated PHWR severe accident progression and consequence assessment code ROSHNI can make a significant contribution to understanding of severe accident progression in CANDU PHWRs, improving significantly on the other PHWR specific computer codes developed decades ago when modeling decisions were constrained by limited computing power and poor understanding of and interest in severe accidents. These codes force gross simplifications in reactor core modelling and do not adequately represent all the right CANDU core details, materials, fluids, vessels or phenomena. But they produce results that are familiar and palatable. They do, however to their credit, excel in their computational speed, largely  because they model and compute so little and with such un-necessary simplifications. 

ROSHNI sheds most previous modelling simplifications and represents the 380 channel, 4560 bundle, 4 ring clad fuel geometry, materials and fluids more faithfully. Each of horizontal PHWR reactor channels with all their bundles, fuel rings, sheaths, appendages, end fittings and feeders are modelled and in detail that reflects large across core differences. While other codes model at best a few hundred core entities,  thermo-chemical transient behaviour of about 73,000 different fuel channel entities within the core is considered simultaneously along with other 15,000 or so other flow path segments. With such detail, ROSHNI is able to provide information on their progressive thermo-chemical contribution to accident progression and a more realistic fission product release source term that would belie the miniscule one (100 TBq of Cs-137 or 0.15% of core inventory)  given now to unsuspecting public and EMOs now in Canada. ROSHNI has an advanced, more CANDU specific consideration of each bundle transitioning to a solid debris behaviour without reverting to a simplified molten corium formulation that happily ignores further vessel failures and energetic interactions. The code is designed to grow into and/or use its voluminous results in a severe accident simulator for operator training.

 

A best effort code such as ROSHNI can be instrumental in identifying the risk reduction benefits of undertaking certain design, operational and accidental management improvements for PHWRs, with some of the multi-unit ones handicapped by poor pressurizer placement and  leaky containments with vulnerable materials, poor overpressure protection, ad-hoc mitigation measures and limited instrumentation common to all CANDUs. Case in point is the PSA supported design and installed  number of Hydrogen recombiners that are neither for the right gas or its potential release quantity.

 

 

The paper presents ROSHNI results of simulations of a postulated station blackout scenario and sheds a Light on the challenges ahead in minimizing risk from operation of these otherwise unique and accident free power reactors. The results challenge the PHWR industry intransigence and unwillingness to accept that these reactors were designed by humans and designed without severe accident within their design basis.

 


Simulation of Severe Accident Progression Using ROSHNI: A New Integrated Simulation Code for PHWR Severe Accidents

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

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