THE NITROGEN INJECTION THREAT IN PWR REACTORS

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1 THE NITROGEN INJECTION THREAT IN PWR REACTORS Weakness of current strategies & ASVAD, the new passive solution. Arnaldo Laborda Rami ASVAD INTL. SL (SPAIN) Tarragona (SPAIN) Web: Abstract The Extended Loss of Ac Power accident (ELAP) is one of the worst accidents that a Pressurized Water Reactor (PWR) plant could face. The plant control is lost for a long time, but it also implies an additional Loss of Coolant Accident (LOCA). The Reactor Cooling System (RCS) depressurization is the next step and the subsequent injection from the safety accumulators. This injection recovers the inventory during some time, but when the water ends, the accumulator shows its ugliest side: Its propellant gas (nitrogen) is injected inside the RCS. This nitrogen can reach the Steam Generators (SG s) tubes. Here, it can interrupt the natural circulation which is the main way to cool the core. And it will remain here during all the recovery time, making the SG s unavailable. Current strategies to deal with this issue are the accumulator isolation or venting. Pressurized Water Reactor Owners Group (PWROG) wrote the Flex Support Guidelines (particularly the FSG-0) to give directions to perform it, and nuclear industry has adapted it into their emergency procedures. Doing this, plants have closed (and forget) this issue. But the sad news is that these current strategies are too weak to be success. It relies on the proper work of a chain of active elements (FLEX generator, cabling and valves) and the operator s effort to deploy it. But this strategy has Time Critical actions which have to be done at the correct moment and simultaneously over all the accumulators. Just one failure in the chain means nitrogen injection. Fortunately now, we have a great alternative to avoid the nitrogen injection: The Automatic Safety Valve for accumulator Depressurization (ASVAD) which has the following advantages: The valve is permanently installed in the accumulator and available all the time. It can avoid completely the nitrogen injection without any human assistance. It's a PASSIVE element and doesn't require any external energy for their main operation. Automatically performs its action at the right time. Just when all the borated water ends. It can be easily installed in the existing facilities. In the author s opinion, Nuclear Industry has to reevaluate again the nitrogen injection issue, because this risk is underestimated and the current strategies can t assure it success avoiding it. Just applying the Defense in Depth concept, all PWR plants with pressurized accumulators should install this valve as the MAIN BARRIER to avoid this risky complication.. INTRODUCTION: THE NITROGEN INJECTION THREAT IN PWR REACTORS Fukushima accident shows us the disastrous consequences of the Extended Loss of Ac Power accident (ELAP). During this accident, the plant power -and its control- is fully lost. Then, the plant evolves following its own physical processes. Only the passive protections are available, as the "natural circulation" or the accumulator s injection. Natural circulation is a physical process that creates flows in the cooling system pipes by the effect of the temperature difference, and is the main way to cool the core in these circumstances.

2 56-IAEA-CN-5 Furthermore the ELAP accident directly induces other risky accident: the Loss of Coolant Accident (LOCA) starts due the loss of cooling in the Reactor Cooling Pumps (RCP) seals. From here, the system depressurization is the natural process. All PWR reactors usually include a passive system to inject borated water to recover the coolant inventory in the Reactor Coolant System (RCS). They are usually called Accumulators, Safety Injection Tank (SIT), or Core Flood Tank (CFT). This safety system consists in several accumulators containing borated water, pressurized with nitrogen at high pressure. These accumulators are connected to the RCS through an isolation valve and at least one nonreturn valve. When the RCS pressure falls under certain level, the accumulator starts injecting their water to the RCS during some time. But when it becomes empty of water, their cover gas begins to be injected into the RCS. This nitrogen is a non-condensable gas, which finally goes to the higher parts of the RCS, first it goes to the top side of the reactor vessel, and finally it reaches the top of the Steam Generators tubes (SG). At this point, -like a bubble in a vein-, the gas can cause the disruption of the natural recirculation flow, which is the best available way to extract the heat outward. This situation greatly complicates the subsequent cooling of the core and substantially increases the chances of core melting, because the gas will remain inside during all the recovery process making the SG s unavailable to cool the core. Many studies have been done about this, but some good ones can be the references [], [], [] and []. All these studies have considered relatively low quantities of uncondensables inside the system, mainly from the dissolved gasses in the liquid phase from the accumulators or even the Hydrogen production from the zircaloy oxidation. Just these small quantities are enough to disturb the proper work of the heat exchangers. But inside the accumulators there are big quantities of nitrogen at high pressure. In one standard accumulator can be around 60 Kg of Nitrogen (@5 After its depressurization to lower pressures this nitrogen can fill the whole volume of the RCS. It s really very important to avoid its injection to the RCS.. CURRENT STRATEGIES TO AVOID THE NITROGEN INJECTION. The Pressurized Water Reactor Owners Group (PWROG) wrote a group of guidelines to cope with severe accidents. These guides are known as "Flex Support Guidelines" (FSG's). These guides describe the strategies to recover and mitigate such accidents. The PWROG plants used these guides to write their own procedures. The PWROG FSG-0 guide [5] specifically describes the strategy to avoid the nitrogen injection. Currently there are three strategies to prevent this threat:. The first strategy is to close the accumulator output isolation valve before the water injection ends.. The second strategy is to vent the residual nitrogen to the atmosphere by means of relief valves.. The last strategy is to keep the RCS pressure over the nitrogen pressure. But the worst news is that ALL these strategies have important drawbacks and weakness: Isolating or venting it s a TIME CRITICAL action. If it s done too soon, part of the water will be wasted. If it s done too late, nitrogen will be injected. And all the actions have to be done at the same time over ALL the accumulators, because all are injecting in parallel to the same RCS pressure. To know the correct moment is not easy. It mainly depends on the leak rate, and it is not linear. ALL the equipment needed ARE ACTIVE ELEMENTS and needs to be powered from a FLEX generator. This generator must be deployed and connected to the valve s circuitry to allow their closure, and valve by valve. This can take a lot of time and organization efforts during the accident. It can spend resources which other important recuperation tasks may need... or even rest unperformed.

3 Even when its closure is achieved, these isolation valves ARE NOT LEAK-PROOF. The valves aren t able to keep the gas isolated due their internal leaks (these valves never are leak-tested). Sooner or later this nitrogen will reach the RCS and the SG tubes, despite its rate can be slower. If the gas reaches the SG s tubes, it will remain inside during ALL the recovery process making the SG s unavailable to cool the core. Operators will be heavily burdened doing all these actions, and with no guarantee of success. Just one failure in the chain of actions, means nitrogen injection to RCS. Keeping high the RCS pressure also needs the emergency equipment work, and it also implies higher RCS leak rates and hard work to the emergency organization. But this is only a TEMPORAL STRATEGY to get more time. Sooner or later, the RCS will be further depressurized, and then the nitrogen injection will happen if no other actions are taken. Therefore, it is evident the need for some automatic (and passive) system, which prevents the injection of this residual nitrogen to the reactor, without requiring any external energy for its operation. Furthermore, the system should automatically recognize the appropriate moment for its actuation. This will allow their unattended operation maximizing the cooling water injection, and avoiding the nitrogen injection into RCS.. ASVAD, THE NEW SOLUTION We were not satisfied with the simple complaint of these weaknesses. We have done a step forward to find an alternative way that can avoid the nitrogen threat, without the previous strategies weaknesses and shortcomings. The result of this effort has been the design of a new passive element (ASVAD) which is very similar to a safety valve, but with a reverse operation. This new element does not vent the pressure when a high pressure is exceeded. It does just when the pressure drops under a desired setpoint. N SUPPLY EXAUST Fig. shows where the ASVAD valve is installed. The valve is installed with an isolation valve in the accumulator s nitrogen side. This isolation valve simply allows servicing the ASVAD valve without disturbing the accumulator system. As it is bearing the internal accumulator pressure, it can detect the end of the water injection and is able to exhaust the residual nitrogen into the atmosphere before it can reach the RCS pipes. Basically the ASVAD valve remains closed while the pressure in the accumulator is normal. After the injection starts, nitrogen will expand inside the tank as the water goes out. This implies a continuous drop in the pressure. M GAS (N) BORATED WATER ASVAD TO RCS FIG.. ASVAD valve installation Once all the borated water has been injected, it only remains the residual gas pressure. When this pressure drops below the valve setpoint, the ASVAD valve suddenly opens and remains opened. This allows the complete accumulator depressurization, and thus the complete avoidance of the nitrogen injection. As a secondary consequence, this vented nitrogen will help to cool and inertize the containment building. Fig. shows the simplified diagram of the ASVAD operation. Its principle of work is the difference between the force made by the inner accumulator pressure (the big red arrow), and the force made by an adjustable spring (the narrow red arrow). While the force done by the pressure inside the accumulator is higher

4 56-IAEA-CN-5 Legend Pressure chamber. Hollow Shut-off obturator. Opening spring. Adjustable disk. Accum. pressure Steady state Opened FIG.. ASVAD simplified operation diagram. than the opening spring force, the valve remains fully closed. Usually the pressure force is three times the spring force. This is its normal steady state. During the water injection, the gas expands and the pressure decreases inside the accumulator. When the pressure drops below the spring mechanical force, the shut-off obturator is displaced from its seal and then, the valve suddenly opens and the gas escapes through its holes and central cavity, and finally through the outlet ports. The valve will remains in this state allowing the accumulator fully emptying. The ASVAD valve has the following advantages: After its installation, the valve IS AVAILABLE ALL THE TIME. It will remain closed until its action will be required. Once tripped, it will remain open allowing the fully accumulator depressurization, FULLY AVOIDING THE NITROGEN INJECTION. It's a PASSIVE ELEMENT and doesn't require any external energy for its main operation. Automatically performs its action AT THE RIGHT MOMENT, when all the borated water ends, and just before the gas injection. It performs its action WITHOUT ANY HUMAN ASSISTANCE. Its use can SAVE organization efforts that can be invested in other recuperation tasks. Its simplicity and robust design, makes the valve immune to harsh accident scenarios. Also its qualified life can be very long. Its installation in existing facilities is very easy. The ASVAD valve is a relatively small valve. The required modification can be minimal. Its simple design also facilitates the maintenance of the valve. You can install and forget it until the next outage. It only needs to be tested from time to time in the same way as a normal safety valve. The valve installation does not induce any new risk to the accumulator system operability, because the normal accumulator pressure always tends to keep closed the valve during all the time.. CONCLUSIONS After an ELAP, a subsequent LOCA will take place. This RCS depressurization can easily lead to the accumulator injection. But when its water ends, the accumulator s nitrogen injection to RCS can be a risky complication to the core cooling due its adverse effects to the natural circulation process and even to the reflux cooling mode also.

5 The current strategies to avoid it are based on the concurrent work of multiple active elements and the (well-trained) organization efforts. These strategies have significant drawbacks and weakness to fully rely on its proper success: Too much human efforts in a bad environment. Time critical actions over many elements. Difficulties to find the correct moment to act. A long chain of active element s operations. Just one failure can compromise its success. The valves weakness to isolate or vent the residual nitrogen. On the other side and using the ASVAD valve, the RCS nitrogen injection threat CAN BE EASILY AVOIDED. It is a fully passive and automatic element. With ASVAD, the operators will not be burdened by the nitrogen injection issue and they can remains focused on the core cooling and other recuperation tasks. It s available from the first moment and it automatically performs its action at the right time. Its simplicity and robust design, results in high reliability of its components and an easy maintenance. Its installation in the current systems can be also very easy. The overall safety of the plant will be greatly improved with its installation. It represents a small investment which can be very profitable. Nuclear community must know and re-evaluate the consequences of the nitrogen injection from the accumulators. A common assumption in the mentioned studies is that no accumulator nitrogen injection is done, assuming the correct valve isolation before its water ends. But what happen if not achieved?. Nuclear Operators must also re-evaluate their current strategies weakness. Nowadays it seems a closed item with little or no attention on it. It seems that having a procedure and trained personnel can be enough to solve this problem and to relegate these strategies into the second order priorities during the accident. Nuclear community must take the appropriate actions to solve this threat definitively. REFERENCES [] Various Authors, WCAP-760-P Rev., Reactor Coolant System Response to the Extended Loss of AC Power Event for Westinghouse, Combustion Engineering and Babcock & Wilcox NSSS Designs, Westinghouse Proprietary Class, January 0. [] Christine Sarrette, Effect of Noncondensable Gases on Circulation of Primary Coolant in Nuclear Power Plants in Abnormal Situations (Thesis for the degree of Doctor of Science (Technology), Lappeenranta University of Technology (Finland), February 00. [] Takashi NAGAE, Toshiaki CHIKUSA, Michio MURASE & Noritoshi MINAMI (007), Analysis of Noncondensable Gas Recirculation Flow in Steam Generator U-Tubes during Reflux Condensation Using RELAP5, Journal of Nuclear Science and Technology, :, [] Noel, B., Deruaz, R., Reflux condenser mode with non-condensable gas: assessment of CATHARE against BETHSY Test 7.C. Nuclear Engineering and Design 9, pp. 9-98, 99 [5] Various Authors, FSG-0 Rev., Passive RCS injection isolation. Background information for Westinghouse Owners Group Emergency Response Guidelines, PWROG, December 0. 5