A scram is a rapid plant shutdown through the insertion of all control rods
- A automatic scram is the type of scram which is triggered by automatically detecting various plant conditions (or even invalid signal).
- A manual scram means that the control rod insertion was trigger by a plant operators.
SCRAM is an acronym for Safety Control Rod Ax Man. Scram refers to the automatic rapid insertion of control rods in response to the detection of undesirable conditions.
Corrosion product spiking in the reactor coolant during the reactor shutdown has been observed in practically all operating boiling water reactors (BWRs or PWRs). However, the spiking magnitude and the total released activities may vary from reactor to reactor, depending on the activity inventory and the characteristics of the deposit on the fuel cladding surfaces. The corrosion product deposit on the fuel cladding surfaces is known to have tow distinguishable layers: the inner tenacious layer and the loosely attached outer layer. The characteristics of the deposit depend largely on the thickness of the deposit and the metallic elemental composition in the deposit (or metallic impurity level in reactor water). There are at least three major causes for the release of the corrosion product deposit from the fuel cladding surfaces during power transients:
- (1) Mechanical and/or hydraulic disturbance.
- (2) Coolant chemistry change caused by temperature and radiation field changes.
- (3) Decreasing temperature which increases the solubilities of some corrosion product oxides in the deposit.
It is obvious that immediately after a reactor shutdown or scram, the crud burst occurs due to mechanical and/or hydraulic disturbance in the core. A portion of the outer layer (made up of fission and corrosion by-products) is believed to be easily shaken off by the mechanical disturbance. Smaller crud spikes can also continue for some time after the reactor shutdown. Such small spikes may be attributed to continued water boiling at lower pressure and/or water chemistry upset at lower temperatures. Since the crud deposition on cladding surfaces may be partially due to a weak interaction between the charged particles and the stationary surface, when a slight change in water chemistry occurs, the weak attraction force may be broken and particles released from the surface.
The corrosion product spiking is not limited to the release of insoluble crud. The spiking of soluble species can sometimes account for the majority of the released activities. The decrease in the reactor water temperature is probably responsible for the release of soluble species, which is normally observed long after the first crud spike and the temperature is about 200°C. Based on the known solubilities of many corrosion product oxides (or mixed oxides) in water, it is predictable that most of the activities in the fuel deposit should become more soluble at lower temperatures. However, there are two factors (thermodynamics and kinetics) that control the dissolution rate of the corrosion product oxide during the cooling down process. At lower temperatures, the solubility is high but the dissolution process may be fast enough but equilibrium is in favor of the insoluble form.
The name Scram isn't an acronym. It is simply a colorful slang term for "run-away".
The name comes from the first nuclear reactor (called a pile, because it was made up of a large square pile of graphite blocks with uranium tubes running through it), which was under the stands at the University of Chicago football stadium. After the reactor went critical (sustained a nuclear chain reaction), if the chain reaction could not be controlled, then everyone was to "scram" (run away) except for a man with an ax, who was to cut through a rope that would allow a boron containing safety control rod to drop into the pile. The control rod would stop the nuclear chain reactor, i.e., shut down the shutdown the reactor. The SCRAM has since been used to mean an automatic or rapid shutdown of a nuclear reactor. <ref>http://public-blog.nrc-gateway.gov/2011/05/17/putting-the-axe-to-the-scram-myth/</ref>
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