Accidents, nuclear power

We discussed the sources of artificial occurrence of " Tc at the beginning of this chapter and demonstrated that the nuclear fuel cycle is the predominant source of Tc in the environment. Other, much less important, sources are the fallout from nuclear weapons testing, the Chernobyl accident, nuclear power production and the radiopharmaceutical use of the metastablc "Tc decaying to ground state Tc. The natural occurrence of Tc formed in the earth s crust by spontaneous fission of and neutron-induced fission of in uranium ores are negligible. 

Nuclear power has achieved an excellent safety record. Exceptions are the accidents at Three Mile Island in 1979 and at Chernobyl in 1986. In the United States, safety can be attributed in part to the strict regulation provided by the Nuclear Regulatory Commission, which reviews proposed reactor designs, processes appHcations forUcenses to constmct and operate plants, and provides surveillance of all safety-related activities of a utiUty. The utiUties seek continued improvement in capabiUty, use procedures extensively, and analy2e any plant incidents for their root causes. Similar programs intended to ensure reactor safety are in place in other countries. 

A technique called probabiUstic safety assessment (PSA) has been developed to analy2e complex systems and to aid in assuring safe nuclear power plant operation. PSA, which had its origin in a project sponsored by the U.S. Atomic Energy Commission, is a formali2ed identification of potential events and consequences lea ding to an estimate of risk of accident. Discovery of weaknesses in the plant allows for corrective action. 

The accident at the Three Mile Island (TMI) plant in Pennsylvania in 1979 led to many safety and environmental improvements (4—6). No harm from radiation resulted to TMI workers, to the pubHc, or to the environment (7,8), although the accident caused the loss of a 2 x 10 investment. The accident at the Chernobyl plant in the Ukraine in 1986, on the other hand, caused the deaths of 31 workers from high doses of radiation, increased the chance of cancer later in life for thousands of people, and led to radioactive contamination of large areas. This latter accident was unique to Soviet-sponsored nuclear power. The Soviet-designed Chemobyl-type reactors did not have the intrinsic protection against a mnaway power excursion that is requited in the test of the world, not was there a containment building (9—11). 

The NRC safety goal can be evaluated by comparison to the risks from accidents incurred from other human activities (Eig. 2) (29). The safety goal and the safety record of the nuclear power industry indicate much lower societal risks from commercial nuclear power than from a wide range of other common human activities. 

Nuclear power plants of the future are to be designed and operated with the objective of better fiilfiUing the role as a bulk power producer that, because of reduced vulnerabiUty to severe accidents, should be more broadly accepted and implemented. Use of these plants could help stem the tide of environmental damage caused by air pollution from fossil-fuel combustion products (64). 

Reactor Safety Study An Assessment of Accident Risks in U.S. Commercial Nuclear Power Plants, Report WASH-1400 (NUREG-75/014), U.S. Nuclear Regulatory Commission, Washington, D.C., Oct. 1975. 

X Bq of Pu has been released, mostiy from bum-up of the nuclear powered sateUite SNAP-9a and that 3.7 X 10 Bqof + ° Pu was released by the Chernobyl accident (167,168). Many studies have been done to determine the cumulative fallout on sods, plants, bodies of water, animals, and humans. For example, the cumulative Pu fallout ia forest and grasslands and ia the Hver of elderly humans ia Bavaria, Germany are approximately... 

Investigation of Potential Hazar ds fr om the Oper ations in the Canvey Island/ Thurrock Ar ea, HMSO, London, 1978. Rasmussen, Reactor Safety Study An Assessment of Accident Risk in U.S. Commer cial Nuclear Power Plants, WASH-... 

State intervention in man s activities to protect the health of the inhabitants goes back to prehistory. The motivation may not have been altogether altruistic the king acted to protect his subjects because he regarded them as his property. Public health protection began for disease control. With industrialization, came the need for control of even more hazardous forces and substances. This extended protection became technological in accident analysis and response. Present efforts in controlling risk, such as from nuclear power, are a continuation of this development. 

One of the products of a nuclear power plant PSA is a list of plant responses to initiating events (accident starters) and the sequences of events that could follow. By evaluating the significance of the identified risk contributors, it is possible to identify the high-risk accident. sequences and take actions to mitigate them. 

Figure 1.4.3-1 from WASH-1400 compares the risk of 100 nuclear plants with other man-caused risks. This is a CCDF that gives the frequency per year that accidents will L-xcccd a value on the abscissa. For example, for 100 fatalities, the frequency that 100 nuclear power plants could do this is lE-4, air crashes to persons on the ground lE-2, chlorine releases 1. IE-2, dam failures 7E-2, explosions SF-2, fires 1. IE-1, air crashes (total) 5E-1, and total man-caused 9E-1,... 

The risk to an average individual in the vicinity of a nuclear power plant of prompt fatalities that might result from reactor accidents should not exceed 0.1% of the sum of prompt fatality ri.sks from other accidents to which members of the U.S. population are generally exposed. ... 

Hazards and Operability (HAZOP) analysis is an accident detection and prevention technique used primarily by the chemical process industry (CPI) (Lees, 1980). Even though the CPI operates in a different regulatory environment from the commercial nuclear power industry, the goals of risk reduction while maintaining productivity are similar. 

Humans control all chemical and nuclear processes, and to some extent all accidents result from human error, if not directly in the accident then in the process design and in the process inadequate design to prevent human error. Some automatic systems such used in nuclear power reactors because the response time required is too short for human decisions. Even in these, human error can contribute to failure by inhibiting the systems. 

Human error contributed to about 50% of the accident sequences m the RSS but none of the human error data came from the nuclear power industry. Furthermore, very high failure rates 0.5 to 0.1/action) were predicted but are not supported by the plant... 

Nuclear power plant systems may be classified as "Frontline" and "Support. . iccurding to their. service in an accident. Frontline systems are the engineered safety systems that deal directly with an accident. Support systems support the frontline systems. Accident initiators are broadly grouped as loss of cooling accidents (LOCAs) or transients. In a LOCA, water cooling the reactor is lost by failure of the cooling envelope. These are typically classified as small-small (SSLOCA), smalt (SLOCA), medium (MLOCA) and large (LLOCA). 

The preceding overviewed the operation and engineered safety features of current and advanced LWRs. Before preceding to describe how PSA is performed on nuclear power plants, two accidents are described that have profoundly affected the industiy... 

On March 28, 1979, the accident began which greatly affected the use of nuclear power in the U.S. and throughout the world, although the effects did not exceed regulations. 

Chernobyl may represent the upper limit that is possible in a nuclear power plant accident. 

In the WASH-1400 analyses of nuclear power accidents, it was calculated that it is possible to overpressure and rupture the containment. Discuss whether this is better or worse than a pressure relief that releases radioactivity but prevents the pressure from exceeding the rupture... 

Suppose an interstate highway passes 1 km perpendicular distance from a nuclear power plant control room air intake on which 10 trucks/day pass carrying 10 tons bf chlorine each. Assume the probability of truck accident is constant at l.OE-8/mi, but if an accident occurs, the full cargo is released and the chlorine flashes to a gas. Assume that the winds are isotropically distributed with mean values of 5 mph and Pasquill "F" stability class. What is the probability of exceeding Regulatory Guide 1-78 criteria for chlorine of 45 mg/m (15 ppm). 

Two studies resolved the Unresolved Safety Issue A-44, "Station Blackout." The first siudy, The Reliability of Emergency AC Power Systems in Nuclear Power Plants," when combined uh die lelevant loss-oToffsite-power frequency, provides estimates of station-blackout frequencies lor 18 nuclear power plants and 10 generic designs. The study also identified the design and operational features most important to the reliability of AC power systems. The second study, "Station Blackout Accident Analysis" (NUREG/CR-3226), focused on the relative importance to risk of laiion blackout events and the plant design and operational features that would reduce this risk. 

On August 8, 1985, the U.S. Nuclear Regulatory Commission (NRCf requested the operators of nuclear power plants in the U.S. to perform Individual Plant Examinations (IPE) on their plants. IPEs are probabilistic analyses that estimate the core damage frequency (CDF) and containment performance for accidents initiated by internal events (including internal flooding, but excluding internal fire). Generic Letter (GL) 88-20 was issued to implement the IPE request to identify any plant-specific vulnerabilities to severe accidents and report the results to the Commission. ... 

Initiating events, in this study, initiate plant scram or setback. Other initiators, such as refueling discharge accidents, do not necessarily cause a reactor shutdown but may lead t< minor fuel damage and radioactive releases. The list of initiators for nuclear power plants has litf ance for HFBR because of size and design differences. A list of HFBR-specific initiators was developed from " st prepared with the HFBR staff, the FSAR, the plant design manual, the procedures manual, techn specifications, monthly operating reports, and the HFIR PRA (Johnson, 1988). 

An Approach to Quantitative Safety Goals for Nuclear Power Plants, October 1980. Rasedag, W. F. et al., Regulatory Impact on Nuclear Reactor Accident Source Term Assumptions, June 1981. 

Emrit, R. et al., A Prioritization of Generic Safety Issues, NUREG-0933 suppliment. Reassessment of the Technical Bases for Estimating Source Terms, Draft, May 1985. Baranowsky, P.W., Evaluation of Station Blackout Accidents at Nuclear Power Plants, May 1985. 

Generic Letter 88-20 IPE for Severe Accident Vulnerabilities at Nuclear Power Plants, NRC, December 1,... 

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