Why are nuclear reactors not invulnerable to loss of coolant accidents?
A previous post, Decisions-1, asked whether or not the power industry anywhere should use the nuclear option to generate power, thus freeing that country from coal or petroleum dependency. Shown (right) is the first commercial reactor station to generate power in the U.S.
PWR means pressurized water reactor,
BWR means boiling water reactor.
Virtually all operating U.S. reactors are either PWR or BWR types; PWR’s account for about 2/3 of the units.
The Fukushima Daiichi plant used a first generation BWR design
How Reactors Work
Decisions-1 brought u the two top-most issues that are associated with nuclear reactors, loss of coolant for the active reactor core, and safe storage of spent fuel rods.
In this post, we discuss accidental loss of coolant. The image opposite is the reactor core from the Swiss KKG plant, shown in Decisions-1. We are looking down, into the top of the pressure containment vessel to the top of the core, the grid-like structure in the center of the “bulls-eye.” The reactor core is seen deeply below the surface of the water used as coolant for the very hot uranium fuel. At least part of the visible blue illumination is generated by radiation resulting from the fissioning of the fuel. The discussion of coolant requires a bit of understanding of how reactors work.
The figure is a very top level diagram of a boiling water reactor.
- The rounded rectangle is the primary containment chamber.
- The reactor core is the block with the fuel rods shown in red. It has 3 functions: (a) it is a rack to hold fuel rods, long thin zirconium pipes (in most PWR and BWR reactors) filled with pellets of fissionable uranium oxide, (b) it provides space for the control rods to be inserted when the reaction is to be slowed or stopped, (c) it provides flow space about the rods to give access to the coolant.
- The control rods in a BWR normally are in the chamber base and are driven vertically to enter the core. Purified water comes into the chamber from the side, flows through the core and becomes high temperature steam. The steam drives turbines which drive the electric generators, which add power to the grid.
Reactor coolant water becomes somewhat radioactive and is kept isolated. It is chilled by a secondary heat exchanger, which uses purified water that loops through an external cooling source: radiators at the base of huge cooling towers, or a natural source of water – a lake, river, or ocean. This final cooler must dissipate a lot of heat energy because inefficient U.S. reactors generate a lot of waste heat. The cooled water returns to the chamber and starts the cycle again.
In a PWR type such as the original Shippingport reactor, the water coolant for the core is kept in an isolated loop at high pressure and stays liquid; it never turns to steam. An additional heat exchanger is required to heat clean water to steam and drive the turbines. The control rods are inserted from the top and in an emergency could naturally fall into the core and turn off the reaction. A PWR plant brochure here.
The LOCA issue – induced core melt down
If for any reason the coolant fails to reach the core (an earthquake-induced crack in the pressure vessel, for example) a runaway condition may develop, the core heats up, the rod sheath can catch fire, the from fission processes would stay in the core and material components will melt. It is hard to envision any material barrier against molten reactor fuel; this is the worst case scenario in any failure mode analysis
LOCA in most reactor types would be considered a disaster – the PWR, BWR, liquid metal cooled, the Soviet implemented forms such as Chernobyl’s RBMK type, all are susceptible. Not a nuclear critic’s phantom, the issue has been demonstrated by all incidents in the Decision-1 post; it is truly frightening.
This should not even be on the agenda!
Core melt-down issues were solved more than 50 years ago.
Really? So why do problems still occur? Short history: President Eisenhower launched his Atoms for Peace initiative in 1953 and the first power generating station came on-line in 1957 at Shipipingport PA. This was a rush job. Its PWR reactor design was similar to what the Navy chose for its small size.
This was followed by a mad rush to cash in on the dollar flow to build real atomic energy power plants – clean, non-polluting, safe power to assure comfort for ourselves and our children.
The core of the problem: Every U.S.-made reactor, up to the last one built, was experimental, a special (i.e. big) modification of one of the designs. As such, each is temperamental, each has/had its own distinct problem set with unique special ways to dealing with them. … This is the message I got from an engineer at the SEFOR Fast Breeder Reactor in Arkansas, when I was considering various professions in physics.
The point: No two units were ever made the same way. This means the set of U.S. reactors pose a threat because standard components are only scattered about here and there. Ok, this may overstate the point a bit to drive it home. But, of our 100+ reactors, each with 100+ special handling techniques, knowledge of a single unit will not necessarily help much when a similar event occurs elsewhere.
There are two parts to solving this LOCA issue:
(A) Intrinsic Safety – build reactors that cannot melt even under LOCA conditions,
(B) Continuous Improvement – build each new unit using small steps in improvement over the previous build. The first unit may be based on the best design possible, but we must be ready to make evolutionary improvements with new builds – not revolutionary jumps.
Small changes, not large jumps is one of the LastTechAge basic themes.
TRIGA (Training, Research, Isotopes, General Atomic) was the first basically safe reactor sold commercially. This is an engineering research tool, not a power generator, and it is meant to be operated by University graduate students, even undergraduates. The first unit was shipped to a university from General Atomic (San Diego, CA) in 1958 and some 66 units from 200 kW to 10 MW have been sold world-wide. The design has continuously evolved as a highly competent, ever-safer reactor type. Continuous design evolution is the key factor that allows safe, trustworthy machines to be made. This is true for automobiles, aircraft, ocean liners and bridges, not just reactors. Although not truly intrinsically safe, TRIGA provides a lot of time for error correction before any safety issues arise. 
CANDU (CANada Deuterium Uranium) is a Canadian power reactor that uses heavy water (heavy deuterium rather than hydrogen in the “H20″ formula) and good design to provide safety. It generated first commercial power in the early 1960’s; sales stand at 32 reactors plus 13 CANDU-types in India. Its unique design (heavy water moderator, horizontal fuel rod, special pressurization scheme) make the reactor a highly versatile power generator, one of the best designs in the marketplace. Most importantly (for this discussion) each is built on the lessons of its predecessors. Note that a CANDU unit is not quite intrinsically safe, but its design provides a lot of time for error correction before any safety issues arise.
HTGR (High Temperature Gas Cooled Reactor) is truly an intrinsically safe reactor design. It is a successful helium gas cooled reactor design that operates with coolant about 800º C, giving the device a nearly 50% thermodynamic efficiency. Coolant leaks in a usual reactor pose the threat of radioactive water in the environment; helium cannot not become radioactive, helium leakage is not an environmental safety issue for an HTGR. Helium also confers many engineering bonuses.
If, for any reason, a LOCA event occurs, the temperature at the core will rise, reducing the reaction rate until a balance point is reached between these two. This is called negative feedback, a term referring to natural processes that dampens the reactor rather than accelerates it. The unit will reach a high but designed-safe temperature and just sit there. Nothing melts. Nothing explodes. The LOCA response is passive – it requires no outside intervention. LOCA was tested and confirmed at a German HTGR of slightly different build.
The modern incarnations of the HTGR could be good concepts; they uses a cluster of modules each of which drives the power turbine directly with the hot helium of the reactor. Called the GT-MHR (gas turbine modular helium reactor), or PB-MHR (for pebble bed etc.), they promise the highest efficiency, safest and simplest design of the competing types.
The company (formally known as) General Atomic made two HTGR units, a small 48 MWe at Peach Bottom near Lancaster Pennsylvania (1967-1974) and a much larger one at Fort St. Vrain (FSV) in Colorado (1977-1989). FSV tried new concepts throughout the entire device. (US practice: each reactor was a major jump in characteristics from predecessors…)
Peach Bottom 1 worked well up to its last day. FSV history is a great poster ad for small-step prototype development. There were no nuclear accidents but the manufacturer discovered “interesting” (but unfortunately unexpected) properties associated with hot helium and both the maker and the power company operator discovered that peripheral issues can destroy even a good idea. In the end, FSV became functional and fully operational, but the Power Company decided it had had enough and closed the plant. (GA withdrew from reactor sales in 1975.)
Why is LOCA yet an issue?
The American experience has been to make of one-of-a-type reactors; this generated a track record of a multitude of near misses, and some not so much “misses” as “caught and contained—this time.”
Why are reactor cores still melting down? The two big builders, Westinghouse and GE, and all the others were bent on BWR, PWR and liquid metal reactors which are intrinsically dangerous.
Was it insatiable lust for personal/family wealth, as some have suggested? Or, was it hubris, as I believe? Here, having hubris means to ignore the essential chaotic nature of natural events. Nothing bad has happened so nothing will. Time between failures was estimated as many thousands of reactor-years. Japanese experts were certain of this. But the base-line truth was that almost every reported major incident has had partial or full core melt-downs.
U.S. nuclear industry operated in its own dream cloud from the 1953 “Atoms for Peace” initiative up until 1979, when the 3 Mile Island excursion permanently ended reactor construction. If the big builders had been able to get development support from the government, might they have built intrinsically safe plants?
LOCA should not be a show stopping issue for reactor builds. The issue causing core melts is human nature, not fundamental technology. There are examples of good practice with well thought-through machines that show that we could have made reliable, safe power generators, if the makers had so wished. We cannot power our society with forest wood or whale oil. We do need some kind high energy density, high power source to keep our cities lit. Let our choice be in the right direction.
Charles J. Armentrout, Ann Arbor
2011 Apr 20, Minor additions, 2011 Jun 03
Listed under Technology … thread Technology > Fission
Have a comment? Click on the title of this post, go to bottom, let us know.
Related posts: Click the INDEX button under the Banner picture
 Personal disclosure: I was a physicist at General Atomic in the 1970’s and 1980’s, working on the Doublet III tokamak fusion test bed. I moved on about the time the “s” was added to the name, the first time in history the adjective atomic (as with energy) was made plural. I worked in plasma fusion physics; I was never involved with fission reactors. Back to Intrinsic Safety