pjs@aristotle.JPL.NASA.gov (Peter Scott) (09/25/90)
Thanks greatly to Jonathan Leech for resurrecting this for me. I think it will be of interest to those who were reading this thread. - - - - - - - I have a report from NUS Corporation, prepared for the DoE, titled Safety Status Report for the Ulysses Mission: Risk Analysis (Book 1). Document number is NUS 5235; there is no GPO #; published Jan 31, 1990. It's too lengthy to reproduce here, so I'll go into some highlights since the RTG issue is surfacing again. Mission phases 0 and 1 are on the launch pad or during early ascent in trhe KSC area. Phase 2 involves the possibility of a failure during later ascent with a reentry and impact along an African ground track. An accident during phases 3 & 4 can result in a reentry anywhere wihtin the 28 deg. latitude bands. No accident cases resulting in release in phase 0. Launch and ascent source terms calculated using LASEP-3 program, using a Monte Carlo approach to simulate RTG response to a given accident environment. 100,000 trials for each scenario or sub-scenario. The LASEP-3 model directs calculations to arrive at fueled clad distortion,... fueled clad crack size, fuel release quantity, and particle size distribution of the release. Integrated risk analysis constructed 5- and 95-%ile bounds on radiological consequences & risks. Based on NCRP report #91 denotes Negligible Individual Risk Level of 1e-7 per yr, == 1 millirem/yr. Also referred to as a "de minimis dose". Dosage considered As Low As Reasonably Achievable (ALARA) is 25 mrem/yr, and 100 mrem/yr as requiring remedial action. (Page missing) ... GPHS (General Purpose Heat Source) module are independent reentry bodies... resulting in broad footprint of impact points. No credit for dose reduction was taken for possible measures such as sheltering, clean-up, food restrictions. Because of the nature of the "weathering" process for ground deposited material, its availability for resuspension inhalation (the dominant long-term dose pathway) decreases dramatically over the first 2 years after an accident. ... Since the dose related deposition criteria of DoE and EPA are intended to ensure population protection over extended periods of time when administrative controls might not be assured, the ground deposition isopleths have been calculated for the second year after an accident when the weathering process has slowed considerably. Phase 1: begins with lift-off and ends with SRB burn-out and jettison. Accidents during this phase include SRB failures, RSS destruct, aft compartment explosion, vehicle breakup, and those leading to crash landing or ocean ditch. These accidents have a total probability of occurrence of 4.36e-3. Analysis shows that given an accident, a release of RTG fuel will occur about 4 times in 100,000. The radiological consequences of Phase 1 accidents are such that individual doses are < 10mrem, no collective dose will exceed de minimis, and therefore no health effects are calculated. No areas would have dose rates exceeding the EPA/DoE criteria, except for localized areas onsite near any ground-level releases. Phase 2: T+128 sec thru MECO, ET sep, until first OMS burn. Accidents occur above 150,000 ft & all scenarios considered result in vehicle breakup and release of GHPS modules. Fuel release can occur if GHPS modules impact hard rock along the trajectory over Africa. This can only occur if the accident occurs during a 5.5-sec interval near the end of the 404-sec phase. Accidents prior would result in RTG or GHPS modules impacting water with no release. The iridium fuel capsules are resistant to corrosion, and the solubility of PuO2 is very small. No measurable concentrations would be expected in water, even over very long time periods. Total initiating accident probability is 1.15e-3. Given an accident, a rock impact and fuel release involving one module would occur about 15 times in 10,000 with 0.834 Ci released. Total probability of 2.31e-6. Radiological consequences: individual doses <~20mrem, collective dose about 0.03 person-rem above de minimis. No areas have dose rates > EPA/DoE draft criteria, except perhaps localized area around impact locations. Calculated health effects essentially zero and risk near zero. An average population density is assumed in determining the separation distance between the release and the nearest people. Phase 3: T+532 sec thru IUS/PAM-S deployment. Accidents result in uncontrolled orbital decay re-entry of Orbiter, followed by Orbiter break-up and independent re-entry of the 18 individual GHPS modules. This could only occur if the failure was of such a nature that a mission abort from orbit to a safe landing was not possible. Fuel release would occur only if one or more GHPS modules impacted hard rock. Modules impacting oceans would not release fuel over long periods of time. The initiating accident probability is 1.58e-4. Given the accident, a rock impact and fuel release involving one module would occur about 4 times in 100 with an average release of 0.477 Ci. Total prob. 6.16e-6. Radiological consequences: individual doses < 40mrem, collective dose above de minimis < 1 person-rem. No areas exceed DoE/EPA, except localized etc. Calculated health effects & risk essentially zero. Consequences are higher due to increase in average population density, even though source term is lower [as shown in tables & failure trees, not included here]. Phase 4: IUS deploy thru escape velocity. Accidents resulting from IUS malfunctions lead to spacecraft reentry, breakup, and independent reentry of GHPS modules. Same as phase 3 with total initiating accident probability for a reentry of 6.16e-3. Given an accident, rock impact and fuel release involving 1 or more modules would occur about 4 times in 100. Identical radiological consequences. Total prob. 2.40e-4. - - - - Elision of scientific justification in the above should not be construed as meaning it wasn't done: I just summarized in a very rough fashion. I think it's interesting that the RTG modules would apparently survive intact any reentry, and only break when they hit rock. I don't expect this will convince the fanatics, but there's some interesting information for the rest of us. -- This is news. This is your | Peter Scott, NASA/JPL/Caltech brain on news. Any questions? | (pjs@aristotle.jpl.nasa.gov)