What is a Reactor Trip and How Does it Protect the Plant?

nppmapThe Salem nuclear power plant’s Unit 1 “tripped” on Dec. 21st. Brown’s Ferry Unit 2 tripped the following day. In both cases, something happened that caused the reactor to automatically shut down to ensure safety. In other words, a trip means a plant is doing what it’s supposed to do. Let’s look at the term a bit more closely.

Key operating parameters of a nuclear power plant, such as coolant temperature, reactor power level, and pressure are continuously monitored, to detect conditions that could lead to exceeding the plant’s known safe operating limits, and possibly, to damaging the reactor core and releasing radiation to the environment.

If any of these limits is exceeded, then the reactor is automatically shut down, in order to prevent core damage. In nuclear engineering terms, the automatic shutdown of a nuclear reactor is called a reactor trip or scram . A reactor trip causes all the control rods to insert into the reactor core, and shut down the plant in a very short time (about three seconds).

How do control rods do their job?

The control rods are composed of chemical elements that absorb neutrons created by the fission process inside the reactor. They are placed methodically throughout the nuclear reactor as a means of control. For example, as the control rods are moved into the reactor, neutrons are absorbed by the control rods and the reactor power is decreased. Inserting them all at the same time shuts down the reactor. Control rods can also be inserted manually, if necessary.

The plant operator then determines the reason for the trip, remedies it and, when it’s determined to be safe, restarts the reactor. So, while not common, a reactor trip is an important way to protect the components in a nuclear power plant from failing or becoming damaged.

Samuel Miranda
Senior Reactor Systems Engineer

Author: Moderator

Public Affairs Officer for the U.S. Nuclear Regulatory Commission

21 thoughts on “What is a Reactor Trip and How Does it Protect the Plant?”

  1. NPPs and SFPs should just be shut down, and the fuel dry-casked and/or treated with Brown’s Gas flame. The normal pollutions in operation and refueling (and the genetic damage to the human genome from exposure not even counting leaks, spills, etc.), the normal risks in operation, the risks of terrorist or foreign commando or guided weapons attack, are too high, considering the catastrophic damage that can be done. We all stand a good chance of dying just from the MOX released by Daiichi #3’s explosions, as it is. Germany is getting rid of its NPPs, and half its daytime electricity is now solar, wind, and hydro, with prices so cheap gas electric plant owners are complaining. Why can’t the owners of US plants just confess it is not worth it, take the loss, and move on to more productive and safe energy production schemes? We have enough weapons grade material to last a very long time, and too much to begin with, and lots of other ways to make WMD’s, why not get out before America has its Fukushima? Before the New Madrid fault zone does an 1811/12 again? Before a Carrington class solar flare? Before a dam break causes the destruction of the Corn Belt?

  2. I’m curious to know what’s meant by some wording in the response above. What is an “intellectual ant-nuke” and what’s the significance of “spotting” said “intellectual?” Further, what does it mean to have an “agenda hidden behind “concerns””? Can’t an “agenda” be a good thing? If there are no “concerns” does one thus not have an agenda? Do people posting here by definition have concerns and agendas, or not? Do you have an “agenda” and have “concerns?”

  3. Garry,
    You might be interested to know (if you don’t already) that there is quite a bit of work going on in the industry currently regarding something we call Human Performance Improvement (HPI). Actually, its been going on for some time and is not restricted to commercial nuclear power production. It is a concept that is being embraced by multiple fields of higher risk work. Hospitals, for example, can make horrible mistakes when decision makers are under too much pressure, for too long, and when operating in business systems and constructs that are not human error tolerant. Most people do not want to make a mistake that could have horrific consequences, but we all know that sometimes real human beings unintentionally do just that.

    But regarding deceit, nuclear security, and personal reliabiltiy within the industry and regulator community. No doubt shameful events and reactions such as you describe have occured. I have read of many, seen and investigated similar situations myself, and of course cannot defend that kind of behavior. You likely observed them in your “military experience”.

    But, sadly, every industry or sector has similar problems and always will – because they involve imperfect people. Hospitals, police departments, fire departments, lawyers, medical professionals, mechanics, business leaders, politiicians, etc. etc etc. all have the same inherent problem.

    So, I would ask what an individual is to do? Should I cease using all hospitals because they may have deceiful doctors, nurses, and administrators? I could, of course, choose to do so. But I don’t think I would like the long term consequences. Should I learn to be a doctor when I get cancer, because I might get a dishonest doctor? Well, maybe I could do so if I have the resources and ability, but I would most likely be dead long before I learned enough to heal myself. Should I cease using mechanics to fix my car, not call the police to report a crime, and not dial 911 to get someone urgent medical help when they need it because of the risk of getting a deceitful responder?

    Of course not. None of us that are rational would make such a drastic overreaction.

    But then, of course, in those examples the consequences are limited to myself or to a few people – not large populations. Yes, nuclear power has potential to cause significant harm when improperly managed. (Can I spell Chernobyll right….!). But I see an important difference in all the nuclear work i have spent my career in – the concept of defense-in-depth.

    Defense-in-depth simply means that when potential consequences are severe, one simply doesn’t rely or only 1 or 2 controls or barriers. And that concept is foundational to nuclear systems, be they used for ship propulsion, for electricity generatuion, or even for weapons. No, in our field we don’t typically allow such conditions to develop, or to exist for long if they do develop. And, we prefer engineered solutions to adminstrative ones, because a machine is more reliable than are fallible human beings. All this could support a long and separate blog to fully describe….

    So, nuclear energy is just as subject to human frailty – intentional or uniuntentional – as any other field. But at least in my experience, willful error (deceit, dishnoest, etc.) are thankfully pretty rare and widespread events in the nuclear field. And, the defense-in-depth concept prevents such behaviors from causing too much damage before a condition is recognized and corrected.

    Again, while acknowledging those facts, I remain convinced that the beneifts available to human society from nuclear energy far outweigh the accompanying risks. Thsoe risks can, and must, be properly managaged and controlled.

    On that, we may have to respectfully disagree! But I certainly can agree with you that;

    “Never should a regulator or nuclear operator deceive the public about safety issues.”

  4. Whether or not Garry is an “intellectual anti-nuke” doesn’t matter to me. First, I have no basis on which to make such an assumption. I know nothing about Garry Morgan, other than what he writes in a blog post. For me, there is just not enough information in a 21 word phrase to make me comfortable drawing such a conclusion.

    But even if Garry may be fundamentally opposed to nuclear energy, it is still very important how we in the field respond to these questions. We should respond thoroughly, carefully, and with respect for the concerns raised. I guess the way we used to mean when we used the the word “professionally”.

    Garry and everyone else have a right to their honest opinions and fears. We don’t have to share them, and we are not likely to change those opinions in a blog. But all kinds of people may read these blogs, including people who are honestly trying to better understand a complex topic that is not well understood by the general population. Garry may well be one of these people. If we fail to respond reasonably and professionally to questions about the technology we support, those people are going to discount what we say because we come across as biased or as a “lackey” of the nuclear power industry.

    We all know there are some people that are not going the be convinced, regardless of the quality or depth of the logic used by the most technically knowledgable person. OK, I can accept that. But I welcome the opportunity such questions provide to emlighten the general public about nucleaqr energy – both the pros and cons. Most people arent stupid. I say tell the facts, answer their questuions with respect, and let them draw their own conclusions.

    By the way – I apologize in advance for any spelling errors! I have become far too dependent on spellcheckers, and I don’t know how to use one in this blog……:)

  5. Well Steven-regarding your Jan.3, 2013 @8:20pm post and ridiculous attack: You and Mitch, do not address any issue, your purpose is intimidation and attacks.

    Lets talk attacks and intimidation. One of my “intellectual anti-nuclear” concerns Steven involves personnel security issues relating to the threats and intimidation by anonymous thugs which have been waged on employees and citizens expressing safety concerns involving the nuclear industry. That includes nooses in nuclear facilities and threats pointed at employees for filing reports of safety violations and fraud.

    “In fact, the TVA has a long and infamous history of ignoring complaints by whistle-blowers and retaliating against them when they persist. Dozens of whistle-blower retaliation complaints have been brought against the TVA in recent decades.” Quote from a Washington, DC law-firm involved in nuclear whistle-blower litigation involving the TVA.

    The threatening activity of the thugs and criminals have cost the TVA ratepayer millions upon millions of dollars.

  6. Questions with loaded phrases like “making an area the size of New York City uninhabitable for hundreds of years in the case of a catastophic accident?” are what lead someone to say you are “praying for disaster”. First, you ought to learn how to spell catastrophic. Mitch has spotted you as an intellectual anti-nuke who thinks his agenda can be hidden behind “concerns”.

  7. Don, your reply is appreciated.

    One of the concerns I have is related to nuclear security and personnel reliability which was part of my “military experience,” I admit that I have a problem with civilian nuclear power. A result of the deceit demonstrated by several within the nuclear industry and the regulator. The incident of the control room fire and its cause is a tip of the larger problem with human factors as they involve safety systems.

    On a positive note, it appears the NRC officials are attempting to correct the problem with “antiquated,” beyond service life electronic parts at Browns Ferry. There is no word on the specific number of parts or systems involved or the time it will take to replace antiquated electronic parts.

    Never should a regulator or nuclear operator deceive the public about safety issues. Unfortunately that has occurred. Worse, employees within the nuclear industry who have reported obvious safety violations have been harassed even to the point of attempted murder with no prosecutions evident. That is a problem and is a reason I cannot support civilian nuclear power. Within the TVA region, this apparent personnel reliability failure has cost the ratepayers millions of dollars. (Reference incidents at Watts Bar and Browns Ferry where employees reporting safety violations were fired or worse, attempted murder.) There was a 2 part special on CBS news about the problems regarding security and threats last year.

    One of the continuous problems at Browns Ferry has been a management related problem connected to correction of identified problems, Corrective Action Programs. For more than 3 years the specific defective part was identified and a corrective action plan initiated to correct the problem. No action was taken to correct the problem until the control room fire in the annunciator (warning) panel. This same sort of inaction to problems occurred resulting in a Red Finding at Browns Ferry concerning a critical valve.

    I’m very concerned with the various design safety reports regarding systems involved in the GE Mark 1 systems. One such report is the Cooling Pool Tornado Safety Report. It is full of unscientific assumptions which attest to the safety of GE Mark 1 cooling pools if hit directly by an F5 tornado. It is flawed but yet the NRC maintains nothing is wrong not only with the report, but the fact the cooling pool design has no overhead or lateral reinforced containment above the cooling pool level to protect the highly radioactive cooling pool contents. Everything is “A OK” according to the NRC, and that is not only wrong, it is deceptively wrong.

    Safety systems problems can be corrected. It is yet to be seen if the human factors problems can be corrected related to deceit and faults regarding safety systems where human interaction is required to insure systems safety. Current and former NRC staff to include former Inspector General officials have expressed these same or similar issues.

  8. Thank you for your informative replies. Let us hope that all “antiquated” or beyond life expectancy electronic parts involved in safety systems are replaced sooner than later.

  9. Garry,

    With regards to annunciators, from an electrical/plant safety perspective, annunciators are isolated (usually with optical isolators) from plant circuitry, to prevent the effects of a fire from propagating back to a safety system. That said, any fire in the control room can become a serious issue, and Browns Ferry is no stranger to control room fires.

    Plants are required to replace safety related and maintenance rule components to prevent failures from ever occurring, and surveillance testing (part of the plants operating license) is required to ensure that any failures are caught before the system is needed to ensure safe shutdown of the plant. Unfortunately passive components like circuit cards, capacitors, transistors, are generally not in that program. The plant I am at now is having issues where we are seeing solid state component circuit cards beginning to fail in our non-safety systems, and we are finding out that we are having to include refurbishment or replacement PMs on these parts as well. As we find failures of particular systems or components, through the industry operating experience program (required for all US nuclear plants), we are required to address failures at other plants to prevent them from reoccurring. Obviously replacing every component in the plant is not a possibility, but the focus is on ensuring that safety-related systems, structures, and components (SSCs) are always operating using equipment which is within its life expectancy.

    My personal thoughts about “beyond life expectancy components”, especially with regards to I&C components (which I personally work with), these ‘passive’ components are starting to get screened into preventative maintenance programs at some sites and should continue to do so. If they are properly placed into the existing maintenance processes are capable of ensuring these components are replaced and refurbished.

  10. First, a disclaimer. While I am an unabashed advocate of nuclear energy for electrical energy production, I have no practical experience in GE Mark 1 reactors, or commercial nuclear power generation at all. My experience (~35 years) has been all US Navy (Pressurized Water Reactors – PWR), and with non-reactor nuclear facility operations and nuclear safety within the US Department of Energy. Finally, my PWR experience is very outdated (last time I operated a PWR was prior to Three Mile island!).

    However, I do have some thoughts that may still be germane to this discussion.

    First, in my opinion HiddenCamper seems to know what he/she is talking about. Reactor shutdown systems are rightfully complex, and factor in a number of input signals. Logic circuits are used to handle the tough job of ensuring the reactor is shut down quickly when needed, while simultaneously screening out signals that come from normal errors, equipment failures, etc. These decisions must be made quickly and without human intervention, to protect against the kind of rapid power excursions that are possible if certain conditions are met.

    One might ask, why not just shut the reactor down on any signal, just to be safe.? Well, the reasons are twofold:

    First, cycling the reactor systems needlessly does stress them (thermally, physically, chemically, etc.). While I would defer to the appropriate qualified experts on Garry Morgan’s specific statement: “Each time a scram occurs the reactor pressure vessel’s primary metal containment becomes embrittled”, he is undoubtedly correct in that these cycles do stress the affected systems. I suspect HiddenCamper is right when he/she states that each reactor vessel is designed for a certain number of scrams in its lifetime.

    Since the GE Mark I’s are older (about 40 years on average, i think) , it is true they likely have components, electrical and otherwise, that have exceeded their design life. Some, perhaps many, of those components have been replaced throughout the lifetime of the reactor plant, as a normal part of operations and maintenance. For other components, the task of replacing would not be so easy. In such cases, I would expect that compensatory measures are required, have likely been implemented, and are under frequent review, testing, and monitoring by the plant operator, as well as the NRC. Part of that monitoring and testing process probably includes consideration of how those components are affected by plant transients such as trips/scrams. I say probably because I don’t know from my own personal experience, but I strongly suspect (based on my similar experience in non-commercial nuclear work) that I am pretty close to being accurate.

    The second reason (lets be honest) is economics. It costs a lot of money to have a reactor plant down, since the utility still needs to meet the power demands of its customers. That is not immoral. Plant operators have a duty to their stakeholders to produce a profit, and tough decisions must be made or there is no reason to have the business at all.

    All this is one reason why, in my opinion, we would be much better served as a nation to move on with building newer reactors – ones whose designs have benefited from multiple decades of operating experience gained from the older designs. In many of these newer designs, concerns about a station blackout are vastly reduced due to increased reliability on natural circulation and other passive safety features.

    Do I believe a station blackout is possible at a US reactor due to a catastrophic disaster – yes. Is it likely? Not so much due to a number of reasons. But if one does occur, I understand the newer designs (the so-called Generation III, III+, IV designs) are much more capable of managing such a situation safely.

    In summary, I believe Gary Morgan has raised a legitimate concern that bears a reasoned and thoughtful reply. We in the nuclear community (yes, I know its a stretch to include me in that community!) are most helpful when we recognize and respect these type of questions, and give them a careful and honest answer. I don’t know if TVA or NRC have acted (or failed to act) as Gary alleges. And, the answer is likely not simple or easy, but rather complex and hard to communicate. But I believe his question is reasonable.

    No energy source is totally without risk. Yes, nuclear can have more significant adverse impacts that other energy sources. Can those risks be properly managed? I believe they can (and must) be. Are the risks associated with nuclear power so high that we should abandon it as an energy source? I don’t believe we should.

    Thoughts from the peanut gallery…. Thanks for reading!

  11. Hiddencamper, thank you for your informative reply and the reply below. Obviously you are an educated individual about the intricate engineering systems of a nuclear reactor and the reactor’s safety systems.

    A concern which has been expressed to the NRC concerning the GE Mark 1 reactor types are the number of beyond life expectancy electronic components in the GE Mark 1 type of reactors. One of these components (a capacitor which had an 8 year life-apparently installed in the late 1970’s) caused a small control room fire in an annunciator panel at Browns Ferry in January of 2012. The NRC has admitted this is a problem but the operators of the plant, TVA, have not identified the specific numbers of “antiquated” components. Apparently these old-beyond life expectancy electronic components could number in the thousands. The problem extends to relays, capacitors, fuses etc. This has a serious implication as to safety of the system and the management of the safety systems and identification of limited life components. Unfortunately this problem is not being assigned a high priority.

    What are your thoughts on beyond life expectancy components in the GE Mark 1 reactors?

  12. “Disappointed with Fukushima”?! What kind of thinking leads one to the conclusion that anyone would think that three melt downs and four threatened spent fuel pools wasn’t enough????

  13. You say “praying for disaster,” shame on you “Mitch B” for your low life attempt at reader manipulation structured in the form of a personal attack. I’m expressing serious concerns regarding aging and defective nuclear reactors.

    Concerns about nuclear power safety and the “personal reliability” of those managing and regulating the industry is also an issue. Your style of attack is an insignificant example of the threats and intimidation wagged against those who bring forward serious issues and questions within the nuclear industry. You have said nothing in regards to the issue brought forward – SCRAMS and safety of aging nuclear reactors.

  14. Standard BWR reactor protection system trip signals.
    High pressure, low water level 3 (the point where steam can skip the steam separator and wet steam can get into the turbine), high containment drywall pressure (indicative of a loca accident), high neutron flux (118%) (instantaneous overpower), high power without enough flow, high scram hydraulic discharge water level (ensures scram hydraulic system will function), turbine control or stop valve closure (loss of heat sink can cause a pressure spike, this limits that spike by reducing power through scram), main steam valve closure, and high power during startup on the IRMs or SRMs. Placing the mode switch into shutdown or arming and depressing the manual scram buttons also works. Scram logic for almost all us bwrs is “1 out of 2 taken twice”. This means there is an a1,a2, b1,b2 channel. If you get any A channel AND any B channel the scram occurs. This means A1 and A2 won’t cause a scram, but A 1 and B2 will. This is to ensure a single equipment failure doesn’t cause unintended scrams, but also protects you from a single equipment failure from preventing a scram to occur when required. Hope some people find this useful or interesting

  15. As part of each plants USAR, if you go into chapter 3, each reactor vessel is rated for a certain number of scrams in its lifetime. Generally the number I’ve ipseen for a BWR is about 120 scrams. This doesn’t put a hard limit on the life of the reactor, but it is what the core is designed for, after which you would need to do extensive analysis to determine embrittlement and shifting of your reactor Tndt (nil ductility temperature). There are limits to nil ductility temperature as part of each plants technical specifications (operating license).

  16. You sound like someone praying for disaster to vindicate your fears, as did many others disappointed with Fukushima.

  17. Do you think a coal/gas/ fuel oil burning power plant has the capability of making an area the size of New York City uninhabitable for hundreds of years in the case of a catastophic accident? Or, do you believe a station blackout as a result of a catastrophic disaster will never occur at a U.S. nuclear power facility?

  18. “Something happened that caused the reactor to… trip” – an over simplification in the attept to downplay the event. Faulty cabling according to Ray Golden, TVA’s Nuclear Spokesperson, caused the trip. Is this like the faulty, numerous antiquated power supplies which caused a control room fire earlier in the year? How many feet of faulty cabling are in Browns Ferry? How many beyond life expectancy analog electronic components are in Browns Ferry and the other defective, dangerous GE Mark 1 type of nuclear reactors around our nation?

    The Browns Ferry nuclear reactors have had many emergency shutdowns, SCRAMS. Each time a scram occurs the reactor pressure vessel’s primary metal containment becomes embrittled, embrittlement is the degeneration of the metal’s molecular structure, as a result of the rapid cool down and neutron flux. How long will it be before the “ax-man cometh” for the final cut in the GE Mark 1’s sortid history? Y’all are playing the odds and the rules of probability say you will eventually “crap out” if you play long enough. Particulary if the play goes beyond the engineered life expectancy of the reactor, that is the case with the Browns Ferry GE Mark 1 defective design. You can not engineer your way out of unexpected and unintended consequences after a disaster.

  19. Good timely article. It’d also help dispel fretful exaggerations of the situation by posting what the equivalent event/status would be in a coal/gas/oil fired plant.

    Have a Happy New Year!

    James Greenidge
    Queens NY

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