THE CURRENT STRATEGIES

Nitrogen complication is not a new issue. Nearly all the plants have procedures to manage it.

But these procedures are based on the existent elements in the safety systems, also supported by auxiliary elements if needed.

The important weakness shared by all these procedures are:

• All these actions are time critical.

• All needs from using active elements (need power to work).

• All needs from a lot of operator actions and vigilance.

As the plants have several accumulators, this work has to be repeated for each one.

As these strategies don’t resolve the root -the nitrogen-, they can easily fail. Just one accumulator is enough to unleash its adverse effects over the core cooling.

Why plants maintain weak strategies?

I’m still surprised that Nuclear Regulators are not still aware of these drawbacks. Probably soon, they will require the plants to develop alternative and SAFER ways to avoid the nitrogen injection.

Plants do not have to wait until these directions come from the Regulator; plants must fix it before!

From my point of view, our nuclear community is still UNDERESTIMATING the Nitrogen Injection threat, and then we’re not prepared to cope with it yet.

One of the main lessons learned from Fukushima was that if you underestimate the possible issues, then sooner or later these issues will pass over you as the tsunami does in Fukushima over the “well-designed” anti-tsunami wall.

Nitrogen is a very risky enemy, but now we can defeat him…

We have the ASVAD valve.

WHAT THE CURRENT STRATEGIES ARE?

To avoid this nitrogen injection, in the PWR plant designs, there are only three strategies:

  • The closure (on time) of the accumulator’s outlet isolation valves.

  • Venting (on time) the accumulator’s nitrogen to the atmosphere.

  • Maintaining RCS pressure above the point of injection of the nitrogen.

 

The first one is to close the accumulator’s output isolation valves before the water injection ends.  The output isolation valves are big valves with a powerful electric motor used to open and close the valve. As we’re suffering the ELAP, we will need the deployment of a FLEX generator. With the power from this generator, the electric-powered valves have to be closed one by one, but at the correct moment.

The second strategy is to vent the residual nitrogen to the atmosphere using the accumulator relief valves. These valves are smaller ones, and usually are pneumatically actuated. This method is quite similar to the previous one, but in this case we need to deploy also a flex air compressor to feed the valve actuators.

The last strategy is to keep the system pressure over the nitrogen pressure. To allow this, it also requires a Flex pump injecting water to maintain the pressure. But this is just a TEMPORARY STRATEGY trying to get more time. Sooner or later, the system will be further depressurized and then, we will have to use one of the two previous strategies.

First and second strategies seem quite simple in concept, but it can be complicated to carry them out in the practice by diverse issues.

The third (and last) strategy of maintaining the pressure in the circuit above the nitrogen injection point could work… for some time. But it requires the correct work of all the active emergency elements to sustain this pressure during long times.

And we must remember that working at higher pressure means higher coolant leaks, higher stress for the emergency equipment, more wear, more fuel consumption, more breakage risk, etc.

It still can take many days to recover the full plant control. While this does not happen, all these active systems have to continue operating at their full capacity… We need to have some luck to avoid these problems.

WHAT THE DRAWBACKS ARE?

This is a short list of the drawbacks and weaknesses of the current strategies:

  • It requires the availability of an “FLEX” AC generator or a “FLEX” pump. If this generator is not available, there is nothing to do.
  • To achieve the isolation, it is needed the correct operation of all (and each one) of the multiple elements involved: FLEX generator, cables, relays, contactors, AC motor, torque limiter, limit switches, the actuator, the valve itself …) to effectively close each valve. If just one element fails, the whole procedure fails.
  • The choice of the adequate moment is critical in time, and it largely depends on the RCS depressurization speed. An inadequate decision may result in either the loss of a portion of the cooling water stored in the accumulator if the isolation is done too soon, or in the nitrogen injection to the RCS if is done too late. On the other hand, if the depressurization speed is fast, operators couldn’t have enough time to perform the isolation, or it may not be fully completed before the nitrogen of some accumulator is injected into the system.
  • It is needed the deployment of a long and heavy power cable from the generator to the electrical cabinets of each one of the isolation valves. If this cable is not preinstalled before, it will need to be deployed by hand through dark and closed rooms (and perhaps uninhabitable ones). This requires a big effort from operators.
  • The correct closure of the valves cannot be guaranteed, because the only information available to know if it is achieved is the closed limit switch. And even with the valve properly closed, these valves are not leak proof. This implies that fully isolation cannot be guaranteed even if the procedure is completed. Remember that despite the initial phase of the accident can be solved, still it will take many days to recover the full control of the plant, and during that time, the nitrogen will be injected slowly, but continuously to RCS.
  • The alternative strategy of venting the nitrogen into the atmosphere containment can definitely eliminate the risk of injection. However, the required valves for venting needs simultaneous air pressure and electric energy available to be actuated. Remember that without electrical power, there will be no air pressure. And even in the case of having it, it is also necessary that the air distribution lines are fully operational. But this cannot be guaranteed because the air distribution system is not a safety-related system, and it is not designed to withstand accidents.
  • The development of the procedure requires a great effort and coordination by the involved personnel. As I said before, this isolation is time-critical, but it is also necessary to do at the same time over all accumulators. The reason: All are seeing the same RCS pressure and all will inject simultaneously their water or their nitrogen.

A SHORT VIDEO ABOUT THE CURRENT STRATEGIES

The FSG-10 Emergency guide

The Flex Support Guideline FSG-10 is an Emergency Guide that was written by the PWROG to cope with the Nitrogen Injection in the Westinghouse-PWR plants.

 

Despite all the work done, this guide still gives very simple directions to isolate or vent the accumulators, leaving to the plants the specific details to proceed with. From my own view, it still underestimates the real work that must be done to achieve this goal. This work is more challenging and “blind” than expected for several reasons:

  • The developing work done by the plants to accommodate this guide to their specific designs and configuration is still weak and without the proper detail to be properly done. They do not include any details about how the work can be done in its best way. It seems to rely on the operator’s ability to improvise the way to do it.

As example, in these guides are not properly described the way how the power from FLEX generators has to be routed and connected to the isolation valves circuitry. If no provision of alternative paths has been made (with a design modification), it could be difficult (or even impossible) to route these cabling across locked (and dark) rooms until reach the valve’s circuitry. All of these valves are 3-phase powered, and then it’s crucial to meet the correct phase rotation to operate the valve.

  • The modifications done by the plants to adapt to the FLEX equipment have been the hydraulic connections to the existing systems, and some electrical connections to the main emergency systems. No modifications have been done to help with the FSG-10 guide.
  • The FSG-10 relies in the FLEX equipment, the operators work, and in the proper operation of the installed elements (mainly the isolation valves and its circuitry). It also need that some instrumentation are available to determine WHEN the isolation must be done. All of these elements have to work properly to achieve the goal. This means that if only one of these elements fails, then the whole procedure fails.

As an example; ALL the electrical components of the valve actuators must works properly (electrical protections, cabling, AC motor, limit switches, torque limiters, gear mechanisms…). Remember than in these situations the ambient in containment building is harsh, and if only one of these elements fails, it’s enough to fail ALL the work.

  • Usually operators don’t have the wide-range accumulator level available (only narrow-range… if available). And its pressure can be very dependent on the ambient temperature around the accumulators. Then, it could be very difficult to know the real level inside the accumulators to be able to determine when this work must be done. In the same way, it could be difficult (or even impossible) to know if the operation has been successful or not. If the closing order is not being fully completed, sooner or later this nitrogen will reach the RCS across the -not perfectly closed- isolation valve. And operators only will realize when the level or pressure inside accumulators still continues their slow drop… But despite they can be aware about it, what can they do to fix it? Probably nothing!
  • Even with all the instrumentation working properly, it’s difficult to find the correct moment (unless the wide-range level of accumulators is available). There are many variables that must be taken into account, such as the ambient temperature in the area of accumulators. The correct moment to perform the isolation must be determined considering if maximizing the water injection is the priority or, on the contrary, avoiding any nitrogen injection. If the isolation/venting is too soon, their injection capabilities are lost. If too late, some nitrogen gets into the system.
  • But the most annoying situation is that this actuation must be done in all of the accumulators of the system at the same time, because all the accumulators are seeing the same RCS pressure, and they inject their water (or nitrogen) more or less at the same moment. There is a lot of work for the operators, and without guarantees of being succeeded. If just one accumulator fails to be properly isolated, then the nitrogen will reach the system.
  • If venting is selected, the problems could be even more. The air-operated valves usually used to vent, can be more difficult to actuate, because they needs not only some AC power (electrovalves), but also enough air pressure and their piping lines available to allow it. In fact, the compressed air piping is even less reliable than the cabling, and usually is not designed to withstand the LOCA environment.

 

  • On the contrary… If this actuation can be done, then the operators can know if the operation has been successful and with guarantees, because even with miss-calibrated instrumentation, the fast pressure drops can be seen. This fact can guarantee that this nitrogen will not reach the system, because it is being exhausted to the containment atmosphere.

A SHORT VIDEO ABOUT THE FSG-10 STRATEGY