Graphite as moderator

The Hanford N Reactor. The Hanford N reactor was built in 1964 for purposes of plutonium production during the Cold War. It used graphite as moderator, pierced by over 1000 Zircaloy 2 tubes. These pressure tubes contained slightly enriched uranium fuel cooled by high temperature light water. The reactor also provided 800 MWe to the Washington PubHc Power Supply System. This reactor was shut down in 1992 because of age and concern for safety. The similarity to the Chemobyl-type reactors played a role in the decision. 

In nuclear technology, graphite is used as a moderator to slow down the neutrons released during fission. This is necessary for continuous fission to be maintained. The Chernobyl accident happened with a reactor using graphite as moderator and ordinary water as coolant This combination caused instability. Better stability is achieved in reactors using a graphite moderator and pressurized COj as the coolant However, the most common reactor type nowadays uses pressurized water as both moderator and coolant. 

HTGR uses helium as coolant and graphite as moderator as well as stmctural material. A single-zone core design was adopted, in which the spherical fuel elements are placed. The cylindrical active reactor core has an outer diameter of 3.0 m and effective height of 11.0 m. The effective core volume is 77.8 m. In the equUibrium core, the reactor core contains 420,000 fuel elements. 

Worldwide reactors continued to be built until the accident at Chernobyl occurred. Several features made the Chernobyl accident unique to a Soviet style reactor. One was the use of graphite as a moderator, which caught fire. Another was the absence of water to contain radioactivity. But, the most important may have been an inadequate containment structure. There were also problems in controlling the stability of the reactor and the control rods had to be changed frequently in order to keep the reactor stable. 

Choice of neutron moderator-light water (protium), heavy water (deuterium), liquid (sodium metal), or solid (graphite) Except in the case of graphite, the moderator also serves as the coolant. In the case of a graphite moderator, gas (He or C02) is used as a coolant. 

In recent years hea w-ater has been used in the field of nuclear chemistry. It is mentioned in the Smyth Report (see Chap. 33) that heavy water can be used instead of graphite as the moderator in a uranium pile. The function of the moderator is to reduce the speed of the fast neutrons emitted when nuclei undergo fission. The Canadian pile at Chalk River is a heavy-water pile. 

The most common moderators are substances of low atomic weight such as heavy water (deuterium oxide) or graphite. Hydrides (binary compounds corrtaining hydrogen), hydrocarborrs, and beryllium and beryllium oxide have also been used as moderators in certain specialized kinds of reactors. 

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