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REGULATORY REQUIREMENTS AND ACTIONS RELATED TO IMPROVE CONTAINMENT VENTING FOR LAGUNA VERDE NPP
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from the core, the containment and the spent fuel pool, in scenarios in which large areas of the plant had suffered fire or explosions that caused impediment in their operation. This requirement follows paragraph 50.54 (hh) (2) of the US NRC 10CFR Code of Regulations.
Following the experience of the Western European Nuclear Regulators Association (WENRA), and through its participation in its equivalent, the Foro Iberoamericano de Organismos Reguladores Radiológicos y Nucleares (FORO), the regulatory body required to the utility to carry out stress tests, in order to assess the safety margins of Laguna Verde NPP.
Additionally, the regulatory body requested to the utility the implementation of a programme that follows the US NRC orders that were developed as a consequence of the Fukushima Accident. This requirement was formalized in a letter dated November 4, 2013.
This document requests that the utility, in order to develop its programme, should consider the following documents: EA-12-049 “Issuance of order to modify licenses with regard to requirements for mitigation strategies for beyond-design-basis external events”, EA-13-109
“Modified NRC Order for Containment Venting Systems”, EA-12-051 “Issuance of order to modify licenses with regard to reliable spent fuel pool instrumentation” [1, 2].
In this context, for the EA-13-109 Order, the regulatory body specified to the utility to consider the US NRC document SECY-12-0157 “Consideration of additional requirements for containment venting systems for boiling water reactors with Mark II containments” of March 19, 2013, and use as guide, the methodology described in the Nuclear Energy Institute document NEI-13-02 “Industry Guidance for Compliance with Order EA-13-109” revision 1 [3].
3. ACTION PLAN TO DESIGN A RELIABLE HARDENED CONTAINMENT VENT SYSTEM (HCVS)
The action plan to design a reliable HCVS considers two phases. Phase 1 is related to the design of the HCVS located at the Wetwell of the Primary Containment. Phase 2 considers the similar process for Drywell of the Primary Containment. Both processes are similar, the inputs, criteria and requirements are described below:
3.1 Design an HCVS to minimize the dependency in the operator actions, considering radiation conditions to determine the impact and capacity of the venting and the analysis of the electrical and pneumatic capacity for long operation of multiple cycles of the valves, and its instrumentation during the first period of 24 hours.
3.2 Design an HCVS to minimize occupational risks such as extreme heat, and radiological conditions that could impede action by the operators, this includes shielding calculations and other actions of radiological control so to have acceptable radiation levels for the operators access, and calculations for the temperature conditions due to the loss of ventilation in parts of the HCVS during Extended Loss of Alternate Current Power (ELAP).
3.3 Calculations for boundary conditions of the instrumentations of the HCVS, controls and indicators must be accessible and functional under a range of plant conditions, including severe accident, ELAP or inadequate cooling of the containment.
3.4 Design an HCVS in order to have the capacity for venting vapor/energy equivalent to 1% of the rated thermal power licensed, this includes the pipe diameter calculation.
3.5 Design an HCVS that has an effluent discharge point above the main structures of the plant, this includes the calculations for HCVS pipe protections against missiles generated by external events.
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3.6 Design the HCVS to include characteristics that minimize the accidental cross flow of vented fluids within a unit and between units; this has to consider the site characteristics.
3.7 Design the HCVS to be operated manually during long operations from Main Control Room or another remote control station. This includes system status monitoring from a specified panel. Calculations for the number of open/close cycles needed during the first 24 hours of operation. This includes the calculations for the temperature and radiological conditions at the primary containment and alternate controls of the HCVS, and the assessment of the pressure indications at the containment and level indication at the suppression pool.
3.8 Design the HCVS capable of being operated manually (from a shielded location, with manually operated valves). This analysis should determine the location for the alternate control of the valves and the support equipment, such as nitrogen bottles, compressors and batteries.
3.9 Design de the HCVS to operate with qualified equipment and permanently installed after 24 hours of the loss of either the electrical or pneumatic power during an ELAP. This design requirement is related with the calculation of the electrical and pneumatic capability of requirement (3.1) already mentioned above.
3.10 Design the HCVS that includes means for an inadvertent actuation. This involves the assessment of the set point of the rupture disc required to avoid the inadvertent actuation.
3.11 Design de HCVS to include means for radioactive effluent monitoring that could be released due to operation. This task is based in calculations in order to develop the selection criteria of the monitors.
3.12 Design the HCVS to sustain severe accident conditions. Design parameters analysis to establish or verify the boundary conditions of the containment during a severe accident.
3.13 Design the HCVS to assure that flammability limits of gases that go through the system are not reached. The system must sustain dynamic loads resulting from hydrogen deflagration or detonation. The system´s piping has to be designed against hydrogen deflagration or detonation. A calculation must be done to measure the purge needed in the HCVS in order to reduce the gases concentration under the flammability limits. An analysis should be done in order to avoid detonation/deflagration phenomena in the piping of the HCVS, this includes the reduction of potential migration of hydrogen from the reactor building or other structures.
3.14 Design of the HCVS should include provisions for operating, testing, inspection and maintenance activities in order to assure its reliable operability. Testing and inspection requirement should be developed.
3.15 The route of the HCVS from end to end, including the second barrier of containment isolation must be designed according to the design basis of the installation. In this sense an analysis should demonstrate the correct safety grade of all HCVS structures and components.
3.16 Development, implementation and updating of the procedures needed for the safe HCVS operation, this includes the system’s operation with non-emergency power, backup power, and during ELAP.
3.17 The utility should provide training programme for the operation personnel in the correct operation of the HCVS. This training should include system’s operation with non-emergency power, backup power, and during ELAP.
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4. TIMETABLE FOR THE IMPLEMENTATION OF THE HCVS
As it was described above, the regulatory process for the actions related to the implementation of the HCVS in Laguna Verde NPP started in November 2013, since then;
there have been various meetings between the regulatory body and utility´s management and technical staff. The design process for the Phase 1 (Wetwell) started in February 2014 and it is expected to end in July 2016. This process for Phase 2 (Drywell) will start in January 2015 and it is expected to end in December 2017 (if there are no changes in the requirements).
The acquisition process for components needed for the implementation of the Phase 1 (Wetwell) started in December 2014, these components are for example: instrumentation equipment needed for pressure and temperature indicators in the HCVS and the Wetwell, level indicators of the suppression pool, equipment for monitoring radiation levels at the HCVS, mechanical components such as valves, rupture discs, piping and structural components for the construction of the system. This process for Phase 2 (Drywell) is expected to start in January 2016.
The design change process related to the elaboration of modification packages, blueprints, construction procedures, for the implementation of Phase 1 (Wetwell) started in December 2014 and it will end by October 2016. This process will be for Units 1 & 2 of LVNPP. For Phase 2 (Drywell) will start in January 2016, for both Units, and will end by October 2016 in the case of Unit 1, and March 2018 for Unit 2.
The construction process which involves the physical implementation of the HCVS will be done during the routine outages used for fuel reloading and large maintenance.
Expected dates are:
Implementation of Phase 1 (Wetwell) for Unit 1, Fuel Reload 18, January 2017. For the Unit 2, Fuel Reload 15, April 2017.
Implementation of Phase 2 (Drywell) for Unit 1, Fuel Reload 19, May 2018. For Unit 2, Fuel Reload 16, September 2018.
This calendar, although subject to changes, is used by the regulatory body staff to develop its own action plan in order to carry out the specific technical evaluations and inspections of the whole process of design and implementation. Any further change or development, whether is regulatory or technical, that comes from the internal or external operational experience, or safety analysis will be attended in order for LVNPP to have a reliable hard venting system.
5. CONCLUSION
This paper described the current status of the requirements established by the Mexican regulatory body and actions underway by the utility in order to improve the containment venting of both units of LVNPP. This process is based in the lessons learned from the international operating experience developed as a consequence of the Fukushima Daiichi accident, and these are based in the Order EA-13-109 developed by the United States Nuclear Regulatory Commission. The design action plan developed by the utility was described for Phases, 1 (Wetwell) and 2 (Drywell); finally, a timetable was presented with the expected dates for the conclusion of these activities related to the improvement of the containment venting.
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REFERENCES
[1] U.S. NUCLEAR REGULATORY COMMISSION, Order Modifying Licenses with Regard to Reliable Hardened Containment Vents Capable of Operation under Severe Accident Conditions, Order EA-13-109, June 6, 2013 (Agency wide Documents Access and Management System (ADAMS) Accession No. ML13130A067).
[2] U.S. NUCLEAR REGULATORY COMMISSION, JLD-ISG-2013-02, Revision 0, Compliance with Order EA-13-109, Order Modifying Licenses with Regard to Reliable Hardened Containment Vents Capable of Operation under Severe Accident Conditions, Interim Staff Guidance, November 14, 2013 (ADAMS Accession No. ML 13304B836).
[3] NUCLEAR ENERGY INSTITUTE, Industry Guidance for Compliance with Order EA-13-109, NEI 13-02, Revision 1, Washington DC (2015).
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STATUS OF SPANISH REGULATIONS AND INDUSTRY ACTIONS RELATED TO FILTERED CONTAINMENT VENT SYSTEMS