SESSION 1
OVERVIEW OF SEVERE ACCIDENTS IN NUCLEAR POWER PLANTS (Impacts of TMI and
Chernobyl)
TMI-2 VESSEL INVESTIGATION: RESULTS, CONCLUSIONS AND
IMPLICATIONS FOR ACCIDENT MANAGEMENT STRATEGIES
T. P. Speis
U.S. Nuclear Regulatory Commission, Washington, DC 20555
In October 1987, the U.S. Nuclear Regulatory Commission proposed that a joint
international cooperative program be formed that would be sponsored by the
Nuclear Energy Agency of the Organization for Economic Cooperation and
Development (NEA-OECD) to conduct further investigations of the potential
damage to the TMI-2 reactor vessel lower head during the accident. This
proposal was accepted by NEA-OECD and the project was initiated in 1988. The
program was completed five years later. This paper discusses the results from
this program and the related conclusions. It specifically addresses the
findings regarding the extent and type of damage to the vessel lower head and
the margin of structural integrity that remained in the vessel during the
accident; the conditions for potential failure (which apparently were never
reached); enhanced cooling mechanisms of the debris and lower head not
currently accounted in severe accident analyses; and insights for severe
accident management strategies. Finally, the results/conclusions from the
program have pointed to a number of areas where further studies, including
appropriate experiments, can add to our understanding of the conditions under
which the reactor vessel can be cooled and its integrity maintained during a
severe accident. Such efforts are already underway and they will also be
addressed in this paper, including their specific goals and significance.
SAFETY ASSURANCE APPROACH TO SEVERE ACCIDENTS AND SEVERE
ACCIDENT MANAGEMENT AT THE NPP WITH WWER
V. Sidorenko, V. Voznesenski*, N. Fil**, E.
Tsygankov*
Ministry of Russian Federation on Atomic Energy, Moscow, Russia
*Russian Research Center Kurchatov Institute, Moscow, Russia
**EDO "Gidropress", Podolsk, Moscow Region, Russia
During 30 years passed since the startup of the first WWER reactor in
Novovoronezh, 55 WWERs have been put into operation at 17 NPPs sited on the
ex-USSR territory, in the East European countries and Finland. As of 01.01.95,
the WWERs of different designs have the total reactor years exceeding 600.
Beginning from the first WWER a great importance was attached to elimination of
phenomena and events that could cause a severe accident of aggravate
development of an accident making it eventually a severe one. The first NPPs
with the WWERs were built in the regions with the least possible external
impacts (in non-seismic regions, far away from the aviation air corridors,
harmful production etc.). In designing the WWERs the materials and structures
resistant to rapid destruction were used. Potentially harmful phenomena such as
reactor vessel embrittlement, initiation of cracks in the SG headers etc., were
under continuous observation during the operation. As the experience showed,
this permitted timely measures to be taken against propagation of failures in
severe accident. The WWER equipment characteristics (secondary, water inventory
in the SG, pressurizer capacity etc.) were chosen so that the accident would
slowly develop, and the operational personnel could take effective measures for
its mitigation. An example of successful employment of the WWER capabilities by
the personnel is the minimization of the consequences of a fire at the Armenian
NPP resulting in complete loss of the SG feed water. A serious impetus to
redouble efforts at investigating severe accidents was the Three Mile Island
and Chernobly accidents. The normative documentation adopted in Russia in the
late 80s stipulates the proof of the low probability of severe accidents in the
designs. For operating NPPs special guides and instructions, determining the
personnel's actions during the beyond-design accidents, were worked out. The
measures for increasing their safety stipulate the modernization of the old
NPPs and introduction of new systems and equipment for of the beyond-design
accidents management. The efficiency of these measures is substantiated by the
use of the PSA methods. The designs of new power units with LWR envisage
special measures both for the elimination of severe accidents and mitigation of
their consequences. In the design of the NPP with WWER-640, which continue the
traditional WWER line, passive systems are used for the long-term core cooling
down during the accident. In spite of the low core melting probability measures
are taken to keep the melt within the reactor vessel or at least inside the
containment. The WPBER-600 design extends the line of safety enhancement, which
has been realized in the design of the Soviet nuclear district heating plant.
The integral layout of the NSSS equipment, low heat loss, second (guard) vessel
make the core melting practically impossible. However, just as in the WWER-640
design, the containment is provided with an additional facility for trapping
the melted core. As one of the measures for the most reliable protection of the
population against the consequences of a severe accident, the underground NPP
siting is considered by the Russian specialists. The Russian investigations of
processes occurring in the severe accidents and measures for their management
are currently closely connected with the international efforts in this
direction. One of the international severe accident projects directed to the
confirmation of the possibility of melt retention inside the reactor vessel
(RASPLAV) is being carried out on the basis of the experimental complex now
under construction at the RRC "Kurchatov Institute". The close connection of
the new engineering approaches undertaken for LWR safety enhancement in each
nuclear country with the international program of investigations permits the
desired results to be reached in an optimal way.
FEATURES OF RBMK REACTORS DURING SEVERE ACCIDENTS AND
RELATED ASSESSMENT TECHNIQUES
E. O. Adamov, Yu. M. Cherkashov, Yu. V. Mironov
and Yu. M. Nikitin
Reactor and Development Institute of Power Engineering, Moscow, Russia
E. B. Burkalov and N. E. Kukharkin
Reactor Scientific Center "Kurchatov Institute", Moscow, Russia
Key features that affect the progression of severe accidents in RBMK reactors
are described. Estimates of plant responses are presented for severe accidents
initiated by a positive reactivity insertion, a loss of electrical power, and a
loss of heat transport circuit integrity. Existing and potential measures to
mitigate the severe accidents are discussed.
It is noted that Probabilistic Risc Assessment techniques and criteria
established for Western Light Water Reactors may not be appropriate for the
channel-type RBMK reactors. Severe accident issues in RBMK are related to
developing preventive and mitigation measures which make effective use of the
operating experience as well as the knowledge gained from international
programs.
CANADIAN INVOLVEMENT IN RBMK SAFETY IMPROVEMENT
PROGRAMMES
R. A. Brown, C. Blahnik and J. P. Karger
Atomic Energy of Canada Limited, 2251 Speakman Drive, Mississauga, Ontario,
Canada
In 1992 at the Munich G-7 Summit Meeting the safety of Soviet-designed reactors
in the Former Soviet Union (FSU) countries was raised as a priority issue and
the G-7 governments established both Bi-lateral and Multi-lateral programmes of
assistance to these countries. A Nuclear Safety Account (NSA) was established
by 13 participating governments to fund a series of grants to selected nuclear
power plants, the fund being administered on their behalf by the European Bank
for Reconstruction and Development. As a member of the G-7 Canada contributed
to this multinational safety fund.
The NSA funds both technical improvements at selected sites as well as the
project management teams needed to develop specifications, contracts and review
bids for these safety improvements. AECL has concentrated its efforts on RBMK
reactors because of its expertise in the design and construction of CANDU
channel type reactors which share certain similarities with the RBMK.
Specifically it is participating in two programmes funded by EBRD for the
Ignalina NPP (INPP) in Lithuania; one related to the design of a diverse
shutdown system and associated activation sensors and logic, the second related
to the preparation of a western styled safety analysis in conjunction with
Vattenfall from Sweden and Stone & Webster Engineering Corporation from the
United States, the RBMK Chief Design organisation and INPP staff. The objective
of this latter study is to apply western type tools and methodology to an
extensive analysis of both the station operation, its safety management
practices and the accident analysis. The work is currently underway and the
paper will describe the process and progress to date.
In 1992 the Canadian Government announced the $30M CDN Canadian Nuclear Safety
Initiative aimed at providing regulatory, design, operational and utility
support for RBMK reactors in Russia and Lithuania. As part of this programme
AECL developed a Nuclear Safety and Engineering Programme (NSEP) which has four
parts.
- An operating team initially residing at one RBMK station and the rotating
to others, aimed at introducing western operating practices and safety culture
to NPPs with RBMK reactors.
- An engineering and safety team located in Moscow to work with with RBMK
design institutes to address some short term safety design improvements and
introduce Canadian expertise in the area of fuel channel inspection techniques.
- A programme which will be carried out both in Canada and Russia.
- An operations staff exchange programme.
As part of the CNSI Canada participated in the 8 nation International RBMK
Safety Review which undertook an extensive review of the design and operation
of an RBMK based primarily upon Unit 3 at Smolensk. The results of this review
were reported in June 1994 and the paper will discuss highlights in various
areas with particular emphasis on the accident analysis.
REGULATORY APPROACHES TO SEVERE ACCIDENT ISSUES: AN
INTERNATIONAL PROSPECTIVE
G. M. Frescura
OECD/NEA
R. J. Barrett
US-NRC
This paper summarizes the results and conclusions of a study conducted by the
OECD/NEA on the approach taken by Regulatory Organizations to deal with severe
accident issues. The information for the study was collected through a
questionnaire distributed to the OECD/NEA Member countries. This information
was then discussed in depth by a committee consisting of senior representatives
of Regulatory Organizations. The paper shows that most countries have taken
substantive steps to reduce the risk resulting from postulated severe accidents
via measures that include prevention, mitigation and accident management.
Practical considerations have resulted in most countries placing greater
emphasis on prevention of severe accidents and implementation of accident
management. However, design modifications have been done or are planned in a
number of countries. For future reactors, most OECD/NEA countries require
specific consideration of severe accidents in the design process, however
severe accidents are not included in the design basis set.
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