Water vs. Gas Cooled Reactors

Round 1

In the period from 1966 to 1964, there were two basic reactor choices being offered for commercial electric power production. American companies were offering reactors that used ordinary water under pressure as the reactor coolant. British and French companies were offering reactors using pressurized CO2 gas as the reactor coolant. There were substantial technical differences between the two which made comparisons difficult for most customers.

In fact, the technological differences were large enough that the question "Which is the best reactor," had to be answered with "It depends on what you are looking for."

In previous issues of AEI we have discussed the theoretical advantages and disadvantages of gas cooled and light water reactors. (See "Water: Best Choice for the 1950s Subs" and "CO2: First Choice for Power Reactors")

Of course, customers do not buy products because of their theoretical advantages. They purchase them based on actual performance and their ability to meet certain specified requirements. Specifically, power plant customers care about steam conditions, construction schedules, proven performance, total construction cost, operating cost and disposal cost. In order to understand the purchase decisions that were made in the first round of reactor orders, one must understand the state of the art.

On the basis of steam conditions there was little difference between the American light water reactor and the European gas cooled reactors. Both produced saturated steam at approximately the same temperature and pressure. For gas cooled and pressurized water reactors, the steam systems were separate, non-radioactive systems, a feature that was a good selling point to customers concerned about the unknown dangers of radioactive contamination. Compared to the superheated steam available from state of the art fossil plants, all first generation nuclear systems were at a disadvantage.

There were gas reactor supporters who pointed out that their reactors would eventually provide better steam conditions as material knowledge improved and as inert gas coolants like helium became more available.

Water cooled reactors, in contrast, were already operating fairly close to their maximum temperature based on the triple point of water. This potential advantage may have helped to convince planners to provide ongoing support for gas reactor cooled research and development, but it did nothing to encourage actual sales.

In the case of the Magnox reactors,a combination of low maximum fuel temperatures and lower coolant heat transfer capability led to the need for reactors that were several times larger than a water reactor with the same power output.

Maxnox reactors required the painstaking construction fo large, high purity graphite structures with tight tolerances. They also required the construction of ver large, high quality pressure vessels. These structures were too large to transport, so they had to be built on site.

Because of these limits, gas cooled reactor economics depended in large measure on the quality and cost of the available local labor force and the quality, purity and cost of graphite available in the area of proposed construction.

The cost of training a specialized construction work force in a new market location could not readily be spread over a large number of total worldwide units.

Water cooled reactors imposed different constraints. Their reactor internals were carefully manufactured components with tight tolerances, but they were small enough to produce in a factory for later transport to the reactor site. The pressure vessel that enclosed the reactor internals was a challenging component that required a large investment in specialized manufacturing equipment, but the final product was small enough to be transported provided there were rail or water routes available.

Because of these characteristics, water cooled reactor economics were more dependent on the unit volume of reactor sales. The high investment cost for production facilities and for training qualified workers would become less important if spread out over more units. This relationship encouraged the reactor manufacturers to work hard to close a sufficient number of deals to make their infrastructure investment pay off.

There appeared to be little substantial difference between most of the operating costs of the two types of facilities. The major operating cost variable between the two was fuel cost. In this area, there were complex and changing relationships that determined which plant had an advantage.

The fuel material used in the Magnox reactors was natural uranium metal clad with Magnox alloy. Initially, the maximum burn-up obtainable from this material was about 3000 MWdays per ton of heavy metal, but that improved to about 6000 MW days per ton fairly quickly. In 1960, the cost per kilogram of uranium with natural enrichment was about $18.00.

The fuel for the light water reactors was uranium oxide with a U-235 concentration of 3 percent (about 4 times higher than found in natural uranium) clad with either stainless steel or zirconium alloy. At first, the maximum burn-up for this fuel was about 5000 MW days per ton, but it improved to about 25,000 MW days per ton within a few years. In 1962, the cost per kilogram of 3 percent enriched uranium hexafluoride (the direct product of the enrichment plants) was listed by the AEC as $254.00.

Based on the differences in materials and fuel design, it also cost a little less to fabricate the natural uranium fuels than it did to fabricate those based on enriched uranium. A representative price was about $75.00 per kilogram of uranium for the enriched cores and about $50.00 per kilogram for the natural cores. (Glasstone, Nuclear Reactor Engineering, 1967 p. 783)

The computation of fuel cost differences were complicated by the fact that several nations, including the U. S., had established policies to purchase fissile materials, including plutonium, that were left over after the fuel had completed its design life in the reactor. This price varied based on the ratio of Pu-239 to Pu-240. It was a significant enough item in the fuel price calculations to encourage some developers to purposely limit burn-up because of the increased value of the discharged fuel.

In order for a purchaser to evaluate the fuel cost differences, he needed to know the expected burn-up, the relative cost of enriched uranium versus natural uranium and the interest rate likely to be charged on the fuel purchase. The decision was based on a significant number of assumptions and unknown costs. Without subsidies, it appeared likely that natural uranium reactor fuel would cost less, but even that depended on the market price of natural uranium at the time the reactor was built.

The natural uranium reactors produced a larger volume of high and medium level waste because of their larger reactors and their lower burn-up fuel material. This tended to raise the cost estimates for decommissioning those reactors. This factor could be made less important by assuming a longer plant life in the cost calculations. This assumption was technically feasible based on the lower stresses and lower neutron irradiation impacting on the pressure vessel. The waste volume could also be reduced by fuel material and moderator recycling.

Large pressurized water reactors, on the other hand, have a significant cost disadvantage compared to gas cooled reactors; their pressure vessels are more highly contaminated and normally will need to be cut up before being transported for disposal. The barges and rail lines that delivered the vessel were frequently at their capacity limits in moving an empty vessel, there is little space or weigh capacity left for adding the shielding.

Gas cooled vessels will also have to be dismantled, but it is far easier to cut a steel wall that is less than one third the thickness of the light water reactor vessels.

Technically, there was no clear winner in the early battle to set the standard for large nuclear electric power stations. Each design had clear advantages and disadvantages. As is often the case in the battle to set a standard, the final outcome was based on a bit of luck, a lot of marketing effort, and substantial help from development partners, including government agencies. As one customer of the era noted "The best one, of course, is the one you donÕt have to pay for!" (Homi J. Bhabha, chairman of the Indian Atomic Energy Commission.)

It should be noted, however, that no technology standard lasts forever, particularly if there are significant competitive disadvantages of the standard compared to new challengers. As we have said several times before, Adams Atomic Engines, Inc. believes that gas cooled reactors using technology developed since the first reactor battle took place have significant technical and cost advantages in certain applications over light water reactors.