Meltdown for Safety – Part 2 Containment Research

SAM-N - The containment that surrounds a nuclear reactor stands as the last barrier preventing releases of fission products.  While the word “containment” simply means “to hold within fixed limits”, a containment is more than a passive system.  Following a severe accident resulting in significant core damage, fission products can leave the confines of fuel elements and be carried out of the core and out into the containment through leak in the reactor coolant system.  Early nuclear power plant designers recognized that should the containment be needed in this “last barrier” role, a combination of both active and passive systems could support the ultimate objective of mitigating the loads challenging the containment’s ability to prevent releases and stabilize fission products. 

To the 1950s and 60s Atomic Energy Commission, the principal focus of containment research and development would be related to the mechanisms of radiological transport and the prevention of unintended leakage.  Subsequently, the AEC sponsored a considerable amount of research. Among these were the Containment Mockup Facility (CMF), the Containment Research Installation (CRI), and the Nuclear Safety Pilot Plant (NSPP) at Oak Ridge National Laboratory; the Aerosol Development Facility (ADF) and Containment Systems Experiment (CSE) program at the Pacific Northwest Laboratories; and the Contamination-Decontamination Experiment (CDE) facility at the National Reactor Testing Station.   

The primary uncertainty that they sought to resolve was how these fission products distribute themselves within the reactor coolant system and the containment following core damage.  The results from Oak Ridge's Containment Mockup Facility showed that the main concern is from gaseous and aerosol-based fission products and interaction and reaction of fission products with structures. The primary elements of interest were iodine, cesium, tellurium, strontium, barium, and ruthenium. From this, analytical models were developed for application in source term calculations.

In all experiments, containment design and configuration was shown to have a large impact on the natural response mechanisms influencing the removal of fission products.  This principally related to the openness of the contained space and how the core and containment residual heat was removed.  In a purely passive arrangement, the transfer of heat to the containment walls promotes natural circulation of the containment atmosphere and condensation of steam at the walls.  The experiments at the Nuclear Safety Pilot Plant and the Contamination-Decontamination Experiment were used in these efforts to study such phenomenon.  In particular, these experiments revealed valuable insight regarding iodine transport and condensation-driven fission product removal, among other phenomena. 

Radioactive iodine is considered the biggest threat to humans from a severe accident because it preferentially deposits itself into the thyroid.  Experiments showed that most of the airborne iodine migrates itself to component surfaces and liquid films or ends up in the vessel sump.  In addition, in an LWR containment, the post-accident environment is very hot and moist.  The relatively cool exterior containment walls will condense steam.  This cooler, wet surface enhances the settling of aerosol-based fission products.  As much as 50% of the total fission product reaching the containment vessel can be removed by this natural process.

While the studies on the natural response of containments generally showed that much of the fission products could be removed from its atmosphere passively, these natural removal mechanisms are inherently slow. If a major release of fission products to a containment occurs, it is desirable to reduce the exposure potential as rapidly as possible. This can be accomplished by means of two interrelated operations: reduction of the driving force for containment leakage and reduction of the airborne inventory of fission products. 

Engineered systems were subsequently developed and tested to accelerate this process.  These included containment spray cooling, recirculating air cooling, heat-removal, and air cleaning and filtering systems.  The cooling systems reduce the pressure and temperature of a post-accident containment atmosphere.  Doing so minimizes the driving forces contributing to the release rate of fission products to the environment. Containment spray systems provide the quickest response by both condensing the steam (thereby, reducing the pressure driving force for leakage) and acting as a filter for removing airborne fission products.  Air cleaning can be used in two ways as an engineered safety feature: 1) recirculated systems in the containment shell reduce the concentration of airborne fission products in the containment atmosphere after an accident and thereby reduce fission-product leakage and 2) once through (single-pass) systems in a secondary containment area collect and retain any fission products that have leaked from the containment shell.  Coincident with the AEC research, the major LWR vendors were developing and testing their own ideas.  The most promising concept from this effort was the pressure suppression water pool that could both reduced containment pressure and filtered aerosol-based fission products. 

Through this early period of research and development, the AEC and the industry that it created were learning how to build an electricity-generating nuclear system and account for all possibilities in the interest of public safety.  Indeed, they had accounted for the breadth of uncertainties as they understood them to be at the time.  Further, events and tests at the EBR-1, BORAX and SPERT facilities, informed them that they could recover from a worst case scenario. Of course, safety requires vigilance and an event occurring in 1961 - one that resulted in casualties - would highlight the most important piece in safety – the human component.  Stay tuned.