Though water was chosen partially because it was a familiar fluid for power engineers, the choice almost doomed the nuclear submarine program. Laboratory testing of the pumps, bearings, valves, and piping demonstrated to Rickover’s team that the simple, familiar fluid was not so simple at high temperatures and pressures, particularly when exposed to neutron radiation. There were a number of unexpected reactions that make water almost unworkable as a reactor coolant.
Seized bearings
In the first round of testing of the pumps and valves selected for use in the pressurized water reactor, several pump bearings froze and valves became stuck after just a short period of time. The failures were caused by the formation of an unusually sticky corrosion film which did not appear in tests at standard water temperatures.
Of course, rapid failures in pumps and valves would have a negative impact on reactor plant safety and operating costs.
A dedicated effort by the team’s chemistry and materials specialists alleviated the problem by adding corrosion inhibitors to the water coolant. The control of reactor plant chemistry continues to be very important consideration in ensuring long term reactor operation and safety.
Crud
Before full scale operation of the land based prototype for the Nautilus, the design team did all they could to test various components in conditions closely matched to those expected in the reactor. One test that was considered to be of key importance was the full scale testing of a fuel element in an operating nuclear reactor.
Most of the materials tests were carried out in a hot water environment with electrical heaters as the means of producing hot water. However, it was known that neutron radiation decomposed water into its constituent hydrogen and oxygen and that this would complicate the chemistry within the reactor.
There was not a water cooled test reactor in the United States large enough to accommodate a full sized fuel element for materials testing. Rickover solved the problem by getting help from the Canadians, who agreed to test a fuel element in their NRX test reactor at their Chalk River facility. Rickover conducted the needed test despite the fact that the Atomic Energy Act of 1946 made it illegal to ship enriched uranium outside of the United States and made it illegal to share atomic information with foreign nationals.
Characteristically, Rickover told Congress what he had done, why he had done it and why they should change their law. As a measure of the respect that he had earned, Congress did exactly as he suggested.
The NRX test revealed that the fuel element became covered with a significant amount of a black deposit that came to be known as CRUD, an acronym for Chalk River Unidentified Deposit. The deposit was troubling because it threatened the ability of the coolant to remove fission heat, it gave a potential for blockage of coolant flow in the flow channels, and it produced the possibility that the crud could become radioactive by exposure to a neutron flux and later flake off, causing contamination problems in the rest of the coolant circuit.
The discovery of crud initiated an intensive effort to solve the problem or to at least minimize its impact on reactor plant operations. The effort to minimize crud formation continues to have an impact on the operation and cost of water cooled reactors.
