Editing Boiling water reactor (section)

=== Start-up ("going critical") ===
<!--Editor is unfamiliar with the details of this section – the Banked Position Withdrawal Sequence – and tried to make sense of the original text – so please correct as necessary if you understand what's going on here better than the text indicates. The original text of the section is as follows:-->
<!--"GE developed a set of rules in the '70s called BPWS (Banked Position Withdrawal Sequence) that help minimize notch worths and going critical with asymmetric patterns."-->
<!--BPWS is a set of criteria for control rod withdrawal sequences designed to minimize how much reactivity a single control rod can introduce into the core should it drop out to the bottom of the core during start up (remember BWR control rods get inserted from the bottom, and its postulated they could fall out to the bottom as well). It is important only during start up where the reactor is very reactive, and local criticality/damage can occur if a single rod controls too much reactivity and moves out of its intended place.-->

Reactor start up ([[critical mass|criticality]]) is achieved by withdrawing control rods from the core to raise core reactivity to a level where it is evident that the [[nuclear chain reaction]] is self-sustaining. This is known as "going critical". Control rod withdrawal is performed slowly, as to carefully monitor core conditions as the reactor approaches criticality. When the reactor is observed to become slightly super-critical, that is, reactor power is increasing on its own, the reactor is declared critical.

Rod motion is performed using rod drive control systems. Newer BWRs such as the [[ABWR]] and [[ESBWR]] as well as all German and Swedish BWRs use the Fine Motion Control Rod Drive system, which allows multiple rods to be controlled with very smooth motions. This allows a reactor operator to evenly increase the core's reactivity until the reactor is critical. Older BWR designs use a manual control system, which is usually limited to controlling one or four control rods at a time, and only through a series of notched positions with fixed intervals between these positions. Due to the limitations of the manual control system, it is possible while starting-up that the core can be placed into a condition where movement of a single control rod can cause a large nonlinear reactivity change, which could heat fuel elements to the point they fail (melt, ignite, weaken, etc.). As a result, GE developed a set of rules in 1977 called BPWS (Banked Position Withdrawal Sequence) which help minimize the effect of any single control rod movement and prevent fuel damage in the case of a control rod drop accident. BPWS separates control rods into four groups, A1, A2, B1, and B2. Then, either all of the A control rods or B control rods are pulled full out in a defined sequence to create a "[[checkerboard]]" pattern. Next, the opposing group (B or A) is pulled in a defined sequence to positions 02, then 04, 08, 16, and finally full out (48). By following a BPWS compliant start-up sequence, the manual control system can be used to evenly and safely raise the entire core to critical, and prevent any fuel rods from exceeding 280 cal/gm energy release during any postulated event which could potentially damage the fuel.<ref>NEDO-21231, "Banked Position Withdrawal Sequence," 
January 1977. General Electric Corporation</ref>