{"id":18368,"date":"2018-08-14T18:12:18","date_gmt":"2018-08-14T18:12:18","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=18368"},"modified":"2023-02-01T07:31:00","modified_gmt":"2023-02-01T07:31:00","slug":"criticality-approach-startup-to-minimum-load","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/reactor-operation\/reactor-startup\/criticality-approach-startup-to-minimum-load\/","title":{"rendered":"Criticality Approach \u2013 Startup to Minimum Load"},"content":{"rendered":"<p>The process of reaching <a title=\"Reactor Criticality\" href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/nuclear-fission-chain-reaction\/reactor-criticality\/\">criticality<\/a> (i.e., k<sub>eff<\/sub> = 1) is known as the <strong>criticality approach<\/strong> or <strong>startup to minimum load<\/strong>. Prior to this process, the boron concentration is usually sufficient (after refueling) to yield ~10% shutdown margin (i.e., k<sub>eff<\/sub> = 0,90), the <a title=\"Control Rods\" href=\"http:\/\/sitepourvtc.com\/nuclear-power-plant\/control-rods\/\">control rods<\/a> are fully inserted into the core. If allowed, the <a title=\"Boric Acid \u2013 Chemical Shim\" href=\"http:\/\/sitepourvtc.com\/glossary\/boron-10\/boric-acid-chemical-shim\/\">boron concentration<\/a> can be adjusted to the required value prior to startup.<\/p>\n<p>For the criticality approach, parameters known as <strong>estimated critical conditions<\/strong> must be known. <strong>Estimated critical conditions<\/strong> are precalculated for each fuel loading pattern and usually consist from:<\/p>\n<ul>\n<li>Estimated critical rod position (e.g., the last control rod bank at 90% of the core height)<\/li>\n<li>Estimated critical boron concentration (e.g., c<sub>B<\/sub> = 9 g\/kg = 1573 ppm)<\/li>\n<li>Estimated critical core inlet temperature (e.g., T<sub>in<\/sub> = 275\u00b0C)<\/li>\n<\/ul>\n<figure id=\"attachment_18153\" aria-describedby=\"caption-attachment-18153\" style=\"width: 290px\" class=\"wp-caption alignright\"><a href=\"http:\/\/sitepourvtc.com\/wp-content\/uploads\/2017\/07\/Subcritical-Multiplication-chart_2.png\"><img loading=\"lazy\" class=\"size-medium wp-image-18153\" src=\"http:\/\/sitepourvtc.com\/wp-content\/uploads\/2017\/07\/Subcritical-Multiplication-chart_2-300x213.png\" alt=\"Subcritical Multiplication\" width=\"300\" height=\"213\" srcset=\"https:\/\/sitepourvtc.com\/wp-content\/uploads\/2017\/07\/Subcritical-Multiplication-chart_2-300x214.png 300w, https:\/\/sitepourvtc.com\/wp-content\/uploads\/2017\/07\/Subcritical-Multiplication-chart_2.png 541w\" sizes=\"(max-width: 300px) 100vw, 300px\" \/><\/a><figcaption id=\"caption-attachment-18153\" class=\"wp-caption-text\">Subcritical Multiplication<\/figcaption><\/figure>\n<p>These three parameters together specifies the estimated critical state. The control rods must satisfy the requirement known as the rod insertion limit, and the rod insertion limit ensures shutdown margin requirements. There are many ways how to reach a critical state, but usually, the following two procedures are used:<\/p>\n<ul>\n<li><strong>Approach by control rods withdrawal<\/strong>. When the existing boron concentration is near the estimated critical, the approach can be performed only by control rods withdrawal. Sometimes the boron concentration can be adjusted to the required value prior to startup, and then this procedure can be used. The control rods are withdrawn from the core in successive steps. The shutdown rod banks are withdrawn in sequence, and then the control banks are withdrawn manually to achieve criticality. Following each step, the neutron count rate is registered. Experimentally the neutron count rate is measured by source range neutron detectors (usually three in the 120\u00b0 symmetry) located at different positions outside the reactor. The neutron level following each addition of reactivity should be allowed to stabilize to obtain an accurate indication of asymptotic multiplication before proceeding with the next reactivity addition. This stabilization time becomes progressively larger as criticality is approached.<\/li>\n<li><strong>Approach by boron dilution<\/strong>. When the existing boron concentration is far from the estimated critical, and the adjustment is not allowed, the critical state must be approached by boron dilution. Prior to boron dilution, the shutdown and control rods are withdrawn to estimated critical condition (e.g., all at ARO position except the last control rod bank, which is withdrawn to 90% of the core height). Then the boron concentration can be lowered by introducing condensate into the reactor coolant system. This process is not performed by steps. Therefore, there is no stabilization to obtain an accurate indication of asymptotic multiplication. Also, in this case, the neutron count rate is measured by source range neutron detectors.<\/li>\n<\/ul>\n<p>See also: <a title=\"Subcritical Multiplication\" href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/reactor-dynamics\/subcritical-multiplication\/\">Subcritical Multiplication<\/a><\/p>\n<p>Evidence of criticality is indicated by a positive, sustained <a title=\"Startup Rate \u2013 SUR\" href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/nuclear-fission-chain-reaction\/reactivity\/startup-rate-sur\/\">startup rate<\/a> or by <a title=\"Inverse Reactor Kinetics \u2013 Reactimeter\" href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/reactor-dynamics\/inverse-reactor-kinetics-reactimeter\/\">reactimeter<\/a> with no rod motion and increasing source range counts. After criticality is achieved, a positive startup rate is established (reactor is slightly <a title=\"Supercritical Reactor\" href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/nuclear-fission-chain-reaction\/reactor-criticality\/supercritical-reactor\/\">supercritical<\/a>), and the power level increases up to \u201c<strong>minimum load<\/strong>\u201d. This power increase is then stabilized at minimum load by control rods insertion. At minimum load, the power level does not influence the<a href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/nuclear-fission-chain-reaction\/reactor-criticality\/\"> criticality (k<sub>eff<\/sub>)<\/a> of a power reactor unless<a href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/nuclear-fission-chain-reaction\/reactivity-coefficients-reactivity-feedbacks\/\"> thermal reactivity feedbacks<\/a> act (operation of a power reactor without reactivity feedbacks is between 10E-8% \u2013 1% of rated power). The reactor behaves as a \u201c<a title=\"Zero Power Criticality vs. Power Operation\" href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/neutron-diffusion-theory\/reactor-thermal-power\/zero-power-criticality-vs-power-operation\/\"><strong>zero power reactor<\/strong><\/a>\u201d.<\/p>\n<p>It is very important to pay attention to safety instructions. Since the reactor is at low power, the reactor is far from <a title=\"Reactivity Coefficients \u2013 Reactivity Feedbacks\" href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/nuclear-fission-chain-reaction\/reactivity-coefficients-reactivity-feedbacks\/\">temperature <\/a><a href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/nuclear-fission-chain-reaction\/reactivity-coefficients-reactivity-feedbacks\/\">feedback<\/a>, improving\u00a0<a title=\"Reactor Stability\" href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/reactor-dynamics\/reactor-stability\/\">reactor stability<\/a>. There is a possibility that automatic shutdown, which can be activated by a reactor protection system in case too much reactivity is inserted.<\/p>\n<p>The inherent power stability is ineffective below the point known as the \u201cpoint of adding heat\u201d. Suppose a power excursion is initiated from a very low power level. In that case, power will continue to rise unchecked until the point of adding heat is reached, and the subsequent temperature rise adds negative reactivity to slow the rise of reactor power.<\/p>\n<p>See also: <a title=\"1\/M Plot\" href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/reactor-dynamics\/subcritical-multiplication\/1m-plot\/\">1\/M Plot<\/a><\/p>\n<p>See also: <a title=\"Source Neutrons and External Source of Neutrons\" href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/reactor-dynamics\/subcritical-multiplication\/source-neutrons-and-external-source-of-neutrons\/\">Source Neutrons<\/a><\/p>\n<div   id="8n9s8rpp6h"     class=\"lgc-column lgc-grid-parent lgc-grid-100 lgc-tablet-grid-100 lgc-mobile-grid-100 lgc-equal-heights \"><div   id="8n9s8rpp6h"     class=\"inside-grid-column\">\u00a0 <div   id="8n9s8rpp6h"    class=\"su-accordion su-u-trim\"><div   id="8n9s8rpp6h"    class=\"su-spoiler su-spoiler-style-default su-spoiler-icon-plus su-spoiler-closed\" data-scroll-offset=\"0\"><div   id="8n9s8rpp6h"    class=\"su-spoiler-title\" tabindex=\"0\" role=\"button\"><span class=\"su-spoiler-icon\"><\/span>References:<\/div><div   id="8n9s8rpp6h"    class=\"su-spoiler-content su-u-clearfix su-u-trim\"><strong>Nuclear and Reactor Physics:<\/strong>\n<ol>\n<li>J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading,\u00a0MA (1983).<\/li>\n<li>J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.<\/li>\n<li>W. M. Stacey, Nuclear Reactor Physics, John Wiley &amp; Sons, 2001, ISBN: 0- 471-39127-1.<\/li>\n<li>Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering,\u00a0Springer; 4th edition, 1994, ISBN:\u00a0978-0412985317<\/li>\n<li>W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467<\/li>\n<li>G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965<\/li>\n<li>Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.<\/li>\n<li>U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.<\/li>\n<\/ol>\n<p><strong><\/strong><strong>Advanced Reactor Physics:<\/strong><\/p>\n<ol>\n<li>K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.<\/li>\n<li>K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.<\/li>\n<li><span style=\"font-weight: 400;\">D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2.\u00a0<\/span><\/li>\n<li>E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.<\/li>\n<\/ol>\n<\/div><\/div><\/div><div   id="8n9s8rpp6h"    class=\"su-divider su-divider-style-dotted\" style=\"margin:15px 0;border-width:2px;border-color:#999999\"><\/div><\/div><\/div><div   id="8n9s8rpp6h"    class=\"su-divider su-divider-style-default\" style=\"margin:15px 0;border-width:2px;border-color:#999999\"><\/div><div   id="8n9s8rpp6h"     class=\"lgc-column lgc-grid-parent lgc-grid-33 lgc-tablet-grid-33 lgc-mobile-grid-100 lgc-equal-heights \"><div   id="8n9s8rpp6h"     class=\"inside-grid-column\"><\/div><\/div><div   id="8n9s8rpp6h"     class=\"lgc-column lgc-grid-parent lgc-grid-33 lgc-tablet-grid-33 lgc-mobile-grid-100 lgc-equal-heights \"><div   id="8n9s8rpp6h"     class=\"inside-grid-column\">\n<h2>See above:<\/h2>\n<p>Reactor Startup<a href=\"http:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/reactor-operation\/reactor-startup\/\" class=\"su-button su-button-style-flat\" style=\"color:#606060;background-color:#ffffff;border-color:#cccccc;border-radius:10px;-moz-border-radius:10px;-webkit-border-radius:10px\" target=\"_self\"><span style=\"color:#606060;padding:7px 20px;font-size:16px;line-height:24px;border-color:#ffffff;border-radius:10px;-moz-border-radius:10px;-webkit-border-radius:10px;text-shadow:0px 0px 0px #000000;-moz-text-shadow:0px 0px 0px #000000;-webkit-text-shadow:0px 0px 0px #000000\"><i class=\"sui sui-link\" style=\"font-size:16px;color:#5d5d5d\"><\/i> <\/span><\/a><\/p><\/div><\/div><div   id="8n9s8rpp6h"     class=\"lgc-column lgc-grid-parent lgc-grid-33 lgc-tablet-grid-33 lgc-mobile-grid-100 lgc-equal-heights \"><div   id="8n9s8rpp6h"     class=\"inside-grid-column\"><\/div><\/div>\n","protected":false},"excerpt":{"rendered":"<p>The process of reaching criticality (i.e., keff = 1) is known as the criticality approach or startup to minimum load. Prior to this process, the boron concentration is usually sufficient (after refueling) to yield ~10% shutdown margin (i.e., keff = 0,90), the control rods are fully inserted into the core. If allowed, the boron concentration &#8230; <a title=\"Criticality Approach \u2013 Startup to Minimum Load\" class=\"read-more\" href=\"https:\/\/sitepourvtc.com\/nuclear-power\/reactor-physics\/reactor-operation\/reactor-startup\/criticality-approach-startup-to-minimum-load\/\" aria-label=\"More on Criticality Approach \u2013 Startup to Minimum Load\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":18316,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"generate_page_header":""},"_links":{"self":[{"href":"https:\/\/sitepourvtc.com\/wp-json\/wp\/v2\/pages\/18368"}],"collection":[{"href":"https:\/\/sitepourvtc.com\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/sitepourvtc.com\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/sitepourvtc.com\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/sitepourvtc.com\/wp-json\/wp\/v2\/comments?post=18368"}],"version-history":[{"count":3,"href":"https:\/\/sitepourvtc.com\/wp-json\/wp\/v2\/pages\/18368\/revisions"}],"predecessor-version":[{"id":37002,"href":"https:\/\/sitepourvtc.com\/wp-json\/wp\/v2\/pages\/18368\/revisions\/37002"}],"up":[{"embeddable":true,"href":"https:\/\/sitepourvtc.com\/wp-json\/wp\/v2\/pages\/18316"}],"wp:attachment":[{"href":"https:\/\/sitepourvtc.com\/wp-json\/wp\/v2\/media?parent=18368"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}