NITROGEN A CLOSED ITEM ? AN STUDY ABOUT THE THREAT OF THE NITROGEN INJECTION TO RCS, AND THE STRATEGIES TO COPE WITH THIS ISSUE IN OUR NUCLEAR PLANTS. by Arnaldo Laborda Rami CEO of ASVAD INT. SL / 1

ABOUT THE PRESENTATION 1. Background. The Nitrogen threat. 2. The FSG-10 guide. 3. The FSG-10 drawbacks and challenges. 4. The alternative: The ASVAD safety valve. 5. Benefits using ASVAD. 6. Conclusions. 7. Questions & Answers. 2

BACKGROUND (1) After Fukushima accident, the nuclear industry starts a wide upgrade in front to the beyond-design-events. The PWROG wrote the FSG’s (Flex (Flex Support Support Guidelines) Guidelines) to cope with these events. The FSG-10 is a specific guide to avoid the nitrogen injection. All PWROG plants had adapted this guide in their emergency procedures, and only with this action they had justified their own ability to cope with this threat. 1 3

BACKGROUND (2) NRC team had revised the plans to cope with ELAP accident. With relation to the issue of the nitrogen injection to RCS, NRC has focused their interest in the calculation of the RCS pressure injection point, than in the ways to avoid it. NRC approved all these plants strategies after a long reviewing process. Nitrogen issue is now a closed item in all the plants strategies. No further actions are expected about it. 4

BACKGROUND (3) But after a deeper examination of these strategies, a lot of important questions arise about the feasibility of the strategy. This presentation discusses the FSG-10, their drawbacks, and why this guide has low possibilities avoiding the nitrogen injection. Hopefully, now there is a new alternative to avoid all these drawbacks: The ASVAD ASVAD valve. is a new passive safety valve specifically designed to automatically perform all the FSG-10 work. 5

M M FIRST STAGE: THE ACCIDENT FIRST HOURS AFTER THE EVENT, OPERATORS USES THE ECA 0.0 GUIDE TO COPE WITH THE ACCIDENT (5”) All the main feed valves are closed. (5”) The main steam valves also are closed. Secondary circuit is lost. (2’- 5’) In the RCS circuit the “natural circulation” mode is established. (15’) Without seal injection, the RCP seals fails, and become a permanent RCS leakage. (3,5h) Until the passive injection from the accumulators starts refilling the RCS… Water CST (40”) The auxiliary feed water turbine supplies water to SG’s. (0”) All the pumps stop, and valves become closed or in position. The control rods stops the nuclear reaction in the core, but residual heat must be removed from the core. (1’) The pressure and the level inside the RCS circuit starts to fall… (12”) Steam Relief Valves controls the SG’s pressure and the RCS cooldown. FLEX Equipment must be deployed and ready to operate after a short time NATURAL CIRCULATION

M SECOND STAGE: DURING THE ACCIDENT (4h – 40h) THE MAIN PROBLEM: THE LOSS OF COOLANT (0h-40h) The “natural circulation” mode still transfers the excess of heat from the core. (3h) The RCP leak decreases, but is still the main RCS leak. (45h-50h) But passive injection from ECCS accumulators is near its end… Water CST Auxiliary feed pump still provides water to SG’s cooling the RCS. The RCS leak fills the containment building with radioactive steam. (2h) Operators control the RCS pressure & temperature with the SG’s PORV. (FSG-2) FLEX Equipment must be ready to refill the RCS (FSG-1). Then, operators must enter into the FSG-10 guide to try to isolate or vent accumulators NATURAL CIRCULATION

M THE PROBLEM IS THE NITROGEN INJECTION INTO THE RCS The RCP seals are leaking. (but not the only leak) Water CST Without enough steam to spin the AFW turbine, their injection to SG’s stops. The nitrogen comes to the upper part of reactor vessel displacing the water… When the water ends, the nitrogen enters into the RCS … The reflux cooling mode begins… but is a less efficient mode and is also disturbed by the nitrogen. The nitrogen reaches the top of SG’s tubes. Now, the “natural circulation” is stopped. If the FLEX Equipment can’t be able to maintain the RCS pressure above the nitrogen pressure, then… 8 NATURAL CIRCULATIONREFLUX COOLING Nitrogen will remain inside RCS disturbing the long term cooling THIRD STAGE: THE ACCIDENT WORSENS (45h - 60h)

THE FSG-10 Purpose: Provides directions to isolate or vent the accumulators (SIT onwards) to allow RCS to be further depressurized for long-term cooling. Entry conditions: Is entered from ECA-0 during a station blackout (SBO or ELAP). From FSG-8 or FSG-9. From other plant specific procedures. 2 9

The FSG-10 in Brief 1.Determine when isolation is required. 2.Determine if isolation valves can be powered from FLEX equipment. 3.Deploy FLEX equipment to Re-energize Accumulator Discharge Isolation Valves. 4.Close the Isolation Valve. 5.Verify the closure of the valve. De-energize it. 6.Repeat the actions 3, 4 & 5 to each valve. 7.If not achieved try to vent accumulators. 8.End of the procedure. 10

THE FSG-10 STEPS: STEP 1: 1: Determine when isolation is needed. SITSIT isolation MUST BE DONE DONE prior to depressurize SG’s below 300* 300* psig. Issues: Operators Operators must be watchful over the SG’s or RCS pressure evolution. Depending Depending on the RCS leak, the depressurization can be faster (and uncontrolled) than expected. If If available, the WR SIT level accuracy will be seriously affected by the temperature stratification (effect on the capillary tubes). started too soon, it will be lost a lot of makeup water wasted inside SIT’s. started too late, it will be nitrogen injection to RCS. The The correct moment is difficult to be known without the proper instrumentation. *plant specific 3 11

THE FSG-10 STEPS: STEP 2: Determine if isolation valves can be powered from FLEX equipment. IfIf not, go to step step 7 (plan B). Issues: AFLEX source of AC power is needed. No Flex, no isolation. It It can be time critical to deploy these elements. Deploy couldn’t be easy. Depending Depending where the valve’s MCC is, a heavy and long cable have to be deployed to feed these circuits. on the accident extension, could not be possible to deploy these elements to all valves. valves. Just one fail, means nitrogen injection. The The weather can help… or difficult the work (night, rain, wind, snow…). 12

THE FSG-10 STEPS: STEP 3: Deploy FLEX equipment to Re-energize. DeployDeploy a cable from the FLEX to the Valve’s MCC. Connect it and energize the circuits. If not achieved, go to Step 7 (Plan B). Issues: Unless Unless using pre-installed cabling, deploying a heavy and long cable across dark and closed rooms can be a big challenge. Correct Correct phase order must be meet. The The cable’s route has to be clearly specified in the procedure. Operators should not improvise on this task. This This task can take a lot of time and operator efforts until the connection is ready. Some padlocks will be broken for sure. 13

STEP 3: A real Flex deployment as example… THE FSG-10 STEPS: 3 SORRY! I FORGOT THE DARKNESS… 14 NO COMMENTS…

THE FSG-10 STEPS: STEP 4: Close the Isolation Valve. Issues: To To be able to close the valve, many elements must work properly to achieve it: The FLEX generator, all the involved electrical elements (switches, relays, protections…), the cabling, the AC motor, torque limiters, limit switches, gears, the valve itself...(don’t forget operators)) If If only one element fail, all the operation fails. In In this situation the containment ambient is harsh, hindering the element’s proper operation. When When the connection is made to a bus duct, the bus itself has to be operable, with the unused equipment stripped. If If the closing order have to be done locally at the valve MCC, in the deep and dark room. Walkies could not work here. Could Could need other actuations as placing jumpers to bypass remote orders, reposition of biestable relays, etc. 15

STEP 3: A REAL diagram as example… THE FSG-10 STEPS: 3 These relays have to be defeated also to allow the valve closure. ANY OF THESE ELEMENTS CAN FAIL. Even the fail of a secondary element can be important in this step (as the lamps indicators). 16 TO BE USED IN THE DARK ROOM ?

THE FSG-10 STEPS: 17 STEP 5: Verify the Valve closed. Deenergize it. IfIf not achieved, try again or go to (plan B). Issues: What What can be done if the isolation fails? Cross the fingers?. Could Could be difficult to verify the closure of the valve. The The simple indication of the limit switches can not guarantee the full closure. No No other process instrumentation signals can determine it. If If the valve remains not fully closed, it will be a slow, -but continuous- nitrogen injection to RCS. Even Even properly closed, the valve can still leak nitrogen… during ALL ALL the long-term cooldown. Remember these valves usually are not leak tested!. This is our best strategy?

THE FSG-10 STEPS: STEP 6: Repeat steps 3 to 5 to all the remaining isolation valves. IfIf achieved, go to end. If not, to 7 (plan B). Issues: All All of these maneuvers must be done (more or less) AT THE SAME SAME time over all the accumulators. All All the accumulators are seeing the same RCS pressure and will be empty of coolant approx. at the same time. If If closure is done simultaneously, Flex generator can fail overloaded. the maneuver is not done quickly, it can be nitrogen injection from the last isolated accumulators. The The valve’s control MCC’s usually are in different rooms. Need to move from one to other. Again, Again, if only one element fail, all the operation fails. 18

THE FSG-10 STEPS: STEP 7 (plan B):Verify B):Verify available the motive force for Accumulator Vent Isolation Valves (AVIV). IfIf not available, go to Step Issues: When When no AC power available, no compressed air is available too. If If the air is available, fully operative pneumatic pipes are also needed (without leaks). Pneumatic distribution is not class 1. These These valves need both electric power and air to operate. valves are closed when de-energized. To exhaust all the nitrogen is necessary to constantly energize the valve during some time. It It is even more challenging to recover these valves than the isolation valves. 19

A REAL procedure from a plant as example… THE FSG-10 STEPS: Have power on valves? Close the valve And deenergize it If not achieved, try to convince someone to go to Containment !!! All the instructions are too simple… Who will be able to fulfill this procedure? 20 YOUR OPINION ABOUT IT ? … OK, but when? Other issue Well explained? This sentence resumes all the FLEX & cabling deployment

THE FSG-10 STEPS: STEP 8: End of the procedure. procedure. No further actions can be taken. IfIf achieved, good work...you’re lucky heroes! not, do not blame yourself. It’s difficult to achieve it. AskAsk your managers why they did not implement the ASVAD ASVAD safety valve to avoid the nitrogen injection. You have burn your options, but the accident still continues!!… And now, with the nitrogen inside… You have burn your options, but the accident still continues!!… And now, with the nitrogen inside… 21

THE FSG-10 FSG-10 RELIES ON: The The Flex AC Power Generator. Their Their deployment on The The proper actuation of ALL the involved elements. proper actuation of the organization. DO YOU RELIES ON FSG-10 ? 22 good time. time.

END OF FSG-10 Fortunately now, we have a good alternative to the FSG-10 and their drawbacks. Is time to know the ASVAD Valve… … and breathe easy and relaxed. … and breathe easy and relaxed. Nobody will go to containment…. ☺ And now we don’t need to rely in the luck. 23

THE ASVAD THE SIMPLESOLUTION TO THE NITROGEN INJECTION ISSUE THE SIMPLE SOLUTION TO THE NITROGEN INJECTION ISSUE  A  Automatic  S  Safety  V  Valve for  A  Accumulator  D  Depressurization 4 24

HOW ASVAD IS INSTALLED? RCS M NITROGEN T ASVAD BORATED WATER EXAUST STANDARD INSTALLATION Accumulator Vessel. Safety Relief Valve. Outlet Isolation Valve. Non-return Valve. Inlet/Vent Valve. Manual Isolation Valve. The ASVAD Valve. ASVAD Is installed in the nitrogen side of the accumulator. 25

WHAT IS INSIDE ASVAD? Pressure chamber. Safety Floater. Upper container. Locking cylinder. Closing piston. Fixing cylinder. Shut-off plug. Opening spring. Spring container. Fixing screws. Opening piston. Upper cover. 26

HOW ASVAD WORKS? The ASVAD operating principle is the balance of forces. At normal pressure, there is a force upwards that overcomes the spring force. This force keeps the shut-off plug closed. This is the normal operating position. During the water injection, the nitrogen expands, and the pressure in the accumulator drops until a certain value is reached. Now, the force of the opening spring is enough to move the shut-off plug off its seat, opening the path from the pressure chamber to the outlet exhaust ports. Once this happens, the pressure drops quickly in the bottom pressure chamber, and the shut-off plug reaches its full opened state. This provides an open path to exhaust all the residual nitrogen in the accumulator. 27

MANUAL OPERATION MODE To unconditionally open the ASVAD, can be done applying pressurized air to the “open” inlet. The air pressure pushes the opening piston downwards, pushing the shut-off subassembly stem until the plug opens. This maneuver can be done to depressurize the accumulators if necessary. To unconditionally close the ASVAD, can be done applying pressurized air to the “close” inlet. The air pressure pushes the closing piston upwards, pushing up the shut-off subassembly until the plug closes. This maneuver can be done after a previous depressurization to allow refilling the accumulator. When the air is exhausted, both pistons return to their initial position, and ASVAD remains armed.

OPTIONAL REDUNDANCY Finally, there is a ASVAD design feature to avoid leaks from RCS even in case the accumulator non-return valve leaks. After actuating, the ASVAD remains open. If there is an in-leakage, the accumulator water level will rise until reaching the ASVAD pressure chamber. Once the water fills the chamber, the security floater starts to float and rises until completely covering the open shut- off plug and seat to stop the leak.

ADVANTAGES OF ASVAD. It does ALL THE WORK of FSG-10 guide. NO EXTERNAL ENERGY is needed. NO OPERATOR assistance is required. Performs its function at the CORRECT MOMENT and over all the accumulators. ASVAD senses the accumulator internal pressure and decides when the accumulator should be vented. Once actuated, it vent COMPLETELY the accumulator. No further delayed nitrogen injections are possible. Their action can be verified by the available instruments (accumulator or containment pressure & temperature). Operators can FORGET ABOUT Nitrogen Injection Issue, and focus on performing other mitigation tasks. 5 30

MORE ADVANTAGES OF ASVAD. HIGH RELIABILITY due its robust and simple design. HARD ENOUGH to bear the post-Loca environment. EASY TO BE INSTALLED in the accumulator system. EASY TO BE LICENSED. Not adding a new failure mode to the system. INTRINSICALLY SAFE. No EMC. No software. Cyber-attack proof INTRINSICALLY SAFE. No EMC. No software. Cyber-attack proof EASY TO BE QUALFIED. It only need a nuclear class 2 qualification. EASY TO BE OPERATED. It can be remotely actuated if needed. EASY TO BE MAINTAINED. No wear in the valve. Easy to be disassembled. Few spares needed. With minimal maintenance cost. EASY TO BE TESTED. It can be functional and leak tested even online. EASY TO BE ADJUSTED. To the desired pressure actuation. HIGH QUALIFIED LIFE. No further investment$ required. 31

CONCLUSION.  Nuclear  Nuclear Industry still relies too much in the human response to accidents. It could be OK, but not as the main (and last) response.  The  The FSG-10 guide has important drawbacks to rely in their effectiveness. Too much human work, too much elements, and time critical actions.  Installing  Installing ASVAD ASVAD in the accumulator, it will stay protected for years against the nitrogen injection (without any human effort).  ASVAD  ASVAD is a simple and reliable valve. It will be ready all the time, time, waiting for the moment to act. It is the first first and the strongest strongest safety barrier.  ASVAD  ASVAD is completely autonomous. It does not need anything more to work. Just enough pressure in the accumulator. 6 32

QUESTIONS? More info at Now is YOUR time to ask