AMONG THE VARIOUS EFFORTS TO IMPROVE OPERATIONAL safety OF Nuclear FACILITIES, THE SYSTEMATIC COLLECTION, EVALUATION AND FEEDBACK OF OPERATIONAL EXPERIENCE ARE CONSIDERED VALUABLE AND EFFECTIVE. SUCH A SYSTEM ENABLES ALL safety RELATED EVENTS TO BE ANALYZED, ROOT CAUSES DETERMINED AND CORRECTIVE AND PREVENTIVE ACTIONS IMPLEMENTED TO AVOID REPEAT EVENTS OR NEW EVENTS ROOTED IN THE SAME CAUSES. A METHOD ESTABLISHED UPON PROBABILITY safety ASSESSMENT CALLED PROBABILISTIC PRECURSOR EVENT ANALYSIS, IS ANALYZED BY INVESTIGATING DIFFERENT ACTIONS LED TO THE SOURCE DESTRUCTION ACCIDENT IN TEHRAN RESEARCH REACTOR (TRR). THE EVENTS AND CONDITIONS BEFORE THE ACCIDENT ARE REVIEWED TO FIND THE PRECURSORS OF THE ACCIDENT.FOR THE ABOVE ACCIDENT A SCENARIO IS DEVELOPED, AND CHAIN EVENTS ENDING UP TO THE ACCIDENT (WHAT) AND CAUSAL FACTORS LEADING TO EVENTS ARE IDENTIFIED. THESE CAUSAL FACTORS AND OTHER AVAILABLE INFORMATION AND DATA ARE ANALYZED TO DETERMINE THE WAY THAT ACCIDENT HAPPENED (HOW). SUBSEQUENTLY, THE CAUSES OF THE ACCIDENT (WHY) ARE IDENTIFIED AND DISCUSSED. FINALLY, APPROPRIATIVE CORRECTIVE MEASURES IDENTIFIED AS THE RESULT OF THIS ANALYSIS TO PREVENTING THE SIMILAR ACCIDENTS AND THESE CORRECTIVE EFFECTS ARE SHOWED NUMERICALLY USING PSA METHOD.

MANY RESEARCH REACTORS ARE USED ALL OVER THE WORLD. THERE IS MORE PROBABILITY OF ACCIDENT IN Nuclear RESEARCH REACTORS DUE TO THEIR BROAD APPLICATIONS IN A WIDE SPECTRUM OF SCIENCES AND INDUSTRIES.MATERIAL TEST REACTORS (MTR) TYPE ARE WIDE SPECIE OF Nuclear RESEARCH REACTORS IN WHICH MODERATOR IS LIGHT WATER AND THEIR FUELS ARE PLATE TYPE. CONSIDERING TO safety CRITERIA IN THE ENTIRE Nuclear RESEARCH REACTOR LIFETIME SUCH AS SITING, DESIGN, COMMISSION, UPGRADE, QUALITY ASSURANCE, REGULATION, MANAGEMENT OF safety AND OPERATION TO SATISFY THE DEFENSE IN DEPTH CRITERIA AND THEN safety INSURANCE IS NECESSARY.ENGINEERED safety FEATURES (ESFS) ARE ONE OF ITEMS CONSIDERED FOR ENHANCING REACTOR safety IN GENERAL AND ESPECIALLY SECOND SHUTDOWN SYSTEM (SSS) IS ONE ESF WHICH IS ABLE TO MAKE REACTOR SUBCRITICAL WHEN IT IS NECESSARY. IN THIS PAPER, THE TEHRAN RESEARCH REACTOR (TRR) IS SELECTED AS A MTR CASE STUDY AND ONE SSS IS STUDIED AND SHOWN THAT IT HAS A LITTLE DEFECT ON DIFFERENT CAPABILITIES AND CHARACTERISTICS OF THIS REACTOR SUCH AS NEUTRONICS CHARACTERISTICS, safety CRITERIA AND PERFORMANCE APPLICATIONS INSTEAD OF EQUIPPING IT WITH ANOTHER SHUTDOWN SYSTEM. THIS STUDY IS DONE USING PROBABILISTIC APPROACH USING MCNPX 2.6.0 CODE.

THE WORLD'S URANIUM RESOURCES IS ENOUGH TO SUPPLY 26 TIMES CURRENT ELECTRICITY CONSUMPTION OF THE WORLD FOREVER, AND THE Nuclear ENERGY WILL BE MORE ECONOMICAL RATHER THAN FOSSIL FUELS, EVEN IF THE COST OF ELECTRICITY PRODUCTION DOUBLES. BUT THE IMPORTANCE OF Nuclear safety AT ALL STAGES SUCH AS INTEGRATED PROGRAM, APPLYING STRATEGIES BEYOND THE PROBABILITY OF INCIDENTS, THE NECESSARY TRAINING FOR STAFF, THE CORRECT ARRANGEMENT OF PERSONNEL ACCORDING TO THEIR EXPERTISE AND THE OCCASION, TO PROVIDE IN TIME DECISION AT CRISIS, MODELING OF COUNTRIES AT THE HIGHER STAGES, ANALYSIS OF ERRORS IN RECENT ACCIDENTS IN ORDER TO AVOID IN FUTURE , OBSERVANCE OF Nuclear CONVENTION PRINCIPLES , AND FINALLY THE MANAGEMENT OF Nuclear ENERGY USE IS INEVITABLE AND MORE CONSIDERABLE THAN THE Nuclear ENERGY ITSELF. DISREGARDING OF THE CASE CAUSES NOT ONLY NO ADVANTAGES, BUT TO REGRESSION AND GOING BACK THE PROCEDURES. SO ITS CONSEQUENCES SUCH AS THOSE Nuclear TRAGEDIES IN RECENT DECADES; FOR HUMAN BEINGS AND LIVING THINGS ARE DEVASTATING AND IRREVERSIBLE. THE SURVEY INTENDS TO OBSERVE safety MECHANISM THAT DISREGARDING OF IT, LED TO 3 SEVERE Nuclear ACCIDENT IN RECENT YEARS. ALSO AT THE END OF THE RESEARCH SOME SOLUTIONS WILL BE PRESENTED.