Rick or somebody that knows,

What is the typical minimum amount of cool-down time required for the shut down of a nuclear reactor?

What is the contingency plan for a nuke and its regional grid blackout out, thus making it impossible to run cooling down pumps around the core? (backup generators at each nuke in the country?)

-- Anonymous, October 19, 1998

Answers

James, I do not know but the attached speaks directly to the subject. This is from Gary North's Links under "The Power Grid" titled "Why the NRC Must Act Decisively on July 1, 1999" posted 10/17.

Gary North: The Nuclear Regulatory Commission has told nuclear power companies that they must be compliant by July 1, 1999. I had not understood why until I received this letter. The NRC faces a technologically imposed deadline: it takes six months to cool the cores. It takes electrical power to do this. In short, it takes the grid to enable the plants to shut down safely.

So, I have been wrong. I have said that the NRC would have to act by late 1999. Now I see that the NRC must act on its official schedule. This will be a crucial deadline. If the NRC backs down, then either (1) the plants will be dangerous in 2000 as they cool down on diesel generator power; or (2) the NRC is blowing smoke; it will not shut down the plants in 2000.

* * * * * * * letter from Nicholas Vrettos follows:

As a nuclear engineer, I read with great interest the article about the NRC mandate for all nuke plants to be compliant or else shut-down. I was discussing the article with a couple of friends of mine who are also nuclear engineers. We were musing about the NRC's deadline---July 1, 1999.

At first we could not understand why the plants needed to be shut-down six months in advance. Then it hit us. A 1000 electric megawatt nuclear plant generates about 3000 megawatts of heat energy. That is 3 BILLION watts of heat energy. When a plant is scramed, the nuclear fission stops almost instantaneously, however the core still generates a tremendous amount of heat. This heat is called residual heat and is a result of the natural cooling-off of the core. under normal circumstances, special pumps called Residual Heat Removal pumps circulate water through the core to keep it cool and remove excess heat. Emergency diesel generators can supply power to these RHR pumps whenever power to the plant is lost. Also under normal circumstances, it takes approximately 4 months (depending on the operating power of the core) to cool a core to the point that loss of cooling will not damage the core. In other words, nuclear plants need six months to ensure their cores are cool enough and won't melt if power to the plant is permanently lost.

Imagine the ensuing mess if nukes can't cool their cores.

Nicholas Vrettos

*********** end of letter

If anyone with the expertise can comment on this it could be helpful.

-- Anonymous, October 19, 1998

Pat,

I claim no particular expertise, but I'm not reading anywhere in the NRC Generic Letter 98-01 that says plants must shutdown July 1, 1999 if they are not Y2K compliant. In fact, the May 11, 1998 NRC letter says the follwing, under "Required Response", Section 2:

"Upon completing your Y2K program, or, in any event, no later than July 1, 1999, submit a written response confirming that your facility is Y2K Ready, or will be Y2K Ready, by the year 2000 with regard to compliance with the terms and conditions of your license(s) and NRC regulations. If you program is incomplete as of that date, your response must contain a status report, including completion schedules, of work remaining to be done to confrim your facility is/will be Y2K ready by the year 2000."

Clearly, from this statement, some plants are still going to have a chance to either operate or be shutdown after that "six months cooling period" has elapsed.

Additionally, and in reference to Dr. North's comments, no one is talking about compliancy here. It's all been Y2K readiness from the beginning. We all know the difference.

-- Anonymous, October 20, 1998

Although I am a Y2K alarmist, with no expertise in nuclear power, I am awaiting the answer to this issue with skepticism about its central concern. Once nuclear fission stops, doesn't the whole system become a passive one? Surely, the core could remain hot for a very long time. But in a passive system, what could make the hottest part hotter?

-- Anonymous, October 21, 1998

I'll try to answer this one as simply as possible. For the nuclear physicists among you, I know this will be simplistic but I'm trying to explain it in lay terms and make a point.

Nuclear fission is a result of unstable atomic level isotopes (typically uranium) "throwing off" excess neutrons in an attempt to have a stable atomic structure. For the sake of example, lets say that when a reactor is started, there's no loose neutrons bouncing around the reactor. Control rods that absorb the loose neutrons are pulled out of the core, and the chain reaction starts - there are now a few loose neutrons bouncing around. These neutrons 'hit' the uranium atoms, creating more loose neutrons. This chain reaction causes heat in the water that surrounds the uranium core, which (depending on the type of reactor) either boils the water directly or through a convective type heat exchanger. Production level of neutrons (and thereby heat generation) is controlled by various means; an ideal controlled chain reaction gets to a certain point where there is a constant number of neutrons producing heated water within a specified pressure and temperature range.

When the reactor is shutdown, the object of the game is to reduce the number of loose neutrons and thereby heat generation in the water. But, there will always be some level of sub-atomic reaction going on, so a continuous flow of cool water is piped into the reactor. Water does two things - it cools the core and also absorbs neutrons. The cooler the water, the more density the water has, and the more neutrons water will absorb. In a reactor core that has been operated, there will always be some amount of neutron production, so even when the core is 'spent', it has to be in a pool of water that is being circulated.

Ok, now armed with this knowledge, it doesn't take much time to cool the core down once the chain reaction is stopped. During a refueling of the reactor, the reactor is actually cooled enough be opened, usually within 48 hours of the plant being shutdown. The trick is keeping it cool, and a typical nuclear plant has many cooling water systems (normal and emergency backup) to do this. The heat generated in this condition is called 'decay heat' (because the level of neutron production is decaying, but still present, and is generating some level of heat).

What the person who posted the email to Gary North was getting at: it takes quite awhile for neutron production to get to the point where very little water is required to keep neutron production rates on a decaying slope. Some amount of circulating water will always be required. What's the point in time at which the core could basically 'sit on its own' without additional cooling water being circulated for a prolonged period of time? It depends. 6 months may be a good number; I don't know.

The time frame is irrelevant, though. Once a reactor is shut down, as long as the control rods remain in the core, and cooling water is circulated, the reactor remains shutdown. There are many systems and backup systems and backups to backups to make sure this happens. That is why I'm not too worried about Y2k impacting the ability of the plants to shutdown and stay shutdown.

At the risk of being absolutely redundant with the euy2k.com website, from a Y2k standpoint, the concern is:

1. The plant operator's ability to monitor, respond to, and understand plant conditions in the event of a problem (the inability to do these things contributed to both TMI and Chernobyl)

2. Ensuring that automatically operated processes respond properly in the event of a problem.

My concern is not cooling the reactor or keeping it cooled once it is shutdown, whether that timeframe is 48 hours or 48 days.

-- Anonymous, October 21, 1998

Nuclear reactors need to install renewable energy back up systems to provide cooling at reactor cores and the irradiated fuel pools in case the grid goes down. (Preferably wind power)

At Davis-Besse in Ohio this summer, a tornado knocked out power to the outside world. The backup generators took some time to start up, and the core temperature went up ... even if the backup generators work, if Y2K disruptions are long term, getting new fuel supplies for the generators could be difficult.

Besides, with the calving of a 2700 square mile iceberg in Antarctica, installing renewable energy that does not alter the climate or generate nuclear wastes incompatible with DNA is good idea.

Mark Robinowitz y2ko -- www.igc.org/icc370/y2k.htm

-- Anonymous, October 23, 1998

Below dupes some info originally sent to Yourdon's site as part of a related question about scram times and cool-down times.

Time to shutdown and cooldown to room temperature (routine maintenance or fuel reload) is 2-3 days. Thermal load after shutdown depends on time and previous power history - but is expontially decreasing not due to neutron flux (fission) but due to decay of the radioactive daughter products left from previous fissions. The number of neutrons goes to essentially zero within miiliseconds of the scram or shutdown.

So heat load (that must be removed) is:

Minutes after shutdown = about 2.0 - 3.0 percent of full previous power - this is the 3000 megawatts mentioned above, or 6 Mwatts.

One day after shutdown = about 0.5 percent.

One week after shutdown = about 0.02 percent of previous power, then more slowly declining. Realistically, use the 0.02 percent as a constant. (0.0002 times max thermal power or only 0.60 Mwatts.) Now, this is still a lot of energy, but the systems are designed to handle it. The operators aren't going to "walk off the job" .... I've been one, my brother is one, etc. Give us a little credit please.

But assume nothing happens - "forever" to a spent fuel pool 100'x60'x48'. For example, 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. It can be replaced from emergency sources, or even from a fire pump, and the spent fuel is safe. Similar if the core is uncovered in the larger refueling pool.

Before startup can begin, and while the reactor is operating, there must be at least two independent backup power sources for cooling - these two sources must be independent of site or off-site power. Normal backup are paired (sometimes triple) diesel gen sets at the site, each driving a different power and control "train" supplying different pumps on site. 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. Various alternative ways of getting cooling water into the pool - up to and including fire pumps.

Note - no fossil power plant has any mandatory backups, nor are any fossil plants legislatively required to have any on-site reserve power to allow a "black" startup. Some do, but they are the exception.

If the grid goes down, it could be very, very difficult to restart a large fossil plant from zero - you must remember to include train unloading stations, barge cranes, coal belts, and coal crushers through exhaust emission and filters in the fossil support systems. And the fossil operators aren't trained and retrained and exercised through disaster and alternative operations like the "nukes" are.

Hope this answers your questions. Do not accept the "four months" time frame for shutdown. It is often repeated, but is equally and as often wrong.

-- Anonymous, January 20, 1999

Sorry, this should be " if this is based on the 3000 megawatts mentioned above, or 60 Mwatts." (Carbon-based life-form Y2Kalculator error.)

-- Anonymous, January 20, 1999

Robert,

You have provided a lot of good information, but I don't think you answered the real question:

"How do the nuke plants keep the spent fuel cool if the grid is down for an extended (months) period of time?"

You said:

"But assume nothing happens - "forever" to a spent fuel pool 100'x60'x48'. For example, 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. It can be replaced from emergency sources, or even from a fire pump, and the spent fuel is safe. Similar if the core is uncovered in the larger refueling pool. "

How do you continually replace this boiled-off water without an outside power source? Remember, we're talking about several months here. Fire pumps presumably need power to run...

Jon

-- Anonymous, January 20, 1999

Where gravity is not available - literally, many (but not all) of the reserve/"emergency alternative" water sources are above the fuel pool and can simply be drained to the pool through temporary or alternative routes - many (again, but not all) fire pumps are direct drive diesel powered. Those that aren't are served by the plant emergency power busses. Their water supply is independent of the plant nuclear grade, emergency reserve, steam and condensate, and cooling water or systems service waterr reserves too. (Each of these different systems can be a cooling source - depending on circumstances and how the plant is arranged.)

But don't forget that the emergency diesel gensets are specifically designed to provide station services power to the plant - so all electrically operated pumps and controls are still available when "grid" power is off. These diesel gen sets (they are the same model diesels that drive locomotives) do not require electricity to start, stop, or run. Fuel? They would need to be refilled after a couple of weeks, if running continuously, but continuous full power loads aren't needed. So fuel use is decreased.

Worse case conditions, back a fire truck up the access door and run a hose inside. All that is needed is reasonably clean water. (Well, completely clean water if you want to save cleanup costs.). The soluable boron remains in the pool, so that is not a factor.

Each operating team will know its own required time-frames, heat history, activation and power history. If you remain concerned, write your local plant, ask for their alternative plans and remediation status. After all, the goal is to continue to safely generate and send electricity to the grid, not shutting down and becoming a load on the grid..

-- Anonymous, January 21, 1999