Document: NUREG-0800
Document ID: 7ed30441-8f7a-44a4-b42a-6bbe8c5bbff4
Document Type: srp
Title: UNCONTROLLED CONTROL ROD ASSEMBLY WITHDRAWAL AT POWER
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0520/ML052070712.pdf
Revision Date: 2023-06
Chapter: 15
Section ID: 15.4.2
CFR Part: 
CFR Title: 

Content:
culation. For a PWR, this is one or two control banks; for a BWR with current modes of control, it is a single control rod (future modifications under consideration may change this to group movement). The review covers a full range of rod or bank withdrawals, up to maximum rod or bank worths and rates of reactivity addition. 15.4.2-5 DRAFT Rev. 3 - April 1996 The exact analysis of the transientAOO would ideally involve a three-dimensional, coupled neutron kinetics, thermal-hydraulics calculation. However, acceptable results may be obtained with suitable approximate calculations. The problem examined and the approximations used differ for a PWR and a BWR. 2. For a BWR, past analyses and reviews have shown that at maximum rod worths and rates of reactivity addition, the reactor power increases slowly and the total increase is relatively small, so that the transientAOO may be approximated by steady-state analyses. Because of changes in local power distribution attributable to rod motion and strong void feedback effects on the power distribution, three-dimensional, steady-state, coupled neutron distribution, thermal-hydraulics calculations that take account of these effects are required. The transientAOO is halted by action of a rod block system, which should block rod withdrawal before fuel safety limits are reached. The review process for a BWR, while recognizing the inherent transientAOO nature of the problem, is concentrated on the steady-state aspects of the transientAOO to assureensure that initial and subsequent power distributions are maximized, that the 29 reactor conditions produce minimum critical power ratio, CPR, and that the response of the rod block system is conservatively calculated considering minimum operation of the associated local power range monitoring system. 3. A PWR analysis, on the other hand, generally involves larger power changes and requires transientAOO calculations. Because power distributions in the course of the transientAOO can