News update #1 for readers of "A serious but not ponderous book about Nuclear Energy"|
In these updates, we try to bring you news of significant current scientific and technological developments in the field of nuclear energy. New, still in development, not yet completely tested, some will be tomorrow's news headlines, some may be obsolete within months or years. Often we have to rely on information from the people who are promoting them, who have a personal or financial interest in them, and who promise results which may or may not materialize. Many numbers that we cite are estimates, and differ from source to source; rarely are all the raw data they are based on available. We do our best to sort out fact from hype and to be accurate and understandable. You'll be the judge.
Walter Scheider Cavendish Press Ann Arbor, PO Box 2588, Ann Arbor, MI 48103
The Pebble Bed Reactor:
The "pebble bed" modular reactor (PBMR) has been around as an experimental technology since the
1960's. So it was something of a surprise to see it featured on the cover of the August 2001 issue of Popular
Science magazine. The cover story, called "Nuclear Energy Comes Full Circle," is less than enlightening.
Uranium fuel microspheres, revival of an old technology
in the search for economy and safety.
Pebble bed technology has been dubbed "old wine in new bottles" by skeptics, who remind us that its graphite moderator and high temperature gas coolant are old, and largely abandoned features in nuclear reactors. Technical failures have caused the shut-down of the original Pebble Bed project, but now some scientists and investors have resurrected the concept. A group headed by Andrew Kadak at MIT, and an energy company called Exelon, are among the new advocates for this technology. A slide show prepared by the Kadak group, with a lot of technical detail, can be viewed on http://web.mit.edu/pebble-bed. A less technical source is the web site of the South African company "PBMR," www.pbmr.com.
How the Pebble Bed Reactor differs from the conventional reactor
The pebble bed reactor is certainly something more than just the resurrected moderator and gas coolant. Instead of 40,000 20-foot long rods, the fuel in the pebble bed core is in 400,000 tennis-ball sized "pebbles." These pebbles are continually removed through a pipeline at the bottom of the core. Pebbles whose fissionable uranium has been used up are discarded as spent fuel. The others eventually join the supply of new pebbles and are fed back into the top of the core. This system allows removal of spent fuel and replenishment of the core on a continuing basis. This not only cleans out spent fuel as it is produced, but also removes the need for fueling the core at the outset with a huge excess of uranium that has to be kept inactive by control rods for months and years until its time comes to be "burned." The core can therefore be smaller, and to that extent safer.
Of the two primary known causes of reactor disasters, there is probably no way to guarantee any reactor based on a chain reaction, to be safe from an "excursion" in energy production. Such an excursion can happen when the control rods are pulled or disabled, accidentally or (as in Chernobyl) deliberately, permitting a runaway nuclear reaction that produces excessive heat and pressure, bursting the structure. The pebble bed module is smaller and operates with less available fuel, but is otherwise no safer than other reactors in this respect. In fact, since it uses carbon as moderator (p130; pp 101 and 111 - page references are from A serious
but not ponderous book about Nuclear Energy), it has the additional risk of graphite fire if air reaches overheated carbon.
Pebble bed does appear to greatly reduce the chance of a disaster of the other kind, from a "Loss of Coolant" (LOCA) accident. The typical LOCA triggers a SCRAM, which stops all fission instantly, so that there is no danger of a runaway nuclear reaction. The danger in LOCA is from heat that continues to be released from accumulated fission products. Coolant is the material that carries the heat from the reactor, cooling the core while providing the hot gas (steam or other gas) that turns the turbines and generates electricity. In a loss of coolant accident, coolant leaks or stops being pumped and the flow of heat from reactor to turbines ceases. With nothing to remove the heat that continues to be produced, not from fission, but from fission fragments, the core can be damaged, even melted. Ultimately the floor of the containment can melt, releasing the core content.
SCRAM: what it does and what it can't do
The first line of defense against a LOCA is the SCRAM (Safety Control Rod Adjustment Mechanism). Either automatically or on a signal from an operator in the control room, the neutron absorbing control rods drop to fill the full height of the reactor core (p114-15). A SCRAM causes all fission to stop in a few seconds. Conventional reactors and pebble beds are both programmed for SCRAM in emergencies.
But even after all fission is stopped by SCRAM (p 208) the fragment nuclei of previous fission continue to produce heat through radioactive dissociation. Immediately after a SCRAM, heat is emitted from these radioactive fission products at 7% of the full power of the reactor. The rate decreases gradually (see Fig 1) to ½% in 24 hours. In a 1 Gigawatt reactor, 7% of full capacity is 70 million Watts; ½% is 5 Million Watts. This after-SCRAM heat is what led to the near melt-down that occurred at Three Mile Island in 1979. Many of the fragments have a short radioactive half life, but others have half-lives of tens of thousands of years, and these form the bulk of what is called "high level nuclear waste."
Here is where the pebble bed technology offers safety that the conventional reactor can't. The total cumulative heat released after SCRAM is large, but not limitless. The carbon that serves as moderator also can absorb the accumulated heat in a LOCA shut-down in a pebble bed reactor. The PBMR is a High Temperature Gas cooled Reactor (HTGR). Unlike the conventional Pressurized Water Reactor, where water serves both as coolant and moderator, the PBMR uses Helium as coolant. No matter what happens to the helium cooling system, the carbon remains throughout the reactor core, absorbing heat from radioactive fragments.
"Microspheres and Pebbles"
The unit of fuel in the PBMR is a microsphere (see left) which has at its center a kernel made up of about 1 milligram of Uranium Oxide, 8% enriched for U235. The kernel is surrounded by a layer of porous carbon which can absorb gases as well as heat, and a layer of silicon carbide, one of the toughest materials known, to withstand high pressure as well as high temperature. The microsphere is not a self-contained reactor (there is not enough uranium, among other reasons), but it is intended to be a self-contained structure for the confinement of pressure and temperature and gases generated by fission. The sturdy microspheres are said to eliminate the need for surrounding the reactor as a whole with a containment structure. It is feared by some that this makes the PBMR vulnerable to unforseen events that could release radioactive material, and to terror attack.
A fuel "pebble" in the PBMR consists of between 15,000 and 100,000 microspheres assembled in a sphere about 60mm in diameter held together by sintered carbon. It is slightly smaller than a tennis ball. (at right). The carbon inside the microspheres, as well as the packing carbon in the pebble and in separate all-carbon pebbles, serves as heat absorber as well as moderator.
Proponents assert that the heat capacity (ability to absorb heat energy) of the carbon in the pebbles is great enough to absorb the total cumulative heat from the fission fragments after SCRAM, even if the helium cooling system fails completely. In the worst case, they claim, the core temperature will never rise above 11000C. This claim checks out, based on rough calculations using as data the amount of graphite present and the heat release graph (Fig. 1). Uranium oxide melts at 25000C.
In the PBMR the only sealed containment is the microsphere shell. The risk of structural failure of the microsphere in production and during normal use, is estimated optimistically at 1 in 15,000 and conservatively at 1 in 300. If the microsphere structure fails, there is no backup containment. Estimates such as these are subject to the unknown effects of long term exposure of the structural materials, such as silicon carbide, to neutrons. (Only after years of operation, was it found that steel in and around the core of conventional nuclear plants loses some of its structural strength and becomes brittle on long-term exposure to neutrons.)
There are also up and down sides to the cycling of pebbles from the drain pipe in the bottom of the core. From the pipeline, the pebbles are fed to a chamber outside the core where the heat from the fission fragments is used to pre-heat the helium before it enters the core. The up side of this feature is that some of the radioactive fission products are used up during the 3 month stay in the pre-heating chamber. At that point the pebbles that still contain significant amounts of unused fissionable uranium are returned to the core. Those whose fuel is depleted are placed in temporary storage, usually under water, as high level nuclear waste. Because the pebbles in the reactor are continually "refreshed" in this manner, the total cumulative energy produced in the core after SCRAM (as a per cent of full capacity) is never quite as large as it would be in a conventional reactor. One down side is what dogged the German project, leading to its shutdown: the pebbles tend to jam the pipelines.
Among other technical difficulties in pebble bed trials are the problems inherent in graphite moderator, and the considerable problems common to HTGR's that have caused HTGR cooling to be largely abandoned. The nature of these problems, and the likelihood of being able to surmount them is not the subject here.
Those who are working on this technology hope to find a way of keeping the advantages of the pebble concept while reducing the risk of malfunction. Searching to find a better and safer reactor, they are limited by the need to be economically competitive with other sources of energy.
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