Successful First Experiment In The Rasplav International Nuclear Reactor
Safety Project In Russia
On 9 October 1996 the first large-scale test involving
the melting of real core material of a light water reactor was successfully
carried out in the Rasplav facility in the Russian Research Centre at
the Kurchatov Institute in Moscow. This experiment was performed within
the framework of the first OECD Nuclear Energy Agency (NEA) sponsored
project set up in Russia in the area of nuclear safety. The project addresses
overheating of the reactor vessel under severe nuclear accident conditions.
It has been designed to ensure that the results generated will be of relevance
to the reactors operating in NEA Member countries as well as to Russian-designed
pressurised water reactors (VVER). Furthermore, the project is applicable
to both current and future reactor designs.
During the test, core material was
heated to 2700C. The test vessel was cooled externally in a controlled
manner to simulate external cooling of the vessel in a severe accident.
All measuring and engineering systems functioned normally during the experiment.
It is the first time that a large mass of core material has been heated
up to such high temperatures under controlled conditions. The test results
confirmed the technical feasibility of conducting further large scale
tests with such core material compositions. The post-test examination
will consist of sectioning the solidified material and performing metallographic
examination to determine the details of test performance, along with a
comparison of the temperature measurements with the pre-test thermal analysis.
The Rasplav Project brings together
14 NEA Member countries and Russia in the first Agency-sponsored joint
nuclear safety project to be carried out in a non-Member country. The
participating Russian organisations are the Russian Research Centre 'Kurchatov
Institute', the Russian Federation Ministry of Atomic Energy and the Ministry
of Science and Technology Policy. Rasplav is a major international nuclear
reactor safety undertaking being carried out over three years. The experimental
work is carried out in the Kurchatov Institute, the analytical work is
done in the Institute of Nuclear Safety, and design work and some testing
work is done by other organisations in the Moscow region.
Rasplav is the latest link in a chain
of international projects aimed at refining strategies for maintaining
the integrity of reactor pressure vessels in the highly improbable event
of a core meltdown. Retaining the integrity of the reactor pressure vessel
for such accidents, which are estimated to have a probability of occurrence
of less than 1 in 10,000 years per reactor, will help prevent the release
of radioactive material to the environment.
A background note on the Rasplav project
is attached.
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BACKGROUND NOTE
Introduction
The Rasplav project is concerned with
behaviour of the lower head of the reactor pressure vessel during severe
accidents leading to core meltdown. The Rasplav project may be regarded
as a successor to the OECD Three Mile Island (TMI) Vessel Investigation
Project which was completed in 1993. That project was set up to examine
and assess the condition of the actual TMI lower head and thus offered
a unique opportunity to study a severe accident in a commercial nuclear
power plant. However, in order to better understand and model the complex
phenomena taking place during interaction of molten fuel with the reactor
vessel lower head, it is also necessary to reproduce this interaction
in a controlled environment where the governing parameters can be varied
over the range of interest. This is the main aim of the Rasplav project.
Technical background
During a core melt accident, the core
debris will relocate to the bottom of the reactor vessel. In the absence
of any cooling of this debris, the heat generated would eventually overheat
the reactor vessel, which would fail, thereby releasing the corium (molten
core material) to the lower containment. There is interest in two aspects
of the issue. First, for existing reactors, when external cooling may
not be practicable, there is a desire for a better understanding of the
phenomena and of the time before the molten fuel penetrates the vessel
and discharges to the reactor containment, to assist in the development
of severe accident management strategies. Secondly, for future designs
where external cooling in severe accidents is being considered at the
design stage, and for some existing reactors, there is a need for a better
understanding of the complex interactions to demonstrate that cavity flooding
is a viable accident management option.
Details of the tests
The Rasplav project uses prototypical
materials (real core material components and vessel steel) in large quantities
and at temperatures representative of a core meltdown accident. This assists
in understanding the chemical reactions, which are very complex, and contributes
to understanding the natural convection processes. The design of this
main integral test facility is a model of the reactor vessel, filled with
200kg corium, in the form of a slice through the lower head. The corium
is electrically heated with graphite induction heaters in the planar side
walls, protected from the corium by layers of tantalum and tungsten. The
vessel section is cooled by water.
There are supporting smaller scale
experiments to study material properties and chemical interactions, and
to investigate the technological aspects of the larger Rasplav experiment.
These have supplied necessary design information for Rasplav.
Several molten fuel structural experiments
are carried out in the Rasplav programme. The technical objectives include:
determining material properties such as emissivity,
viscosity, and density to aid predictions of the volume, the composition,
and the temperature of the core melt.
evaluating the interaction of molten core material
with the vessel lower head and determining the heat fluxes imparted
to the lower head.
exploration of the effect of scaling, in order
to assure that the experimental data will be suitable for application
to full size vessels.
Measurements made in conducting the experiments include:
Melt temperature measurements in the Rasplav
furnace and in the vessel lower head.
Heat flux measurements.
The information from Rasplav complements
the database obtained from other experiments and projects that provide
information related to vessel failure, such as the OECD TMI Vessel Investigation
Project, and the European Union sponsored the Melt-Vessel Interaction
Project.
The following OECD countries with nuclear
power programmes participate: Belgium, Canada, Finland, France, Germany,
Italy, Japan, Republic of Korea, the Netherlands, Spain, Sweden, Switzerland,
the United Kingdom and the United States.
The budget is $6.9 million in total,
over a three year period. The project started on the 1st of July 1994.
