The ACS consists of a set of structures and equipment, whose main functions are to confine radioactive releases in case accidents and to provide a source of water for emergency water injection to the primary circuit i n case of LOCAs. In this last case, part of the steam lost from the break, after condensation, can be used for restoring the water source for ECCS. The geometry of the ACS does not permit a similar reuse of the liquid lost from the break, which is collected in drains and then reused after clean up. As in the other RBMK plants of the most recent generation, the confinement envelopes only parts of the pressure boundary, mostly the parts filled with liquid or located in lower positions. In the design stage, it was decided to envelope only those pipes whose rupture was expected to result in the most significant radiological releases. A description of the ACS is given in Section 6.3. The main design functions of the ACS are:
The primary support and service systems relevant to the ACS that are not mentioned above are:
A detailed Engineering Assessment and separate Single Failure Analysis were performed for the ACS in the SAR. Separate assessments were performed for connected and support systems. In general, the ACS and its support systems were found to be adequately capable of performing their safety function. Testing of all active components is performed with acceptable test intervals, and is governed by test instructions. Visual inspections both during shutdown periods, when all parts of the ACS and compartments are acceptable, and of critical parts during power operation, are carried out with acceptable scope and frequency. The design of the ACS permits critical parts and components to be maintained as required both during outages and during normal operation. Reliability records shows that the reliability of critical components is consistent with testing performed, and with the test and maintenance intervals.
The main limitation of the ACS in performing the radioactive releases confinement function, as compared to Western compartments, is the limitation of the envelope to part of the primary circuit. This means that ruptures outside of the ACS envelope lead to easy release of radioactive isotopes to the environment. The deficiency in the mitigating capability and in the defense-in-depth concept, is demonstrated to be acceptable for design basis accidents, but does not leave margins for mitigating accidents beyond the design basis, involving possible loss of integrity of pressure boundary outside ACS and multiple failures in ECCS.
Another important limitation is the high leak rate of the ACS, first of all unit 1, mainly attributable to the complex geometries and to the absence of metallic liners on some boundaries. This limitation affects the mitigation capability during design basis accidents and beyond DBAs. Even accidents amongst DBAs might unduly challenge the confinement function due to the leaktightness limitations. Although the limitations outlined restrict its performance, ACS design requirements have to be met in order to avoid exceeding limits to external doses during the loss of coolant accidents inside it.
The significant deficiencies found by assessments are in the area of structural integrity tests and leak rate tests. There have been no structural integrity tests of any of the compartments at pressure equal to either the design pressure or maximum accident pressure. Leak rate tests performed at a pressure of about 2 kPa are too low to permit accurate extrapolation to leak rates at design pressure or maximum accident pressures. It may not be practical to perform structural integrity or leak rate tests at higher pressure, due to leakage from the ACS. Nevertheless, confidence in the ability of the ACS to perform its function under accident conditions needs to be demonstrated. Additional findings and recommendations have been identified both by the SAR and RSR teams include:
The accident confinement system was not built according the recent regulatory requirements. The need to demonstrate the structural integrity of the ACS to withstand expected peak pressure during design basis accidents still remains. The steam distribution pipes and pools were never verified to withstand dynamic loads.
The above mentioned ACS deficiencies have been recognized as highly safety important and Ignalina Safety Panel recommends to perform safety cases for the ACS before licensing. It was also recommended that before either units restarts from its 1997 maintenance outage planning should be completed and development started on a safety case for the Accident Confinement System.
Ignalina NPP has placed a contract with Lithuanian Energy Institute to assist in preparation of the safety case for Accident Confinement System. The intention of the developed work plan is basically in line with the recommendations of the Ignalina Safety Panel. The plan has been reviewed by Western experts and further recommendations were made to address relevant technical matters in sufficient detail. The work has started and will be finished by the end of October 1998.