I have read it could require between 1 - 6 months of an outside source of electricity (i.e., from the grid) in order to guarantee a safe cold shutdown of a nuclear power plant. What is the correct period of time required? > > Also, I understand that the fuel rods are often kept on-site > in a pool and that some percentage of the fuel rods are > removed at regular intervals and put into pool storage. > Do these pools containing spent fuel rods REQUIRE AN OUTSIDE > SOURCE OF ELECTRICITY to keep the water flowing through them? > IF SO, FOR HOW LONG?

-- Anonymous, November 23, 1998

Answers

I saw your question on euy2k.com re how long the cooling pools at N plants need electricity - I have heard that if they do not have electricity for 48 hours they will explode. I called the NRC phone hotline for y2k info and they said that the generators will cover them if the electric goes off. Recent tests have shown some generators not to work. Or at least not to be dependable. And how LONG do they work for? These are questions not easy to get an answer for. My feeling is VAST irresponsibility here. The situation is extremely hazardous. And FEW are talking about it. Glad you are -- Best Sara

-- Anonymous, May 02, 1999

I asked for and got an answer for how long the diesel generator at Maine Yankee will be able to power the cooling of the spent fuel pool if there is a power loss. Pat Dostie, Maine's Nuclear Safety Inspector told me in a letter, "The diesel generator has a fuel capacity for 72 hours of contiuous operation". Maine Yankee is shut down but the spent fuel pool is full of rods and even the reactor core. It must be kept cool. Is this time frame common in other plants?. Sounds scary to me.

-- Anonymous, May 03, 1999

Wendy, Sarah and Nancy, your questions have been the subject of several threads on this forum, and there is a lot of input and information to be found. At the top of the New Questions page on this forum you'll find the SEARCH link. If you put the words, "cooling and nuclear and days" in the search box you'll get lots of hits, including one thread titled, "Cool Down Time For Nukes", another called, "What about spent fuel rods?!" and another titled, "Nukes need power to stay off line?"

It will take you some time to read all the various threads and answers, but you'll get some industry opinions and also background information and explanations of how cooling systems work. There's so much there, I don't think anyone can summarize it all for you. And as usual, there are variations in viewpoints!

-- Anonymous, May 03, 1999

A measure of guilt has caught up with me and I apologize for not taking the time to compile and summarize an answer here. My priorities were amiss in not easing people's concerns immediately.

The following is summarized from a variety of sources. Thanks to Rick Cowles, Robert A. Cook, and Rob Piper for previous posts they have made on this forum and which I have taken the liberty of including parts of in my summary, as well as others who have provided me personally with info. Any errors are mine only, although I have tried to be as factual as possible with the information I have.

I'll start from the beginning and try to give the simplest explanation I've gleaned out of all I've read. First, there is a difference if we're talking about a loss of offsite power for an online (operating) nuclear plant versus one that is already shutdown. For instance, a plant which is shut down has already gone through the initial cooling process and it does not require very much to keep a spent fuel pool cooled. For some spent fuel pools, we don't have to be talking about running a generator continuously to pump cooler water in. You might only have to run it an hour or so each day. (So three days worth of fuel for continuous running would go a long time.) We're also not necessarily talking about really high temperatures, depending on how long the spent fuel has been in the pool. Sometimes the water temps being talked about are in the 110 degree range.

For an online nuke, if there is a loss of offsite power (the local utility goes down, or the grid goes down), then diesel generators come on to provide power to the coolant pumps (primary and secondary loops, meaning even the diesal generator systems have backups). At the same time that the diesels are coming on-line, the reactor scrams (emergency shutdown).

What that means is control rods (usually made of galodinium or europium which will absorb neutrons & stop the reaction--control rods are not made of graphite.) will drop into place forcing the nuclear fission process to STOP. This is a "Design Basis" accident, so called because nuclear power plants are designed with this type of accident (and a lot of others) in mind. Actually, if you read the Daily Incident Reports to the NRC, you'll discover that emergency shutdowns are not all that uncommon and can happen in various places every year. Severe weather brings down power lines all the time or otherwise interrupts offsite power. Also, depending on the Licensing requirements for each individual nuclear plant, things like the flooding of an access road which would impede emergency vehicles from getting to the plant can require a shutdown just on general principles.

So it's good to keep in mind that emergency shutdowns are part and parcel of the normal operating procedures at nuclear plants. Now, for an ACTIVE nuclear plant, I'm pretty sure that NRC regulations require having a 30-day fuel supply for backup generators and initial primary cooling. After the shutdown, with rods inserted, what you have to worry about is decay heat in the fuel after the actual fission process has been stopped. You will still have appreciable heat in the core for 5-6 days after the reactor has shut down. However, after about 6 days (or less) the core will be cool enough where there isn't any real danger of fuel melt.

That's when the spent fuel cooling comes into play, which is to keep the fuel covered (water absorbs decay neutrons) and to cool the small residual heat output. As time goes on, the heat output continues to decrease. The estimates people hear about four or six months to cool spent fuel comes from that period of time AFTER the fuel is not in danger of any meltdown.

As one nuclear worker put it, even assuming no generators are operating to cool a spent fuel pool, and if, after several days, the water begins to boil and still nothing is done about cooling, less than 12" of water is lost a day from the pool. And the pools are 100'x60' and 48 feet deep. Water can be replaced from emergency sources other than generator pumping. To refill a "fuel pool" with cooling water - if the level ever gets low, there are up to six other sources of clean water on site and various alternative ways of getting cooling water into the pool - up to and including fire pumps.

Many (but not all) of the reserve/"emergency alternative" water sources are above the fuel pool so gravity can work, and they can simply be drained to the pool through temporary or alternative routes. A fire truck backed up to the access door with a hose run inside will do the trick. All that is needed is reasonably clean water. Also, every nuke plant has at least two backup systems which have to be independent of site or off-site power.

There has been concern expressed that the generator systems can fail since sometimes they're not tested on a regular basis. However, I've seen evidence that leads me to believe that making absolutely sure any generators are operable and good to go is a priority for nuke plants right now and part of a prudent Year 2000 project. In fact, reading NRC reports, you'll find that the emphasis in on a nuke plant's capacity for a safe shutdown and not it's capacity to keep generating electricity through the rollover. Yes, they want the nukes up and running because the nation needs their generation, but industry people do recognize that being able to shut down safely is top priority -- good times, bad times, or in between.

I do have to say that some other countries' nuclear plants are not built along the same safety parameters as those in the U.S. I do have concerns in that direction -- I think the risks are higher in Russia, for instance. Does all the above mean I don't think anything can go wrong here in this country? After Three Mile Island, nobody can say that. (Actually, the Three Mile incident caused a lot of extra safety measures to be mandated in all nuclear plants.) What I do think, though, is that the risks always inherent in using nuclear power are as low as they can be - short of shutting down every single nuke plant before the 2000 rollover. I personally don't think that is going to happen, because there are some pretty big risks in doing without the electric power that nukes generate, too. And it is true that nothing has been found to date (Y2K wise) which would stop nukes from doing an emergency shutdown. There are things which might cause a shutdown if they're not fixed, but I have confidence they will shutdown safely if it comes to that. At least here in the U.S. and several other countries using similar safety designs.

I have plenty of concerns over various risks inherent in the Year 2000 computer problem, but not being able to cool shutdown reactors is way, way far down on my list.

If I have made any errors in this summary, or misinterpreted the info I have, I welcome corrections.

-- Anonymous, May 04, 1999

Here are 2 previous IMPORTANT posts dealing with that same question... -------------------------------- About two months ago, I spoke with Matt Chalama (sp?), who is the NRC's person in charge of Y2K-related issues (NOT "PR"). According to him, the following scenario is what happens:

Once the reactor is SCRAMmed, you need a high volume of coolant water for the next 4-8 hours, in order to dissipate the massive amount of "waste heat" generated. After that, the "low pressure" cooling system is all that's required. This system needs only ONE low-volume pump to function, and uses very little electrical power.

HOWEVER, from the time the low-volume pump is started until the fuel needs NO COOLING at all, is 2 YEARS. So, this 4-6 month stuff, according to the NRC, is an "urban myth". You need power to at least ONE small pump for 2 years. Period.

Dennis

concernedbuthopeful Member posted May 29, 1999 04:47 PM --------------------------------------------------------------------- ----------- Dennis is right, The reactor needs this low pressure pump for two years to cool to a point of no need for cooling. The heat generated in a reactor core continues to be generated long after the reaction is shut down. If the reactor does not get this cooldown, the result is what you saw with Chernobyl, a reactor core meltdown. Of course our reactors have safegaurds to prevent this, some of which need electricity to operate, some of which do not. I used to work for Duke power company at the Lake Norman reactor north of Charlotte n.c. and we had lake Norman which could be drained through the reactor core in a dire emergency!!!!!!!!!, but of course even under these conditions the core would be fatally damaged, let alone the damage done to the environment. Urban myth indeed.!!!!!!!

-- Anonymous, July 08, 1999

River - it is an urban legend.

Cooling down (operating temperature to room temperature) requires no more than 3 days; I've regularly done it in less than 3 shifts.

Once cooled, the reactor (after about 4 days) is generating roughly 0.005% of its original heat. This amount of heat does require a small cooler, unless the top of the core is opened to atmosphere in the containment - as is usually done for normal refueling outages.

If the core cover is removed, the only thing needed is to periodically add clean water. No other power is needed, though all utilities are required by law to keep two alternate sources available.

The claims that backup power generators are unreliable (these have been repeatedly spread by a MA democratic congressman's office whose staffers hate and distruct nuclear power) are simply false. There is no other description nor excuse needed, these claims are simply wrong. Testing of these backup power supplies is also mandated in the operating license, and all plants regularly operate them.

-- Anonymous, July 08, 1999

So, maybe you can explain to me why they had such a big problem at the nuke plant in Ohio last summer, when the tornado rolled through.

From the sounds of the news report, the only thing that saved them was the offsite power being restored. Perhaps the news story is wrong, I don't know.

Jon

-- Anonymous, July 08, 1999